The objective of this paper is to present a low cost control system for servohydraulic load frames to perform variable amplitude fatigue crack growth rate testing. The system consists of a relatively low cost personal computer equipped with two combination 12 bit A/D-D/A cards. A four channel simultaneous A/D card is used for reading load and CMOD from the specimen while a second card provides real-time function generation of the command signal. Depending on the loads and specimen size, variable amplitude loading can be applied at high speeds of up to 100 Hz, thereby minimizing the time required for threshold type testing. This system can be connected easily to existing hydraulic equipment and will provide variable amplitude capability that is both low in cost and flexible enough to perform specialized testing.
The system is capable of automatically shedding loads under both constant R-ratio and constant Kmax conditions, and could be adapted easily to other specialized types of testing. Both crack length and ΔK are calculated continuously using the compliance of the specimen. In addition, crack closure analysis is performed from the compliance data to calculate ΔKeff. The results of tests performed on high strength-low alloy structural steel used for highway bridge construction are presented to demonstrate the capabilities of the system.