Evolution of bridging fiber stress in titanium metal matrix composites at elevated temperature

This paper deals with the determination of stress evolution in bridging fibers during fatigue crack growth in a SM1240/Timetal-21S composite using the finite element method. Several parameters affecting this evolution were considered, namely, the process-induced residual stress, the creep characteristics of the matrix layer surrounding the fiber, the test temperature, and the loading frequency. In support of these calculations, a series of elevated temperature fatigue crack growth tests was conducted to identify the crack growth behavior of the composite when subjected to different temperatures at both high and low loading frequencies. Results of this numerical/experimental work were then utilized in conjunction with a postulated fiber fracture criterion based on the notion that a competition exists between the increase in the axial fiber stress and the continuous degradation of the fiber strength due to cyclic wear induced by the interface frictional shear stress. The conclusions of this study show that the axial stress in the bridging fibers increases with an increase in temperature and with a decrease in both the loading frequency and the matrix grain size. A combination of high-temperature, low-frequency, and small-matrix grain size would enhance creep deformation of the matrix, thus leading to an increase in the rate of the load transfer from the matrix to the bridging fibers. Furthermore, the presence of a compressive residual stress state in the bridging fibers retards the time-dependent increase of their axial stress. The fatigue strength of the bridging fibers was estimated to range from 720 to 870 MPa within the temperature range of 500 to 650°C. This strength was found to depend on both the temperature and the loading frequency

Fatigue damage mechanisms of bridging fibers in titanium metal matrix composites

This paper investigates the fatigue damage mechanisms of SiC fibers bridging a fatigue crack in unidirectional reinforced titanium matrix composites. For this purpose, an experimental/computational fiber fracture model is developed on the basis of the occurrence of two damage events taking place along a bridging fiber. These events are the time-dependent evolution of axial stresses and the simultaneous strength degradation of the fiber due to cyclic-related damage processes. The stress evolution in a fiber is calculated using the finite element method employing a cylinder model of a fiber embedded in a cracked matrix phase. The model considers the visco-plastic behavior of the matrix phase at elevated temperature loadings. The failure strength of the as-received SiC fiber are determined through a series of monotonic tension, residual fatigue strength and fatigue-life tests performed on SiC fibers at different temperatures. In order to take into account the notch-like effects resulting from the presence of fiber coating cracks and possible deflection of fiber/matrix interfacial cracks, the fatigue strength of the as-received SiC fiber was modified using elastic stress localization. The resulting fatigue strength of bridging fibers was found to be about 56 percent less than the corresponding strength of as-received fibers. The fiber stress evolution curve and the modified fatigue strength curve were then combined to predict the life of bridging fibers. Results of the model are compared with those obtained experimentally for bridging fibers in SiC/Timetal-21S composite subjected to load conditions including low and high loading frequency at 500 and 650 °C

Fracture criterion for bridging fibers in titanium metal matrix composites at elevated temperature

In this paper, a fracture criterion for bridging fibers in titanium metal matrix composites at elevated temperature is proposed. This criterion is based on the notion that in a composite subjected to cyclic loading, a competition exists between the evolving axial stress of a bridging fiber and its continuously decreasing fatigue strength. The stress build-up in the bridging fiber during fatigue crack growth of SM1240/Timetal-21S composite under different temperatures and loading frequencies is simulated using the finite element method. The life of bridging fibers was estimated from the knowledge of the fatigue crack growth behavior of the composite subjected to loading conditions similar to those used in the numerical simulation. Effects of the process-induced residual stress, test temperature and loading frequency were included in this simulation. Results of the study show that the build-up of the fiber stress is a time-dependent process while the degradation of the bridging fiber strength is both temperature and loading frequency dependent

Fiber damage mechanisms in titanium metal matrix composites

The fatigue damage mechanisms of SiC fibers were examined to interpret the observed variations in composite strengths tested at different temperatures. A series of monotonic tension, residual fatigue strength and fatigue-life tests were conducted on SCS-6 fibers at 500 and 650 °C. The surface of the fatigued fibers were examined by scanning electron microscopy for damage features such as the distribution of cracks and the spalling of the carbon-rich coating. Both the static and fatigue strength of SiC SCS-6 fibers were unaffected by the test temperature of 650 °C and below. The mean strength of the fiber was found to be 3.05 GPa with one standard deviation of 0.6 GPa

Interphase behavior of titanium matrix composites at elevated temperature

This paper examines the effect of temperature and thermal exposure on the interphase behavior of continuous fiber reinforced titanium metal matrix composites. The system considered is SCS-6/Timetal-21S. Elevated temperature fiber push-out tests were conducted to determine the effect of test temperature on interphase shear properties. Corresponding variations of debonding shear strength and frictional shear stress with test temperature are presented and discussed. Thermal exposure, both in a vacuum and an air environment, has been conducted on specimens, with temperatures up to 650 °C and exposure times of up to 100 h. The resulting size and composition of the interphase have been examined. Fiber push-out tests were carried out at room and elevated temperature on the aged specimens. Results are discussed in terms of the influence of relaxation and oxidation on the debond shear strength. Using the experimentally determined interphase shear properties, the interphase toughness has been calculated and discussed in relation to interface decohesion models

Time-dependent behavior of continuous-fiber-reinforced metal matrix composites: Modeling and applications

A time-dependent approach employing a four-phase concentric cylinder model has been developed to predict the response of titanium-based metal matrix composites subjected to thermomechanical loadings in which both plastic and creep responses of the composites are considered. The progressive development of plasticity in the matrix phase is determined using the deformation theory of plasticity while the creep deformation of this phase is estimated using the Bailey-Norton equation with an Arrhenius-type expression for the time-dependent creep coefficient

Interphase shear strength of titanium metal matrix composites at elevated temperatures

A series of fiber pushout tests on thin-slice samples of a SCS-6/Timetal-21S composite were carried out to determine the load values at which partial and full debonding occurs. Finite element calculations of the stress field in the specimen were employed to assess the interphase strength of the composite as function of temperature. In these calculations, the semi-infinite thickness and the traction-free surface effects of the thin-slice samples on the corresponding stress field are considered. For each of these specimens, the distribution of shear stress along the fiber/matrix interface is determined in order to identify a region of stress localization which is taken in this study to be a measure of the interphase shear strength. This strength is then identified as the balance of forces at this localized field due to the traction-free surface of the composite section. Both contributions from process-induced residual stress and geometry-induced constraint of the traction-free surface to the strength are considered. The results of this study showed that the interphase shear strength decreases with an increase in temperature and processing-related residual stress contributes about 35 % to the interphase shear strength at room temperature. Furthermore, the interphase shear strength as calculated in this paper was found to be larger than that determined by considering uniformly distributed shear stress along a pushout fiber

Influence of interfacial properties on fiber debonding in titanium metal matrix composites

The influence of fiber/matrix interfacial properties on both the initial fiber/matrix debonded length and the stress associated with a bridging fiber at elevated temperatures was studied, including the process-induced residual stress, surface roughness and shear strength. The fiber bridging process was simulated using finite element method applied to concentric three-phase cylinders representing the fiber, interphase and matrix phases. Three temperatures applied to the fibers were also investigated. Both the initial debonded length and the bridging fiber traction range increased with increasing temperature and with decreasing coefficient of friction

Processing-related interface properties in titanium metal matrix composites

This paper deals with the influence of processing-related residual stresses on the shear strength of the fiber/matrix interphase in a SiC/Ti MMC. For this purpose, three identical SCS-6/Timetal-21S composites were consolidated with different sets of processing variables and post-processing heat treatments. The evolution of residual stress fields in each composite throughout initial cool down from consolidation to room temperature is calculated using finite element method. In this analysis, the relaxation characteristics of the residual stresses at elevated temperature and their effects on the stress field at room temperature are identified. A series of fiber pushout tests on thin-slice samples of each composite were carried out to determine the load values at which partial and full debonding occurs. Finite element calculations of the stress field were employed to assess the interphase strength of the composite as function of temperature. In these calculations, the semi-infinite thickness and the traction-free surface effects of the pushout specimens on the corresponding stress field are considered. For each of these specimens, the distribution of shear stress along the fiber/matrix interface is determined in order to identify a region of stress localization which is taken in this study to be a measure of the interphase shear strength. This strength is then identified as the balance of forces at this localized field due to the traction-free surface of the composite section. Both contributions from process-induced residual stress and geometry-induced constraint of the traction-free surface to the strength are considered. The results of this study showed that the interphase shear strength decreases with an increase in temperature and processing-related residual stress contributes about 35 % to the interphase shear strength at room temperature. Furthermore, the interphase shear strength as calculated in this paper was found to be larger than that determined by considering uniformly distributed shear stress along a pushout fiber