Effects of thermal and mechanical fatigue on flexural strength of G40‐600/PMR‐15 cross‐ply laminates

Abstract

AbstractThe effects of thermal and mechanical fatigue on the flexural strength of G40‐600/PMR‐15 cross‐ply laminates with ply orientations of (02, 902)2s and (902, 02)2s are examined. The relative improtance of shear and tensile stresses is examined by varying the span‐to‐depth ratios of flexural test specimens from 8 to 45 Acoustic emission singals are measured during the flexural tests in order to monitor the initiation and growth of damage. Optical microscopy is used to examine speciments for resin cracking, delamination, and fiber breaks after testing. Transverse matrix cracks and delaminations occur in all specimens, for regardless of ply orientation, span‐to‐depth ratio, or previous exposure of specimens to thermal and mechanical fatigue. A small amount of fiber tensile fracture occurs in the outer 0° ply of specimens with high span‐to‐depth ratios. Because of the complex failure modes, the flexural test results represent the “apparent” strengths rather than the true flexural or shear strenghts for these cross‐ply laminates. Thermal cycling of specimens prior to flexural testing does not reduce the apparent flexural strength or change the mode of failure. However, fewer acoustic events are recorded at all strins during flexural testing of specimens exposed to prior thermal cycling. High temperature thermal cycling (32–260°C, 100 cycles) causes a greater reduction in acoustic events than low temperature thermal cycling (‒85 to 85°C, 500 cycles). Mechanical cycling (0–50% of the flexural strength, 100 cycles) has a similar effect, except that acoustic events are reduced only at strains less than the maximum strain applied during flexural fatigue. © 1994 John Wiley & Sons, Inc.This article is a US Government work and, as such, is in the public domain in the United States of America.