Fracture toughness measurements on SiC/Al2O3 composite

Three test techniques are addressed for the measurements of plane strain critical stress intensity factors,K _IC, on monolithic Al₂O₃ and SiC-whisker/Al₂O₃ composite: a four-point test on chevron-notched bend bars; a four-point test on single edge-notched bend bars; and a fractometric test on chevron-notched short bars. The tests were performed on 99.80% Al₂O₃ and 30 vol % SiC whisker-reinforced Al₂O₃. Bend bar test techniques yielded more realistic stress intensity factors,K _IC, on the SiC-whisker/Al₂O₃ composite than the short-bar test results. Chevron-notched bend-bar tests yielded relatively higher critical stress intensity factors, on both Al₂O₃ and SiC/Al₂O₃, possibly due toR-curve effects, suggesting the use of stress intensity factor as a function of crack length instead of using the minimum value. Ambiguous results,K _IC, obtained from short-bar tests on SiC/Al₂O₃ composite, strongly suggests the need to run compliance calibration tests on ceramic composites to determine an appropriateK-factor.     

Fracture toughness and fatigue crack growth of grey cast irons

The fracture behaviour of alloys with a pearlitic matrix and lamellar and spherulitic graphite has been compared. Fatigue crack growth functions were measured and various critical stress intensities obtained from load-displacement curves. An analysis of microstructure in the uncracked and cracked state served as the base for a discussion of quantitative models for the relation between microstructure and bulk fracture mechanical properties. In addition to volume fraction and shape of graphite, crack branching (and eventually transformation of residual austenite) are required to explain the resistance of grey cast irons against stable and unstable crack growth. It is concluded that the standard methods for the evaluation of fracture mechanical properties are not satisfactory especially for cast irons with lamellar graphite.     

The fatigue crack growth behaviour of 2524‐T3 aluminium alloy in an Al2O3 particle environment

The effect of the Al₂O₃ dust environment on the crack propagation behaviour of 2524‐T3 Al alloy was investigated. The results show that the Al₂O₃ dust environment reduces the fatigue crack growth rate (FCGR) of alloy especially at low ΔK. Many Al₂O₃ particles are deposited and stuck in the crack during fatigue loading which promotes crack closure, while this effect is gradually weakened with the increase of ΔK. The deposited Al₂O₃ particles induce the disorderly arranged slip bands (SBs) ahead of the crack tip which deflects the crack path making it more tortuous in the Al₂O₃ dust.     

Rheological properties of Al2O3-SiC whisker composite suspensions

The rheological properties of both aqueous and non-aqueous composite suspensions were investigated together with a characterization of the colloidal stability of the separate components. The colloidally stable, concentrated aqueous SiC_w and Al₂O₃-SiC_w composite suspensions displayed strong shear thinning followed by a severe, sometimes discontinuous, shear thickening at a critical shear rate. The non-aqueous composite suspensions, containing weakly flocculated SiC_w, only showed a continuous shear thinning behaviour. The variations in the steady shear behaviour were related to the differences in the colloidal stability and possible shear induced effects on the suspension structure. It was possible to fit the volume fraction dependence of both the SiC_w and the Al₂O₃ suspensions to a modified Krieger-Dougherty model which yields values of the maximum volume fraction; _m(Al₂O₃)=0.61, _m(SiC_w)=0.28. The viscosity of the composite suspensions were successfully predicted from the Farris theory using the rheological data for the separate components.     

3Y-TZP/mullite-Alumina复合陶瓷的韧性及增韧机制

为了研究3Y TZP为基体的3Y TZP/mullite Alumina复合陶瓷的断裂韧性及其增韧机制,将3Y TZP、mullite、Alumina3种粉料球磨混合,经干压、等静压成型,在1480℃,4h无压烧结,通过改变Alumina/mullite体积比,得到了不同断裂韧性的陶瓷复合材料,利用XRD与SEM技术分析了复合材料的成分及微观结构.研究结果表明:Al2O3/mullite体积比影响复合材料中四方氧化锆(t ZrO2)向单斜氧化锆(m ZrO2)转变的相变量、复合材料的微观结构和t ZrO2晶面间距,进而影响材料的断裂韧性;用单边切口梁法测试复合材料断裂韧性(KIC)为9 26～10 4MPa·m1/2;此系统中存在ZrO2相变增韧、非相变第二相颗粒增韧等机制.     

Fracture behaviour of composites based on Al2O3-TiC

The critical stress intensity factor and other related fracture parameters have been measured in three-point bending for pure Al₂O₃-TiC composites containing 4 to 35 volume fractions of TiC. An increase has been observed for all the parameters with increasing volume fraction of TiC. Following a study of the mode of fracture, the results are explained in terms of a linear variation of the fracture energy with the volume fraction of TiC.     

Microstructure-related fracture toughness and fatigue crack growth behaviour in toughened, anhydride-cured epoxy resins

Fracture toughness and fatigue crack propagation (FCP) of plain and modified anhydride-cured epoxy resin (EP) were studied at ambient temperature. Liquid carboxyl-terminated acrylonitrile-butadiene (CTBN) and silicon (SI) rubber dispersions were used as tougheners for the EP. The morphology of the modified EP was characterized by dynamic mechanical analysis (DMA) and by scanning electron microscopy (SEM). The fracture toughness, K_c, of the compositions decreased with increasing deformation rate. K_c of the EP was slightly improved by CTBN addition and practically unaffected by incorporation of the SI dispersion when tests were performed at low cross-head speed, v. Both modifiers improved K_c at high v, and also the resistance to FCP, by shifting the curves to higher stress intensity factor ranges, ΔK, by comparison with the plain EP. It was established that both fracture and fatigue performance rely on the compliance, J_R, at the rubbery plateau, and thus on the apparent molecular mass between crosslinks, M_c. The failure mechanisms were less dependent upon the loading mode (fracture, fatigue), but differed basically for the various modifiers. Rubber-induced cavitation and shear yielding of the EP were dominant for CTBN, whereas crack bifurcation and branching controlled the cracking in SI-modified EP. The simultaneous use of both modifiers resulted in a synergistic effect for both the fracture toughness at high deformation rate and the FCP behavior.     

Fracture and fatigue crack growth behaviour of PMMC friction stir welded butt joints

The paper is focused on the evaluation of the fracture and Fatigue Crack Growth (FCG) properties of butt joints of particulate metal-matrix composite (PMMC) obtained by friction stir welding (FSW). The materials considered are two aluminum alloy matrix/alumina particle PMMCs (AA6061/Al₂O₃/20p and AA7005/Al₂O₃/10p). Tests were conducted on unwelded and welded PMMCs using CT and Extended CT (ECT) specimen geometries, respectively. The crack growth rate was monitored by means of compliance with a strain gage attached on the back of the specimen. FCG experiments were carried out both at the centre and in the Thermo-Mechanically Altered Zone (TMAZ) at the side of the weld. The comparison between unwelded and welded PMMCs showed that FSW influences fracture toughness and FCG rate in a different fashion depending on the material. In particular, the FSW AA6061/Al₂O₃/20p butt joint exhibited comparable fracture toughness and higher FCG threshold with respect to the unwelded material, while in the case of AA7005/Al₂O₃/10p the behaviour is the opposite. The interpretation of this trend has been carried out by optical analysis of the crack path roughness and its correlation with the FCG rate. The dynamic recrystallization of the aluminum matrix and particle shaping operated by the FSW tool are at the ground of the explanation.     

Fracture surface roughness and toughness of Al2O3-platelet reinforced glass matrix composites

The correlation between fracture surface roughness and fracture toughness of alumina platelet reinforced borosilicate glass matrix composites was investigated. With increasing volume fraction of platelets both fracture toughness and fracture surface roughness increase. The fracture roughness parameter Ra was determined by the profilometric technique. A linear correlation was found between the fracture toughness and the Ra values of the composites. Scanning electron microscopy observation of fracture surfaces confirmed that surface roughness is related to a crack deflection process. Crack deflection is thus one of the toughening mechanisms acting in these composites.     

Fracture toughness and fatigue crack propagation behavior of 8090 Al-Li alloy

We present results of fatigue tests of high-strength 8090 Al-Li alloy and data on its fatigue crack growth resistance. High strength combined with fairly high crack growth resistance and endurance limit results in much better service characteristics compared to other high-strength aluminum alloys. We discuss results of tensile and impact tests of Charpy specimens and the critical values of theJ-integral andK _1c for 10-mm-thick specimens in the T-L and L-T orientations subjected to complete and partial aging. The experimental results are compared with published data for 8090 and other high-strength aluminum alloys. We suggest a numerical method for the evaluation of fatigue strength according toda/dN-K diagrams.Published in Fiziko-Khimicheskaya Mekhanika Materialov, Vol. 31, No. 1, pp. 45–58, January – February, 1995.     

Al/Al2O3复合材料的伪半固态压缩变形力学行为

对37%Al/Al2O3复合材料坯料进行高温压缩,研究其伪半固态压缩变形力学行为.结果表明,该材料在伪半固态温度下的高温压缩过程中,真实应力-真实应变曲线存在四个变化阶段:急剧上升-下降-平稳-缓慢上升,且稳态流动应力及峰值应力随着温度的提高和应变速率的降低显著下降,说明该材料具有应变速率和变形温度敏感性.随着变形量的增加,微观组织中的孔洞逐渐增多.     

Pressureless sintering of Al2O3-SiC whisker composites

High-density compacts, up to 88% theoretical density, of Al₂O₃-SiC whiskers were prepared by a pressure casting and impregnation technique. Starting with these green bodies, composites of Al₂O₃–20 vol% SiC whiskers were pressureless sintered to higher than 95% theoretical density. They were further densified by hot isostatic pressing up to 99% theoretical density, resulting in a rupture strength of 680 MPa and a fracture toughness of 4.70 Mpa m^1/2.