NISTIR 7902

Fatigue Flaw NDE Reference 

Standard Development Phase I—

Feasibility Study

Mark D. Richards 

J. David McColskey 

Timothy S. Weeks

http://dx.doi.org/10.6028/NIST.IR.7902
NISTIR 7902

Fatigue Flaw NDE Reference 

Standard Development Phase I—

Feasibility Study

Mark D. Richards 

J. David McColskey 

Timothy S. Weeks

Applied Chemicals and Materials Division

Material Measurement Laboratory

http://dx.doi.org/10.6028/NIST.IR.7902

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
ii

Abstract

A feasibility study was performed to evaluate a novel method of accurately fatigue-cracking steel plates to generate proposed 

fatigue-crack reference standards. Fatigue cracks were introduced into low-carbon steel reference plates and the actual size of 

the fatigue cracks were accurately predicted by the fatigue-crack introduction technique. Flaw sizing of the fatigue cracks in 

the plates was conducted by industrial collaborators through use of various nondestructive evaluation (NDE) techniques. As 

part of the feasibility study, a limited round robin analysis was conducted that included NDE sensor manufacturers and users, 

that demonstrated the fatigue cracks in the reference plates could be located and sized. The round robin demonstrated 

significant scatter in the NDE data, and the crack depth and surface breaking length values (a × 2c) were both over-and underpredicted as compared to the actual values of the reference plates. The NDE community acknowledges a genuine need for a 

fatigue crack reference standard by which NDE sensors and technologies can be calibrated and verified for the accurate sizing 

of fatigue flaws in structural components.

Keywords

Artifact, fatigue crack, flaw sizing, NDE, non-destructive evaluation, phased array ultrasonic, reference standard.

Table of Contents
iii

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

2. Methods.................................................................................................................................................................................2

2.1 Materials......................................................................................................................................................................2

2.2 Notching ......................................................................................................................................................................2

2.3 Fatigue Crack Introduction ..........................................................................................................................................3

2.3 Reference Standard Preparation...................................................................................................................................7

3. Results.................................................................................................................................................................................10

3.1 Liberation of the Fatigue Crack Surface ....................................................................................................................10

3.1 Round Robin NDE Analysis......................................................................................................................................12

4. Discussion ...........................................................................................................................................................................14

5. Conclusions.........................................................................................................................................................................14

Acknowledgements.....................................................................................................................................................................14

Bibliography................................................................................................................................................................................15
1

1. Introduction

Nondestructive evaluation (NDE)

1 techniques are widely used to detect flaws in critical structures in chemical 

processing facilities, nuclear reactors, oil and gas pipelines, bridges and other infrastructure. If these flaws are undetected, 

they can compromise the integrity of the structure and potentially lead to catastrophic failure, extensive property damage, 

injury, and in many cases, loss of human life. More recently, structural designers have adopted a “damage-tolerant” design 

with use of engineering critical assessment (ECA). In this paradigm, components having known defects can continue in 

service as long as models predict that the defects will not grow to a critical, failure-producing size. As a result, the simple 

detection of defects is insufficient; accurate flaw sizing is critical for predicting the remaining service life of a given structure 

[1], and determining structural reliability and prediction of structural lifetime [2].

Reference standards

2 are used as calibration tools for NDE systems and are critical both to establish a general level of 

consistency in measurements and to help interpret and quantify the information contained in the received signal. They ensure 

that the equipment and the setup provide similar results from one day to the next and that similar results are produced by 

different systems and different inspectors. They can also help the inspector estimate the size of flaws. For example, 

ultrasonic testing (UT), the most common of the inspection techniques described here, employs high-frequency (typically 

from 2 × 105 Hz to a maximum of 107 Hz) sound energy to detect flaws and make measurements. In a UT pulse-echo type 

setup, signal strength depends on both the size of the flaw and the distance between the flaw and the transducer. The inspector 

can use a reference standard to produce a signal from an artificially-induced flaw of known size and distance from the 

transducer. By comparing the signal from the reference standard to that received from the actual flaw, the inspector can detect 

the flaw and gain some insight about its size. However, precise flaw sizing is not possible with this technique.

An example of a current reference standard is a block described by the International Institute of Welding. This block 

is machined from a material that is acoustically similar to the material to be tested. The block has several “defects” that are 

machined into it for calibration purposes, including machined right circular cylindrical holes, notches and surfaces. While 

these “defects” are precisely machined, they are not representative of true flaws in a structure and do not provide a calibration 

standard for NDT equipment sufficient for accurate flaw sizing. In most cases, artificially induced defects more efficiently 

reflect sound energy (due to their flatter, smoother, machined surfaces) and produce indications that are larger than those that 

a similar sized actual flaw (fatigue crack, weld defect, wall thinning due to corrosion, etc.) would produce. Accurate 

characterization of fatigue cracks is difficult, because the contact of surface asperities during crack closure makes the crack 

nearly transparent to the ultrasonic wave [2]. A significant challenge in quantifying the performance of NDE techniques for 

their accuracy and reliability for probability of detection and flaw sizing is the development of reference artifacts that contain 

representative cracks or discontinuities [3]. For this reason, the industry requires reference standards that contain 

representative, accurately characterized “cracks” to enable methods of accurately sizing of actual flaws in the field.

Despite the fact that calibration of NDE instrumentation has been noted as a serious challenge for inspectors, there 

are no “true-crack” reference artifacts available for instrument calibration that are certified or traceable to standards of known 

crack dimensions. Such artifacts must be representative of the material to be inspected and precisely characterized with 

regards to crack size (width, depth and shape). Researchers [4-6] have demonstrated the resulting considerable improvement 

in flaw-sizing accuracy when a natural flaw was used as a reference standard. However, a reliable method of production of 

more realistic defects has not been available, and the development of a fatigue crack reference standard has been considered 

cost prohibitive. Based on techniques developed for growing well-characterized fatigue cracks in machined test specimens

1 NDE includes, but is not limited to: ultrasonic methods [ultrasonic testing (UT), electromagnetic acoustic transducer 

(EMAT), phased array ultrasound, and long-range guided wave (LRGW)]; electromagnetic methods [eddy current (EC) and 

magnetic flux leakage (MFL)]; radiographic methods; thermographic methods; and dynamic methods (resonant frequency and 

attenuation testing).

2 The leading standards organizations in this area include the International Standards Organization (ISO), the American 

Welding Society (AWS) and the American Society for Testing and Materials, International (ASTM). The responsible ISO 

committee is TC135/SC3, “Ultrasonic Testing”; and the primary standard relative to the proposed SRM is ISO 12715, 

“Nondestructive testing - Ultrasonic inspection - Reference blocks and test procedures for the characterization of contact 

search unit beam profiles.” AWS provides a related reference block called the IIW (International Institute of Welding) block, 

and ASTM provides several recommended standards useful in the calibrations of NDT equipment including ASTM E164, 

E127, E428, E2491, and E797.
2

[7], the development of standard reference materials (SRMs) containing real cracks can be manufactured at a reasonable cost; 

and their use will enable more accurate, cost-effective sizing of flaws in pipelines, structural components such as bridges, 

nuclear power plants and pressure vessels.

2. Methods

2.1 Materials

Two materials were selected and procured for the present standard reference material (SRM) development program. 

The first material is an API-5L (American Petroleum Institute) X80 linepipe steel [551 MPa (80 ksi) nominal yield strength] 

that was received in 1.067 m (42 in) outer diameter and 14.5 mm wall thickness condition. Strips of base metal were 

sectioned from the pipe and made into 254 mm (10 in.) × 254 mm (10 in.) square specimens. The second material is an

ASTM A709 grade that has a nominal chemical composition of 0.20 % C, 1.09 % Mn, 0.18 % Ni and 0.52 % Cr by mass with

V microalloying additions. The A709 grade was received in two geometries; both were flat plate with a nominal thickness of

25.4 mm, widths of 101.6 mm and 152.2 mm and lengths of 305 mm. The A709 material was supplied with a nominal yield 

strength of 384 MPa and tensile strength of 562 MPa. Room temperature tensile engineering stress versus strain data for the 

A709 material are shown in Figure 1.

Figure 1 – Tensile engineering stress versus engineering strain data for A709 alloy tested at room temperature at a 

constant engineering strain rate of ė = 5.75 × 10-5 s

-1

.

2.2 Notching

Fatigue crack starter notches were introduced into the surface of the pipe and plate specimens by use of a sinker type 

electrical discharge machining (EDM) process. A starter notch is a manufactured stress concentration artifact through which 

the higher stress intensity generated at the periphery of the starter notch allows for the initiation of fatigue cracks at low 

applied force levels and allows for the guidance of fatigue crack shape. In the sinker EDM process, an electrode was

fabricated and was plunged into the work piece; the resultant arc ablated the work piece material, and a void was formed in the 

shape of the electrode.

EDM starter notches were machined at the centers of six flat plates, three each from the 4 in. and 6 in. widths. Figure

2 shows the EDM electrode geometry for the 1.4 mm × 23.2 mm (a × 2c) notch geometry used for the 25.4 mm thick flat plate
3

specimens. The electrode was composed of three parts, two pieces that had a square profile, between which is the smooth arc 

profile electrode from which the starter notch derives its shape. The two square electrodes were included to provide a 

sufficient slot for instrumentation that was used for the fatigue-cracking step. This electrode arrangement allowed for the 

notch to be machined in one step using electrodes 0.20 mm (0.008 in) thick. The notches were cut to a depth of 1.4 mm from 

the surface and produced a surface-breaking length (2c) of 23.2 mm.

Figure 2 – EDM electrode geometry for the 1.4 mm x 21.2 mm notch geometry for the 4 in. and 6 in. wide, flat plate 

specimens (units in mm).

EDM starter notches were machined at the center of one pipe segment, while three more segments are awaiting 

notching, pending analysis by a collaborating NDE analysis company. The electrode geometry for the pipeline is illustrated in 

Figure 3. Two electrode thicknesses were used for the pipe EDM starter notches, 0.41 mm (0.016 in) thick to a notch depth of

1.55 mm and 0.05 mm (0.002 in) thick to a notch depth of 1.9 mm. The two electrode method allowed for the creation of a 

wide slot into which a crack mouth opening displacement transducer could be affixed, while the starter notch was finished 

with a thin slot to achieve the high-stress concentration effect of a small notch-root radius.

Figure 3 – EDM electrode geometry for the 1.9 mm × 48.3 mm notch geometry for the curved pipe specimens (units in mm).

2.3 Fatigue Crack Introduction

The first step in generating a reference standard for NDE evaluation of true “crack-like” features in structural 

materials was the development of a procedure to generate accurate and repeatable fatigue cracks in a component. The 

development of the fatigue-crack introduction procedure followed that outlined by Richards et al. [7]. The procedure depends 

upon the correlation between the change in a component’s stiffness and therefore compliance (the inverse of stiffness, the 

change in crack mouth opening displacement per unit of applied force) with a change in crack length. The compliance was 

measured by use of the applied force in the four-point bending arrangement by use of a load cell, and the crack mouth opening 

displacement (CMOD) was measured by use of an extensometer. Calibration specimens were generated by fatigue cracking 

notched specimens under specific conditions. During fatigue cracking, the applied loads were periodically changed to 

introduce characteristic fatigue striation marks on the fatigue crack surface to serve as characteristic artifacts on the fracture 

surface. These artifacts, termed “marker bands,” were used to correlate crack length with known compliance measurements. 

Upon completion of the calibration specimen run, the specimens were fractured in a brittle manner to expose the fracture 

surface. Figure 4 shows an example of a fatigue fracture surface with the characteristic marker bands as described below.
4

Figure 4 – Fatigue fracture surface of a fatigue crack calibration specimen. From the bottom of the image, progressively are 

shown (a) the surface of the thick EDM notch, (b) the thin EDM notch, (c) the fatigue crack fracture surface in which the thin, 

horizontal dark lines as indicated by (d) are the fatigue marker bands, and (e) finally at the top is the brittle fracture surface 

generated during the procedure to expose the fracture surfaces.

Closed form solutions for the stress-intensity factor, K, for a surface crack in a flat plate under pure bending were 

determined based upon ASTM E740-03, annex A2. Richards et al. [7] demonstrated by use of a finite element modeling that 

the closed form solutions for K, which are based upon linear elastic fracture mechanics (LEFM), satisfactorily predicted the 

stress intensity generated in a curved plate in four-point bending for the geometry shown in Figure 5. The four-point bend 

loading configuration for fatigue cracking of the flat plates is shown in Figure 6. The applied force levels were determined by 

use of a generalized relationship for steels, based upon the Paris law, in which the microstructure is composed of ferrite and 

pearlite as [8]:

da = 6.89 × 10−9 ∆K3.0, (1) dN

where the fatigue crack growth rate (FCGR) da/dN is in units of millimeter per cycle and the stress intensity factor range ΔK

is expressed in MPa√m. The ΔK levels were determined from Equation 1 in order to achieve a FCGR of approximately 0.032

μm per cycle to 0.041 μm per cycle, which results in approximately 0.5 mm of crack extension in 10,000 cycles. The selected

ΔK levels were 16.7 MPa√m for the pipe and 18.2 MPa√m for the flat plates. The ratio of the minimum load or stress 

intensity factor to the maximum load or stress intensity factor, R = 0.4 for both the pipe and flat plate specimens.
5

Figure 5 – Schematic of contoured loading support and straight-roller loading configuration used in four-point bending fatigue 

of pipe specimens.

Figure 6 – Four point-bend loading configuration for fatigue crack introduction in a flat plate.

The thin, horizontal dark lines in Figure 4 are called marker bands. Marker bands are a way to introduce a 

characteristic feature on a fracture surface that can be used during fracture surface evaluation to identify the position of the 

crack front at a given point in the fatigue crack growth process. One way of producing fatigue marker bands is to change the 

stress intensity level during the test, which will alter the FCGR and therefore the appearance of the fatigue fracture surface 

generated. The fatigue marker bands were introduced through a reduction of the applied ΔK levels to 60 % of those used for 

the fatigue crack growth procedure listed above, i.e. ΔKmarker band = 0.6 × ΔKfatigue crack growth, while maintaining an R = 0.4, and 

were cyclically loaded between 50,000 cycles to 150,000 cycles per marker band. Fatigue marker bands were introduced 

every 10,000 cycles of fatigue crack growth at the fatigue crack growth stress intensity factor amplitude, and compliance was
6

0

measured every 1,000 cycles. Post-test, the fracture surfaces were exposed and the marker bands were measured, normal to 

the surface at the center of the starter notch and these data were corroborated with compliance measurementstaken at the 

onset of the marker band introduction procedure as shown in Figure 7. Typically, three crack depth measurements were 

conducted for each marker band within the field of the fractograph as shown in Figure 4, and the average value was reported.

From Figure 7, the characteristic slope values relating the change in compliance with an extension of the fatigue 

crack are listed for the 1.067 m diameter pipe specimens having 1.9 mm x 48.3 mm notches and for the 101.6 mm and

152.2 mm wide flat plate specimens having 1.4 mm x 23.2 mm notches. The linear regression analysis of the slope of the 

offset compliance versus crack extension data for the 6 in wide plate data in Figure 7 was limited to a crack extension of 

approximately 3 mm, as the data began to deviate from linearity beyond this limit. The linear relationship between

compliance change and crack extension allows for the use of a simple equation to predict the instantaneous crack length ai in a 

component based upon the instantaneous compliance Ci and the initial crack length a0 and the initial compliance, C0 [7]:

��i = C��−��0 + a . (2) M

Based upon Equation 2, an initial evaluation of the ability to predict crack length from compliance data was 

performed. The compliance data from the calibration specimens were used to generate the compliance offset versus crack 

extension curves shown in Figure 7. The compliance data, in turn, were used to generate the predicted crack length versus 

measured crack length plot shown in Figure 8 by use of Equation 2.

Figure 7 – Offset compliance versus crack extension measured optically from exposed fracture surfaces for the 1.067 m. 

diameter pipe (1.9 mm x 48.3 mm starter notch) and the 101.6 mm and 152.2 mm wide flat plates (1.4 mm x 23.2 mm starter 

notch).
7

Figure 8 – Predicted versus optically measured crack length data for the 1.067 m diameter pipe (1.9 mm × 48.3 mm starter 

notch) and the 101.6 mm and 152.2 mm wide flat plates (1.4 mm × 23.2 mm starter notch). The dashed lines are ± 0.1 mm 

from the 1:1 solid line.

The data in Figure 8 demonstrate an excellent correlation between the predicted and measured crack lengths from 

Equation 2. The data maintain a 1:1 relationship, demonstrated by the solid line in Figure 8, and the dashed lines represent 

deviation limits of ± 0.1 mm from the 1:1 ratio. The data for the 1.4 mm × 23.2 mm starter notch in the 152.2 mm wide plate 

show a deviation from the linear response in Figure 8 at crack lengths greater than 5.5 mm, this corroborates with the 

deviation in the response of the compliance change with crack extensions greater than 3 mm in Figure 7.

2.3 Reference Standard Preparation

Two flat plate specimens, one 101.6 mm wide and one 152.2 mm wide, and labeled NDE SD 4-003 and

NDE SD 6-002 respectively, which had EDM starter notches as shown in Figure 9, were fatigue-cracked with a target crack 

length of 4.4 mm. The final predicted crack length of the 102.6 mm wide plate was 4.38 mm, which was achieved at

72,000 cycles. The final predicted crack length of the 152.2 mm wide plate was 4.40 mm upon initial cracking at

61,000 cycles. However, after further evaluation of the compliance change versus crack extension data in Figure 6 for the

152.2 mm wide plates, the data at crack extensions beyond 3 mm were excluded. The result was an increase in the 

characteristic slope used in Equation 2. The adjusted predicted crack length for the 152.2 mm wide plate was 4.25 mm.
8

Figure 9 – Flat-plate reference standard development specimens in the EDM notched condition.

After the specimens were fatigue cracked, the thickness of the plates was reduced by Blanchard grinding, a surface machining 

process typically used for creating flat and parallel surfaces. The Blanchard grinding process removed the EDM starter notch 

through a reduction of the plate thickness. Table 1 lists the initial and final thicknesses for the 102.6 mm and 152.2 mm wide 

plates, along with the predicted fatigue crack length (depth from the surface) before and after Blanchard grinding.

Table 1 – Specimen thickness and predicted fatigue crack depth (a) values before and after Blanchard grinding

Initial

Thickness

Finish

Thickness

Material

Removed

Predicted Fatigue

Crack Depth a

Before Grinding

Predicted Fatigue

Crack Depth a

After Grinding

Specimen mm mm mm mm mm

NDE SD 4-003 24.51 22.99 1.524 4.279 2.855

NDE SD 6-002 24.38 22.61 1.778 4.254 2.476

After Blanchard grinding, the specimen surface condition was such that no indication of a fatigue crack was visible. The 

specimens were labeled with the specimen numbers listed in Table 1, and the two faces of the specimens were labeled “A” and 

“B”. The specimens were wrapped in anti-corrosion paper and packaged in a hard plastic shipping crate. Along with the plates,

an instruction sheet accompanied the crate to each of the collaborators providing inspection instructions. The

following is the attached letter that accompanied the plates to each of the industrial collaborators.
9

UNITED STATES DEPA RTMENT OF COMMERCE 

National Institute of Standards and Technology

325 Broadway

Boulder,Colorado 80305-3328

Thank you for participating in Phase I of the NDE reference sta ndard development progra m.

The objecti ve of Phase I is to determine the feasibility and effecti veness of usi ng actual cracks as reference 

standards in the cali bration of NDE sensors, methods and analysis. NIST acknowledges that flaw sizing is a 

complex problem with many va riables for different measuremen t systems. The approach is to eliminate variables 

incrementally to obtain the highest fidelity cali bration possible.

You will fi nd two plates packaged in anti-corrosion paper. Both plates are ASTM A709 spec steel that were hot 

rolled to 1" thick. NDE SD-003 is nominall y 4" wide and NDE SD-002 is nominally 6" wide. Both plates were 

ground flat and parallel. An EDM notch was put in the surface of both plates. A fatigue crack was grown in the 

through thickness direction and then the plates were ground again to remove the EDM starter notch.

You art: invilt::d to ust:: any NDE ntt::lhod at your disposal lo d t::Lt::rrnint:: tht:: flaw loca tion and gt::omt::try. Tht::rt:: art: 

Side A and Side B reference marks and there is also an indication of which corner to use as an x-y datum for 

referencing the location of the flaw in each pla te.

Do not use any chemicals on the plates, dye penetrant is strictl y not allowed, cou pli n g gel is permitted. Non-waterbased is preferred and if surface cleani ng is necessa ry then etha nol is accepta ble. Take care to avoid exposure to 

water including exposure to air with more than 30% RH. Keep the plates wrapped in the anti-corrosion paper and in 

the shipping box when not activel y bei ng used. You are not permi tted to make any permanent marks on any surface 

of the plate.

Make as many measurements with various sensors, methods and analysis that you feel are a ppropriate, these plates 

will be destroyed and this is your one opportunity to measure it in any way you can. NIST has provided a return 

shippi ng label to send the plates back to NIST. Along with the plates, send preli minary data on the location and 

geometry of the flaw. If your technique is capable of determini ng shape or volumetric characteristics of the flaw, 

please i nclude those results as well. Your results will not be shared with other industrial colla borators and will only 

be used internally as a reference to determine feasibility and effectiveness of the cali bration reference standards.

When Phase I is complete, the plates will be destroyed to liberate the flaw surface. At that time the surface 

geometry will be optically measu red. NIST provide you the actual measurements when that task is complete. You 

are encou raged to use that data to "calibrate" your sensors, methods and analysis in anticipation of Phase II.

For tech nical questions or comments please contact: 

Dash Weeks

303-497-5302(office)

timdash @boulcler.nist.gov

Mail Stop 853
10

3. Results

The plates were shipped to each industrial collaborator with an included label for shipment back to NIST, at which 

point the plates were shipped to the next collaborator. The collaborators were asked to provide crack geometry data such as 

crack depth, a, and width, 2c, when possible by use of any NDE flaw detection equipment and techniques available. Phased 

array ultrasonic analysis and eddy current analysis were the two methods evaluated in the present round robin study. All of 

the collaborators were told that the fatigue cracks were centered on the plates after that information was requested by one of 

the collaborators. Various sensor arrays, configurations and analysis methods were used in the Phased Array analysis. In 

order to retain the confidentiality of the industrial collaborators, specific details of the analysis type, sensor configuration and 

screenshots of the software are not presented.

3.1 Liberation of the Fatigue Crack Surface

After each of the industrial collaborators had analyzed the reference plates shown in Figure 9, the specimens were

prepared for exposure of the surfaces of the fatigue crack. The sides of the specimens were saw-cut to within 25 mm of the 

fatigue crack edges at the surface to reduce the cross-sectional area of the specimen. The specimens were then submerged into 

liquid nitrogen and allowed to cool to approximately 77 K. Once thermal equilibrium was achieved, the specimens were loaded

in four-point bending until catastrophic failure. The two sides of the fracture were then immersed in ethanol and allowed to

warm to ambient temperature. The fractured pieces were then dried and optical fractography of the fatigue crack fracture

surfaces was performed. Figure 10a shows the fatigue fracture surface of the 4 in. NDE SD specimen, and Figure 10b shows a 

closeup of the fatigue crack region. Figure 11a shows the fatigue fracture surface of the 6 in. NDE SD specimen, and Figure

11b shows a closeup of the fatigue crack region.

Crack depth a and surface breaking length 2c values were measured optically by use of the scale bars in Figures 10 

and 11 as a reference, in which the smallest dimensional increment in the scale bar is 0.5 mm. The two sides of each broken 

specimen were photographed and imaged separately. The average of the two values is listed in Table 2.

Table 2 – Fatigue crack geometry estimates and optically measured values for the two reference plates.

NDE SD 4-003 NDE SD 6-002 NDE SD 4-003 NDE SD 6-002

Crack Depth a Crack Depth a Crack surface 

breaking length 2c

Crack surface 

breaking length 2c

Initial Notch Depth mm 1.34 1.37

Predicted Crack Depth mm

Prior to Notch

Removal

4.379 4.254

Final 2.855 2.476

Measured Crack Depth mm

Final 3.016 2.487 18.9 17.7

Deviation from

Predicted Depth

(abs)

0.161 0.011
11

(a)

(b)

Figure 10 – Optical fractographs of exposed fatigue crack from the NDE SD 4-003 specimen, showing (a) the full specimen 

thickness and (b) a closeup of the fatigue crack region.

(a)

(b)

Figure 11 – Optical fractographs of exposed fatigue crack from the NDE SD 6-002 specimen, showing (a) the full specimen 

thickness and (b) a closeup of the fatigue crack region.
12

3.1 Round Robin NDE Analysis

Table 3 lists the estimated and measured crack geometry values for the two reference plates along with the reported

crack depth a, and surface breaking length 2c from the industrial collaborators. The industrial collaborators are listed as #1 

through #4, and the respective measurement methods are separated by numerical increments a, b, c, etc. The resolution and 

accuracy of the respective NDE equipment and analysis methods were not initially requested; therefore the reported values are 

listed with the reported number of significant figures by the respective industrial collaborator. For instances in which the 

values were reported in U.S. customary units (inches) the number of significant figures listed in Table 3 were selected to 

represent the significant figures of the reported value converted to SI units (mm). If there was no reported value for the crack 

depth a or the surface breaking length 2c a dash was listed for that respective position in Table 3.

Table 3 – Crack dimension values from the NDE round robin analysis for the two fatigue cracked reference plates.

NDE SD 4-003 NDE SD 6-002 NDE SD 4-003 NDE SD 6-002

Crack Depth a Crack Depth a Crack surface 

breaking length 2c

Crack surface 

breaking length 2c

Initial Notch Depth mm 1.34 1.37

Predicted Crack Depth mm

Prior to Notch

Removal

4.379 4.254

Final 2.855 2.476

Measured Crack Depth mm

Final 3.016 2.487 18.9 17.7

Deviation from

Predicted Depth

(abs)

Crack Length Predicted 

from NDE

Collaborator

1 Method

0.161 0.011

a mm 2.44 -

b mm 3.35 -

a mm 3.63 1.93

b mm 3.58 1.19

2

a mm 2.08 1.93 12.7 12.7

3

a mm 2.1 1.9 18.5 14.5

b mm - - 11.5 17

4

a mm - 2.6 16 18

b mm - - 14 20

c mm - - 13 12

d mm - - 14 12

The omitted dimensional data in Table 3 are a result of the industrial collaborators either not being able to make the 

measurement due to a technical limitation of the test equipment, a technical limitation of the measurability of the reference 

specimen due to surface preparation or due to oversight of the collaboration statement above. Future standards development 

in this area will undergo different surface-preparation processes to attempt a resolution of the technical aspects of the 

measurability of reference specimens. Such changes include a switch from finishing the surface of the reference specimen
13

with Blanchard grinding processes, which are thought to impart sufficient surface deformation to “smear” the surface of the 

fatigue crack, to a method that has much lower surface deformation such as surface grinding. Issues associated with this 

change will be addressed in a future research program.

Figure 12 shows a bar chart of the deviation in crack depth measurement (aNDE-aOptical) versus the measurement 

number (measurement number represents each measurement method by each collaborator sequentially) for the two reference 

plates. Figure 12 shows that the various NDE analysis methods and collaborators both over and under-predicted the actual 

crack depths of the two reference plates. Figure 13 shows “bullseye” plots of the absolute value of the deviation in the values 

between the predicted NDE value and the optically measured value for crack depth a in Figure 13a and surface breaking 

length 2c in Figure 13b.

Figure 12 –Deviation of the predicted crack depth by NDE analysis to the optically measured crack depth for the two 

reference plates. Zero deviation indications are a result of no available data as indicated in Table 3.
14

(a) (b)

Figure 13 – “Bullseye” polar plots of radius and angle, where the radius represents the deviation between the NDE predicted 

and actual values for crack depth a in Figure 13a and surface breaking crack length 2c in Figure 13b for the two reference 

plates. The angles are arbitrary. The radius scale is shown in the lower left, which represents the dimensional increment 

between the solid circles in each figure.

4. Discussion

In Figure 13a, the NDE analysis typically resulted in a measurement that deviated from the actual crack depth value 

by just over 0.5 mm, with a few measurements above this limit and a couple below. The most troubling finding here, is that 

for engineering critical assessment (ECA), which uses crack sizing techniques made possible by NDE technologies such as 

phased array ultrasonic and eddy current, the results of the round robin analysis presented here demonstrate that the NDE 

analysis both over-predict and under-predict the actual fatigue crack depths of the two reference plates, in some cases by 

substantial amounts of deviation. An over-predicted crack depth may lead to unnecessary mitigation such as repair or 

replacement of components, whereas an under-predicted crack depth may lead to no action, potentially leading to a 

catastrophic failure.

5. Conclusions

A feasibility study was successfully completed in which proposed fatigue-flaw standard-reference artifacts were 

developed and manufactured and were evaluated by NDE experts in collaboration with the present study. The results from the 

round-robin NDE analysis illustrated the need by the NDE community for such a standard, which was previously anecdotal, 

based upon experience with sizing real flaws by use of artificial flaws as a standard. The benefits of a representative calibration

standard by which NDE sensors can be calibrated and verified should be self-evident, in that improvement of the accuracy of

flaw sizing through application of verification through reference standards will decrease unnecessary costs and prevent

unpredicted failure by improving structural reliability of components. The accuracy and reproducibility of the proposed fatigue

crack reference standard, based upon statistical analysis, will be the focus of future work.

Acknowledgements

The Authors gratefully acknowledge the contributions of Ross Rentz and Ken Talley of NIST to this research.
15

Bibliography

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