A microcomputer-based system has been developed to perform near-threshold fatigue crack growth testing. The crack length is measured by both potential-drop and compliance techniques. The potential-drop technique has a resolution of 0.0005 W and is used for real-time load control. The compliance technique has a resolution of 0.002 W and is used for comparative reference. The potential-drop system uses d-c current and has multiple voltage probes (located remote from current input points) to minimize the effects of current variation and resistance changes. The potential-drop measurements are made with and without excitation to minimize thermal offset problems. Compliance and potential-drop measurements are determined at specified load points during a cycle to minimize the effects of crack closure.
The system is programmed to control the rate of ΔK change (for increasing or decreasing ΔK tests) to a user-selected value. As the applied load is changed, the sample points (for measuring compliance and potential drop) during the cycle are changed automatically. A real-time sliding-seven-point fatigue crack growth rate calculation is performed for each sample. The sampling frequency is adjusted by the system during the test to maintain measurement accuracy. The microcomputer records crack lengths, loads, cycle number, and specimen temperature on floppy disks for post-processing.
Near-threshold fatigue crack growth results are presented for Type 304 stainless steel at both 24 and 538°C.