The variation of resonance frequency in fatigue tests as a tool for in-situ identification of crack initiation and propagation, and for the determination of cracked areas

This paper assesses the potential of using the observed variations of the resonance frequency in fatigue tests for identifying crack initiation and propagation, and for determining total cracked areas, during fatigue crack growth. Tests were performed on commercially pure (99.5%) aluminum plates using mode I stress controlled fatigue loading with R=0.1 in a commercial resonant testing machine. The resonance frequency was found to increase with increasing number of cycles due to material hardening, but also to peak and subsequently decrease through a reduction in cross-sectional area caused by crack growth. It has been found that the point where the resonance frequency peaks correlates very well with the beginning of the propagation stage, as determined through analysis of specimen photographs extracted from video recordings of the tests and by digital image correlation. Analysing changes in resonance frequency allows one to estimate the beginning of the crack initiation and propagation stages even in smooth specimens, where the initiation site is not localized a priori and therefore obtaining the required photographic information may be difficult. In this situation, other crack monitoring techniques such as those based on replicas or potential drops are almost useless with large specimens. The proposed technique greatly facilitates the analysis of stress intensity factor thresholds and allows one to establish whether a crack is growing at a given time simply from the variation of the resonance frequency. Using a commercial resonant testing machine instead of a custom-built machine makes the technique more flexible as regards materials, geometry and loading (tensile, torsional) conditions; also, it facilitates implementation at an industrial research level.