【作者】 孙德平；

【导师】 湛从昌；

【作者基本信息】 武汉科技大学 ， 机械电子工程， 2004， 硕士

【摘要】 本课题的目是探讨起重机梁声发射技术无损检测的可行性及声发射信号的特征。迄今尚无有关本课题的文献报导。 本课题所采用的技术路线表示于以下框图： 首先进行常规无损检测(超声波和表面磁粉检测)以确定起重机梁是否已有裂纹存在，如果没有现存裂纹，就在适当位置人工制造一裂纹作为测试点，然后用液压脉冲疲劳试验机加载作为起重机梁的工作荷载，在加载过程中进行声发射监测。最后，分析所获得的声发射信号，并评价裂纹的危险级别。 试验所用的主要设备有：CTS-23超声波检测仪，CDX-Ⅲ磁粉探测仪，Spartan AT-24通道声发射监测仪(美国)，液压脉动疲劳试验机，YJ-18静态应变仪。 起重机梁的主要参数为：最大工作荷载7.1t，材质16Mn，跨度10000mm，主梁宽255mm，高500mm。 取7.1×1.1=7.8t作为最大试验荷载，进行两个加载循环，在第一循环中以2.5t／min的速度分5档(5t，6.25t，7.0t，7.5t，7.8t)进行加载，每档保载5或10分钟(7.8t时)，随后卸载至0t，在第二循环中以2.5t／min的速度分4档进行加载(5t，6.25t，7.0t，7.5t)，其余同第一循环，与加载同时进行声发射信号采集。 围绕裂纹布置了两个探头阵，即线性阵(7，5，1，2，6及8号探头)和三角阵。第n页武汉科技大学硕士学位论文 利用日产HBO.snnn铅笔芯的折断声校准探头阵。 衰减测量显示:信号的衰减随探头至声源距离的变化而变化，当距离在1米以内时，信号能量急剧衰减，当距离为l一3米时，衰减接近常数，当距离为3一5米时，信号不衰减反而放大，这一事实说明，声发射信号在起重机梁内的传播机理是非常复杂的。 常规无损检测表明起重机梁内无现存裂纹，在这种情况下，将起重机梁加载至最大试验载荷，没有可检测的声发射信号，制造人工裂纹之后，加载过程中检测到大量的声发射信号。 定位显示，所有声发射源都位于裂纹尖端，这证明起重机梁的声发射技术无损检测是可行的，它可准确确定起重机梁内裂纹的位置，按照有关部门推荐的评价标准，所发现的声发射源都为高强度活性源。 随着载荷的增大，撞击计数和声发射能量都明显增大，这证明用声发射技术进行起重机梁的安全评价是完全可靠的，它可准确地评价裂纹的危险级别和起重机梁的安全可靠性。 所检测到的声发射信号的主要参数是:幅度多70dB，能量计数65535一27，撞击计数(振铃数)O一65355，上升时间0一671 ps，维持时间63704一919686 ps，这些参数在很大范围内变化，除撞击数外，其它参数，例如幅度，能量计数，频响等都明显取决于监测设备的特性及探头的位置，作者认为，声发射源的危险评价标准应以撞击数为基础。 文中对人工神经网络模式识别原理作了简要介绍。通过给BP神经网络输入声发射信号进行模式识别。复杂的数据组成，低信噪比，数据重复性差，环境不确定是声发射信号的固有特征，因此难以获得大量的典型训练数据。所以，用单一组典型数据训练之后，通过测试相对误差进行模式识别可能是一种较好的方法。 本课题首次将声发射监测技术应用于起重机梁的安全评价，从而扩展了声发射技术的应用领域。更多还原

【Abstract】 The aim of this subject is to discover the feasibility of crane beam’ AET NDT and the characteristic of AE signal, which has never been reported in literature until today. The technique way adopted in this subject is shown as following block diagram.First, conventional NDT (UT & MT) is carried out determining whether there are existing crack in the beam. If there are no existing cracks, a crack is made artificially in suitable location as the point to be tested; secondly, the crane beam is loaded by hydro-pluses tired-test machine, during the cycle of loading AE signal is monitored. At last, I analyze the AE signal obtained and assess the critical degree of the crack.Main equipments used in the test are: CTS-23 ultrasonic tester, CDX-III magnetic powder detester, Spartan AT-24 channel AE monitor (US), hydro-pluses tired-test machine, YT-18 static resistance strain measurers.Main parameters of the crane beam are: max load 7.1t,material 16Mn, Span10000mm,width 255mm, and height 500mm.For example, take 7.1x1.1=7.8t.as the max test load. Two loading cycle has been carried out. In the first loading cycle at velocity 2.5t/min by 5 steps (5t, 6.25t, 7.0t, 7.5t, 7.8t), load is persisted constantly at leak step 5 min or 10min (at step 7.8t), then unloading to 0t,In the second cycle, loading progress at velocity 2.5t/min by 4 steps (5t, 6.25t, 7.0t, 7.5t), the others are same the as the first cycle. AE signals are picked simultaneously with loading.There are two transducer arrays being arranged around the crack, i.e. linear array (No:7,5,1,2,6& 8 transducer) and triangle arraySound created by breaking off an HB0.5mm pencil core (Japan) is used to calibrate the transducer array.Decay measurement reveal that signal decay varies as the distance from transducer to sound source change. When the distance is less than 1m signal energy decay sharply, when the distance is l-3m it keep constant nearly, when the distance is 3-5m, it does not decay but amplifies. This fact indicates that the propagation mechanism of AE signal in the crane bean is very complex.Conventional NDT indicate that there are no existing cracks in the crane beam, under this condition when loading the beam to max test load, no AE signal can be detected. After the artificial crack is made, numerous AE signal emitted during loading process.Location calculation revealed that all AE sources locate at the crack top, it proves that AET is feasible for NDT of crane beam. It cans accurately locate the crack in the beam. According to assessment criteria recommended by relative authority, all AE sources discovered are high intense active source.As the loading increases count of hit and AE energy increases markedly. This proves that safety assessment of crane beam by AET is fully creditable. It can accurately evaluate the critical degree of crack and the safety reliability of the crane beam.Main parameters of AE signal detected are:Amplitude >70dB, energy count 65535-27, hit count ( ring down count ) 0-65355, rise time 0-671 us, duration 43704-919656 u s.These parameters vary in large range. Except hit count, other parameters, such as amplitude energy count; frequency response, etc. are depending obviously on the characteristic of the monitor apparatus and location of the transducer. In author’s opinion, the critical evaluation criteria of AE source should base on hit counts.The principle of nervous network mode discrimination of AE signal is introduced in this paper briefly. Inputting AE signal to BP nervous network carries out mode discrimination. Complex data form, low signal -noise ratio, bad data repetitiveness, uncertain environment are the inherent characteristic of AE signal, so it is difficult to obtain numerous topic training data. Therefore, after training with a signal set topic data, carrying out mode discrimination by testing relative error may be a better method.This subject applies AET to safety assessment of crane beam first, so extend the application area of AET.更多还原