6 - Stochastic loading-dependent nonlinear damage behavior of ceramic–matrix composites

7 - Time-dependent nonlinear damage behavior of ceramic–matrix composites

9 - Effect of thermal fatigue on nonlinear damage behavior of ceramic–matrix composites

In this paper, the stress-dependent matrix multiple fractures of fiber-reinforced ceramic-matrix composites (CMCs) with fiber oxidation and fracture is investigated. The shear-lag model is combined with the fiber oxidation and fracture models, and the fiber/matrix interface debonding criterion is adopted to determine the micro-stress field of the damaged fiber-reinforced CMCs. The effects of the fiber volume, interface shear stress, interface debonding energy, oxidation temperature and time on the stress-dependent matrix multiple fractures, interface debonding and oxidation, and fiber fracture are discussed. When the fiber volume fraction and the interface shear stress in the debonding region increased, the matrix first cracking stress and saturation cracking stress increased, the saturation matrix cracking space decreased, and the matrix multiple fracture evolution rate increased. When the interface debonding energy and the interface shear stress in the oxidation region increased, the matrix cracking density increased. When the oxidation temperature and oxidation duration increased, the matrix cracking density decreased. The experimental matrix multiple fractures of unidirectional C/SiC, SiC/SiC, mini-SiC/SiC, SiC/CAS, SiC/CAS-II, and SiC/borosilicate composites are predicted.

In this chapter, the time-dependent first matrix cracking stress of fiber-reinforced ceramic-matrix composites (CMCs) is investigated using the energy balance approach. The shear-lag model combined with the interface oxidation model, fiber oxidation model, and fiber failure model is adopted to analyze the microstress distributions in fiber-reinforced CMCs. The relationships between the first matrix cracking stress, interface debonding and slip, fiber fracture, and oxidation time and temperature are established. The effects of the fiber volume, the interface shear stress, the interface debonding energy, the fiber Weibull modulus, the fiber strength on the first matrix cracking stress, the interface debonding length, and the fiber broken fraction are analyzed. The first matrix cracking stresses of C/SiC with strong and weak interface bonding after unstressed oxidation at 700 °C in air atmosphere are predicted for different oxidation time.

In this paper, tensile damage and fracture of 2D SiC/SiC composites are investigated using damage monitoring of acoustic emission (AE) and natural frequency. Nonlinear damage and fracture are mainly attributed to damage mechanisms of matrix cracking, interface debonding, and fibers fracture. Monotonic tensile stress-strain curves are divided into three stages based on the analysis of AE count, amplitude, and energy. Under cyclic loading/unloading, hysteresis loops appear due to internal frictional slip between the fiber and the matrix, and the natural frequency and composite modulus are obtained for different peak stress. A micromechanical tensile and cyclic loading/unloading constitutive model is adopted to predict the tensile curves. Micro damage parameters of interface debonding ratio and broken fibers fraction are used to characterize tensile damage and fracture. Relationships between natural frequency, composite modulus, interface debonding, and fibers fracture are established. Under tensile loading, the AE signal is large and concentrated mainly in the stress range of 50 -100 MPa and 150 -180 MPa, which corresponds to matrix cracking and fiber fracture, respectively. When the degradation rate of natural frequency approaches 0.01, matrix cracking and interface debonding occur, however, fibers failure does not appear; when the degradation rate of natural frequency approaches 0.04, the composite modulus decreases approximately 47 % and the interface debonding ratio approaches 0.8 and the broken fibers fraction is approximately 2.2 %.

The fatigue hysteresis behavior of unidirectional, 2D cross-ply and woven, and 2.5D woven fiber-reinforced ceramic-matrix composites (CMCs) are analyzed. Based on the fiber/matrix interface debonding and sliding behavior, the fiber/matrix interface debonding and sliding lengths are determined using the fracture mechanics approach. The fiber/matrix interface debonding ratio and interface sliding ratio are determined for different interface slip cases. The effects of fiber volume fraction, peak stress, matrix crack spacing, interface shear stress, interface debonded energy, fibers failure, fiber Poisson contraction, fiber strength, fiber Weibull modulus, matrix cracking mode, applied cycle number and fiber/matrix interface wear on the fatigue stress–strain hysteresis loops and the fiber/matrix interface debonding and sliding are discussed. The experimental cyclic fatigue stress–strain hysteresis loops of unidirectional SiC/CAS, SiC/1723 and C/SiC, 2D cross-ply SiC/CAS and woven SiC/SiC, and 2.5D woven C/SiC composites under cyclic loading/unloading tensile and tension–tension fatigue loading are predicted.

In this paper, the effect of the fiber/matrix interface properties on the tensile and fatigue behavior of 2D woven SiC/SiC ceramic‐matrix composites (CMCs) is investigated. The relationships between the interface parameters of the fiber/matrix interface debonding energy and interface frictional shear stress in the interface debonding region and the composite tensile and fatigue damage parameters of first matrix cracking stress, matrix cracking density, and fatigue hysteresis‐based damage parameters are established. The effects of the fiber/matrix interface properties on the first matrix cracking stress, matrix cracking evolution, first and complete interface debonding stress, fatigue hysteresis dissipated energy, hysteresis modulus, and hysteresis width are analyzed. The experimental first matrix cracking stress, matrix cracking evolution, and fatigue hysteresis loops of SiC/SiC composites are predicted using different interface properties.

This paper describes a creep rupture model considering the interfacial debonding in broken fibers in unidirectional polymer matrix composites reinforced by glass fibers. Interfacial debonding was accompanied with fiber breaks under the tensile load, depended on the fiber stress at the simultaneous fiber breaks. The interfacial debonding length, fiber strength and an interfacial shear stress were investigated by the fragmentation test. A vinylester/glass fiber interface's properties were investigated by the fragmentation test. Subsequently, the debonding length and a stress recovery length were quantitatively studied and it was found out that the relation between the strength and the length of fiber was linear. The debonding length was calculated by a measure of the distance between the fiber breaking faces. The interfacial debonding length was taken into the Curtin's fracture model. Moreover, the creep test of the unidirectional composites, it was vinylester resin matrix reinforced by glass fibers, was performed to investigate the creep constants. Then, the prediction in the use above parameters of the creep rupture time was discussed.

Under stress-rupture loading, stochastic loading affects the internal damage evolution and lifetime of fiber-reinforced ceramic-matrix composites (CMCs) at intermediate temperatures. The damage mechanisms of the matrix cracking, fiber/matrix interface debonding and oxidation, and fiber fracture are considered in the analysis of stochastic loading. The strain, fiber/matrix interface debonding and oxidation length, and the broken fibers fraction versus the time curves of SiC/SiC composite under constant and three different stochastic loading conditions are analyzed. The effects of the stochastic loading stress level, stochastic loading time, and time spacing on the damage evolution and lifetime of SiC/SiC composite are discussed. When the stochastic loading stress level increases, the stress-rupture lifetime decreases, and the time for the interface complete debonding and oxidation decreases. When the stochastic loading time and time spacing increase, the stress-rupture lifetime decreases, and the time for the interface complete debonding and oxidation remains the same.

Fiber-reinforced polymer (FRP) composite materials have been widely used in the field of retrofitting. Theoretical analysis of FRP plate- or sheet-strengthened cracked concrete beams is necessary for estimating service reliability of the structural members. In previous studies, the effect of a perfectly bonded FRP plate or sheet was equivalent to a cohesive force acting at the bottom of crack to delay the crack propagation in concrete and reduce the crack width. However, delamination between FRP and cracked beam is inevitable due to interfacial shear stress concentration at the bottom of crack. The intention of this paper is to present an analytical solution for fracture analysis of carbon FRP (CFRP) sheet–strengthened cracked concrete beams by considering both vertical crack propagation in concrete and interfacial debonding at CFRP-concrete interface. The interfacial debonding is modeled as the interfacial shear crack propagation in this paper. Four different stages are discussed after initial cracking state of the concrete. At the first stage, only fictitious crack propagation occurs in the concrete. At the second stage, macrocrack propagates in the concrete without interfacial debonding. At the third stage, both vertical macrocrack propagation in the concrete and horizontal shear crack propagation at the CFRP-concrete interface occur in the strengthened beam. The tensile stress in the CFRP sheet and interfacial shear stress along the span are formulated based on the deformation compatibility condition at the CFRP-concrete interface at this stage. Finally, macroshear crack propagates at the interface until the CFRP sheet is completely peeled out from the beam, and then the member is fractured. The applied load is determined as a function of the referred two crack lengths at different stages. At the beginning, the applied load increases to one peak value with the full propagation of fictitious crack at the first stage. At the third stage, the applied load is improved to another peak value due to the relatively high cohesive effect of the CFRP sheet. Then the two peak values are determined by the Lagrange multiplier method. The validity of the proposed analytical solution is verified with the experimental results and numerical simulations. It can be concluded that the proposed analytical solution can predict the load-bearing capacity of CFRP sheet-strengthened cracked concrete beams with reasonable accuracy.

The effect of water on fiber bridging of polymer matrix composites (PMCs) has been studied for stress-corrosion cracking (SCC). The fiber bridging during crack propagation in the PMC was observed directly using a transparent polymer matrix. Specimens were immersed and weighed just before conducting the crack propagation test. The contribution of mechanical degradation caused by the water to the bridging was measured as a function of weight gain due to water absorption. The critical energy release rate of the matrix decreased with increase of the water absorption. The energy release rate of a bridged crack varied with the water absorption. To investigate the variation in the bridging mechanism under the water environment, we calculated the bridging stress the σt and the bridging contribution to crack propagation resistance ΔG based on a single-fiber bridging mechanism with the interfacial debonding length and the interfacial debonding energy. The debonding length was measured directly, and the debonding energy was obtained in a fragmentation test. The calculation showed that the increase in the interfacial debonding length influenced the fiber bridging mechanism.

In this chapter, stress-rupture of fiber-reinforced ceramic-matrix composites (CMCs) with stochastic loading at intermediate temperature is investigated. Four different stochastic loading sequences under stress-rupture are considered with a different loading time and time interval. Microstress field of the damaged CMCs and different damage models consider the effect of stochastic loading and time-dependent interface and fiber oxidation. Damage evolution of stress-rupture strain, interface debonding and interface oxidation ratio, and the broken fibers fraction versus time curves of SiC/SiC composite subjected to four different stochastic loading sequences are analyzed. Effects of composite constituent properties, damage state, and environmental temperature on damage evolution and lifetime of SiC/SiC composite are discussed. Experimental damage evolution and lifetime of SiC/SiC composite under stress-rupture with stochastic loading are predicted.

Orientation-dependent tensile deformation and damage of a T700 carbon fiber/epoxy composite: A synchrotron-based study

This paper proposes a strength prediction method for unidirectional glass fiber reinforced plastics (GFRPs) after hydrothermal aging: immersion in deionized water at 80°C. First, the strength degradation of the constituents (i.e., the glass fiber and the fiber/matrix interface) of unidirectional GFRP after hydrothermal aging was evaluated from the fiber strength and the interfacial shear stress by using a single-fiber composite (SFC). Both the fiber strength and the interfacial shear stress had a tendency to decrease in the early stage of hydrothermal aging and to saturate toward certain values with long-term aging. In addition, the tensile strength of the unidirectional GFRP was measured after hydrothermal aging. The residual strength of the unidirectional GFRP also had a tendency to decrease sharply in the early stage of hydrothermal aging and to saturate toward a certain strength with long-term aging. Finally, the residual strength of the unidirectional GFRP after hydrothermal aging was predicted using the global loading sharing (GLS) model, by considering the degradation of both the glass fiber and the fiber/matrix interface. The predicted results indicated good agreement with the experimental data while considering not only the degradation of the fiber reinforcement but also the fiber/matrix interface adhesion. It was concluded that the GLS model applied considering the degradation of the GFRP constituents would be a suitable and a simple model to predict the residual strength of the unidirectional GFRP after hydrothermal aging.

Shear behavior of fiber reinforced concrete beams with high fly ash content