Porosity, defined as voids in the matrix material, is a recurring anomaly in fiberglass and more advanced composite laminates. Although a variety of dependable nondestructive testing (NDT) methods are available for detecting porosity, reliable analytical techniques for assessing the fatigue strength reduction imposed by porosity are not currently available. Instead, manufacturers and analysts have historically relied on simple “rule of thumb” strength reduction factors based on monotonic tension tests, a typical reduction being 20%.
With an increasing emphasis on the effects of age and fatigue on high time aircraft, the question arises as to the adequacy of static strength tests alone to assess the effects of porosity. This paper discusses the results of a testing program in which porous and nonporous [0,90]s fiberglass specimens are cycled in zero to maximum (R = 0) loading over life ranges from a few hundred to a few million cycles. The degree of porosity is estimated from visual examination of the edges of each specimen. Flat specimens approximately 2.8 mm thick were prepared by laser cutting with the edges then sanded to remove the heat-affected region. For comparison, several specimens were also milled using a template. Results of the tests are presented in terms of maximum stress versus number of cycles to complete separation. The data indicate that fatigue strength reduction can be significantly greater than 20% for lower fatigue lives and close to 20% at lives of a few million cycles. If the voids are assumed to constitute stress concentration factors that accelerate the breakdown of the matrix material, then a fatigue strength reduction factor of about 1.5 to 1.2 may be inferred from the data. This compares favorably with the results of a finite element analysis on a typical internal void.