Acoustic emission during fatigue crack propagation in SiC particle reinforced Al matrix composites

The acoustic emission (AE) behavior during fatigue propagation in aluminum 6061 and aluminum 6061 matrix composites containing 5, 10, and 20 wt pct SiC particle reinforcement was investigated under tension-tension fatigue loading. The purpose of this investigation was to monitor fatigue crack propagation by the AE technique and to identify the source(s) of AE. Most of the AEs detected were observed at the top of the load cycles. The cumulative number of AE events was found to correspond closely to the fatigue crack growth and to increase with increasing SiC content. Fractographic studies revealed an increasing number of fractured particles and to a lesser extent decohered particles on the fatigue fracture surface as the crack propagation rate(e.g., †K) or the SiC content was increased. This article is based on a presentation made in the symposium entitled “Creep and Fatigue in Metal Matrix Composites” at the 1994 TMS/ASM Spring meeting, held February 28–March 3, 1994, in San Francisco, California, under the auspices of the Joint TMS-SMD/ ASM-MSD Composite Materials Committee.     

Aspects of fracture in particulate reinforced metal matrix composites

The tensile deformation and fracture behaviour of the aluminium alloy 6061 reinforced with SiC has been investigated. In the T4 temper plastic deformation occurs throughout the gauge length and the extent of SiC particle cracking increases with increasing strain. In the T6 temper strain becomes localised and particle cracking is more concentrated close to the fracture. The elastic modulus decreases with increasing particle damage and this allows a damage parameter to be identified. The fraction of SiC particles which fracture is less than 5%, and over most of the strain range the damage controlling the tensile ductility can be recovered, indicating that other factors, in addition to particle cracking are important in influencing tensile ductility. It is suggested that macroscopic fracture is initiated by the SiC particle clusters that are present in these composites as a result of the processing. The matrix within the clusters is subjected to high levels of triaxial stress due to elastic misfit and the constraints exerted on the matrix by the surrounding particles. Final fracture is then produced by crack propagation through the matrix between the clusters.     

SiC颗粒增强铝基复合材料的制备工艺和性能研究

采用粉末冶金法制备SiCp/6061Al复合材料,研究热压温度、球磨工艺参数和SiC颗粒(SiCp)体积分数对SiC颗粒增强铝基复合材料性能的影响,测试其力学性能及物理性能,用扫描电镜对材料的微观组织和断口进行观察。结果表明:540℃是较适合的热压温度;随着SiCp含量的增加,复合材料的致密度、热膨胀系数下降,抗拉强度先提高后迅速降低。     

Residual stresses in silicon carbide particulate reinforced aluminum composites

In metal matrix composites (MMCs) residual stresses are unavoidable during cooling from high temperature in fabrication or heat treatment because of the difference in the thermal expansion coefficients between the matrix and the reinforcement. In particle reinforced MMC the residual stresses have been proved to be hydrostatic in this study by both experiments and mathematical analysis. A very slight surface effect on the measured stresses was predicted in the case Cu Kα radiation was used. The residual stresses were determined to be tensile in the Al matrix and compressive in the reinforcement. A reduction in residual stress magnitudes of both the matrix and reinforcement was observed after the sample was cooled into liquid nitrogen and heated back to room temperature, which is believed to be caused by plastic deformation of the matrix in low temperature treatment.     

Thermoelastic analysis of matrix crack growth in particulate composites

We examine the conditions under which differences in thermal expansion between a particle and the matrix lead to crack growth within the matrix. Using linear elasticity fracture machanics, we obtain closed-form, analytical results for the case of a penny shaped crack present in the matrix interacting with a spherical inclusion which is misfitting with respect to the matrix. A simple and direct relationship is established between the strain energy release rate, the crack size, the crack orientation with respect to the inclusion, the crack/inclusion separation, the degree of thermal expansion mismatch and the elastic properties of the medium. We also analyze the size to which these cracks can grow and find that for a given misfit strain and material properties, crack growth is inhibited beyond a certain critical crack size. We find that beyond this critical size, the elastic strain energy released upon crack growth is no longer sufficient to compensate for the energy expended in extending the crack, since the crack is growing into the rapidly decreasing stress field. The modification of the above conditions for crack growth due to the superposition of an external stress field has also been analyzed. The preferred orientation of these cracks as a function of misfit strain is predicted. The implication of these results for thermal cycling are analyzed.     

Mode III fatigue crack propagation in low alloy steel

To provide a basis for estimating fatigue life in large rotating generator shafts subjected to transient oscillations, a study is made of fatigue crack propagation in Mode III (anti-plane shear) in torsionally-loaded spheroidized AISI4340 steel, and results compared to analogous behavior in Mode I. Torsional S/N curves, determined on smooth bars containing surface defects, showed results surprisingly close to expected unnotched Mode I data, with lifetime increasing from 10⁴ cycles at nominal yield to 10⁶ cycles at half yield. Fatigue crack growth rates in Mode III, measured on circumferentially-notched samples, were found to be slower than in Mode I, although still power-law related to the alternating stress intensity(△K _III) for small-scale yielding. Mode III growth rates were only a small fraction (0.002 to 0.0005) of cyclic crack tip displacements(△CTD _III) per cycle, in contrast to Mode I where the fraction was much larger (0.1 to 0.01). A micromechanical model for Mode III growth is proposed, where crack advance is considered to take place by a Mode II coalescence of cracks, initiated at inclusions ahead of the main crack front. This mechanism is consistent with the crack increment being a small fraction of △CTD_III per cycle. Formerly with Massachusetts Institute of Technology, Cambridge, MA Formerly with M.I. T.     

CNTs增强铝基复合材料研究现状

综述制备CNTs增强铝基复合材料的主要方法,分析各种制备方法的工艺特点并综合讨论各种制备方法的优点与不足之处,列举CNTs增强铝基复合材料包括力学性能在内的多个主要特性相较传统铝合金的优势所在,展望CNTs增强铝基复合材料的发展前景.      

GH864合金组织特征对裂纹扩展速率的影响

利用光学显微镜、场发射扫描电镜等手段,研究了GH864合金三种轧制态组织经热处理后的演变过程,分析了热处理后不同组织对室温冲击韧性及高温650℃裂纹扩展速率的影响.结果表明,GH864合金由初始轧制态组织经标准热处理后(1020℃,4h/空冷→845℃,4h/空冷→760℃,16h/空冷)获得的晶粒组织,其晶粒组织演变具有明显的一致性,而合金的晶界碳化物分布及基体γ'强化相没有明显差别,其分布状态及尺寸大小基本一致.热处理后的晶粒尺寸越大,抗裂纹扩展能力越好,合金的室温冲击韧性越低;热处理后形成的项链状组织,对合金冲击韧性及裂纹扩展速率有较好的影响.     

Plastic work of fatigue crack propagation in steels and aluminum alloys

The plastic work per unit area of fatigue crack propagation,U, is one of the parameters controlling the rate of fatigue crack propagation,dc/dN. The equation,dc/dN = A ΔK ⁴/(σf_y ²μ U), was previously shown to fit the data for 7 iron and aluminum base alloys for the range of thedc/dN vs ΔK curve where the Paris relation is valid. Values ofU are now available for 6 additional alloys covering a much wider range of σ_y 42 to 868 MN/m². For the total populationA = (2.8 ± 0.9) X 10^-3 where 2.8 is the mean and 0.9 is the standard deviation. In this equation, σ_y is the 0.2 pct offset cyclic yield stress and μ is the shear modulus. The parameterU is related to microstructure and should be of interest to the metallurgist. Generally,U varies oppositely to σ_y due to decrease in the plastic zone size; however, the plastic strain amplitude and degree of localization of the plastic strain in the plastic zone are also important.     

Creep behavior of fiber reinforced metal matrix composites

A theoretical model of the creep behavior of metal matrix composites having strong fiber-matrix interfaces is described in terms of creep parameters of the matrix and fibers. The available experimental data, obtained from the unidirectionally solidified aluminum-nickel eutectic containing 10 vol pct Al₃Ni fibers, are in good agreement with the theoretical model. The creep activation energy of the composite is described in terms of the creep activation energy of fibers and the matrix. The experimentally de-termined data of (Co, Cr)-(Co, Cr)₇C₃ and Al-Al₃Ni eutectics are in agreement with those values as predicted. Formerly a Visiting Scholar, Materials Department, University of California, Los Angeles.     

Creep behavior of fiber reinforced metal matrix composites

A theoretical model of the creep behavior of metal matrix composites having strong fiber-matrix interfaces is described in terms of creep parameters of the matrix and fibers. The available experimental data, obtained from the unidirectionally solidified aluminum-nickel eutectic containing 10 vol pct Al₃Ni fibers, are in good agreement with the theoretical model. The creep activation energy of the composite is described in terms of the creep activation energy of fibers and the matrix. The experimentally de-termined data of (Co, Cr)-(Co, Cr)₇C₃ and Al-Al₃Ni eutectics are in agreement with those values as predicted.     

Interfacial fatigue in a fiber reinforced metal matrix composite

An experimental investigation of interface fatigue in a fiber reinforced metal matrix composite has been conducted. For this purpose, the cyclic traction law (the relationship between the fiber stress and the pullout displacement) was measured using fiber pullout tests. On the first loading cycle, the traction law was found to be parabolic, in accord with predictions of a micromechanical model based on a constant interface sliding stress. Upon subsequent unloading and re-loading, the relationship changed, following trends which suggest that the sliding resistance degrades with cyclic sliding. Such effects have been confirmed through SEM examinations of the fiber coatings following fatigue testing. Furthermore, the degradation was found to be greatest near the plane of the matrix crack. The results are consistent with the notion that the degradation in sliding stress occurs most rapidly in regions where the relative sliding distance (fiber/matrix) is greatest. A phenomenological model incorporating such degradation is presented and compared with the experimental measurements.     

碳纤维增强铝基复合材料的研究现状

碳纤维增强铝基复合材料同时具备了增强材料和金属材料的优良特性,具有高强度、高模量、高耐磨性等特征,并且可以在导热、导电和高温下提供高强度、高弹性系数和高尺寸强度,在航空航天、汽车等行业的应用方面表现出巨大的发展空间.介绍了几种制备碳纤维增强铝基复合材料方法,从制备工艺、微观组织、力学性能等方面评述了制备的关键问题和研究...     

SiC颗粒增强铝基复合材料的研究进展

简要介绍了SiC颗粒增强铝基复合材料的优点及几种制备方法,包括搅拌法、浸渗法、喷射法、粉末冶金法和固液分离法;并对其后热变形加工参数对复合材料的性能影响进行了论述;最后,展望了粉末冶金法制备铝基复合材料的发展前景。     

Prediction of thermomechanical fatigue lives in metal matrix composites

To identify the role of silicon carbide participate reinforcement on high-temperature thermomechanical fatigue behavior of Al 2xxx-T4, experiments have been conducted under thermomechanical out-of-phase and in-phase loading conditions. A general constitutive representation, based on Eshelby’s inclusion theory, is used for the determination of volumetric average stresses and strains under cyclic loading of the metal matrix composite. This constitutive representation is used with a life prediction model, based on the matrix stress-strain behavior, which predicts contributions of fatigue, creep, and environmental damages to failure under both isothermal and thermomechanical fatigue loading. In isothermal fatigue experiments at 200 °C and 300 °C, pure fatigue damage and creep damage are the dominant damage mechanisms in the short-life regime. In the long-life regime, however, the stress levels are too low to induce considerable creep damage; so, oxidation damage becomes dominant. When fatigue damage is dominant, the model predicts a decrease in life, based on strain range, with increasing volume fraction of reinforcement. Based on stress range, improved fatigue lives are predicted with increasing volume fraction of reinforcement. The reinforced alloy exhibits longer lives when compressive hydrostatic stresses in the matrix at the high-temperature end of the cycle reduce the creep damage. Temporarily Director, Mechanics and Materials Program, National Science Foundation, Washington, DC 20550     

Metallography of fatigue crack initiation in an overaged high-strength aluminum alloy

Crack initiation was observed by optical microscopy using Nomarski interference contrast during fatigue cycling of an overaged 2024 aluminum alloy. The number of cracks more than five microns long at any given fraction of the fatigue life, and the distribution of cracks among various possible initiation sites, both depend on the applied stress amplitudeσa). The crack density at failure falls from approximately 300/mm² when σ^a is 90 pct of the yield strength, to less than I/mm² when σ^a is less than 60 pct of the yield strength. Cracks may begin in the matrix, in grain boundaries, or at constituent particles. At all stress amplitudes, however, the most common initiation sites areβ (Al₇Cu₂Fe) constituent particles. At low stress amplitudes in particular, fatigue cracks develop from the interface between closely-spaced fragments of β particles broken during prior processing (cluster sites). The stress-raising effect of voids which often occur at cluster sites may be responsible for their effectiveness in initiating fatigue cracks. Formerly Graduate Students in the Department of Metallurgical Engineering at Wayne State University During 1982–83 he is on leave as Associate Director of the Metallurgy Program, Division of Materials Research, National Science Foundation, Washington, DC 20550.