The use of a second harmonic correlation to detect damage in composite structures using thermoelastic stress measurements

A novel development of the thermoelastic stress measurement technique using the second harmonic of the load test frequency is presented. Clear identification of significant subsurface damage is demonstrated using the new technique, and test results are presented from a small scale wind turbine blade test which indicate that some quantitative assessment of damage criticality might be possible from a single full-field measurement. A hypothesis is presented as to why the new technique is successful, based on complex movements of the surface caused by local buckling (itself a consequence of the sub-surface damage). Strain gauge evidence is presented to support this hypothesis.