Controlled energy dissipation in fibrous composites. II: Microscopic failure mechanisms

Mechanical properties and microscopic fracture mechanisms of continuous fiber reinforced polymer composites were investigated. Perforated polyimide films (e.g. Kapton®) were added between composite prepreg layers to modify the interlaminar bonding strength. Addition of highly perforated films can increase the toughness of unidirectional glass/epoxy composites without an appreciable reduction in strength. The fibrous composites studied exhibit two fracture modes (compressive and tensile) when failed by three-point bending. In general, the compressive failure mode preceded the tensile failure mode. Real-time acoustic emission (AE) analysis was found to provide more fracture information which is otherwise not discernible from mechanical testing alone. The crack initiation stress level and the subsequent crack propagation mode were identified by real-time AE during deformation and by post-failure scanning electron microscopy fracture surface analysis.     

Damage and failure analysis of a SiCf/SiC ceramic matrix composite using digital image correlation and acoustic emission

The analysis of failure behaviors of continuous fiber-reinforced ceramic matrix composites (CMCs) requires the characterization of the damage evolution process. In service environments, CMCs exhibit complex damage mechanisms and failure modes, which are affected by constituent materials, meso architecture, inherent defects, and loading conditions. In this paper, the in-plane tensile mechanical behavior of a plain woven SiC_f/SiC CMC was investigated, and damage evolution and failure process were studied in detail by digital image correlation (DIC) and acoustic emission (AE) methods. The results show that: the initiation of macro-matrix cracks have obvious local characteristic, and the propagation paths are periodically distributed on the material surface; different damage modes (matrix cracking and fiber fracture) would affect the AE energy signal and can be observed in real-time; the significant increase of AE accumulated energy indicates that serious damage occurs inside the material, and the macroscopic mechanical behavior exhibits nonlinear characteristic, which corresponds to the proportional limit stress (PLS) of the material.     

FRP复合材料拉伸过程的声发射特性实验研究

用声发射技术研究了FRP复合材料的拉伸损伤与断裂行为。宽带传感器记录了FRP复合材料试样在拉伸破坏过程中的声发射信号,运用声发射参数分析方法对单向FRP复合材料的声发射历程图进行分析,得出复合材料在拉伸过程中的损伤类型以及各损伤阶段所呈现出来的特性。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。     

UHMWPE/LDPE复合材料拉伸破坏的声发射特性研究(Ⅰ)

本文试验研究了UHMWPE/LDPE复合材料拉伸破坏的声发射特性。利用声发射仪结合参数分析得出了增强纤维和基体在拉伸破坏过程中声发射参数特征。对后续研究UHMWPE/LDPE复合材料声发射特性和拉伸破坏机理具有参考价值。     

An acoustic emission study on the fracture behavior of continuous glass fiber/polypropylene composites based on commingled yarn

The fracture behavior of continuous glass fiber reinforced polypropylene composites made of commingled yarn in the form of biaxial (±±45°) noncrimp warp‐knitted fabric, twill woven fabric, and swirl mat, respectively, was investigated by virtue of single edge notched tensile (SEN‐T) specimens. These composite laminates were manufactured by compression molding and cooled at two different rates (1°C/min and 10°C/min) during the last processing phase of the laminates. The failure mechanisms were studied by acoustic emission (AE) analysis. AE amplitude ranges corresponding to the individual failure modes have been identified. For biaxial noncrimp fabric reinforced materials, the failure mechanisms involved in the fracture procedure are governed by the interface related failure events. Higher cooling rate, which is accompanied by better fiber/matrix adhesion, results in not only the increase in the relative proportion of high‐amplitude failure events, but also the occurrence of a large quantity of fiber fracture events. For woven fabric and mat reinforced composites, fiber‐dominated failure mechanisms result in the higher fracture toughness when compared with biaxial noncrimp fabric composites. Under this circumstance, the change in cooling rate only results in the difference in the relative frequency of the individual failure modes. In addition, it is found out that the initiation fracture toughness of SEN‐T specimens can be easily assessed by marking the load value which corresponds to the first point of AE signals emitted stably in AE events‐displacement curves. POLYM. COMPOS., 2008. © 2008 Society of Plastics Engineers     

Combining in-situ synchrotron X-ray microtomography and acoustic emission to characterize damage evolution in ceramic matrix composites

In-situ synchrotron X-ray microtomography and acoustic emission (AE) were combined to study the behavior of ceramic matrix composite laminates subjected to in-plane tensile or flexural loading at room temperature. A detailed characterization of the initiation and progression of two key damage modes (matrix cracking and fiber breaks) is obtained from microtomography, and the relationship between damage and AE is directly observed. A graphical representation of AE data, which has potential for real-time use, is employed to reveal differences in damage progression due to fiber architecture or loading mode. In addition, strong empirical relationships are observed between matrix crack area and AE energy, as well as between fiber breaks and number of AE events.     

Fracture Process of Silicon Carbide Fiber-Reinforced Glasses

This paper evaluates the fracture process for fiber-reinforced glasses under tensile loading. Two types of unidirectionally aligned Nicalon SiC-fiber-reinforced glass with different fiber coatings were examined. One channel acoustic emission (AE) measurement was employed during the tensile tests. Probabilistic fracture analysis as well as the replication technique were used to investigate the relation between the AE signals and fracture processes. The AE technique proved to be an effective method for observing fracture processes of the material systems studied. The fracture process could be distinguished in terms of the AE amplitude. AE signals with high amplitudes corresponded to fiber breaking; AE signals with low amplitudes corresponded to matrix cracking, interfacial debonding, and fiber pullout. In the well-toughened material studied the reinforcing fibers would break extensively over 75% load of the ultimate strength.     

Interpreting acoustic energy emission in SiC/SiC minicomposites through modeling of fracture surface areas

The relationship between acoustic emission (AE) and damage source areas in SiC/SiC minicomposites was modeled using insights from tensile testing in-scanning electron microscope (SEM). Damage up to matrix crack saturation was bounded by: (1) AE generated by matrix cracking (lower bound) and (2) AE generated by matrix cracking, and fiber debonding and sliding in crack wakes (upper bound). While fiber debonding and sliding exhibit lower strain energy release rates than matrix cracking and fiber breakage, they contribute significant damage area and likely produce AE. Fiber breaks beyond matrix crack saturation were modeled by two conditions: (i) only fiber breaks generated AE; and (ii) fiber breaks occurred simultaneously with fiber sliding to generate AE. While fiber breaks are considered the dominant late-stage mechanism, our modeling indicates that other mechanisms are active, a finding that is supported by experimental in-SEM observations of matrix cracking in conjunction with fiber failure at rupture.     

Mechanical and fracture investigation of magnesia refractories with acoustic emission-based method

In this study, the fracture behaviour of magnesia, magnesia chrome and magnesia spinel (MgAl₂O₄ and FeAl₂O₄) refractories under wedge splitting test are qualitatively and quantitatively investigated with the acoustic emission (AE) and digital image correlation (DIC). First of all, the concepts of characteristic widths are proposed for estimating the brittleness of refractory materials according to the shape of load-displacement curve and validated by their good correlation with the characteristic length. Besides, the AE data are analyzed with AE parameter-based approaches and offer new insight into the fracture behaviour of refractory materials, including the classification of the cracking events in grains and in matrix, the distinction between the tensile mode and shear mode damage, and the visualization of the fracture process zone development. It confirms that the pre-existing micro-crack networks in refractories are favourable for the brittleness reduction, which enhance their nonlinear fracture behaviour and thermal shock resistance.     

Fatigue damage identification of SiC coated needled C/SiC composite by acoustic emission

The fatigue damage process of SiC coated needled C/SiC composite specimen was monitored by acoustic emission (AE) under tension-tension cyclic loading. By analyzing the collected AE parameters of the composite, it is found that Kaiser effect enhances with the increase of stable cycles in the fatigue process. Moreover, multivariate K-means cluster analysis of AE parameters was carried out after the standardization of energy, amplitude, peak frequency and duration of AE signal. By comparing the objective function values of different number of clusters, and referring to the intra group variance and the variance between groups, the damage modes of the needled C/SiC composite are finally divided into four clusters, and the characteristics of AE parameters with different damage modes can be obtained. Furthermore, by referring to the microstructure characteristics of needled C/SiC composite, various damage modes at different fatigue stages were analyzed. In addition, the fracture morphology of the specimen was also observed by scanning electron microscope after fatigue fracture.     

Fracture characterization of C/C composites under various stress modes by monitoring both mechanical and acoustic responses

C/C composites with different porosities, produced by chemical vapor infiltration have been mechanically tested under quasi-static loading in bending modes using uniform and notched specimens. The acoustic emission (AE) method was used to monitor the damage accumulation profile during loading up to fracture, supported by optical and scanning electron microscope characterization. Three stages in the damage buildup up to fracture were observed: Stage I, with no AE activity, Stage II, gradual growth in AE counts up to an abrupt jump and Stage III, sharp increases in AE counts. Moreover, the similarity in the profile between the cumulative AE counts vs. strain data and the predicted crack density vs. strain by the micro mechanical model suggested for interlaminar cracking, indicates the importance of AE in monitoring the damage evolution in composites in terms of AE counts. Fast Fourier transform analysis of the AE waves revealed three characteristic frequencies in Stage III, which is a sign of three main micro-mechanisms of failure which control the failure progress: fiber fracture, debonding and matrix cracking seem to be the active mechanisms.     

Fracture behavior of carbon fiber reinforced plastics determined by the time–frequency analysis method

Fourier transform has been used to study the frequency characteristics of a signal. However, based on the Fourier transform and power spectrum alone, it is hard to tell whether or not the frequency content of a signal evolves over time, even though the phase of the Fourier transform relates to time shifting. On the other hand, except for a few special cases, the frequency content of the majority of signals encountered in the real world change with time. Recently, to overcome the problem that Fourier transform is unable to represent a nonstationary signal, time–frequency analysis methods that can simultaneously represent information about the time and frequency of a signal have been developed. In this study, the damage process of cross‐ply carbon fiber reinforced plastics (CFRP) during a monotonic tensile test was characterized by acoustic emission (AE). Different laminated types of CFRP were used to determine the characteristics of the AE signal and frequency. The time–frequency analysis method was found to be useful for the determination of the fracture mechanism in CFRP (such as, matrix cracking, debonding–delamination, and fiber fracture). © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 88: 1659–1664, 2003     

Damage analysis of a SiCf/SiC ceramic matrix composite under stepwise fatigue loading with acoustic emission

Continuous fiber-reinforced ceramic matrix composites (CMCs) exhibit different damage mechanisms at multiple scales under cyclic loading. In this paper, the tension-tension fatigue behavior of a plain woven SiC_f/SiC CMC was investigated, and damage accumulation and evolution process were studied in detail via acoustic emission (AE) method. With the increase of cycles, the material exhibits obvious hysteresis behavior affected by interfacial slip and wear mechanisms. Most of the fibers with radial fracture characteristic have relatively high strength, showing excellent toughening property. In the stepwise cyclic loading process, the Kaiser effect of AE determines the initiation of AE activities at each initial loading moment, which shows obvious nonlinear damage accumulation behavior of the material. High-energy events are related to significant matrix cracking and fiber fracture, and the evolution process of material damage initiation and propagation is monitored in real time.     

Tensile fracture and failure behavior of technical flax fibers

The apparent tensile strength of technical flax fibers was determined in single‐fiber tests at various clamping lengths (20, 40, and 80 mm) and the outcome was compared with literature data. It was demonstrated that the strength of flax at each clamping length obeyed the two‐parameter Weibull model. The failure mode and sequence were studied in situ (i.e., during loading) by SEM and acoustic emission (AE). The failure sequence (axial splitting of the technical fiber along its elementary constituents, radial cracking of the elementary fibers, multiple fracture of the elementary fibers) concluded reflected the hierarchical build‐up of the flax bast fibers. To the above failure events AE amplitude ranges were assigned. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 3638–3645, 2003     

Monitoring by acoustic emission the mechanical behavior of polystyrene submitted to stress cracking at different temperatures

Environmental stress cracking (ESC) is one of the main phenomena that limit the use of polymers, being responsible for an expressive number of premature failures in service. This occurs when mechanical loading is combined with a certain kind of chemical agent, causing surface cracks and, eventually, catastrophic failure. This issue is not widely reported in the literature, and the usual procedures for investigation involve conventional mechanical testing. Alternative tools, such as, the technique of acoustic emission (AE) to monitor the stages of fracture, are rarely used. In this work, injection molded polystyrene were submitted to stress cracking conditions, using two types of active fluids and different exposure temperatures (25, 40, and 55°C). The AE technique was applied simultaneously with the mechanical testing to monitor parameters like intensity of the hits and energy released during the deformation and fracture. The results showed that the failure by ESC was related to the fluid interaction with the polymer and was very dependent on the exposure temperature. The use of acoustic emission technique was very useful to differentiate the effects of the various exposure conditions and to explain certain fracture characteristics observed by visual inspection and by scanning electron microscopy.     

Acoustic emission in single filament carbon/polycarbonate and Kevlar®/olycarbonate composites under tensile deformation

An analysis of envelope signals of acoustic emission (AE) produced from carbon/polycarbonate and Kevlar®/polycarbonate composites undergoing tensile deformation has been carried out to identify the sources of emission. The Kaiser effect was reproduced to validate the AE technique. Two different fiber failure mechanisms, i.e., fiber fracture and fiber degradation in Kevlar®/polycarbonate composite have been identified. A one-to-one correspondence between acoustic emission signals and fiber fracture and degradation has been established. It is shown that the critical length of the fiber, evaluated by using acoustic emission signals, is helpful in understanding the fracture behavior of the composites, as affected by surface treatments of the fibers.     

Interfacial properties of glass fiber/brittle-ductile dual-matrix composites using micromechanical techniques and acoustic emission

The interfacial adhesion and microfailure modes of glass fiber-reinforced brittle unsaturated polyester/modified epoxy composites were investigated via micromechanical techniques and acoustic emission (AE). Various silane coupling agents caused different degrees of interfacial adhesion and subsequent microfailure modes. In the brittle matrix layer, the number of matrix fragments was significantly influenced by the type of silance coupling agents. The more cracks, the higher the interfacial adhesion under both dry and wet conditions. This is attributed to the chemical and hydrogen bondings in two interphases. The results obtained from microdroplet and fragmentation tests were correlated by associating with the AE technique. The sequential occurrence of mainly three groups of AE were as follows: the first group originated mainly from brittle matrix cracking. The second and the third groups resulted in fiber breakage and ductile matrix cracking and debonding. For dual-matrix specimens the micromechanical tests provide reliable information with regard to the interfacial adhesion and characterize the microfailure modes when combined with the AE technique.     

Fracture and acoustic emission characteristics of glass fiber reinforced plastics panels for hot water

This study deals with the fracture process and acoustic emission (AE) characteristics of a randomly oriented E-glass fiber mat reinforcement with a crosslinked polyester. These panels were evaluated after they were immersed in hot water. The fiber volume content of the panel was 19%. Glass fiber reinforced plastics (GFRP) panels wer immersed in water at 81°C. Bending and AE monitoring tests were Performed and after bending, the cross-section of the specimen was observed by an optical microscope and SEM. The influence of degradation, due to water immersion, on the changes of fracture process of GFRP is discussed. The dominant fracture mode of the virgin specimen was matrix cracks, whereas that of the immersed specimen was debondings at the fiber bundle/matrix and fiber/matrix interfaces. This change was caused by reduction of the bonding strength at the interface. The scale of fracture can be estimated by both AE amplitude and AE energy and this estimation method was used to estimate the fracture mode changes of GFRP panels immersed in hot water.     

Low temperature fracture behaviour and AE characteristics of autoclaved aerated concrete(AAC)

For a quantitative understanding of freezing damage on Autoclaved Aerated Concrete(AAC) and fiber reinforced AAC(i.e., RAAC), the influence of water and temperature (R.T. ∼ −20°C) on those materials have been studied by the investigation of AE characteristics, the fracture mechanics J-integral test and SEM observation. Furthermore, using the AE frequency analysis based on the frequency energy density distribution ratio (EDDR), the micro-fracture process for various test conditions has been interpreted.The AE activities and fracture toughness showed a large difference depending on the water content and temperature. All the AE events emitted during the fracture toughness tests could be classified into 6 groups. Also, the AE sources were considered paying particular attention to the micro-crack formation, the friction of inter-matrix and the fiber breaking behaviors at fracture. Noting that the AE is emitted during the drying process, the drying shrinkage damage was discussed.     

Acoustic emission analysis using Bayesian model selection for damage characterization in ceramic matrix composites

Acoustic emission (AE) during tensile testing of three-dimensional woven SiC/SiC composites was analyzed by a statistical modeling method based on a Bayesian approach to quantitatively evaluate the fracture process. Gaussian mixture models and Weibull mixture models were utilized as candidate models describing the AE time-series data. After fitting AE time-series data to these models with Markov Chain Monte Carlo (MCMC) methods, the model selection was conducted by stochastic complexity. Among the candidate models, the two-component Weibull mixture model was automatically selected. It was confirmed that the component distributions in the two-component Weibull mixture model were corresponding to the evolution of matrix cracking and fiber breakage, respectively. Since the proposed AE analysis method can determine the number of component distributions without the decision of researchers and inspectors, it is expected to be useful for an understanding of the fracture process in newly developed materials and the reliability assessment in service.