【作者】 郑鑫；

【导师】 温卫东；

【作者基本信息】 南京航空航天大学 ， 机械设计及理论， 2012， 博士

【摘要】 汽车轻量化设计已经成为降低汽车自重、提高燃油经济性的重要手段。铸铝合金以其较高的比强度和优良的综合性能已经逐渐取代传统的铸铁成为汽车发动机部件的主要材料。汽车发动机部件如气缸盖和气缸体等在工作状态下承受着复杂的高低周复合疲劳载荷历程，传统的常幅载荷高、低周疲劳设计方法不再适用于高低周复合疲劳载荷条件下材料与结构的耐久性分析与设计。铸铝合金材料的高低周复合疲劳行为研究将为新一代高性能轻量化汽车发动机部件的耐久性设计提供必要的理论基础。学术界和工程界对于高低周复合载荷作用下的疲劳裂纹扩展、疲劳损伤机理及疲劳寿命预测模型的研究仍然不够充分。本文针对Al-Si-Cu系铸铝合金，采用试验研究和理论建模的手段，开展高低周复合载荷条件的裂纹扩展试验和光滑试件疲劳寿命试验，研究疲劳裂纹扩展规律和疲劳损伤机制，并建立了疲劳裂纹扩展数值建模方法和高低周复合疲劳损伤模型。本文的主要工作和结论如下：（1）对铸铝合金E319-T7开展了常幅载荷疲劳裂纹扩展速度曲线试验和含拉伸或压缩过载的裂纹扩展试验。观察到单个压缩过载后的裂纹扩展加速现象：在单个压缩过载之后，裂纹扩展速度立即提高，然后很快回落至常幅载荷下的稳态水平。裂纹扩展试验结果表明，高低周复合载荷谱的压缩过载作用使裂纹发生额外长度的扩展，对材料与结构的耐久性具有一定的危害性。提出了裂纹长度增量ΔaUL来定量评估不同压缩过载水平和不同常幅载荷ΔK下的裂纹扩展加速水平。ΔaUL越大，表示压缩过载对裂纹扩展的贡献越大。本文的裂纹扩展试验和理论模型都指出：在相近的常幅载荷ΔK下，压缩过载越大导致ΔaUL越大；而当压缩过载相同时，ΔaUL随着常幅载荷ΔK的增大而增大。（2）提出并建立了基于临界距离理论的疲劳裂纹扩展数值建模方法。该模型基于裂尖临近距离处的塑性耗散能密度决定裂纹扩展的思想，具有明确的物理意义。开发了模拟裂纹扩展的有限元模型（包括相关子程序UEL和UVARM）和计算裂纹扩展速度的MATLAB后处理程序，基于R=0.1裂纹扩展试验曲线建立了裂纹扩展判据（即临界距离和临界塑性耗散能密度），定量预测了裂纹在不同应力比条件和含拉伸/压缩过载条件下的扩展速度，预测结果与试验结果基本一致。有限元分析表明压缩过载后的裂尖钝化作用是导致裂纹张开应力水平下降的原因；而裂纹张开应力的下降导致裂尖区域（包括临界距离处）塑性耗散能密度增量的增大，最终使裂纹扩展加速。压缩过载越大，裂纹张开应力水平下降越多，裂尖区域塑性耗散能密度增量越大，裂纹扩展加速越显著。此外还发现，裂纹扩展数值建模方法的临界距离与铸铝合金材料的微观组织特征和宏观力学性能相关，即临界距离、微观组织特征长度（SDAS）以及ΔKth对应的循环塑性区尺寸等三者比较接近，处于同一数量级。（3）分别建立了常幅高、低周疲劳寿命回归模型，拟合得到了概率疲劳寿命曲线。断口分析指出，近自由表面处的铸造缺陷为主要裂纹萌生源，其中以氧化膜最多。裂纹萌生源大小不一是导致常幅高、低周疲劳寿命分散性的主要原因。裂纹萌生源大小（等效直径）近似符合对数正态分布规律。裂纹源等效直径的累积概率与疲劳寿命在任意载荷水平的的累积概率具有明确的45度线性关系，即裂纹源等效直径的累积概率与疲劳寿命的累积概率相加等于1。考虑了试验本身分散性对高低周复合疲劳损伤计算精度的影响，提出并采用了概率疲劳寿命作为计算高低周复合疲劳损伤所需的常幅高、低周疲劳寿命。概率疲劳寿命由常幅载荷疲劳寿命回归模型结合高低周复合疲劳裂纹源等效直径的累积概率计算得到。（4）提出并建立了高低周复合疲劳损伤指数衰减模型。对铸铝合金AS7GU-T64光滑试件开展了高低周复合疲劳试验。研究了各种载荷条件对复合疲劳损伤的影响，载荷条件涉及高周应力幅值（σHa），低周相对应力范围(σ HLmin σmin)以及每个载荷块的高周循环数（η）。结果表明：复合疲劳损伤随着高周应力幅值和低周相对应力范围的增大而增大。η对复合疲劳损伤的影响可分为三个区间，每个区间都有明显特征。低周循环后裂纹张开应力的降低及回复为高周循环复合疲劳损伤产生与衰减的主要原因。当η较小时（小于约30），频繁施加的低周循环易于促使裂尖循环塑性区内的Si颗粒提前发生损伤，从而为裂纹在高周循环期间的扩展提供弱化的路径，加快裂纹扩展速度，η值越小，复合疲劳损伤量越大。在这个阶段，复合疲劳损伤是裂纹张开应力水平降低和周期性低周循环共同作用的结果。基于以上结果，通过拟合光滑试件高低周复合疲劳试验结果，提出并建立了高低周复合疲劳损伤指数衰减模型，该模型能够比较全面的描述各种载荷条件对复合疲劳损伤的影响。相比线性累积损伤模型，包含高低周复合疲劳损伤模型的累积损伤模型能够明显改进高低周复合疲劳寿命的预测精度。更多还原

【Abstract】 The lightweight design in the automotive industry has been growing rapidly because of theincreasingly need to reduce weight and increase fuel efficiency. Automotive engine components, suchas cylinder heads and cylinder blocks, which are increasingly manufactured by cast aluminum alloysinstead of cast iron, are subjected to complex high-cycle and low-cycle (HCF-LCF) interaction loadhistories. The traditional durability design based on high-cycle and low-cycle fatigue alone might benon-conservative and not appropriate. Study on the HCF-LCF interaction fatigue behavior for castaluminum alloys would provide fundamental understandings for the development ofhigh-performance&light-weight engines in future. Unfortunately, the HCF-LCF interaction fatiguehas received limited investigation in the literature. The present thesis aims to get a betterunderstanding of the HCF-LCF interaction fatigue mechanisms using experimental and theoreticalapproaches. Fatigue crack propagation model which is able to predict fatigue crack propagation ratesunder HCF-LCF interaction loading are developed. The HCF-LCF interaction fatigue damage modelis also built based on the experimental results.The main work of this thesis is summarized as follows,(1) Fatigue crack propagation tests including the fatigue crack propagation curve testing andoverload/underlaod testing have been carried out. The crack propagation acceleration has beenobserved after the application of single underload. Specifically, crack propagation rates increaseimmediately after the application of the underload and then recover to the steady-state crackpropagation rate under constant amplitude loading. The traditional growth of crack under HCF-LCFinteraction loading would be detrimental for the durability of materials and structures. The incrementin crack length after the application of an underload, ΔaUL, has been proposed as a suitable factor toevaluate the magnitude of the crack acceleration. It has been found that ΔaULincreases with both theapplied underload level and the constant amplitude loading ΔK level at which the underload isapplied.(2) A numerical modeling approach based on the theory of critical distances has been developedto quantitatively simulate the fatigue crack propagation for cast aluminum alloys. In the proposedapproach, the fatigue crack advances by one element length when the plastic energy density at thecritical distance point ahead of crack tip accumulates to a critical level. A finite element model hasbeen developed to simulate the fatigue crack propagation and the calculation of fatigue crack propagation rates are carried out by a post-process program. The crack propagation criterion has beenestablished by fitting the simulated crack propagation curve to the R=0.1experimental data. Theeffects of different R-ratios and overload/underload are then accurately predicted using the establishedpropagation criterion. The proposed FE model has also successfully simulated the plasticity-inducedcrack closure. The findings from the present FE analyses indicate that crack opening stress intensity isreduced after an underload is applied, which directly affects the plastic energy dissipation ahead of thecrack tip, which has been assumed to be the main damage mechanism controlling the fatigue crackpropagation rate. For the W319-T7cast aluminum alloy, the secondary dendrite arm spacing (SDAS),the cyclic plastic zone size for ΔKthand the critical distance have been found to be of the same orderof magnitude.(3) Constant amplitude loading (CAL) HCF and LCF tests have been performed on the AS7GUcast aluminum alloy. The probabilistic fatigue lives are estimated by fit the life regression models tothe experimental S-N data. The scatter in fatigue life is related to the fatigue crack initiation sites.Fractographic studies indicated that fatigue cracks in most specimens initiated from casting defectslocated near specimen surface. Special attention has been paid to the fatigue crack initiator size, whichis the major cause of the scatter in fatigue life. The Lognormal distribution provides appropriate fit tothe fatigue initiator sizes. For both HCF and LCF tests, the cumulative density function (CDF) offatigue life has been found to be approximately equal to the complementary value of the CDF of thenear-surface fatigue initiator size. To take into consideration the influence of the inherent scatter infatigue life on the accuracy of interaction damage calculation, probabilistic fatigue lives are used asthe estimates of the CAL HCF and LCF lives for the calculation of HCF-LCF interaction fatiguedamage.(4) A HCF-LCF interaction damage model based on exponential decay law has been proposed.HCF-LCF interaction tests have been carried out under various load conditions. The effects of HCFstress amplitude (σHa), LCF relative stress range(σ H σLminmin), and number of HCF cycles perblock (η) are investigated. The interaction damage increases with HCF stress amplitude and LCFrelative stress range. The impact of the number of HCF cycles per block, η, can be divided into threeregions, each of which has distinct feature. The evolution of crack opening stresses following anunderload has been proposed as the major mechanism. The additional interaction damage caused bythe frequent applications of LCF cycles (η<30) is explained by the fracture or debonding of Siparticles within the LCF reversed plastic zone, which facilitates subsequent crack propagation. Aninteraction damage model based on exponential law has been proposed to account for the effects of the three loading parameters mentioned above. The proposed model can correctly characterize theinteraction damage for the load conditions investigated in this paper. The damage summation lawmodified by including the HCF-LCF interaction damage model has been found to achieve moreaccurate fatigue life predictions compared with the Miner’s linear damage summation law.更多还原