Cyclic behavior of unidirectional and cross-ply titanium matrix composites

Relatively simple and efficient micromechanical models are used to obtain the uniaxial response of SCS-6/Timetal 21S with [0]₄ and [0/90]_s laminates when subjected to isothermal and thermomechanical fatigue (TMF) loadings. Features of the modeling that are required to obtain the accurate deformation behavior for this class of materials under these loadings are highlighted. To this end, a comparison is made between the concentric cylinder model and the uniaxial stress model for representing the [0] laminate. The axial stresses from the two models are very similar under mechanical loading. The greatest differences appear under thermal loading alone. The differences on the composite response between a time-independent elastic-plastic and a viscoplastic matrix constitutive model are also examined. The latter is based on the Bodner-Partom unified constitutive model. The [0/90] laminate is treated by adding a parallel element with smeared [90] ply properties to the [0] model and invoking axial strain compatibility as well as stress equilibrium. The proposed constitutive law for the [90] ply includes both matrix viscoplasticity and fiber/matrix separation damage and is based on damage mechanics concepts. The effect of cyclic frequency on TMF behavior is examined. The in-phase TMF life is shown to be very sensitive to frequency due to the relaxation of matrix stress and the attendant increase in fiber stress.     

Hygric characterization of composites using an antisymmetric cross-ply specimen

A novel experimental method is developed to improve the sensitivity in measuring the hygric properties of a composite material. The technique is based on measuring the curvature of an unbalanced laminate introduced by the unbalanced interlaminar resultant forces. A theoretical foundation is established for evaluating the coefficients of moisture expansion (in the longitudinal and transverse directions) and the stress-free temperature from a single set of measurements. The measurement scheme is validated with a set of experiments using two antisymmetric cross-ply laminates [O₂/9O₂] and [O₅/9O₅]. The experimental results agree with the measurements reported elsewhere. This study reveals that the sensitivity of the technique is greater than that of traditional techniques.     

Effect of matrix cracking in cross-ply ceramic matrix composite beams on their mechanical properties and natural frequencies

The present paper illustrates the effect of matrix cracks in longitudinal and transverse layers of cross-ply ceramic matrix composite (CMC) beams on their mechanical properties and vibration frequencies. Even in a geometrically linear problem considered in the paper, the physical non-linearity is introduced by matrix cracks and interfacial fiber-matrix friction in longitudinal layers. A closed-form solution for mechanical properties of a cross-ply CMC beam with matrix cracks is developed in the paper. The frequency of free vibrations of a simply supported beam is derived as a function of the amplitude, accounting for the effect of matrix cracks. As shown in the paper, the prediction of the natural frequencies of cross-ply CMC beams with matrix cracks in both longitudinal and transverse layers is possible using simple, yet accurate, approximate equations.     

Damping in ceramic matrix composites with matrix cracks

The paper presents an analytical solution capable of predicting the effect of matrix cracking in ceramic matrix composites (CMC) on damping. The cracking scenarios considered in the paper include through-the-thickness cracks and cracks terminating at the layer interfaces. The increase in damping associated with matrix cracking is mostly due to the frictional energy dissipation along the damaged fiber–matrix interfaces adjacent to the bridging cracks whose plane of propagation intersects the fiber axis. Damping increases with a higher density of matrix cracks. The loss factor is affected by the angle of the lamina relative to the direction of the applied load. The loss factor is also influenced by the frequency and magnitude of local dynamic stresses. Examples of distributions of the local loss factor along the axis of a CMC beam subject to pulsating loads of various frequencies are shown in the paper.     

Analysis of elastic crack bridging in ceramic matrix composites

A micromechanics analytical model based on the consistent shear lag theory is developed for predicting the failure modes in fiber reinforced unidirectional stiff matrix composites. The model accounts for a relatively large matrix stiffness and hence its load carrying capacity. The fiber and matrix stresses are established as functions of the applied stress, crack geometry, and the microstructural properties of the constituents. From the predicted stresses, the mode of failure is established based on a point stress failure criterion. The role of the microstructural parameters of the constituents on the failure modes such as self-similar continuous cracking, crack bridging and debonding parallel to the fibers is assessed.     

On photoelastic response of composites

Using the theory of the unitary system of retarders, it is shown that two seemingly different theories of photoelasticity (Sampson’s phenomenological theory and Dally and Prabhakaran’s stress-proportioning concept) are identical if the heterogeneous nature of the composites is respected. It is also shown that the optical isoclinic parameter can be accurately predicted even if the initial birefringence is present in the unloaded specimen.     

纤维增强陶瓷基复合材料的D失效判据

经典唯象强度理论适用于正交各向异性线弹性体.对于非线性纤维增强复合材料,通过加卸载试验和损伤力学的分析方法,可以得到一种虚拟的线性化应力-应变关系;依据损伤等效假设,针对线性损伤和非线性损伤,对基于应力的经典二次失效准则进行变换,建立了一种基于损伤的强度理论,即"D失效判据",这一强度理论可以作为经典判据的补充和扩展.针对平纹编织C/SiC复合材料的拉/剪组合试验,进行了实例计算,结果表明:利用D失效判据预测的失效包络线比蔡-希尔准则的预测曲线低,而且,失效曲线的形式与材料的损伤演化规律相关.     

Modeling oxidation damage of continuous fiber reinforced ceramic matrix composites

For fiber reinforced ceramic matrix composites(CMCs),oxidation of the constituents is a very important damage type for high temperature applications. During the oxidizing process,the pyrolytic carbon interphase gradually recesses from the crack site in the axial direction of the fiber into the interior of the material. Carbon fiber usually presents notch-like or local neck-shrink oxidation phenomenon,causing strength degradation. But,the reason for SiC fiber degradation is the aw growth mechanism on its surface. A micromechanical model based on the above mechanisms was established to simulate the mechanical properties of CMCs after high temperature oxidation. The statistic and shearlag theory were applied and the calculation expressions for retained tensile modulus and strength were deduced,respectively. Meanwhile,the interphase recession and fiber strength degradation were considered. And then,the model was validated by application to a C/SiC composite.     

The behavior of cracked cross-ply composite laminates under shear loading

An interlaminar-shear-stress analysis developed earlier by Tsai et al. (1990, Micro-cracking-Induced Damage in Composites) for a [φ_m/θ_n], bi-directional composite laminate is used to solve the case of a cross-ply [0_m/90_n]_x laminate with the 90° layer only or both layers cracked under pure shear loading. Strains, forces and laminate shear modulus reduction due to matrix cracking were obtained. Experimental results for shear modulus as a function of crack densities were obtained by a simple shear test and they agree very well with the theoretical prediction.     

Frequency-dependent random fatigue of panel-type structures made of ceramic matrix composites

The panel-type structures used in aerospace engineering can be subjected to severe highfrequency acoustic loadings in service. This paper evaluates the frequency-dependent random fatigue of panel-type structures made of ceramic matrix composites(CMCs) under acoustic loadings. Firstly, the high-frequency random responses from the broadband random excitation will result in more stress cycles in a deinite period of time. The probability density distributions of stress amplitudes will be different in different frequency bandwidths, though the peak stress estimations are identical. Secondly, the fatigue properties of CMCs can be highly frequency-dependent. The fatigue evaluation method for the random vibration case is adopted to evaluate the fatigue damage of a representative stiffened panel structure. The frequency effect through S-N curves on random fatigue damage is numerically veriied. Finally, a parameter is demonstrated to characterize the mean vibration frequency of a random process, and hence this parameter can further be considered as a reasonable loading frequency in the fatigue tests of CMCs to obtain more reliable S-N curves.Therefore, the inluence of vibration frequency can be incorporated in the random fatigue model from the two perspectives.     

Effects of non-uniform strains on tensile fracture of fiber-reinforced ceramic composites

Effects of non-uniform strains on tensile fracture of fiber-reinforced ceramic–matrix composites have not been satisfactorily explained by existing mechanics-based models. In this paper, we use an exact model of fiber fragmentation under global load sharing conditions to predict fracture in three model problems in which non-uniform strains occur: (i) an end-constrained plate subject to a linear transverse temperature gradient; (ii) an internally-pressurized cylindrical tube with a linear through-thickness temperature gradient; and (iii) a rectangular beam under combined bending and tension. Fracture is assumed to occur when the global load reaches a maximum value. Approximations to the exact fragmentation model are also assessed, with the goal of decoupling the effects of two important parts of the computed stress–strain response: the rate of post-peak strain softening and the magnitude of the plateau “flow” stress once fiber fragmentation is complete. We find that for cases in which the fiber Weibull modulus is low and hence its plateau strength is high relative to its peak and the loading yields a sufficiently high strain gradient, the failure strain lies in the plateau regime. Consequently, the results can be predicted with good accuracy using a perfectly-plastic representation of the post-peak response. In contrast, for cases in which the fiber Weibull modulus is high, the failure strain lies in the softening portion of the curve. Here a linear-softening model is found to yield accurate results. A preliminary assessment of the model has been made by comparing predicted and measured bending/tension strength and failure strain ratios for one specific composite. The correlations appear good, though additional experiments are required in order to critically assess the model predictions over a range of loading scenarios.     

Suppression of edge cracking in layered ceramic composites by edge coating

A strategy is proposed to reduce surface tensile stresses that develop in ceramic microlaminate structures. As a specific example, surface stresses, which can lead to unwanted edge cracking, appear within a thin alumina/mullite layer bounded by thicker alumina layers after fabrication at some elevated temperature and subsequent cooling. A stress analysis is performed using the finite element method on a geometry in which the (edge) surface of the ceramic sandwich structure has been coated with an overlayer of the alumina/mullite material with thickness proportional to the initial buried layer thickness. These analyses show there is a critical thickness of the overlayer at which the surface tensile stresses can be reduced to zero. Stress concentration factors are calculated numerically for cracks propagating through this structure, and thus the resistance to surface cracking is quantified.     

Numerical simulation on crack growth of multilayered ceramic/metal composites

The present paper develops a numerical technique named FSMS for simulating the crack growth of multilayered composites. Numerical simulations for the crack growth of multilayered ceramic/metal composites are carried out. The effects of some factors such as thickness ratio, initial crack length, material properties and dimensions of the structure on the crack growth are investigated. Numerical results show good agreement with experiments. FSMS is also a simple numerical method to solve crack problems of complex composite structures.     

A bar impact tester for dynamic fracture testing of ceramics and ceramic composites

A bar impact test was developed to study the dynamic fracture responses of precracked ceramic bars, Al₂O₃ and 15/29-percent volume SiC_w/Al₂O₃. Crack-opening displacement was measured with a laser-interferometric displacement gage and was used to determine the crack velocity and the dynamic stress-intensity factorK _I ^dyn . The crack velocity andK _I ^dyn increased with increasing impact velocity while the dynamic-initiation fracture toughness,K _Id, did not vary consistently with increasing impact velocities.Paper was presented at the 1992 SEM Spring Conference on Experimental Mechanics held in Las Vegas on June 8–11.     

Virtual testing applied to transverse multiple cracking of tows in woven ceramic composites

The present paper proposes a method of virtual testing with a view to investigating the local response of tows within textile ceramic matrix composite (CMC) under various loading conditions. The method was developed on 2D woven SiC/SiC composites. It capitalizes on knowledge on mechanical damage phenomenology and data established in previous works. It is applied to isolated transverse tows subjected to uniaxial loading by parallel longitudinal tows. The transverse tows contain heterogeneities like matrix voids, fibres and interphases. Mesh for finite element analysis is constructed from micrographs of composite cross section. Cracks were introduced into the mesh for simulation of multiple cracking. Transverse tow tensile behavior and data on distributions of flaw populations were derived from finite element computations of stress-state. Results were compared to experimental observations.     

Virtual testing applied to transverse multiple cracking of tows in woven ceramic composites

The present paper proposes a method of virtual testing with a view to investigating the local response of tows within textile ceramic matrix composite (CMC) under various loading conditions. The method was developed on 2D woven SiC/SiC composites. It capitalizes on knowledge on mechanical damage phenomenology and data established in previous works. It is applied to isolated transverse tows subjected to uniaxial loading by parallel longitudinal tows. The transverse tows contain heterogeneities like matrix voids, fibres and interphases. Mesh for finite element analysis is constructed from micrographs of composite cross section. Cracks were introduced into the mesh for simulation of multiple cracking. Transverse tow tensile behavior and data on distributions of flaw populations were derived from finite element computations of stress-state. Results were compared to experimental observations.     

The influence of microstructural arrangement on the response of polymer composites in planar impact

In recent years there has been an increased demand for advanced materials that can sustain rapid dynamic loadings. To this end, we simulate the transient response of composites with nonuniform arrangements of their microstructures. First, a constitutive model that reproduces experimentally measured response of a glass-fiber composites is identified and adjusted. This involves a Mie–Grüneisen equation of state for the dilatational response together with a Voigt model for the isochoric behavior which is modified to include damage effects from void nucleation and growth. Then, with the aid of this constitutive model, a sequence of simulations of composites with nonuniform distributions of the reinforcement are executed. We find that composites with increasing volume fraction of the reinforcement along the impact direction tend to attenuate the intensity of the propagating waves. This attenuation delays the initiation of failure mechanisms to higher impact velocities and improves the composite’s sturdiness.     

短切碳纤维C/SiC陶瓷基复合材料的动态劈裂拉伸实验

为了探究C/SiC陶瓷基复合材料的动态断裂力学行为和破坏形态,利用分离式霍普金森压杆(split Hopkinson pressure bar,SHPB)装置对3种不同短切碳纤维体积分数的C/SiC陶瓷基复合材料进行了动态劈裂实验,并利用扫描电子显微镜扫描了C/SiC复合材料试件的破坏界面,分析了C/SiC陶瓷基复合材料的失效特征和增韧机理。实验结果表明：C/SiC复合材料在冲击劈裂实验过程中,同一短切碳纤维体积分数下试件的动态抗拉强度随着冲击气压的增大而增大; 短切碳纤维体积分数为16.0%时, 材料的抗拉强度最低; 冲击后,试件的整体破坏情况与冲击气压、短切碳纤维体积分数有关。

     

Creep buckling of cross-ply symmetric laminated cylindrical panels

A creep buckling analysis of cross-ply symmetric laminated cylindrical panels is given in this paper.By means of theoretical analysis,a method to determine the critical load of creep buckling of the panels with simply supported boundary conditons is obtained.