【作者】 王冠；

【导师】 李落星；

【作者基本信息】 湖南大学 ， 机械工程， 2013， 博士

【摘要】 薄壁梁具有质量轻，强度高，变形稳定等特点，被广泛的应用于现代汽车车身结构中。汽车薄壁梁结构的典型变形工况为弯曲变形与轴向压缩变形。本文针对两种典型变形工况，系统研究了铝合金薄壁梁的变形行为及设计方法，为汽车铝合金结构的设计开发提供理论支持。基于显式动力学有限元分析软件，以混合元胞自动机(HCA)作为优化计算模型，对铝合金前碰撞横梁的结构进行优化设计。针对拓扑优化结果，采用模拟退火法优化横梁壁厚尺寸，获得薄壁、中空且带有加强筋结构的铝合金前碰撞横梁设计方案。通过三点弯曲、摆锤冲击与台车碰撞实验与数值仿真验证，铝合金前碰撞横梁比原钢质横梁质量减轻了25%，抗弯曲性能提高52%，吸能性能提高了45.6%。结果表明，采用元胞自动机优化设计得到的铝合金横梁满足法规要求，具有良好的耐撞性，能够对车身起到较好的防护作用。以6063铝合金多胞截面挤压型材为研究对象，通过准静态压缩试验，研究了铝合金薄壁梁结构的轴向压缩变形行为。铝合金薄壁梁试样在180℃条件下时效0min-540min，使用WAW-E600型万能试验机沿轴向压缩薄壁梁试样，研究不同时效状态下铝合金薄壁梁结构的轴向压缩性能。结果表明，随着时效时间的增加，铝合金薄壁梁试样的轴向压缩变形由欧拉模式逐渐转变为手风琴模式，试样的名义载荷与吸能性能也随之增加。时效540min试样以手风琴模式变形，其吸能性能较未时效试样提高99%。研究发现，材料的弹性模量与切线模量对于薄壁梁结构的轴向压缩变形模式有较大的影响。提出使用变形模式参考系数φ判断薄壁梁结构的变形行为。随着φ的降低，试样的变形由欧拉模式转变为手风琴模式。同时，铝合金薄壁梁结构的变形模式会随着材料弹-塑性过渡阶段应力-应变曲线变化率的降低逐渐转变为手风琴模式。通过在铝合金薄壁梁结构一端合理开设单排诱导孔，可以使试样的第一峰值载荷降低7%-12%，其变形由欧拉模式转化为混合模式，由钻石模式转化为手风琴模式。与相同时效状态的原始薄壁梁试样相比，开设诱导孔后，试样的变形过程更加规律稳定。在原变形为欧拉模式和钻石模式的铝合金薄壁梁上开设诱导孔后，试样压缩变形的名义载荷增加，吸能性能提高；在原变形为混合模式和手风琴模式的薄壁梁结构上开设诱导孔后，试样的吸能性能略有降低。在180℃条件下时效30min的铝合金薄壁梁试样上开设多排诱导孔，进行了一系列薄壁梁轴向压缩试验，结果表明，诱导孔的位置及孔径对薄壁梁试样的变形行为有较大的影响。当诱导孔直径过大，薄壁梁诱导孔45°对角线位置首先发生失稳变形，试样的压缩变形模式为灯笼模式，是一种全新的薄壁梁压缩变形模式。在该变形模式下，试样的变形载荷波动较小，压溃稳定，但由于可压缩距离减少，试样的总吸能性能有所降低。采用万能试验机进行了标准试样准静态拉伸，S型试样准静态拉伸和薄壁梁试样的轴向压缩试验。在准静态轴向压缩试验过程中，6061铝合金薄壁梁试样发生开裂，而6063铝合金试样未发生失效，但是拉伸试验结果表明两种材料具有相同的延伸率。通过光学显微镜观察拉伸试样表面可知，6061铝合金试样表面出现橘皮状组织，而6063铝合金试样表面光滑。使用金相显微镜观察试样的显微组织，6063铝合金为典型的等轴状再结晶组织，而6061铝合金存在大量弥散的金属间化合物，晶粒组织较粗大。使用Cockcroft-Latham韧性断裂准则预测铝合金材料的失效行为。通过单向拉伸试验和数值仿真相结合的方法，得到了铝合金发生韧性断裂的临界应变能，并通过S形试样的拉伸试验及薄壁梁结构的压缩试验与数值仿真结果对比，验证了Cockcroft-Latham韧性断裂准则的准确性。使用该失效准则预测铝合金的失效行为，具有参数求解方便，计算精度高等优点，非常适用于铝合金车身以及零部件在碰撞过程中失效行为的预测。更多还原

【Abstract】 Thin walled structure with light weight, high strength and good deformationstability are widely used in modern automotive body structure. Typical deformation ofthin-walled structure are bending and axial compression. In this paper, deformationbehaviors and design methods of aluminum thin walled structures were studied. Theresults could be used for the design and optimization of automotive aluminiumstructures.The finite element analysis software based on explicit dynamic method wasapplied to the topology optimization of aluminum bumper using hybrid cellularautomata (HCA) as an optimizing model. The results of topology optimization showthat the simulated annealing method can be used for the optimization design of thebumper thickness. The simulation and experiment of bending, charpy impact andcrash were performed. The results indicate that comparing with steel bumper, themass of aluminum bumper was25%lighter, the flexural strength was52%higher andthe crash energy absorption was45.6%higher. The aluminum bumper designed withHCA topology optimization could meet regulatory requirements with goodcrashworthiness and improve the safety performance of vehicles.The deformation mode and energy absorption of6063aluminum beam wereinvestigated by quasi-static axial compressions. The aluminum samples were aged attemperature of180℃from30min to540min, respectively. The axially compressedexperiments of aluminum thin-walled beam were studied by using WAW-E600universal testing machine. The load was recorded by computer automatically. Theaxial compression performance of aluminum beam with different heat treatment wereevaluated. The results indicate that the deformation mode of the6063beam graduallychanges from Euler mode to Concertina mode, the mean load and energy absorptionincreases with the increasing of aging time. The sample aged for540min deforme s inconcertina mode, the energy absorption increased about99%as compared with theNo-HT sample. The elastic modulus and tangent modulus of thin-walled structure hassignificant impact on the deformation mode during compression. The referencecoefficient of deformation modes φ was proposed to evaluate the deformationbehavior of thin-walled structure. With the reducing of φ, deformation mode of thespecimen changes from Euler mode to Concertina mode. The deformation of aluminum thin-walled structure gradually changes to Concertina mode with thereducing of strain harden rate.By reasonably inducing a single row of holes at the end of aluminum beam, thefirst peak load could be reduced about7%-12%, and the deformation mode is alsochanged. The deformation behaviors change from Euler mode to Mixed mode, or fromDiamond mode to Concertina mode with inducing holes. Comparing with the originalsample in the same aging state, the deformation process of thin-walled beams withinducing hole is more regularity and stability. A series of axial compression tests withmulti-rows of induced holes were studied for30min aged aluminum beams at180℃.The results showed that the position and diameter of induced holes ha s greater impacton the deformation behavior of specimens. Especially, when induced holes diameterare too large, the position at45°diagonal of holes occurred bucking distortion. Thethin-walled beams deforms in a new mode, the Lantern mode. In this deformationmode, the fluctuation of deformation load is low. Because of the less compressiondistance, the total energy absorption of aluminum beams reduced in Lantern mode.Tensile tests with traditional samples and S-shaped samples were carried out. Theaxially compressed experiments of aluminum beam were studied by WAW-E600tester.In the compression process,6061aluminum beams fracture along the specimens, and6063samples deforms without failure. However, the tensile test results show that thetwo materials have the same elongation. Tension specimens was observed by opticalmicroscope, the results showed that the surface morphology of6061alloy was orangepeel, and the surface of6063alloy was smooth. The microstructural of deformedspecimens were observed on MM-6metallographic microscope, it was found that6063was typical equiaxed recrystallized microstructure, while6061was coarse grain.The Cockcroft-Latham ductile fracture criterion was used to predict the deformationcracking. According to uniaxial tensile test results and numerical simulations, thecritical strain energy of aluminum fracture was obtained. Comparison between thepredition and experiment results indicates that Cockcroft-Latham criterion is verysuitable for predicting vehicle structures failure during collision with high accuracy.更多还原