Traditional computational approaches for predicting the life and
long-term behavior of materials rely on empirical data and are
neither generic nor unique in nature. Also, those approaches are
not easy to implement in a design procedure in an effective, integrated
manner. The focus of ongoing research at the NASA Lewis Research
Center has been to develop advanced integrated computational methods
and related computer codes for a complete reliability-based assessment
of composite structures. These methods--which account for uncertainties
in all the constituent properties, fabrication process variables,
and loads to predict probabilistic micromechanics, ply, laminate,
and structural responses-have already been implemented in the
Integrated Probabilistic Assessment of Composite Structures (IPACS)
(ref. 1) computer code. The main objective of this evaluation
is to illustrate the effectiveness of the methodology to predict
the long-term behavior of composites under combined mechanical
and thermal cyclic loading conditions.
A unified time-, stress- and load-dependent Multifactor Interaction
Equation (MFIE) model developed at NASA Lewis (ref. 2) has been
used to simulate the long-term behavior of polymer matrix composites.
The MFIE model evaluates the magnitude of degradation and properties
of constituent materials (including possible impending failure
modes) at every cycle step at the temperature that will be used
for micromechanics and laminate analysis.
The deterministic part of the methodology has been implemented
in the in-house computer code Integrated Composite Analyzer (ICAN)
(ref. 3). NASA Lewis has demonstrated a methodology to compute
the fatigue life for different applied-stress to laminate-strength
ratios on the basis of first-ply failure criteria and thermal
cyclic loads. (First-ply failure criteria assumes that a laminate
has failed when any stress component in a ply exceeds its respective
allowable.) Degradation effects due to long-term environmental
exposure and thermomechanical cyclic loads are considered in the
simulation process.
In reference 4, an application of MFIE is illustrated by considering a [0°/±45°/90°]s graphite fiber epoxy matrix composite. The fatigue life cycles were computed for different thermal cycles and for different magnitudes of applied-stress-to-laminate-strength ratios based on first-ply failure criteria (ref. 4). These curves can be used to assess the fatigue life of a component subjected to mechanical cyclic loading for a given reliability (as in the figure above). Cumulative probability distribution functions for mechanical fatigue due to different cyclic stress magnitudes and the respective sensitivity factors are shown in the following figure.1
Previous articleLast updated April 30, 1997
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