声弹性效应测试金属材料内部平面应力

【作者】 罗琅；

【导师】 董师润；

【作者基本信息】 江苏科技大学 ， 材料物理与化学， 2010， 硕士

【摘要】 声弹性技术可以作为一种应力测试技术,而且可作为一种材料的无损评估技术,可用于零部件(或装置)从制造到服役过程的各个阶段,其应用前景广阔。本文对平面应力状态下声弹性公式做了归纳总结。经推导得到横波检测理论公式,为平面应力测试打好了理论基础。并自行设计和加工了四点弯曲加载夹具和固定探头的夹具。试验表明,四点弯曲夹具符合设计要求,试验现象稳定,探头夹具固定效果良好,实验数据准确可靠。在此基础上分别对LY12和45钢自然状态下不同偏振方向的横波声速进行了测试,结果表明LY12铝合金和45钢都具有正交各向异性材料的特征,即材料存在声速快轴和慢轴(声主轴),且声速快轴和慢轴互相垂直;横波声速随其偏振方向变化而改变,在LY12铝合金检测时当横波探头与辊轧方向成60°或120°夹角附近时,声速急剧增大;当横波探头与辊轧方向平行时,声速出现极大值;LY12铝合金横波声速与偏振夹角的关系表明,声速随偏振夹角变化会出现非连续现象,即声速跳变。经过对实验现象进行分析和查阅相关资料,其原因是与其微观择优取向有关。为了测试材料中的平面应力,本文对LY12铝合金和45钢的长方体和八面体进行了单轴标定试验,得到了材料的相关声弹性常数,并分别测试它们在自然状态和四点弯曲状态下的横波声速分布。研究结果表明在处于平面应力状态下弹性介质中传播的超声横波会产生声弹性现象,这与经典的声弹性理论是符合的。计算和实验都表明,当LY12铝合金和45钢受到平面载荷应力时,在其中传播的不同偏振方向的超声波声速都发生了变化,其中45钢声主轴发生明显的偏转。利用声速变化与应力之间的依赖关系——声弹性效应,测试得到了LY12铝合金和45钢四点弯曲状态下的各测试点的正应力和剪切应力。将测试得到的应力分量与有限元模拟的结果进行了比较,证明了利用声弹法测试弱织构效应的LY12铝合金方法的可行性,并且测试结果可靠、准确;文中对强织构效应材料45钢,测试结果不理想,可能与材料的织构及残余应力等因素有关,值得进一步研究。在测试过程中本文还考虑了温度对LY12铝合金声速的影响。本文主要研究了四点弯曲状态下的声弹性现象及其应用。可进一步研究二维轴向拉伸状态下,弱各向异性材料声速与应力的变化关系,使得理论与实验更完备;同时在模拟方面还有许多工作需要做,比如开发单独声弹性模拟单元,直接利用声速信息得到材料应力分布;由此可见超声波检测应力的方法有着广阔的应用前景。更多还原

【Abstract】 The acoustoelastic phenomenon not only as a technique for experimental stress analysis but also as a nondestructive technique for evaluation in metals, which can be used to evaluate metals all period from be made up to serve, has been extensively attracting related research people domestically and abroad.The present paper has generalized and summarizes the acoustoelastic theory in the plane stress. It has studied the temperature influence to LY12 aluminum alloy. In this paper, This self-developed a set of four-point bending loading fixture, and two sets of transducers holder, it is showed that four-point bending fixture comply with design requirements, the experimental phenomenon of stability, one of which transducers holder works well, stable and reliable experimental data.Shear wave velocity changes as the polarization angle (or rotation angle of sensor) changes in unstressed condition;The results indicate that when the oscillation direction of the shear wave makes an angle of about 60 degrees or 120 degrees with rolling direction, the shear wave velocity decreases sharply and the relationship between shear wave velocity and rotation angle tends to become discontinuous in LY12 aluminum alloy; When the oscillation of the shear waves occurs in the anisotropic direction of the rolling direction, the shear wave velocity exhibits a peak value; After researching to LY12 aluminum alloy, can conclude that maybe the Microcosmic Preferred Orientation cause the shear wave velocity cups.In this paper, it is consistent with that the present paper deals with the classical acoustoelasticity in plane stressed and elastically deform media when they are submitted bending stresses. Numerical and experimental evaluations of the resulting change in the ultrasound velocities as a function of bending loads are described. Through detecting the shear wave velocity of different polarization direction in unstressed condition, obtained that the LY12 aluminum alloy and 45 steel sample are the characteristics of orthotropic materials, ie, they had fast axis and slow axis of shear wave velocity, which have an Characteristic that the lower velocity transverse the faster velocity.It detected the plane stress under four-point bending in LY12 aluminum alloy and 45 steel samples by acoustoelastic method, comparing the detecting result with the Computation simulation by the finite element, the acoustic elasticity detecting and the analogue result tallies well in the magnitude. But partial test point’s result in 45 steel and the simulation value had discrepancies, maybe because the anisotropic of 45 steel and the modeling selection of finite element software, this is worth further studying. Ultrasonic birefringence phenomena under four-point bending and its application are studied in this paper. In the following work, the relationship between ultrasonic velocities and the applied stress under two dimensions tensile or compression will be futher studied; the stress evaluation by ultrasonics will be studied and will be applied to pratical production. So, stress determination by ultrasonics has a broad application.更多还原