陶瓷材料破坏的声发射特性

采用先进的多通道声发射系统监测了AD95氧化铝陶瓷在三点弯曲加载下的破坏过程.对有缺口和无缺口两种试件的声发射能量数,事件数,振铃数随时间变化特征的对比分析,讨论了预制缺口对材料损伤破坏过程和断裂的影响.声发射幅值随时间的变化曲线直接反映了材料内微裂纹成核前期,成核,扩展至宏观裂纹形成和断裂过程.实验结果表明,声发射参数可以表征材料的内部损伤演化过程.     

基于小波变换降噪的声发射源定位方法

选取最优小波基对声发射信号进行小波变换降噪处理,采用阈值法确定不同传感器接收声发射信号时间以提高各向异性材料中声发射源定位精度。在钢板和木板上进行断铅模拟声发射源实验,利用Geiger算法分别计算出降噪前后的声发射源定位结果以验证该定位方法的有效性。结果表明该定位方法能有效减小噪声信号的影响,提高各向异性材料中声发射源定位精度。采用该定位方法对氧化铝陶瓷试件压缩破坏过程中的声发射源进行定位,定位结果有效地反映了材料内微裂纹产生和扩展而导致试件破坏的动态过程。     

金属/玻璃钢柱壳胶接头拉伸实验声发射监测

为研究胶接长度对风电叶片柱壳胶接接头力学性能及声发射响应特征的影响,对金属/玻璃钢柱壳试件的拉伸实验全过程进行声发射监测。结果表明:柱壳试件拉伸过程基本为线性破坏,胶接接头的失效模式为界面破坏,损伤多发生在胶层边缘应力集中处。加大胶接长度,能有效增强金属/玻璃钢柱壳试件的胶接强度。较大的胶接长度对应的声发射信号幅度高、相对能量高、撞击累积数多。因此,在风电叶片复合材料叶根部位的安全评估时,可将声发射信号的动态变化特征作为评判依据。     

混凝土静态轴拉声发射试验相关参数研究

摘要：对声发射采集系统的硬件参数设置、滤噪参数设置以及声发射信号特征参数及其相关性进行了系统的试验研究。共进行了11组断铅人工激发源试验和13个混凝土试件的单轴静态拉伸试验,采用全数字化的参数-波形式声发射采集系统和三种不同型号的传感器同步采集并存储了试验过程中的声发射特征参数和波形,应用参数和波形分析相结合的方法以及波形事后提取分析等技术手段对数据进行分析。结果表明：前置放大器增益、阈值、波形采样率和采样长度、带通滤波器等有一合理参数取值;幅度、振铃、持续时间、声发射信号能量、绝对能量、信号强度这6个参数能够较好地体现混凝土轴拉损伤过程的阶段性特征;声发射幅度、振铃数、持续时间、上升时间和信号强度5个参数之间存在显著的相关性。研究成果可为采用声发射技术研究混凝土的损伤破坏过程提供参考依据。     

基于K熵和关联维数的金属疲劳损伤过程的声发射信号特征分析

深化对金属疲劳损伤过程中声发射信号的特征认识,是运用声发射信号对金属结构损伤过程进行监测预测的重要基础热点问题。针对金属疲劳损伤经历裂纹萌生阶段、裂纹缓慢扩展阶段、裂纹快速扩展阶段、临近破坏等四个阶段产生大量的声发射信号,采用K熵和关联维数等混沌特征量来分析海量的声发射信号;通过对45号钢试件进行三点弯曲疲劳试验、测试得到试件疲劳损伤过程的声发射信号,分别估算不同时间段所产生声发射信号的K熵和关联维数,分析结果表明金属疲劳损伤过程的声发射信号具有混沌特征,其K熵和关联维数的变化趋势与金属疲劳损伤过程的四个阶段具有较清晰的对应关联,表明K熵和关联维数可以较好的揭示金属疲劳损伤过程的动力学特性,这将为运用声发射信号实现金属结构疲劳损伤在线监测及预测提供了一种新思路。     

风电叶片复合材料拉伸损伤破坏声发射行为

通过风电叶片单向和多向复合材料拉伸力学性能实验, 结合声发射技术, 研究复合材料损伤演化特性及纤维预断缺陷对复合材料力学性能的影响。复合材料单向和加卸载拉伸实验时, 采用声发射实时监测整个损伤破坏过程, 获取复合材料试件的拉伸力学性能、 损伤破坏特征及相应的声发射响应特征。结果表明: 由于纤维预断缺陷的存在, 单向复合材料加载到约30％破坏载荷时, 缺陷位置及相邻区域的基体和界面开始出现明显损伤; 加载到约60％破坏载荷时, 含缺陷层和相邻的层出现明显的层间剪切破坏, 导致刚度的急剧缩减, 声发射撞击累积数明显高于无缺陷试件。含纤维预断多向复合材料加载到约60％破坏载荷时, 纤维预断处树脂基体出现明显损伤; 随相对应力水平的提高, 多向复合材料的Felicity比下降较为平缓。     

含不同埋深分层损伤复合材料弯曲破坏声发射监测

为研究风电叶片复合材料弯曲损伤破坏特性及声发射响应特征,对含分层缺陷风电叶片复合材料进行四点弯曲试验,并用声发射技术进行实时监测。结果表明:缺陷位于近表面时的撞击累积总数和最高声发射相对能量明显高于缺陷位于中间层时的对应值。因此缺陷位置对试件的承载能力有较大影响,随着分层缺陷距试件表面埋深的减少,试件的承载能力降低,试件破坏程度加剧。分层试件的损伤破坏与对应声发射信号幅度、撞击累积数、相对能量、定位源等特征参量密切相关。     

风电叶片复合材料拉伸损伤破坏声发射行为

通过风电叶片单向和多向复合材料拉伸力学性能实验,结合声发射技术,研究复合材料损伤演化特性及纤维预断缺陷对复合材料力学性能的影响.复合材料单向和加卸载拉伸实验时,采用声发射实时监测整个损伤破坏过程,获取复合材料试件的拉伸力学性能、损伤破坏特征及相应的声发射响应特征.结果表明:由于纤维预断缺陷的存在,单向复合材料加载到约30％破坏载荷时,缺陷位置及相邻区域的基体和界面开始出现明显损伤;加载到约60％破坏载荷时,含缺陷层和相邻的层出现明显的层间剪切破坏,导致刚度的急剧缩减,声发射撞击累积数明显高于无缺陷试件.含纤维预断多向复合材料加载到约60％破坏载荷时,纤维预断处树脂基体出现明显损伤;随相对应力水平的提高,多向复合材料的Felicity比下降较为平缓.     

板状三维四向编织复合材料压缩细观破坏机理的实验研究

通过对三维四向编织复合材料薄板试件的宏观压缩破坏实验及其声发射信号的分析, 研究了该材料的抗压力学性能及其失效机理。实验结果表明: 材料的编织角对其压缩力学性能的影响很大, 随编织角的变化, 编织复合材料的压缩破坏机制发生了变化, 编织角小时, 材料表现为脆性特征; 当编织角大于某个临界角时, 材料的应力-应变曲线趋于非线性, 更多地表现为屈曲破坏。试验过程中采集到的声发射信号能有效地监测材料内部损伤演化过程。      

基于声发射的钢筋混凝土梁承载能力评估

在钢筋混凝土断裂过程中能量平衡基础上,结合声发射与断裂力学参数的耦合关系,得到裂纹稳定拓展过程中,声发射振铃数与钢筋混凝土梁抗弯刚度间的关系,并通过钢筋混凝土梁静、动态弯曲破坏试验进行验证。结果表明：在能量平衡原理基础上建立声发射参数与钢筋混凝土梁承载能力的关系是可行的。在宏观裂纹稳定拓展阶段,通过声发射传感器实时监测获得破坏过程的声发射信号,能准确评估钢筋混凝土梁的抗弯刚度。     

Acoustic emission and damage characteristics of alumina

Advanced multi-channels acoustic emission (AE) system is used to study the fracture process of alumina material subjected by three-point-bending loading. Using AE counts and AE hits, the location of damage and damage characteristics are discussed. AE energy, AE counts, AE amplitude changing with loading time are analyzed for the notched alumina specimen. It is indicated that AE characteristic parameters reflect the damage process and fracture of material.     

Assessment of stress corrosion cracking susceptibility by a small punch test

The object of this study is to establish a new test method for evaluating stress corrosion cracking (SCC) susceptibility of high-strength steel using a small punch (SP) test and acoustic emission (AE). A miniaturized specimen (10 × 10 × 0.5 mm) is adopted for SCC evaluation. The experiments are conducted at various loading rates and at various orientations of the specimen. The cumulative average amplitude of the AE signal per unit equivalent fracture strain ( ε _qf ) increases as the SCC susceptibility increases. Through the load–displacement behaviour, the fracture energy ( E _SP ), the SEM fractographs, and the correlation between the SCC susceptibility and the AE characteristics, it is proved that the small punch test method combined with AE measurements is a useful method to evaluate the SCC susceptibility of high-strength steel.     

Monitoring the fracture of wood in torsion using acoustic emission

Acoustic emission (AE) was used to monitor the failure process of hardwood and softwood test-pieces under static and fatigue torsional loading. In static torsional-loading tests, acoustic activity indicated some microcrack initiation before the visible cracking in both hardwood and softwood test-pieces. Hardwood produced more AE counts than softwood during testing, and the grain angle of test-pieces influenced the total AE counts. During torsional fatigue fracture, increased acoustic activity indicated the onset of microcrack formation. Fatigued test-pieces produced more total AE counts during fracture than static test-pieces, provided the angle of twist exceeded a minimum value. The results show that it is possible to monitor and analyze the failure process in wood when under torsional loading using acoustic emission techniques.     

Fracture analysis of ceramic femoral head in hip arthroplasty based on microdamage monitoring using acoustic emission

Damage accumulation during compression testing of the alumina femoral head used in hip arthroplasty was monitored using an acoustic emission (AE) technique. Because a number of mechanical noises due to friction disturbed the AE measurement during the test using a conventional testing configuration standardized in ISO 7206, the testing apparatus was modified so that AE signals from microdamage were detected successfully. During the compression tests of femoral heads, a rapid increase in AE energy was observed before the final fracture. Dye penetration observations demonstrated that the rapid increase in AE energy correlated with the formation of maincrack which leads to the final fracture. The stress distribution in femoral heads was analyzed using finite element analysis (FEA). The critical stress for maincrack formation and the final fracture strength were determined. Finally, fundamental insights into the development of a technique for assessing the long-term reliability of ceramic femoral heads were obtained.     

基于声发射技术的非标准自密实混凝土三点弯曲梁动态断裂特性

对不同初始缝高比的自密实混凝土(Self-compacting concrete,SCC)非标准三点弯曲梁开展不同加载速率下的断裂试验,获得其断裂的荷载-裂缝嘴张开口位移曲线及峰值荷载、断裂韧度、临界缝高比增量、弹性模量和柔度系数等断裂参数,结合Pearson相关性检验公式及加载速率效应模型,定量分析初始缝高比、加载速率与断裂参数间的相关性强弱及SCC断裂参数的加载速率效应。结果表明峰值荷载、断裂韧度及弹性模量均存在一定的加载速率效应,柔度系数仅与初始缝高比强相关,弹性模量和断裂韧度是材料的固有属性,不受初始缝高比影响。同时,基于声发射(Acoustic emission,AE)技术对SCC的损伤断裂过程、断裂边界效应及裂缝扩展模式进行分析,结果表明,AE参量能较好地反映混凝土断裂的三阶段特性及边界效应。裂缝的扩展首先以拉伸裂缝为主,剪切裂缝占比随着裂缝扩展过程逐渐增大。      

Acoustic emission during the tensile deformation of Incoloy 901 superalloy

Observations have been made on the acoustic emission (AE) response related to the deformation-damage mechanisms during tensile tests of a common engine material, Incoloy 901 superalloy. Results show that dislocation motion, twinning and inclusion fracture cooperated to generate acoustic emission during tensile deformation of Incoloy 901. Based on AE recorded results and microstructural examination, a dislocation-saturation model was developed to describe AE activity during elastic and plastic deformation, and to distinguish between the AE response in the yield region and in the work-hardening region. Furthermore, the effects of strain rate and loading methods on AE outputs were examined. The dependence of acoustic emission on dislocation motion and saturation, deformation twinning, and decohesion and fracture of inclusions and secondary particles are discussed.Formerly a postdoctoral student, Faculty of Mechanical Engineering, Technion, Israel.     

Effectiveness of nonlinear ultrasonic and acoustic emission evaluation of concrete with distributed damages

Nonlinear ultrasonic (NLU) and acoustic emission (AE) techniques are used for nondestructive evaluation of concrete, damaged under compression loading. Experiments were carried out in 18 cubic specimens (150 × 150 × 150 mm) cast with three different w/c (six specimens for each w/c). Three specimens at each w/c were used for AE monitoring and three others for NLU evaluation. The NLU evaluation is based on measuring the change in fundamental amplitude with increasing damage and output power level. In acoustic emission testing technique four sensors were used to listen to the wide range of events under various loading and unloading cycles. An increase in AE hits was observed with increasing damage. Each loading and unloading stage was carefully examined for Kaiser and Felicity effects in order to assess the concrete deterioration. It was proposed to measure Felicity ratio at three different loading levels, corresponding to AE hits at 3%, 5%, and 10% of the AE hits at the previous maximum load, respectively. Normalized values of Felicity ratio were plotted and compared with the NLU test data. Correlation between acoustic emission and nonlinear ultrasonic techniques in assessing damage growth in concrete was investigated.     

聚乙烯自增强复合材料损伤过程的声发射特征

复合材料在承受外载时, 声发射可产生于基体破裂、纤维-基体界面脱粘和纤维断裂等。测定了U HMWPE/ HDPE 复合材料在拉伸载荷作用下的声发射(AE) 振幅信号。对特殊试样, 即预测到断裂有明确方式, 如纤维-基体界面脱粘、基体破裂、纤维断裂和分层等的试样, 实施加载直至破坏。用扫描电子显微镜(SEM) 观测试样的断裂表面, 对产生于若干特殊损伤类型的AE 信号进行了鉴别。在相同加载条件下, 完成了不同种类的U HMWPE/ HDPE 准各向同性层合板声发射检测。结果在特殊试样损伤类型与声发射信号事件振幅之间建立了对应关系, 揭示了上述各种准各向同性层合板损伤扩展过程的AE 特征与损伤破坏机制。各种准各向同性层合板试样的声发射事件累计数对拉伸应力关系曲线相异, 其相同损伤类型发生时所对应的拉伸载荷水平不等, 表明它们的铺设角度和铺设顺序对损伤演变过程有显著的影响。结果证实了它们的最终破坏由严重层间分层造成。      

Universal Educational and Research Facility for the Study of the Processes of Generation and Propagation of Acoustic Emission Waves

The design and the algorithm for the use of an approved universal educational and research facility is described. The facility is designed to simulate a wide range of model controlled objects (vessels, pipelines, tube furnaces, reservoirs, etc.) and the main acoustic emission (AE) sources: cracks, leaks, corrosion damage, sites damaged by an aggressive medium, etc. The AE simulators installed at the facility are equipped with waveguides to provide the acoustic connection and protect the main elements of the facility in case of fracture of the simulators. The AE simulators are controlled using a loading device to preset the fracture initiation point and the simulator fracture rate. The facility is designed so as to provide low-cost repeated simulation of diverse situations that occur during the operation and examination of dangerous industrial objects. The facility can be used for research, education, and certification of personnel, techniques, and AE hardware; thus, it is useful for nondestructive testing laboratories.     

基于声发射信号的三维针刺C/SiC复合材料拉伸损伤演化研究

本研究对三维针刺C/SiC(3-dimension needled C/SiC, 3D-N C/SiC)复合材料进行室温单调拉伸和拉伸加载-卸载试验, 利用声发射技术对试样损伤演化进行动态监测。采用K-均值聚类分析方法对小波降噪后的声发射信号进行了损伤模式识别, 结合试样断口扫描电镜观测, 发现3D-N C/SiC复合材料在拉伸载荷作用下主要存在五类损伤模式: 基体开裂、界面脱粘、界面滑移、纤维断裂和纤维束断裂。通过快速傅里叶变换(FFT)方法对小波降噪后的信号进行频谱分析得出: 3D-N C/SiC复合材料在拉伸载荷作用下主要存在240、370和455 kHz三种频率的损伤信号, 分别对应于界面损伤、基体损伤和纤维损伤。结合单调拉伸试验过程声发射信号能量柱分布和加卸载过程累积能量曲线特征, 分析了试样损伤演化机理。