Effect of loading parameters on the fatigue behavior of impact damaged composite laminates

The long-term mechanical fatigue of AS4/3501-6 graphite/epoxy quasi-isotrop ic laminates has been investigated to determine the influence of loading pa rameters on impact-induced delamination growth during constant amplitude, b lock, and spectrum fatigue loading. From the observed fatigue damage growth it is suggested that conservative damage tolerance criteria in the presenc e of barely visible impact damage should be based on static loading tests a nd a limited number of fatigue tests to determine the load levels that init iate delamination growth. For the type of impact damage and laminate config uration investigated in this study the fatigue limit is found to be 60% CSA I (compression strength after impact) in constant-amplitude compression-com pression loading, and 65% CSAI under the spectrum loading conditions. Furth ermore, the results of spectrum loading tests suggest that the durability t esting can be accelerated by omitting from the standardized test sequence l oad levels in which the stress range is less than 20% CSAI. (C) 1999 Elsevi er Science Ltd. All rights reserved.