超声冲击对材料力学性能的影响及冲击过程的有限元模拟

发布时间：2018-10-31 19:00

【摘要】：虽然大量的研究结果表明超声冲击处理对消除焊接残余应力、提高接头的疲劳性能有明显的效果,但其对材料力学性能的影响却鲜有研究。本文以海洋平台上常用的EQ56大厚度高强钢作为研究对象,使用微冲剪方法测试了超声冲击处理前后焊接接头不同位置的微区力学性能,以此来研究超声冲击对材料力学性能的影响。测试结果表明,由于超声波所引入的声软化作用,经过超声冲击处理之后,焊接接头内部在一定程度上发生了软化,材料的抗拉强度、屈服强度降低而塑性升高;但不同位置处的软化程度并不相同,就材料的抗拉强度而言,母材区域的最大降低幅度为11%,热影响区为9%,焊缝区域为19%,焊缝区域软化效果较为明显的原因是由于冲击时间过长所导致的。此外,超声冲击处理会在材料表面形成一层晶粒较细的强化层,与未经超声冲击处理的接头表面材料相比,该层的屈服强度、抗拉强度和塑性均有所增高;并且在该强化层内,焊接接头的力学非均匀性得到了改善,焊缝与母材的屈服强度差值从超声冲击处理前的30MPa降低到超声冲击处理后的15MPa。此外,考虑到超声冲击处理过程复杂,工艺参数较多,实际测试操作性不好的原因,本文以ABAQUS商用有限元软件为基础,使用Python语言对其进行了二次开发,建立了可以真实反映超声冲击过程的有限元模型,并通过实际应力测试试验对其准确性进行了验证。在此基础上,以无应力状态的304L不锈钢板为对象,使用不同工艺参数计算分析了其对超声冲击应力结果的影响。计算结果表明,冲针初始速度对冲击效果有较为明显的影响,当其从1m/s增大到3m/s时,残余压应力的最大值从412MPa增大到460MPa,而且残余应力最大值的所在的位置从距工件表面1.74mm处增加到2.46mm处;此外,随着冲击时间的增加,残余压应力的最大值逐渐从433MPa增加到449MPa,但增加的趋势逐渐变缓,由此说明超声冲击处理时,冲击时间存在一个最佳值,超出该最佳时间后,冲击效果变化不明显;但冲针与工件之间的摩擦系数对冲击效果的影响不明显,使用不同摩擦系数计算所得到的应力分布结果基本相同,无太大变化。
[Abstract]:Although a large number of research results show that ultrasonic shock treatment has obvious effect on eliminating welding residual stress and improving fatigue properties of joints, but the effect of ultrasonic shock treatment on mechanical properties of materials is seldom studied. In this paper, the micromechanical properties of welded joints at different positions before and after ultrasonic impact treatment were tested by using micro-impact shear method, which is a common EQ56 steel with large thickness and high strength on offshore platform. The effect of ultrasonic impact on the mechanical properties of materials was studied. The test results show that due to the effect of acoustic softening introduced by ultrasonic wave, after ultrasonic impact treatment, the internal of welded joint softens to a certain extent, the tensile strength, yield strength and plasticity of the welded joint decrease and increase. In terms of the tensile strength of the material, the maximum reduction in the base metal area is 11, the heat affected zone is 9, and the weld area is 19 percent. The obvious reason for the softening effect of weld zone is that the impact time is too long. In addition, ultrasonic impact treatment will form a fine grain strengthening layer on the surface of the material. Compared with the joint surface material without ultrasonic impact treatment, the yield strength, tensile strength and plasticity of the layer are increased. The mechanical inhomogeneity of welded joint was improved and the difference of yield strength between weld and base metal was reduced from 30MPa before ultrasonic shock treatment to 15MPa after ultrasonic impact treatment. In addition, considering the complex process of ultrasonic shock treatment, the process parameters are many, and the practical testing is not easy to operate, this paper uses the ABAQUS commercial finite element software as the foundation, and uses Python language to carry on the secondary development to it. The finite element model which can truly reflect the ultrasonic impact process is established, and the accuracy of the model is verified by the actual stress test. On this basis, the influence of different process parameters on the results of ultrasonic shock stress was calculated and analyzed with 304L stainless steel plate without stress as an object. The results show that the initial velocity of punching needle has a significant effect on the impact effect. When the initial velocity of punching needle increases from 1m/s to 3m/s, the maximum residual compressive stress increases from 412MPa to 460 MPA. Moreover, the position of the maximum residual stress is increased from 1.74mm to 2.46mm on the surface of the workpiece. In addition, with the increase of impact time, the maximum value of residual compressive stress gradually increased from 433MPa to 449 MPA, but the increasing trend gradually slowed down. The change of impact effect is not obvious; However, the influence of friction coefficient between punching needle and workpiece on the impact effect is not obvious. The results of stress distribution calculated by using different friction coefficients are basically the same and have no great change.
【学位授予单位】：江苏科技大学
【学位级别】：硕士
【学位授予年份】：2016
【分类号】：TG407;TB559

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