Stitch Cracks in Constraint Plies Adjacent to a Cracked Ply

Abstract

A high density form of matrixmicrocracking in constraint plies adjacent to a cracked ply, and here called stitch cracking, was found to form readily in awide variety of [+. n/–. n/902n] s laminates. Stitch cracks appear to form instead of interply delamination at the tip of the crack in the constrained ply, more typical of cross ply laminates, when the included angle is greater than 50°. Stitch cracks do not require fatigue loading to form, but appear due to thermal residual stress and monotonic loading. Stitch cracks were observed to instantly form in –. ° constraint plies next to a cracked 90° ply, apparently when the included angle of the plies was less than about 50°. In addition, they formed in –15° and –60° plies next to corresponding cracked +. ° surface plies. Data on stitch crack length and spacing is included. This paper seeks to raise awareness of stitch cracks by systematically demonstrating that they not only exist, but occur in awide variety of laminates. The data suggest that stitch cracks should not be overlooked during development of composite damage mechanics models.

Keywords

matrix crack microcrack cracks in constraint plies

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