Title page for ETD etd-08292008-063445
|Type of Document
||Halverson, Howard Gerhard
||Improving fatigue life predictions :theory and experiment on unidirectional and crossply polymer matrix composites
||Master of Science
|Curtin, William A. Jr.
|Hajj, Muhammad R.
|Reifsnider, Kenneth L.
|Date of Defense
A method is presented by which fatigue life predictions of polymer matrix composites may
be improved. First a "critical element", whose failure defines global failure of the material.
is identified. The global stiffness changes of the specimen during a fatigue test are
monitored and taken to be inversely proportional to the increase in applied stress on the
critical element. Using a cumulative damage model, the complicated stress history of the
critical element is reduced to a "critical element SN curve", which defines the fatigue
response of the critical element. The residual strength of the critical element may then be
continuously evaluated to predict failure. The statistical nature of material strength is
accounted for by forcing the critical element SN Curve to yield a specimen initial strength
distribution in fatigue which is equivalent to the quasi-static tensile distribution. In
contrast to most methods, the predictions are based on the stiffness history of the
specimen in question, rather than on generalized phenomenological models. The critical
element SN curve is then applied to (90°/0°) crossply materials to evaluate their fatigue
response. Simulations and the variation of experimental parameters are examined for their
effect on the predictions.
The unidirectional fatigue predictions were vastly improved over the traditional SN Curve.
While the crossply predictions were not as good, they still demonstrated the applicability
of the critical element SN curve to a material with a different geometry. Additionally.
such a method may have application in real-time durability evaluation of composite
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