Life prediction of fiber-reinforced composites: Macro- and micro-mechanical modeling

With the recent emphasis on cost, the role of material durability has taken on greater significance in present material system design philosophy. With isotropic materials the life of a component is defined by monitoring the growth of the dominant crack using fracture mechanics. The loading of composite materials does not result in a single dominant crack but rather in damage that is widely distributed. The life prediction of composite materials is thus covered under the domain of damage mechanics. The two common approaches to damage mechanics are, macro- and micro-mechanical modeling. The former is favored by engineers because homogeneity is assumed at the lamina level where stiffness can be tailored while, the latter is favored by material designers as all failure originates at the fiber-matrix level.; In the first part of this study a life prediction methodology (MRLife{dollar}sp{lcub}rm TM{rcub}{dollar}) based on the remaining-strength-degradation approach is presented. In MRLife{dollar}sp{lcub}rm TM{rcub}{dollar} representation of the damage phenomena measured at the macroscopic (lamina) level as a function of time or cycles is used as inputs. Remaining strength, the damage metric, is monitored and failure assumed to occur when it falls below the local level of stress. A Fortran code of this remaining strength life prediction methodology, modified for ceramic composites (CCLife), is integrated into the finite element package CSTEM, to create an integrated design tool for ceramic matrix composites. Using this tool a case study to predict the life of a notched Nicalon/Silicon Carbide 2-D woven laminated composite coupon subject to an isothermal fatigue loading is performed. Global failure of the notched plate is predicted based on a Whitney-Nuismer type average strength criterion.; In spite of it getting good results in the case study, it is recognized that the macro-level methodology has severe limitations. In the second part of this study, simulation of events occurring at the fiber-matrix level are used to develop micro-mechanical models for the time-dependent behavior of fiber-reinforced composites. The two mechanisms simulated are shear creep of the interface and fiber degradation by slow crack growth. The simulations are performed on a modified Monte-Carlo model, used earlier by Curtin to accurately predict fast-fracture strength. The results of the simulations are then used to validate analytical models for the two mechanisms developed. The results from the shear creep model are compared with those from Du and McMeeking while the fiber degradation model results are compared with data from Brennan.; In the final part of the study an analytical model for the time-dependent failure of a composite due to the combined effects of shear creep at the interface and fiber degradation is presented. The potential for including the time-dependent failure model into CCLife is evaluated by comparing the time-dependent results with those form CCLife results under the same conditions.