Measurements of interlaminar deformation along the cylindrical surface of a hole in laminated composite materials by Moiré interferometry

The deformation along cylindrical surfaces of holes in tensile-loaded laminated composite specimens was measured by means of new moiré interferometry techniques. Two composite tensile specimens, fabricated from IM7/5250-4 pre-preg with ply lay-ups of [04/904]3s and [+302/−302/904]3s, were examined by newly developed moiré interferometry techniques. Circumferential and thickness direction displacement fringe patterns, recorded in 3° segments around one quadrant of the hole periphery, were assembled into 90°-wide mosaics for both composite specimens. Distributions of strain were calculated with high confidence on a sub-ply basis at select angular locations. Measured strain behavior was complex and displayed ply-by-ply trends. Large ply-related variations in the circumferential strain were observed at certain angular locations around the periphery of the hole in both composites. Extremely large ply-by-ply variations of the shear strain were also documented in both composites. Residual viscoelastic shearing strains were recorded in regions of large load-induced shearing strains. Large ply-group related variations in the thickness direction strain were observed in the [+302/−302/904]3s specimen. An important large-scale trend was observed in which the thickness direction strain tended to be more tensile near the outside faces of the laminate than near the mid-ply region. The measured strains were compared with a spline-based three-dimensional numerical technique, resulting in a very close match.