Monitoring Fatigue Crack Initiation and Propagation in Cruciform Joints Using Resistance-Type Gages

Abstract

The monitoring of fatigue crack initiation and propagation in cruciform joints is very complex and time consuming. The amount of technical information generated by a single fatigue test can vary from a terse noting of the cycles to the point till the specimen breaks, to a continuous observation of crack lengths. A wide variety of experimental methods are available to study the changes in bulk properties taking place in a test specimen (with simple geometry) while it is subjected to alternating stresses. However, in many fatigue test situations (for instance, in cruciform joints), it is very difficult to monitor the changes due to multiple possible crack initiation points. In this paper, a method has been developed to monitor the crack initiation for cruciform joints with different geometry by resistance-type strain gages. Also, the application of a relatively new, thin-film bondable transducer, commercially available under the name ‘crack propagation gage,’ was evaluated for fatigue crack propagation tests for cruciform joints at room temperature. Additionally, the strain gage and the crack propagation gage instrumentation system can be used for direct test machine control, permitting completely automated fatigue crack growth testing.