延性金属材料力学性能评价新方法-撕裂试验

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	延性金属材料力学性能评价新方法-撕裂试验
	李翠红
学位类型	博士
导师	张哲峰
	2012
学位授予单位	中国科学院金属研究所
学位授予地点	北京
学位专业	材料物理与化学
关键词	延性金属拉伸强度塑性撕裂试验撕裂韧性裂纹扩展 Ductile Metals Tensile Test Strength Ductility Trousers Tearing Test Tearing Toughness Crack Propagation
其他摘要	" 在材料科学与工程应用中，高强度是人们一直追求的目标，但是，强度的提高往往伴随着塑性的损失，工程应用中的材料需要具有良好的综合性能，但强度与塑性之间出现的这种倒置关系给工程选材带来了很大的困扰。如何选择综合性能更好的材料？这对材料力学性能测试方法提出了更高要求，使得材料力学性能测试的评价、优化成为亟待解决的问题。另一方面，延性金属材料的断裂，因其过程复杂且对试样形状及加载方式都有依赖，断裂韧性KC或GC已经无法适用，尤其对于延性金属薄板来说，尺寸上更是无法满足要求，这需要新的力学性能测试方法、参数指标来表征。本论文从材料力学性能评价、优化的角度出发，来尝试探索新的试验方法或性能指标、参数，用以表征延性金属材料的撕裂韧性与断裂行为。在延性金属材料的撕裂裂纹扩展过程中，裂纹尖端所处的应力状态是一样的，假设在撕裂过程中载荷是恒定不变的，则裂纹就稳定扩展。根据这一假设，尝试用“撕裂韧性”这一概念来表征延性金属材料抵抗裂纹稳定扩展的能力，用符号g表示，表达式为g =F/t。以这一研究思路为前提，采用不同厚度冷轧铝、铜薄板作为研究对象，分别进行了自由型、限制型、疲劳型、裤型加载撕裂等形式的撕裂实验，发现当采用三腿裤型加载的方式进行撕裂试验时，撕裂过程中撕裂载荷基本维持稳定；在实验基础上提出撕裂韧性的概念，即裂纹稳定扩展单位面积所需要外力做的功，以此来表征延性金属材料抵抗裂纹稳定扩展能力的大小，单位则为J/m2，将描述撕裂韧性的表达式修正为g =FS/2t，并将其定义、公式应用于几种常见金属的撕裂中。将定义的撕裂韧性用于不同强度20钢的性能评价中，发现20钢的三腿裤型撕裂试验表现出稳定的撕裂过程，相应的撕裂韧性并不是材料常数，与其微观组织和拉伸性能有密切联系。而且，20钢在920 °C水淬再400 °C回火后具有最高的撕裂韧性，高的撕裂韧性对应着高的强度和好的塑性，是强度和塑性的综合表现。20钢的裤型撕裂试样的缺口宽度Wn影响着撕裂行为，撕裂裂纹扩展过程中向内汇聚还是向外发散是由试样总宽度W和其缺口宽度Wn决定的。如果缺口宽度远小于试样宽度的一半，即WnW/2时，撕裂裂纹会向外发散扩展，待撕裂的部分会沿着顺时针方向转动。利用三腿裤型加载方式对铜及铜合金进行了撕裂试验，并计算了合金元素铝对铜铝合金撕裂韧性的影响。结果显示：撕裂韧性与厚度相关，材料越厚，撕裂越难进行，相应的撕裂韧性数值也就越大，但不同厚度下材料撕裂韧性的变化规律相同；缺口宽度改变时，得到的撕裂韧性数值也会不等，不同缺口尺寸下撕裂韧性的变化趋势也是一致的。铜及铜合金的撕裂韧性也不是材料常数，随着加入的合金元素不同，对撕裂韧性的影响也不相同，这种影响与材料的微观组织、力学性能相关。对于Cu-Al合金，通过降低层错能或增加铝含量的方式，可以同时提高Cu-Al合金的强度、塑性和撕裂韧性；撕裂韧性和强度、塑性的这种同步增长趋势说明高的撕裂韧性对应着高的强度和好的塑性。对于Cu-2%Be合金来说，撕裂韧性则随强度的增加呈现先上升后下降的趋势，撕裂韧性出现的这种极值点，为寻找材料的强度和塑性配合提供了线索。撕裂裂纹由剪切方式引起的萌生，需要越过一定的能垒，故而撕裂载荷—位移曲线上有极值出现；由颈缩和剪切同时作用引起的裂纹萌生，可以通过能量的缓慢积累而实现，撕裂载荷—位移曲线上就没有载荷极值的出现。改变材料的加工状态，进行不同锰含量高锰钢的裤型加载撕裂试验，结果表明：高锰钢的撕裂韧性仍然不是恒定的材料常数，随着强度的升高，撕裂韧性的数值总体呈现先上升后下降的趋势；随着锰含量的增加，撕裂韧性数值总体也是先上升后下降的趋势，且热轧处理后高锰钢的综合性能得到很好的提高。15Mn钢的撕裂断口上有明显的分层、解理现象，在此条件下所得到的撕裂韧性数值不能真实反映出材料撕裂裂纹扩展面上能量的情况，不适于用撕裂试验和撕裂韧性来表征。根据已有数据，延性金属的撕裂韧性数值可依材料类别而有不同范围，其中钢的撕裂韧性范围一般在~250 KJ/m2到~350 KJ/m2之间，铜及铜合金的撕裂韧性在140 KJ/m2到225 KJ/m2的范围内，铝及铝合金的撕裂韧性在70 KJ/m2上下。延性金属材料的撕裂韧性与其力学性能相关，撕裂韧性的变动也伴随着强度的变化。" ; " Throughout history of materials science and engineering, there has been a never-ending effort to develop new materials with high strength. However, the increase in strength is usually accompanied with a loss of ductility, of which the trade-off between strength and ductility has made great trouble for the selection among various engineering materials, because the strength is one of the important parameters and usually a material must provide comprehensive properties. How does the strength match the ductility becomes the key of the problem, and this indicates that the mechanical properties of materials should be evaluated more rationally. On the other hand, for high-strength or brittle metals, the fracture toughness (KC or GC) has been widely employed to characterize their properties, and yet for those ductile metals, the fracture processes are more complex and strongly depend on the specimen geometry and loading configuration. Undoubtedly, the fracture toughness is invalid to evaluate the toughness of ductile materials; besides, for these thin sheets of ductile metals conventional testing geometries are difficult to use. To solve these problems, attempt has been made to use new methods, or parameters to describe the fracture process of ductile metallic materials. During tearing crack propagation of the ductile metals, the stress states of the crack tip are always the same, then it is assumed that if the load in the whole tearing processing is constant, the tearing crack will propagate steadily, accordingly, a concept of tearing toughness, g =F/t, is proposed to describe the resistance to the crack propagation. Based on the assumption, free form, constrained form, fatigued form, and trousers tearing tests were used to tear cold-rolled aluminum, and copper sheets with different thickness. The applied load will maintain steady in the tearing processing when the trousers tearing tests are used, and assumption can be achieved in the three-leg trousers tearing processing. Based on the experiments, concept of tearing toughness, with a unit of J/m2, can be defined to represent the fracture work dissipated per unit area of a tearing crack in evaluating the resistance to the crack propagation of ductile materials, and formula can be modified as: g =FS/2t. Consequently, definition and formula of tearing toughness can be used for several ductile metals. Concept of tearing toughness is used to evaluate the properties of 20 steel with different strengths. It is revealed that three-leg trousers tearing processing of 20 steel sheets is steady, and tearing toughness of 20 steel is not material constant, it depends on its microstructure and tensile properties. Furthermore, tearing toughness is the highest when the 20 steel was quenched at 920 °C then tempered at 400 °C, and higher tearing toughness represents the better comprehensive property, including strength and ductility. Notch width Wn of tearing specimens could affect tearing behavior of the 20 steel. Converging or diverging of tearing cracks depends on the tearing specimen width Wand notch width Wn. If the notch width Wn is less than half of the sample width (WnW/2), the cracks will diverge outside, and the untorn part of the sheet will rotate clockwise. Therefore, the notch width plays an important role in the tearing processing. Copper and copper alloys are also tested by using three-leg trousers tearing test, and effects of aluminum on tearing toughness of those Cu-Al alloy with different aluminum contents was calculated. It is found that the tearing toughness relies on material thickness, the thicker, the more difficult to tear, correspondingly, tearing toughness is much higher, though thickness is different, the variation tendency of tearing toughness is the same; as well as the notch width, variation tendency of tearing toughness is changeless when the notch width varies. Tearing toughnesses for copper and copper alloys are also not materials constant, effects of alloying elements on tearing toughness vary with elements: With increasing the Al contents or decreasing the stacking fault energy, the uniform elongation and tensile strength of Cu-Al alloys display a synchronously increasing trend; meanwhile the tearing toughness also rises rapidly in Cu-Al alloy with higher Al content, the synchronous increase indicates the combination of higher strength and better ductility. Tearing toughness of Cu-2%Be alloy rises first then drops, there is a peak point for tearing toughness. Peak point of the tearing toughness provides clue for the combination of strength and ductility. Energy barrier has to cross for those cracking initiation induced by shearing, so there will be a peak point on the tearing-load curve; for those cracking initiation induced by necking and shearing, energy accumulation can be achieved slowly, energy barrier will disappear and no peak point appears on the tearing load-displacement curve any more. Effects of processing state on the tearing toughness of ductile metals are also considered. Hot-rolled and solution-treated high manganese steel sheets with different manganese contents were tested using three-leg trousers tearing test. Similarly, it is shown that tearing toughness of high manganese steel is not material constant, and will increase first then decrease with increasing strength and manganese contents; comprehensive properties are much better for the hot-rolled high manganese steel. There are delaminating and cleavage grains on the tearing fractures of 15Mn steel, calculation of tearing toughness can not represent the tearing energy on the cracking surface, so using trousers test and tearing toughness to describe the fracture processing of 15Mn steel are not feasible. According to the results tested by trousers tearing test, tearing toughness of different types of ductile metals can be classified, tearing toughness of steels may be in the range of ~250 KJ/m2 to ~350 KJ/m2, between 140 KJ/m2 and 225 KJ/m2 for copper and copper alloys, about 70 KJ/m2 for aluminum and aluminum alloys. Tearing toughness of ductile metals is dependent on the mechanical properties, and varies with their strength."
文献类型	学位论文
条目标识符	http://ir.imr.ac.cn/handle/321006/64420
专题	中国科学院金属研究所
推荐引用方式 GB/T 7714	李翠红. 延性金属材料力学性能评价新方法-撕裂试验[D]. 北京. 中国科学院金属研究所,2012.