Interlaminar deformation along the cylindrical surface of a hole in laminated composites - Experimental analysis by moire interferometry

The deformation along cylindrical surfaces of holes in tensile-loaded speci mens was measured using new moire interferometry techniques. The techniques were developed and validated using an isotropic, homogeneous aluminum spec imen. Two composite tensile specimens, fabricated from M7/5250-4 pre-preg w ith ply lay-ups of [0 degrees (4)/90 degrees (4)](3s) and [+30 degrees (2)/ -30 degrees (2)/90 degrees (4)](3s), were then examined using the new techn iques. Circumferential and thickness direction displacement fringe patterns (each 3 degrees wide) were assembled into 90 degrees -wide mosaics around the hole periphery for both composite specimens. Distributions of strain we re calculated with high confidence on a sub-ply basis at selected angular l ocations. The measured strain behavior was complex. Ply-by-ply trends were revealed. Large ply-related variations in the circumferential strain were o bserved at certain angular locations around the periphery of the holes in b oth composites. Extremely large ply-by-ply variations of the shear strain w ere also documented in both composites. Peak values of shear strain approac hed 30 times the applied far-field axial strain. Residual viscoelastic shea ring strains were recorded in regions of large load-induced shearing strain s. Large ply-group related variations in the thickness direction strain wer e observed in the [+30 degrees (2)/-30 degrees (2)/90 degrees (4)](3s) spec imen. An important large-scale trend was observed in which the thickness di rection strain tended to be more tensile near the outside faces of the lami nate than near the raid-ply region.