【作者】 谭柱华；

【导师】 盖秉政； 庞宝君；

【作者基本信息】 哈尔滨工业大学 ， 固体力学， 2007， 博士

【摘要】 高体积分数颗粒增强金属基复合材料由于其具有高比刚度、高比强度、低密度和优越的热学性能,使得其被广泛地应用于航空航天、军工、装配和电子封装等领域。其中,在很多领域涉及到动态载荷的作用,而材料在准静态载荷和动态载荷作用下的力学性能存在着显著的差异。研究其动态力学性能及其机理,为该类复合材料的材料设计与开发以及工程应用等方面具有重要意义。本文针对高体积分数颗粒增强金属基复合材料SiCp/2024Al的动态力学性能进行了实验和数值模拟研究。主要研究内容包括以下几个方面:分别采用霍普金森压杆(SHPB)动态压缩加载装置和Instron材料试验机研究了铸造、退火和T6热处理的三种颗粒体积分数40%、45%和50% SiCp/2024Al复合材料的压缩力学性能,其中应变率测试范围为0.001/s-2500/s。由于高体积分数颗粒增强金属基复合材料具有一定的脆性,通常的SHPB加载方法表现出一定的局限性。为此,采用了波形整形的方法,通过对实验结果的比较与分析,选择尺寸为Ф5mm×1mm的黄铜垫片作为整形器,以实现试件在变形过程中处于应力平衡和常应变率变形状态,满足了SHPB工作原理应力和应变均匀性的假设,保证了实验结果的可靠性。准静态和动态压缩实验结果表明,不同体积分数SiCp/2024Al复合材料表现出明显不同的变形行为。其中,与准静态相比,高应变率下复合材料表现出较高的延伸率;颗粒体积分数为40%的复合材料在高应变下从腰鼓状变形逐渐至剪切破坏,具有较大的延伸率和较好的塑性,而颗粒体积分数为45%和50%的复合材料则从剪切破坏到劈裂破坏,表现出明显的脆性;结合文献中体积分数40%以下的颗粒增强金属基复合材料在高应变率不发生破坏的实验结果,本文的实验结果表明,对于SiCp/2024Al复合材料,40%左右颗粒体积分数为该复合材料由塑性至脆性特性转变的临界体积分数。通过单轴压缩实验得到了SiCp/2024Al复合材料在应变率范围为0.001-2500/s的应力应变曲线,实验结果表明,应变率对复合材料的流动应力有明显的影响。其中,颗粒体积分数为40%的复合材料的流动应力随着应变率的增大而提高,而颗粒体积分数为45%和50%复合材料的流动应力则是随着应变率的增大先增大后减小,这一趋势可能是由于高应变率下绝热压缩过程中生成的热所引起的软化效应所导致的,本文还讨论了颗粒体积分数、热处理、应变硬化和应变率硬化对复合材料动态力学性能的影响。通过对压缩试件断口的SEM照片分析,观察到了基体在高应变率下发生熔化的现象。这一现象可能是由于绝热压缩是塑性变形转换生成的热所引起的。结合此现象,以及试件的宏观变形和不同应变率下的应力应变曲线结果表明,高应变率下,颗粒增强金属基复合材料SiCp/2024Al的变形机理为:低体积分数复合材料是由基体变形承受、传递载荷,而高颗粒体积分数则是由颗粒相接触形成的网状结构承受载荷。针对上述动态压缩实验还进行了数值模拟分析。其中,利用有限元软件LS-DYNA建立了含有不同长径比椭球形颗粒的二维轴对称和三维的单胞模型,根据SEM照片中发现熔化现象,为了分析和比较热软化效应的影响,本文分别采用了Cowper-Symonds和Johnson-Cook材料模型进行了数值计算。计算结果表明,带有温度软化的Johnson-Cook模型的计算结果与实验结果比较吻合,还利用从隐式到显示计算分析了动态冲击载荷下的热残余应力的影响。在此基础上,利用有限元LSDYNA软件计算分析了不同颗粒体积分数、长径比和颗粒形状等对SiCp/2024Al复合材料动态力学性能的影响。数值计算结果表明,颗粒体积分数10-30vol.%时长径比对复合材料的流动应力影响小于对40-50vol.%复合材料流动应力的影响;以及复合材料的应力应变曲线在体积分数为30%以下明显的线性硬化,而在40 vol.%和50vol.%时复合材料的应力应变曲线则表现出随着应变增大而下降的趋势,与实验结果得出的40%左右颗粒体积分数为复合材料性能转变的临界体积分数的结论相吻合。更多还原

【Abstract】 Metal matrix composites (MMCs) reinforced with high content particles were extend applied in aerospace, military, assemble and electric packaging fields due to its specific stiffness and strength, low density and advanced thermal properties. Some of those applications were involved dynamic loading. There is great different behavior between under the quasi-static and dynamic loading. It is necessary to study the dynamic properties to provide the reliable data for numerical calculation, engineering application and materials design.The dynamic properties of SiCp/2024Al with high particle content were investigated by experiment testing and numerical calculation method in present paper; it mainly includes content as following:The compression mechanical properties of SiCp/2024 reinforced with 40vol.%, 45vol.% and 50 vol.% particles were investigated by using split Hopkinson pressure bar(SHPB) and Instron materials testing machine, respectively. Before testing, samples were with casting, T6 and annealing solution-heat treated. The experiments were carried out in comparison at stain rates ranging from 0.001/s to 2500/s. The common SHPB technique showed limitation due to the brittle properties of metal matrix composites reinforced with high content particles. The pulse shaper technique was employed in dynamic compression. In order to obtain the reliable data, the copper disk with dimension ofФ5mm×1mm was chosen as pulse shaper to ensure the Samples are in dynamic stress equilibrium and have nearly constant strain rate over most of the test duration.The experiment results showed the composites display obviously different deformation mechanism at various strain rates. The composites show better elongation at high strain rate compared with under quasi-static loading. And at high strain rate, 40vol.% SiCp/2024Al deformed with good elongation from drug shape to shear fracture step by step, however, 45 and 50vol% SiCp/2024Al deformed with obvious brittle properties from shear fracture to split fracture. Compared with lower particle content (<30%) MMCs with good plastic deformation, the results showed that 40% as a critical volume fraction for SiCp/2024Al between plastic properties and brittle properties.The stress-strain curves at strain rates ranging from 0.001/s to 2500/s were obtained. The results showed that the composites exhibited high strain-rate sensitivity. The flow stress kept increase for 40 vol.% SiCp/2024Al and showed increase-decrease tendency for 50 vol.% SiCp/2024Al at strain rates ranging from 1250 to 2500/s, which maybe was caused by heat generated during adiabatic compression.The fracture surfaces were characterized by scanning electron microscopy. It is obvious that the matrix was softened /melted by heat generated during adiabatic compression. Combining the results of macro-deformation and the stress-strain curves, the results showed that the deformation mechanism of 40 vol.% SiCp/2024Al was controlled by matrix, and 50 vol.% SiCp/2024Al was controlled by net shape structure consisted by particles.Compressive properties of MMCs reinforced with high content partilces were investigated by using LS-DYNA. Axi-symmetric and three dimensional unit cell models with ellipsoids particles were employeed in numerical simulation. The effects of temperature rising caused by heat generated during adiabatic compression with Johnson-Cook model compared to Cowper-Symonds model and the thermal resident stress on the flow stress of SiCp/2024Al with high particle content were discussed. The results with Johnson-Cook model agreed well with the experiment results.And the effects of particle volume fraction, aspect ratio and particle shape on SiCp/2024Al dynamic properties were also discussed by using LS-DYNA. The numerical results showed that the effect of aspect ratio on flow stress of composites with lower particle content (10-30 vol.%) was less than on composites with high particle content. And the stress/strain curves of composites with low particle content was linear strain work hardening, however, the stress/strain curves of composites with 40% and 50% particle content showed increase-decrease tendency with increasing of strain. This agreed well with the experiments results of 40% is the critical volume fraction for properties of SiCp/2024Al.更多还原