16MnR钢板冲击试验的研究

一、前言用冲击试验方法来判断材料的韧性,简单而又方便,在一定程度上反映材料缺陷尖端处抵抗塑性变形的能力,广泛应用于材料的验收和评定。冲击试样的缺口形状有多种。美、英、法、西德、日本等国以及国际标准“ISO”中冲击试验均采用夏比 V 试样。我国以前均采用 U 型缺口,即梅氏试样。经大量使用证明:梅氏试样缺口太钝,不能确切地模拟实际结构中可能出现的缺陷尖端所产生的应力状态,来评定材料的韧性。因此,近年来用 V 型缺口代替 U 型缺口的冲击试验,愈来愈得到重视和应用。国内曾有很多     

TC21钛合金缺口疲劳试样的应力分布计算

采用有限元程序计算TC21钛合金圆形缺口试样不同载荷下缺口根部的应力分布，以及循环载荷下缺口根部的循环应力比的变化。结果表明，缺口根部的轴向应力集中因子Kσ和应变集中因子Kε与载荷大小有关，但变化趋势不同。缺口根部区处于三向应力状态，尽管三向应力的最大值的位置随载荷变化而变化，但等效应力最大值在表面。根部位置切向应力与轴向应力的比值随着载荷的变化而变化。由于缺口根部的塑性变形，随载荷升高，缺口根部的应力比降低，当名义应力超过1100MPa时，应力比反而增加。在疲劳过程中，高循环应力时，缺口根部的应力比也有一定变化。     

利用红外热像法测定AZ31B镁合金应力集中系数

利用红外热像法测量AZ31B镁合金双V型缺口试样的应力集中系数,并与Abaqus模拟结果进行对比。结果表明,红外热像法得到的双V型缺口试样的应力集中系数为2.14,而利用有限元模拟得到的应力集中系数为2.20,较红外热像法的误差为2.8%。利用红外热像法测得的应力集中处的温度下降值为-0.36 K;而有限元模拟在不考虑热传导时的结果为-0.37 K,考虑热传导的结果为-0.20 K,与红外热像法所得结果的误差分别为2.8%和44.4%。利用红外热像法能快速且精确的预测AZ31B镁合金应力构件的集中系数。     

不同尺寸试样冲击功等效比值的研究

通过夏氏Ⅴ型缺口试样冲击试验,揭示了结构钢小尺寸试样冲击功与标准试样冲击功的比值在系列温度冲击试验中的变化规律,研究分析了确定不同尺寸试样冲击功的各种等效比值的基本原理。主要结果为,在冲击试验的上平台温度,小尺寸试样与标准试样的冲击功比值是一与钢号无关的常数项;在不同冲击试验温度下,小尺寸试样与标准试样平均冲击功的比值将趋于一个定值。     

残余应力集中及其对疲劳极限和短裂纹扩展的影响

缺口根部的残余应力存在应力集中效应,应力集中值不仅取决于缺口几何形状而且和材料的屈服强度有关。残余应力集中系数是一个分布值,其数值甚至可超过理论应力集中系数。软试样喷丸后缺口处的残余应力易于松弛及喷丸时的表面损伤使其对疲劳极限的贡献小。残余压应力使缺口处裂纹闭合力增大甚至可出现非扩展裂纹。     

管线钢及其焊接接头示波冲击试验中E_i和E_p的关系

本文通过WX52管线钢及其焊接接头的Charpy V 型缺口(CVN)试样和预制疲劳裂纹的Charpy V 型缺口(PCVN)试样的示波冲击试验,讨论了区分E_i 和E_p 的意义及其与E_t 的关系(E_i 和E_p 分别为载荷—挠度曲线上最大载荷前后的能量,E_t 为总冲击能量,E_t=E_i+E_p),并研究了应力集中以及试验温度等因素对E_i/E_t 及E_p/E_t 值的影响。结果表明,在本试验条件下,E_p与E_t 成线性关系,这一关系式对材料的显微组织、材料的状态(轧制态或铸态)以及材料的断裂模型不敏感。当缺口前端应力集中程度增大时,E_i/E_t 值减小而E_p/E_t 值增大;在E_i/E_t—T℃曲线上T_d 温度(高于该温度,缺口延性起裂)时,F_i/E_t 有一峰值,而E_p/E_t 为谷值。     

缺口对TC17钛合金拉伸性能和低周疲劳性能的影响

研究了不同缺口(缺口应力集中系数)对TC17钛合金拉伸性能和低周疲劳性能的影响。结果表明,缺口试样抗拉强度随着缺口应力集中系数的增大而提高,伸长率随着缺口应力集中系数的增大而减小;低周疲劳寿命随缺口应力集中系数的增加而降低,当应力集中系数大于1.92时,疲劳寿命急剧下降。断口分析表明,试样拉伸断口由正应力拉断区和剪切断裂区两个典型区域构成。正应力拉断区为穿晶和沿晶相混合的断裂,剪切断裂区为穿晶断裂,断口上均为韧窝形貌。低周疲劳断口由疲劳断裂区、正应力拉断区和剪切断裂区三个典型区域构成。疲劳断裂区比较平坦,有明显的疲劳条带和平行的二次裂纹;低周疲劳断口的正应力拉断区和剪切断裂区的形貌特征与拉伸断口类似。     

双态组织高Nb-TiAl合金的缺口敏感性

通过对圆棒状缺口试样和无缺口光滑试样进行单向拉伸试验,研究了双态组织高Nb-TiAl合金的缺口敏感性。研究表明,缺口根半径R≥0.5 mm的U型试样和缺口角度60°、R≥1 mm的V型试样的抗拉强度RmN都大于光滑试样的抗拉强度Rm,缺口敏感度NSR均大于1,合金对缺口不敏感。当缺口类型相同时,缺口敏感性随缺口根半径R的减小而增大;V型缺口敏感性大于U型缺口;对于V型缺口试样,当R=0.5 mm时,其NSR在0.98~1.03之间,RmN在Rm附近波动,易出现缺口敏感。     

夏比冲击试验方法对比

从适用范围、试样要求、设备要求、试验过程、试验结果评定等方面对现行三项冲击试验标准中的夏比冲击试验方法进行对比.对比发现:ASTM A 370只规定了夏比冲击V型缺口,ASTM E 23和GB/T 229规定的夏比冲击缺口类型包括V型缺口、U型缺口以及无缺口试样;ASTM A 370和ASTM E 23规定摆锤刀刃半径...     

铝合金在两种应力状态下损伤机理研究

铝合金汽车构件在撞击的过程中,构件的应力状态各点均不相同,而且在撞击的过程中各点的应力状态随着时间变化而变化.为了研究铝合金构件在不同应力状态下损伤机理,采取了两种代表性的三轴应力状态,即缺口拉伸与纯剪切所产生的应力状态.研究结果表明缺口拉伸试验中,缺口根部产生相对较高的三轴应力,而且缺口根部明显产生微孔洞,随着应力的不断升高,微孔洞的体积分数不断增大.当达到材料的临界孔洞体积分数时,试样断裂;纯剪切试验中,三轴应力几乎等于0,在材料内部几乎没有产生微孔洞而产生了剪切带.剪切带在切应力的作用下变形不断增大,当到达材料的等效塑性断裂应变时,试样发生断裂;用Gurson损伤模型和剪切失效分别模拟缺口拉伸和剪切试验,试验的工程应力-应变曲线与模拟的工程应力-应变曲线拟合的很好.     

Notch effect on the tensile properties of cold-rolled Ni3Al foils

Notch effect on the tensile properties of the Ni₃Al foils has been investigated as a function of notch geometry and foil’s thickness. Tensile tests along the rolling direction (RD) and the transverse direction (TD) showed that notch weakening occurred with introduce of a notch. Plastic deformation was observed locally at notch root and the effective stress concentration factor (k_e) was much lower than the theoretical stress concentration factor (K_t). Contrary to the anisotropy in fracture stress along the RD and the TD, k_e fell on the same curve. Crack initiation mechanism was different between two tensile directions, that is, cracks initiated along the shear band in the RD tension, while cracks initiated homogenously on the slip planes in the TD tension.     

Damage and fracture mechanism of 6063 aluminum alloy under three kinds of stress states

To study the damage and fracture mechanism of 6063 aluminum alloy under different stress states,three kinds of representative triaxial stress states have been adopted,namely smooth tensile,notch tensile,and pure shear.The results of the study indicate the following.During the notch tensile test,a relatively higher stress triaxiality appears in the root of the notch.With the applied loading increasing,the volume fraction of microvoids in the root of the notch increases continuously.When it reaches the critical volume fraction of microvoids,the specimen fractures.During the pure shear test,the stress triaxiality almost equals to zero,and there is almost no microvoids but a shear band at the center of the butterfly specimen.The shear band results from nonuniform deformation constantly under the shear stress.With stress concentration,cracks are produced within the shear band and are later coalesced.When the equivalent plastic strain reaches the critical value（equivalent plastic fracture strain）,the butterfly specimen fractures.During the smooth tensile test,the stress triaxiality in the gauge of the specimen remains constant at 0.33.Thus,the volume of microvoids of the smooth tensile test is less than that of the notch tensile test and the smooth specimen fractures due to shearing between microvoids.The G-T-N damage model and Johnson-Cook model are used to simulate the notch tensile and shear test,respectively.The simulated engineering stress-strain curves fit the measured engineering stress-strain curves very well.In addition,the empirical damage evolution equation for the notch specimen is obtained from the experimental data and FEM simulations.     

Determination of the critical hydrogen concentration for delayed fracture of high strength steel by constant load test and numerical calculation

The critical hydrogen concentration for hydrogen induced delayed fracture of the AISI 4135 steel at 1320 and 1450 MPa has been determined by constant load tests in combination with numerical calculations, and thus the concept of a critical hydrogen concentration has been verified. The time to fracture was obtained for circumferentially notched round bar specimens under a constant load after electrochemically pre-charged with various hydrogen contents. A numerical model was then developed for calculating the accumulated hydrogen concentration in the vicinity of the notch root, taking into account the driving effect of the hydrostatic stress on hydrogen transport. The results showed that the delayed fracture of the steel occurred when a critical hydrogen concentration at the location of the stress peak was reached by accumulation, and that the time to fracture was related to the stress-driven hydrogen accumulation process. The critical hydrogen concentration was dependent not only on the strength level, but also on the stress concentration factor of the specimens.     

Effect of hydrogen on the fracture behavior of high strength steel during slow strain rate test

The effect of hydrogen on the fracture behavior of the quenched and tempered AISI 4135 steel at 1450 MPa has been investigated by means of slow strain rate tests on smooth and circumferentially-notched round-bar specimens. Hydrogen was introduced into specimens by electrochemical charging and its content was measured by thermal desorption spectrometry (TDS) analysis. Results showed that the steel had high hydrogen embrittlement susceptibility. For both smooth and notched specimens, the fracture mode was changed from microvoid coalescence (MVC) to brittle intergranular (IG) fracture after the introduction of a small amount of diffusible hydrogen. Fracture initiated in the vicinity of the notch root for notched specimens, while it started from around the center in smooth specimens. The fracture stress decreased with increasing diffusible hydrogen content, and the decreasing trend was more prominent for specimens with a higher stress concentration factor. Taking into account the stress-driven hydrogen diffusion and accumulation in the vicinity of the notch root, the local diffusible hydrogen concentration and local fracture stress in notched specimens have been calculated. According to numerical results, the relationship between the local fracture stress and local diffusible hydrogen concentration was independent of stress concentration factor, which could account for the effect of hydrogen on the fracture stress of the steel.     

Damage Progression and Stress Redistribution in Notched SiC/SiC Composites

The stress fields adjacent to machined notch roots were examined during tensile tests of woven SiC/SiC composites using thermoelastic stress analysis. As expected, the stress-concentration factor (SCF) at the notch roots increased with increasing notch lengths. Damage-induced stress-relief was not evident in these composites. In fact, the redistribution of stresses onto isolated fiber tows adjacent to the notch root caused an increase in the apparent SCF. Lastly, the SCF increased with mean stress. This was assumed to occur as a result of opening matrix cracks that are typically closed due to residual compressive stresses in the matrix material at near zero loads.     

Measuring the stress field around an evolving crack in tensile deformed Mg AZ31 using three-dimensional X-ray diffraction

The stress field around a notch in a coarse grained Mg AZ31 sample has been measured under tensile load using the individual grains as probes in an in situ high energy synchrotron diffraction experiment. The experimental set-up, a variant of three-dimensional X-ray diffraction microscopy, allows the position, orientation and full stress tensor of each illuminated grain to be determined and, hence, enables the study of evolving stress fields in coarse grained materials with a spatial resolution equal to the grain size. Grain resolved information like this is vital for understanding what happens when the traditional continuum mechanics approach breaks down and fracture is governed by local heterogeneities (e.g. phase or stress differences) between grains. As a first approximation the results obtained were averaged through the thickness of the sample and compared with an elastic–plastic continuum finite element simulation. It was found that a full three-dimensional simulation was required to account for the measured transition from the overall plane stress case away from the notch to the essentially plane strain case observed near the notch tip. The measured and simulated stress contours were shown to be in good agreement except at the highest applied load, at which stress relaxation at the notch tip was observed in the experimental data. This stress relaxation is attributed to the initiation and propagation of a crack. Finally, it was demonstrated that the measured lattice rotations could be used as a qualitative measure of the shape and extent of the plastic deformation zone.     

Notch sensitivity of heavily cold-rolled Ni3Al foils

The notch sensitivity of 95% cold-rolled Ni₃Al foils was examined using double U-notched tensile specimens. The foils were found to be essentially insensitive to the presence of the notch. Plastic deformation occurred locally at the notch root to relieve the stress concentration, resulting in the notch insensitivity.     

THEORETICAL AND EXPERIMENTAL STUDIES ON FRACTURE PLANE CONTROL BLAST WITH NOTCHED BOREHOLES

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应力/应变场对插销试验焊接接头氢扩散的影响

采用ABAQUS有限元分析软件及有关焊接接头中氢扩散与集聚的计算模拟技术计算分析了不同外加载荷条件下插销试件的能力、应变场及其对氢扩散与聚集过程的影响。计算结果表明，静载拉伸条件下插销缺口前沿有明显的应力应变集中，随着外加载荷的增加，缺口尖端应力逐渐增大，但应力集中系数逐渐减小；在相同初始氢条件下，随着外加载荷的增加，缺口前沿应力应变集中部位氢的峰值浓度逐渐提高，氢浓度达到峰值的时间也逐渐增长。分析认为，插销缺口附近产生的应力应变集中是导致氢在该区域产生聚集的重要原因。     

恒幅载荷下棒料V型槽尖端裂纹起始寿命的估算

针对带V型槽金属棒料下料中需要较精确计算下料时间的问题,获得了包含V型槽几何参数在内的V型槽尖端理论应力集中系数的计算公式,并在此基础上,建立了恒幅载荷下V型槽尖端裂纹起始寿命的完整的数学表达式。分析了理论应力集中系数和棒料的材质对该始裂寿命的影响规律。45号钢棒料下料实验证明,利用该文提出的V型槽尖端裂纹起始寿命公式所计算的下料时间和实际的下料时间十分吻合。