Phenomenology of Composite Fatigue

For polymer-matrix composites, fatigue failure is primarily a matrix-dominated event.

Fatigue damage begins with microcrack accumulation in the polymer. These microcracks accumulate most rapidly in the early stages of fatigue life, with the accumulation rate slowing with increasing number of cycles. Ultimately, the microcracks can cause a macroscopic crack that can quickly cause a catastrophic failure event. Thus, accurate fatigue life prediction requires capturing the accumulation of microcracks per cycle.

Unidirectional Materials

Fatigue failures of unidirectional composite materials typically fall into one of three categories: off-axis failure, on-axis failure, and delamination. Off-axis failure occurs when composite tensile loading is more than a few degrees from the fiber axis [22].

1. Failure is characterized by matrix cracking parallel to the fiber [23-24] or by debonding between the fiber and matrix interface.

2. On-axis failure occurs when a tensile fatigue load is applied in the direction of the fibers. As with the off-axis failure, microcracking occurs first in the matrix, but any matrix crack that may form cannot easily propagate through the fibers. Rather, cracks are arrested at the fibers causing an increase in number and magnitude of stress concentration points in the fiber [25].

3. In the case of a composite laminate, delamination is possible. Again, delamination is a failure event that results from accumulated matrix microcracking. Thus, accounting for the accumulation of matrix microcracks is sufficient for predicting fatigue failure due to delamination.

   Note: When delaminations are present before initiation of fatigue loading, built-in FEA tools for crack propagation should be used rather than the fatigue feature within Helius PFA.

Woven Materials

The failure mechanisms of woven composite lamina can be separated into distinct categories identified as [26-28]:

1. Transverse cracking in the tows during the initial loading. This is identified as a "knee strength", as the matrix constituent within the tows is failing and causes a stiffness change of the lamina.
2. During a low applied cycle range, the matrix pocket begins to microcrack, and the composite softens slightly.
3. During the larger applied cycle range, the tows perpendicular to the loading direction begin to macrocrack further, causing a further softening of the lamina.
4. Simultaneously with (3), delaminations between the fill and warp tows cause a decoupling of the tows and a further reduction in modulus.
5. Final fatigue failure of the tows parallel to the loading direction occurs, and stiffness of the composite lamina is lost.