颗粒增强铝基复合材料冲击疲劳裂纹扩展的行为和机理

IMECH-IR > 力学所知识产出(1956-2008)

	颗粒增强铝基复合材料冲击疲劳裂纹扩展的行为和机理
	董雁瑾
导师	白以龙
	2000
学位授予单位	中国科学院研究生院
学位授予地点	北京
学位类别	博士
学位专业	固体力学
关键词	颗粒增强铝基复合材料基体材料冲击疲劳常规疲劳裂纹扩展速率加载速率履带板 Particulate-reinforced Al Composites Matrix Impact Fatigue (Repeated Impulsive Loading) Sine Fatigue (Cyclic Loading) The Crack Extension Rate Loading Rate
英文摘要	本文针对发展新一代步兵战车复合材料履带板所面临的关键问题，结合其实际受载特点，设计制备了冲击疲劳实验加载装置，并着重从实验设计及机理分析上进行细致深入的探索，揭示了Al_2O_3/LC_4复合材料冲击疲劳破坏的微观过程和机理。首先分别对SiC_P/LC_4、Al_2O_(3P)/LC_4 及基体 LC_4 进行了显微组织的观察与定量分析，并对其拉伸、三点弯曲破坏过程进行了在位观察，结合其断裂形貌的观察与分析，揭示出颗粒增强铝基复合材料断裂破坏的根本原因是颗粒的聚集及脆性相在晶界的严重偏聚。针对这一结论，给材料制备单位提出工艺改进意见。对工艺改进后制备的复合材料进行常规力学性能的测试，结果表明，其拉伸性能明显优于改进前制备的相应材料。为了进行冲击疲劳的实验研究，在分析步兵战车履带板实际受载特点的基础上，自行设计制备了冲击疲劳实验的加载装置。主要包括主体框架和测量系统，前者与小型振动系统配合使用可以实现冲击能量为 0.3J、冲击频率为 1Hz、冲击速度为 0.6m/s 的多次冲击实验；后者可以准确记录下任意时刻的冲击载荷波形及冲击疲劳载荷的循环数。为了考察颗粒与加载速率对复合材料疲劳机理的影响，实验研究了 Al_2O_3/LC_4 复合材料和 LC_4 纯基体材料在冲击疲劳和常规疲劳过程中裂纹的扩展过程及扩展速率。综合结果发现：与LC_4纯基体材料相比，Al_2O_3/LC_4复合材料疲劳裂纹扩展得更为迅速。复合材料中，由于颗粒的加入，两种疲劳方式下袭纹都发生严重偏转；裂纹经过颗粒时，多数是绕过，少数是切过颗粒；冲击疲劳裂纹扩展速率明显高于常规疲劳裂纹扩展速率。纯基体材料中，两种加载方式下，裂纹基本都以穿晶的方式扩展，裂纹常常表现为小锯齿状；冲击疲劳裂纹尖端的塑性变形程度比常规疲劳更大；冲击疲劳裂纹比常规疲劳裂纹更曲折，表现出多尺度的锯齿状（Zig-Zag）特征；冲击疲劳裂纹扩展速率高于常规疲劳的裂纹扩展速率。在基本实验的基础上，进一步对断口及裂纹扩展途径进行了微观观察和定量分析，最后综合全文的实验和统计结果，讨论了颗粒增强铝基复合材料的冲击疲劳机理。复合材料疲劳裂纹扩展速率的提高主要与裂纹的偏转有关，裂纹更倾向于沿着颗粒与基体的界面扩展；两种材料的疲劳裂纹扩展速率均随加载速率的增加而增加，呈现加载速率的反作用。加载方式的改变，一方面，由于冲击情况下载荷持续时间降低，使裂纹扩展速率降低；另一方面，加载速率的提高使得断裂韧性值降低，材料变脆，裂纹扩展速率升高。这两个方面相互影响，相互竞争，决定实际的裂纹扩展速率。两种材料中，不同加载速率下的疲劳裂纹扩展的微观机制基本一致，没有明显的本质区别。; This thesis is aimed at the understanding of the mechanism underlying crack extension in particulate-reinforced LC_4 Al composites under impact-fatigue loading. For this sake, an experimental system to perform the impact-fatigue tests was designed and processed. Then the fatigue crack extension in two materials, i.e. Al_2O_3/LC_4 composites and pure LC_4 alloy, subjected to two types of loading, i.e. impact-fatigue (repeated impulsive loading) and sine fatigue (cyclic loading), were studied. In order to examine the failure mechanisms of the particulate-reinforced Al composites, observations of microstructures and fracture surfaces of SiC_P/LC_4 , Al_2O_(3P)/LC_4 and pure LC_4 specimens in both in-situ tensile and three points bending tests were carried out. The results demonstrate that the particle cluster and the brittle phase precipitated on grain boundaries are the key factors responsible for the failure of the composites. Based on these observations, some suggestions to improve the materials process were made for the producer. Significantly, the tensile properties of the materials produced by the new process are higher than the old ones. In order to perform impact fatigue tests, an experimental system was designed and processed. This system consists of two parts: a cyclic loading device and a monitoring system. The former one equipped with a vibration system can be applied to perform impact fatigue tests with 0.3J impact energy, 1Hz frequency and 0.6m/s impact speed; the latter to measure and record impact stress and the number of cycles simultaneously. To reveal the effects of particles and loading rate on the fatigue mechanisms of particulate-reinforced Al composites, the crack extension rate of Al_2O_3/LC_4 and pure LC_4 subjected to repeated impulsive loading and cyclic loading, were studied, respectively. The experimental results indicate that the fatigue crack in Al_2O_3/LC_4 propagates much faster than that in pure LC_4. In Al_2O_3/LC_4 composites, the cracks are deflected by the reinforcement under both repeated impulsive and cyclic loadings. The interface failure between particles and matrix was observed quite often. Sometimes, the crack transects particles. The crack extension rate under repeated impulsive loading is much higher than that under cyclic loading in Al_2O_3/LC_4 composites. In pure LC_4 alloy, the fatigue cracks propagate transgranularly and show "Zig-Zag". The crack under repeated impulsive loading is more "Zig-Zag" than that under cyclic loading and shows multi-scale "Zig-Zag". The crack extension rate under repeated impulsive loading is also much higher than that under cyclic loading. Furthermore, quantitative analysis of the crack propagating characteristics on fracture surfaces was performed. On the basis of all above experimental results, impact fatigue mechanisms in particulate-reinforced Al composites were discussed. The increase of crack extension rate in Al_2O_3/LC_4 composites appears to be related to crack deflection. The crack is inclined to propagate towards the interfaces between particles and matrix. the fatigue crack extension rate in both pure LC_4 alloy and Al_2O_3/LC_4 composites increases with increasing loading rate. The effect of loading rate on the fatigue crack growth rate in Al_2O_3/LC_4 composites is more apparent than in pure LC_4 alloy. Under the two different loading modes, on the one hand, the loading durative time δ of impulse is much shorter than that of sine wave, which may make the crack extension rate decrease; on the other hand, the fracture toughness decreases with increasing loading rate, which make the crack extension rate increase. The two opposite factors interact and compete with each other. The result of competition determines the fatigue crack growth rate. In addition, the micro-mechanisms of crack extension under different loadings in Al_2O_3/LC_4 composites or pure LC_4 alloy are similar.
索取号	29936
语种	中文
文献类型	学位论文
条目标识符	http://dspace.imech.ac.cn/handle/311007/22962
专题	力学所知识产出(1956-2008)
推荐引用方式 GB/T 7714	董雁瑾. 颗粒增强铝基复合材料冲击疲劳裂纹扩展的行为和机理[D]. 北京. 中国科学院研究生院,2000.

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