Going to Extremes Meeting the Emerging Demand for Durable Polymer Matrix Composites (2005) / Chapter Skim
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2 Composite Properties and Behavior
2 Composite Properties and Behavior
Pages 16-25
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From page 16... ...
In laminated PMCs, these phenomena can include microcracking, local crazing of polymers in the vicinity of cracks, and localized fiber failures. It is useful in this discussion to differentiate among damage mechanisms, such as oxidation, time of service, moisture absorption; failure modes, such as fiber delamination or matrix cracking; and stress factors, such as load, temperature, and chemical species available at the surface.
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From page 17... ...
Or did the designers neglect the environmental cracking potential of trace sulfur compounds in natural gas, as was actually the case? In the presence of a small stress concentration, such as a small starter crack, and an applied stress, the trace environmental agent was sufficient to transform a relatively benign crack in an ostensibly tough material into a potentially lethal crack growing at impressively low stress levels and very slow rates!
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From page 18... ...
When the damage product is obvious, as it is in matrix cracking, it is possible to define protocols and perform calculations to ascertain the degree to which various cracks can form and grow while still maintaining structural integrity. When the damage product is the degradation and weakening of material in the immediate vicinity of a crack tip and in the presence of an applied load, the protocols have yet to be established and the calculations are at best crude.
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From page 19... ...
2004. An energy criterion to predict delayed failure of multi-directional polymer matrix composites based on a non-linear viscoelastic model.
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From page 20... ...
· Mechanical models attempt to represent durability as a mechanical process -- that is, in terms of balance of forces, mass, momentum, energy; transport; and constitutive equations. Macroexamples are structural analysis methods such as fracture mechanics, strain energy release methods, and some energy dissipation schemes.18-20 These include composite-specific models, such as those for statistical fiber breakage,21 matrix microcracking,22 or interfacial failure.23 Indeed, some of these phenomena are often relatable to more complex loading conditions, including fatigue.24 Microexamples include dislocation or slip models (and diffusion models in some cases)
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From page 21... ...
,36 which has shown ability to predict failure under simple loadings accurately,37 knockdown factors are still used in conjunction with this theory in predicting fatigue lifetime (absent the data needed to provide validation)
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From page 22... ...
Reducing dependence on knockdown factors in lifetime estimation will not require anything so grand as development of completely interlinking and multiscale models for a large variety of materials. Indeed, the recent exercise that compared leading failure theories in laminated PMCs39 revealed that the head-to-head comparison of models thought of as classical show wide variation in both their predicted failures and in their correlation with experimental results, when the results are plotted as points on traditional failure envelopes with axes of principal stresses or strains.
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From page 23... ...
Polymer matrix composites allow doing this, and their use in conformal composite fuel tanks offers lower weight and lower cost fabrication. The X-33 program entailed a one-fourth scale unmanned prototype of a reusable launch vehicle.
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From page 24... ...
Thus, strain invariant failure theory is nothing more than an appropriate and proper framework in which to describe strains in three dimensions within an inhomogeneous and anisotropic material. Within this framework, it may be possible to analyze failure data obtained with a particular composite stacking sequence subjected to a particular set of stress components and then transfer that analysis in a solid-mechanics-based way to another composite stacking sequence with a different set of stresses.
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From page 25... ...
In certain copolymers, for example, the microstructure can change dramatically (the matrix and distributed phases can be inverted, for example) as a function of hydration and temperature, greatly altering stiffness, fracture strain and stress, creep rates, and failure times.
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