Interlaminar Normal Stiffness and Strength of Thick Orthotropic Laminates: An Experimental Study

Abstract

A test techique for measuring the interlaminar (in the thickness direction) tensile stiffness and strength of orthotropic laminates is discussed. A special gripping technique is used to eliminate the end bending of the specimens due to any misalignment of the load frame grips. The measured values of the interlaminar normal stiffness, Poisson's ratio, and strength of AS4/3501-6 composite laminates ([0/90]100s and [0/90/ ± 45]50S), both in tension and compression, are presented. It is observed, espe cially for the [0/90/ ± 45] laminate, that the interlaminar stiffness decreases with increas ing specimen aspect ratio (ratio of specimen gage length to thickness) of greater than 15, due to the free-edge stress effect as confirmed by a finite element analysis. The average values of the interlaminar tensile stiffness for [0/90] and [0/90/ ± 45] laminates are found to be respectively 18 and 23 percent higher than the transverse stiffness of the constituent ply. Analysis of failure strength indicates that the interlaminar compressive failure is a highly interactive failure mode, whereas the tensile failure is controlled by the in terlaminar normal stress, a non-interactive failure mode.