Electron emission and acoustic emission from the fracture of graphite/epoxy composites

In past studies it has been shown that the fracture of materials leads to the emission of a variety of species, including electrons, ions, neutral molecules, and photons, all encompassed by the term fracto-emission (FE). In this paper we examine electron emission (EE) from the fracture of single graphite fibres and neat epoxy resin. We also combine measurements of EE with the detection of acoustic emission (AE) during the testing of graphite-epoxy composite specimens with various fibre orientation. The characteristics of these signals are related to known failure mechanisms in fibre-reinforced plastics. This study suggests that by comparing data from AE and FE meausrements, one can detect and distinguish the onset of internal and external failure in composites. EE measurements are also shown to be sensitive to the locus of fracture in a composite material.     

An acoustic-emission characterization of the failure modes in polymer-composite materials

A new methodology for the analysis of failure modes in composite materials by means of acoustic emission techniques has been developed. A single-carbon-fiber composite based on a polyester matrix, has been used as a simple model. The occurrence of fiber-breakage during tensile loading tests has been observed by a polarized light microscope and concurrently detected by a resonant acoustic probe. The resonant probe has been used as a trigger for the reading of fiber failure events. Single acoustic emission events from a wide-band probe has been recorded for FFT Analysis. The single-fiber specimen, having a unique failure mode, has advantages for the standardization of AE techniques for the quantitative analysis of failures in polymer-composite materials.

The same procedure can be exploited to investigate other failure modes namely, fiber matrix solidus debonding and matrix cracking.     

Schallemissionsuntersuchungen an polymeren Verbundwerkstoffen. Teil II: Experimentelle Ergebnisse

Acoustic Emission Monitoring of Polymeric Composite Materials. Part II: Experimental results The technique of acoustic emission monitoring of polymeric composite materials is described. It is a high sensitive, quasi-nondestructive testing method that indicates the origin and the behaviour of flaws in such materials, when submitted to different load histories. In the usage of sophisticated signal-analysis-methods it is possible to distinguish between different types of failure mechanisms, such as fiber fracture, delamination or fiber pull-out. Imperfections can be detected while monitoring complex composite structures by acoustic emission measurements.     

复合材料高压气瓶声发射检测研究

芳纶纤维缠绕气瓶水压及爆破过程声发射特性表明，费利西蒂比是评价气瓶内部损伤的重要参数，它直接影响爆破强度，高的幅度分布值是气瓶爆破的先兆。     

A study on the failure mechanisms of carbon fiber/epoxy composite laminates using acoustic emission

The complex failure mechanisms that are commonly considered as the distinctive characteristic of composites are being amenable to nondestructive test advance. This research adopts the acoustic emission technique to study the failure mechanisms and damage evolution of carbon fiber/epoxy composite laminates. Effects of different lay-up patterns and hole sizes on the acoustic emission response are studied to set up the mapping between the failure properties and the acoustic signal features such as the energy, counting and amplitude. Moreover, the microscopic properties of different composite specimens after fracture are watched and analyzed by scanning electron microscope (SEM). Based on the mapping conception, the controlling microscopic failure mechanisms of composites including the splitting matrix cracking, fiber/matrix interface debonding, fiber pull-out and breakage as well as delamination are identified. It is expected the influence of complex lay-up patterns and sizes on the damage and failure properties of composites is represented by creating true mapping based on the acoustic emission technique.     

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.     

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.     

Effect of moisture absorption on the micromechanical behavior of carbon fiber/epoxy matrix composites

Carbon fiber/epoxy material in the form of a single fiber unidirectional composite was subjected to controlled humidity environments. Moisture uptake in polymer composites has significant effects on the mechanical properties of the matrix as well as on the final performance of the composite material. Diminishing of the mechanical properties of the matrix is attributed to a decrease of its glass transition temperature (T _g). The quality of the fiber–matrix interphase was assessed using the single fiber fragmentation test and the fiber-fragment length, considered as an indicator of interfacial quality. In order to measure the fiber fragment lengths and indentify failure mechanism at the interface optical observation and acoustic emission technique were used. The speed of propagation of an acoustic wave in the material was also determined. A comparison is made of interfacial shear strength values determined by acoustic emission and optical techniques. Excellent agreement between the two techniques was obtained. By means of a micromechanical model, it was possible to determine from the fragmentation lengths a measure of the interfacial shear strength between the fiber and the matrix. The role of moisture uptake swelling of the matrix on the residual stresses is considered to be important when considering the effect deterioration of interfacial shear properties. Both the contribution of the radial stresses and the mechanical component of fiber–matrix adhesion are seen to decrease rapidly for higher moisture contents in the matrix and/or interface.     

Experimental Analysis of the Influence of Structural Parameters on the Behavior of Glass-Fiber Reinforced Polypropylene Composites

In a composite material reinforced by short random fibers, damage results from different elementary failure mechanisms such as matrix microcracking, fiber pull out, failure of the fiber/matrix interface, failure of fibers, etc. These damages influence greatly the macroscopic behavior of composite materials. To obtain good mechanical performance of a composite material, it is important to optimize the fiber ratio and the quality of the fiber/matrix interface, which have a direct influence on the damage mentioned above. The main objective of this study is to determine the influence of structural parameters on the evolution of damage for two types of polypropylene glass-fiber reinforced composites. In parallel with the classical approach of the mechanical theory of damage, which consists in load–unload tensile tests, the use of acoustic emission allows one to follow in real time the character and the importance of damage mechanisms in the course of loading. In addition, fractographic analysis makes it possible to confirm different assumptions and conclusions from this study.     

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。     

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。      

玻璃纤维复合材料静载荷声发射试验研究

通过对玻璃纤维复合材料试件静力载荷条件下的声发射试验,研究了该种材料在静力试验条件下的损伤破坏过程的声发射特性,分析了该材料损伤各阶段的声发射特性和对应的载荷比,系统而完整地得到了该材料损伤类型特征及声传播特性。试验证明:对玻璃纤维复合材料进行静力载荷条件下的声发射试验研究,可有效并清晰地揭示该材料在静力试验条件下损伤破坏过程中的声发射特性及材料损伤类型特征,为该材料的寿命健康监测、缺陷判定提供评价依据。     

The characteristics of acoustic emission from a hole in a composite laminate

Holes in composite laminates are sources for the onset of delamination and matrix cracking. Early discovery of delamination growth makes it possible to repair the damage. Monitoring of acoustic emission and analyzing the signals may enable the recognition of the onset of delamination and its location by using a few sensors and calculation of arrival times. It is shown that acoustic emission signals originating from holes can be characterized by using a ‘pattern recognition’ scheme. Thus the acoustic emission which results from the presence of a hole can be differentiated from that generated by fiber breakage.     

复合材料随机渐进失效分析与声发射监测

结合随机渐进失效分析方法和声发射监测对复合材料单向拉伸试件进行损伤分析。结果表明: 随机渐进失效方法能很好地反映复合材料失效的随机性和渐进性特征。受载初期, 复合材料失效的随机性特征明显, 在整个试件内均有失效产生。随着载荷的增加, 损伤不断累积, "随机临界核"形成, 复合材料很快失效, 且一旦有失效产生, 纤维断裂数的增加与声发射事件数的累积具有很好的一致性。比较基体开裂和界面脱粘对复合材料拉伸性能的影响: 界面脱粘比基体开裂更容易导致复合材料拉伸性能的下降, 当不存在基体开裂和界面脱粘时, 纤维断裂呈现"集簇"特征, 复合材料断裂的脆性特征较为明显。      

In-Situ Monitoring of Damage Evolution in Glass Matrix Composites during Cyclic Loading using Nondestructive Techniques

Infrared thermography is a powerful non-destructive testing technique which can be used for the detection of damage in advanced materials such as ceramic matrix composites. The purpose of this study is to apply a non-destructive methodology for analyzing, in real-time, the thermal effects in ceramic matrix composites caused by cyclic loading. Mechanical stresses induced by cyclic loading cause heat release in the composite due to failure of the interface, which results in increasing the material’s temperature. The heat waves, generated by the thermo-mechanical coupling, and the intrinsic energy dissipated during mechanical cyclic loading of the specimen, were detected by an infrared camera. The results were correlated with acoustic emission events that occurred during the damage accumulation process of the material.     

Simulation of Acoustic Emission in Planar Carbon Fiber Reinforced Plastic Specimens

The simulation of acoustic emission waveforms resulting from failure during mechanical loading of carbon fiber reinforced plastic structures is investigated using a finite element simulation approach. For this investigation we focus on the dominant failure mechanisms in fiber reinforced structures consisting of matrix cracking, fiber breakage and fiber-matrix interface failure. To simulate the failure process accurately, we present a new acoustic emission source model that is based on the microscopic source geometry and micromechanical properties of fiber and resin. We demonstrate that based on this microscopic source model these failure mechanisms result in excitation of macroscopic plate waves. The propagation of these plate waves is described using a macroscopic three-dimensional model geometry which includes contributions of reflections from the specimen boundaries. We further present a model of the acoustic emission sensors used in experiments to simulate the influence of aperture effects. To enhance the understanding of correlation between macroscopically detectable acoustic emission signals and microscopic failure mechanisms we simulate the response to different source excitation times, crack surface displacements and displacement directions. The results obtained show good agreement with fundamental assumptions about the crack process reported by various other authors. The simulated acoustic emission signals obtained are compared to experimentally measured waveforms during four-point bending experiments of carbon fiber reinforced plastic structures. The simulated signals of fiber-breakage, matrix-cracking and fiber-matrix interface failure show systematic agreement with the respective experimental signals.     

Wavelet transform of acoustic emission signals in failure of model composites

The fundamental characteristics of acoustic emission (AE) signals, such as the attenuation, and frequency dependency of AE signals, were investigated and the fracture process of the single fiber composite (s.f.c.) was examined. As a result, the frequencies of AE signals were almost unchanged, while the amplitudes attenuated greatly with the increment of the propagation length. This proved that the frequency analysis is an effective way in processing AE signals of composite materials. In the fracture process of the s.f.c., the number of AE events was in a good agreement with the number of fiber breakages, and the sources of AE signals were the failure modes at fiber breakages. Using the proposed time-frequency method of wavelet transform (WT) to process AE signals, the microfailure modes at a fiber breakage and the microfracture mechanism, such as the sequence of each failure mode and their interaction, were made clearer. These indicated that both processing methods of AE signals, fast-Fourier transform and WT, were powerful for identifying the microfailure modes and for elucidating the microfracture mechanisms in composite materials.     

聚合物基复合材料界面剪切强度的测试

用声发射技术确定纤维断裂的位置,从而决定纤维断裂段的长度分布。采用含有单纤维丝的聚合物基复合材枓试件,应用细观力学模型,根据纤维断裂段的长度,确定纤维和基体之间的界面剪切强度。这种方法适用于不透明的复合材料基体,有可能用来测定金属基和陶瓷基复合材料的界面剪切强度。     

无机颗粒形状对高储能密度有机复合材料介电性能的影响

通过在有机基体内添加无机陶瓷颗粒形成二相复合材料是当前研究高储能密度的热点和难点,材料的静电储能特性由其内部电场分布决定。对于纯高聚物材料在均匀外电场环境中其内部电场分布均匀,但当填充无机颗粒形成复合材料时,材料局部电场会发生畸变,进而影响复合材料的介电性能。本文通过有限元方法系统研究了不同形状颗粒,包括球型、纤维状和圆片状颗粒及其空间分布的电响应特性,进而分析其对复合材料储能特性的影响。结果表明,颗粒形状及空间分布的不同均会产生不同的局部电场分布,对于球型颗粒其顶端和低端会出现明显的电场集中现象;对于纤维状颗粒,当其长径比较小时,其端部束缚电荷产生的电场畸变不能被忽略。最后,本文建立了不同形状颗粒填充复合材料三维有限元模型,计算结果表明,在相同填充浓度下,一维纤维状颗粒填充复合材料的介电常数最大,二维圆片状颗粒填充复合材料介电常数最小,而球型颗粒填充复合材料介于二者之间。本文对理解复合材料储能特性的微观机制具有重要的意义。      

Comparison of nondestructive microfailure evaluation of fiber-optic Bragg grating and acoustic emission piezoelectric sensors using fragmentation test

Nondestructive evaluation of microfailure mechanisms in two-diameter SiC fibers/epoxy composites is investigated using a directly embedded fiber-optic sensor attached with an acoustic emission piezoelectric (AE-PZT) sensor. Interfacial shear strength by fragmentation test, and optical failure observation inside microcomposite can contribute to analyze two sensors quantitatively. Although fiber Bragg grating (FBG) sensor exhibits sudden wavelength shift due to plastic deformation by larger diameter SiC fiber breakage, AE-PZT monitors much more precise microfailure process, such as the fiber break or matrix cracking. Since the FBG sensor can measure the strain at only a single point, whether it can detect a fiber break in single-fiber composite specimen depends on its proximity to the failure location. In addition, micro-strain measurement at one single point may not provide enough information on the whole microfailure process including multiple fiber breakage and matrix crack. It can be considered that FBG sensor can be somewhat effective in measuring the continuous micro-strain change due to the internal disturbance such as resin curing, whereas AE-PZT sensor can be effective in detecting the microfailure by elastic wave propagation through the composite materials.