轻薄型金属反平面撕裂机理研究

#x0201c;轻薄型金属(厚度#x02264;10mm)反平面撕裂机理研究#x0201d;旨在为报废汽车及家用电器的破碎回收处理提供理论依据。通过对轻薄型金属破碎过程分析及试验研究,发现#x02162;型裂纹起到了主要破坏作用;根据拉伸试验应力-应变关系曲线变化趋势与理想弹塑性材料基本一致的现象,将材料简化为理想弹塑性模型;裤形撕裂试验所获得的载荷-位移曲线变化趋势表明试样起裂前后所需载荷较大,扩展过程中载荷逐渐减小;对反平面撕裂过程研究可知,在载荷作用下裂尖塑性区逐渐向外扩展,当其达到最大尺寸后,随着载荷继续增加,裂纹则开始扩展,而塑性区也逐渐向前推移直至试样断裂。裂纹尖端场的解表明起裂前及扩展过程中尖端应力-应变场均存在奇异性,且扩展过程中尖端场奇异性比起裂前弱,即起裂前裂尖的应力-应变集中程度比扩展过程中要大,这表明裂纹在起裂阶段比扩展阶段所需载荷更大。     

加工硬化对304不锈钢应力腐蚀裂纹裂尖力学性能的影响

压力管道中应力腐蚀开裂(SCC)是奥氏体不锈钢的主要失效形式之一,同时冷加工变形对材料的力学性能和裂纹的萌生及扩展会产生一定影响。本工作首先利用疲劳拉伸机获取304不锈钢不同冷加工硬化下的材料本构参数,同时利用有限元仿真软件ABAQUS建立了SCC裂纹裂尖宏观分析模型及子模型,研究不同加工硬化下304奥氏体不锈钢材料的SCC裂纹裂尖应力应变、J积分及裂纹扩展速率的影响。结果表明,材料在20%冷加工率变形内,随着材料加工硬化程度的增加,SCC裂纹裂尖Mises应力、J积分逐渐增大,裂纹裂尖应变(PEEQ)减小,一定程度加工硬化会促进和加速304不锈钢发生应力腐蚀开裂。     

载流金属材料裂纹尖端电磁应力的数值模拟

用数值模拟方法研究了金属裂纹尖端电磁应力的分布情况,给出了电流分布、磁场分布和电磁应力的分布.模拟结果表明,金属材料裂纹尖端受的电磁应力是最大的,并且这个力的大小随着电流密度的增加而增大,裂纹尖端的电磁力指向金属的内部.通过具体算例表明,在金属能承受的电流密度下,金属材料裂纹尖端的电磁应力约能达到1MPa的数量级.因此,在研究电磁场处理金属裂纹时,不能忽略电磁应力.     

高温合金CT试样裂纹前沿应力分布及第二相影响的模拟计算

利用有限元方法分析了IN718合金CT试样裂纹区域的应力应变分布、裂纹尖端应力强度因子及第二相状态对它的影响，计算结果与实测数据基本一致，表明采用有限元方法研究含裂纹体构件的蠕变行为是一种较好的研究手段。通过计算认为，IN718合金在时效初期虽然有α-Cr相的析出而不至于促进蠕变裂纹扩展，是由于α—Cr于δ相周围γ″贫化区(微塑性区)内析出而处于微塑性区内使得裂纹尖端钝化，不利于裂纹扩展。     

应变速率对X80管线钢应力腐蚀的影响

利用慢应变速率拉伸试验研究了应变速率对X80管线钢在土壤模拟溶液中的应力腐蚀的影响。采用的模拟溶液以我国西北部碱性土壤的化学成分为基础,在不同应变速率条件下进行试验。样品断裂后利用扫描电镜对断口形貌以及断口侧面二次裂纹进行观察。研究结果表明:X80钢在1.0×10-6s-1应变速率下表现出最高的应力腐蚀敏感性。低于该应变速率下,应力腐蚀敏感性略有降低;而高于该应变速率下,应力腐蚀敏感性明显减小。不同应变速率下应力腐蚀敏感性的差异主要是由应力腐蚀过程中腐蚀和力学作用的影响程度不同造成。应变速率低于1.0×10-6s-1时,腐蚀作用影响更大,较长的腐蚀时间造成裂纹被腐蚀,裂纹扩展受到影响,因此应力腐蚀敏感性略有降低。当应变速率高于1.0×10-6s-1时,力学作用主导整个过程,形成的裂纹没有受到足够腐蚀的情况下,在力学作用下发生快速机械扩展、断裂,因此产生了明显降低的应力腐蚀行为。     

外加电位对X80钢在玉门土壤模拟溶液中应力腐蚀的影响

采用慢应变速率拉伸实验研究X80钢在土壤模拟溶液中的应力腐蚀敏感性。以玉门地区的碱性土壤为基础,分析不同电位对应力腐蚀的影响。用扫描电镜对断口及二次裂纹形貌进行观察。结果表明:阳极电位下X80钢应力腐蚀敏感性不高。高阳极电位下,阳极溶解在一定程度上抑制了应力腐蚀。阳极电位较弱以及开路电位下,阳极溶解较弱,裂尖和其他表面存在溶解性差异,这些因素有利于裂纹扩展。但是较慢的溶解速率以及相对高的应变速率使得裂纹没有足够时间发生有效扩展,应力腐蚀敏感性仍然较低。当外加阴极电位时,裂尖发生阳极溶解而其他位置受到阴极反应抑制,应力腐蚀敏感性增加。随着阴极电位降低,不断增加的氢影响裂纹萌生和扩展,应力腐蚀敏感性随外加电位的降低而增大。     

7A52铝合金焊接件应力腐蚀性能评价

应用慢应变速率拉伸应力腐蚀实验方法和恒载荷拉伸应力腐蚀实验方法评价了7A52铝合金焊接试样的应力腐蚀(SCC)敏感性,并对断口微观形貌进行了分析.结果表明:使用5A56焊丝,采用金属焊条惰性气体焊接(MIG)工艺双面焊制成7A52焊接件应力腐蚀敏感性比较低,具有较好的抗应力腐蚀开裂性能;但当使用环境温度较高、施加应力大于90％σp0.2时,也有可能发生应力腐蚀开裂.断口微观分析表明焊接部位普遍存在气孔;高温或高应力下产生SCC开裂的断口存在明显的二次裂纹,并且随着应力水平的增加,二次裂纹增大.     

梯度复合材料应力强度因子计算的梯度扩展单元法

推导了一种适用于梯度复合材料断裂特性分析的梯度扩展单元,采用细观力学方法描述材料变化的物理属性,通过线性插值位移场给出了4节点梯度扩展元随空间位置变化的刚度矩阵,并建立了结构的连续梯度有限元模型.通过将梯度单元的计算结果与均匀单元以及已有文献结果进行对比,证明了梯度扩展有限元(XFEM)的优越性,并进一步讨论了材料参数对裂纹尖端应力强度因子(SIF)的影响规律.研究结果表明:随着网格密度的增加,梯度单元的计算结果能够迅速收敛于准确解,均匀单元的计算误差不会随着网格细化而消失,且随着裂纹长度和属性梯度的增大而增大;属性梯度和涂层基体厚度比的增大导致涂覆型梯度材料的SIF增大;裂纹长度的增加和连接层基体厚度比的减小均导致连接型梯度材料的SIF增大.     

梯度复合材料应力强度因子计算的梯度扩展单元法

推导了一种适用于梯度复合材料断裂特性分析的梯度扩展单元, 采用细观力学方法描述材料变化的物理属性, 通过线性插值位移场给出了4节点梯度扩展元随空间位置变化的刚度矩阵, 并建立了结构的连续梯度有限元模型。通过将梯度单元的计算结果与均匀单元以及已有文献结果进行对比, 证明了梯度扩展有限元(XFEM)的优越性, 并进一步讨论了材料参数对裂纹尖端应力强度因子(SIF)的影响规律。研究结果表明: 随着网格密度的增加, 梯度单元的计算结果能够迅速收敛于准确解, 均匀单元的计算误差不会随着网格细化而消失, 且随着裂纹长度和属性梯度的增大而增大; 属性梯度和涂层基体厚度比的增大导致涂覆型梯度材料的SIF增大; 裂纹长度的增加和连接层基体厚度比的减小均导致连接型梯度材料的SIF增大。     

基于振动时效仿真的流变塑性模型

振动时效是一种局部循环塑性现象,当循环载荷与材料内部残余应力叠加超过材料的局部屈服强度时就会发生残余应力释放。采用流变塑性模型对振动时效进行仿真,分析了振动时效过程中应力幅、应变幅、振动频率、振动周期和材料屈服应力等对振动时效的影响。结果表明,振动时效中应力释放很大程度上取决于应力幅或应变幅,振动频率和材料参数也是关键因素,而振动周期或时间对振动时效没有很大影响。将此振动时效模型应用于7075铝合金试样机械载荷下的应力松弛实验,所得结果与仿真较一致。     

2D modelling of inclined intergranular stress corrosion crack paths

Inclined high pH stress corrosion cracking (SCC) is a type of intergranular environmental cracking in gas pipelines, which differs from typical SCC by propagating at an angle from the wall direction. Investigations of Australian and Canadian inclined SCC colonies have not provided a clear indicator of a cause for the abnormal crack growth direction. This paper addresses the possibility of crack tip strain enhanced electrochemistry causing the inclination. Potentiodynamic tests were conducted to quantify the influence of strain on the electrochemistry, and strain was found to increase current density up to 300% in the SCC region. A model was developed that incorporates crack tip strain driven SCC growth, which showed good agreement with field grown cracks, and the aspect ratio of the grains was shown to have an effect on the inclination angle. The results indicate that crack tip strain enhanced electrochemistry is a plausible cause for inclined SCC.     

Effect of hydrogen on stress corrosion cracking of copper

The effects of hydrogen on electrochemical behavior and susceptibility of stress corrosion cracking (SCC) of pure copper were studied. SCC susceptibility of pure copper in a 1 M NaNO₂ solution was increased by pre-charged hydrogen. The effect of hydrogen on the susceptibility is more obvious in the low stress region due to the longer fracture time, which resulted in a longer time for more hydrogen to diffuse toward the crack tip. Synergistic effects of hydrogen and stress on corrosion and SCC processes were discussed. The results showed that an interaction between stress and hydrogen at the crack tip could increase the anodic dissolution rate remarkably.     

ESEM observations of SCC initiation for 4340 high strength steel in distilled water

The stress corrosion cracking (SCC) initiation process for 4340 high strength steel in distilled water at room temperature was studied using a new kind of instrument: an environmental scanning electron microscope (ESEM). It was found that the applied stress accelerated oxide film formation which has an important influence on the subsequent SCC initiation. SCC was observed to initiate in the following circumstances: (1) cracking of a thick oxide film leading to SCC initiation along metal grain boundaries, (2) the initiation of pits initiating SCC in the metal and (3) SCC initiating from the edge of the specimen.All these three SCC initiation circumstances are consistent with the following model which couples SCC initiation with cracking of a surface protective oxide. There is a dynamic interaction between oxide formation, the applied stress, oxide cracking, pitting and the initiation of SCC. An aspect of the dynamic interaction is cracks forming in a protective surface oxide because of the applied stress, exposing to the water bare metal at the oxide crack tip, and oxidation of the bare metal causing crack healing. Oxide crack healing would be competing with the initiation of intergranular SCC if an oxide crack meets the metal surface at a grain boundary. If the intergranular SCC penetration is sufficiently fast along the metal grain boundary, then the crack yaws open preventing healing of the oxide crack. If intergranular SCC penetration is not sufficiently fast, then the oxidation process could produce sufficient oxide to fill both the stress corrosion crack and the oxide crack; in this case there would be initiation of SCC but only limited propagation of SCC. Stress-induced cracks in very thin oxide can induce pits which initiate SCC, and under some conditions such stress induced cracks in a thin oxide can directly initiate SCC.     

On the mechanism of intergranular stress corrosion cracking of sensitized stainless steel in tetrathionate solution

Intergranular stress corrosion cracking (SCC) mechanism in sensitized stainless steel (Type 304) was investigated experimentally. A tetra-thionic potassium (K₂S₄O₆) chemical solution was used to mimic polythionic acid SCC which the most aggressive SCC type. During the SCC test, the steel specimen was subjected to three-point bending with constant strain at room temperature, and simultaneous monitoring of acoustic emission and corrosion potential were employed to monitor SCC initiation and progression. At the early stage, transient phenomenon of local anodic dissolution was observed. Upon initiation of SCC, passivation film fracture and dissolution of metal at specimen surface take place. Through microscopic observation of SCC tip, it was found that the SCC tip advanced along the grain boundary with further mechanical loading. This suggested that the stress component plays a significant role of SCC propagation, in addition to the effect of the localized metal dissolution along Cr-depleted grain boundaries.     

Effect of hydrogen on stress corrosion cracking of copper

The effects of hydrogen on electrochemical behavior and susceptibility of stress corrosion cracking (SCC) of pure copper were studied. SCC susceptibility of pure copper in a 1 M NaNO₂ solution was increased by pre-charged hydrogen. The effect of hydrogen on the susceptibility is more obvious in the low stress region due to the longer fracture time, which resulted in a longer time for more hydrogen to diffuse toward the crack tip. Synergistic effects of hydrogen and stress on corrosion and SCC processes were discussed. The results showed that an interaction between stress and hydrogen at the crack tip could increase the anodic dissolution rate remarkably.     

The mechanism of hydrogen embrittlement: the stress interaction between a crack, a hydrogen cluster, and moving dislocations

Mechanisms of dissolvent anodic chemical reaction and hydrogen embrittlement have been proposed as stress corrosion cracking (SCC) mechanisms. The former is feasible for the case of plastic deformation dominant metals (low-yield stress), and the latter is for high-strength metals such as high-strength steels. However, in spite of low-yield stress, a discontinuous cleavage-like fracture is sometimes observed during SCC for ductile fcc alloys, which concerns the interaction between dislocations and the hydrogen cluster. The problem of when these mechanisms will be dominant remains. In this paper, the stress corrosion cracking model on the basis of hydrogen diffusion and concentration toward the elastic-plastic stress field around a crack and the interaction of dislocations and hydrogen around a crack tip are proposed to clarify the mechanism of stress corrosion cracking for ductile and brittle materials. We conducted numerical analyses using these proposed models.     

Study on cracked plates, shells and three dimensional bodies

This paper presents a summary of the authors' recent work in following areas: (1) The stress-strain fields at crack tip in Reissner's plate. (2) The calculations of the stress intensity factors in finite size plates. (3) The stress-strain fields at crack tip in Reissner's shell. (4) The calculations of the stress intensity factors and bulging coefficients in finite size spherical shells. (5) The stress-strain fields along crack tip in three dimensional body with surface crack. (6) The calculation of stress intensity factors in a plate with surface crack.     

Investigation of plastic zones near SCC tips in a pipeline after hydrostatic testing

Stress corrosion cracking (SCC) is an important failure mechanism for oil and gas pipelines. In the past, hydrostatic testing has been frequently used to assess and mitigate stress corrosion cracking. It is commonly agreed that an effective hydrostatic test not only eliminates critical crack-like flaws, but also blunts the sub-critical crack tip thereby suppressing further SCC propagation. However, little study has been done on the plastic deformation that results from the high stress intensity at the crack tip due to hydrostatic testing pressure and its possible role in subsequent SCC propagation. In this study, microstructural details were examined of an API 5L X52 SCC-containing pipe removed from field service. Plastic deformation generated by the hydrostatic testing pressure was revealed by using high-resolution imaging of a focused ion beam (FIB) microscope. The existence of the microscopic plastic zones around some crack tips suggests that caution should be taken when setting up pipeline hydrostatic tests.     

Two-Parameter (J-M) Description of Crack Tip Stress-Fields for an idealized weldment in small scale yielding

The crack tip stress-field in a trimaterial finite element model has been examined. The model represents an idealised steel weldment with a crack located at the fusion line. The model was loaded with a K _I displacement field to simulate small scale yielding conditions. The effect of changing the weld metal plastic properties and the HAZ layer thickness on the crack tip stress-field was studied, keeping the material properties of the HAZ and base metal constant. The results show that the calculated J-integral remains path independent in the trimaterial model. It is confirmed that the crack tip stress-fields can be normalised by the J-integral. The mismatch constraint can be characterised by a difference field, which is independent of the normalised distance from the crack tip. The results show that changes of HAZ thickness only have a small effect on the stress-fields close to the crack tip. The hardenability of the weld metal influences on the slope of the crack tip stress distribution, but for small changes in hardenability, this effect can be neglected. The results indicate that the difference fields show some radial dependence when a homogeneous reference field is used, but the radial dependence was removed by introducing an inhomogeneous reference field. The effect of changes in the weld metal yield strength has been described with a two parameter (J-M) formulation using the inhomogeneous reference field.