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Fatigue damage mechanisms have been investigated using a series of tension fatigue tests on several laminates of boron/aluminum (6061-0). This study focuses on four aspects of the fatigue response. First, in laminates with 0-deg fibers on the outside, an analysis that identifies “shakedown” conditions is shown to predict the stress amplitude below which no fatigue damage accumulates. Second, a simple fatigue damage accumulation model which relates matrix fatigue cracking and the overall laminate properties is described. A model for the saturation damage stage development is presented. Third, data illustrate that identical laminates, tested in directions 90-deg apart (such that one layup has 90-deg outer plies and the other 0-deg), have different fatigue behaviors due to the stacking sequence. The 90-deg plies on the surface develop cracks earlier than predicted by shakedown. An attempt is made to explain this stacking sequence effect. Finally, variable load history effects on the fatigue damage response are investigated by simple tests. These tests reveal that for a given stress ratio the specimen seeks the saturation damage state for the largest stress range to which it is subjected. It also was found that little damage is generated by shifting a given stress range down, whereas significant damage may be created by shifting it upward.
boron-aluminum composites, metal matrix composites, fatigue damage mechanisms, fatigue, stacking sequence, saturation damage state, load history, shakedown analysis, composite materials
Research engineer, National Aeronautics and Space Administration, Langley Research Center, Hampton, Va.