NIST Technical Note 1768

Measurement of Temperature, 

Displacement, and Strain in Structural 

Components Subject to Fire Effects: 

Concepts and Candidate Approaches 

Therese McAllister 

William Luecke 

Mark Iadicola 

Matt Bundy 

http://dx.doi.org/10.6028/NIST.TN.1768
NIST Technical Note 1768 

Measurement of Temperature, 

Displacement, and Strain in Structural 

Components Subject to Fire Effects: 

Concepts and Candidate Approaches 

Therese McAllister 

Materials and Structural Systems Division 

Engineering Laboratory 

William Luecke 

Mark Iadicola 

Metallurgy Division 

Material Measurement Laboratory 

Matt Bundy 

Fire Research Division 

Engineering Laboratory 

http://dx.doi.org/10.6028/NIST.TN.1768

November 2012 

U.S. Department of Commerce 

Rebecca Blank, Acting Secretary 

National Institute of Standards and Technology 

Patrick D. Gallagher, Under Secretary of Commerce for Standards and Technology and Director 
Certain commercial entities, equipment, or materials may be identified in this 

document in order to describe an experimental procedure or concept adequately. 

Such identification is not intended to imply recommendation or endorsement by the 

National Institute of Standards and Technology, nor is it intended to imply that the 

entities, materials, or equipment are necessarily the best available for the purpose. 

National Institute of Standards and Technology Technical Note 1768

Natl. Inst. Stand. Technol. Technical Note 1768, 73 pages (November 2012) 

http://dx.doi.org/10.6028/NIST.TN.1768

CODEN: NTNOEF 

i 

Executive Summary 

For the last forty years, NIST has led the world in fire metrology through 

research conducted at the Large Fire Laboratory, which is being expanded to 

enable experiments on real-scale structures under combined structural and fire 

loads. These two capabilities will be joined in the National Fire Research 

Laboratory (NFRL), which is expected to be completed in 2013. 

Measurements of temperature, displacement, and strain at hundreds of points 

on a structural system in the fire zone are needed to validate analytical tools 

for structural response in fire conditions. However, the ability to measure the 

performance of structures during realistic fire exposures is severely limited 

due to a significant gap in measurement science. 

At present, temperatures are measured with thermocouples and strains are 

measured with high-temperature strain gages. Each of these sensors requires a 

separate signal path for data collection during the experiment. Further, hightemperature strain gages are unreliable and often do not perform as expected 

during fire tests. Significant improvements to structural measurement in fire 

conditions are needed to advance the validation of analytical tools and 

performance based design methodologies. 

A set of desired performance criteria for structural-fire measurements 

were established to guide the assessment and development of candidate 

technologies and measurement systems. Rather than having one measurement 

per sensor/data line, measurements over an entire surface or multiple sensors 

per data line are desired. The ability to record tens to hundreds of data points 

per line will produce the quantity and quality of data needed for 

understanding structural behavior and validating analytical tools. 

Measurements are necessary at temperatures as high as 750 °C, and they must 

be insensitive to heating rate. Displacement measurements can range from 

1 mm to 1 m. Elastic strain measurements up to e = 0.002 with a resolution of 

±200 μm/m and plastic strain measurements up to e = 0.2, with a resolution 

better than ±2000 μm/m are the desired measurement capabilities. 

Candidate methods that could meet these performance requirements were 

reviewed. While technologies that could provide multiple measurements at 

elevated temperatures were the primary focus, technologies that could provide 

single-point measurements at elevated temperatures were also considered. 

Candidate methods for temperature measurement include 

thermocouples, infrared imaging sensors, and fiber-optic sensors. 

Candidate methods for displacement measurement include linear 

transducers, laser distance measurement, and digital image 
ii

correlation techniques. 

Candidate methods for strain measurement include traditional 

high-temperature strain gages, digital image correlation 

techniques, and fiber-optic sensors. 

A demonstration test that employed a natural gas burner in the Large Fire 

Facility evaluated the potential of digital image correlation and hightemperature strain gages to measure thermally induced strains. Digital image 

correlation successfully measured the resulting thermal expansion strain of a 

plate heated by fire; the strain gages did not provide meaningful data in fire 

conditions. 

The technology review and the outcome of the demonstration test indicate 

that digital image correlation and fiber-optic methods have great promise for 

temperature, displacement, and strain measurement in the NFRL, but many 

challenges must be overcome. For digital image correlation, the challenges 

include ensuring the integrity of the patterned coating, forming usable images 

in spite of the optical radiation from the fire, processing to minimize image 

changes due to turbulence, and coping with smoke and soot deposition. For 

fiber-optic sensors challenges include preserving the integrity of the sensor at 

high temperature and ensuring that the strain in the member is transferred to 

the fiber. A method to deconvolve the contributions of thermal, elastic, and 

plastic strains is needed for both measurement technologies. 

A four-stage development plan is proposed to overcome these challenges. 

Two early stages develop the measurement science with simple heating and 

deformations. The intermediate stage employs more realistic fires and more 

complicated deformations. In the final stage, the measurement uncertainty of 

both methods will be quantified. 

If the proposed research is successful, the resulting structural metrology in 

fire will greatly enhance the research capabilities of the NFRL. By starting 

this new line of foundational research, structural measurements in the fire 

zone could be propelled from virtually nonexistent to full-field measurements. 

Keywords: structural fire tests; strain measurement; displacement 

measurement; temperature measurement; fiber-optic; digital image 

correlation. 
iii 

Acknowledgements 

Many colleagues, both inside and outside NIST contributed to this report. 

NIST colleagues include Francine Amon, Peter Bajcsy, Rodney Bryant, 

Stephen Cauffman, Artur Chernovsky, Ashu Gola, Sae Woo Nam, Long Phan, 

Kuldeep Prasad, Richard Rhorer, and Eric Whitenton. Colleagues outside of 

NIST include Pedro Calderon at the Polytechnic University of Valencia, 

Gustave Fralick at NASA, Otto Gregory at the University of Rhode Island, 

Alex Sang at Luna Technology, Anbo Wang at Virginia Tech University, and 

Vince Wnuk at HiTec Products. 
iv

Contents 

1. Introduction.............................................................................................. 1

2. Measurement Methods in Structural-Fire Test Laboratories ................... 4

2.1. Michigan State University................................................................. 4

2.2. Purdue University ............................................................................. 4

2.3. National Research Council ................................................................ 5

2.4. University of Coimbra ....................................................................... 5

2.5. Centre Technique Industriel de la Construction Metallique ............ 6

2.6. British Research Establishment, Cardington Facility ........................ 6

2.7. Architecture and Building Research Institute ................................... 6

2.8. Korea Institute of Construction Technology ..................................... 6

2.9. Summary of Current Practice for Structural Fire Measurement 

Methods ....................................................................................................... 7

3. Structural Measurement Needs for Fire Conditions ................................ 9

4. Candidate Measurement Technologies and Concepts for Fire Conditions

12

4.1. Temperature Measurements .......................................................... 12

4.1.1. Thermocouples ....................................................................... 12

4.1.2. Infrared Sensors ..................................................................... 12

4.1.3. Fiber-optic Sensors ................................................................. 13

4.1.4. Summary of Temperature Measurement Technology............ 15

4.2. Displacement Measurements ........................................................ 16

4.2.1. Linear Transducers.................................................................. 16

4.2.2. Lasers ...................................................................................... 17

4.2.3. Digital Image Correlation (DIC) Systems ................................. 17

4.2.4. Summary of Displacement Measurement Technology .......... 19
v

4.3. Strain Measurements ..................................................................... 20

4.3.1. Strain Gages ............................................................................ 20

4.3.2. Digital Image Correlation ........................................................ 21

4.3.3. Fiber-optic Sensors ................................................................. 21

4.3.4. Summary of Strain Measurement Technology ....................... 21

5. Digital Image Correlation and Strain Gage Demonstration Test ............. 23

5.1. Background and Goals .................................................................... 23

5.2. Summary of the Demonstration Test ............................................. 23

5.2.1. Test Configuration .................................................................. 23

5.2.2. Results .................................................................................... 25

5.3. Summary of Demonstration Test Findings, Conclusions, and 

Recommendations ..................................................................................... 26

6. Candidate Technologies for Next-generation Measurements of 

Structural Response in Fire............................................................................. 28

7. Proposed Development Plan .................................................................. 31

8. Summary ................................................................................................ 34

9. References .............................................................................................. 36

Appendix A. Results and Analysis of the Demonstration Test......... ....40 

1

1. Introduction 

Temperature, displacement, and strain measurements of structural 

components and systems are needed to fully understand the performance of 

structural systems in fire and to validate analytical models of structural 

response to fire effects. Present-day tools, such as the prescriptive ASTM 

E119 standard, provide comparative fire ratings between assemblies, but no 

useful measured data about structural performance. Measurements of 

structural performance in fire conditions are needed to validate analytical 

tools and to develop performance-based design methodologies. 

Most structural fire resistance experiments are conducted either in 

furnaces or for compartment fire conditions. For most experiments, structural 

displacements and strains are not measured in the heated zone of a fire test, 

where transient gas temperatures may reach 1400 °C. Instead, structural 

displacements are measured outside the heated zone at unheated slab surfaces 

or at unheated column ends, and strains are not usually measured. 

Measurement techniques for structural response at room temperature and 

with thermocouples for temperature are well developed, but measurement 

techniques for structural response in the fire zone at elevated temperatures are 

lacking. For example, Figure 1 shows a composite floor truss system 

constructed to replicate the World Trade Center (WTC) towers floor system 

that was tested according to the ASTM E119 standard. Heat flux, gas 

temperatures, steel surface temperatures, and vertical displacement at nine 

locations on the unheated side of the floor slab were measured; no local 

deformation or strain measurements were obtained. These state-of-the-art 

measurements are inadequate for understanding structural behavior in fire 

conditions and for validating analytical tools. 

Measurements under realistic fire conditions require instrumentation that 

can withstand elevated temperatures and the evolving fire environment (for 

example varying states of smoke, convective air distortions, and soot), while 

providing accurate, reliable, and repeatable measurements. Additionally, 

strain measurements need to be resolved into thermal and mechanical 

components. Total strain measurements may comprise thermal strain, elastic 

strain, plastic strain, and creep strain, each of which depends on temperature. 

Room-temperature strain and displacement measurement devices 

primarily measure response at discrete points which must be selected a priori, 

based on either experience or a predictive analysis model. The discrete 

measurement devices are located at select locations, and may miss critical 
2

data, reducing the value of expensive, complex laboratory experiments and 

potentially result in erroneous conclusions. The likelihood of missing critical 

locations during fire tests is even greater, due to thermally induced forces that 

evolve during the fire exposure. In such situations, field measurement systems 

that record structural response data across surfaces (for example a camerabased measurement system) or along/through components (for example a 

fiber-based measurement system) would provide a much richer data set. Such 

systems have been developed for room-temperature applications and would 

provide a tremendous leap in structural-fire measurement capabilities. 

Figure 1. Scaled model of the composite floor system in the WTC Towers 

during an ASTM E119 test. Note deformations in truss web members, which 

were observed but not measured. 

The ability to measure displacement, strain, and temperature across 

structural components during fire exposure will: 

• provide comprehensive experimental data on the structural performance 

of components and connections in systems under realistic fire conditions 

and gravity loads, 

• support the validation of physics-based analytical tools, 
3

• provide the technical basis for the development of performance-based 

standards for the design of fire resistant structures, 

• foster innovation in design and construction, and 

• lead to the development of improved building materials, standards, design 

practice, and building codes. 

The specific challenges of developing structural response measurements 

for fire conditions are primarily to develop, through adaptation or innovation, 

field measurement technologies for ambient applications to the harsh 

environment of fire and to quantify measurement resolution and uncertainty 

as a function of temperature. 

The NIST National Fire Research Laboratory (NFRL) will be available 

for conducting full-scale structural-fire tests with realistic fire and boundary 

conditions in 2013. The utility of the facility and test data will be greatly 

increased with accurate measurements of the structural response 

(displacements and strains at known temperatures) in fire. 
4

2. Measurement Methods in Structural-Fire Test 

Laboratories 

The review of structural measurement methods under fire conditions 

included determining which, if any, methods are used at other structural fire 

test facilities. Table 1 lists the displacement and strain measurement methods 

used at Michigan State University, Purdue University, University of Coimbra, 

Portugal, Centre Technique Industriel de la Construction Metallique 

(CTICM), France, the National Research Council (NRC) of Canada, the 

British Research Establishment (BRE) Cardington Facility, the Architecture 

and Building Research Institute (ABRI) Fire Research Facility, Taiwan, and 

the Korean Institute of Construction Technology (KICT), Fire Research 

Center. This is a representative list of structural fire testing capabilities around 

the world. A recent NIST publication [1] summarizes the capabilities of other 

fire test facilities. All of the test facilities listed in Table 1 measure gas and 

structural temperatures with thermocouples inside the furnaces, as they must 

meet the standard fire heating requirements. 

2.1. Michigan State University 

The furnace housed at the Civil Infrastructure Laboratory at Michigan 

State University is capable of testing loaded structural assemblies such as 

columns, beams and floor systems under fire conditions [2]. It consists of a 

steel framework supported by four columns with the furnace chamber of 

about 2.5 m by 3 m. Two small view ports on either side of the furnace wall 

allow visual monitoring of the fire-exposed surface during tests. 

Several measurement methods have been used for obtaining structural 

displacement and strain data [2,3,4]. Displacements are measured with Linear 

Variable Differential Transformer (LVDT) devices outside of the heated zone. 

Laser-based devices did not work in a fire environment due to fire spectrum 

interference at the laser frequency. Strain measurements of steel components 

were attempted with high-temperature strain gages. They failed in the first 

20 minutes of an ASTM E119 fire exposure, when temperatures reached 

400 °C to 600 °C. Bonding adhesion of the strain gages to steel surfaces was 

also a problem at elevated temperatures. The strain gages needed to be welded 

for temperatures up to 800 °C. Concrete strain gages worked until the 

concrete cracked, which released the local strain. 

2.2. Purdue University 

Purdue University researchers designed a system of heating panels to 
5

simulate the heating effects of fire. The panels have electrical coils, and are 

placed close to surfaces being tested. The heating system is being used to test 

full-scale steel columns at the Bowen Laboratory for Large-Scale Civil 

Engineering Research. Test structures can also be subjected to forces with 

hydraulic equipment to simulate loads experienced in real structures [5]. 

A Complementary Metal Oxide Semiconductor (CMOS) digital camera is 

used for close range photogrammetry to measure displacement and strain 

during tests [6]. The camera-based method was calibrated to determine its 

accuracy and reliability for a monochrome 8-bit camera (640 by 480) pixels 

with a 75 mm lens, a view distance of 0.4 m (15 in.), and a field of view of 

25 mm by 7 mm (1 in. by 0.75 in.). Displacements were obtained with an 

accuracy of 5e-3 mm (2e-4 in.). Strain measurements were derived as average 

strains using three target points and relative displacements between the points. 

A strain gage was also applied to the back of the test samples at the target 

points. Overall, the stress-strain curve was captured quite well, including the 

yield point and plastic strains up to 0.04. The accuracy of the strain 

measurements was a function of the displacement measurement accuracy and 

the spacing of the points. 

2.3. National Research Council 

The National Research Council (NRC) in Canada has a Fire Resistance 

and Performance of Structures program with fire test facilities that include 

column, wall, and floor test furnaces, and a large burn hall/smoke tower 

complex that is 55 m long, 30 m wide, and 12.5 m high. Fire test facilities 

include a column test furnace with hydraulic jacks that can load along three 

principal axes and a three-story wall test in the burn hall [7]. NRC is 

exploring the use of camera-based and fiber-optic systems for measuring 

structural displacements and strains under fire conditions [8]. 

2.4. University of Coimbra 

A large experimental program on the fire resistance of steel and composite 

steel and concrete columns with restrained thermal elongation [9] was carried 

out at the Laboratory of Testing Materials and Structures of the Department of 

Civil Engineering of the Faculty of Sciences and Technology of the 

University of Coimbra in Portugal. Their fire test facility has a furnace with a 

reaction frame for testing structural components and assemblies. 

Displacements are measured outside the furnace. A refractory element for 

high-temperature conditions is welded to the test assembly, and a commercial 

transducer at the furnace window measures the deformation. Structural strains 
6

are not measured inside the furnace. The use of high-temperature strain gages 

was abandoned after poor outcomes during the Cardington tests (Section 2.6).

2.5. Centre Technique Industriel de la Construction Metallique 

The Centre Technique Industriel de la Construction Metallique (CTICM) 

conducts research to support the steel construction industry in France. The 

Fire Research Section conducts research projects to improve fire safety and 

develop computational tools based on fire test programs. CTICM has seven 

furnaces for fire testing, but also conducts full scale fire tests as needed. 

CTICM measured temperatures in the heated zone and displacements outside 

the heated zone during fire tests in a full-scale parking structure; strains were 

not measured [10]. 

2.6. British Research Establishment, Cardington Facility 

In 1995 and 1996, seven fire tests were carried out on an eight-story, steelframed building with composite metal deck floors at the Building Research 

Establishment (BRE) at the Cardington Large Building Test Facility. The 

displacement and strain response of the structural components subjected to 

fire conditions were measured primarily outside of the heated zone [11]. 

Vertical displacements were measured at the top (unexposed) surface of 

the concrete slab to determine primary beam deflections. Column 

displacements were measured relative to other (unheated) columns in the 

building. Rotations at each of the main connections within the test 

compartments were also measured. Strains were measured with a mix of 

ambient temperature gages outside of the heated zone and high-temperature 

gages within the heated zone. 

2.7. Architecture and Building Research Institute 

The Fire Research Facility of the Architecture and Building Research 

Institute (ABRI), Taiwan has a large-scale furnace, 4 m × 8 m in cross-section 

and 5 m in height, to test beam-column assemblies and floors [12]. The 

furnace is equipped with a 500-ton hydraulic system to apply axial loads to

the specimen while it is exposed to fire. Temperatures, axial loads, and axial 

deformations outside the heated zone are monitored during fire exposure [13]. 

2.8. Korea Institute of Construction Technology 

The Fire Resistance Laboratory of the Korea Institute of Construction 

Technology (KICT) has several furnaces for building and tunnel fire 
7

research [14]. The column furnace can test a column up to 5 m in height and 

apply a 1000-ton axial load. The furnace for floor systems (slab and beams) 

can test specimens up to 10 m long and apply loads up to 100 tons. The wall 

furnace can test wall specimens that are 4 m by 4 m and has a 100 ton load 

capacity. Temperatures and deformations outside the heated zone are 

measured during fire exposure [15]. 

2.9. Summary of Current Practice for Structural Fire 

Measurement Methods 

In structural-fire test facilities around the world, quantitative structural 

measurements are made outside the heated area, with the exception of a few 

strain measurements at pre-selected points. The Cardington full-scale fire 

experiments used many room-temperature and high-temperature strain gages 

with varying degrees of performance. Other researchers reported that hightemperature gages began to fail at temperatures of 300 °C—temperatures that 

are well below temperatures reached in fire conditions. 

Structures exposed to fire may also have thermally induced loads and 

failure mechanisms in addition to those that occur at room temperature. 

However, there is little experimental data to validate analytical models of 

structural behavior in fire conditions, as there are no reliable methods for 

measuring displacements and strains across components and subsystems. To 

advance understanding of how structural systems perform under fire 

conditions, methods to obtain quantitative measurements of temperaturedependent structural system response need to be developed. 
8

Table 1. Displacement and strain measurement methods in structural fire test 

facilities.

Test Facility Displacement 

Measurements

Strain Measurements

Michigan State University 

Furnace with applied 

structural loads

LVDT devices outside 

of the heated zone

Laser-based devices 

did not work in a fire 

environment 

Not measured, have 

tried hightemperature strain 

gages and concrete 

strain gages

Purdue University 

Heating elements applied to 

structural components

CMOS digital camera 

for 0.4 m (15 in.) focal 

length

Computed from 

displacements

NRC, Canada 

Multiple furnaces; fire 

laboratory (55 m x 30 m x 

12.5 m) 

Measured outside 

heated zone

High-temperature 

strain gages, fiberoptic sensors

University of Coimbra, 

Portugal 

Furnace with reaction frame

Refractory element 

and camera system

Not measured, have 

tried hightemperature strain 

gages

CTICM, France

Seven furnaces; parking 

structures with real fire

Measured outside 

heated zone

Not measured

BRE Cardington, England

Eight-story steel structure 

with real fire in 

compartments

Measured outside 

heated zone

High-temperature 

strain gages

ABRI, Taiwan

Beam/column furnace (7 m 

x 5 m x 9 m) with a 500 ton 

actuator

Measured outside 

heated zone

None listed

KICT Fire Research Center, 

Korea 

column, floor, and wall 

furnaces with axial load 

capabilities

Measured outside 

heated zone

None listed
9

3. Structural Measurement Needs for Fire Conditions 

A list of desired performance criteria was developed to guide the review 

of existing technology and the search for possible innovations that meet the 

demands of a fire environment. The rationale for the criteria is provided here, 

and summarized in Table 2. These criteria may not be fully achievable, but 

even partial fulfillment of the criteria listed in Table 2 will significantly 

advance structural response measurements in fire. 

Table 2. Desired performance criteria for temperature, displacement, and 

strain sensors.

• Gas temperature measurements from 20 °C up to 1400 °C for a heating 

duration of four hours with a known resolution and uncertainty

• Structural response measurements in steel and concrete components up 

to component temperatures of 750 °C 

• Method is insensitive to rate of heating and cooling and has the ability to 

measure in fire environment with smoke and or soot

• Continuous measurement over a surface area or along a line preferred 

over point measurements

• Displacement measurements that range from 1 mm to 1 m with a 

resolution of 0.02 mm (0.001 in) to 10 mm (0.4 in)

• Elastic strain measurements that range from 10 μm/m to 2 000 μm/m 

(0.2 % strain) with a resolution of 1 μm/m to 200 μm/

• Plastic strain measurements from 2000 μm/m (0.2 % strain) up to 

200 000 μm/m (20 % strain) with a resolution of 20 μm/m to 

2 000 μm/m 

Gas temperatures from ambient conditions to 1400 °C should be measured 

with a known resolution and uncertainty. An upper bound of 1400 °C for gas 

temperatures is based on expected peak temperatures while burning liquid 

fuels. Gas temperatures for combustible cellulosic and plastic materials 

produce peak temperatures in the range of 900 °C to 1100 °C in postflashover enclosure fires. Pool fires with liquid fuels typically produce 

1200 °C temperatures, with an upper bound of 1400 °C. Flame-impingement 

temperatures may reach 1800 °C, but since the duration is on the order of 

seconds, there is little effect on the temperature of structural components. 
10 

The response of structural components should be measured up to 750 °C 

for steel and concrete materials. Above 700 °C, structural steel undergoes a 

eutectoid phase that affects stiffness and yield strength properties. Less than 

20 percent of room temperature strength remains above 700 °C [16]. Concrete 

dehydrates as its temperature rises and retains approximately 20 to 40 percent 

of its original strength at 750 °C [16]. 

Measurements of structural response to fire should be insensitive to the 

rate of heating and cooling, so that the measurement accuracy is only a 

function of temperature. Fire environments vary during experiments, 

depending on the fuel type, ventilation, and whether smoke and hot gases are 

retained in compartments or other enclosed areas. A clean source of heat, such 

as natural gas, produces a fire without soot or smoke particles. Other fuels, 

such as heptane, or incomplete combustion of combustible materials produce 

smoke and soot particles. Hot soot particles re-radiate within the smoke layer 

and create a hot upper gas layer in compartments. Re-radiation can modify the 

gas temperatures within the compartment, and consequently, the heating of 

structural members. Thus, an enclosure fire with cellulosic and plastic 

contents differs from a quasi-steady-state natural gas or pool fire in the 

heating rate, peak temperatures, and heating duration of structural 

components. Therefore, continuous measurements over a surface area or 

along a line across a component, such as a beam or slab section, would 

provide a tremendous advantage over point measurements in understanding 

structural system response to fire. 

The range of measurements that may be needed at elevated temperatures 

is based on a combination of typical ambient responses and possible damage 

or failure mechanisms. For instance, a floor section with a 10 m to 15 m floor 

span may initially deflect on the order of a centimeter under gravity loads and 

then sag at elevated temperatures on the order of a meter. Therefore, the 

capability to measure displacements from 1 mm to approximately 1 m with a 

resolution of 0.02 mm (0.001 in) to 10 mm (0.4 in) is desirable. Similarly for 

strain measurements, linear elastic and plastic strains up to approximately 

0.02 are expected for structural response for ambient conditions. Plastic 

strains may be greatly increased as structural components heat. While the 

extent of possible plastic strain is not known, an upper bound is expected to 

be on the order of 0.20. Therefore, the capability to measure linear strain 

should range from 10 μm/m to 2 000 μm/m (0.2 % strain) with a resolution of 

1 μm/m to 200 μm/m. Plastic strain measurements should potentially range

from ~2 000 μm/m (0.2 % strain) up to 200 000 μm/m (20 % strain) with a 

resolution of 20 μm/m to 2 000 μm/m.

Impediments to measuring temperatures, displacement, and strains in a 
11

fire environment include issues related to visibility, attachment, and radiation 

effects. Interference with visual methods of measurement (e.g., camera, laser, 

infrared, etc.) occurs when wavelengths are reflected or absorbed by smoke 

particles, distorted by thermal gradients and air column turbulence, or masked 

by soot deposition on surface marking or tags. Many methods of sensor 

attachment to structural components at elevated temperatures do not perform 

well at elevated temperatures. A key issue is the characterization of shear 

transfer from the structural component to the sensor, through the attachment 

mechanism. Epoxy adhesives fail above approximately 300 °C. While 

ceramic adhesives are reported to have higher operating temperatures 

(1000 °C), they need to be tested in a fire environment. Sensor attachment by 

welding to steel components or by embedment in a concrete section may be 

required. Methods may also be needed to thermally protect instrumentation 

for a range of fire size, duration, and radiative effects. 

Equipment costs and total experimental costs should also be considered 

when developing and evaluating sensors. For instance, disposable (one-timeuse) sensors may be desirable for conditions where heat exposure cannot be 

controlled or is expected to exceed sensor capacity. Ideally, sensor pricing 

should be moderate compared to the total experimental costs. 
12 

4. Candidate Measurement Technologies and 

Concepts for Fire Conditions 

Candidate techniques for measuring temperatures, displacements, and 

strains in fire conditions are presented. The measurement techniques are 

described as point, field, or surface methods. Point methods obtain data at a 

single location with a dedicated data line. Field methods obtain data at 

multiple points along a data line. Surface methods obtain measurement data 

across a surface or area. 

4.1. Temperature Measurements 

4.1.1. Thermocouples 

Thermocouples are a well-established technology with known operating 

temperature ranges and measurement uncertainty. A drawback to 

thermocouples is that each thermocouple must be individually wired to 

recording instrumentation. It is not unusual to require hundreds of 

thermocouples to reasonably measure gas and structural temperatures during 

a fire test. This method is not scalable to more than several hundred 

individual measurement points. 

4.1.2. Infrared Sensors 

Infrared sensors are available as either pyrometers for point measurements 

or as thermal imagers for surface measurements. Infrared sensors are often 

used to identify thermal gas plumes during experiments, but such 

measurements are considered qualitative rather than quantitative. 

Most imaging technologies for defense purposes are directed at seeing 

through smoke, haze, or darkness to identify threats. In general, these threats 

are warmer than their surroundings, e.g. soldiers at night, or exhaust from 

vehicle engines obscured by smoke on the battlefield. The requirements for 

full-scale structures in fire create a different problem—measuring deflections, 

strains, and temperatures of objects that may be cooler than the surrounding 

environment with optical distortions due to convection currents of varying 

temperature and gas density. 

Thermal infrared cameras detect certain gases that emit radiation within 

their spectral range. Long wave infrared (LWIR) detectors detect wavelengths 

within the (8 to 14) μm bandwidth, which includes most common combustion 

products. Soot and dust deposition and water deposition/condensation on the 

thermal imaging optics and target surfaces will affect temperature 
13 

measurements, and they are difficult to protect against. Soot deposition on the 

target surfaces will change the emissivity of the surface and, thus, change the 

temperature measurement. Emissivity also depends on temperature. Nitrogen 

curtains have been used with limited success. The gases associated with some 

fuels may interfere with the thermal infrared view of the surface. This is less 

likely to happen in the LWIR range, as smoke becomes increasingly 

transparent as the infrared wavelength increases. Very heavy smoke may 

disturb surface temperature measurements because the thermal infrared will 

be reflected by a percentage of smoke particles. Water and dust pose similar 

problems. Flames will probably disturb measurements if they enter the view 

field [17]. 

Measurement requirements, such as temperature accuracy, size of the 

measurement field, or output data type will identify appropriate thermal 

imaging instruments. Many thermal imagers have a selection of dynamic 

temperature ranges; it may be necessary to switch between them as the target 

temperature increases. 

Research is being conducted with thermal imaging methods during fire 

exposure to measure thermal properties of materials, such as emissivity [17]. 

Such research may lead to thermal methods for quantitative measures of 

surface temperatures. 

4.1.3. Fiber-optic Sensors 

Temperature and strain measurements are discussed in this section to 

avoid repetition, because the same measurement technology is used for both. 

Fiber-optic sensors are being developed for many applications, including 

temperature measurements for aerospace applications and strain 

measurements of dams and bridges. The Naval Research Laboratory (NRL) 

and Naval Surface Warfare Centers at Norfolk and Carderock have used fiberoptic sensors to monitor strains for detection of cracking in the aluminum 

superstructure of naval vessels at ambient temperatures. However, there are 

no reports of research at NRL for fiber-optic sensors at elevated temperatures. 

Most of the fiber-optic high-temperature applications started with the 

National Aeronautics and Space Administration (NASA) in the late 1990s to 

monitor strains in aircraft and spacecraft for temperatures up to approximately 

300 °C. Fiber-optic sensors are also used on bridges and other structures to 

monitor temperatures (less than 300 °C) or strains during construction or for 

health monitoring. Examples include monitoring temperatures in large 

concrete sections while curing, measuring cable and pavement temperatures 

in bridges, and monitoring strains of pipelines in landslide areas [18]. 
14 

Optical fibers are silica glass doped with small amounts of germanium 

and theoretically can function in temperatures up to the glass transition 

temperature of 1200 °C. A cladding is formed around the optical fiber that 

protects it from damage and moisture and forms a waveguide around the fiber 

with a lower refractive index. Coatings are applied over the cladding to 

provide service life durability and strength. Most coatings in use today are 

limited to temperatures of approximately 300 °C. There are specialty 

coatings, such as gold-based coatings, that reportedly can reach 700 °C.

However, in a laboratory setting, fiber-optic coatings may not be needed for 

one-time experimental applications [19]. 

Fiber-optic sensors that measure changes in temperature or strain are 

based on three forms of scattered light. A laser light at a specific wavelength 

is pulsed into an optical fiber, and is scattered as it travels down the fiber by 

interactions between the photons and the crystalline structure of the glass 

fiber. This scattering takes three forms: Rayleigh, Brillioun, and Raman 

scattering. Rayleigh scattering is the result of elastic collisions, and the 

scattered (reflected) signal has the same wavelength as the incident light. 

Brillioun and Raman spectra scattering are both inelastic events, and result in 

scattered signals comprising the red-shifted Stokes and the blue-shifted antiStokes components. In Brillioun scattering, the amplitudes of the Stokes and 

anti-Stokes signals are predictable given the amplitude of the incident light, 

but the wavelengths of the resulting signals are variable. In contrast, the 

Raman spectra Stokes and anti-Stokes signals have predictable wavelengths, 

but the relative amplitude of the two signals varies with temperature [20]. 

Fiber-optic sensing mechanisms used for structural measurements include 

Extrinsic Fabry-Perot Interferometer (EFPI), Fiber Bragg Gratings (FBG), 

Long Period Fiber Grating (LPFG), and Distributed Sensing System (DSS). 

EFPI sensors have a gap along the fiber length and are a point measurement 

method. As the gap length changes due to temperature or strain effects, the 

interference pattern for the reflected light signals changes. The maximum 

temperature for EFPI sensors is approximately 1000 °C, since the sensor is 

based on a gap in the fiber [21]. FBG sensors have a series of spaced lines 

etched into the optical fiber to reflect the light signal at a given frequency. 

Changes in fiber length, whether due to temperature or strain, cause a shift in 

the reflected frequency. The maximum temperature for FBG sensors is 

approximately 600 °C, due to the germanium doping typically used with 

optical fibers. If the FBG area is heat treated (annealed), the sensor may 

perform up to 1100 °C [22]. LPFG sensor gratings are fabricated by exposing 

the fiber core to ultraviolet (UV) radiation. This technology has been used to 

make Bragg-type fiber gratings (wavelengths less than a micrometer) and 

LPFG gratings (wavelengths in the hundreds of micrometers) [23]. The 
15 

gratings couple the guided fundamental mode in a single-mode fiber to 

forward propagating cladding modes. These modes decay rapidly as they 

propagate through scattering losses at the cladding-air interface. Since the 

coupling is wavelength-selective, the fiber grating acts as a wavelength 

dependent loss element [24]. DSS sensors measure temperature or strain 

anywhere along the fiber length. The measurement technique is based on low 

level backscattering of the laser signal at minor variations in the fiber density. 

DSS techniques that are based on Raman or Rayleigh scatter measurement 

typically employ optical frequency domain reflectometry (OFDR) for 

applications requiring higher spatial resolution. A commercially available 

fiber-optic system based on Rayleigh OFDR measures temperatures (and 

strains) at user-selected points along a 70 m fiber, with reported spatial 

resolution on the order of 1 cm and uncertainties of ±1 °C [25]. A second 

commercially available fiber-optic system based on Raman OFDR [26] 

measures temperatures along a fiber up to 2 km long with possible resolutions 

of spatial sampling (0.25 m minimum) and measurement time per fiber (5 s 

minimum), depending on several parameters, such as the length and type of 

fiber. 

DSS techniques based on Raman and Brillouin scatter measurements 

typically employ optical time domain reflectometry (OTDR) and may not be 

well suited for applications requiring high spatial resolution. Brillouin 

techniques are often used for long distances on the order of kilometers to 

monitor temperatures over a region on the order of meters [27]. However, a 

new Brillioun measurement technique has high-spatial-resolution and longrange distributed temperature sensor, based on experimental results [28]. It 

uses a differential pulse-width pair Brillouin optical time-domain analysis 

(DPP-BOTDA), which detects differential Brillouin gain instead of Brillouin 

gain itself. As a result, the spatial resolution of the Brillouin technology is 

comparable to Rayleigh OFDR for a much longer fiber length. The reported 

resolution of BOTDA technology for the combined sensing length and spatial 

resolution is 2 cm spatial resolution over a 2 km sensing length [28]. 

4.1.4. Summary of Temperature Measurement Technology 

Table 3 summarizes current technology for temperature measurements. 

Thermocouples, infrared-based methods, and fiber-optic techniques have been 

used to measure gas and structural temperatures, and are well established 

technologies for non-fire environments. The measurement accuracy and 

uncertainty of each method needs to be determined for a fire environment. 

Lonnermark, et. al. [29] compared thermocouple and fiber-optic temperature 

measurements during an experimental tunnel fire with gas temperatures up to 
16 

250 oC. Gas temperatures measured by thermocouples with two bead 

diameters (0.5 mm and 2 mm) and by a FBG sensor were compared with a 

gas temperature history based on a steady-state heat transfer model for two 

fire tests in the model-scale tunnel. The temperature measured with the FBG 

sensor and the calculated gas temperature were within approximately 25 oC, 

while the temperatures measured by the thermocouples were significantly 

lower by up to 75 oC. These results demonstrate the need for further research 

of thermocouple and fiber-optic gas temperature measurements in real fire 

conditions with time-varying gas temperatures. 

Table 3. Technology for temperature measurement.

Measurement 

Type

Technology Technical Issues for Fire Conditions

Point Thermocouples Established technology with known 

operating temperature ranges and 

measurement uncertainty. Measurement 

uncertainty may vary with temperature.

Point Infrared 

pyrometer

Infrared gun aims with laser, measures up 

to 1600 °C. Generally used for qualitative 

measurements. Subject to optical 

interference.

Surface Thermal 

Imaging

Thermal imaging with video, 3 frames/s, 

measures up to 1000 °C. Generally used 

for qualitative measurements. Subject to 

optical interference.

Field Fiber-optic

Sensor

Need to characterize measurement 

resolution and uncertainty for varying 

temperature and fiber conditions.

Field Fiber-optic

Sensor

Need to characterize measurement 

resolution and uncertainty for varying 

temperature and fiber conditions. 

4.2. Displacement Measurements 

4.2.1. Linear Transducers 

Traditional structural displacement measurement techniques include a 

linear position transducer (LPT, also referred to as draw wire or string pot 

devices) and a linear variable differential transformer (LVDT). The LPT 

measures the amount of wire drawn from a spool during structural 
17 

deformations and displacements. The LVDT measures translational 

displacements through induced voltage changes as the core moves through 

electromagnetic coils. Use of LPT and LVDT in fire conditions would require 

thermal protection or materials suitable for exposure to elevated temperatures 

(e.g., no significant thermal expansion of draw wire for temperature range of 

interest). 

4.2.2. Lasers 

Lasers are widely used for noncontact measurements of distances to 

objects and changes in position. Laser rangefinder devices, with appropriate 

software, can define three-dimensional objects such as buildings or terrain. 

However, lasers perform poorly in fire conditions because the light spectrum 

for flames distorts or masks the laser light frequencies. Suitable laser 

frequencies and signal processing techniques for their use in fire 

environments need to be identified. 

4.2.3. Digital Image Correlation (DIC) Systems 

Digital image correlation (DIC) systems process sequential images to 

compute surface measurements of displacement and strain. Digital images can

record the displacement of thousands of data points over a component 

surface. The recorded data history can be reprocessed after the experiment to 

correct for rigid body motions and out-of-plane curvatures. Measurement over 

areas rather than at discrete points could reduce the need for additional 

experiments. 

For digital image correlation measurements, a coating with a random 

pattern is applied to a component surface. The digital camera takes images 

during the test. Software processes the images for relative movement of the 

random pattern, and surface strains are computed between two points based 

on relative displacements. Coatings have been tested that adhere up to 800 °C 

in a furnace. However, smoke or soot may interfere with imaging of the 

coated surface. 
18 

NIST researchers have applied this technique for gusset plates at room 

temperature, as shown in Figure 2, in collaboration with the Federal Highway 

Administration to measure strains at ambient conditions [30]. The figure 

demonstrates the state of the art in structural strain measurement for ambient 

conditions. In the figure, the red color denotes regions of high strain. The 

heterogeneous distribution of strains would be impossible to capture or 

understand if only a dozen strain gages had been applied to the plate. Because 

image-based techniques do not choose discrete locations for measurements, it 

is possible to analyze any area in the field of view after the test. 

Digital Image Correlation (DIC) is an image analysis technique, that is not 

limited to optical wavelengths. Any set of images, not just those formed by 

visible light, is amenable to the technique. However, infrared wavelengths are 

probably not feasible, since as the temperature increases, the intensity of the 

background radiation increases. Some success has recently been reported with 

short wavelength illumination (λ = 450 nm) and band-pass filtering all 

wavelengths outside the range (425 < λ < 470) nm [31]. This approach 

attempted to measure the coefficient of thermal expansion of stainless steel in 

a resistance-heated (i.e., no flames) furnace. A significant challenge in 

extending the technique to the fire facility will be creating sufficient short 

wavelength illumination on the target. Most previous research on hightemperature digital image correlation has focused on imaging small parts in 

furnaces, rather than a building-sized element. 

Figure 2 Example of DIC computed strains on a bridge plate 

with surface markers and superimposed strains.
19 

Use of DIC techniques in fire conditions will require identifying methods 

to increase the contrast of the image under fire conditions. The contrast of 

surface coating patterns can be increased by developing markings that 

fluoresce in ultraviolet wavelengths and by filtering the reflected light to 

include only the wavelengths from the fluorescent markers. The technical 

challenge lies in developing coatings or paints that both fluoresce and adhere 

to the specimen in fire conditions. 

Image processing methods developed for inspections of underwater 

structures, such as pipelines and offshore platforms where visibility is poor 

due to turbidity and debris, may be applicable to post-processing of images 

with smoke and visual disturbances in a fire environment [32]. Innovations 

based on other digital imaging technologies could also be employed, such as 

adaptive optics. Adaptive optics have been applied to a number of imaging 

applications to remove the effect of disturbances in the visual medium, such 

as astronomical telescopes, greenhouse gas detection, and free-space 

communication. 

4.2.4. Summary of Displacement Measurement Technology 

Table 4 lists technologies for displacement measurements and technical 

issues requiring research for use in fire conditions. Noncontact technologies, 

such as digital imaging or laser-based systems, provide many measurements 

without wiring from each measurement location. Noncontact technology can 

potentially save considerable labor and time in the construction and setup of 

an experiment, and provide a richer data base for validation of analysis 

models. Depending on the number of components and complexity of the 

structural system, more than one of these methods may be required in a given 

test. 

The following challenges must be addressed to use digital image 

correlation (DIC) techniques in fire conditions: 

(1) Develop coating systems that increase image contrast with coatings 

or paints that both fluoresce and adhere to the specimen. 

(2) Determine optimal imaging measurement methods (e.g., short 

wavelength illumination on large targets, filtered wavelengths, laser, 

x-ray) and image quality for multiple angles, complex shapes, and 

fields of view. 

(3) Optimize digital image processing for measurement of local 

deflections and strains that resolve interference issues that include 

air turbulence, smoke and soot obstruction of target markings, and 
20 

fire spectrum interference with image lighting and resolution. 

(4) Quantify measurement uncertainty as a function of temperature. 

Table 4. Measurement issues for displacement measurements.

Measurement 

Type

Technology Measurement Issues for Fire Conditions

Point LPT 

LVDT

Thermal protection or material modifications 

to sensor is needed for exposure to gas 

temperatures up to 1400 oC.

Point Lasers Laser frequencies and signal processing 

techniques need to be developed for use in fire 

environments.

Surface DIC Methods to increase image contrast, short 

wavelength illumination on large targets, and 

image processing for smoke and turbulence in 

fire conditions are needed. Also need to 

characterize measurement resolution and 

uncertainty for varying fire conditions.

4.3. Strain Measurements 

4.3.1. Strain Gages 

Strain gages measure linear strains at discrete points. Measurement 

science for strain gages is well understood, but still has significant 

performance issues in high-temperature applications. Research is being 

conducted to identify materials that are thermally stable at elevated 

temperatures [33]. Examples of high-temperature strain gages for steel 

components include an iron-chromium-aluminum alloy wire resistance strain 

gage of the Hoskins 875 alloy that has a temperature range up to 850 °C and a 

0.01 strain limit. Users are cautioned that bonding the strain gage to a 

material with a higher thermal expansion rate will induce tensile stresses and 

to avoid rapid heating or cooling (rates greater than 14 oC/s) that will produce 

transient apparent strains [34]. This type of strain gage was used in a digital 

image correlation demonstration test; see Section 5. 

Epoxy bond materials that attach strain gages to steel surfaces can be used 

up to temperatures of approximately 400 °C. At higher temperatures, either 

ceramic bond materials or welds must be used. Strain gages for concrete 

sections can be bonded to the concrete surface (for crack detection), bonded 
21 

to the reinforcement, or embedded in the concrete section for strain 

measurements up to 100 °C.

As noted in Section 5, the performance of high-temperature strain gages 

in furnace and fire conditions is highly variable and unreliable. Further 

research is needed to identify the sources of poor performance and develop 

improved strain gages for fire conditions. 

4.3.2. Digital Image Correlation 

Digital image correlation (DIC) systems (see Section 4.2.3) compute 

surface strains using measurements of relative displacement between points. 

All the limitations of displacement measurement apply to strain measurement. 

4.3.3. Fiber-optic Sensors 

Fiber-optic sensors measure strains along a fiber length using FBG, EFPI, 

or LPFG sensors or DSS sensing (see Section 4.1.3). The optical fiber must 

be either attached to the surface of a steel component or embedded in a 

concrete section. Similar to strain gages, attachment methods other than 

epoxy bonds are required at elevated temperatures. Attachment methods 

between the fiber-optic sensor and/or fiber and the steel or concrete 

component are not well developed for fire conditions. Instead, each 

application requires development of an attachment method that considers 

issues such as material properties and differential thermal expansion at 

elevated temperatures [19]. The attachment method must be developed so that 

the transfer of strain from the steel or concrete component through the 

bonding agent and cladding to the core can be characterized. 

Simultaneous strain and temperature measurements were investigated for 

a LPFG sensor with an experimental strain transfer mechanism designed for 

large strains [19]. The strain transfer mechanism included both gauge length 

change and shear lag effects, and measured strains up to 15 200 μm/m in 

temperatures up to 700 °C. At higher temperatures, different adhesives must 

be used to bond the optical fiber, the protective sleeve, and the substrate. 

4.3.4. Summary of Strain Measurement Technology 

Table 5 lists technologies for strain measurements and technical issues 

requiring research for use in fire conditions. Strain gages can provide point 

measurements to confirm surface or distributed strain measurements if their 

performance issues in fire conditions are resolved. Digital image correlation 

challenges are the same as those given in Section 4.2.3. 
22 

Three challenges must be addressed to use fiber-optic sensor techniques in 

fire conditions: 

(1) Determine the best fiber-optic sensing method for temperature and 

strain measurements across large (beam length) and small 

(connection) scales in fire conditions. Possible fiber-optic sensors 

include Fiber Bragg Gratings (FBG), Extrinsic Fabry-Perot 

Interferometer (EFPI), Long Period Fiber Grating (LPFG), and 

Distributed Sensing System (DSS).

(2) Develop methodologies for bonding, embedment, or encasements of 

fiber-optic sensors that measure large strain measurements in fire 

conditions. Analytically determine and experimentally validate the 

strain transfer from the component to the sensor.

(3) Quantify measurement uncertainty as a function of temperature.

Table 5. Measurement issues for strain measurements.

Measurement 

Type

Technology Measurement Issues for Fire Conditions

Point Strain 

Gage

Further research is needed to identify the 

sources of poor performance and develop 

improved strain gages for fire conditions.

Point Concrete 

Strain 

Gage

Thermal protection for concrete strain gages 

or new measurement techniques at elevated 

temperatures need to be developed. Strain 

transfer from the component to the sensor 

need to be characterized.

Surface DIC Methods to increase image contrast, short 

wavelength illumination on large targets, and 

image processing for smoke and turbulence 

in fire conditions are needed. Also need to 

characterize measurement resolution and 

uncertainty in for varying fire conditions.

Points along a 

fiber; 

Continuous 

along fiber

Fiber-optic

Sensor

Sensor integrity and attachment methods in 

fire conditions need to be developed. Strain 

transfer from the component to the sensor 

need to be characterized.
23 

5. Digital Image Correlation and Strain Gage 

Demonstration Test 

5.1. Background and Goals 

To evaluate the suitability of digital image correlation (DIC) for strain and 

displacement measurement, a demonstration test in the NIST Large Fire 

Facility was conducted. The demonstration test had three goals: 

1. Evaluate the suitability of digital image correlation for strain 

measurement under fire conditions. 

2. Compare digital image correlation to traditional high-temperature 

strain-gage measurements. 

3. Understand the sensitivity and uncertainty of both digital image 

correlation and high-temperature resistance strain gages in real 

fire conditions, as opposed to furnace conditions. 

Appendix 1 details the configuration and results of the test. This section 

summarizes the important results and major findings; see Appendix 1 for 

more details. 

5.2. Summary of the Demonstration Test 

In preparation for the test, a suitable paint and method for applying the 

paint to a test specimen of mild steel plate was identified. Three candidate 

paints and preparation techniques were first evaluated in a one-hour furnace 

exposure test. Two survived exposures of 600 °C, with different amounts of 

discoloration. None survived for an hour at 700 °C. The most durable and 

color-fast coating was then tested under direct flame exposure at temperatures 

up to 700 °C for short times to demonstrate that the paint adhered. 

A three-phase test with the prepared test plate was conducted in the Large 

Fire Facility. Rigid body displacements were used to determine the DIC 

measurement uncertainty at ambient conditions, and thermal strains were 

measured with DIC methods and strain gages. No mechanical loads were 

applied to the test specimen during the tests. 

5.2.1. Test Configuration 

The test specimen consisted of a 30 cm x 45 cm x 1 cm mild steel plate 

instrumented with two high-temperature strain gages and eight Type K 

thermocouples. The side opposite the strain gages was patterned with black 

overspray on a white background for strain measurement by digital image 
24 

correlation. The plate was suspended about 1 m above a natural gas burner 

capable of 70 kW output. Figure 3 shows the instrumented plate with its black 

and white patterning, suspended above the gas burner. The vantage point is 

just behind one of the two cameras. The thermocouples and strain gages are 

on the opposite side of the plate. 

Figure 3. Configuration during the demonstration test seen from just behind 

the cameras, looking at the random paint pattern applied to the test specimen. 
25 

5.2.2. Results 

Phase 1 of the test produced an estimate of the absolute uncertainty in the 

strain. In phase 1, the plate was translated left and right (x-direction) as well 

as up and down (y-direction) with no fire present. Since rigid-body motion 

should produce zero strain, this test established the uncertainty in strain 

measurements with DIC at ±50 μm/m. This strain corresponds to the strain 

produced by a room-temperature elastic loading of 10 MPa or a thermal 

expansion strain produced by a 4 °C temperature change. 

In Phase 2 of the test, the burner was placed between the plate and the 

cameras, to assess the effect of air turbulence on the DIC strain uncertainty. 

Again, the plate was translated in the x- and y- directions. The reading-toreading scatter in strains was 150 times larger than in Phase 1, but the mean 

strain calculated was still zero. 

In Phase 3, the plate was located directly over the flame. A major 

limitation of the test was that the 70 kW fire was only able to heat the plate to 

about 200 °C in the open configuration (no compartment to collect heated 

gases), which is well below the expected temperatures in structures during 

fire conditions. Figure 4 shows DIC computed thermal strains in the 

horizontal (x-x) and vertical (y-y) directions of the plate as a function of 

temperature during phase 3 of the test. The dashed line is a linearization of the 

accepted value [35] for thermal expansion based on temperature. The dashdot line is the linear regression of the data. Over the range of the data, the 

slopes of the lines differ by less than 1.9 %, and the maximum difference 

between the literature and fitted strain values is less than 11 %. 

The high-temperature strain gages are designed to self-compensate for 

thermal strains; see Appendix 1 for details. Unfortunately, both gages 

displayed large negative apparent strains during all phases of the test. The 

magnitude of the apparent strain-gage strain was about 25 percent of the DICmeasured thermal strain. Subsequent testing of the strain gages in a furnace 

demonstrated that these apparent strains were quite repeatable and 

proportional to temperature. Extensive discussions with the strain gage 

manufacturer did not lead to any method for compensating for these strains. 

The demonstration test confirmed the reported difficulties with obtaining 

reliable measurements with high-temperature strain gages. 
26 

Figure 4. Comparison of the thermal strains, e, measured by DIC as a function 

of temperature near the two strain gages during Phase 3 of the test. Dashed 

line is the computed value for thermal expansion based on temperature. 

Dash/dot line is the linear regression of the data 

5.3. Summary of Demonstration Test Findings, Conclusions, and 

Recommendations 

Although the test plate did not reach the temperatures that steel 

components can reach in fire conditions, the results of the demonstration test 

encouragingly support the feasibility of digital image correlation methods for 

measuring strain in fire conditions. The demonstration test produced four 

significant findings. 
27 

1. Commercial high-temperature paint can survive up to 600 °C for 

an approximately 1 h furnace exposure. 

2. Rigid body translations produced no apparent strain for a plate 

viewed through air turbulence over a fire. 

3. Digital image correlation measured the thermal strain of a plate 

heated by fire to within 11 % of the accepted value. 

4. Conventional high-temperature strain gages produced apparent 

compressive strains on heating. 

Based on these findings, digital image correlation methods show 

significant promise for measuring strains and deformations in fire, and could 

revolutionize structural fire measurements. The data acquisition and analysis 

of the images employed no special illumination techniques or image postprocessing. Both of these approaches have significant potential to improve the 

quality of the measurements. Despite decades of development, hightemperature strain gages are still difficult to use, and have poorly understood 

thermal response. 

Recommendations for developing digital image correlation as a strainmeasurement tool for the National Fire Research Laboratory include the 

following steps: 

1. Conduct tests that heat the plate to more than 600 °C to further 

study the adherence of commercial paints in fire conditions. 

2. Evaluate methods for post-processing the digital image 

correlation data to reduce the uncertainty in strains. 

3. Add mechanical loads and/or deformations to the fire tests. 

4. Develop quantifiable uncertainties for strain-measurement 

methods. 
28 

6. Candidate Technologies for Next-generation 

Measurements of Structural Response in Fire 

Based on the review of available technology and their measurement 

challenges, and the demonstration test with digital image correlation and 

high-temperature strain gages, digital image correlation and fiber-optic 

technologies appear to be the most promising technologies for temperature, 

displacement, and strain measurements of structural systems in fire 

conditions. Fiber-optic and digital image correlation measurement capabilities 

are complementary technologies. Digital image correlation methods measure 

surface displacements and strains. Fiber-optic sensors measure strain along a 

fiber on any exterior surface or in the interior of concrete sections. Parallel 

fiber-optic sensors measure corresponding temperatures for strain 

measurements. 

Advances in computer processing have led to distributed systems that 

employ image techniques or fiber-optic sensors to measure strains and 

deformations at hundreds to thousands of points on a structure. Although 

these techniques have been successfully applied at room temperature, 

significant challenges remain to apply them to the extremely hostile 

environment of a fire (e.g., flames, smoke, soot, high temperatures, radiation, 

and air turbulence). 

Image-based techniques, using digital image correlation, measure the 

strain or displacement between successive images of a suitably patterned 

structure. Because image-based techniques do not choose discrete locations 

for measurements, it is possible to analyze any area in the field of view after 

the test. Because fire tests are expensive and time consuming, this post-test 

analysis capability is particularly significant. Table 6 lists the challenges and 

possible solutions for applying digital image correlation methods for 

measurements in fire conditions in increasing order of difficulty. Issues with 

smoke and soot are likely to be the greatest challenge. 

Today, fiber-optic sensors are used to monitor the structural health of 

buildings and infrastructure. NASA started work in the 1990s to use fiberoptic sensors at elevated temperatures (up to 300 °C) to monitor strains in 

aircraft and spacecraft. In contrast, very little work has been done at 

temperatures found in fires because the measurement science challenges are 

significant. For example, one commonly used fiber-optic sensor is based on a 

Fiber Bragg Grating (FBG). In this type of sensor, the grating reflects a 

particular wavelength of light. However, the wavelength can shift due to 

changes in both temperature and stress on the grating. In a fire scenario to 

measure structural performance, these two effects cannot be separated. 
29 

However, using a Raman scattering technique to measure the temperature of 

the fiber, it may be possible to extract the strain-induced component from the 

FBG signal. Other fiber-optic sensors that could be investigated for strain 

measurements include Extrinsic Fabry-Perot Interferometers (EFPI), Long 

Period Fiber Gratings (LPFG), and Distributed Sensing Systems. In addition 

to potential signal confusion at elevated temperatures, fiber-optic sensors 

must overcome the effects of differential thermal expansion between 

materials, large strains that exceed the fiber strain limit, and possible 

measurement artifacts from various attachment methods due to hightemperature effects. Table 7 lists the challenges and possible solutions for 

using fiber-optic sensors for measurements in fire conditions. 

Table 6. Challenges and possible solutions for digital image correlation.

Challenge Possible solutions

Integrity and adherence of the 

pattern at high temperature and 

under fire assault

High-temperature paints

Anodized coatings 

Flame- or plasma-sprayed ceramic 

coatings

Optical radiation from fire 

overwhelms image contrast; need 

to increase image contrast

Infrared filtering

Short wavelength/UV illumination

Monochromatic, high-intensity 

illumination

Turbulent variable-density air 

currents change the optical path 

and distort the image, leading to 

spurious strain measurements

Integration of IR and visible spectrum 

images 

Electro-magnetic wave propagation using 

Mie scattering and Kolmogorov 

turbulence theory

Data-driven modeling from integrated 

images to correct distorted measurements

Adaptive optics

Smoke, flame, and soot obscure 

pattern

Image post-processing techniques, for 

example Retinex

Point measurement technologies should also be evaluated and developed 

to provide a secondary level of measurements. However, there are significant 

disadvantages to relying solely on point measurement methods. Discrete 

measurement devices require separate data lines for each sensor, and there can 
30 

be significant associated labor and costs. More significantly, point 

measurements may miss critical displacement or strain locations, reducing the 

usefulness of expensive laboratory experiments, and potentially resulting in 

misleading conclusions. 

In summary, traditional room-temperature structural experiments measure 

displacements and strains at a few "critical" nodes, selected a priori. In fire 

conditions, existing measurement methods fail due to either high-temperature 

effects on materials or optical saturation from the fire. Notably, hightemperature strain gages have shown a lack of reliability, based on reports 

from other laboratories and the experiment at NIST. Rather than improve 

point measurement methods, research should focus on developing innovative 

distributed measurement techniques for fire environments based on fiberoptic sensors and digital image correlation. The ability to measure 

deformations and strains inside, rather than outside, the fire zone would 

greatly enhance structural performance measurements in the newly 

commissioned NFRL. Experimental data are critical for designing safer 

buildings, bridges, and other infrastructure using validated next generation 

computational tools. 

Table 7. Challenges and possible solutions for fiber-optic measurements.

Challenge Possible solutions

Ensuring fiber and sensor 

integrity and performance 

at large strains using 

materials with different

thermal expansion

behavior, over a large range 

of temperatures

Characterize performance of existing sensors at 

elevated temperatures (e.g., FBG, EFPI , LPFG, 

DSS).

Novel, modified, or hybrid fiber sensor types to 

measure temperature and strain in fire conditions 

that optimize fiber and sensor characteristics, 

such as an external Fabry-Perot cavity

Shielding fiber-optic

sensors from fire radiation 

and gas temperatures

Insulated covers with a small foot print to 

minimize interference with the heating of 

components (mineral wool, ceramic, etc.)

Deconvolving strain and 

temperature measurements

Accurate measurements of temperature along 

fiber to separate thermal and mechanical strains

Developing attachment 

methods 

Ceramic adhesives

Welded or mechanical attachment
31 

7. Proposed Development Plan

The following research plan is outlined for the development and/or 

adaptation of existing technologies for measurements in fire conditions with 

fiber-optic and digital imaging systems. The successful measurement 

methodology must address the effects of high temperatures on material 

properties and optical distortion or obscuration by thermal plumes, soot, and 

smoke. 

Four critical challenges for digital image correlation (DIC) techniques in 

fire conditions have been identified. 

1. Developing coating systems that increase image contrast with 

coatings or paints that both fluoresce and adhere to the specimen. 

2. Determining optimal imaging measurement methods (e.g., short 

wavelength illumination on large targets, filtered wavelengths, 

laser, x-ray) and image quality for multiple angles, complex 

shapes, and fields of view. 

3. Optimizing digital image processing for measurement of local 

deflections and strains that resolve interference issues that include 

air turbulence, smoke and soot obstruction of target markings, and 

fire-spectrum interference with image lighting and resolution. 

4. Quantifying measurement uncertainty as a function of 

temperature and fire conditions (e.g., fuel types). 

Three critical challenges for fiber-optic sensor techniques in fire 

conditions have been identified: 

1. Determining the optimal fiber-optic sensing method for 

temperature and strain measurements across large (beam length) 

and small (connection) scales in fire conditions. Possible fiberoptic sensing technology includes Fiber Bragg Gratings (FBG), 

Extrinsic Fabry-Perot Interferometer (EFPI), Long Period Fiber 

Grating (LPFG), and Distributed Sensing System (DSS). 

2. Developing methodologies for bonding, embedment, or 

encasements of fiber-optic sensors for large strain measurements 

in fire conditions. Analytically determine and experimentally 

validate strain transfer from the structural component to the 

sensor. 

3. Quantifying measurement uncertainty as a function of 

temperature. 
32 

The proposed research starts with tests of existing sensors and technology 

to determine their limitations, bounds of performance, and measurement 

uncertainty for ambient and fire conditions. The research will be iterative in 

nature, with progressive development of successful methodologies, 

technologies, and materials suitable for fire conditions. 

Stage 1: Develop measurement science in quiescent, high-temperature 

conditions for simple deformations and identify uncertainty sources. 

Investigate the technical challenges listed in Table 6 and Table 7 and develop 

solutions for accurate, reliable sensor measurements in a fire environment. 

Use small-scale, clean-burning fires or furnaces in the early stages of 

research. For digital image correlation, develop methods to integrate infrared 

and visible spectrum images to model distortion of the optical path to ensure 

metrological quality of the visible images for rigorous digital image 

correlation. 

Research goal: Develop the supporting science and technology for 

measuring deformations of a small, simple test structure with quantified 

uncertainty under elevated temperature conditions. 

Stage 2: Extend measurement technology to structural systems in clean 

fires. Extend the measurement technology to a model structure, such as a steel 

or concrete frame with realistic loads, under clean fire conditions (no 

significant smoke or soot). Study the quality of thermal and load 

measurements with digital image correlation and fiber sensors, as well as air 

column distortions for digital image correlation. 

Research goal: Extend the supporting science and technology for 

measuring deformations of a realistic test structure with quantified 

uncertainty under clean fire conditions. 

Stage 3: Extend measurement technology to structural systems in dirty 

fires. Continue developing measurement technology to experiments under 

dirty fire conditions (shown in Figure 5), adding varying levels of smoke or 

soot to digital image correlation tests. Continue developing fiber-optic sensors 

with a focus on robust attachment approaches. Apply electro-magnetic wave 

propagation models using Mie scattering and Kolmogorov turbulence theory 

to reduce the influence of smoke on soot in the image quality. 

Research goal: Develop the supporting science and technology for 

measuring deformations of a realistic test structure with quantified 

uncertainty under dirty fire conditions. 

Stage 4: Evaluate refined fiber-optic and digital image correlation 

measurement techniques and methodology to quantify sources of uncertainty. 
33 

Evaluate measurements of several steel and concrete beams that have 

mechanical loading, thermal restraint at the end supports, and staggered 

placement that creates difficult surfaces for viewing, and are located inside a 

compartment to allow for assessment of compartment fire conditions (e.g., 

hot zone, smoke, and soot). 

Research goal: Measure the deformation of a real structural system with 

complicated load paths, restraints, and internal areas with quantified 

uncertainty in realistic fire conditions. 

Figure 5. NIST compartment fire experiment with a sooty 4 MW fire. 
34 

8. Summary 

At present, temperatures, displacements, and strains of structures in fire 

conditions are measured with single point sensors, such as thermocouples, 

transducers outside the heated zone, and high-temperature strain gages. 

Significant advancement of measurement science for structures in fire 

conditions are needed to support the validation of analytical tools and 

performance-based-design methodologies. 

A set of desired performance criteria for structural-fire measurements 

were established to guide the assessment and development of candidate 

technologies and measurement systems: 

Measurement methods that capture the response of an entire surface. 

Sensors that function at temperatures as high as 750 °C, and are 

insensitive to heating rate. 

Displacement measurements ranging from 1 mm to 1 m. 

Elastic strain measurements up to e = 0.002 with a resolution of 

±200 μm/m and plastic strain measurements up to e = 0.2, with a 

resolution better than ±2000 μm/m. 

Two innovative measurement techniques, distributed fiber-optic sensors 

and digital image correlation field measurements, will enable full 

characterization of structural elements, assemblies, and systems experiencing 

mechanical and transient thermal loads if the significant technical challenges 

identified in this report can be resolved. A high density of high-quality test 

data is essential for validating the next generation of design tools for the 

practicing engineer. 

A four-stage development plan is proposed to overcome these challenges. 

The early stage develops the measurement science with simple heating and 

deformations. The intermediate stage employs more realistic fires and more 

complicated deformations. In the final stage, the measurement uncertainty of 

both methods will be quantified. 

The ability to measure strain and displacement of structural components 

and systems during fire experiments is critical for (1) obtaining experimental 

data to validate structural analytical models and (2) developing performancebased design methods for structures in fire. The successful completion of the 

proposed work will: 

• Advance the state-of-the-art in structural fire measurements from a few 

unreliable point measurements to full-field measurements with 
35 

documented uncertainty. 

• Provide a new measurement capability for other industries, such as 

aircraft, spacecraft, ships, and nuclear facilities, with fire hazards or 

elevated temperatures that does not presently exist. 

• Lead to advances in safer buildings, bridges, and other infrastructure with 

quantified structural fire resistance and safety factors for fire. 

• Lead to new experimental methods, validated design and computational 

tools, and best practices for structural fire laboratory measurements. 

• Advance the capabilities of the NFRL. 
36 

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40

A Results and Analysis of the Demonstration Test

A.1 Introduction and Motivation

The demonstration test had three goals.

• Evaluate the suitability of digital image correlation (DIC) for strain measurement under fire conditions.

• Compare digital image correlation to traditional high-temperature straingage measurements.

• Understand the sensitivity and uncertainty of both digital image correlation

and high-temperature resistance strain gages in a real fire, as opposed to

furnace, condition.

A.1.1 Background on Digital Image Correlation for High- Temperature Strain

Measurement

Digital image correlation has been used for full-field strain measurement for more

than twenty-five years. [1]. After several proof-of-concept demonstrations in the

mid 1990s, [2, 3] recently researchers have been applying it to high-temperature

strain measurement. [4–7] Most recent reports have also approached the problem

as an experimental proof of concept, and are confined to methods for overcoming

the significant problems of image distortion and illumination. They typically only

report the successful measurement of thermal expansion strains in a test coupon in

a furnace [4, 5, 7] or other bench-top environment. [6]

Digital image correlation is an image matching technique [8]. As implemented

in most systems for measuring displacements and strains of solid bodies it relies on

being able to compute the mapping (i.e. the correlation) of specific subset regions

of the specimen, identified by their signature grey intensity level, between an initial and a deformed state. Uniquely identifying each region in an image through its

grey level requires a random speckle pattern. Of course, if the specimen deforms,

the coating must move with the surface to indicate strain in the specimen. It can be

naturally occurring, etched into the surface, or applied as a coating. The preferred

pattern has a minimum of three light-dark transitions in every direction within the

subset of the image to be correlated. The displacements are determined in an average sense for each subset, so the subset size determines the footprint of a single

independent displacement measurement. Reducing the subset size improves spatial

resolution of the displacement field, but reduces the quality of the correlation. The

correlation subset position is rastered across the images in the region of interest to

determine a grid of displacements. As a rule of thumb, the minimum feature size
41

of a pattern should be three pixels wide, resulting in a minimum correlation subset

on nine pixels. In real applications, obtaining this idealized pattern for the entire

surface is not possible, so patterns often have features larger than the three-pixel

size and the subset for correlation is two to three times this minimum size.

Traditional two-dimensional digital-image correlation measures a displacement

field on the surface of a flat specimen, and derives the strain field from this displacement field. All of the displacements and strains are based on the change from

a reference image. The method uses a single camera to image a high contrast surface pattern on a flat specimen held at a fixed distance normal to the surface and

at a fixed magnification. Out-of-plane motion must be minimized or compensated,

since it will be seen as uniform expansion or contraction of the specimen. Out-ofplane bending produces a similar effect, but compensation is much more complex.

Good quality images require a clear line of sight, low digital-image noise, and minimal shadowing, specular reflection, blurring, and degradation of the pattern, for

example flaking of an applied coating.

Three-dimensional digital image correlation combines stereo-photogrametry

and traditional DIC, and is used in the tests that this report describes. In this

method, two digital cameras image the surface of interest from slightly different

angles. After calibration of the orientation of the cameras, the stereo images of the

surface can be used to see the surface pattern in three dimensions. This 3D pattern

can then be correlated from the reference state to the current state to measure the

3D displacement field. From these displacements the surface strain field is calculated. Similar requirements on the pattern and image quality as mentioned above

apply here, but three-dimensional surface shapes are now acceptable, as well as out

of plane displacements. The only additional requirement is that both cameras must

be able to image the pattern in the region to be measured. The pattern selection

can be more complex, since the surface is not a a single effective magnification in

either image even for a flat specimen, because each camera is angled to the surface

normal, which results in a perspective distortion in each image.

Extending digital image correlation to high-temperature for strain measurements adds six challenges.

1. Convective air currents blur the image and refract individual rays of light.

The latter can cause apparent specimen translation as well as apparent strain.

2. The index of refraction changes between the calibration phase and the test

phase, and it varies with fire intensity and temperature.

3. Heating of DIC setup changes the camera orientation.

4. Flames licking the test specimen block the imaging, and general lighting

changes from fire affect the measurement.
42

5. The fire assault may degrade the pattern, which may peel off, or soot buildup may obscure it.

6. Radiation from the specimen at high temperature may change the contrast of

the image.

A.1.2 Background on High-Temperature Strain Gages

Resistance strain gages capable of operating at temperatures above 500 ◦C were

developed in the 1960s, primarily to serve the aerospace industry. [9–11]. The

basic operation and materials of the contemporary resistance strain gage are similar

to those developed forty years ago. The field is sufficiently mature that ASTM

International publishes a Standard Practice for using high-temperature strain gages.

[12]

High-temperature strain gages suffer from some important limitations. Because their resistance depends on the microstructure of the metal that forms the element, oxidation and annealing of the wire can cause the gage resistance to change

with time, producing apparent strain drift. This effect is generally a larger problem

in configurations where strain measurement is made over days, rather the few hours

envisioned for the measurements made in fire conditions. In addition, although the

half-bridge configuration offers the possibility of self-compensation, some apparent strain as a function of temperature has proven to be unavoidable.

A.1.3 Outline

Section A.2 describes the evaluation and selection of suitable coatings for the digital image correlation measurements. Section A.3 describes the configuration and

results of the demonstration test in the Large Fire Facility. Section A.4 describes

several subsequent tests undertaken to understand the performance of the hightemperature strain gages.

A.2 Evaluation of Coatings For Digital Image Correlation

Digital image correlation requires that the area of interest be covered with a highcontrast, random pattern that produces regions with characteristic gray-level signatures, that the analysis software can track from image to image. For hightemperature digital image correlation, the black and white colors of the pattern

must be stable against the temperature and fire. Other investigators have used different methods for producing the pattern, for example spattering the surface with

different oxide pigments [5], or abrading the surface with SiC paper. [4].
43

Table 9: High-temperature paints evaluated

Coating Paint Manufacturer Form

01 Thurmalox 230 Dampney Co. liquid (white)

aerosol (black)

02 Thermal Kote Superior Ind. aerosol

03 Zynolyte Hi-Temp Extreme

Heat Enamel

Aervoe Ind. aerosol

For this demonstration test, we chose to produce the pattern by painting the

specimen with a matte white, high-temperature paint, and overspraying with a

similar matte black high-temperature paint. These paints are sold commercially

for painting wood stoves and automobile exhaust manifolds, as well as for protective coatings in the chemical process industry. We evaluated the performance of

three types∗ of high-temperature paint, Table 9, using a coupon test. Both exposure temperature and surface preparation were evaluated. Exposure to open flame

and the very high temperatures planned for the demonstration test are well outside the recommended use conditions of all three paints. Their performance in

the demonstration test should not be construed as evaluation of their suitability for

their intended use. All three coatings are proprietary formulations. The material

safety data sheet (MSDS) for Coatings 01 and 02 specifically mention crystalline

silica (CAS 14808-60-7 and CAS 7631-86-9 respectively). Coating 02 also contains aluminum flake (CAS 7429-09-5). The MSDS for Coating 03 lists only the

hydrocarbon propellants.

A.2.1 Applying the Random Pattern for Digital Image Correlation

Digital-image correlation relies on the random, high-contrast pattern of light and

dark regions that define, through their gray level, a region that can be located in

each image. Based on the size of the test plate, the lenses used for imaging, the

geometric arrangement of the cameras, and the potential for image blurring due to

convective currents, a minimum black spot size of about 1.5 mm was determined

for the demonstration test, Section A.3. All of the test coupons and the plates for

the demonstration test were first painted with the white background over which

black spots would be applied. We evaluated three methods for producing the black

∗Certain commercial equipment, instruments, or materials are identified in this paper in order to

specify the experimental procedure adequately. Such identification is not intended to imply recommendation or endorsement by the National Institute of Standards and Technology, nor is it intended

to imply that the materials or equipment identified are necessarily the best available for the purpose.
44

portion of the high-contrast pattern with optimal spot size: partial spray, adhesive

stencil, and particle masking. In the partial spray method, aerosol paint is oversprayed onto the test piece. Larger droplets fall onto the test piece. This method

produced black regions that were too small. A second method used an adhesive

stencil with 2 mm diameter holes, but it produced patterns with insufficient density

of black. This result, in addition to the difficulty making a stencil large enough for

patterning a full-size plate, suggested a third patterning method. Since the plates

being tested were nominally flat, a random distribution of non-adhering particles

on the white paint coating could be used as a mask to prevent full coverage by the

black paint. After removal of the particles, areas of the white base coat (slightly

larger than the particles used) would remain on the plate. Seven types of particles

ranging in aspect ratio from approximately 1:1 to 1:4 were tested; all had a smaller

dimension < 2 mm. Sample patterning and evaluation suggested the aspect ratio

of the best particle was near 1:1 and its diameter was between 0.5 mm and 1.0 mm.

Although that particle size is smaller than the preferred feature size, the particles

naturally clustered, and produced features very close to the desired 1.5 mm size.

After the patterned plate was dry to the touch, the coating was cured for three heating and cooling cycles of 215 ◦C for about 40 minutes, followed by cooling to room

temperature for about 60 minutes. Some yellowing of the white coating occurred

during the first heating cycle, but most of this yellowing disappeared by the end of

the curing process.

A.2.2 Furnace Coupon Test

Procedure The performance of the coatings was first evaluated in a furnace

coupon test at different temperatures. The most promising coating was then tested

to determine the robustness against direct contact with flame using a propane torch,

and to evaluate the digital image correlation system in a more realistic scenario with

flames and convection currents. Fifty-four mild steel coupons (50 mm × 25 mm

× 6 mm) were prepared for a preliminary test of three brands of high-temperature

paint and different surface preparation techniques. Coupons were engraved to indicate the factors tested. Codes were assigned according to brand of paint (01, 02,

03), surface Treatment (S, M, L), and test temperatures: (300, 400, 500, 600, 700,

800) ◦C.

The coupons were sandblasted using abrasive of three grit sizes: 100-170 (S),

60-100 (M), and 40-60 (L). The preparation instructions for Coating 2 recommended abrading the surface with a 60-100 sanding disk and curing the coating

at 204 ◦C for 3 h before use. The other two coatings contained no surface preparation or pre-treating instructions, but all three coatings were prepared with the

sandblasting and 204 ◦C cure.
45

t /s

Specimen Temperature, /°C

0

200

400

600

800

0 2000 4000 6000 8000

Furnace Temp.

300 °C

400 °C

500 °C

600 °C

700 °C

800 °C

Figure 7: Temperature-time traces for the six coupon exposure tests.

To test the response of the paint to an elevated temperature, nine samples comprising three distinct surface treatments and three coating types were tested in air in

a resistance-heated box furnace. The coupon temperature was monitored by peening a K-type thermocouple into a hole drilled to half the thickness of a dummy

specimen. For the 300 ◦C and 800 ◦C tests the thermocouple was inserted in one

of the actual test coupons. The furnace was set to the test temperature and allowed

to stabilize before the test coupons, generally sitting on end in a rack, were inserted.

For the tests with T > 400 ◦C, the coupons reached 95 % of the maximum temperature in less than 12 minutes. The two lower-temperature tests reached the 95 %

level in 17 minutes and 26 minutes, respectively. Figure 7 shows the temperature-
46

time traces of the specimens during the coupon tests. The coupons remained in

the furnace for about one hour after their temperatures had stabilized, and were

then removed and allowed to cool on the lab bench. Immediately after removal

from the furnace, the coupons were photographed while still hot. They were also

photographed again several hours later after cooling to room temperature.

Results For exposure temperatures less than 700 ◦C, the different surface preparations had no effect on the adherence of the coatings. Figure 8 is a montage of

the results of five of the exposure tests, and shows before and after images. The

images have been partially color corrected to facilitate comparison, but the images

are not completely color consistent.

300 ◦C All the coupons were slightly discolored to a brownish hue, and Coating 02 developed a white speckle pattern in regions up to 1 cm in diameter. This

report does not address the question whether this speckling is an inherent quality

of the coating or if it represents a reaction with contaminants in the furnace, which

had been used for ceramic research with volatile oxides for many years.

400 ◦C All coupons were moderately discolored, and the Coating 02 discoloration was more pronounced. In addition, the white speckle pattern on Coating 02

that developed in the 300 ◦C exposure became more pronounced. All discoloration

and speckling occurred during the exposure. Unlike some of the higher temperature tests, the appearance of the coupons did not change between removal and

cooling.

500 ◦C The results were identical to the 400 ◦C exposure.

600 ◦C Immediately after being removed from the furnace, the samples had a

yellow tinge, possibly from the glow produced by the steel under the coat surface.

All of the coatings adhered to the coupons, and there was no evidence of blistering

or peeling. There was no speckling on the Coating 02 coupons. After cooling,

all three coatings were white, and similar to the color before exposure. At room

temperature, Coating 01 began to peel off the coupon in flakes up to 1 cm across.

The other two coatings adhered to the coupons.

700 ◦C Immediately after removal, the samples had a orange tinge, possibly

from the glow produced by the steel under the coat surface. All of the coatings adhered to the coupons, and there was no evidence of blistering or peeling. There was
47

(a) 300 ◦C: before exposure (b) 300 ◦C: after exposure

(c) 400 ◦C: before exposure (d) 400 ◦C: after exposure

(e) 500 ◦C: before exposure (f) 500 ◦C: after exposure

(g) 600 ◦C: before exposure (h) 600 ◦C: after exposure

(i) 700 ◦C: before exposure (j) 700 ◦C: after exposure

Figure 8: Images of the test coupons taken just before and several hours after the

furnace test.
48

Table 10: Summary of coating exposure tests.

T

◦C Coating 01 Coating 02 Coating 03

300 whiter tint browner tint; 5 mm

white speckling

whiter tint

400 whiter tint browner tint; 5 mm

white speckling

whiter tint

500 whiter tint browner tint; 5 mm

white speckling

whiter tint

600 peeled after cooling adhered after cooling adhered after cooling

700 all coatings flaked off coatings mostly adhered some coatings flaked off

800 all coatings flaked off all coatings flaked off all coatings flaked off

no speckling on the Coating 02 coupons. After cooling, the paint on all samples began to peel. Coating 01 peeled the most severely. Handling dislodged more of the

coating, and all of Coating 01 fell off. Coating 02L and Coating 03L (sandblasted

with the coarsest grit) adhered the most.

800 ◦C None of the coupons retained any coating. The coupons were oxidized under the coatings. In this test, the coupons had been supported upright by

wires which in some cases had touched the coatings. In these areas the oxidation

and coating failure was more pronounced.

Conclusions from exposure tests Table 10 summarizes the results of the

furnace exposure tests. None of the coatings performed well at 700 ◦C or above.

Because it adhered well and did not discolor or develop the white speckling, which

would certainly interfere with the digital image correlation, the demonstration test

used Coating 03.

A.2.3 Flame-lick Test

To gain experience prior to the full-scale demonstration test, we evaluated the performance of Coating 03, with two different spot patterns, as well as the performance of the digital image correlation cameras and filters, in bench-scale tests. In

this test, two propane torches heated a thermocouple-instrumented coupon. The Ktype thermocouple was peened into a hole drilled to half the depth of the thickness

on the backside of the coupon. The tip of the flame of each torch projected into
49

the field of view of the cameras, and directly hit the coated surface in the region

of the thermocouple. Within ten minutes, the temperature of the coupon exceeded

700 ◦C. Because the cameras were unfiltered, the infra-red radiation from the

coupon overwhelmed the auto-level capability of the camera. To the naked eye, the

black spot pattern was clearly visible, but the camera imaged only a white center

region. Presumably the proper filtering used in the phase 3 of the demonstration

test would remove this problem. Although the test lasted for about fifteen minutes,

the coating did not flake off either during or after the test. This behavior suggests

that it was the formation of the oxide layer, rather than the exposure to temperature,

that caused the coatings to delaminate in the furnace exposure tests.

A.3 Demonstration Test

The demonstration test took place in the National Fire Research Laboratory on

2011-09-15 in three phases. The test evaluated the displacement and strain resolution of the digital image correlation system by imposing known rigid-body translations of the plate and comparing the computed in-plane strains, which should

remain identically zero. In phase 1, the plate was translated vertically and horizontally known amounts to determine the sensitivity of the digital image correlation

system with no heat applied. In phase 2, the burner was placed about 600 mm

in front of the plate, to test the effect of convection with minimal heating to the

plate. In phase 3, the plate was suspended by an arm directly over the natural gas

burner to test the combined effects of convection, heating, and flame lick. Figure 9

shows the general layout of the test, with the plate suspended above the gas burner

during phase 3. Figure 10 is a schematic diagram of the test configuration, viewed

from above, that shows the relationship of the plate, the burner, the digital image

correlation camera system, and the coordinate system. During phase 3 of the

test, with the burner directly under the plate, the heat output, P, was in the range

(0 < P < 70) kW with specific holds at 30 kW, 40 kW, 50 kW, and 70 kW.

A.3.1 Test Configuration

The original experimental plan was to use a natural gas fire to heat a instrumented

mild-steel plate, ultimately to realistic fire temperatures. The 30 cm x 45 cm x 1 cm

plate is instrumented with eight type K thermocouples that are peened into holes

drilled roughly half the depth of the plate. Thermocouples 2 and 3 are mounted

adjacent to gage 1, and thermocouple 7 is mounted adjacent to gage 2. Thermocouple 8 broke off during installation. The front side of the plate is painted with

a special high-temperature paint to produce a random black-and-white pattern to

enable strain measurement by digital image correlation. The back side of the plate
50

Figure 9: Experimental configuration during the Phase 3 of the demonstration test,

with the burner in position 2.
51

camera 1 camera 2

illumination illumination

burner position 1

x

z

plate

burner 

position 2

burner 

position 1

2330 mm

1378 mm

600 mm

400 mm

280 mm

Figure 10: Layout of the digital image correlation camera system and instrumented

plate. View from the top.
52

X

Y

(7.5,7.75)

(356,292)

3/8” THRU 3/8” THRU

(9,8)

Spot Mark on Reverse Side

Thermocouple Location

TC# 9

TC#8 TC#7

TC#4 TC#5 TC#6

TC#2 TC#3

TC#1

SG#1

SG#2

PLATE ID

(102,292)

(229,267)

#52 3.18 deep

(19,254)

(260,203)

#52 3.18 deep #52 3.18 deep #52 3.18 deep

#70 3.18 deep

#70 3.18 deep

#70 3.18 deep

#70 3.18 deep

(330,152) (228,152) (127,152)

(260,102)

(190,95) (229,102)

(229,19) (19,19)

#52 3.18 deep

(438,19)

Figure 11: Location of gages and thermocouples on the plate, as viewed from the

back side.

is instrumented with the eight thermocouples and two strain gages, which were installed by their manufacturer. Figure 11 shows the locations of the gages and the

thermocouples, viewed from the back side of the plate. The reference marks shown

in Figure 11 at 19 mm from the plate edge appear in the pattern on the front side

of the plate. These marks are used to rotate the DIC results to the plate coordinate

system shown in Figure 11.

A.3.2 Digital Image Correlation Setup

Figure 10 shows the orientation of the cameras to the plate for the demonstration

test , where the plate and burner are under the fume hood and the cameras and illumination are just outside of the hood. Two 5 MP Flea 2 CCD cameras are mounted

to an aluminum extrusion bar and are angled approximately ±16.5 degrees to the

surface normal, resulting in a 33 degree included angle. The mounting bar is about

2330 mm from the face of the plate at a height that places the cameras’ optical

centerlines at the initial centerline of the plate. Each individual image is recorded

as a 2448 pixel x 2048 pixel 8-bit gray-scale tagged image file (TIF). Both cameras
53

Z (mm)

1.500

1.375

1.250

1.125

1.000

0.875

0.750

0.625

0.500

0.375

0.250

0.125

0.000

-0.125

-0.250

450 337.5 225

112.5 0 X /mm

Y /mm

280

210

140

70

0

-0.250

0.188

0.625

1.060

1.500

Z /mm

Figure 12: Shape of the plate as measured by digital image correlation.

use Schneider 35 mm F1.9 C-mount Xenoplan compact lenses with locking focus.

Each lens was fitted with a UV-IR cut off filter (IF486). These filters pass more

than 90 % of the visible light in the range of (390 < λ < 690) nm. Based on this

setup, the average effective magnification of the plate was 0.243 mm/pixel. Two

500 W incandescent photographic lights illuminated the plate, with the intention to

overpower the potential light from the flame. Using this lighting and an aperture of

f/7 on the lenses, an exposure time of 15 ms was needed to achieve a good-quality

high-contrast image.

After initial set up, the camera, lenses, and their orientation to one another was

calibrated using 75 image pairs of a calibration grid of points (spaced at 38.1 mm).

These images were processed using the VIC-3D 2007 software from Correlated

Solutions Inc. An initial image pair of the hanging plate was processed using the

VIC-3D software and the calibration that was developed. The DIC 3D plot in

Figure 12 shows that the plate is slightly curved. It also illustrates the sensitivity

of the digital image correlation measurement to changes in shape.

Image acquisition during the demonstration test was performed using VIC

Snap 2007 software from Correlated Solutions Inc. During phases 1 and 2 of the

demonstration test, at each plate position, approximately 60 sequential image pairs

were acquired at intervals of 200 ms. During the phase 3 test, the reference image

pair was taken during phase 1, before the fire was lit. Then image pairs were acquired at 200 ms intervals until the fire was extinguished, after which time image

pairs were taken at 30 s intervals for another 4400 s. Post-processing of the image
54

Table 11: Characteristics of the HBWAH-12-250-6-2CB-HB weldable strain gage

Parameter Code Value

Shim thickness 0.13 mm (0.005 in)

Gage resistance 12 120 Ω

Shim material 6 Hastelloy X

Gage factor, G 2.42

Gage length 250 6.3 mm (0.25 in)

Cable 2CB braided Nextel 2 ft (610 mm) long

Extension wires HB Hoskins alloy 875

Figure 13: Bridge configuration

pairs used a subset of 31 pixels (approximately 7.5 mm) with a raster step size of

15 pixels (approximately 3.6 mm) resulting in about 9000 node points across the

face of the plate. These parameters were selected based on experience, and were

not optimized.

A.3.3 Strain Gages

The two strain gages were HITEC model HBWAH-12-250-6-2CB-HB gages. Table 11 summarizes the characteristics of the gages. These gages have an active

gage with R = 120 Ω attached to a Hastelloy X shim that is in turn spot welded

to the plate. A second compensation gage wired as the other leg in the Wheatstone

bridge, RT in Figure 13, is attached to a mild-steel “compensation block” that the

vendor supplied. The compensation block is welded to the shim at only one end, so

it expands freely with temperature. In this way, the thermal expansion of the plate

is compensated or nulled. If the temperature of the active and compensation gages
55

Figure 14: Schematic diagram of the strain gages used in the demonstration test.

changes the same amount, the changes in their resistance should also be nulled, and

produce no change in the output of the gage. In a non-isothermal test, it is possible

that the active and compensating gages will not be at the same temperature, and the

gage may register an apparent strain. The entire gage assembly is protected by a

stainless steel cover also welded to the plate. Figure 14 is a schematic diagram of

the structure of the gages, redrawn from the manufacturer’s literature. During both

tests that this report describes, the two gages were conditioned by Measurements

Group, Inc model 2210 signal conditioners, with 5.00 V excitation and 10x gain of

the resulting signal.

A.3.4 Strain Calculation from Gage Output

Figure 13 shows the generic half-bridge strain gage circuit. In the figure, RG is the

active gage, and RT is the temperature compensating gage. The ratio of the output

to input voltages, ignoring any gain, A, applied to the output in this circuit, is

V0

Vex

=

RT

RT + RG

−

R2

R1 + R2

(2)

The change in resistance with strain, , in a gage is defined as

∆Rx = GRx (3)
56

where the proportionality constant G, is termed the gage factor.

The compensating gage, RT is meant to correct or compensate for temperatureinduced changes in the resistance of the strain gage material and for thermal strains

in the material being measured, in this case the plate. If the strains in the active and

compensating gages are different, the bridge will have a non-zero voltage output

at V0. Assume that the resistances of the bridge-completion resistors are equal:

R1 = R2. Then Eq. (2) becomes

V0

Vex

=

1 + GT

2 + G(T + G)

−

1

2

(4)

If the strains in the active (“G”) and compensating (“T”) elements are equal, the

bridge output remains zero. If the strain in the active gage (“G”) is greater than

the strain in the compensating (“T”) gage, the first term is less than 1/2, and the

resulting output is negative. To first order, if the temperatures of the active and

compensating gages are identical, this configuration nulls the temperature-induced

change in resistance as well as the thermal expansion strain of the plate.

A.3.5 Demonstration Test Results

The results of each of the three phases of testing are described below in regards to

the specific effects of interest in each phase. Some comparisons are made across

different phases of testing where appropriate.

Effect of rigid body motion in the absence of heating or turbulence In phase 1

of the demonstration test, the plate was moved to nine different positions and the

resulting apparent plate strains, which should remain identically zero, were computed by digital image correlation. This set of results provides a baseline on the

uncertainty in the measurement in the absence of both heating and distortion of the

images caused by the turbulent atmospheric convection caused by the fire. Figure 15 shows the computed exx strain across the surface of the plate for an image

from phase 1 of the test. The contours of strains are superimposed on the actual

image of the plate. The strains, which should be identically zero, lie in the range

±150 μm/m.

Figure 16 shows the computed exx and eyy strains evaluated at a single position near strain gage 1 for nine different plate positions in space. The path of the

plate, computed from the digital image correlation image data is shown in Fig. 16a.

Although the individual determinations of exx and eyy are statistically distinguishable, the mean values range over less than 50 μm/m. In perspective, this uncertainty

corresponds to the elastic strain that a 10 MPa stress produces, or to the thermal

strain that a 4 ◦C temperature change produces.
57

Figure 15: Computed exx strain across the plate during phase 1.

Effect of convection on measurements by digital image correlation In phase 2

of the demonstration test, the burner was in position 1, between the digital image

correlation camera system and the plate, see Fig. 10. From these results it is possible to asses the effects of the turbulent column of air rising from the fire, in the

absence of significant heating to the plate. During this test segment the plate was

moved to nine different positions in a nominal x-y plane relative to the focal plane

of the camera system. The z coordinate is nominally out of the plane defined by

the plate. Since the translation of the plate amounts to a rigid-body motion, the

digital image correlation system should not measure any strain in the plate.

Figure 16d shows the computed strains in the x (horizontal) and y (vertical) directions on the plate, evaluated at a point directly opposite strain gage 1. Each determination at a given position consists of approximately 60 images, spaced nominally 0.2 s apart. The solid line is a moving regression to the entire data set. The

scatter in the data is 150 times as large as in phase 1, with no fire and minimal convection, Figure 16b and c, but the mean value at each position is not significantly

different from zero in all but two of the eighteen cases.

The two data sets depicted in Figure 17 illustrate the effect of the convection

on the measurement of strain and displacement in the absence of plate motion.

Figure 17 compares the x and y strains and the U, V , W displacements (corre-
58

Nominal plate X position /mm

Nominal plate Y position /mm

0

5

10

15

20

-40 -20 0 20 40

P 1

P 2 P 3 P 4 P 5

P 6

P 7

P 8

P 9

(a) plate path during phase 1 and 2

Position

strain / 

μ

m

m

-300

-200

-100

0

100

200

300

P1 P2 P3 P4 P5 P6 P7 P8 P9

exx

eyy

(b) phase 1 x and y strains, exx and eyy

Position

strain / m

m

-0.04

-0.02

0.00

0.02

0.04

P1 P2 P3 P4 P5 P6 P7 P8 P9

exx

eyy

(c) phase 1 x and y strains, exx and eyy

Position

strain / m

m

-0.04

-0.02

0.00

0.02

0.04

P1 P2 P3 P4 P5 P6 P7 P8 P9P9NF

(d) phase 2 x and y strains, exx and eyy

Figure 16: DIC-computed strains during phase 1 and phase 2. (a) path of the plate

in space during the tests; (b) x and y strains in phase 1; (c) x and y strains in

phase 1, same scale as in (d); (d) x and y strains in phase 2.
59

sponding to the x, y, and z directions relative to the plate) for the fire on and off

conditions. The scales for the vertical axes for the strains and the positions are

identical. Although the scatter in the strains exx and eyy are much larger with

the fire, and presumably with convection and resulting image distortion present,

the mean values are indistinguishable from zero. Figures 17c-e compare the DICdetermined position of strain gage 1 for the two conditions, on identical scales.

The U and V positions, corresponding to the x and y position in the plane of the

plate, are unchanged, though the presence of the fire increases the scatter. The W

position, essentially the position in the direction normal to the plane of the plate

changes between the two conditions. The most significant result of this phase of

the demonstration test is the invariance of the computed average in-plane strains

exx and eyy to the presence of the convective currents in front of the plate.

Effect of direct heating of the plate Figure 18 shows the output of the eight

thermocouples on the plate during phase 3, in which the burner heated the plate

directly. The plate reached a maximum temperature of about 200 ◦C during the test.

After the peak temperature was reached, the fire was extinguished and the plate

cooled naturally back to room temperature. Figure 19 shows the calculated thermal

expansion strain near the two gages (dashed lines). The strains were calculated

from a polynomial expression

∆L

L0

= l0 + l1T + l2T

2 + l3T3

(5)

The data for this expression, Table 12, come from the curve labeled “Provisional”

in Figure 261 of Touloukian’s [13] compendium of thermal expansion of metals

and alloys. Table 13 summarizes the regression of Eq. (5) on the data. Figure 19

also shows the strains (solid lines) computed from the output of the two strain

gages, using Eq. (4). The gages are designed to correct for thermal expansion

strains and for apparent strain caused by the change in temperature of the gage,

so their EMF should not change. Instead they each read an apparent compressive

strain.

Figure 20 plots the strains reported from the two strain gages, computed from

Eq. (4), the strains computed from the digital image correlation image data in the

region of gages 1 and 2, and and the thermal strains computed from the thermal

expansion data, Eq. (5). during a 100 s window near the time of the peak plate temperature. The lines for the DIC-computed strains are a moving fit to the individual

values for each image, since the digital image correlation strains and the temperature and strain-gage data are not on common time bases. The agreement between

the digital image correlation data and the predicted thermal expansion strain is

striking. Figure 21 plots the measured strains in the horizontal and vertical direc-
60

t /s

ex x

-0.02

-0.01

0.00

0.01

0.02

0.03

990 1000 1010 1020 1030

Fire on

Fire off

(a) x strain, exx

t /s

ey y

-0.02

-0.01

0.00

0.01

0.02

0.03

990 1000 1010 1020 1030

Fire on

Fire off

(b) y strain, eyy

t /s

U /mm

0.5

1.0

1.5

2.0

990 1000 1010 1020 1030

Fire on

Fire off

(c) x position, U

t /s

V /mm

-1.0

-0.5

0.0

0.5

990 1000 1010 1020 1030

Fire on

Fire off

(d) y position, V

t /s

W /mm

-0.5

0.0

0.5

1.0

990 1000 1010 1020 1030

Fire on

Fire off

(e) z position, W

Figure 17: DIC-computed strains and positions during the portion of the test with

the fire in burner position 1 (“between”) showing effect of convection “fire on”

(980 < t < 995) s vs. “fire off” (1025 < t < 1035) s.
61

time /s

Temperature /°C

50

100

150

200

0 2000 4000 6000

1

2

3

4

5

6

7

9

Figure 18: Temperature of the plate during phase 3 of the demonstration test.
62

time /s

strain / 

μ

m

m

-500

0

500

1000

1500

2000

0 2000 4000 6000

Strain gage 1

Strain gage 2

thermal strain near gage 1

thermal strain near gage 2

Figure 19: Computed thermal strain, , Eq. (5), and computed strain reported by the

two strain gages during the demonstration test.
63

time /s

strain / 

μ

m

m

-500

0

500

1000

1500

2000

1700 1720 1740 1760 1780 1800

Strain gage 1

Strain gage 2

thermal strain near gage 1

thermal strain near gage 2

strain (DIC) near gage 1

strain (DIC) near gage 2

Figure 20: Comparison of the x − x strains measured by DIC and the two strain

gages with the thermal expansion strain computed from Eq.(5) during the time of

the peak temperature.
64

T

Strain

0.0000

0.0005

0.0010

0.0015

0.0020

0 50 100 150 200

literature thermal strain (polynomial)

literature thermal strain (linearized) 

fit

Figure 21: Thermal strain measured by DIC as a function of temperature in the

regions of gages 1 and 2. The solid gray line is the thermal expansion strain computed from Eq. (5). Data for phase three of the demonstration test. In the region of

the data, that polynomial expression has been linearized (dashed line). The green

dash/dot line is the linear regression of the data.
65

Table 12: Data used to estimate the parameters of Eq. 5

T ∆L/L0

◦C

201 -0.0010339

265 -0.0003330

333 0.0003679

393 0.0010796

467 0.0020976

530 0.0029733

587 0.0037505

656 0.0048449

735 0.0060161

809 0.0072417

883 0.0083909

Table 13: Summary of the non-linear regression to estimate the parameters of Eq. 5

Parameter Estimate Standard Error Units t

l0 −2.4661×10−3 1.5289×10−4 −16.13

l1 4.8662×10−6 1.0132×10−6 ◦C

−1 4.80

l2 1.2640×10−8 2.0187×10−9 ◦C

−2 6.26

l3 −4.7712×10−12 1.2353×10−12◦C

−3 −3.86

RSD: 3.046 × 10−5 on 7 degrees of freedom

tions measured from the digital image correlation system in the region of gages 1

and 2 for two additional 100 s windows during the heating portion of phase 2, as

well as for one portion during the cooling portion, with the burner off. The solid

gray line is the thermal expansion strain computed from Eq. (5) referenced to the

temperature, T = 24.6

◦C . The dashed line is a linearization of that polynomial

expression in the range of the data. The dash/dot line is the fit to the data. The

slopes of the lines in the region of the data are quite similar: αfit = 1.30 × 10−5

and αlit = 1.28 × 10−5, a difference of less than 1.9 %In addition, the fit to the

strain data differs from the literature value by less than 11 % everywhere in the

range of the data. . Table 14 summarizes the data plotted in Figure 21. The temperature uncertainties in Table 14 represent the range of the temperatures during

the 100 s window. The strain uncertainties are the usual standard uncertainty of the
66

Table 14: Comparison of mean strains,  ̄ measured by digital image correlation

and computed from thermal expansion, Eq. (5), during four time increments during

phase 3 of the demonstration test. Data are plotted in Fig.21

Gage T e ̄xx e ̄yy ∆L/L0

◦C μm/m μm/m μm/m

1 126 ± 4 1258 ± 79 1422 ± 87 1208

1 87 ± 1 599 ± 772 713 ± 634 724

1 127 ± 2 1064 ± 1493 1292 ± 977 1217

1 181 ± 3 1915 ± 1628 2086 ± 1504 1926

2 123 ± 3 1255 ± 72 1317 ± 72 1171

2 84 ± 1 489 ± 706 543 ± 618 691

2 122 ± 2 840 ± 863 940 ± 951 1156

2 170 ± 3 1360 ± 1344 1575 ± 974 1778

mean value. In computing the mean values, strain points whose value was more

than two standard deviations from the mean value were excluded. These generally came from images in which a flame lick appears. In general, not more than

10 % of the images were excluded. The uncertainty in the data for temperatures

about 125 ◦C originate from a portion in the cooling phase, with no flame present.

To within the accuracy of the original expression for thermal expansion strain, the

digital image correlation system has measured the thermal expansion strains of the

plate.

A.4 Subsequent Gage Verification Tests

The strain gages were not fully temperature compensated during phase 3, see Figure 19, and produced apparent compressive strains on heating. Two other tests

provided information about the source of this discrepancy: a mechanical verification test at room temperature, and an isothermal heating test in an oven.

A.4.1 Gage Verification Test

After the demonstration test, we checked the proper room-temperature operation

of the two strain gages by verifying against the strain measured in the plate using

digital image correlation. We elastically bent the plate, while it was still suspended

in the test fixture, and measured the output of the gages on the compression side and

simultaneously measuring the elastic strain near the gage using the digital image
67

Strain measured using DIC

Strain from gages

0e+00

1e-04

2e-04

3e-04

4e-04

0e+00 1e-04 2e-04 3e-04 4e-04

Gage

1

2

Figure 22: Strains computed from the gage output compared to strains measured

using digital image correlation during elastic bending of the plate.

correlation setup.

Figure 22 shows the result of that test. The sign of the strain from the gages

has been reversed, since the gages were on the compression side of the plate, and

the DIC-measured strains were on the tensile side of the plate. The slope of the

solid line is unity. Given the simple bending fixture and all the approximations,

the agreement is excellent. The results of this experiment show that the gages are

operating as expected for mechanical deformations at room temperature.
68

Figure 23: The instrumented plate in the convection oven.

A.4.2 Oven Test

To test the possible contribution of the non-isothermal temperature field on the output of the strain gages, we conducted a heating and cooling test on the instrumented

plate in a laboratory convection oven with isothermal holds at 150 ◦C, 200 ◦C, and

250 ◦C. Figure 23 shows the plate mounted in the convection oven.

Figure 24 summarizes the results of the oven test to determine the response

of the strain gages to temperature. Figure 24a shows the plate temperature during the oven test. The third, fourth, and fifth holds could have been longer, but

the plate temperature was constant to within a degree during those holds as well.

The leading curve labeled “monitor” corresponds to a thermocouple mounted in

the airspace above the plate. Figure 24b shows the output of gage 2 during the

oven test. The negative offset at each temperature is reproducible and depends linearly on temperature, see Figure 24c. Figure 24d shows the voltage output data

of Figure 24b transformed to strain, after including the A = 10 gain of the output

signal.

The linearity and reproducibility of the gage voltage with temperature, Figure 24c, demonstrate that the apparent strain from the gage is an inherent limitation

of the gage and is not driven by differences in temperature between the active and
69

t /s

T /°C

50

100

150

200

250

0 20000 40000 60000

TC2

TC3

TC7

MonitorTC

(a) Plate temperatures

t /s

Gage emf /V

-0.015

-0.010

-0.005

0.000

0 20000 40000 60000

(b) Gage 2 output

T /°C

V

o /V

-0.015

-0.010

-0.005

0.000

50 100 150 200

(c) Gage 2 output during isothermal portions

t /s

Gage 2 strain /

μ

m

m

-600

-500

-400

-300

-200

-100

0

0 20000 40000 60000

(d) Gage 2 apparent strain

Figure 24: Summary of oven test to determine the response of the gages to temperature.
70

compensating elements.

Apparent strains with heating in compensated and uncompensated high-temperature gages have been commonly reported [10, 11, 14]. Under the heading “Precautions for static strain measurements,” the “Strain Gage Users’ Handbook” states

[14],

In order to achieve accurate strain measurements at high temperature,

the investigator must pre-calibrate the bonded strain gage over the test

temperature range and record a zero-stress apparent strain curve.

Pre-calibrating the actual gage used in a high-temperature fire test will be very

difficult, since the structure cannot be isothermally heated in a furnace. Some

calibration curve could be made from a sample taken from the structural steel, but

the individual gages may not be completely identical.

A.5 Findings, Conclusions, and Recommendations

Although the temperature of the plate did not reach the temperatures that steel connections reach in fire, the results of the demonstration test encouragingly support

the feasibility of digital image correlation for high-temperature strain measurement

in fire. Specific, significant findings include

• Commercial high-temperature paint can survive up to 700 ◦C for short times.

(Section A.2.2)

• Rigid body translations produced no apparent strain for a plate behind a fire.

(Section A.3.5)

• Digital image correlation successfully measured the thermal expansion strain

of a plate heated by fire. (Section A.3.5)

• Conventional high-temperature strain gages were not fully temperature compensated, and produced apparent compressive strains on heating. (Section A.3.5

and Figure 19)

These four findings lead to four conclusions about the measurement of strains in

fire conditions.

1. Commercial high-temperature paints can survive fire exposure long enough

to be used in digital image correlation measurements in fire.

2. The data acquisition and analysis of the images employed no esoteric measures. The test did not employ any out-of-the ordinary illumination techniques or special image averaging, filtering, or transformations. Both of
71

these approaches have significant potential to improve the quality of the measurements.

3. Digital image correlation has significant promise for measuring strains and

deformations in fire, and could revolutionize structural fire measurements.

4. Despite decades of development, high-temperature strain gages are still difficult to use, and have poorly understood thermal response.

The conclusions of this demonstration test lead to four recommendations for developing digital image correlation as a strain-measurement tool for the National Fire

Research Laboratory.

1. A follow-on test that employs a fire powerful enough to heat the plate to

more than 600 ◦C should be conducted. This test will probe the adherence

of the paint.

2. Further tests should explore methods for post-processing the digital image

correlation data to reduce the uncertainty in strains.

3. Further tests should add actual mechanical deformation deformation to the

thermal expansion measured in the initial test.

4. Laboratory-scale tests in controlled environments should be used to develop

strain-measurement methods for deformation so that quantifiable uncertainties can be established for the method.

References

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