A3 Fatigue in Composites (Distributed Stiffness Degradation)

Topic

Under cyclic fatigue loading, this composite does not develop a single dominant crack that grows in isolation; instead, it accumulates micro-damage distributed throughout its entire volume. The matrix cracks at numerous scattered points, while multiple zones of debonding—local separation between the fiber and the surrounding matrix—also occur throughout the composite. This distribution of micro-damage across the composite's volume, rather than its concentration at a single point, distinguishes it from an isolated dominant crack.

This distributed micro-damage—comprising matrix cracking and scattered debonding—is not quantified point-by-point but rather through a single accumulated damage variable. This variable aggregates the combined effect of all matrix cracking and scattered debonding accumulating cycle after cycle into a single, monotonically increasing value. As fatigue cycling continues, this accumulated damage variable rises steadily, never reverting to a lower value.

The increase in this accumulated damage variable causes the composite's residual stiffness to degrade: each matrix crack and debonding zone accounted for by the variable further reduces the composite's ability to elastically transfer load between its fibers and matrix. A higher accumulated damage variable results in lower residual stiffness, whereas keeping the variable low allows the residual stiffness to remain closer to that of the undamaged composite.

As the composite progresses along its stress-life curve, this residual stiffness does not drop uniformly but rather logarithmically with the number of cycles: it drops rapidly during the initial cycles, degrades slowly and gradually throughout the majority of the composite's fatigue life, and drops rapidly again in the final cycles, just before ultimate failure. The fact that residual stiffness follows this logarithmic pattern—rather than a uniform decline—means that the majority of the composite's fatigue life unfolds with residual stiffness degrading very slowly.

Consequently, this model—characterized by distributed micro-damage—differs explicitly from the equivalent model for metal fatigue; in the latter, a single dominant crack concentrates all damage at one point rather than distributing it throughout the material's volume, and the growth of that single crack governs fatigue life. In this composite, however, the distribution of damage throughout the volume means that the accumulated damage variable—comprising matrix cracking and dispersed debonding—is what drives the degradation of residual stiffness along the stress-life curve.

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