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.     

Tensile damage evolution of 2D SiC(f)/BN(i)/(SiC-B4C)(m) composites based on acoustic emission pattern recognition

The tensile test of 2D SiC_(f)/BN_(i)/(SiC-B₄C)_(m), with acoustic emission (AE) online monitoring, was carried out to study the mechanical properties and failure mechanism. Using the pattern recognition technique to find out the relationship between AE signals and damage mechanisms, an improved sentry function was proposed by choosing the AE energy of fiber breakage instead of the AE energy in its definition. Combined with the cumulative trend of each class of the AE signals, the damage evolution process was identified. It is found that the entire loading process includes four stages: (a) the linear elastic stage, there have sporadic AE events; (b) the initial damage stage, the matrix cracking contributes 80.4% of the AE energy and leads to the non-linear behavior of the stress-displacement curve; (c) the damage development stage, in which all types of damage continuously happen; and (d) the damage acceleration stage, dominated by fiber breakage that accounts for 60.1% of the AE energy. The improved sentry function has a decreasing trend during the fourth stage, which provides an early warning before failure, and gives a reliable ultimate strength.     

Measuring effective interfacial shear strength in carbon fiber bundle polymeric composites

Adhesion in composite materials is often quantified using the single fiber fragmentation (SFF) test. While this method is believed to provide accurate values for the fiber–matrix interfacial shear strength (IFSS), these may not accurately reflect the macroscopic mechanical properties of specimens consisting of tows of thousands of tightly spaced fibers embedded in a resin matrix. In these types of specimens, adhesion may be mitigated by fiber twisting and misalignment, differences in the resin structure in the confined spaces between the fibers and, most importantly, by any incompleteness of the fiber wetting by the resin. The present work implements fiber band fragmentation (FBF) testing to obtain effective interfacial shear strengths, whose values reflect the importance of these factors. The fiber fragmentation in these specimens is tracked through the counting and sorting of acoustic emission (AE) events occurring during the tensile testing of the specimen and yields the average critical fiber fragment length. AE results, in conjunction with stress-strain data, show that fiber breakage events occur at acoustic wavelet amplitudes substantially greater than those generated by fiber/matrix debonding. Kelly–Tyson analysis is applied, using the measured critical fiber fragment length together with known values for the fiber diameter and tensile strength to yield the effective IFSS. FBF tests are performed on carbon fiber/poly(vinyl butyral) (PVB) dog-bone fiber-bundle systems, and effective IFSS values substantially lower than those typically reported for the single fiber fragmentation testing of similar systems are obtained, suggesting the importance of multi-fiber effects and incomplete fiber wetting.     

Oxidation effects on failure mechanism of plain-woven SiC/SiC Z-pinned joints subjected to push-out loading

Plain-woven SiC/SiC Z-pinned joints were prepared using chemical vapour infiltration. The oxidation behaviours and failure mechanism were investigated using push-out tests before and after oxidation. Microstructural analysis indicated that the degree of oxidation of the pin near the surface was more severe than that in the middle region. Internal damage was monitored using an acoustic emission (AE) system. Pattern recognition was introduced to study the evolution of failure during the test. Combined with the failure morphology, all AE signals were labelled as matrix cracking, fibre–matrix debonding, matrix peeling, and fibre break. The results suggest that the failure mechanism of the Z-pinned joints under axial loading is dominated by fibre–matrix debonding and matrix cracking.     

Effect of multiple loading sequence on time-dependent stress rupture of fiber-reinforced ceramic-matrix composites

In this paper, the effect of multiple loading sequence on time-dependent stress rupture of fiber-reinforced ceramic-matrix composites (CMCs) at intermediate temperatures in oxidative environment is investigated. Considering multiple damage mechanisms, a micromechanical constitutive model for time-dependent stress rupture is developed to determine damage evolution of matrix crack spacing, interface debonding and oxidation length, and fiber failure probability under single and multiple loading sequences. The relationships between multiple loading sequence, composite strain evolution, time, matrix cracking, interface debonding and oxidation, and fiber fracture are established. The effects of fiber volume, matrix crack spacing, interface shear stress in the slip and oxidation region, and environment temperature on the stress/time-dependent strain, interface debonding and oxidation fraction, and fiber broken fraction of SiC/SiC composite are analyzed. The experimental stress rupture of SiC/SiC composite under single and multiple loading sequences at 950°C in air atmosphere is predicted. Compared with single loading stress, multiple loading sequence affects the interface debonding and oxidation fraction in the debonding region, leading to the higher fiber broken fraction and shorter stress-rupture lifetime.     

Study on interfacial debonding stress and damage mechanisms of C/SiC composites using acoustic emission

Among ceramic matrix composites (CMCs), carbon fiber-reinforced silicon carbide matrix (C/SiC) composites are widely used in numerous high-temperature structural applications because of their superior properties. The fiber–matrix (FM) interface is a decisive constituent to ensure material integrity and efficient crack deflection. Therefore, there is a critical need to study the mechanical properties of the FM interface in applications of C/SiC composites. In this study, tensile tests were conducted to evaluate the interfacial debonding stress on unidirectional C/SiC composites with fibers oriented perpendicularly to the loading direction in order to perfectly open the interfaces. The characteristics of the material damage behaviors in the tensile tests were successfully detected and distinguished using the acoustic emission (AE) technique. The relationships between the damage behaviors and features of AE signals were investigated. The results showed that there were obviously three damage stages, including the initiation and growth of cracks, FM interfacial debonding, and large-scale development and bridging of cracks, which finally resulted in material failure in the transverse tensile tests of unidirectional C/SiC composites. The frequency components distributed around 92.5 kHz were dominated by matrix damage and failure, and the high-frequency components distributed around 175.5 kHz were dominated by FM interfacial debonding. Based on the stress and strain versus time curves, the average interfacial debonding stress of the unidirectional C/SiC composites was approximately 1.91 MPa. Furthermore, scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDXS) were used to observe the morphologies and analyze the chemical compositions of the fractured surfaces. The results confirmed that the fiber was completely debonded from a matrix on the fractured surface. The damage behaviors of the C/SiC composites were mainly the syntheses of matrix cracking, fiber breakage, and FM interfacial debonding.     

二维机织碳纤维/碳化硅陶瓷基复合材料损伤分析

利用声发射技术全程监测二维机织C／SiC复合材料拉伸实验,通过声发射多参数分析法和断口显微观察,结合材料拉伸应力-变曲线,分析了二维机织C／SiC复合材料拉伸损伤演化过程和损伤机理。结果表明：材料拉伸损伤演化经历3个阶段：第一阶段为无损伤阶段,材料无损伤发生;第二阶段为损伤初始阶段．损伤主要为微裂纹开裂．并且微裂纹开裂基本上均匀发生在样品工作段;第三阶段为损伤加速阶段,损伤主要为宏观基体、界面开裂和纤维束断裂．井且集中发生在断口区域。损伤第二阶段与第三阶段的转换点在拉伸强度的76％左右,转换点的确定对二维机织C／SiC复合材料工程应用有重要意义。     

风电叶片复合材料在三点弯曲过程中的声发射研究

为研究风电叶片复合材料损伤破坏的声发射特性以及复合材料的力学性能,对含有纤维预断试件和无纤维预断的完好试件分别进行了三点弯曲力学性能试验,并在加载过程中采用声发射检测仪实时监测整个损伤破坏过程。对采集到的声发射信号处理分析,便可获得风电叶片复合材料的弯曲力学性能和损伤破坏的声发射特性。试验结果表明:玻璃纤维复合材料在弯曲载荷作用下出现典型的破坏特征包括纤维断裂、纤维/基体脱胶和纤维分层。纤维预断试件的声发射信号波形最高幅度达到2.5 V,频带分布在20~300 k Hz范围;无纤维预断试件的声发射信号波形最高幅度为0.07 V,频带分布在10~180 k Hz之间。     

Effects of cyclic tensile loading on the rupture behavior of orthogonal 3-D woven SiC fiber/SiC matrix composites at elevated temperatures in air

This study examined the rupture mechanisms of an orthogonal 3D woven SiC fiber/BN interface/SiC matrix composite under combination of constant and cyclic tensile loading at elevated temperature in air. Monotonic tensile testing, constant tensile load testing, and tension–tension fatigue testing were conducted at 1100 °C. A rectangular waveform was used for fatigue testing to assess effects of unloading on the damage and failure behavior. Microscopic observation and single-fiber push-out tests were conducted to reveal the rupture mechanisms. Results show that both oxidative matrix crack propagation attributable to oxidation of the fiber–matrix interface and the decrease in the interfacial shear stress (IFSS) at the fiber–matrix interface significantly affect the lifetime of the SiC/SiC composites. A rupture strength degradation model was proposed using the combination of the oxidative matrix crack growth model and the IFSS degradation model. The prediction roughly agreed with the experimentally obtained results.     

Prefailure damage processes in highly filled glass/mica/epoxy composites

In the present study, the prefailure damage processes of a series of short glass fiber/mica/epoxy composites under three-point bending were elucidated using acoustic emission (AE) coupled with in situ scanning electron microscopy (SEM) observations. This study consisted of a detailed investigation of the damage tolerance of composite systems that had constant inorganic content of 75% by weight with a varying ratio of glass fibers to mica. The flexural strength was found to increase from 11 to 21 ksi as the glass fiber content increased (mica content decreased), while the flexural modulus decreased from 5.0 to 2.5 Msi. By monitoring the AE during flexural deformation of the glass fiber-to-mica ratio composites, it was determined that low amplitude (0–42 db) AE events, which occurred throughout the deformation process, were caused by matrix cracking, whereas the high-amplitude (43–100 db) AE events, which occurred just prior to failure, were caused by a fiber-related mechanism. In situ SEM observations of the composites during flexural deformation allowed a correlation between the AE and the damage mechanisms as a function of strain. In the all-mica composite, microcracking initiated in the linear region at preexisting flaws, on the order of 10 μm, located at the mica interface. These microcracks grew along the mica contours over the majority of the deformation process, emitting low-amplitude events, until final fracture occurred at relatively low strains. In the glass fiber-containing composites, microcracking initiated in the linear region at preexisting flaws and voids, on the order of 10 μm. These microcracks grew slowly, emitting low-amplitude events, as the strain increased, but were prevented from causing failure at low strains because of the toughening effect of the glass fibers. At sufficiently high strains, however, fiber breakage and fiber pull-out occurred that corresponded to the high-amplitude events detected by the AE. At strains just prior to failure, catastrophic crack growth occurred, producing a rapid increase in both low-and high-amplitude events, causing ultimate failure.     

A synergistic model of stress and oxidation induced damage and failure in silicon carbide-based ceramic matrix composites

A micromechanics-based modeling approach that allows for the simultaneous consideration of deformation, damage, and oxidation associated with each constituent of silicon carbide (SiC)-based ceramic matrix composites (CMC), including the fiber, fiber coating, and matrix, is described. Chemical kinetics models from the literature are combined with a progressive damage model. Rupture predictions of unnotched and notched stress-hold (creep) specimens are compared with experimental measurements from a SiC/SiC CMC to assess the efficacy of the modeling approach. Techniques of improving creep rupture life are explored using the model.     

Interfacial properties of two SiC fiber–reinforced polycarbonate composites using the fragmentation test and acoustic emission

Interfacial shear strengths (IFSS) between the fiber and the matrix in two SiC fiber–reinforced polycarbonate (PC) composites (TFC) were investigated through the fragmentation method and the acoustic emission (AE) technique. Statistical analysis of SiC fiber tensile strength was performed mainly in terms of a Weibull distribution. The tensile strength and elongation for SiC fiber decreased with increasing gauge lengths, because of the heterogeneous distribution of flaws on the fiber surface. Using an amino-silane coupling agent, the IFSS showed significant improvement, in the range of 150%, under dry conditions. On the other hand, in the aspect of the environmental effect, the IFSS was improved about 170% under wet conditions (immersed in hot water at 85°C for 75 min). This is probably due to chemical and hydrogen bonds in the two different interphases in the SiC fiber/silane coupling agent/PC matrix system. In-situ monitoring of AE during straining of microspecimens showed the sequential occurrence of two distinct groups of AE data. The first group may result from SiC fiber breakages, and the second probably results from mainly PC matrix cracking. Characteristic frequencies coming from the failures of the fiber and the PC matrix were shown via fast Fourier transform (FFT) analysis. By setting an appropriate threshold level, a one-to-one correspondence between the number of AE events and fiber breakages was established. This AE method could be correlated successfully to the IFSS via the fragmentation technique, which can also applied to nontransparent specimens.     

Damage development of a woven SiCf/SiC composite during multi-step fatigue tests at room temperature

The monotonic tensile and multi-step fatigue tests of 2D woven SiC_f/SiC composite were performed to explore the damage development, respectively. The acoustic emission-based technique was used to analyze the damage state and select the peak stresses for fatigue tests. The damage evolution after specific mechanical tests was characterized by optical microscopy and scanning electron microscopy. Cracks are prone to occur in the vicinity of flaws and boundaries of different matrix components under relatively low fatigue stress. The cyclic fatigue stress can do much harm to the interfaces and facilitate the interfacial debonding. The damage characteristics of five types of cracking, fiber breakage and pull-out, and interfacial debonding of the composite after specific mechanical tests are concluded and discussed in detail, which can offer help for deeper analysis of the oxidation mechanism in service and more reasonable design of SiC_f/SiC composite.     

Modeling and experimental study on electrical impedance response to damage accumulation in 2D C/SiC composites

The electrical properties of C/SiC composites could be used for online and in-situ damage monitoring. To investigate alternating current (AC) impedance response to damage in the C/SiC composites, monotonic and incremental cyclic tensile tests were performed. Both AC impedance and acoustic emission (AE) techniques were applied to clarify the damage evolution during the tests. The relationship between damage and electrical impedance response was investigated and validated via macroscopic equivalent circuit models. The effects of longitudinal deformation and damage on AC impedance characteristics, including impedance magnitude and phase angle, were obtained from the models. Results showed that the longitudinal deformation increases the impedance magnitude and the phase angle, and the damage causes the impedance magnitude to increase and the phase angle to decrease. The phase angle is significantly sensitive to fiber breakage, which makes the AC-based method more suitable for online damage monitoring and final failure warning.     

Crack Branching in All-Oxide Ceramic Composites

The existence of oxidation embrittlement in previously developed ceramic-matrix composite (CMC) systems at intermediate temperatures (500–900°C) has motivated the development of all-oxide CMCs. Studies of the damage tolerance of an all-oxide CMC suggest that it relies on distributing fiber bundles heterogeneously in a porous matrix with a thin matrix-only region surrounding each bundle. For such composites, loading in the direction of the fibers causes cracks to first commence in the fiber bundles, breaching both the matrix and fibers within each bundle. These fiber-bundle cracks then branch in the matrix-rich region, parallel to the load, forming stable H-shaped cracks. The mechanics behind one mechanism controlling the competition between branching and continued extension of a preexistent fiber-bundle crack in a unidirectional all-oxide CMC are provided in this paper. Both plane-strain and axisymmetric cases are studied using an integral equation method as well as a numerical method based on finite elements. With no elastic mismatch assumed between the fiber and matrix, a single nondimensional combination, E Ω/σ_∞, emerges as the key parameter that governs the tendency of the crack to branch (where E is Young's modulus, Ω is the thermal misfit strain between the fiber bundle and matrix, and σ_∞ is the stress applied parallel to the fiber axis). Quantitative estimates of the conditions under which the branching of the bundle crack is favored are presented.     

High-velocity impact behaviors and post-impact tension performance of 2D-C/SiC composite beams

2D-C/SiC composites have widely been used in aeronautical and aerospace engineering, but their mechanical behaviors under small-mass and high-speed impact have not been thoroughly studied yet. In this paper, 2D-C/SiC beam specimens were impacted by a single-stage light-gas gun and the fracture processes were captured by a high-speed camera. Post-impact internal and surface damage morphologies were scanned by a CT and a SEM, respectively. Similar damage modes were revealed by high-speed images. Subsequently, quasi-static post-impact tension tests were conducted to understand the residual mechanical properties. Acoustic emission (AE) signals of specimens were detected during the tests and then classified by the K-means algorithm. Therefore, evolutions of matrix damage, interfacial delamination and fiber fracture were recognized. At the same time, strain value was obtained by digital image correlation (DIC) method and main crack propagations were obvious in strain contours. A combination of the AE and DIC methods very well monitored the real-time damage during post-impact testing, which further revealed the damage during impact phase.     

A chemo-mechanical coupled constitutive model applied to the oxidation simulation of SiC/SiC composite

Herein, a chemo-mechanical coupled constitutive and failure model is proposed to predict the tensile behavior of SiC/SiC composites under oxidizing environments. The diffusion of O₂ through the oxide scale and the oxidation reaction of SiC/O₂ are modeled and implemented in finite element software, through a user-defined element. Numerical validation studies and tests are conducted on a domestic SiC fiber. An orthotropic constitutive model for reinforcements, which considers modulus reduction due to oxidation damage, and a continuum damage model associated with O₂ diffusion along the micro-cracks in the SiC matrix are subsequently presented. The developed framework is used to simulate the mechanical behavior and oxidation process of a single fiber SiC/SiC composite.     

Identification of damage mechanisms of carbon fiber reinforced silicon carbide composites under static loading using acoustic emission monitoring

The damage mechanisms of carbon fiber reinforced silicon carbide (C/SiC) composites under static loading are investigated using the acoustic emission technology. The C/SiC sample is subjected to compressing static load, and acoustic emission is used to monitor the cracking process. In addition, the digital image correlation technique is also applied to enhance the comprehension of the damage mechanisms of C/SiC composites. To evaluate their extent of damage, the main acoustic emission characteristic parameters and indexes are extracted. The k-means clustering method is used to analyze the acoustic emission (AE) signals, identify the three damage modes, and determine the central values of the AE parameters of these modes. The time–frequency energy of some typical signals is analyzed by using the wavelet packet transform. Thereafter, the damage evolution is described by analyzing the cumulative number of acoustic emission events and the cumulative energy change with loading time. Moreover, the digital imaging results show that the strain in the structure increases with the increase in loading magnitude, especially in the area around the fault zone, where the strain level is evidently higher than those in other locations. Accordingly, this necessitates effective methods for investigating damage in C/SiC composites. Among the two different technologies implemented in this work, the extraction of AE events at several stages of the test allows the classification and analysis of crack evolution in C/SiC structures; this technique also provides an effective methodology to monitor the damage at the microscopic scale.     

基于深度学习的平纹Cf/SiC复合材料原位拉伸损伤演化与断裂分析

为揭示平纹C_f/SiC复合材料的拉伸损伤演化及失效机理,开展了X射线CT原位拉伸试验,获得材料的三维重构图像,利用深度学习的图像分割方法,准确识别出拉伸裂纹并实现其三维可视化。分析了平纹C_f/SiC复合材料损伤演化与失效机理,基于裂纹的三维可视化结果对材料损伤进行了定量表征。结果表明:平纹C_f/SiC复合材料的拉伸力学行为呈现非线性,拉伸过程中主要出现基体开裂、界面脱黏、纤维断裂及纤维拔出等损伤;初始缺陷易引起材料损伤,孔隙多的部位裂纹数量也多;纤维束外基体裂纹可扩展至纤维束内部,并发生裂纹偏转。基于深度学习的智能图像分割方法为定量评估陶瓷基复合材料损伤演化与失效机理提供了有效分析手段。     

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.