Nondestructive Evaluation of Fiberglass Reinforced Plastic Subjected to Combined Localized Heat Damage and Fatigue Damage Using Acoustic Emission

The effect of fatigue damage to unidirectional fiberglass composite specimens with prior contact heat damage was investigated. After damaging the specimens by contacting them to a hot tip at 360°C, the specimens were subjected to fatigue loading at cyclic stress amplitude corresponding to 65% of the specimens' ultimate tensile strength. The fatigue experiments was halted after 3,000 cycles. The specimens were then subjected to tensile tests while monitoring their acoustic emission (AE) activity. In addition, acoustic emission activities of undamaged and contact heat-damaged specimens were monitored during tensile tests for comparison with specimens with combined fatigue and heat damage. AE activities of all specimens can be categorized into three regions: an early rise in activity, a relatively dormant period in activity, and a high exponential activity before failure. The early rise in activity did not appear on the specimens with combined contact heat and fatigue damage. For undamaged and contact heat-damaged specimens, the period of the dormant activity was independent of the contact heat duration of less than 15 minutes. However, the period was a function of the contact heat duration for combined contact heat and fatigue damaged specimens. Analyzing event duration distribution identified micro-mechanisms of the damage growth upon tensile loading. AE-stress delay concept was used to predict the state of the damage in the composite. A correlation between stress delay parameter and damage parameter was obtained for all of the specimens. Fatigue life of contact heat damaged specimen was also studied. It was found that localized heat damage reduced the fatigue life significantly. Loss of matrix to transfer the load to the fibers uniformly was believed to be responsible for the reduction in the fatigue life.     

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。      

Fracture behavior and acoustic emission in bending tests on single-fiber composites

The bending fracture mechanisms and interfacial behavior of single-fiber composites (s.f.c.) with different fiber surface treatments and embedded fiber positions were investigated in three-point bending with simultaneous acoustic emission monitoring. Microfractures occurring at fiber breakages were examined by AE parameters and observations by a polarized microscope. As a result, it was found that AE signals in a bulk resin specimen were almost not detected, while many AE events were monitored in the s.f.c. bending specimens. The number of AE events was in good agreement with the number of fiber breakages, except for specimens with an embedded fiber near the compressive surface. Using AE parameters, especially the peak frequency and its power energy obtained by a power spectrum analysis, failure modes can be identified. A transition of failure mode from fiber break accompanied by a matrix crack and debonding to buckling is observed when the stress in the embedded fiber changes from tension to compression. The debond length is very long near the loading point for the specimens with a fiber near the tensile surface, but it decreases with increasing distance from the loading point. The debond length is small for the specimen with an embedded fiber near the neutral plane since the strain in the fiber decreases. Furthermore, a model for debonding failure is proposed and the maximum interfacial shear stress is derived. It is confirmed that fiber fragment lengths for the specimens with a fiber near the tensile side can be also expressed by the Weibull distribution as done in s.f.c. tensile tests.     

Fracturing behaviour of sandstone specimens with a cavity formed by intersecting excavations under compression: Experimental study and numerical modelling

To investigate the mechanical properties, damage characteristics, and fracturing behaviour of specimens with a cavity formed by intersecting excavations, a series of uniaxial compression tests were conducted incorporating digital image correlation (DIC) and acoustic emission (AE) techniques. PFC^2D modelling was conducted to further study the failure modes and crack evolution under biaxial loading. The results showed that the mechanical properties are significantly weakened by the cavity and influenced by its shape. The failure of the specimens containing a cavity under uniaxial compression can be considered as a progressive process of crack initiation, propagation, and coalescence of different cracks with each other, leading to forming macrofractures, which can be visually displayed by the DIC technique. A new method for determining the crack closure stress is proposed, and the crack initiation stress and the crack damage stress of specimens are also obtained by the AE measurements. The failure mode of the intact specimen changed from the tensile–shear failure mode under the uniaxial compression to the shear‐dominated failure mode under the biaxial compression. Failure of the specimens with a cavity is dominated by shear cracks rather than tensile cracks. Under high confining stresses, almost no macrotensile cracks appeared on the roof or floor of the cavity; instead, several spalling fractures were visible on the two sides of the cavity. The fracturing mechanism is well explained by the evolution of the internal stresses in the specimens.     

Investigation of damage mechanisms in self-reinforced polyethylene composites by acoustic emission

The objective of this study is to investigate the damage mechanisms in self-reinforced polyethylene composite laminates (UHMWPE/HDPE) under monotonic tensile loading by the acoustic emission (AE) technique. Fracture surface examinations were conducted using a scanning electron microscope (SEM). Using model specimens exhibiting a dominant failure mechanism, correlations were established between the observed damage growth mechanisms and the AE results in terms of the events amplitude. These correlations can be used to monitor the damage growth process in the UHMWPE/HDPE composite laminates exhibiting multiple modes of damage. Results from this study revealed that the AE technique is a viable and effective tool for identifying damage mechanisms such as fiber–matrix debonding, matrix cracking, fiber pull-out, fiber breakage and delamination in the UHMWPE/HDPE composite materials.     

Effects of loading and sample geometry on acoustic emission generation during fatigue crack growth: Implications for structural health monitoring

The reliability of traditional non-destructive methods for crack detection is well understood and characterised using Probability of Detection (POD) curves. Structural Health Monitoring (SHM) techniques in contrast remain largely unquantified. The performance of the Acoustic Emission (AE) technique for damage detection and location in potential SHM applications is underpinned by the intensity of AE signal generation from the damage site. In this paper, factors influencing the rates of emission of Acoustic Emission (AE) signals from propagating fatigue cracks were investigated. Fatigue cracks were grown in specimens made from 2014 T6 aluminium sheet while observing the effects of changes in crack length, loading spectrum and sample geometry on rates of acoustic emission. Significant variation was found in the rates of AE signal generation during crack progression from initiation to final failure with a number of distinct phases identified in that progression implying different failure mechanisms operating at particular stages in the failure process. A new ‘probability of hit’ method for quantifying crack detecting capability using AE is also presented.     

基于声发射信号模式识别的 UHMWPE/ LDPE复合材料损伤机制分析

测定了 U HMWPE/ LDPE复合材料在准静态拉伸作用下的声发射 (AE) 信号 , 用无监督模式识别方法对预处理后的 AE信号进行分类 , 据此分析了几种试样 (0° 、90° 和 [ + 45° / - 45° ]) 的损伤机制。研究表明 ,模式识别 (PR) 方法能识别出试样中基体开裂、 纤维2基体界面脱粘、 纤维抽拔和纤维断裂等损伤模式 , 识别结果与利用扫描电子显微镜 (SEM) 对破坏断面观察得到的结果一致。U HMWPE/ LDPE复合材料的 AE信号特征只受损伤模式的影响而与试样类型无关 , PR方法能有效地区分不同损伤模式的 AE信号 , 每种损伤模式的 AE信号累计数对应变的关系曲线能清楚地反映复合材料的损伤进程。AE信号的 PR分析为复合材料的损伤机制分析提供了准确依据。      

Effect of impact loading on damage and Residual Compressive Strength of CFRP laminated beams

The effect of flexural impact loading on damage characteristics and on Residual Compressive Strength (RCS) of CFRP composite beam specimens was investigated. Detection and follow-up of interlaminar damage initiation and propagation processes were conducted via video inspection of specimens subjected to quasi-static loading. It was found that the shear delamination mechanism is the most effective in the reduction of RCS. The edge delamination process, on the other hand, seems to have only a minor effect on residual failure characteristics. Compressive failure was mainly controlled by sub-laminate buckling, which seems to be dependent on delamination length, damage location and layer sequence. The presence of tough adhesive interlayers was found to delay and even prevent interlaminar damage formation and growth under flexure and compressive loading. The dependence of RCS on initial flexural energy seems to be similar under both quasi-static and impact loading modes. It was concluded that the inter-relationship of flexural energy, damage characteristics and RCS, as derived via beam specimen testing, may provide a sensitive tool for comparative damage tolerance evaluation of different composite material configurations.     

碳纤维增强环氧树脂复合材料螺栓连接结构在拉伸载荷下损伤过程的声发射分析

采用声发射技术对不同几何尺寸的碳纤维增强环氧树脂复合材料（CFRP）螺栓连接结构在静力载荷下破坏行为进行了试验研究,比较了不同几何构型下的连接结构的破坏行为与声发射信号之间的映射关系。采用声发射技术对结构损伤过程中的声发射信号进行全程采集与转换,结合CFRP螺栓结构的载荷-位移曲线和宏/细观破坏形貌,分析了幅值、熵曲线和Andrews曲线与破坏行为之间的关系。结果表明:挤压与剪切破坏试件的载荷-位移曲线均呈现出较明显的塑性特征。结构发生挤压和剪切破坏时,声发射信号以中幅值信号为主,并伴随少量高幅值信号;结构发生拉伸破坏时对应的幅值为中幅值信号。根据熵曲线特征将CFRP连接结构破坏过程分为四个阶段,在损伤演化阶段发生纤维断裂、分层等失效模式,在结构失效阶段以分层失效为主。基于Andrews曲线分析得到挤压和拉伸失效模式在损伤演化阶段会出现多种损伤类型,剪切失效模式在结构失效阶段会出现多种损伤类型。      

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.     

Fracture behavior and damage mechanisms identification of SiC/glass ceramic composites using AE monitoring

Large Scale Bridging in SiC/MAS-L (ceramic glass matrix) composites was investigated by using DEN specimens under tensile loading conditions with in situ Acoustic Emission monitoring. The AE data were successfully classified using Unsupervised Pattern Recognition Algorithms and the resulted clusters were correlated to the dominant damage mechanisms of the material. The evolution in time of the different damage mechanisms is feasible after the pattern recognition classification. Microscopic examination was used to correlate the clusters to the damage mechanism they correspond and thus to provide the failure mode identification based on AE data.     

Comparative study of the fracture behavior of flow-molded GMT-PP with random and chopped-fiber mats

The fracture toughness, K_Q, of hot-flow compression-molded glass-mat-reinforced thermoplastic polypropylene (GMT-PP) with random (R) and chopped (C) fiber mats was determined on single-edge-notched specimens loaded in tension (SEN-T) at ambient temperature. The development of damage was assessed by location of the acoustic emission (AE) activity in specimens cut from the side and central parts of the molded plaques and loaded along (L) and transverse (T) to the flow direction. The molding-induced anisotropy was clearly reflected in the K_Q values, which were considerably higher in the L than in the T orientation, especially for specimens cut from the side of the plaque. The toughness response of GMT-PPs with R and C mats was similar however. It was found that the fracture toughness calculated by considering non-linearity in the force vs. displacement curve according to ASTM E 399 is the best estimate for the initiation value. The damage zone and its development was estimated by considering the located AE events. AE served also to trace the differences in the failure mode as a function of mat type and loading ranges.     

Tensile failure of laminates containing an embedded wrinkle; numerical and experimental study

The failure of a quasi-isotropic composite laminate containing an embedded out-of-plane fibre wrinkle defect was investigated under tension loading. Laboratory test specimens with controlled severity of fibre waviness were manufactured. Along with recording load–displacement data, high resolution camera images were taken at regular intervals which monitored the initiation and interaction of different damage mechanisms during test. Three-dimensional FE models were built following the geometry of actual test specimens. The information obtained from the tests was used to develop user material subroutines, implemented in Abaqus/Explicit as continuum damage and cohesive zone models for intra- and inter-ply failure respectively. The results of the simulations showed very good correlation with test observations in terms of failure load, location of damage initiation and interaction between different damage mechanisms for a range of waviness cases tested.     

基于声发射检测技术的PE/PE自增强复合材料破损机理分析

用声发射(AE)技术研究了聚乙烯自增强复合材料的拉伸损伤与断裂行为.宽带传感器记录了不同角度纤维铺层的复合材料试样在拉伸破坏过程中的声发射信号,用扫描电子显微镜(SEM)观察了试样的几种典型的损伤破坏断面,对比分析了不同类型的损伤机制.实验分析表明,拉伸过程中破坏机制对声发射信号的特征具有显著影响,不同损伤模式的信号频谱特征存在明显的差异.声发射检测能有效提取热塑性复合材料损伤破坏信息,在材料的结构损伤主动监测中有良好的应用潜力.     

ESEM investigations for assessment of damage kinetics of short glass fibre reinforced thermoplastics - Results of in situ tensile tests coupled with acoustic emission analysis

The damage mechanisms of short glass fibre reinforced polypropylene (PP) and polybutene-1 (PB-1) materials were investigated. For this purpose, in situ tensile tests were conducted in the environmental scanning electron microscope (ESEM) while simultaneously recording the acoustic emission (AE). To be able to observe damage mechanisms directly during loading, notched specimens were used. This method allows the direct correlation of the recorded load - elongation data with observed damage mechanisms, as well as correlations with acoustic emission data. Hence, it is possible to describe the damage kinetics of short glass fibre composite.It was found that different bonding conditions of the two investigated materials result in different damage mechanisms as well as in different AE behaviour. For fibre reinforced PP with excellent bonding conditions of the fibres in the polymeric matrix, fibre fracture, slipping of fibres in the delamination area, debonding and pull-out with matrix yielding was observed. The determined AE parameter amplitude A_p and energy E_AE for the PB-1 material are lower because of the weak bonding of the fibres to the PB-1-matrix. Hence, energy dissipative damage mechanisms like pull-out with matrix yielding can occur only in a limited part of such materials.     

Investigating the static response of hybrid fibre–metal laminate doublers loaded in tension

Experimental and numerical studies have been undertaken on hybrid fibre–metal (aluminium–Glare) laminate (FML) doubler joints to investigate their static response under tension loading. The specimens either have the fibres parallel to the loading direction (spanwise) or perpendicular to the loading direction (chordwise). Inevitably sheets in these laminates butt together and these butts can affect the joint strength. The effect of the butt position was investigated. The spanwise specimen was found to have the highest strength followed by chordwise specimens without butts and finally chordwise specimens with butts. The most critical position for a butt was found to be adjacent to the doubler end. The ultimate strength for spanwise and chordwise specimens without butts was controlled by the failure in the Glare layer, however, for specimens with butts, the butts control the ultimate strength. A progressive damage numerical analysis was undertaken using cohesive zone elements to model failure in the Glare and in the adhesive layers. This modelling was found to be in good agreement with the experiment data both in terms of the strength and the failure mechanisms.     

Numerical simulation on the acoustic emission activities of concrete

Acoustic emission (AE) is a very important tool to study the cracking process and to evaluate the integrity of structures. The numerical simulation on AE activities is helpful for us to explain the AE data observed during the AE test of concrete and to therefore clarify the associated failure mechanism. Based on the knowledge that the AE events of concrete under external loading are closely related to its internal damage, it becomes possible for us to simulate these AE activities of concrete at meso-level with a numerical tool. In this paper, a numerical model is used to simulate the AE activities of mortar and concrete beams under three-point bending. Based on numerical simulation, the load–displacement curves and corresponding AE activities of mortar and concrete specimens are obtained and are calibrated against the corresponding experimental results. Subsequently, this numerical model is utilized to simulate the AE activities of mortar specimen when it is subjected to cyclic loading, and the Kaiser effect of AE is reproduced.     

纤维增强复合材料声发射的Felicity效应

本文对正交铺层的层间剪切试样和含表面裂纹试样的玻璃纤维增强复合材料板进行了声发射试验研究,记录了在单向拉伸下声发射的总数,计数率和幅度分布曲线。结果表明,两种试样的声发射幅度分布几乎相似,只是在加载的最后阶段才显示出由纤维破坏而引起的差别。这说明含表面裂纹板的破坏方式主要是以层间剪切破坏为主。两种试样的声发射总数—载荷关系曲线表明:在低载荷水平时,试样显示Kaiser效应,而在高载荷水平时则显示Felicity效应。      

Acoustic Emission Study on Effect of Stress Triaxiality on Damage Micromechanisms in Near Alpha Titanium Alloy

Acoustic Emission (AE) studies have been performed on smooth and notched specimens of a near alpha titanium alloy under monotonic loading condition to study the effect of stress triaxiality on damage micromechanisms. The damage in the notched specimen was found to be of cleavage type as against the ductile type of damage in smooth specimen. This difference was distinguished in the AE energy evolution with notched specimen producing higher energy rate with lower total energy than that of smooth specimen.     

Acoustic Emission as a Tool for Damage Identification and Characterization in Glass Reinforced Cross Ply Laminates

Loading of cross-ply laminates leads to the activation of distinct damage mechanisms, such as matrix cracking, delaminations between successive plies and fibre rupture at the final stage of loading. This study deals with the investigation of the failure of cross ply composites by acoustic emission (AE). Broadband AE sensors monitor the elastic waves originating from different sources of failure in coupons of this material during a tensile loading-unloading test. The cumulative number of AE activity, and other qualitative indices based on the waveforms shape, were well correlated to the sustained load and mechanical degradation as expressed by the gradual decrease of elastic modulus. AE parameters indicate the succession of failure mechanisms within the composite as the load increases. The proposed methodology based on Acoustic Emission for the identification of the damage stage of glass reinforced cross ply laminates is an initial step which may provide insight for the study of more complex laminations.