A computer-controlled automated test system for fatigue and fracture testing
Abstract: A computer-controlled system consisting of a servohydraulic test machine, an in-house designed test controller, and a desktop computer has been developed for performing automated fracture toughness and fatigue crack growth testing both in the laboratory and in hot cells for remote testing of irradiated specimens. Both unloading compliance and dc-potential drop can be used to monitor crack growth. The test controller includes a dc-current supply programmer, a function generator for driving the servohydraulic test machine to required test outputs, five measurement channels (each consisting of low-pass filter, track/hold amplifier, and 16-bit analog-to-digital converter), and digital logic for various control and data multiplexing functions. The test controller connects to the computer via a 16-bit wide photo-isolated bidirectional bus. The computer, a Hewlett-Packard series 200/300, inputs specimen and test parameters from the operator, configures the test controller, stores test data from the test controller in memory, does preliminary analysis during the test, and records sensor calibrations, specimen and test parameters, and test data on flexible diskette for later recall and analysis with measured initial and final crack length information. During the test, the operator can change test parameters as necessary. 24 refs., 6 figs.
Summary (2 min read)
- The automation of materials testing equipment is certainly not a recent concept.
- This is evident from the relatively narrow technical field for which this symposium was developed.
- The characteristics of automated systems are as diverse as the applications, reflecting the particular needs of the user.
- The Fracture Mechanics Group of the Metals and Ceramics Division at Oak Ridge National Laboratory began a computer automation activity in 1978 for the purpose of conducting elastic-plastic fracture mechanics tests.
- The system has evolved markedly since then, particularly in terms of speed.
- Automated testing for evaluation of fracture resistance was largely spurred by developments in elastic-plastic fracture mechanics.
- The unloading compliance test procedure requires excellent test control, high-precision data acquisition capability, and rapid calculation.
- The extensometer must be carefully calibrated and must be seated in such a way that effects of error sources such as friction and vibration are minimized.
- Dc-pd is an important aspect of the testing system and analysis procedures described herein.
- The dc-current programmer consists of a 12-bit digital-to-analog converter and a transformer-isolated output amplifier and can set the output of the dc-current supply over a range of 0 to 100 A.
DESCRIPTION OF TEST SYSTEM
- Function generator ramp outputs (used in J-R testing) are obtained by using a 12-bit digital-to-analog converter to set input current to an operational amplifier integrator; this results in ramp outputs that are smooth and stepless even at low ramp rates.
- The test controller incorporates photo-isolation on all input/output lines to the computer and programming lines to the dc-current supply, the extensometer is electrically isolated from the specimen by using Teflon spacers between the razor blades and the specimen, and the specimen grips are electrically isolated from the load frame by ceramic inserts in the load train ball joints.
- The single-point ground chosen is that of the servohydraulic test machine electronics.
- Comparison of the fits revealed that first order coefficients for low and high temperatures varied less than 4% from that at room temperature.
ELASTIC-PLASTIC FRACTURE MECHANICS TESTING
- The specimen is similarly cycled a third time to obtain the initial value of compliance which, together with the previously entered estimate for initial crack length, is used to calculate an effective modulus (as specified in ASTM E 813).
- The initial slope of the load-displacement trace is used to determine the 95% secant offset line as described in ASTM E 399.
- During the hold period, the computer acquires and stores 60 readings of both pd-active and pd-reference voltages with full current and zero current.
- The amount of unloading is dependent on the accuracy of the compliance determined during the previous unloading (a statistical analysis of the unloading data is performed).
- The length of the fatigue precrack and the final crack length are measured, as prescribed in ASTM E 813, using a digital measuring microscope.
- The computer analysis routine uses the measured initial crack length value and the compliance determined at the initial test unloading to calculate an effective modulus (the equation in ASTM E 813 is used) which is used for the balance of the analysis.
- This procedure differs from that described by Lowes and Fearnehough, 12 and used by others, 13 " 15 and investigations to determine which of the procedures gives the more reliable results are continuing.
- Final measured crack length is compared to those predicted by unloading compliance and dc-pd analyses, and the differences in crack extension printed.
- The routine displays the results and the power-law fit on the CRT and prompts the operator to accept the fit; if the fit is rejected, any suspect test results may be excluded and the regression repeated.
FATIGUE CRACK GROWTH TESTING
- For each pd-voltage sampling, the load, pd-active and pd-reference voltage waveforms are read twice, once with full current and once with zero current.
- The software contains logic to determine if a data point is significant based on the change from the last crack extension value.
- The testing performed so far has yielded very promising results.
- Software for the test system and analytical techniques described is being developed for various types of FCG tests (constant load, constant K, decreasing K, constant K^-increasing R ratio, etc.), and specific test results will be reported separately.
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