An integrated study of defects in composite materials using brittle strain-sensitive coatings and acoustic emission

A technique for an integrated study of defects in composite materials using brittle strain-sensitive coatings and acoustic emission is described. Strain-sensitive coatings make it possible to detect hidden defects that affect the stress–strain state of the product at the early stages of loading. Acoustic emission monitoring conducted during further loading provides a more accurate determination of the position of potentially dangerous defects. The technique is optimized using flat samples with a central hole simulating a defect under conditions of uniaxial tension.     

Strength failure and crack coalescence behavior of brittle sandstone samples containing a single fissure under uniaxial compression

Uniaxial compression experiments were performed for brittle sandstone samples containing a single fissure by a rock mechanics servo-controlled testing system. Based on the experimental results of axial stress-axial strain curves, the influence of single fissure geometry on the strength and deformation behavior of sandstone samples is analyzed in detail. Compared with the intact sandstone sample, the sandstone samples containing a single fissure show the localization deformation failure. The uniaxial compressive strength, Young’s modulus and peak axial strain of sandstone samples with pre-existing single fissure are all lower than that of intact sandstone sample, which is closely related to the fissure length and fissure angle. The crack coalescence was observed and characterized from tips of pre-existing single fissure in brittle sandstone sample. Nine different crack types are identified based on their geometry and crack propagation mechanism (tensile, shear, lateral crack, far-field crack and surface spalling) for single fissure, which can be used to analyze the failure mode and cracking process of sandstone sample containing a single fissure under uniaxial compression. To confirm the subsequence of crack coalescence in sandstone sample, the photographic monitoring and acoustic emission (AE) technique were adopted for uniaxial compression test. The real-time crack coalescence process of sandstone containing a single fissure was recorded during the whole loading. In the end, the influence of the crack coalescence on the strength and deformation failure behavior of brittle sandstone sample containing a single fissure is analyzed under uniaxial compression. The present research is helpful to understand the failure behavior and fracture mechanism of engineering rock mass (such as slope instability and underground rock burst).     

Acoustic emissions during tensile test of DC06 and HCT600X

In this paper the acoustic emission behaviour during tensile tests of the materials DC06 and HCT600X is studied. Two steels with different characteristics (mild deep‐drawing steel DC06 and high‐strength steel HCT600X) are consciously chosen to show the influence of the material properties on the generated acoustic emissions. The acoustic emission behaviour and the corresponding signals differ clearly from each other. In addition, the effect of the strain rate as well as the rolling direction (0°, 90°) on the acoustic emission behaviour is investigated. Both parameters have a significant influence on the resulting acoustic emissions during tensile deformation. Furthermore, a new criterion based on the acoustic emission parameter FCOG (centroid frequency) for detection of damage beginning in dual‐phase steels is developed. The criterion supports the assumption that during tensile deformation of dual‐phase steels two failure mechanisms, ferrite/martensite interface decohesion and martensite phase fracture, exist.     

Using acoustic emission for the analysis of wear processes during sliding friction

The acoustic response of two friction pairs, in which brittle fracture and intense plastic deformation take place during friction, has been studied. The acoustic signals have been analyzed by calculating median frequencies using the window Fourier transform technique.     

Grain size effect on high-speed deformation of Hadfield steel

The influence of the microstructure on the tensile properties and fracture behavior of Hadfield steel at high strain rate were studied. Hadfield steel samples with different mean grain sizes and carbon phases were prepared by rolling at medium temperatures and subsequent annealing. A sample with an average grain size larger than 10 μm, and a small number of carbides shows ductility with local elongation (post uniform elongation) at a high-speed tensile deformation rate of 10³ s⁻¹. In addition, the fracture surface changes from brittle to ductile with increasing strain rate. In contrast, a fine-grained sample with carbides undergoes brittle fracture at any strain rate. The grain size dependence is discussed by considering the dynamic strain aging as well as the emission of dislocation from cracks. The accelerated diffusion of carbon due to grain refinement is considered as one of the important reason for brittle fracture in the fine-grained Hadfield steel.     

SUS304不锈钢拉伸过程中的声-热像特征

测量了含有Ⅰ型裂纹的SUS304不锈钢试样的单轴拉伸过程中的塑性变形和断裂。分析了裂纹尖端区域的塑性变形和断裂过程。结果表明:SUS304的各向异性在断裂阶段对声发射信号影响较大;红外热图像中的温度分布与塑性应变率有关;通过声发射参数和红外热图像可以从微观和宏观两方面分析裂纹尖端区域的塑性变形和断裂。     

基于声发射的TiB/TC4复合材料塑性变形缺陷检测

目的 钛基复合材料塑性差,断裂机制复杂,研发准确的缺陷在线检测技术对控制其服役过程的破坏风险和成形过程的零件质量具有重要意义。方法 提出了基于声发射的TiB/TC4复合材料塑性变形缺陷检测方法,首先进行3种不同应力状态TiB/TC4复合材料试样的单轴拉伸试验,以采集声发射信号,通过断口形貌分析其断裂机制,并使用谱聚类方法对声发射信号进行聚类分析,探寻声发射信号与断裂机制之间的关系。结果 通过谱聚类可将TiB/TC4复合材料塑性变形过程中的声发射信号分为低频连续和高频突发2类,其质心频率以600~650 kHz为分界。根据断口形貌分析将其分别对应为TC4基体晶粒内位错运动与晶界处增强相TiB颗粒断裂。TiB颗粒断裂的声发射信号占比随着应力三轴度的增大而提高。TC4基体晶粒内位错运动的声发射信号为低频、连续信号,而TiB断裂的声发射信号为高频、突发信号。结论 TiB/TC4复合材料塑性变形过程中的声发射信号源机制包括TC4基体晶粒内的位错运动与晶界处增强相TiB颗粒的断裂,通过谱聚类不仅能有效检测TiB/TC4复合材料塑性变形缺陷的产生,还能识别缺陷的形成机制。     

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...     

On a possible mechanism of instability of plastic deformation of metals and alloys at 4.2 K

The fine structure of spontaneous deformation jumps and jumps initiated by both the shock action on a deformation device and a strong fast-changing magnetic field (up to 2.7 MA/m) has been investigated in metals and alloys at liquid-helium temperatures. Oscillography of the signals of changes in the load on the sample and of acoustic emission has made it possible to reveal the relationship between types of jumps and postulate on the mechanism of their appearance.     

A correlation between acoustic emission and the fracture toughness of 2124-T851 aluminum

The paper reports on an experimental study of the possible application of acoustic emission as an inspection technique, to determine whether 2124-T851 aluminum plate meets certain, minimum, fracture toughness specifications. Unflawed specimens, taken in the three principal directions, from three separate plates are tested in compression and tension. The-true-mean-square or the root-mean-square voltage of the continuous acoustic emission generated during the tests is recorded as a function of specimen strain. The acoustic emission, chemical analysis, fracture surfaces, and large (1–20 μm) second-phase particles are studied in relation to fracture toughness. Results indicate that the generated acoustic emission from a longitudinal compression test, long-transverse tension test and short-transverse tension test all correlate with changes in plate fracture toughness. Based on the experimental results, the monitoring of acoustic emission during relatively simple tension or compression tests of aluminum may be useful in checking whether the material meets a fracture toughness specification. Also, the results indicate that acoustic emission is useful in studies to develop fructure toughness models.     

Deviations from the Gutenberg-Richter law

The energy distributions of acoustic emission (AE) signals have been analyzed on two scaling levels corresponding to (i) deformation of granite samples and (ii) processes on a commercial mining enterprise. It is established that, in cases of localized fracture, the AE energy distribution has a commonly accepted shape described by a power law, while a dispersed fracture is characterized by an exponential energy distribution of the AE signals. Analysis of the functional form of the energy distribution performed at the early stage of loading allows one to recognize a spatial region in the sample where localization of the defect formation will subsequently take place.     

Quantification of failure mechanisms in mode-I loading of fiber reinforced plastics utilizing acoustic emission analysis

Acoustic emission signals originating from interlaminar crack propagation in fiber reinforced composites were recorded during double cantilever beam testing. The acoustic emission signals detected during testing were analyzed by feature based pattern recognition techniques. In previous studies it was demonstrated that the presented approach for detection of distinct types of acoustic emission signals is suitable. The subsequent correlation of distinct acoustic emission signal types to microscopic failure mechanisms is based on two procedures. Firstly, the frequency of occurrence of the distinct signal types is correlated to different specimens’ fracture surface microstructure. Secondly, a comparison is made between experimental signals and signals resulting from finite element simulations based on a validated model for simulation of acoustic emission signals of typical failure mechanisms in fiber reinforced plastics. A distinction is made between fiber breakage, matrix cracking and interface failure. It is demonstrated, that the feature values extracted from simulated signals coincide well with those of experimental signals. As a result the applicability of the acoustic emission signal classification method for analysis of failure in carbon fiber and glass fiber reinforced plastics under mode-I loading conditions has been demonstrated. The quantification of matrix cracking, interfacial failure and fiber breakage was evaluated by interpretation of the obtained distributions of acoustic emission signals types in terms of fracture mechanics. The accumulated acoustic emission signal amplitudes show strong correlation to the mechanical properties of the specimens. Moreover, the changes in contribution to the different failure types explain the observed variation in failure behavior of the individual specimens quantitatively.     

界面结构对SiCf／Al复合材料性能和声发射行为的影响

运用高分辨场发射扫描电镜（ＦＥ－ＳＥＭ）和声发射（ＡＥ）测试研究了ＳｉＣ纤维铝基复合材料的不同界面组成和界面产物及其对复合材料性能和ＡＥ行为的影响发现富碳自理的ＳｉＣｆ／Ａｌ生成Ａｌ４Ｃ３脆性界面,在伸过程中界面脆断产生许多中幅ＡＥ信号,而富ＳｉＯ２处理的ＳｉＣｆ／Ａｌ生成韧工较高的富氧产物,界面强度较高,在形变过程中不易发生界面断裂,不产生中幅ＡＥ信号。     

Acoustic emission monitoring of split formation during Charpy impact testing of high strength steel

Abstract

Acoustic emission (AE) monitoring has been used to detect split formation during room temperature low blow Charpy impact testing of high strength thick strip steels. The AE signal analysis identified separate signals originating from hammer impact, plastic deformation (verified using Charpy impact testing on mild steel with no splits) and split initiation/growth. The presence of splits was confirmed by sectioning and fractography, and the splits were brittle in nature. A possible correlation between the AE signal features and fracture mode is presented.     

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.     

Fatigue residual strength of circular laminate graphite–epoxy composite plates damaged by transverse load

The research dealt with the relation between damage and tension–tension fatigue residual strength (FRS) in a quasi-isotropic carbon fibre reinforced epoxy resin laminate. The work was organized in two phases: during the first one, composite laminates were damaged by means of an out-of-plane quasi-static load that was supposed to simulate a low velocity impact; in the second phase, fatigue tests were performed on damaged and undamaged specimens obtained from the original composite laminates. During the quasi-static transverse loading phase, damage progression was monitored by means of acoustic emission (AE) technique. The measurement of the strain energy accumulated in the specimens and of the acoustic energy released by fracture events made it possible to estimate the amount of induced damage and evaluate the quasi-static residual tensile strength of the specimens. A probabilistic failure analysis of the fatigue data, reduced by the relative residual strength values, made it possible to relate the FRS of damaged specimens with the fatigue strength of undamaged ones.     

Acoustic emission in hafnium under tensile deformation

The paper addresses the dependence of acoustic emission produced in rolled hafnium GFé-1 under tensile deformation on the material’s structural state. A correlation has been established between the material structure, the level of mechanical properties and the values of acoustic parameters. Acoustic emission in non-recrystallized hafnium under tensile deformation is recorded only at the stages that precede fracture. Upon recrystallization annealing at temperatures of 1123 and 1373 K acoustic emission occurs at all the stages of tensile deformation. The highest level of acoustic emission activity in hafnium is observed during the transition from elastic to elastic-plastic strain.     

Correlation of the acoustic emission and the fracture toughness of ductile nodular cast iron

Salzbrenner has recently determined the fracture toughness of a series of ductile cast iron samples which were heat treated to produce a fully ferritic matrix. His results indicated that the fracture toughness is strongly dependent upon the average spacing between (or equally the diameter of) the spherical graphite nodules in the ferrite matrix. The acoustic emission generated during the uniaxial compressive deformation of nodular cast iron also depends strongly on the average diameter of the graphite nodules in the test sample. The present investigation was carried out to determine the correlations, if any, between the fracture toughness and the acoustic emission generated during compression of ductile cast iron. The acoustic emission generated during compression was determined using sample materials identical to those used by Salzbrenner. Excellent correlations between certain features of the measured acoustic emission and the fracture toughness were obtained. Data indicate that it should be possible to determine both the fracture toughness and the average size of the graphite nodules from the acoustic emission and load curve generated during a compression test of ductile cast iron.     

Micromechanics and acoustic emission analysis of the failure process of thermoplastic composites

The finite element method (FEM) and acoustic emission technique (AE) were applied to the micromechanics analysis of the failure process of composites with thermoplastic matrix materials. FEM calculations to local stress-strain distribution and the influence of very different intermediate layer properties are interpreted with regard to microscopic failure mechanisms in composite materials. The strongly differing AE behaviour of both chalk-filled Polyvinylchloride and high density polyethylene and short-glass-fibre reinforced polypropylene, polyamide, PBTP, SAN and ABS in tensile test experiments is demonstrated. Representative loading limits are derived from the nature and extent of the dominating failure mechanisms by comparison of theoretical and experimental results. The influence of critical strain, shear strength and fracture toughness properties of the modified matrix as well as the composite morphology and phase adhesion on significant deformation and failure stages is discussed. Finally some conclusions are drawn about a possible critical long-term strain of composites.     

Impact behavior of different stainless steel weldments at low temperatures

A comparative study was made of the fracture behavior of austenitic and duplex stainless steel weldments at cryogenic temperatures by impact testing. The investigated materials were two austenitic (304L and 316L) and one duplex (2505) stainless steel weldments. Shielded metal arc welding (SMAW) and tungsten inert gas welding (TIG) were employed as joining techniques. Instrumented impact testing was performed between room and liquid nitrogen (?196 °C) test temperatures. The results showed a slight decrease in the impact energy of the 304L and 316L base metals with decreasing test temperature. However, their corresponding SMAW and TIG weld metals displayed much greater drop in their impact energy values. A remarkable decrease (higher than 95%) was observed for the duplex stainless steel base and weld metals impact energy with apparent ductile to brittle transition behavior. Examination of fracture surface of tested specimens revealed complete ductile fracture morphology for the austenitic base and weld metals characterized by wide and narrow deep and shallow dimples. On the contrary, the duplex stainless steel base and weld metals fracture surface displayed complete brittle fracture morphology with extended large and small stepped cleavage facets. The ductile and brittle fracture behavior of both austenitic and duplex stainless steels was supplemented by the instrumented load–time traces. The distinct variation in the behavior of the two stainless steel categories was discussed in light of the main parameters that control the deformation mechanisms of stainless steels at low temperatures; stacking fault energy, strain induced martensite transformation and delta ferrite phase deformation.