Crack detection in adhesive joints: use of strain gages in aggressive environments

A large three-dimensional finite element model was used to predict the response of miniature resistance strain gages to the initiation and growth of center and corner cracks in the fillet of an asymmetric single lap-shear joint. Experiments were conducted to assess the feasibility of the technique under conditions of accelerated durability testing. Drift in the strain-gage output complicated the interpretation of the data and placed restrictions on the slowest rate of crack growth which could be detected. The finite element model was also used to predict the strain-gage response to two-dimensional cracking under conditions of cyclic loading. This was seen to eliminate the experimental problem of signal drift and to create a more robust technique for the detection of crack initiation and growth. Using this method, it is predicted that cracks as small as 0.2 mm can be detected by comparing the best-fit slopes of the strain-gage output vs. the applied load in subsequent loading cycles.