Fatigue Failure Behavior And Life Prediction Method Of Fiber-reinforced Ceramic Matrix Composites

Fiber-reinforced ceramic matrix composites(CMCs)have been widely used in the aerospace engineering due to their high specific strength and stiffness,corrosion resistance,and low density.These composite structures inevitably undergo dynamic alternating loads during actual service,such as vibration and noise,which could lead to fatigue failure.Therefore,research on the fatigue behavior of fiber-reinforced CMCs is of great importance.In addressing these issues,this paper conducts research in the following four aspects:First,the fatigue behavior and damage mechanisms of CMCs are investigated.Monotonic loading,loading/unloading,and cyclic loading tests were performed on CMCs to analyze their fatigue behavior,including hysteresis,deformation,and energy dissipation.Optical microscopy and scanning electron microscopy were utilized to observe the fracture morphology of the specimens and analyze their microscopic damage mechanisms.The results reveal that under monotonic tensile loads,the failure mechanisms of the CMCs include matrix cracking,fibermatrix interface debonding,fiber fracture,and fiber bundle extraction.Under high-level stress loading/unloading conditions,the hysteresis phenomenon becomes more pronounced,the elastic modulus decreases,and residual strength decreases as fiber bundles are extracted in clusters,indicating the appearance of damage.Under cyclic loading,the strain of the CMCs increases gradually with the increase of the cycle number,and the elastic modulus decreases,while the hysteresis dissipated energy decreases first and then increases.The fracture morphology shows that the majority of fiber extraction is scattered,and a large amount of debonding occurs within fiber bundles,more fibers are detached from the matrix,and the fiber extraction length increases,indicating that cyclic loading causes deterioration of the interface between fibers and matrix.Secondly,a micromechanics-based fatigue life prediction model for CMCs is proposed.The model takes into account processes such as matrix cracking,fiber debonding and sliding,and fiber fracture.The Weibull distribution is employed to statistically describe the fiber strength variability.Furthermore,fatigue damage mechanisms,such as fiber degradation and interface wear between fibers and matrix,are considered to analyze the damage evolution of the material under cyclic loading.Results indicate that as the increase of cycle number,the gradual degradation of fiber and interface properties leads to an increase in the stress borne by fibers,and the probability of fiber failure constantly increases,which lead to the fatigue fracture.Additionally,a comparison between the predicted fatigue life and experimental results shows that around half of the experimental results fall within a 2-fold fatigue life error interval,and all experimental results fall within a 4-fold fatigue life error interval,indicating the high accuracy of the micromechanics model.Finally,sensitivity analysis is conducted on micromechanics parameters,such as fiber characteristic strength,Weibull modulus,and interfacial shear stress between fibers and matrix.The influences of uncertain micromechanics parameters on the fatigue behavior of CMCs are studied.Thirdly,the strain rate effects on the mechanical behavior of CMCs under dynamic loading are investigated.By conducting monotonic tensile tests at different loading rates,the influence of strain rate effect on the failure mechanism and load-bearing capacity of CMCs was analyzed.The results show that compared to quasi-static loading,the strength of materials is slightly improved,and brittle strength is enhanced under dynamic loading.The fracture surface is rougher and the multiple failure features are more pronounced.Then,the strain rate effect and anisotropy are incorporated into the von Mises strength criterion.A dynamic strength criterion considering the strain rate effect is proposed.The effectiveness of the criterion is verified by comparison with experimental results.Finally,a method for predicting the fatigue life of CMCs considering strain rate effects is proposed.Fatigue tests under different loading frequencies were carried out to investigate the influence of strain rate effects on mechanical behaviors such as hysteresis,deformation,energy dissipation,and fatigue life.A micromechanical model was developed to incorporate the effects of strain rate on stress distribution and damage evolution,including fiber degradation and interface wear.Based on this model,a fatigue life prediction method considering the strain rate effect was established.The results showed that the increase in strain rate could accelerate fiber degradation and interface wear,leading to a reduction in fiber characteristic strength and interface shear stress under the same cycle number.,thus leading to the increasing probability of fiber failure and reducing the fatigue life of CMCs.Comparison between the experimental and predicted values of fatigue life showed that all experimental results fell within the 4-fold error interval,and approximately 60% of them fell within the 2-fold error interval.These findings demonstrate that the proposed strain rate-related fatigue life prediction method has good prediction ability and can effectively support the design and evaluation of composite structures.