Single-specimen J-integral testing is performed commonly with the unloading compliance technique. Use of modern instrumentation techniques and powerful desktop computers have made this technique a standard. However, this testing technique is slow and tedious, with the loading rate fixed at a slow quasi-static rate. For these reasons the dc potential drop technique was investigated for crack length measurement during a J-integral test. For direct comparison, both unloading compliance and potential drop were used simultaneously during a J-integral test. The results showed good agreement between the techniques. However, the potential drop technique showed an offset in crack length due to plastic blunting processes. Taking this offset into account, J/sub Ic/ values calculated by both techniques compared well

McGowan, J. J.

Nanstad, R. K.

UNT Digital Library

By acceptance o' !*>'• a'ticie. in«ih« U.S. Government* fight 10retain a nonexcluuwe, royalty-trwlicense m jnd to «ny eopytghtcowering the article./Lol- " '?COHF-84 111 04—1DE85 001550A DIRECT COMPARISON OF UNLOADING COMPLIANCE AND POTENTIALDROP TECHNIQUES IN ./-INTEGRAL TESTINGJ. J. McGowan and R. K. NanstadABSTRACTSingle-specimen ./-integral testing is performed commonly with the unloading compliance technique. Useof modern instrumentation techniques and powerful desktop computers have made this technique a standard.However, this testing technique is slow and tedious, with the loading rate fixed at a slow quasi-static rate.For these reasons the dc potential drop technique was investigated for crack length, measurement during a./-integral test. For direct comparison, both unloading compliance and potential drop were used simultaneouslyduring a ./-integral test. The results showed good agreement between the techniques. However, the potentialdrop technique showed an offset in crack length due to plastic blunting processes. Taking this oflset intoaccount, Jjc values calculated by both techniques compared well.INTRODUCTIONThe unloading compliance (UC) technique is routinely used to determine ./-integral resistance carves froma single specimen. The UC technique determines the amount of crack growth by using small unloadings (<15% ofmaximum load) conducted at regular intervals throughout the test. The load-versus-displacement slope duringthis elastic unloading is then used to calculate the crack length from compliance relationships. Through useof modern signal conditioning units and sensitive analog-to-digital converters, the UC technique has beensuccessfully married to a desktop computer. The union has resulted In a system for performing accurate./-integral resistance curve tests with a crack length resolution of 0.03 mm. However, due to the manyunloadings required, these tests require 30 to 40 mln, and the loading rate is limited to low values. Also,the UC technique tends to reflect the crack length on the outside of the specimen instead of the mean value.For these reasons the dc potential drop (PD) technique was investigated for use in ./-integral testing. Thistechnique can give Instantaneous readings of crack length without regard to loading rate. Recent developmentsin microvolt dc amplifiers and high-current dc power supplies make the PD technique an attractive alternativeto the UC technique. A direct comparison of the two crack length measurement systems was required to compareadvantages, disadvantages, and idiosyncrasies. This paper describes the dc potential drop calibration, thedesktop computer test control system, and a direct comparison of the UC and PD techniques for crack lengthmeasurement as applied to a (/-integral test.dc POTENTIAL DROP CALIBRATIONFor accurate determination of crack length by the PD method, an accurate calibration is essential.Several calibrations exist in the literature [1—3] for various fracture mechanics specimens. However, thereare no calibrations of sufficient accuracy for the complex Jj0 specimen modified for load-line displacement.Three separate calibrations were performed: two experimental and one numerical. The current input andvoltage probe locations are shown in Fig. 1. Note that both active and reference probes were used. The probelocations were selected for maximum output and minimum sensitivity to errors in probe location. The currentInput location was selected for maximum output. The ADINAT finite-element program, with eight-noderectangular isoparametric elements, was used to perform the numerical calibration. The finite-element grid,shown in Fig. 1, utilized 85 elements and 299 nodes. Two experimental calibrations were performed:(1) 0.7-mm-thick type 304L stainless steel sheet model and (2) 0.03-ma-thlck aluminum sheet model. Eachmodel was precisely machined to the planar dimensions of a 50.8-mm-thick «/jc specimen. Current was appliedto the models, and the dc potential was measured at the desired locations for each crack length selected.Research sponsored by the Office of Nuclear Regulatory Research, U.S. Nuclear Regulatory Commission,under interagency agreements DOE 40-551-75 and 40-552-75 with the U.S. Department of Energy under contractDE-AC05-84OR21400 with Martin Marietta Energy Systems, Inc.J. J. McGowan and R. K. Nanstad are research engineers in the Metals and Ceramics Division, Oak RidgeNational Laboratory, Oak Ridge, Tennessee 37831.OlSTMBUTiON OF THIS D K W W ISDISCLAIMERThis report was prepared as an account of work sponsored by an agency of the United StatesGovernment. Neither the United States Government nor any agency thereof, nor any of theiremployees, makes any warranty, express or implied, or assumes any legal liability or responsi-bility for the accuracy, completeness or usefulness of any information, apparatus, product, orprocess disclosed, or represents that its use would not infringe privately owned rights. Refer-ence herein to any specific commercial product, process, or service by trade name, trademark,manufacturer, or otherwise does not nscessarily constitute or imply its endorsement, recom-mendation, or favoring by the United States Government or any agency thereof. The viewsard opinions of authors expressed herein do not necessarily state or reflect those of theUnited States Government or any agency thereof.The results of the three models are shown in Fig, 2. Note that the results are presented as a ratio ofdc potentials. Thin ratio is a property only of the geometry and does not depend upon input current orresistivity. The agreement among the three models is quite good, with a difference in a/a of 1% for0.5 < a/u < 0.9.DESKTOP COMPUTER TESTING SYSTEMThe system for performing cA-integral resistance curve tests by the UC technique utilized a Hewlett-Packard (???) 9836 desktop computer, a custom-designed interface, and a standard KTS 800 testing system. Tothis existing system, the following components were added for the PD method: a 90-A dc power supply, twohigh-gain microvolt amplifiers, two HP-3478A 5 l/2~digit digital voltmeters, and a 12-bit digital-to-analogconverter. The desktop computer controls the test by clip gage displacement and unloads the specimenautomatically through the custom interface (consisting of two 16-bit analog-to-digital converters and adigitally programmed analog ramp generator). The computer also controls the power supply to optimize thedc potential output. The dc power supply is cycled off and on to permit correction for any thermal voltagefrom the dc potential probes. A schematic of the computer testing system is shown in Fig. 3. The computertesting system controls the test, provides printed and graphical output during the test, and records data ondisk for future analysis.DIRECT COMPARISON OF UNLOADING COMPLIANCE AND POTENTIAL DROP TECHNIQUESTo compare the two crack langth measurement systems, a standard single-specimeu unloading compliance^-integral test [4] was performed. The material chemistry was nominally ASTM A-508 forging steel but was heattreated to a 740-MPa room-temperature ultimate tensile strength. A standard 25.4-mm Jr0 specimen was usedwith side grooves of depth equal to 10J of specimen thickness (20% total). The test was performed at 104°C ata load-line displacement rate of 0.2 mm/rain. The trace of load versus load-line displacement is shown inFig. 4. At regular intervals the specii'.jn was unloaded 152 to determine the crack length by the UC method.The crack length was also determined continuously by the PD method. A comparison of the (/-resistance curvesis shown in Fig. 5. The ./-integral was calculated by the modified-Ernst method [5]. The PD crack length usedin this figure was the value measured Immediately before the unloading. The figure shows a positive offset inthe PD crack length at the beginning of the test. The reason for this offset is the sensitivity of the PDtechnique to local crack tip changes in resistivity and geometry prior to initiation of crack extension, adirect result of plastic blunting. This blunting behavior is illustrated in Fig. 6, which shows the threestages of crack growth. This behavior has been reported previously by others, e.g., Hilkowski and Maxey [6].This offset, however, can be calculated easily through the power law J^a procedure [7J. In this procedure, acurve fit Aa « C + AJB is determined by nonlinear regression techniques for all J-ha pairs up to the 1.5-mo-offset line (as shown in Fig. 7). The value of C is then used to offset the PD crack length values. Theresulting J x0 (power law method) for the PD technique (158.4 kj/m2) is quite close to J xc (power law method)value for the UC technique (124.1 kj/m2); this represents a difference of 16Z, within the characteristicvariance for this parameter. The good agreement between Jja values obtained by ASTM E813 and the power lawmethod is not affected by the use of the PD technique.CONCLUSIONSA desktop computer-controlled testing syjtem, developed for unloading compliance ^-integral testing, hasbeen modified to also use dc potential drop techniques for crack length measurement. The results of a testusing both methods indicate that either method of crack length measurement can be used with equal confidence.However, the offset in potential drop measured crack length must be determined and eliminated before Jjacalculations are performed.ACKNOWLEDGMENTSThe authors extend their gratitude to Julia Bishop for preparation of the manuscript, Ron Swain forunloading compliance testing, Tom Huttoti and David Thomas for unloading compliance instrumentation and soft-ware development, Richard Gwaltney for assistance with the ADINAT finite-element code, and Bob Swindeman andRey Berggren for their helprul manuscript reviews.REFERENCES1. H. H. Johnson, "Calibrating the Electric Potential Method for Studying Slow Crack Growth," Mater.Res. Stand.. 5, 422 (1965).2. R. O. Ritchie and K., J. Bathe, "On the Calibration of the Electrical Potential Technique forMonitoring Crack Growth Using Finite Element Methods," Int. J. Fract. 15'1), 47-55 (1979).3. B. L. Freenan and G. J. Neate, "The Measurement of Crack Growth During Fracture at ElevatedTemperatures Using the dc Potential Drop Technique," in The Measurement of Crack Length and Shape DuringFracture and Fatigue, ISBN 0-9506820-04, Materials Advisory Services, Ltd., Marley, England, 1980.4. "Standard Test for <//<., A Measure of Fracture Toughness," ASTM E813-81, yp. 810-28 in Annual Book ofASTM Standards, Pt. 10, Anerican Society for Testing and Materials, Philadelphia, 1981.5. H. A. Ernst, "Material Resistance and Instability Beyond J-Controlled Crack Growth," ScientificPaper 81-107-JINTF-F6, Westinghouse Research and Development Center, Pittsburgh, 1981.6. G. M. Wilkowskl and H. A. Maxey, "Review and Applications of the Electric Potential Method forMeasuring Crack Growth in Specimens, Flawed Pipes, and Pressure Vessels," pp. 11-266-94 in Fracture •'•leahanias:Fourteenth Symposium, ASTM STP 791, American Society for Testing and Materials, Philadelphia, 1983.7. F. J. loos, ed., Structural Integrity of Water Reactor Pressure Boundary Components, Annual Report,Fiscal Year 1979, NUREG/CR-1128, N8L Memorandum Report 4X22, Naval Research Laboratory, Washington, D.C., 1979.REFERENCE PROBEdc CURRENT INPUTFig. 1. j£C specimen geometry and probe location.Ur-Ld\LJLJO ..03 mm HL SHEETD . 7 mm SS SHEET• FE RNflLYSIS.2 .4 .8Fig. 2. Results of calibration models.5 \SlDIGITnvnHEWLETT-PRCKRRD 9836DESK TOP COMPUTER5 \/tDIGITDVMCUSTOM INTCTffiCEI IIS BIT IS SIT «J»-OCRSO I fly-D i ftlVif*1LORD! CDISP) I GENRTR\DIGITflLSYSTEM12 IITDflRNHLOGSYSTEMFig. 3. Schematic of desktop computer control system.E 4 6 8DISPLRCEMENT (mm)Fig. 4. Load-versus-displacement trace for Jja test.E\.KHL[NTECI>->1688.288888488ao•--_/- //wUNLOADINGPOTENTIAL/ 1. . /, . .COMPLIANCE DELTR-RDROP DELTA-A*. i . . . i . . . i . . .-.2 l.B 3.B 5.8 7.8 9.8CRRCK EXTENSION (mm)Fig. 5. Comparison of (/-resistance curves for theunloading compliance and potential drop techniques.ELJrLJUCE_ Ja.inaY>8E42n-— BLUNTING— TUNNELING* STABLE CRHCK GROWTH/^. INITIRTION2 4 E 8CRRCK EXTENSION (mm)18Fig. 6. Trace of load-line displacement versus crackextension for potential drop technique (note that the tracesduring unloading have been omitted for clarity).^ 2888E 1600w 1288 |-Q/ 688Ul7 488•: / /: /- / /MODIFIED-ERNSPOWER LAW FITOFFSET LINESBLUNTING LINE////• . .1 J_ 0-INTEGRHL• JICCE-B13)• JICCPMR LAM)• ACTUAL DELTA-Aoca z 4 6 aCRRCK EXTENSION (mm)Fig. 7. (/-resistance curve for potential droptechnique showing power law curve fit.18

Direct comparison of unloading compliance and potential drop techniques in J-integral testing

https://digital.library.unt.edu/ark:/67531/metadc1211640/m2/1/high_res_d/6505372.pdf

Direct comparison of unloading compliance and potential drop techniques in J-integral testing

Abstract

Similar works

Full text

Available Versions

UNT Digital Library