Speckle interferometric damage investigation of fibre-reinforced composites

With the aid of the recently reported technique of adding up phase images modulo 2π that correspond to stepwise applied load increments, the fringe density that can be achieved in electronic speckle pattern interferometry (ESPI) has been substantially improved. This technique also allows the measurement of much larger deformations than were hitherto possible with ESPI. The analytical power of the method is demonstrated in the measurement of high local displacement gradients in carbon fibre-reinforced plastics (CFRPs). In-plane and out-of-plane displacement field measurements, performed with one single optical instrument, are compared with finite-element models. This paper reports how the technique is used to detect and quantify damage in fatigued CFRP laminates via its effect on the surface displacement field. Moreover, the measured displacement fields are used to validate a finite-element damage model. The correctness of the delamination measurement is verified with the aid of ultrasonic C-scan reference results.