Effects of fiber orientation on the acoustic emission and fracture characteristics of composite laminates

The effects of fiber orientation on acoustic emission (AE) characteristics have been studied for various composite laminates. Reflection and transmission optical microscopy were used to investigate the damage zone of specimens. AE signals were classified through short time Fourier transform (STFT) as different types: AE signals with a high intensity and high frequency band were due to fiber fracture, while weak AE signals with a low frequency band were due to matrix cracking and/or interfacial cracking. Characteristic feature in the rate of hit-events having high amplitudes showed a procedure of fiber breakages, which expressed the characteristic fracture processes of notched fiber-reinforced plastics with different fiber orientations. As a consequence, the behavior of fracture in the continuous composite laminates could be monitored through nondestructive evaluation (NDE) using the AE technique.     

Wet hydrogen sulfide cracking of steel monitoring by acoustic emission: discrimination of AE sources

In the petroleum industry, it is known that equipments which operate in wet hydrogen sulfide (H₂S) media can be subjected to damages like Hydrogen Induced Cracking (HIC) and Sulfide Stress Cracking (SSC). In this study, Acoustic Emission (AE) technique is used to monitor the cracking of steels immersed in sour media. The aim is to establish a methodology for HIC and SSC detection by AE and therefore to get more detailed information on the cracking mechanisms in steels during standard tests. The main focus of this article is the preliminary identification of the different AE sources involved during the tests performed in sour media. The methodology of identification of the different AE signals and the monitoring of HIC and SSC tests performed on different steel grades are described. The results indicate that AE can provide an early detection of cracking (HIC and SSC) when the various AE sources are identified.     

Acoustic emission characterization of steel in H2S solution subjected to tensile load

Abstract

This paper investigates the characteristics of acoustic emission (AE) signals generated from hydrogen induced cracking (HIC) and sulfide stress corrosion cracking (SSCC) for a medium strength steel in hydrogen sulfide (H₂S) solution. The experiments performed in this study include the constant load test (CLT) and the constant extension rate test (CERT).

From the results of the CLT, the AE count rate caused by SSCC was found to be much higher than that resulted from the HIC; and the increase of the AE count rate was observed to be approximately proportional to the increase of the applied loading. In addition, the AE frequency distribution diagrams obtained from these tests were found to contain potential information for distinguishing the mechanisms of HIC and SSCC. From the results of CERT, the AE signals detected from the specimens tested in the H₂S solution were compared with those obtained from the specimens tested in the air. In addition, the AE count rate detected from the tensile specimens was studied according to the different deformation stages of the specimens. From this study, the variation of the AE count rate in each deformation stage was described.     

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。      

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。      

Unsupervised and supervised classification of AE data collected during fatigue test on CMC at high temperature

This paper aims at giving a better understanding of damage mechanisms that control lifetime of C_f/SiC composites at high temperatures (700-1200 °C) under static and cyclic fatigue. Acoustic emission (AE) signals were analysed with a view to identify classes corresponding to a specific damage mode. An unsupervised classification method allowed differentiating signals resulting from the following damage mechanisms: collective or individual fibre breaks, matrix cracking, fibre/matrix debonding, yarn/yarn debonding and sliding at fibre/matrix interfaces or matrix cracks closing after unloading. Then, a supervised classification method was developed. It allows real-time identification of damage mechanisms regardless of testing conditions (temperature, applied load and loading mode).     

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。     

Quantitative experimental measurements of matrix cracking and delamination using acoustic emission

Quantitative measurements of the amplitude and angular variation of acoustic emission (AE) events due to matrix cracking and delamination in large quasi-isotropic composite plate specimens are reported. A procedure for determining the minimum specimen size necessary to make quantitative measurements is presented. The amplitude of AE events is quoted as the absolute surface displacement of different guided wave modes and can therefore be used as the input to forward models of the AE process. Matrix cracking events are found to be dominated by the S₀ guided wave mode and have a pronounced amplitude variation with angle. Events due to delamination growth are dominated by the A₀ guided wave mode and have no clear angular dependence.     

Acoustic emission and precision force-displacement observations of pointed and spherical indentation of silicon and TiN film on silicon

Simultaneous measurements have been recorded of the force, displacement and acoustic emission (AE) during the loading and unloading of a spherical indentor on to a silicon and a physical-vapour deposited 2.7 m thick TiN film on silicon. The AE signals were able to detect the formation of small cracks, such as Hertzian cracks, lateral cracks in silicon and to detect the onset of film cracking during loading and film delamination during unloading in TiN film. The measure data were also compared with SEM cross-section observations of the indented region. Excellent correlation between AE, force-displacement and SEM observation was found.     

Acoustic emission from stress corrosion cracks in aligned GRP

Acoustic emission (AE) produced by the propagation of stress corrosion cracks in an aligned glass fibre/polyester resin composite material has been recorded. Tests have been carried out over a range of crack growth rates and the variation of AE with crack velocity/applied stress intensity has been examined. The main source of AE is fibre fracture and there is a one-to-one relationship between the number of fibre fractures and the number of high-amplitude AE signals. This enables crack growth to be monitored directly from acoustic emission. The amplitude of AE signals produced by fibre failure appears to be proportional to the fracture stress of the fibres, although further analysis requires a greater understanding of the generation, transmission and detection of AE signals. This work demonstrates that stress corrosion cracking is an ideal source for the study of AE produced by fibre fracture without complications caused by interface effects, such as fibre debonding or pullout.On leave from the Technical University of Wroclaw, Wroclaw, Poland.     

Use of acoustic emission to determine static cracking resistance in cast titanium alloys

Summary Measurements have been made on cracking resistance in cast aluminum alloys by means of acoustic emission AE. Determining the crack start instant is important. Tests on prismatic and platey specimens made of Ti3Al and Ti5Al alloys have been made with three-point bending and stretching correspondingly. AE signals were recorded throughout the loading period up to failure. The stress intensity coefficient K_Is at the start of cracking has been determined, as has that at failure K_c. Detailed relations have been established between K_Is and K_c on the one hand and the AE signal parameters on the other. K_Is is invariant with respect to specimen thickness. Data are given on the AE excitation mechanisms for titanium alloys on failure.Translated from Fiziko-Khimicheskaya Mekhanika Materialov, Vol. 26, No. 1, pp. 103–107, January–February, 1990.     

Determination of the damage threshold in woven-ply thermoplastic laminates at T > Tg: Acoustic emission and microscopic damage analysis

An original in situ measurement of acoustic emission (AE) was applied to monitor damage progress in discrete steps during gradual load/unload tensile tests on [±45°]₇ C/PPS laminates at temperatures T > Tg, when matrix ductility is enhanced. In order to understand the specific damage behavior of such materials under severe environmental conditions, AE analysis was accompanied by microscopic observations to detect the damage initiation threshold as well as the damage mechanisms within the composite material. Once the AE source mechanisms have been separated into classes thanks to the pattern recognition software Noesis, they have been identified to match physical phenomena. Earliest cracks events occur at the crimps where the rotation of warp/weft fibres takes place, followed by the intra-bundles splitting on free surface. It is observed that the onset of intralaminar cracking and debonding is affected by the presence of matrix-rich regions between the plies, because of an extensive plasticization of the PPS matrix. The study of the specific acoustic activity of neat PPS resin specimens confirms that the local plastic deformation in matrix-rich areas contributes to delay the initiation of damage, and subsequent AE signals. Finally, AE proved to be a relevant technique to investigate damage mechanisms and to determine accurately the damage threshold in TP-based composites to be used in aeronautical applications at T > Tg.     

Acoustic emission in tests on materials in hydrogenous and corrosive media

Summary Acoustic-emission studies are surveyed for cracking resistance parameter determination in corrosive media, which have been made by the authors and by others. Acoustic emission AE has been applied in research on processes occurring in hydrogen and corrosion cracking. AE signal analysis enables one to identify crack initiation and the times between steps more accurately than do traditional methods, and also enables one to predict working life. However, further research is required on methods of identifying AE signals from various sources during failure in corrosive media.Translated from Fiziko-khimicheskaya Mekhanika Materialov, Vol. 26, No. 5, pp. 26–36, September–October, 1990.     

Damage Mode Identification for the Clustering Analysis of AE Signals in Thermoplastic Composites

An objective analytical procedure for the investigation of damage mechanisms in the thermoplastic self-reinforced polyethylene (UHMWPE/PE) composites under quasi-static tensile load has been established, using Unsupervised Pattern Recognition (UPR) technique for the clustering task of Acoustic Emission (AE) signals. Focus is on the correlating between the obtained classes and their specific damage mechanisms. This was carried out by waveform visualization and Fast Fourier Transform analysis. Pure resin and fiber bundles were tested to collect typical waveforms of matrix cracking and fiber fracture respectively, in order to label the signal classes in the composites. The evolution process of various damage mechanisms in the composites revealed that the correlating method was effective. The AE characteristics of different damage modes found out in this study can be used as the reference for identifying unknown AE signals in the UHMWPE/PE composites. The established procedure is also potential in the investigation of failure mechanisms for composite materials with UPR technique.     

Detection of Acoustic Emission (AE) from Zinc Emibrittlement Cracking During Welding Using Optical Fiber AE Monitoring System

We have developed an optical fiber acoustic emission (AE) monitoring system equipped with phase and polarization control systems. The phase control system has an integration-type feedback loop with a phase lead process that maintains the phase difference between the sensing and the reference beam at ??/2 and compensates for environment noise in the frequency range of less than one hundred hertz. Additionally, we controlled the polarization of the sensing beam corresponding with the reference beam to keep the system highly sensitive. We applied the developed system to the detection of zinc embrittlement cracking in an AISI type 304 plate during a simulated welding process (melt-run) at elevated temperatures. We succeeded in detecting a significant amount of AE from the cracking during welding process.     

The Use of Acoustic Emission Intensity Analysis for the Assessment of Cover Crack Growth in Corroded Concrete Structures

In this paper, acoustic emission (AE) intensity analysis was utilized to assess the concrete cover cracking due to steel corrosion in reinforced concrete structures. A total of 30 reinforced concrete prism samples were tested under an accelerated corrosion test coupled with continuous AE monitoring using attached AE sensors. The samples were cast with three concrete cover thicknesses (20, 30 and 40 mm) around steel bars and were exposed to five percentages of steel mass loss: 1, 2, 3, 4, and 5 %. The cover cracking was monitored daily by visual inspection to detect and measure crack widths. Different AE signal parameters were continuously recorded during the tests, including number of hits, signal strength, energy, and amplitude. The acquired AE events were subjected to an intensity analysis of signal strength to estimate historic index (H (t)) and severity (\(S_r)\). In addition, a b value analysis was conducted on all AE data and the results were compared to those obtained from the intensity analysis. The results showed that increasing the cover thickness had no significant impact on AE parameters (number of hits, cumulative signal strength, cumulative energy, amplitude, H (t), and \(S_{r})\) at similar values of crack growth. Nonetheless, varying the cover thickness from 20 to 40 mm resulted in lower crack widths and slightly higher b values at the same levels of steel mass loss. It was also found that both H (t) and \(S_r\) showed a more evident correlation with the values of crack growth than did b values, regardless of cover thickness or percentage of steel mass loss. Finally, an intensity classification chart was developed to quantify the cover crack growth based on the values of H (t) and \(S_{r}\).     

Correlation of photon and acoustic emission with failure events in model composites

The photon emission (phE) and acoustic emission (AE) accompanying the deformation and failure of model composites were investigated, and related to various failure events. Unidirectional composites containing from 1 to 300 fibers were strained in tension, and photon and acoustic emission (in some cases, electron emission) monitored before, during, and after fracture. Fiber-matrix bond strength was varied with different surface treatments, thereby altering the mode and locus of fracture, and the resulting failure events were correlated with the emission signals. AE identified cohesive failure of fibers and matrix and provided a reference for the phE bursts. The photon signals were observed to be significantly more intense in poorly bonded specimens, and were shown to arise from adhesive failure between fibers and matrix. Well bonded specimens displayed predominantly cohesive failure, with correspondingly less intense photon emission. In the cases where electron emission was recorded, signals were seen only during the following ultimate failure as opposed to phE seen prior to failure.     

Fracture damage prediction in fissured red sandstone under uniaxial compression: acoustic emission b‐value analysis

In this paper, uniaxial compression tests are conducted on fissured red sandstone specimens to predict fracture damage (large‐scale events). The acoustic mission (AE) coupled with digital image correlation (DIC) technologies are used to monitor and record the real‐time cracking process of tested specimens. The AE characteristics are analysed during the cracking process. Moreover, three types of b‐value methods based on the AE parameters are adopted to predict the occurrence of large‐scale events (macro‐cracking). The results show that every macro‐cracking leads to a rapid decrease in three types of b value. When the fissured specimens reach to ultimate failure, all three types of b value reach to the minimum. The b value based on the AE parameters can be used as a predictor of large‐scale events during the cracking process of fissured rocks.     

Mechanism of fracture of short glass fibre-reinforced polyamide thermoplastic

The mechanism of fracture of short glass fibre-reinforced polyamide 6.6 thermoplastic was studied by means of optical and electron microscopy and acoustic emission methods. It was found that there were three stages in the failure, i.e. initiation of the interfacial cracks at fibre ends, propagation of the interfacial cracks along fibre sides, and propagation of the crack into the matrix leading to the failure of the composite. On the fracture surface, fibres were almost pulled-out from the matrix, not broken. The close correspondence between the crack initiation and propagation and the amplitude of AE signals was observed. The AE signals of lower amplitude occurring under a relatively low stress were considered to be made in association with the initiation and propagation of the interfacial cracks. The AE signals of higher amplitude observed prior to the failure of the composite were considered to be made in association with the occurrence of the matrix cracks. Furthermore, in order to analyse the effect of the stress state in the composite on crack occurrence and propagation, the stress levels in matrix, fibre and interface were estimated for the composite stressed to the failure stress. The calculation was based on the equivalent inclusion method proposed by Eshelby and on an assumption of a perfect bond between the matrix and the fibres. The result was found to be consistent with the mechanism of the fracture, the occurrence of the interfacial cracking in the initial stage and the matrix cracking in the final one.