The mechanical behaviour of a cross-weave ceramic matrix composite

In order to find out whether or not a cross-weave ceramic composite, graphite fibre/SiC matrix, would be prone to fatigue failure, tests in pulsating tension and pulsating compression have been carried out with the weave oriented in the 0/90 configuration. Both types of testing (at fairly high fractions of the ultimate monotonic failure load) cause creep strain, which is frequency dependent in tension, and ultimately complete failure can occur. Damage in pulsating tension is found to consist of cumulative microcracking and spalling, with the final failure mechanism broadly similar to that in monotonic deformation. Damage in pulsating compression appears dominated by delamination.     

Mechanical behaviour of a cross-weave ceramic matrix composite

The deformation and fracture processes of a cross-weave carbon fibre/SiC composite prepared by a chemical vapour deposition process has been explored by interrupted-loading tests and SEM examination of cracking and fracture processes. The tensile stress-strain curves show non-linear behaviour associated with progressive matrix cracking and spalling, and the occasional fracture of a fibre. Re-loading curves and compressive stress-strain curves show linear behaviour. The fracture process does not involve cracking by a single dominant crack but occurs by the development of multiple damage sites operating around the transverse fractures of groups of four to eight fibres followed by longitudinal cracking at their fibre-matrix interfaces and temporary arrest of the cracks, until specimen failure occurs and there is massive fibre debonding and pull-out.     

Tension-compression fatigue behaviour of fibre-reinforced ceramic matrix composite with circular hole

The tension-compression fatigue behaviour of a silicon carbide fibre-reinforced glass ceramic matrix composite, SiC/1723, with a circular hole was investigated at room temperature. Two laminate lay-ups were studied: cross-ply, [0/90]_2s, and unidirectional, [0]₈. At first, the fatigue limit based on one million cycles was established for the tension-tension fatigue condition. Then, the fatigue response under fully reversed (tension-compression) cycling loading with a maximum stress equal to the tension-tension fatigue limit was investigated. This tension-compression loading resulted in an increased amount of damage and ultimately led to the specimen failure well before one million cycles. In the cross-ply laminate, the damage mechanisms in the 90° plies involved transverse cracks only during tension-tension cycling, and transverse and longitudinal cracks during tension-compression cycling. In the unidirectional laminate, the longitudinal cracks which initiated at the hole periphery grew longer in tension-compression fatigue than in tension-tension fatigue. On the other hand, no damage and consequently no effect on fatigue life was observed during the compression-compression fatigue condition only.     

Cyclic shearing behaviour of a unidirectional glass/epoxy composite

Results from an experimental program consisting of monotonic and fatigue shear tests on a unidirectional glass–epoxy composite are presented in this paper. Unnotched and tabbed specimens were subjected to interlaminar and intralaminar shearing modes using an appropriate testing device called “Cube Test”. Shear mechanical characteristics are determined and cyclic resistance is analyzed. Fatigue results are in good agreement with three-point-bending tests with predominant shearing studied elsewhere on the same material. Modified Basquin relationship is established to characterize the shearing fatigue strength. Microstructural observations of the fracture surfaces by scanning electron microscopy show that hackle formation is the predominant damage mechanism.     

单向纤维增强陶瓷基复合材料界面滑移规律

建立了分析单向纤维增强陶瓷基复合材料力学特性的界面摩擦模型.采用基体应变能准则对基体损伤状态进行预测;由Weibull分布模型拟合出纤维断裂分数;将界面磨损处理为纤维/基体相对滑移历程的函数τi=τi(△δ),很好地表征了不同位置纤维/基体相对滑移历程不同所引起的界面磨损程度的区别.运用该模型分析了准静态加载和拉-拉循环载荷下的应力-应变特性,预测结果与实验数据吻合较好.最后采用此模型研究了任意载荷历程下界面的滑移规律.     

单向纤维增强陶瓷基复合材料界面滑移规律

建立了分析单向纤维增强陶瓷基复合材料力学特性的界面摩擦模型。采用基体应变能准则对基体损伤状态进行预测; 由Weibull分布模型拟合出纤维断裂分数; 将界面磨损处理为纤维/基体相对滑移历程的函数τ_i=τ_i (Δδ), 很好地表征了不同位置纤维/基体相对滑移历程不同所引起的界面磨损程度的区别。运用该模型分析了准静态加载和拉-拉循环载荷下的应力-应变特性, 预测结果与实验数据吻合较好。最后采用此模型研究了任意载荷历程下界面的滑移规律。     

Modeling of crack bridging in a unidirectional metal matrix composite

The effective fatigue crack driving force and crack opening profiles were determined analytically for fatigue tested unidirectional composite specimens exhibiting fiber bridging. The crack closure pressure due to bridging was modeled using two approaches; the fiber pressure model and the shear lag model. For both closure models, the Bueckner weight function method and the finite element method were used to calculate crack opening displacements and the crack driving force. The predicted near crack tip opening profile agreed well with the experimentally measured profiles for single edge notch SCS-6/Ti-15-3 metal matrix composite specimens. The numerically determined effective crack driving force, K _eff, was calculated using both models to correlate the measured crack growth rate in the composite. The calculated K _eff from both models accounted for the crack bridging by showing a good agreement between the measured fatigue crack growth rates of the bridged composite and that of unreinforced, unbridged titanium matrix alloy specimens.     

Dual-stage tertiary creep of a ceramic matrix composite

The creep behaviour of an alumina fibre/silicon carbide matrix composite has been studied. The creep curves are characterised by a short primary stage followed by one or two tertiary stages. The secondary regime of this composite is limited to a single point. The occurrence of one or two tertiary stages in the creep behaviour is discussed, and some theoretical considerations are invoked to explain this behaviour. Creep in this composite is controlled by two mechanisms, namely viscoplastic creep of the alumina fibres and damage accumulation within the composite. The two tertiary stages differ in the damage mechanisms occurring, the first one being related to fibre–matrix debonding only, whereas successive fibre failure dominates in the second part. The second tertiary regime occurs only at low creep stresses, for which a non-catastrophic rupture of the composite is observed.     

Fatigue behaviour of a phenolic matrix composite

This paper presents results of a fatigue life investigations carried out in plate specimens of a fibre-glass-reinforced phenolic matrix composite. Tensile and Young's modulus data were obtained at four different testing temperatures (room temperature, 100, 150 and 200 °C). The fatigue S−N data were obtained at room temperature only and for two stress ratio values (R=0 and 0.4). Fatigue and tensile behaviour was assesesed in the composite with the fibres aligned in the longitudinal loading direction. The results were obtained for two values of volume fraction (0.28 and 0.42) and three different glass surface treatments. A detailed comparison of fatigue results is given taking into account several fatigue parameters and also the testing variables. Results of observations of SEM fracture surfaces are also presented.     

Creep behaviour of a unidirectional SM1140+/Ti-6242 composite

The creep behaviour of a unidirectionally reinforced SM1140+/Ti-6242 composite at 500 °C in a vacuum atmosphere is investigated. Results are presented for creep tests performed in longitudinal and transverse direction, respectively. Microstructural observations show that the main damage mechanisms are filament fracture in the longitudinal direction and interface debonding in the transverse direction. In both cases, creep life can be estimated with the help of models taking into account the damage mechanisms.     

Analysis of a unidirectional composite containing broken fibers and matrix damage

An analytical solution is developed for the determination of the stresses and displacements in a unidirectional fiber-reinforced composite containing an arbitrary number of broken fibers as well as longitudinal yielding and splitting of the matrix.The solution is developed using a “materials-modeling” approach which is based on a shear-lag stress transfer mechanism. The equilibrium equation in the axial direction gives a pair of integral equations which are solved numerically.Excellent agreement is shown to exist between the solution and experimental results for notched unidirectional boron/aluminum laminates without splitting. For brittle matrix composites (i.e. epoxy) equally good results are indicated for both matrix yielding and splitting.For yielding without splitting the fracture strength is found to depend on crack length while for large splitting it is crack length independent.     

单向纤维增强陶瓷基复合材料单轴拉伸行为

采用细观力学方法对单向纤维增强陶瓷基复合材料的单轴拉伸应力-应变行为进行了研究。采用Budiansky-Hutchinson-Evans（BHE）剪滞模型分析了复合材料出现损伤时的细观应力场,结合临界基体应变能准则、应变能释放率准则以及Curtin统计模型三种单一失效模型分别描述陶瓷基复合材料基体开裂、界面脱粘以及纤维失效三种损伤机制,确定了基体裂纹间隔、界面脱粘长度和纤维失效体积分数。将剪滞模型与3种单一失效模型相结合,对各个损伤阶段的应力-应变曲线进行模拟,建立了准确的复合材料强韧性预测模型,并讨论了界面参数和纤维韦布尔模量对复合材料损伤以及应力-应变曲线的影响。与室温下陶瓷基复合材料单轴拉伸试验数据进行了对比,各个损伤阶段的应力-应变、失效强度及应变与试验数据吻合较好。     

Thermal shock cracking in a metal-particle-reinforced ceramic matrix composite

We study thermal crack shielding and thermal shock damage in a double-edge cracked metal-particle-reinforced ceramic matrix composite subjected to sudden cooling at the cracked surfaces. Under severe thermal shocks, the crack will grow but will be bridged by the plastically stretched metal particles. A linear softening bridging law is used to describe the metal particle bridging behavior. An integral equation of the thermal crack problem incorporating the bridging effect is derived and the thermal stress intensity factor at the bridged crack tip is calculated numerically. It is found that the thermal stress intensity factor is significantly reduced by the metal particle bridging. While the crack growth in thermally shocked monolithic ceramics is unstable, the composite can withstand sufficiently severe thermal shocks without failure.     

An alternative model for nonlinear stress-strain behaviour of composite materials

An alternative model for the Jones-Nelson material model is developed, in which the secant mechanical property is assumed to be a function of the plastic strain energy density of an equivalent linear elastic system which replaces the total strain energy term in the Jones-Nelson model. The present model is capable of treating multiple mechanical property non-linearities which are generally exhibited by fibre-reinforced composite material. The new model is represented in two forms; the basic model and the iterative model. A comparison is carried out in order to correlate strains predicted by the present model with experimental data and other theoretical models cited from the literature. What makes the new model practical is that the plastic strain energy due to loading at any fibre-orientation is not allowed to exceed the fibre direction value obtained from the uniaxial loading test. Hence, the model does not require an extension of behaviour beyond the defined range of strain energy.