Degradation modelling mechanical properties

In this chapter, existing models for describing the change of mechanical properties of FRP composites under elevated temperatures and fire have been reviewed. On the basis of a kinetic description of the involved physical and chemical processes, the modeling approach developed in Chapters 2-4 has been further extended to predict the degradation of mechanical properties. Those mechanical properties... 

Thermomechanical models for FRP materials were first developed in the 1980s. One of the first thermomechanical models for FRP materials was introduced by Springer in 1984 [1], where the degradation of mechanical properties was empirically related to the mass loss. In 1985, Chen et al. [2] added a mechanical model to the thermochemical model presented by Griffis et al. in 1981 [3] mechanical properties at several specified temperature points were assembled into a finite element formulation. Griffis et al. [4] introduced an updated version of Chen s model in 1986, whereby an extrapolation process was used to obtain the data in a higher temperature range. 

In recent years, poly(p-phenylene benzobisoxazole) (PBO) fibers have become prominent in high strength applications such as body armor, ropes and cables, and recreational equipment. However, degradation of PBO fiber mechanical properties following exposure to moisture has been documented by the manufacturer (7) and at least one failure of PBO-based body armor in the field has occurred (2). The objectives of this study were to compare changes in mechanical and chemical properties of yams extracted from PBO-based body armor that was penetrated by a bullet in the field approximately 6 months after being deployed, with PBO-based body armor panels of the same model aged in the laboratory under elevated temperature and moisture conditions. 

Oxidative degradation dominates when Estane/NP is thermally aged in the absence of water and oxygen. Through NMR analysis, the oxidation of the bridging methylene group was quantified. These data contributed to the development of kinetic models for prediction of oxidative degradation. We observed a correlation between oxidation and mechanical properties. 

Gibson et al. [8] presented a temperature-dependent E-modulus model in 2004. Mechanical properties were assumed to degrade during the glass transition as described by Eq. 5.3 ... 

In 1992, McManus and Springer [5, 6] presented a thermomechanical model that considered the interaction between mechanically induced stresses and pressures created by the decomposition of gases within the pyrolysis front. Again, temperature-dependent mechanical properties were determined at several specified temperature points as stepped functions. The issue of degradation of material properties at elevated temperatures was considered in Dao and Asaro s [7] thermo-mechanical model in 1999. The degradation curves used in the model were, once again, obtained by curve fitting of limited experimental data. 

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