scope:

This report is a discussion and evaluation of several crack length measurement techniques at elevated temperature. Two of the methods allow direct recording of the crack length signals on analog recorders or on computer peripherals. The same two methods (Compliance determined crack length requiring the MTS Crack Correlator; and an electrical potential drop method) can be utilized at high frequency, do not require visual access to the specimen, nor load cycling interruption and give essentially the same output. Two novel developments have resulted from this work. Firstly, a clip gage measure of specimen mouth COD was developed that functions well at frequencies of up to 60 Hz. Secondly, a dummy specimen was mounted above the test specimen and the electrical potential drop (PD) of the specimen to the dummy was ratioed to eliminate most of the extraneous PD signals and give a linear output of PD to crack length. An electronic amplifier-divider circuit is described that performs this function automatically. Finally these methods were employed to measure the crack growth curves and log da/dN vs log K for 304 stainless steel at 1000° F for stress ratios from 0.01 to 0.7 for both cold rolled and annealed conditions. A minicomputer was utilized in conjunction with this work. The PD method gives the greatest sensitivity of crack length increase but MTS Crack Correlator is also a convenient system to use to obtain accurate and reproducible data.