Abstract
Polymer matrix composites (PMC’s) are widely used in critical aerospace structures due to their numerous advantageous mechanical properties. Recently, PMC’s have been considered for high temperature applications where viscoelasticity arising from the time dependent nature of the polymer matrix becomes an important consideration. This inherent viscoelasticity can significantly influence deformation, strength and failure response of these materials under different loading modes and environmental factors. With a potentially large number of plies of different fiber directions and perhaps material properties, determining a fatigue failure criterion of any degree of generality through experiments only, may seem to be an unrealistic task. This difficult situation may be mitigated through the development of suitable theoretical micro or macro mechanical models that are founded on considering the fatigue failure of the constituting laminas. The micro‐approach provides a detailed examination of the individual failure modes in each of the constituent materials i.e. fiber, matrix. In this work, a micromechanical approach is used to study the role of viscoelasticity on the fatigue behavior of polymer matrix composites. In particular, the study examines the interaction of fatigue and creep in polymer matrix composites. The matrix phase is modeled as a vicoelastic material using Schapery’s single integral constitutive equation. Taking viscoelsticity into account allows the study of creep strain evolution during the fatigue loading. The fatigue failure criterion is expressed in terms of the fatigue failure functions of the constituent materials. The micromechanical model is also used to calculate these fatigue failure functions from the knowledge of the S‐N diagrams of the composite material in longitudinal, transverse and shear loadings thus eliminating the need for any further experimentation. Unlike the previous works, the present study can distinguish between the strain evolution due to fatigue and creep. The results can clearly show the contribution made by the effect of viscoelasticity to the total strain evolution during the fatigue life of the specimen. Although the effect of viscoelsticity is found to increase with temperature, its contribution to strain development during fatigue is compromised by the shorter life of the specimen when compared to lower temperatures.
Highlights
The thermo‐mechanical viscoelastic response of a high temperature polymer matrix composite system made up of T650‐35 graphite fibers embedded in polymerization of monomer reactants (PMR)‐15 resin is studied through the Simplified Unit Cell Micromechanical (SUCM) model within a temperature range of 250 to 300o C corresponding to aerospace engine applications
Once σT is obtained from Equation 16, its corresponding Nf i.e. the number of cycles to failure can be calculated from the S‐N curve of the composite material in transverse direction, which serves as the first guess to be input into the convergence algorithm of figure 6
‐ The stress‐strain response of the composite material under a monotonically increasing stress was seen to be affected by temperature in transverse and shear loadings which were dominated by matrix properties
Summary
High strength‐to‐weight ratios of composites compared to metallic materials has won them numerous applications in different industries. One important feature expected of polymer matrix composites in today’s applications especially in advanced aerospace systems, is to sustain high temperatures and mechanical loadings while maintaining their light weight and flexibility. These applications usually involve long term exposure of the composite material to high temperatures such as in the propulsion systems and other engine components of commercial and military aircrafts. Development of a predictive model to study the response of high temperature PMC’s to both static and dynamic loading, in particular the contribution of the time dependent effect of the matrix material to the failure of the composite system, poses a serious challenge
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