Investigation of the basic creep of concrete by acoustic emission

The physical origins of the basic creep of concrete are still poorly understood even though some researchers have proposed hypotheses concrening the physical mechanisms that govern this delayed behaviour of concrete. A very complete summing up of these hypotheses was produced, in the past, by, Neville; he shows that none of them, provides a satisfactory explanation of the bulk of basic creep. More recently, Rossi has proposed a new hypothesis, which has been the subject of several articles. This article concerns the use of the acoustic emission technique as a ‘tool’ to provide information on the pertinence of the physical hypothesis advanced by Rossi. The counting of the total number of acoustic events together with a frequency analysis of the acoustic emission signals indicate that there is a strong correlation between the basic creep of concrete and the creation of microcracks in the material.     

Damage Classification of Sandwich Composites Using Acoustic Emission Technique and k-means Genetic Algorithm

In this study acoustic emission (AE) technique was used for monitoring mode I delamination test of sandwich composites. Since, during mode I delamination test various damage mechanisms appear, their classification is of major importance. Hence, integration of \(k\) -means algorithm and genetic algorithm was applied as an efficient clustering method to discriminate different failure modes. Performing primary experiments to find the relationship between AE parameters and damage mechanisms, the AE signals of obtained clusters were assigned to distinct damage mechanisms. Also, the dominance of damage mechanisms was determined based on the distribution of AE signals in different clusters. Finally SEM observation was employed to verify obtained results. The results indicate the efficiency of the proposed method in damage classification of sandwich composites.     

基于微缺陷成核序列的岩石微裂纹生长和损伤演化模型

岩石在承载之初,由于微缺陷无序成核和有限生长,在材料内部形成大量分布性微裂纹。在该文中,这种演化机制被归结为:微缺陷随机、孤立成核生成最小微裂纹和微缺陷无重叠聚集成核、排列生长形成大尺度微裂纹,裂纹尺度生长是微缺陷成核数的函数。利用微裂纹尺度-频数分布分形以及微裂纹粗糙表面分形,建立基于微缺陷累计成核数序列的裂纹尺度生长模型和损伤演化模型。通过对二维岩石试件破坏过程的微裂纹尺度统计和损伤测试表明,模型的预测结果与观测值符合较好。由于微缺陷成核与声发射源机制具有相似性以及微缺陷成核数序列与声发射数序列具有相似性,所以该模型可用于通过声发射参数序列跟踪微裂纹生长和损伤演化。裂纹尺度生长对于完整认识材料宏观力学性质演化和预测材料灾变具有重要意义。     

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.     

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.     

基于温度-应力耦合作用的岩石时效蠕变模型

基于岩石变形与热力学基本理论,建立了温度-应力耦合作用下脆性岩石时效蠕变损伤模型。根据此模型,在有限元数值软件COMSOL的基础上进行二次开发,考虑岩石介质的非均匀性,并以最大拉应力准则以及摩尔库仑准则为岩石单元的破坏准则,给出了温度-应力耦合作用下岩石时效蠕变损伤模型的数值求解方法。并结合室内实验结果验证了该模型方法的可行性和合理性。数值模拟结果表明该模型能准确描述不同温度条件下花岗岩典型蠕变全过程三个阶段,即初始蠕变阶段、稳态蠕变阶段和加速蠕变阶段。数值模拟也表明声发射累计数和岩石轴向应变的演化趋势整体上具有一致性,且在初始蠕变阶段和加速蠕变阶段出现较多的声发射现象。     

Health monitoring of carbon/carbon,woven reinforced composites. Damage assessment by using advanced signal processing techniques. Part I: Acoustic emission monitoring and damage mechanisms evolution

The present work, first of two parts, deals with three types of woven carbon/carbon (C/C) composites having differentiations during the manufacturing procedure, which influences their fibre/matrix interface. All material types were tested under tensile loading in a load–unload–reload configuration, with online acoustic emission monitoring. Unsupervised pattern recognition algorithms were utilized to classify the acoustic emission (AE) data recorded during the tests. The resulted clusters, concluded by the analysis of AE hits, are associated with the damage mechanisms of the material, activated at the different load levels, and significant remarks were extracted regarding the damage evolution and its differentiation according to the different fibre/matrix interfaces. Emphasis is given on the impact of the different interface types upon the total mechanical behavior and damage accumulation at the test coupons. A qualitative evaluation of the interfaces using non-destructive testing data is also attempted. This first part intends to propose methodologies and procedures to analyze data from online acoustic emission monitoring in order to extract useful information regarding the damage evolution within C/C materials.     

Experimental study of creep-damage coupling in concrete by acoustic emission technique

In order to design reliable concrete structures, prediction of long term behaviour of concrete is important by considering a coupling between creep and damage. An experimental investigation on the fracture properties of concrete beams submitted to creep bending tests with high levels of sustained load is reported. The influence of creep on the residual capacity and the fracture energy of concrete is studied. The progression of fracture is followed by the measurement of the crack mouth opening displacement during a three-point bending test. The sustained loading seems to increase the flexural strength of concrete, probably because of the consolidation of the hardened cement paste. The acoustic emission (AE) technique is used to perform the characterization of the influence of creep on the crack development. Results give wealth information on the fracture process zone (FPZ) and the propagation of the crack. A decrease in the amplitude distribution of AE hits is observed in the post-peak region for creep specimens. The width of the FPZ also decreases in this later indicating that the material has a more brittle behaviour which may be due to the development of microcracking under creep and the prestressing of the upper zone of the beam.     

Near-surface delamination induced local bending failure of laminated composites monitored by acoustic emission and micro-CT

Aiming to investigate the effects of the near-surface delamination on buckling response behavior of carbon fiber reinforced laminated composites under different bending modes, acoustic emission (AE) data analysis and X-ray micro-computed tomography (micro-CT) imaging method were promoted to characterize the mechanical properties, acoustic responses and damage visualization. Due to the existence of the artificial embedded delamination, when subjected to local compression induced by bending loads, the laminated composites showed a strong tendency to buckling behavior. The mechanical properties indicated that under different bending modes, the size of delamination had little influence on the relative change ratio of ultimate bearing capacity, but the thickness of specimen had a significant influence on the relative change ratio of ultimate load. AE monitoring results showing the characteristics of energy release for composites were related to the mutation rate of load curve. Moreover, cluster results indicate that matrix failure, interfacial failure and fiber failure are the main damage mechanisms. Micro-CT results illustrated that as the thickness of composites increases, there is a reduction in crack density. AE monitoring can reflect the initiation and evolution process of damage, and damage mechanism identification can be realized by clustering analysis. Besides, the internal damage morphologies acquired by micro-CT can directly verify the damage mechanisms. The cross-validation of AE and micro-CT can provide a basis for structural health monitoring of composites.

     

利用声发射信号与速率过程理论对混凝土损伤进行定量评估

混凝土性能退化过程伴随着混凝土材料中的孔隙、微裂缝和裂缝的发展,而声发射是材料不均匀变形或裂缝开裂及扩展过程的伴生现象。在损伤力学和声发射速率过程理论的基础上建立了在单轴受压状态下混凝土材料的声发射特征参数与损伤演化间关系的方程,从而实现运用测量得到的声发射特征参数最终量化评估混凝土的损伤大小。对某实际桥梁混凝土芯样进行了单轴压力下的声发射试验,得出了混凝土的实际损伤量;通过对声发射Kaiser效应和Felicity效应的分析,评价了混凝土芯样的受力历史;试验结果证明了声发射技术在混凝土损伤评估应用中的有效性和准确性。     

Damage monitoring and identification in SiC/SiC minicomposites using combined acousto-ultrasonics and acoustic emission

The acoustic emission (AE)-based technique is considered to be a promising way to real-time monitoring of microstructural changes and damage evolution in Ceramic Matrix Composites (CMCs). The present paper proposes a testing protocol that combines acousto-ultrasonics (AU) and acoustic emission (AE) monitoring, with a view to obtain both global and local definite characteristics on damage modes and kinetics. It is developed and assessed on SiC/SiC minicomposites, which are appropriate test specimens to establish sound relations between mechanical behavior and damage modes. AU wave velocity measurements provide a global measure of matrix cracking damage and the relations between crack growth and damage characteristics. AE monitoring allows accurate localization of AE sources taking into account wave velocity dependence to damage as well as differentiation of the damage modes, which control the mechanical behavior. Finally, multivariate analysis of AE data allowed classification of signals into clusters, which were successfully associated to the various damage modes.     

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.     

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.     

Corrosion Process and Mechanisms of Corrosion‐Induced Cracks in Reinforced Concrete identified by AE Analysis

Abstract: Concrete structures could suffer from the corrosion of reinforcing steel bars (rebars) because of the penetration of chloride ions. For crack detection and damage evaluation in concrete, acoustic emission (AE) techniques have been extensively applied to concrete and concrete structures. In the corrosion process of reinforced concrete, it is demonstrated that continuous AE monitoring is available to identify the onset of corrosion and the nucleation of concrete cracking because of the expansion of corrosion products. At the latter stage, the expansion of corrosion products generates corrosion‐induced cracks in concrete. The generating mechanisms of these cracks are studied in accelerated corrosion tests of reinforced concrete beams. Kinematics of microcracks are identified by SiGMA (Simplified Green’s functions for Moment tensor Analysis) analysis of AE. It is demonstrated that AE activity at the onset of corrosion and at the nucleation of corrosion‐induced cracks is in remarkable agreement with the phenomenological model of the corrosion process in steel. Then, mechanisms of corrosion‐induced cracks are visually and quantitatively investigated by the SiGMA analysis.     

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.     

Moisture effects and acoustic emission characterization on lap shear strength in ultrasonic welded carbon/nylon composites

Carbon/nylon composites were ultrasonically welded under controlled welding pressure and time. The optimized pressure and time for the highest joining strength are found by conducting the lap shear test. The acoustic emission (AE) technique used during the test is able to detect the first-damage load (FDL) and identify the damage mechanisms. Fiber breakage contributes to higher lap shear strength of the specimen, while debonding and pull-out lead to lower strength. Furthermore, the configurations of the AE curves provide the judgment of the magnitude of lap shear strengths of composites. The moisture absorption in welded composites follows Fick's law of diffusion; and lap shear strength of the composite decreases with increasing moisture content, the correlation of which follows an exponential decay function. The reduction of the strength is due to weaker hydrogen bond in the matrix connected between water and amide groups, and damage caused by the swelling of the matrix.     

Principal Component Analysis of Acoustic Emission Signals From Landing Gear Components: An Aid to Fatigue Fracture Detection

Abstract: This work forms part of a larger investigation into fatigue crack detection using acoustic emission (AE) during landing gear airworthiness testing. It focuses on the use of principal component analysis (PCA) to differentiate between fatigue crack propagation (FCP) signals and high levels of background noise. An artificial AE fracture source was developed and additionally five sources were used to generate differing artificial AE signals. Signals were recorded from all six artificial sources in a real landing gear component subject to no load. Furthermore, artificial FCP signals were recorded in the same component under airworthiness test load conditions. PCA was used to automatically differentiate between AE signals from different source types. Furthermore, successful separation of artificial FCP signals from a very high level of background noise was achieved. The presence of a load was observed to affect the ultrasonic propagation of AE signals.     

Recovery of microcrack parameters in mortar using quantitative acoustic emission

A three-dimensional quantitative acoustic emission (AE) analysis of microcracking in unreinforced mortar beams was conducted. In order to facilitate the analysis of the large amounts of data generated by an AE test, a simplified method for the inversion of AE signals was developed. By applying the theoretical Green's function for an infinite space, the multichannel deconvolution normally required of AE data inversion reduces to a nonlinear curve-fitting problem. Using this procedure, microcracking in a mortar specimen was evaluated using a seismic moment tensor representation. Source-time functions for the microcracks were also recovered. The locations of the AE events were calculated, and damage localization was observed. The moment tensor analysis showed the dominant mode of microfracture to be mode II, with a limited number classified as mixed mode. A microstructural mechanism for this behavior is 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。