Strip chart recorders and data loggers have numerous shortcomings for monitoring sustained-load cracking (SLC) and fatigue tests, principally because they record at fixed rates. During a long test this affords low resolution during rapidly changing events and creates large amounts of mostly useless data. For over two years the authors have used a personal computer to monitor tests and record data for SLC and fatigue experiments. The computer stores specimen identification and test parameters, converts raw data into usable form, displays current test status, notes and acts on various test status parameters, periodically stores significant data on a floppy disk system, and automatically terminates tests as specified by the operator. The decision-making capability of the computer greatly reduces the amount of nonsignificant data to be stored while permitting more rapid data acquisition when the signals begin to change rapidly. A fast-switching battery-inverter system provides standby power for up to two and a half days in the event of power failure. If both primary and standby power fail, the computer's autostart feature allows it to resume data collection when power resumes.
The use of BASIC permits software to be produced in-house and allows revision as operational needs change. The input routines, start-up, and running of experiments are completely interactive, and designed to prevent omissions and errors by the operator. Presently four experiments can be conducted simultaneously, but a newly acquired 16-channel, 12-bit analog-to-digital converter will allow for considerable expansion. Because system response is inadequate to measure and record fatigue load signals directly, they are filtered to produce an average value which is recorded. The analysis of data stored on disk files is done using another personal computer to avoid interference with data acquisition. A unique feature of the analytical method is the use of the decrease in load with increasing crack length in the stiff, displacement-controlled test configuration to calculate crack lengths, instead of using a clip gage across the notch.