A capacitance-based solution to monitor absolute crack length in four-point bending test: Modelling and experiments

Four-point bending tests are widely used to characterise the flexural strength or fatigue behaviour of adhesive joint and multilayer structure. The present study reports a capacitance-based method to monitor accurately and in real-time the crack propagation at the interface of sandwich-beam specimens. The specimen behaves like a built-in sensor, where the capacitive coupling between the upper and lower beams is used as a gauge for the crack propagation monitoring. A fully-analytical model of the specimen under test is developed. A simple analytical expression is obtained that establishes a one-to-one correspondence between the measured capacitance and the crack length at a given load. This expression allows the direct retrieval of absolute crack length. The model is then compared to experimental data, showing consistent agreement without any fitting parameters. Estimates of the sensor model accuracy are also given.