On the fracture toughness of ceramic matrix composites

Based on the resistance curve (R-curve) behaviour of ceramic matrix composites (CMCs) determined under either quasi-static or cyclic loading, the crack-face fibre bridging stress field is determined for the compact tension (CT) test specimen geometry. Two different methods have been used for the analysis of the bridging stresses. The first considers a compliance approach. Using the difference in compliance calibration curves with and without bridging and assuming a power-law relation between bridging stress and crack opening displacement, the bridging stress field was calculated. The second approach uses the existence of an invariant stress reversal point in the CT geometry and assuming that the material exhibits linear elastic fracture behaviour, yields a recurrence relation for the bridging stresses resulting in a piece-wise constant stress function. Both models are applied to the experimentally determined fracture behaviour of a 2D carbon/carbon (C/C) composite, and the resulting bridging stress distributions are discussed.     

Short-fiber/particle hybrid reinforcement: Effects on fracture toughness and fatigue crack growth of metal matrix composites

We have studied the effects of short-fiber/particle hybrid reinforcement on fracture toughness and fatigue crack growth in metal matrix composites. Reinforcement hybridization was achieved by a hybrid preform process, and composites were fabricated by the squeeze casting method. Al6061 matrix alloy and four composites having different short-fiber/particle ratio were tested. The fracture toughness (K_IC) and the fatigue threshold (ΔK_th) increased with increasing particle contents, whereas the Paris’ exponent (m) was insensitive to the short-fiber:particle ratio. These results emerged as a shift of the crack growth curve which implies on enhanced crack resistance over the entire stress intensity factor range. The positive aspect of particulate reinforcement is advocated by comparison of microstructural variables, and by observation of the crack path and surfaces. The characteristics of hybrid composites in damage tolerance are emphasized.     

RMS matrix strains in transformation toughened alumina

The Warren Averbach technique was used to determine the RMS strain profile in the Al₂O₃ matrix surrounding ZrO₂ particles in zirconia toughened alumina. The X-ray domain size was found to be 100nm in all but the most severely microcracked sample. The RMS strain, averaged over all domains, decreased nearly linearly from the edge of the domain (presumably from the ZrO₂ particle), except when microcracking was detected and then the RMS strain level was reduced near the surface of the domain. The maximum RMS strain level increased with increasing ZrO₂ particle size up to the point where microcracking occurred then decreased. The results were explained by noting that the maximum RMS strain was directly related to the monoclinic content of the sample.     

Cure path dependency of mode I fracture toughness in thermoplastic particle interleaf toughened prepreg laminates

The effect of cure cycle on fracture behaviour of a commercial thermoplastic particle interleaved prepreg system was investigated. Laminates were manufactured at 700 kPa in an autoclave using eight different thermal cycles that included both raising the cure temperature above the standard 180 °C cure cycle and incorporating an intermediate dwell stage between 150 and 170 °C prior to reaching the 180 °C cure temperature. Double cantilever beam tests were conducted on specimens from the cured laminates. The stick–slip crack behaviour, observed in samples manufactured using the standard cure cycle, changed to stable crack growth when processing deviated by 10 °C. The mode I fracture toughness values were reduced by 11–22% when incorporating an intermediate dwell stage before the final cure temperature. Scanning electron microscopy inspection of the fracture surfaces showed differences between samples made by standard cure cycles and those made using process deviations.     

Controlling fracture toughness of matrix for ductile fiber reinforced cementitious composites

An experimental study was carried out to find material parameters for making fiber reinforced cementitious composites (FRCC) more ductile. One of the dominant factors to control the ductility might be hidden in fracture property of matrix as well as the interface property between fiber and matrix. Therefore this study varied air content and water-binder ratio as the parameters to change the fracture property of matrix and experimentally examined their influence on the ductility of FRCC by three-point bend test with notched beams. As a result, it is concluded that fracture toughness of the matrix could be one of key parameters to control the ductility of FRCC. In case of a polyethylene fiber used in this study, the optimum value of the fracture toughness (critical strain energy release rate): G_IC of the matrix was obtained to be 7.5-8.0 N/m.     

Microstructural efficiency and fracture toughness of short fiber/thermoplastic matrix composites

This paper outlines the fracture behavior of composites with thermoplastic matrices of different fracture toughness K_cm (increasing in the order PPS → PET (I) → PET (II) → ETFE → PC). In particular, the way in which the fracture toughness of these composite systems, K_cc, is affected by the volume fraction, orientation and distribution of short glass fibers across the plaque thickness (fiber length ≈ 200 μm, fiber diameter ≈ 10 μm), and by the quality of their interfacial bonding to the matrix is discussed. SEM studies were carried out to define the microstructural details and the dominant mechanisms of energy adsorption during breakdown of the composites.In general, an increase in composite toughness can be expected with increasing extent of reinforcement if the matrix is in a brittle condition (here also verified by K_c-tests at lower temperatures) and if the fibers are well bonded and mostly oriented perpendicular to the crack front. An opposite tendency may occur for matrices which behave in a highly ductile manner even in the presence of fibers. The probability of this behavior is favored in poorly bonded systems. The results are discussed in terms of a ‘microstructural efficiency factor’ M, which mainly considers the relative contributions of fiber and matrix related mechanisms to energy dissipation during breakdown of a composite (‘energy absorption ratio’ n) as well as the reinforcement content and its arrangement in the matrix (‘reinforcing effectiveness parameter’ R).     

Plane strain fracture toughness test procedures for particulate metal matrix composites

Abstract

Procedures for plane strain fracture toughness tests on a number of particulate reinforced aluminium alloy metal matrix composites (MMCs) have been examined. Measurements of toughness are reported on a range of particulate aluminium alloy MMCs and the results are compared with validity criteria in standards for metallic materials. In particular, the effect of testpiece thickness was studied in a 6000 and a 2000 series aluminium alloy containing, respectively, 25 and 30% silicon carbide. The results are compared with other published work on the toughness of particulate reinforced MMCs.

MST/1225     

Effect of barrier layer thickness and composition on fracture toughness of layered zirconia/alumina composites

Composites of yttria or ceria-partially-stabilized zirconia with layers of either alumina or a mixture of 50% by volume of alumina and zirconia were fabricated by sequential centrifuging of powder suspensions. This method allowed formation of layers with thickness of 10 to 70 m. In both cases (Y-ZrO₂ and Ce-ZrO₂ matrices), a significant increase in fracture toughness, work of fracture and bending strength was observed only for composites with barrier layers made of a pure alumina. A crack deflection in alumina layer was found to be the main mechanism responsible for an increase in mechanical properties. For confirmation this thesis, no increase in the transformation zone width was observed. As it was shown, crack deflection angle was dependent on alumina layer thickness. Higher deflection angles for a thicker alumina layers were found. Explanation of this phenomenon was given by determination of residual stress distribution in barrier layers made by piezospectroscopy. A correlation between the crack deflection angle and the difference of stress between the layer boundary and the centre of the layer was noticed. The residual stresses observed are a result of thermal expansion mismatch between alumina and zirconia and thermal anisotropy of alumina. Shrinkage mismatch, especially in the case of Ce-ZrO₂ and Al₂O₃, as a third source of stress is suggested.     

Fracture toughness and fracture of WC-Co composites

Critical stress intensity factor, and related parameters have been measured in three-point bending for 18 different combinations of different volume fractions of cobalt (5 to 37%) and grain size of tungsten carbide (0.7, 1.1 and 2.2 m). In particular, a study was made of the correlations between the strength and mechanical and microstructural parameters, such as ¯L _Co,C _WC, ¯L _Co/¯D _WC, ¯L _Co ² /¯D _WC,H _V and wear resistance. A hypothesis for the mechanism of fracture has been proposed following an analysis of these results and a study of the mode of fracture.     

Some recent developments in numerical modelling of fracture toughness in brittle matrix composites

Finite element analysis was used to study the fracture toughening of a ceramic by a stress induced dilatant transformation of second phase particles. The finite element method was based on a continuum theory which modelled the composite as subcritical material. Transient crack growth was simulated in the finite element mesh by a nodal release technique. The crack's remote tensile opening load was adjusted to maintain the near-tip energy release rate at the level necessary for crack advance. The transformation zone surrounding the crack developed as the crack propagated through the composite. Resistance curves were computed from the analysis; and the results show that during crack advance maximum toughness is achieved before a steady state is reached. The toughening effect of a crack-bridging ductile phase in a brittle material may be predicted if ligament deformation is characterized. A plastically deforming ligament constrained by surrounding elastic matrix material is modelled using finite elements and the relevant toughness enhancement information extracted. Comparison is made to model experiments as well as to toughness measured for technologically important materials. The results suggest that debonding along the interface between the ligament and the matrix may enhance the toughening effect of a ductile phase.     

Microstructure and room temperature fracture toughness of cast Nbss/silicides composites alloyed with Hf

The effect of Hf addition on microstructure and room temperature fracture toughness of cast Nb-16Si alloy was investigated. The Hf addition changes significantly the microstructural morphology of Nb-16Si alloys, which includes microstructure refinement and disappearance of eutectic colonies. Fracture toughness of the alloys improves with increasing Hf content. The improvement in fracture toughness is mainly attributed to the microstructural change by Hf addition. The Hf addition leads to a transition of Nb solid solution fracture manner from brittle cleavage to plastic stretching.     

片状氧化铝晶种对氧化铝陶瓷断裂韧性的影响

以片状氧化铝晶种作为第二相,采用无压烧结制备了氧化铝陶瓷,分析了片状氧化铝含量对氧化铝陶瓷微观结构的影响,采用扫描电子显微镜（SEM）观察分析试样的断口形貌;采用压痕法计算试样的断裂韧性（KIC）值;研究了不同含量的晶种引入量对氧化铝陶瓷断裂韧性的影响。结果表明烧结温度为1575℃时,相对致密度可以达到96.7%;片状氧化铝晶种的引入能够显著提高氧化铝陶瓷的断裂韧性;其片晶的裂纹偏转、片晶拔出效应等增韧机制发挥了主导作用;随着片状氧化铝含量的提高,氧化铝陶瓷的力学性能逐渐提高,当掺杂含量达到35%（质量分数）时,KIC达到6.4MPa.m1/2,当含量继续增加,KIC呈现逐渐降低的趋势。     

Effect of composite layering on the fracture toughness of brittle matrix/particulate composites

Glass matrix/Ni particulate composites, ranging from 0 to 25% particulate phase, were prepared both as single-volume-fraction composites and as multi-volume-fraction layered composites. Fracture toughness (K_c) measurements were made on all composites using the applied moment double cantilever beam technique. The measured toughness values for the layered composites were found to be equivalent to those of the single-volume-fraction composites. The fracture toughness measured for the layered composites was found to be dependent on the volume of composite phase tested and ultimately on the number of crack-particle interactions which occurred. R-curve like behaviour was observed in the layered composites.