Moiré interferometry measurements of composite laminate repair behavior: Influence of grating thickness on interlaminar response

Bonded scarf-lap and step-lap specimens representative of fully repaired composites were fabricated from two quasi-isotropic sixteen ply panels made from IM6/3501-6 material. The full-field optical displacement measuring technique, moiré interferometry, was used to gather detailed information about the distribution of interlaminar strain on the edge of these specimens. Highly detailed 3D numerical models were constructed for both specimen geometries and results obtained. Instead of modeling idealized bondline geometries, the geometries were obtained through digitization of micrographs of the actual test specimens. As a further enhancement on a second set of models, a thin resin layer representing the moiré interferometry diffraction grating was added to each model. Comparisons between the model and experimental results showed that a localized effect over the bondline was more accurately represented when the grating layer was modeled. Surprisingly, it was found that in regions away from the bondline, the through-thickness interlaminar normal strain was found to be amplified by the presence of the modeled grating. A series of parametric models were conducted on a laminated composite while varying grating thickness and modulus to further examine this phenomenon. A mechanism for the strain amplification is proposed.