Crack Growth Monitoring with Phased-array Total Focusing … · 2020. 11. 15. · The test setup used a 50 kip servo-hydraulic test frame, a FlexTest GT Controller Version 3.5C 1815

Crack Growth Monitoring with Phased-array Total Focusing Method (TFM)

Oleg Volf, EWI, Columbus, OH USA, [email protected]

Abstract

Standard laboratory fatigue tests typically require monitoring of the crack length at various

intervals to obtain an engineering relationship between applied loading and the rate at which the

crack propagates. The technique that has proven to be the most efficient in way of data

acquisition, analysis, and accuracy, has been termed the ultrasonic phased array technique. Until

recently, no other technique provided high sizing accuracy was possible. With the advent of

phased-array technology and faster computers, a new approach is feasible that can augment or

even replace the older phased array technique. This paper describes how the Full Matrix Capture

(FMC) and Total Focusing Method (TFM) have advanced to provide superior displays and

analysis tools for crack sizing. Examples are provided illustrating simplicity of analysis and

sizing capabilities.

Keywords: Crack Growth Monitoring, Phased Array, PA, Total Focusing Method, TFM, Crack

Sizing, Diffraction Signals

Introduction

Standard laboratory fatigue tests typically require monitoring of the crack length at various

intervals to obtain an engineering relationship between applied loading and the rate at which the

crack propagates. This allows calculation of the fatigue crack growth rate, which can be used in

engineering assessments to predict the remaining service life of high-consequence, cyclically-

loaded structures such as cranes, bridges, aircraft wing spars, power generation turbines, etc.

Crack growth rate can be material- and/or and geometry-dependent, and thus can require some

sophistication in monitoring crack size particularly at low crack growth rates or where

environmental factors (i.e. corrosion) influence fatigue properties.

Conventional laboratory practice for monitoring crack size during fatigue testing involves the use

of instrumentation such as clip gages, cameras, electric-potential drop (EPD) sensors, etc. While

they have high sensitivity and resolution, these types of instruments usually measure an indirect

property as a function of crack size rather than the crack itself. For example, a clip gage

measures a change in compliance or stiffness of a test sample as the crack propagates. EPD

sensors measure a change in the electric resistance during cracking and reduces the cross-

sectional area of the specimen as a low-amperage electric current passes through the non-cracked

portions. Cameras are useful for measuring crack length in thin materials but are far less accurate

in quantifying crack length in thicker sections due to increased tendencies for crack tunneling.

Consequently, while these techniques have been widely used in fatigue testing, they require

careful setup, calibration and, in some cases, complicated data analysis to generate crack size

information. EWI has evaluated the feasibility of using advanced NDE methods as an alternative

approach for crack length measurement to simplify the process and gather better information

about the nature of the crack during propagation (i.e. detect non-planar growth, tunneling, etc.).

Mor

e in

fo a

bout

this

art

icle

: ht

tp://

ww

w.n

dt.n

et/?

id=

2554

2The e-Journal of Nondestructive Testing - ISSN 1435-4934 - www.ndt.net

Approach

The test setup used a 50 kip servo-hydraulic test frame, a FlexTest GT Controller Version 3.5C

1815 to apply and control the cyclic loads, and an MTS 5 mm clip gage Model 632.02F-21 to

monitor crack length using compliance methods on a standard, single-edge notched bend

(SENB) fracture toughness specimen with a starter notch per ASTM E1820. Figure 1 illustrates

the typical SENB rectangular geometry with a machined notch and fatigue crack that emanates

from the notch bottom. As part of the conventional specimen preparation for fracture toughness

tests, a fatigue crack is grown to a prescribed length from the starter notch. Monitoring the crack

length is done using a clip gage under standard practice. In our study, the initiation and growth of

a fatigue crack was monitored by three different ultrasonic testing (UT) methods along with the

conventional clip-gage method. For most SENB specimens, the final length of the fatigue crack

at the completion of the cyclic loading is 3-5 mm (0.12 to 0.20 in.). However, in this program,

the crack was grown to more than 25 mm (1.0 in.) to comprehensively assess UT techniques for

monitoring crack growth over an extended period of time and for much longer fatigue cracks.

The SENB test specimen was 300mm long, 40mm wide, and 45.9mm high. The notch, 11.2mm

deep, was generated by electro-discharge machining (EDM) with a width of 1mm (Figure 2).

The recorded test parameters during the cyclic loading included number of load cycles,

maximum applied load (to permit calculation of applied stress intensity factor) and crack length

as predicted by compliance methods using the clip gage measurements. (See ASTM E1820 for

more information about these parameters and how they are measured or calculated).

The cyclic loads were applied under three-point bending with the loading concentrated at the root

of the notch. Figure 2 shows the three-point bend rig where the center roller at the top of the

figure is positioned in-line with the machined notch during cyclic loading. A cyclic loading rate

of about 5-10 hertz was applied at a predetermined load range to initiate fatigue cracking and

growth to the target length, at which point the cyclic loading was stopped. The applied loads

were based on the size of the test specimen, the material yield strength, and the depth of the

machined notch to ensure that the resulting cyclic stresses at the notch tip were elastic and

appropriate for fatigue crack initiation and growth (i.e. to prevent plastic deformation or ductile

tearing during cyclic loading).

Figure 1. Illustration of a conventional SENB test specimen geometry

Figure 2. Fatigue test setup on a servo-hydraulic test machine using an MTS clip gage

Description of the UT Setup

To achieve high resolution and sizing accuracy, an Olympus 7.5L60 PWZ1 7.5MHz linear array

probe containing 60 elements at a pitch of 1 mm was placed on a Rexolite SPWZ1 N55S wedge

to generate shear waves at natural refracted angles 55 degrees (Figure 3). The ultrasonic data was

collected using an Olympus OmniScan X3 instrument. To prevent interference with the test

fixture, the SENB sample was removed after each scan. After the UT measurements were made,

the three-point bend fixture was re-positioned and the cyclic loading re-initiated to continue

crack propagation. Three different UT measurement techniques were evaluated: conventional

phased-array ultrasonic testing (PAUT), PAUT full matrix capture /total focusing method in the

T-T mode (PAUT FMC/TFM-TT), and PAUT FMC in the TT-T mode (PAUT FMC/TFM TT-

T).

Figure 3. The phased array ultrasonic technique setup to monitor the notch

Results

UT readings were taken at 20-30 minute intervals throughout the fatigue test. The test images

display the growth of a crack using the different UT approaches. To optimize the measurement,

the sensitivity of the ultrasonic system was set high enough to be able to detect weak diffraction

signals from fatigue cracks in the pulse-echo mode.

Table 1. Periodic Crack Measurement Values Using Various Measurement Techniques

Analysis

The data presented can be plotted on a graph of crack size from the original position of the notch

tip against the number of cycles as shown in Figure 4. These results provide valuable insight into

the nature of the material and the variable rate of the crack growth as input to failure models.

Number of Cycles 0

TFM images (left – TT Mode, right – TT-T Mode)

PAUT image (Sectorial Scan)

Number of Cycles 44914





Figure 4. Graphical representation of fatigue crack growth measured using each technique.

At the conclusion of testing, sample 17279-13 was opened to measure the crack physically.

Table 2 lists nine post-test measurements starting from the left side of the fatigue crack profile as

viewed in Figure 5. The table also includes the average, minimum, and maximum measurements

of the crack.

Figure 5. Image of Sample 17279-4-13 used for detailed crack measurement.

Conclusions and Next Steps

Results using conventional PAUT and TFM techniques demonstrated very similar sizing

accuracy as shown in Table 3. The TFM-measured crack length nearly matched the visual

confirming crack length measurements and was slightly more accurate than PAUT and the

compliance-based clip-gage methods.

Table 3: Comparison between the four different crack measurement techniques in this

study

The UT measurement techniques show comparable results to the clip gage during the fatigue test

and the gradient of this plot gives the growth rate of the crack at any given time during its

propagation towards failure. Since UT is a nondestructive technique, this process shows promise

for structural health and crack growth applications in the field. EWI plans to complete more

testing to verify the accuracy of this technique for structural health monitoring.

Additionally, the FMC/TFM technique shows a significantly improved visual presentation of the

crack including the orientation of crack propagation (crack angle). As shown in Figure 6, the

TFM crack image is visualized as a cross-section of the sample, which provides more accessible

data and eliminates the need for specially trained UT expertise to evaluate the images.

Figure 6. SENB sample showing fatigue crack and corresponding FMC/TFM image of the crack.

The laboratory feasibility work described here demonstrates the advantages of FMC/TFM over

other conventional NDE and non-NDE methods commonly used to detect and monitor crack

growth. Not only is crack sizing improved, but information regarding the nature and orientation

of cracking can be visualized with FMC/TFM (i.e. crack orientation, degree of branching, etc.).

Moreover, the improved resolution and accuracy obtained with FMC/TFM methods for

inspection of fatigue-sensitive structures offers the potential for increased accuracy in fatigue

crack detection and sizing, which would correspondingly improve the accuracy of engineering

life assessments based on those crack measurements. To provide a basis for establishing field

inspection protocols, additional work should consider more complex specimen crack geometries

such as corner cracks, branched cracking, and buried cracks (i.e. cracks that do not extend to a

free surface).

About the Author

Oleg Volf, Principal Engineer, leads the EWI nondestructive evaluation (NDE) team. His work

involves developing technical scopes of work for projects, building partnerships with other

technical organizations and stakeholders, and promoting EWI’s technical capabilities and

expertise in in-process monitoring, nondestructive testing (NDT) and inspection, and quality

measurement. Oleg holds multiple professional certifications, including Professional Engineer

(P.E.), ASNT Central Certification for NDT UT Level III, CSWIP AUT Data Interpretation

Instructor Certification, and PCN ToFD and PA Certifications. Contact Oleg Volf at

[email protected]

Documents

Crack Growth Monitoring with Phased-array Total Focusing … · 2020. 11. 15. · The test setup used a 50 kip servo-hydraulic test frame, a FlexTest GT Controller Version 3.5C 1815