【作者】 张慧；

【导师】 邢鸿雁；

【作者基本信息】 天津科技大学 ， 工程硕士（专业学位）， 2017， 硕士

【摘要】 应力集中一直是工程界普遍关注的问题。构件存在应力集中会造成其承载能力降低,严重时会使运行中的承载构件突然断裂,导致灾难性事故的发生。因此,最好的解决办法是可以对工作中的构件进行实时监测,及时发现应力集中及早期损伤,降低突发性事故的发生率。传统的无损检测技术虽然已经在工程中得到了广泛的应用,但主要的检测对象是构件上已经存在的缺陷,对于那些由于应力集中等引起的早期损伤并不能检测出来。金属磁记忆检测技术的出现,使铁磁构件的早期诊断成为可能,该技术是利用地磁场环境,通过检测构件由于应力集中引起的表面漏磁场变化来判断损伤部位以及损伤程度。由于发展时间较短,该技术还不是很成熟,且检测过程中受到的影响因素较多,目前只是作为判断铁磁构件应力集中位置的一种初步检测方法,还需要其他检测方法进行复检,且不能提供量化结果。在该技术中,常用的磁参数是自有漏磁场法向分量以及其在长度方向上的梯度值。在现阶段大多数的实验研究中,主要是对试件表面应力集中区域上所作的一些测量线进行检测并研究磁记忆信号变化情况,并通过法向磁记忆信号曲线过零点来判断应力集中位置,但一些实验研究也表明,通过此方法来判断试件的应力集中位置不是十分准确。针对此问题,本文首先对不同直径的中心小孔试件加载并进行磁记忆检测,研究试件表面小孔附近测量线上法向磁记忆信号变化过程,并进行力学仿真分析磁记忆信号与应力集中的关系;其次,分别对预制中心圆孔和两半圆槽试件进行磁记忆检测,提出了通过法向磁记忆信号在两个方向上的梯度来判断试件应力集中位置的方法,主要内容和结论包括:(1)对一系列不同直径中心小孔试件拉伸加载并进行磁记忆检测,观察试件经拉伸后法向磁记忆信号变化情况,并进行力学仿真得到受载试件在不同拉伸载荷下,不同位置应力集中系数相同而磁记忆信号过零点位置不同的现象,得出仅根据法向磁记忆信号过零点判断受载试件应力集中位置方法欠妥的结论。(2)在弹性阶段内,对中心圆孔试件表面划分网格后进行拉伸,对不同载荷下试件表面上的网格点进行磁记忆信号采集,提出了通过法向磁记忆信号在检测平面长和宽两个方向上的梯度来判断试件应力集中位置的方法。此外,利用COMSOL软件对试件在弹性阶段内的受力情况进行了仿真,发现应力与法向磁记忆信号梯度成正比关系,得出可通过法向磁记忆信号在两个方向上的梯度判断受载试件的应力集中位置。(3)对两半圆槽试件加载并进行磁记忆检测,该实验与中心圆孔试样的实验方法完全相同,进一步验证了通过法向磁记忆信号在试件长和宽两个方向上的梯度来判断应力集中位置方法的可行性。更多还原

【Abstract】 The stress concentration are always common concern issues to engineering. If components have stress concentration, it would reduce it’s bearing capacity, what’s worse,stress concentration may make the running load components fracture suddenly, this may lead to catastrophic accidents. So, the best solution is to real-time monitor the components in the working, discovering the stress concentration and early damage in time, reducing the incidence of sudden accidents. Although the traditional nondestructive testing technology has been widely used in the engineering, they are mainly better used for defecting existing defects,it’s can’t used for those early damage by stress concentration and so on. The emergence of metal magnetic memory testing technology, make it possible to make early diagnosis for the ferromagnetic components. This technology is the use of the magnetic field environment, determining the damage location and damage degree of components due to the surface leakage magnetic field change caused by stress concentration. Because the technology development time is shorter and not very mature,also have more influence factors in the process of testing, this technology just as a preliminary test methods for judging the stress concentration position of ferromagnetic component at present, and also need to use other detection methods to re-inspection the components and can’t provide quantitative results. At present, in magnetic memory testing technology, the commonly used magnetic parameters is leakage magnetic field of normal component and it’s gradient in the length direction .In most experimental study, it’s mainly testing some measuring line on the surface of specimens and studying the magnetic memory signal changing, determining the stress concentration position by a zero curve of normal magnetic memory signal. But some experiment show that through this method to determine the stress concentration of the specimens is not very accurate. Aiming at this question, In this paper, first of all, testing tensile specimens of different diameter center hole by magnetic memory testing technology,study on the change of normal magnetic memory signal on the line near the center small hole of the test specimen surface, and make a mechanical simulation, analysis of the relationship between magnetic memory signal and stress concentration;secondly testing the center hole and two semicircles specimens by magnetic memory testing technology respectively, proposing the method of using normal magnetic flux leakage signals gradient of two directions to judge the stress concentration of tensile specimens, the main contents and conclusions including:(1)Testing a series of different diameter of center hole tensile specimens by magnetic memory testing technology, observing the change of normal magnetic memory signals, and making a mechanical simulation,getting specimen under different tensile load,there are the same stress concentration coefficients on different locations but magnetic memory signal passing zero have different phenomenons.It is concluded that the method of according to magnetic memory signal passing zero position determining the stress concentration position is inappropriate.(2)In the elastic stage, using the demarcation mesh method for the surface of center hole specimens, testing the magnetic memory signals of grid point on the surface under different tensile load, putting forward to the method of using the normal magnetic signals gradient on length and width two directions of detection plane to judge stress concentration position. In addition, simulating mechanics of the center hole specimen in it’s elastic stage by COMSOL, comparing the stress distribution chart with the normal magnetic memory signals gradient nephogram, founding that stress is proportional to the gradient of magnetic memory signal,that is the stress concentration location of the loaded specimen can be determined by the gradient in two directions of the magnetic memory signal .(3)Testing the magnetic memory signal of two semicircles specimen tensile specimens,It’s experimental method is the same as the method of center hole sample exactly, further validation, according to the method of the gradient in length and width two direction of the magnetic memory signal to judge the stress concentration of specimen is feasible.更多还原