Application of an advanced piezoelectric strain sensor for crack closure measurement

Abstract

It is well known that fatigue crack growth in airframe structures is governed by sequence effects in aircraft load spectrums, which typically include millions of load cycles of varying amplitude. In predicting the fatigue life of airframe structures, one advanced method used to account for these effects considers the concept of crack closure. This concept requires the measurement or prediction of the load required to open the crack tip, otherwise known as the crack opening load. However, traditional methods used to directly measure the level of crack closure for each cycle tend to be prohibitive for large load sequences. This is mainly due to sensor limitations and large scatter using traditional measurement techniques. The current study proposes a new piezoelectric strain sensor for detection of the crack opening load to addresses these issues. In this work, a comparison is conducted using three sensors on a compact fatigue test specimen: an extensometer (clip gauge), a conventional back face strain gauge, and the piezoelectric strain sensor. It is shown that the piezoelectric strain sensor exhibits far superior performance in measuring the crack opening load relative to the back face strain gauge and the extensometer. At minimum, for all cases considered the piezoelectric strain sensor displayed 100% or better reduction in measurement scatter than the next best sensing method considered. In some cases where the stress ratio is high and the crack length is small, the improvement seen using the piezoelectric strain sensor approached 200%. Crack tip opening loads were detected in almost all cycles across a wide range of crack sizes and load ratios; while the efficiency of the traditional sensors reduced significantly for relatively short cracks and high load ratios. The outcomes of this work will help to better understand the load sequence effects on crack growth rates, improve fatigue life predictive methods and develop effective procedures of truncation/compression of load spectrums for accelerated testing of aircraft components.