NASA CASE NO. LAR 14559-1

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PRINT FIG. 1 /

NOTICE

The invention disclosed in this document resulted from research in aeronautical and

space activities performed under programs of the National Aeronautics and Space

Administration. The invention is owned by NASA and is, therefore, available for licensing

in accordance with the NASA Patent Licensing Regulation (14 Code of Federal

Regulations 1245.2).

To encourage commercial utilization of NASA-owned inventions, it is NASA policy to grant

licenses to commercial concerns. Although NASA encourages nonexclusive licensing to

promote competition and achieve the widest possible utilization, NASA will provide the

necessary incentive to the licensee to achieve early practical application of the invention.

Address inquiries and all applications for license for this invention to NASA Patent

Counsel, Langley Research Center, Code 143, Hampton, Virginia 23665-5225.

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Serial No.: 07/878,631

Filed: 05-05-92 LaRC

( N'ASA-C ase-LA_-1455_-] ) mETHE)O AND

APPA_ATI-J5 Fn_R THrRMOGRAP_[CALLY AND

_UANTIIATIV_LY ANALYZING A STRUCTURF

C]'SqL_'_L,q, AN.F]/OR INCLUSIONS Patent

ADpI ic_ition (_ASA) 19 p

FOR

N92-29829

Unc] as

03/38 0111099
METHOD AND APPARATUS FOR THERMOGRAPHICALLY AND 

QUANTITATIVELY ANALYZING A STRUCTURE FOR DISBONDS 

AND/OR INCLUSIONS 

Awards Abstract 

NASA Case No. LAR 14559-1 

Laminated structures are used in numerous applications such as aerospace 

vehicles. The nondestructive detection of disbonds and/or inclusions in the laminate is 

paramount to ensure mission safety. Current techniques are hampered by an inability to 

inspect inaccessible or difficult geometries and by subjective qualitative comparison. 

The present invention employs a heat source such as a magnetic induction/eddy 

current generator to remotely heat a region of a surface of a test structure to a desired 

depth. For example, the frequency of the heating source can be varied to heat to the 

desired depth. A thermal sensor senses temperature changes in the heated region as 

a function of time. A computer compares these sensed temperature changes with 

calibration standards of a similar sample having known disbond and/or inclusion 

geography(ies) to analyze the test structure. A plurality of sensors can be arranged 

linearly to sense vector heat flow. 

Novel aspects of the present invention include its quantitative analysis of 

temperature changes as a function of time. 

Inventors: Joseph S. Heyman William P. Winfree 

Address: 130 Indian Springs Road 131 Cherwell Court 

Williamsburg, VA 231 85 

SS: - SS: 

Employer: NASA LaRC 

Initial Evaluator: Jag J. Singh 

Serial No.: 07/878,631 

Filed: May 5, 1992 
LAR 14559-1 -1- PATENT APPLICATION

METHOD AND APPARATUS FOR THERMOGRAPHICALLY AND

QUANTITATIVELY ANALYZING A STRUCTURE FOR DISBONDS

AND/OR INCLUSIONS

5 Origin of the Invention

10

The invention described herein was made by employees of the United

States Government and may be used by or for the Government for

governmental purposes without the payment of any royalties thereon or

therefor.

Background of the Invention

15

2O

o

m

Technical Field of the Invention

The present invention relates generally to the nondestructive evaluation

of thermal properties and more particularly to a method and apparatus for

quantitatively and thermographically analyzing a structure for disbonds and/or

inclusions.

o

n Description of the Prior Art

The use of laminated structures in aerospace applications has increased

in recent years. It has been demonstrated that thermographic techniques such

25 as laser scanners, infrared (IR) cameras and contact temperature sensors such

as thermocouples for inspection of these structures for disbonds has significant

advantage in portability, expense and adaptability to field use compared to

other methods. Many of these thermal techniques use full field noncontacting

imaging and accordingly require a significant amount of equipment, making the

30 inspection limited to areas of easy access. Many of the test structures currently
5

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15

technique of Adams et

encompass the entire

determination.

LAR 14559-1 -2- PATENT APPLICATION

in use are located in a somewhat inaccessible area, or their geometry is such

that it is impractical to attempt to use a full field IR imager for inspection.

U.S. Patent Nos. 4,647,220 and 4,854,724 to Adams et al disclose IR

analysis of corrosion spots and spot welds. The test area is heated with a

pulsed IR heat input and problem areas identified after cooling as localized hot

spots, i.e., a relative measure is obtained with respect to surrounding areas

rather than a quantitative value related to thermal properties over time. The

al is unable to recognize imperfections which

heated area since it relies on a comparative

U.S. Patent No. 4,950,990 to Moulder et al discloses a system for

photoinductive imaging for material flaw detection. A laser heats a specified

area of the material and a probe detects the resulting eddy currents which are

indicative of flaws or holes.

Objects of the Invention

2O

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30

It is accordingly an object of the present invention to detect disbonds in

a laminated or composite structure.

It is another object of the present invention to detect disbonds in a

laminated or composite structure quantitatively.

It is another object of the present invention to differentiate between

various inclusions in a laminated or composite structure.

It is a further object of the present invention to differentiate between

various inclusions in a laminated or composite structure quantitatively.

It is a further object of the present invention to accomplish the foregoing

objects for relatively inaccessible laminated or composite structures.

Other objects and advantages of the present invention are apparent from

the specification and drawings which follow.
LAR 14559-1

Summary of the Invention

-3- PATENT APPLICATION

5

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The foregoing and additional objects are obtained by a method and

apparatus according to the present invention. A heat source such as a

magnetic induction/eddy current generator remotely heats a region of a surface

of a test structure to a desired depth. For example, the frequency of the

heating source can be varied to heat to the desired depth. A thermal sensor

senses temperature changes in the heated region as a function of time. A

computer compares these sensed temperature changes with calibration

standards of a similar sample having known disbond and/or inclusion

geography(ies) to analyze the test structure. A plurality of sensors can be

arranged linearly to sense vector heat flow.

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Brief Description of the Drawin.qs

FIG. 1 is a side view of a thermographic disbond detection system

according to the present invention;

FIG. 2 is a front view of the thermographic disbond detection system of

FIG. 1;

FIG. 3 is a block diagram of the thermograph disbond detection system

of FIG. 1;

FIG. 4a is an exposed side view of a magnetic induction heating source

used in the disbond detection system of FIG. 1;

FIG. 4b is a front view of FIG. 3;

FIG. 5 is a front view of the disbond detection system using three linearly

arranged _ ,mal sensors;

Fi is a front view of the disbond detection system using the three

sensor at, gement of FIG. 5 plus an additional thermal sensor;

Fl_. / graphs the normalized temperature change over time for a steelrubber-aluminum laminate sample with a disbond region and a bonded region;
5

10

LAR 14559-1 -4- PATENT APPLICATION

FIG. 8 graphs the mean value of twenty measurements taken of each

region of the sample of FIG. 7;

FIG. 9 graphs the normalized temperature change over time for a

graphite-epoxy composite sample having a no inclusion region, a rubber

inclusion region, and a grease inclusion region located in the sample midplane;

FIG. 10 is an electrical analog circuit for the pyroelectric detector used

as a thermal sensor of the thermographic disbond detection system of the

present invention;

FIG. 11 graphs the modeled and experimental normalized response over

time of the pyroelectric detector of FIG. 10; and

FIG. 12 graphs the normalized temperature change over time for a

numerical simulation of the sample of FIG. 7.

15

Detailed Description

The present invention is shown it1 reference to FIG. 1, wherein a

thermographic disbond detection system analyzes a sample S containing a flaw

F. The detection system presents a quantitative digital display 12 of the

effective thermal properties of the sample as discussed below. Heat is injected

20 into the sample by the heating source 14 and is detected by the thermal sensor

16.

FIG. 2 shows a detail of the front of detection system consisting of the

heat source 14, thermal sensor 16 and three individual contact sensors 18. For

example, the thermal sensor for the detection is a thin film, pyroelectric detector

25 such as the Model PPF-110 available from Microwatt Applications. The

detector element is made of polyvinylidene fluoride (PVF2) and is 1.0 mm in

diameter. This detector contains a low noise FED impedance matching

preamplifier within the element housing. The bandwidth for the spectral

response of the detector is limited to the 8-14 micron wavelength range by an

30 AR coated filter/window. The output of the thermal sensor 16 is then amplified
LAR 14559-1 -5- PATENT APPLICATION

with a simple, two-stage buffered amplifier (not shown) constructed using

ultralow-noise precision operation amplifiers suchas Model AD OP-27 available

from Analog Devices. This amplifier provides a signal gain of 100. Further, a

0.1 to 45 Hz bandpass filter (not shown) was incorporated intothe second state

5 of the amplifier to eliminate any DC offset and drift and to help reduce high

frequency noise. This configuration provided a noise floor for the output signal

of +20 mV with a dynamic signal range of +13 V. The thermal sensor 16 was

positioned so that its field of view was restricted to a small region, e.g.,

approximately 6 mm in diameter, in the center of the heating area.

10 Alternatively, the thermal sensor is an IR detector.

The block diagram in FIG. 3 outlines the operation of the _y_,tem. A

logic microprocessor/bus 20 of a computer controls the entire system and

through the bus communicates to all the system parts. The sequence is started

by firing a charged energy capacitor 22 to drive the heat source 14, which can

15 be a flash tube having a rapid discharge. The energy capacitor 22 is recharged

by a recharge circuit 24. The result of the discharge is to heat the sample

which is measured by the thermal sensor 16. The signal produced by the

detector is digitized by the signal digitizer 26 and stored in memory 28 of the

computer. The output of the detector circuit was digitized for analysis and

20 archival with the personal computer and a 12 bit analog-to-digital converter

board which allowed for data collection at rates up to 142 KHz.

The heating source 14 can be a magnel,c inductance device or eddy

current heater, wherein the energy lost in the sample through eddy currents

heats the sample region. The eddy current heater was constructed from a

25 horseshoe-shaped ferrite core wrapped with transformer wire and driven by an

RF power amplifier. An associated power amplifier (not shown) was modulated

at the resonant frequency of the wrapped core coupled to the sample. This

frequency was determined through manual tuning of the system to provide

maximum power input into the sample. The inductive device can be of variable

30 frequency so as to heat the near surface at a high frequency or the bulk at low
5

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LAR 14559-1 -6- PATENT APPLICATION

frequency to differentiate the thermal properties of the bulk or the surface of the

tested sample. For example, at 1MHz,the skin depth in iron is about 0.02mm

while at 10KHz,the depth is -0.2ram. The heating source 14can be controlled

by the microprocessor 22 to allow for synchronization between the start of

heating and the start of data collection.

The center frequency of the magnetic source is determined by the

resonance of the magnetic heating source 14, when coupled to the output of

the energy capacitive source or driver 22, as a LC resonator governed by:

f = 1 (2_ (q'-_--C)), where f is the resonance frequency, L is the inductance and

C the capacitance. The resonance circuit can be driven for a number of cycles

as a tone burst, or can be pulse excited.

The capacitive recharge circuit 24 can switch to different capacitors to

vary the resonance frequency. With this technique, one can perform a first

measurement at high frequency heating only at the near surface. The radiated

energy just after the excitation is a measure of the emissivity of the surface for

a calibration factor. For high frequencies the material thermal properties have

the appearance of extending to infinity. The surface temperature of the

material is relatedto the emissivity of the sample, the input flux and the thermal

conductivity of the sample. This factor is a normalizing constant which appears

in all subsequent analytical representations of the temperature. A second

heating at a lower frequency results in a measurement of the interior of the

material.

The thermal sensor 16 can be alternatively radiometric based on

balometers or photon statistics, or can be contact based on any of the many

temperature sensors such as thermoelectric effect, optical fiber sensors or

junction electronic devices.

More than one sensor can be set into the front of the detector 10 as is

shown in FIG. 5. The thermal sensors are indicated by 16a, 16b, and 16c.

These three sensors permit a measurement of the vector flow of heat from the

central heating source out to the periphery of the detector 10, i.e., the
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LAR 14559-1 -7- PATENT APPLICATION

outermost thermal sensor. The measurement of the vector heat flow in the

material enables a determination of the diffusivity tensor in the material. By

comparison of the different components of the diffusivity tensor important

parameters such as fiber direction or fiber volume fraction in graphite fiber

reinforce composites is possible. The detector is rotated to indicate diffusion

anistropy in the sample as is found in composite materials. The detector 10 is

self-aligning via the three contact sensors 18 which permit the detector 10, and

particularly thermal sensor 16, to be repeatably positioned with respect to the

sample S. The detector 10determines vector heat flow through a linear pattern

of sensors or a two-dimensional pattern such as shown in FIG. 6. Three

thermal sensors 16a, 16b, and 16c are arranged in a line and a fourth thermal

sensor 16d is arranged non-linearly. In the specific embodiment depicted,

fourth thermal sensor 16d is arranged linearly with the center sensor 16b of the

three sensors along a line perpendicular to the line of the three sensors.

The operation of the present invention will now be discussed in greater

detail. A steel-rubber-aluminum laminate sample S was fabricated with a

known defect or flaw F in an inner layer. The sample consisted of a sheet of

steel 0.16 cm thick, backed by a layer of rubber which was 0.63 cm thick, which

was backed by a layer of aluminum 2.54¢_n thick The entire sample was 19.0

cm square. The layers were l_n_:led together with a slow curing two-part

epoxy. A disbond was produced by removing a known portion of the middle

rubber layer before assembly of the sample.

The sample was heated for five seconds using an eddy current or

magnetic induction heating source 14. Measurements were made at a rate of

1.7 KHz via the pyroelectric thermal sensor 16 and every twenty consecutive

measurements were averaged to increase the signal-to-noise ratio.

Measurements were first performed over a bonded region, then the sample was

repositioned so that the detector and heater were over a disbonded region.

FIG. 7 shows the results of this data collection.

FIG. 8 shows the means value of twenty measurements taken on the
LAR 14559-1 -8- PATENT APPLICATION

sample over tile course of approximately three hours. Measurements were

randomly alternated between bonded and disbonded regions. The error bars

indicated one standard deviation from the mean. Further analysis indicates that

after three seconds the difference between the two measurements is greater

5 than two standard deviations. Therefore there is less than a 5% probability of

mistaken identification of bonding in that region.

Another sample which was examined was a graphite-epoxy composite.

This 16-ply composite sample was manufactured with various midplane

inclusions between the eighth and ninth plies. This 30.48 cm square composite

10 had plies laid up at 0°, +45 °, -45 °, and 90°. Measurements were made over

three regions. The first was a region of no inclusion. The second was over a

region containing a rubber inclusion. The last was over a region where silicon

grease was included between the plies. FIG. 9 shows the results of these

measurements. Again data was collected for five seconds with the pyroelectric

15 thermal sensor 16 at a sampling rate of 1.7 KHz with twenty samples being

averaged. As can be seen, the regions of inclusion are clearly distinguishable

from the fully bonded region, having greater temperature changes than a no

inclusion region over the same period. Further there is some separation

between the two types of inclusions, though slight because of the similarity in

20 thermal properties between the grease and the rubber.

The injection of heat into the surface of a laminated structure with a

disbond results in a surface temperature profile which increases in the region

of disbond. By heating over only a small region of the surface and then

recording the rate of temperature change at the center of the heating area, a

25 determination of the bond integrity of the laminate at that location can be made.

An investigation of this technique for disbond detection was done by

computational simulation of the thermographic technique. A finite element heat

transfer algorithm developed at Lawrence Livermore National Laboratory and

described by Arthur Shapiro in "TOPAZ2D-A Two-Dimensional Finite Element

30 Code for Heat Transfer Analysis, Electrostatics, and Magnetostatics Problems,"
LAR 14559-1 -9- PATENT APPLICATION

UCID-20824, Lawrence Livermore National Laboratory, Livermore, California

94550, July, 1986, was used to mode the three-layer laminated structure. The

laminated sample was constructed with a 0.16 cm thick front surface of steel,

bonded to a 0.318 cm thick layer of rubber, finally bonded to a 2.54 cm thick

5 layer of aluminum. This sample was 19.0 cm square. A two-dimensional

simulation was performed onthis sample configuration using a 25 x 25 element

grid clustered around the disbond where the temperature gradients are the

largest. The clustering was done using a hyperbolic sine transformation

equation, described by Patricia H. James, Christopher S. Welch, William P.

10 Winfree in "A Numerical Grid Generation Scheme for Thermal Simulations in

Laminated Structures," Review of Progress in Quantitative Nondestructive

Evaluation, Vol. 8A, Plenum Publishing Corp., New York, New York, 1989,

pp. 801-809. The front surface boundary condition was flux heating and

convective cooling.

15 The output of the finite element model gives the surface temperature of

the sample as a function of time. To determine the response of the system of

interest, this time evolution must be convolved with the response of the

detector.

As a pyroelectric detector, the thermal sensor is sensitive to variations

20 in temperature. The response of the detector is a function of both its thermal

and electrical responses. These contributions are shown in an electrical analog

circuit for the detector depicted in FIG. 10.

The electrical response is due to the physical capacitance of the sensor

element and the internal resistance provided by the manufacturer for coupling

25 to the input of the FET preamplifier. A potential difference arises across the

detector element when a temperature differential is present. This electrical

response can be thought of as a high pass filter with a time constant "_1 equal

to ReC. in the equivalent circuit. The output of such a filter to some arbitrary

input wave V_(t) is given by:

30
LAR 14559-1 -10- PATENT APPLICATION

_ (_.t)

Vo,( t). Vl( t)_ Vl( O) e ",

- (_)

li.[otVi(t__) 8 ,1 d.c

(1),

where the input voltage V_(t) is in reality the surface temperature of the sample

5 viewed by the detector.

The second component of the thermal sensor response is thermal in

nature. It arises from the physical properties of the detector element. This

thermal response is analogous to a low pass filter, providing some maximum

rate of temperature change that the detector can respond to. As was done with

10 the electrical response, the response of the thermal component can be

expressed as

_(_t) _-_ [°t _(__,)

Vo(t ) - Voo(O)e "" - ( ) Voe(t-_)e " d._

(2)

15

20

where % is the time constant for the low pass filter and equal to RtCt in the

equivalent circuit. The input function (Vo,(t) is the output of the electrical

response component given by equation (1).

Equations (1) and (2) can be evaluated numerically quite easily for some

discrete set of points V_(t). The points used for the input would be the time

history of the surface temperature as calculated by the finite element model.

Before the thermal sensor response can be calculated, it is necessary

to determine the time ¢onCtants for each component of the response. This was

done experimentally. A mechanical shutter was placed between the detector

and a blackbody source. The shutter was opened for five seconds, and the
LAR 14559-1 -11- PATENT APPLICATION

output of the detector was measured. These measurements were normalized

to give a maximum output of 1. These normalized measurements were

compared to a simulation of the response of the detector by sequentially

applying equations (1) and (2) to a five seconds square pulse input with an

5 amplitude fixed to give a maximum detector response of 1. The time constants

(_1 and _2) were varied to locate a minimum squared deviation between the

simulated and measured response of the detector.

A comparison of the simulation response with the minimum squared

deviation and measured response of the thermal sensor is shown in FIG. 11.

10 The electrical and thermal time constants determined for this simulation are

2.32 and 0.15 seconds respectively. These values fall within the range cited

by the manufacturer Microwatt Applications of the thermal sensor.

Two finite element simulations were then performed--one with heating

over the disbonded region of the model, the second with heating over a bonded

15 region. The results of each of these runs were then convolved with the

simulated detector response. The sample used was the steel-rubber-aluminum

laminate described above.

FIG. 12 shows the simulation results for both simulation runs. The

model results have been normalized to the maximum temperature change for

20 the bonded region. As can be seen from a comparison of this figure to FIG. 7,

the numerical simulations agree with the experimental results. The average

deviation of the experimental results from the numerical simulations is

approximately four percent of the full scale normalized response.

The requirements and responses for a prototype thermal detection

25 system have been presented. This system has been shown to detect the

presence of subsurface disbonds in laminated structures. This detection has

been shown by statistical analysis to be at least 95% accurate. Application has

been shown to both multilayered laminates and to composite structures.

Further, an analysis method has been presented for modeling the response of

30 the detector, thus allowing comparison of the experimental results with results
LAR 14559-1 -12- PATENT APPLICATION

from numerical simulations. This comparison shows good agreement thus

verifying the technique used by the system.

Calibration standards are determined by generating data curves as

discussed in reference to FIGS. 7-9, 11 and 12, i.e., are either experimentally

5 determined or numerically simulated for a laminate sample having a known

construction and disbond geography and/or inclusion geography. A test

laminate structure is "then tested as described above and the obtained

temperature changes over time are correlated with the previously determined

calibration standards, whereby the disbond and/or inclusion geography of the

10 test structure are determined. All necessary calculations and comparisons can

be performed by the personal computer. The particular program employed can

either display a quantitative number on the digital display indicative of

disbond(s) and/or inclusion(s), e.g., "100" indicating a complete

disbond/inclusion of the region at the depth of interest and "0" indicating a good

15 bond with intermediate values indicating corresponding intermediate conditions,

or display a specific conclusion based on the quantitative assessment of the

sensed temperature changes, e.g., "satisfactory" or "unsatisfactory",

"disbonded" or "bonded", "further testing", etc.

Many modifications, improvements and substitutions will be apparent to

20 the skilled artisan without departing from the spirit and scope of the present

invention as described herein and defined in the following claims.
LAR 14559-1 _ PATENT APPLICATION

METHOD AND APPARATUS FOR THERMOGRAPHICALLY AND

QUANTITATIVELY ANALYZING A STRUCTURE FOR DISBONDS

AND/OR INCLUSIONS

5 Abstract of the Disclosure

10

15

A heat source such as a magnetic induction/eddy current generator

remotely heats a region of a surface of a test structure to a desired depth. For

example, the frequency of the heating source can be varied to heat to the

desired depth. A thermal sensor senses temperature changes in the heated

region as a function of time. A computer compares these sensed temperature

changes with calibration standards of a similar sample having known disbond

and/or inclusion geography(ies) to analyze the test structure. A plurality of

sensors can be arranged linearly to sense vector heat flow.
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