SiCw/MB15复合材料断裂行为研究

扫描电镜下采用动态拉伸法,原位观察SiCw/MB15复合材料复合材料在动态受载条件下裂纹形成、扩展至断裂的全过程,并对断口进行了分析。结果表明,裂纹主要在折断的晶须处萌生,在主裂纹前端应力集中区内长大、连结,在与主应力垂直方向上形成宏观裂纹。     

起始原料对氨化铝粉末合成的影响

本文用碳还原法合成氮化铝粉末，通过对使用不同起始原料，如活性碳、碳黑、α－Ａｌ２Ｏ３、Ａｌ（ＯＨ）３等合成的氢化铝粉末性质的比较，发现活性碳和Ａｌ（ＯＨ）３有助于加快反应速率，提高产物氮含量．另外发现α－Ａｌ２Ｏ３－活性碳系统中添加少量的氟化物具有催化功效，可在１６５０℃保温５ｈ条件下获得氮含量达３３．５４％的ＡＩＮ粉末．本文还对ＡｌＮ生成机理作了一定探讨．     

Fatigue crack growth and crack closure in an AlMgSi alloy

This study reports an experimental investigation of fatigue crack propagation in AlMgSi1-T6 aluminium alloy using both constant and variable load amplitudes. Crack closure was monitored in all tests by the compliance technique using a pin microgauge. For the constant amplitude tests four different stress ratios were analysed. The crack closure parameter U was calculated and related with Δ K and the stress ratio, R . The threshold of the stress intensity factor range, Δ K _th , was also obtained. Fatigue crack propagation tests with single tensile peak overloads have been performed at constant load amplitude conditions. The observed transient post overload behaviour is discussed in terms of the overload ratio, Δ K baseline level and R . The crack closure parameter U trends are compared with the crack growth transients. Experimental support is given for the hypothesis that crack closure is the main factor determining the transient crack growth behaviour following overloads on AlMgSi1-T6 alloy for plane stress conditions.     

铝及铝合金与熔融钠反应的研究

研究了纯铝和铝硅合金在３５０～４５０℃与熔融钠的反应,采用光镜（ＯＭ）和Ｘ射线衍向射分析（ＸＲＤ）观察和分析腐蚀产物组织形貌,分布和腐蚀产物的动力学生长机制,。实验结果表明,试样受熔融钠腐蚀的程度与铝中硅含量多少有关,铝硅合金在熔融钠中的腐蚀是受扩散控制的固相反应。     

轧辊断裂原因分析

某轧辊直径由840mm减少到780mm时发生断裂,未达到其正常使用寿命,采用化学成分分析、断口分析、金相检验以及硬度检测等方法对轧辊断裂原因进行了分析。结果表明：轧辊辊颈与辊身交界处的表面存在微裂纹是导致其发生断裂的根本原因;由于套密封环时操件不当使得该处的表层显微组织发生淬火,形成薄且脆的淬火组织,加之该处为应力集中处,从而导致该处产生微裂纹;已经形成的微裂纹在轧辊工作时所承受压力和扭转力的作用下,由于应力集中裂纹尖端不断向轧辊中心扩展,当裂纹达到一定深度后,辊颈处无法承受外力的作用时便会发生瞬间失稳扩展,导致轧辊断裂。     

Fracture Resistance of Residually-Stressed Ceramic Laminated Structures

We have studied the effect of residual stresses on fracture resistance and crack arrest behavior of asymmetric ceramic laminated Si ₃ N ₄/Si ₃ N ₄–TiN structures. Using the compliance method, we assessed the technique of R-curve construction for laminar composites. For laminar structures with layers varying by their elastic characteristics we developed an analytical method for calculation of fracture resistance – crack length dependence. The method applicability is verified by calculation of stress intensity factors for laminar specimens with an edge crack. The calculated results are compared to the experimental data.     

Resistance Against the Intrinsic Rate of Fracture Mechanics Parameters for Polymeric Materials Under Moderate Impact Loading

This study contributes towards understanding crack toughness as resistance against the intrinsic rate of fracture mechanics parameters. Up to now only few investigations have been done under moderate impact loading conditions. Based on experimental investigations using the crack resistance (R) concept, it has been shown that the stop block method combined with the multiple-specimen technique is a unique method for polymers under impact loading conditions in comparison with different R-curve methods. Other methods for the determination of R curve such as the low-blow technique are normally not applicable for polymers due to their time-dependent mechanical properties. The crack-tip opening displacement (CTOD) rate is a measurement of the rate sensibility of stable fracture process depending on the type of deformation, which can provide deep insights into the micromechanics and activation mechanisms during the fracture processes. In the polymeric materials mostly investigated, one can understand the stable crack propagation with three-stage processes; crack-tip blunting/crack initiation, non-stationary stable crack growth and steady-state stable crack growth (an equilibrium state). In this stable crack propagation, the values of normalized CTOD rate converge rapidly to a ‘matrix’-specific threshold. The stop block method in the multiple-specimen technique assures the criteria of the time-independent strain field around the crack tip and constant crack speed therewith and the J-integral is a valid toughness parameter.     

不同添加剂对钛酸铝热膨胀系数影响的研究

钛酸铝是一种优异的高熔点、低膨胀无机非金属材料．为提高其强度、抑制其热分解,往往在钛酸铝中引入添加剂．本文研究了单一添加剂和复合添加剂对钛酸铝膨胀系数的影响．结果表明：引入少量的稀土元素氧化物能显著降低钛酸铝的热膨胀系数．引入1～4mol％CeO₂或2mol％La₂O₃可使钛酸铝的膨胀系数降至接近于零．这就为开发零膨胀材料提供了新的方法．在钛酸铝中引入1～4mol％Fe₂O₃,其膨胀系数保持不变．在含有4mol％Fe₂O₃的钛酸铝中再引入SiO₂,钛酸铝的膨胀系数增大,而加入另一种稀土元素氧化物则能降低其膨胀系数．三种添加剂的适当配比能将钛酸铝的膨胀系数调整至最小．这对于引入复合添加剂对钛酸铝的改性是很有意义的．     

Fracture propagation using the radial point interpolation method

This work presents a crack path prediction algorithm combined with the radial point interpolation method (RPIM), a meshless method. To allow easier implementation in existing structural analysis software, this algorithm is numerically compatible with finite element method (FEM) formulations. The proposed RPIM formulation uses the triangular elements of a FEM mesh as the background integration grid, allowing also to combine both formulations more easily. Thus, with the developed methodology, the RPIM can be integrated directly in a FEM software or use the same CAD tools to build the discretization meshes. Because the RPIM shape functions possess the delta Kronecker property, all the numerical techniques available for the FEM can be applied to enforce the natural and essential boundary conditions. The developed algorithm uses the maximum tangential stress criterion to determine the crack propagation direction, and the crack paths obtained with it corresponded well to previous research.     

Fatigue crack initiation and growth in AlMg4.5Mn butt weldments

The fatigue life of full-penetration and partial-penetration 5 and 25  mm thickness AlMg4.5Mn (AA5083) aluminium alloy butt weldments was investigated under ( R = 0 and R = − 1) constant amplitude loading. The fatigue lives of the tested specimens were predicted using an analytical model which estimated both the crack initiation and crack growth portions of the total fatigue life. The fatigue life of partial-penetration weldments was found to be substantially less than that of full-penetration weldments because of the greater stress concentrations of the incomplete joint penetration and the consequent absence of a substantial crack-initiation life period. Tensile mean stresses ( R = 0 versus R = − 1-test conditions) markedly reduced the fatigue life of the weldments studied and greatly diminished the duration of the fatigue crack growth period. The extra material provided by the weld reinforcement noticeably increased the fatigue life of the partial-penetration weldments. Weld angular distortion-induced bending stresses greatly affected the smaller thickness (5  mm) full-penetration weldments offsetting the fatigue strength bonus anticipated for small-size weldments. Except for the predictions for R = − 1 full-penetration weldments at long life, which the analytical model underestimated, the agreement between experiment and analytical prediction was within a factor of 2, that is, as good as can be generally expected.     

纳米Al膜在可见和红外频段吸收率的尺寸效应

纳米A1膜在可见的红外频段的吸收率随薄膜厚度变化的关系表明,金属薄膜吸收率具有尺寸效应,并具有极大值,对比纳米A1膜直流电导率的实验结果,结构特征变化是导致吸收率尺寸效应的重要原因。     

Effect of crack closure on non-linear crack tip parameters

Crack closure concept has been widely used to explain different issues of fatigue crack propagation. However, some authors have questioned the relevance of crack closure and have proposed alternative concepts. The main objective here is to check the effectiveness of crack closure concept by linking the contact of crack flanks with non-linear crack tip parameters. Accordingly, 3D-FE numerical models with and without contact were developed for a wide range of loading scenarios and the crack tip parameters usually linked to fatigue crack growth, namely range of cyclic plastic strain, crack tip opening displacement, size of reversed plastic zone and total plastic dissipation per cycle were investigated. It was demonstrated that: (i) LEFM concepts are applicable to the problem under study; (ii) the crack closure phenomenon has a great influence on crack tip parameters decreasing their values; (iii) the ΔK_eff concept is able to explain the variations of crack tip parameters produced by the contact of crack flanks; and (iv) the analysis of remote compliance is the best numerical parameter to quantify the crack opening level. Therefore the crack closure concept seems to be valid. Additionally, the curves of crack tip parameters against stress intensity factor range obtained without contact may be seen as master curves.     

B5后桥A356铝合金支承座早期断裂失效分析

针对某B5后桥A356铝合金支承座台架试验早期断裂的问题,采用宏观分析、化学成分分析和扫描电镜断口观察等方法对支承座的断裂原因进行了分析.结果表明:由于该支承座受到意外挤压而造成其在加强筋处表面产生一较小的凹坑,由此形成的应力集中使零件在随后的台架试验中很快在此处萌生裂纹源;同时由于该支承座在裂纹源附近存在大面积的疏松...     

The Influence of Finite Geometry and Material Properties on Mixed-Mode I/II Fracture of Aluminium

Numerical calculations have been carried out to assess the influence of both finite geometry effects as well as material properties on mixed mode fracture of aluminium. These effects have been studied in close connection to experimental data for two aluminium alloys found in the literature. Interactions between the crack tip and the outer boundary have, for one of these alloys, been quantified in two ways. Firstly, by evaluating a number of non-singular stress on mixed mode fracture have been examined within the framework of a recently suggested effective plastic strain criterion. The other alloy was addressed in order to furnish a limited investigation concerning the sensitivity of this criterion with respect to material properties. The main conclusions arrived at in this paper are: (i) Boundary induced constraints may relocate the transition between different operative fracture modes and hence be responsible for scatter of experimental achieved under different testing conditions. (ii) The two alloys under consideration were predicted to behave very differently due to variations in the flow behaviour. Different behaviour was also confirmed by the experimental results. This revised version was published online in August 2006 with corrections to the Cover Date.     

A New Contribution to the Theory of Anode Effect in Aluminium Electrolysis

This paper discusses the general characteristicsof anode effect in aluminium electrolysis and pre-vious theories about the mechanism of anode effect.On the basis of laboratory experiments,the authorsuggests a new contribution to the primary and di-rect reason for anode effect.     

Fatigue properties of particle-reinforced metal-matrix composites

Fatigue-lifetime behaviour has been examined for extruded 6061 aluminium alloy composites reinforced with 15 vol% SiC and 10 vol% Al₂O₃ particles. The peak particle sizes are at about 4.5 and 6 μm. Within measured S- N curves the fatigue lifetime at given stress amplitudes of SiC_p/AA6061 is superior to that of Al₂O_3p/AA6061 in the low-cycle fatigue region as well as in the high-cycle fatigue region. These results are discussed by consideration of theoretically evaluated crack propagation curves.     

纳米铝粉颗粒分散稳定性的研究

以六偏磷酸钠作分散剂,采用吸光度实验来考察分散介质、六偏磷酸钠的用量及超声时间对铝基纳米流体稳定性的影响,并对其分散机理进行初步的探讨。结果表明:当乙二醇和去离子水按照体积比为1∶1时混合液作为分散介质得到的纳米流体的分散稳定性最好。六偏磷酸钠能够有效地分散纳米铝粉颗粒,得到均匀、稳定的铝基纳米流体;当加入六偏磷酸钠的质量浓度为0.5 g/L、超声分散时间为50 min时,分散效果最佳;六偏磷酸钠对纳米铝粉颗粒的稳定分散作用主要是通过颗粒间的静电作用来实现的。     

固溶体类型对钛酸铝热稳定性的影响

本文采用添加剂与钛酸铝形成不同类型的固溶体的方法,研究了固溶体类型对钛酸铝热稳定性的影响。结果表明,无论是在未稳定化的钛酸铝中,还是在用Ｆｅ２Ｏ３稳定的钛酸铝中,形成连续固溶体对抑制钛酸铝热分解的效果最佳,而填隙固溶体和有限置换固溶体的稳定效果不如连续固溶体的好,相比之下,形成不等价态置换固溶体的钛酸铝,其稳定性最差。     

泡沫铝复合材料的研究

泡沫铝材料具有优越的综合性能,已被广泛应用于各个领域.系统阐述了国内外泡沫铝材料的应用现状,指出泡沫铝材料复合化是进一步完善其结构与功能的有效途径,并在此基础上详细介绍了国内外泡沫铝复合材料的研究情况.     

Biological Structures Mitigate Catastrophic Fracture Through Various Strategies

Gao et al. (PNAS, 100, 5597–5600 (2003)) have argued that load-bearing mineralized hard tissues, including bones, shells, and teeth, are nanocomposites, in which the mineral phase has nanoscale dimensions that ensure optimum strength and flaw tolerance. In particular, it has been claimed that the thickness of these brittle building blocks, being smaller than a critical size, h^*, of the order of tens of nanometers, renders them insensitive to the presence of crack-like flaws and enables them to achieve near-theoretical strength, which is why Nature employs nanoscale features in mineralized biological composites. We find this point of view, which Gao et al. and others have quoted in subsequent publications and presentations, unpersuasive and present several counterexamples which show that biological structures, as a result of being comprised of relatively fragile constituents that fracture at stress levels several orders of magnitude smaller than the theoretical strength, adopt various strategies to develop mechanical responses that enable them to mitigate catastrophic failure. Nanoscale structural features are not a result of an innate resistance to very high stresses.