IDENTIFICATION OF FATIGUE CRACKS FROM VIBRATION TESTING

Vibration measurements can offer an effective, inexpensive and fast means of non-destructive testing of structures. This work is a further investigation of the effect of crack closure on the frequency changes of cracked cantilever beams. This closure causes the frequency response function to exhibit non-linear characteristics. The study has been conducted by using both computer simulations and experimental modal analysis. Crack flexibilities for open and closed positions have been identified for several crack depths. The effect of static preloading on crack closure also has been studied. It is shown here that this preloading can cause the fatigue crack to remain open. Also computed are bounds of frequency changes due to continuously open cracks and cracks that open and close. Both simulations and experimental results lead to the conclusion that, relying on the drop in the natural frequencies alone, especially the higher modes, may lead to serious underestimation of the crack severity. These results, on the other hand, prove the reliability of the vibration testing method in detecting the presence and nature of the crack. © 1990.