含裂纹钢试样在拉伸过程中的声发射特性

对于带有I型裂纹的SM490A钢和SUS304钢试样,通过测量其在拉伸断裂过程中的声发射特征参数及裂纹尖端特定点的温度,分析了裂纹尖端和塑性变形区域的力学特征.结果表明:声发射能量计数、振铃累积计数在裂纹扩展的各个阶段都有着明显的曲线特征,且曲线特征和屈服、强化阶段的应力-应变曲线基本吻合,因此可以用声发射能量计数、振...     

316LN不锈钢断裂过程的声发射特性

用声发射技术监测316LN母材和焊缝的断裂过程得到了材料断裂破坏的声发射特征,并对声发射信号进行分类,研究了316LN不锈钢材料的断裂韧性。结果表明:316LN母材和焊缝的断裂是韧性断裂,母材的塑性优于焊缝;断裂过程在时域上可划分为5个阶段:裂尖张开阶段、弹性变形阶段、塑性变形阶段、裂纹萌生阶段以及裂纹稳定扩展阶段;使用上升时间-持续时间关联分析法可将母材和焊缝断裂过程的声发射信号进行分类,可区分出噪声信号和有效断裂信号。     

基于声发射技术的30CrMnSi钢断裂机理研究

采用三点弯曲试样加载过程声发射监测实验,结合断口SEM观察及X射线能谱分析,考察了30CrMnSi合金钢延性断裂的整个物理过程.结果表明:30CrMnSi钢的断裂过程从宏观上可以分为裂尖塑性变形、裂纹亚临界扩展和裂纹失稳扩展三个阶段.其机理如下:裂尖塑性变形过程是位错的滑移、堆积而引起的孔洞形核和长大;裂纹亚临界扩展过...     

钛镍形状记忆合金拉伸过程的声发射特征研究

对奥氏体状态下的钛镍形状记忆合金(SMA)的拉伸过程进行了声发射监测,通过声发射特征参数分析了SMA拉伸过程中的马氏体相变、塑性变形等细观力学行为。结果表明,马氏体相变过程中声发射源活动性最强,塑性变形过程的声发射源活动性较差;马氏体相变所产生的声发射信号振铃计数和持续时间分别小于100次和1 000μs;塑性变形所产生的声发射信号振铃计数和持续时间分别达到800次和4 000μs。上升时间-持续时间关联分析结果表明声发射源可分为马氏体相变和塑性变形两类,振铃计数-幅值关联分析可将声发射源分为马氏体相变、塑性变形起始以及屈服断裂3类。     

带缺口塑性材料的声发射特性研究

以带线切割加工缺口的碳钢在拉伸试验时的断裂过程为研究对象,通过对其产生的声发射信号进行检测和分析,研究其开裂时的声发射特性。试验结果表明:带缺口塑性材料在拉伸断裂过程中会产生幅值在47dB￣90dB范围内的声发射信号;信号的振铃累计数在材料的塑性变形段与裂纹稳态扩展段均与应变成线性关系,但后者斜率大于前者;试样缺口深度越大,材料开裂时间越早,振铃计数率越高,且信号能量累计数与应变成线性关系。     

金属中裂纹尖端的无位错区(DFE)

本文对材料断裂研究中的裂纹尖端形变行为,特别是裂纹尖端无位错区的研究作了简要评述。其中包括:一些薄膜金属,如 Al,Cu,Nb,Fe,W 和 Mo 等,裂纹尖端在变形时的位错发射和无位错区存在的实验事实;裂纹尖端位错的屏蔽与反屏蔽概念的引进;材料断裂研究中无位错区模型的引进和描述;延-脆断裂转变的判据。     

DISLOCATION-FREE ZONE AHEAD OF CRACK-TIP IN METALS

本文对材料断裂研究中的裂纹尖端形变行为,特别是裂纹尖端无位错区的研究作了简要评述。其中包括:一些薄膜金属,如Al,Cu,Nb,Fe,W 和Mo 等,裂纹尖端在变形时的位错发射和无位错区存在的实验事实;裂纹尖端位错的屏蔽与反屏蔽概念的引进;材料断裂研究中无位错区模型的引进和描述;延-脆断裂转变的判据。     

以声发射信号的特征诊断敏化304不锈钢沿晶应力腐蚀

采用声发射检测技术研究了304不锈钢在0.5 mol/L Na2S2O3溶液中的沿晶应力腐蚀过程,根据声发射信号的平均频率及幅值等特征参数,按声发射源将其分为易感晶界的溶解、裂尖的塑性变形及韧带的机械断裂3个阶段,并采用参数关联法及时频法聚类分析了3个阶段的信号特征。试样现场断口形貌及信号特征参数分析结果表明,声发射技术能够用于监测沿晶应力腐蚀演化进程,辨别其微观发展,为其微观机制研究提供依据,实现应力腐蚀在线精确监测。     

高周疲劳载荷下6061-T6铝合金的温度演化

为了研究金属材料在疲劳载荷下的温度变化,采用红外热像系统对高周疲劳载荷下6061-T6铝合金的温度演化进行分析,用热像图对疲劳裂纹尖端的塑性区进行测量.结果显示,疲劳加载作用下,循环次数达到107次时6061-T6铝合金试样表面温度的变化分为四个阶段:初始温升阶段、温度缓降阶段、温度二次缓慢上升阶段和温度快速上升阶段.结合热弹性理论、铝合金塑性变形的微观机制分析并预测疲劳载荷下温度的演化和宏观裂纹扩展时裂纹尖端塑性区域大小.宏观裂纹开始扩展时,裂纹尖端的塑性区域可达3.6 mm2,红外热像仪测得结果为3.46 mm2,测试结果与理论结果吻合.     

含边裂纹试样弯曲断裂声发射特征研究

采用类岩石材料研究了含边裂纹试样在三点弯曲加载条件下的裂纹扩展过程，并利用声发射（AE）测试技术分析了含裂隙试样弯拉应力作用过程中预制裂纹萌生、起裂、扩展和断裂破坏的全过程的声发射特征。试验结果表明，声发射的阶段性突发特征反映了张拉裂纹阶段性扩展的规律，但边裂纹的拉破裂特征不同于压剪应力作用下的声发射特征。预制裂纹在拉破裂过程中的声发射特征表现为AE数的渐进增长、快速增长、突发式增长和逐渐下降4个阶段。声发射源的带状分布规律表明，在弯拉应力作用下，裂隙的张性断裂不是孤立的，而是有若干个小断裂面聚集而成宏观断裂，探讨了预制裂隙受拉破坏的内在机理。     

Evaluation of in situ fracture toughness of ceramic coatings at elevated temperature by AE inverse analysis

This paper describes acoustic emission (AE) measurement at elevated temperature by laser techniques. AE signals originated from microfracture of Al₂O₃/SUS304 plasma sprayed coatings were detected using a laser interferometer during thermal cycling. The radii of microcracks were evaluated by the inverse analysis of AE signals. Microcrack radii were also studied by the numerical analysis based on the delamination model for coating materials with various fracture toughness and an initial crack size at elevated temperature. In situ fracture toughness for microcracking was evaluated by combining the results of AE inverse analysis with those of numerical analysis. This approach revealed that the in situ toughness of the coatings is 40–50 J/m², consistent with the results of a double cantilever beam test.     

DEPENDENCE OF ACOUSTIC EMISSION FOR LOW STRENGTH STEELS UPON THE EMBRITTLEMENT AND THE PLASTIC ZONE REDUCTION AT THE CRACK TIP DURING CORROSION FATIGUE CRACK PROPAGATION

Acoustic emission signals were continuously monitored during fatigue crack propogation for two kinds of low strength steel placed in either laboratory air or 3.5% NaCl solution. The plastic deformation of the materials at the crack tips were compared by determining the changes in specimen geometry at the fracture surface in the thickness direction while the hydrogen distribution from the crack tip material was analysed by SIMS. The results showed that acoustic emission was less active in an aqueous solution than in air and that the plastic zone size was reduced in a corrosive medium. It was considered that the entrance of hydrogen into the crack tip during corrosion fatigue results in a decrease of material ductility, which lead to a low activity in acoustic emission.     

Experimental investigation of fracture damage of notched granite beams under cyclic loading using DIC and AE techniques

Fracture experiments of three‐point bending notched granite beams were performed under cyclic loading using digital image correlation (DIC) and acoustic emission (AE) techniques. The damage evolution process of the specimen under cyclic loading was analysed on the basis of AE ring count and b value. The strain and displacement fields and the fracture process zone (FPZ) ahead of the crack tip were revealed by DIC. The results showed that the AE characteristics of rock fracture indicated a noticeable Kaiser effect in the stage of cyclic loading and unloading. Moreover, when the loading force reached 70% of its peak value, the AE characteristics showed the Felicity effect. The damage produced during the loading‐unloading process contributed to the development of the cracks leading to the catastrophic fracture. Besides, a relatively high loading rate was found to help to suppress the development of the FPZ at the crack tip.     

Failure assessment of a hard brittle coating on a ductile substrate subjected to cyclic contact loading

Hard brittle films and coatings are often employed as a protective coating for metallic ductile substrates. In use, such coatings are generally subjected to cyclic/repeated contact loading and sliding over long periods of time. This study investigated the monotonic and cyclic contact fracture mechanism of hard coatings on ductile substrates (an electroplated Ni–P coating on a stainless steel substrate, SUS304) in order to evaluate their mechanical durability. In the experiment, both monotonic and cyclic indentation tests using a ball indenter with large contact force were performed. The fracture nucleation process was identified using the acoustic emission method. For monotonic contact loading (single indentation), coating cracks are produced by the excessive plastic deformation of the substrate, itself caused by contact loading, which makes the bending curvature of a coating a critical moment. Subsequently, cyclic contact loading (cyclic indentation) was applied to the coating in order to investigate the cycle number of film cracking. It was found that the critical contact force for coating fracture decreases, compared with that of monotonic loading. This critical force is dependent on the number of loading cycles. This may be due to the fact that cyclic contact loading encourages large plastic deformation of the SUS304 substrate owing to cyclic plasticity. Therefore, the cyclic plastic deformation behavior of the substrate was investigated using cyclic microindentation tests and the finite element method. In the computation, the Chaboche model was employed to compute the cyclic plastic deformation of the substrate, since it simulates cyclic plasticity. We clarified the cyclic contact fracture mechanism of electroplated Ni–P coating on an SUS304 substrate. Based on this, we finally predicted the coating lifetime (i.e., mechanical durability) under cyclic contact loading. Therefore, the present study is useful for obtaining information about film/coating fracture properties under both monotonic and cyclic contact loadings.     

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.     

The influence of specimen geometry on crack tip plasticity

The effect of panel width and thickness on the crack tip plasticity of center-cracked fracture panels of 7075-T6 aluminum alloy was examined. The plastic zone formation was continually monitored photographically during the fracture test and the zone size and shape determined by an image distortion technique. The influence of specimen width and thickness was examined to determine its effect on the plastic zone behavior and the mode (stable tear or pop-in) of initial crack extension. Plastic zone models were then compared with the characteristic plastic zone size parameters measured experimentally and the models evaluated. From these results, the effect of panel thickness and width on the fundamental characteristics of crack tip plasticity and initial crack extension was assessed. The importance and implications of these findings on methods of evaluating material toughness are examined. In addition, the applicability of these zone size models in linear elastic fracture mechanics applications are discussed.     

Application of fracture surface topographic analysis technique to thermal fatigue crack extension in stainless steel vessel

The fracture surface topographic analysis (FRASTA) technique is applied to investigate the propagating process of fatigue crack in a model vessel made of type 304 stainless steel due to repetitive thermal shocks for 1000 cycles. FRASTA is a procedure for reconstructing the process of crack extension by comparing topographic features of coupled areas of opposing fracture surfaces via computer. Based on the result, fracture mechanics analysis is made to estimate the relationship between crack depth and applied number of thermal cycles.

During the thermal transient test, acoustic emission (AE) is monitored using four channel AE sensors. The estimated number of initiating cycles and propagating behavior of macroscopic crack by fracture mechanics analysis is compared with the result of AE monitoring examination, and the usefulness of the FRASTA technique is discussed.     

On the stress corrosion cracking mechanisms of austenitic stainless steels

In this paper, experimental results on stress corrosion cracking in austenitic stainless steels are described. Crack growth data in sodium chloride solution for AISI 304 steel obtained for different metallurgical conditions, acoustic emission data recorded during crack growth and fractographic observations have been discussed with a view to identifying the operating mechanism. Some of the experimental observations such as crack propagation occurring in discontinuous jumps of the order of a few microns, lowering of the threshold stress intensity andJ-integral values on sensitization and cold working, typical transgranular fractographic features, transition in mode of fracture from transgranular to intergranular in sensitized conditions and activation energies of the order of 50 to 65 kJ/mol can all be accounted by hydrogen embrittlement mechanism. Hydrogen generated at the crack tip by corrosion reaction diffuses ahead of the crack tip under hydrostatic stress and influences the deformation process at the crack tip and also leads to the brittle component of the crack advance in jumps.