Effect of long-term high-temperature atmospheric exposure on damage progress and mechanical properties for carbon fiber/polycyanate 90° unidirectional composites

This study focuses on the effect of long-term exposure to high-temperature atmosphere on mechanical properties of carbon fiber reinforced plastic (CFRP) laminate with polycyanate resin system. Transversely oriented unidirectional laminate of carbon fiber/polycyanate (T700SC/FSD-M-08178) CFRP was employed on isothermal aging at 180 °C in air up to 4000 h. Matrix crack extension during isothermal aging was observed by soft X-ray radiography. Tensile and compressive tests were conducted on both non-aged and aged (500, 1000, 2000, and 4000 h) specimens with an acoustic emission (AE) measurement in order to evaluate mechanical properties and damage behavior due to thermo-oxidative degradation. After tensile tests, the fracture surface observation with a scanning electron microscopic (SEM) apparatus was also carried out to characterize damage extension due to thermo-oxidative degradation. The results show that thermo-oxidation induced matrix cracks preferentially in perpendicular area along the fiber direction and the matrix crack propagated along the fiber direction. Due to the crack onset, tensile strength was sharply dropped, even though compression strength was slightly changed. From the AE measurement result, the failure mode change was detected for the tension test. Comparing the soft X-ray radiography result with the SEM observation result, the fracture for tension test did not occur at the maximum crack length location for the longer thermo-oxidative aged specimen.     

Interfacial properties and microfailure degradation mechanisms of bioabsorbable fibers/poly-l-lactide composites using micromechanical test and nondestructive acoustic emission

Interfacial properties and microfailure degradation mechanisms of the bioabsorbable composites for implant materials were investigated using micromechanical technique and nondestructive acoustic emission (AE). The tensile strength of absorbable fibers with hydrolysis was analyzed statistically using either uni- or bimodal Weibull distribution. As hydrolysis time increased, the tensile strength, the modulus and the elongation of poly(ester-amide) (PEA) and bioactive glass fibers decreased, whereas those of chitosan fiber almost did not change. Interfacial shear strength (IFSS) between bioactive glass fiber and poly-l-lactide (PLLA) was much higher than PEA or chitosan fiber/PLLA systems using dual matrix composite (DMC) specimen. The decreasing rate of IFSS was the fastest in bioactive glass fiber/PLLA composites whereas that of chitosan fiber/PLLA composites was the slowest. Work of adhesion, W_a between bioactive glass fiber and PLLA was the highest, and the wettability results were consistent with the IFSS. AE energies of PEA fiber decreased gradually, and their distributions became narrower than those in the initial state with hydrolysis time. In case of bioactive glass fiber, AE energies in tensile failure were much higher than those in compression. In addition, AE parameters at the initial state were much higher than those after degradation under both tensile and compressive tests. Interfacial properties and microfailure degradation mechanisms can be important factors to control bioabsorbable composite performance.     

Fracture process of thermally shocked discontinuous fibre-reinforced glass matrix composites under tensile loading

Fracture mechanisms of discontinuous carbon-fibre-reinforced glass matrix composites were experimentally studied for specimens with initial damage induced by thermal shock. First, matrix cracking due to thermal shock was observed using both optical microscopy and scanning acoustic microscopy (SAM) to reveal the damage state. Secondly, tensile stress-strain behaviour and acoustic emission during tensile tests were measured for specimens with and without thermal shock. The progress of microscopic damage during tensile loading was also investigated using both replica and in-situ SAM techniques. Finally, macroscopic transient thermal stresses during thermal shock were calculated using finite-element analysis. It is proved that the fracture process of the composite specimen with thermal-shock-induced cracks is different from that of the virgin specimen. This difference in fracture processes is attributed to the difference in the evolution of matrix cracking, which is affected by pre-existing microcracks in the matrix. This revised version was published online in November 2006 with corrections to the Cover Date.     

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。      

三维编织碳/环氧复合材料力学性能测试及破坏机制

通过宏观拉压试验, 研究了三维正交编织碳/环氧复合材料的拉伸和压缩力学性能。对试验过程进行了声发射分析, 对断口进行了扫描电镜观察分析, 给出了该类材料的拉伸和压缩破坏机制。结果表明: 三维正交编织碳/环氧复合材料有良好的拉伸和压缩力学性能; 三维正交编织复合材料在拉伸和压缩载荷作用下的断裂均为脆性断裂, 拉伸试验的主要破坏现象是纤维断裂拔出, 而压缩试验则是纤维剪切破坏; 通过声发射参数分析可以基本判定该类材料损伤过程中的损伤类型。     

Fatigue-life prediction of SiC particulate reinforced aluminum alloy 6061 matrix composite using AE stress delay concept

A study of the residual fatigue life prediction of 6061-T6 aluminum matrix composite reinforced with 15 vol % SiC particulates (SiC_p) by using the acoustic emission technique and the stress delay concept has been carried out. Fatigue damages corresponding to 40, 60 and 80% of total fatigue life were stimulated at a cyclic stress amplitude. The specimens with and without fatigue damage were subjected to tensile tests. The acoustic emission activities were monitored during tensile tests. It was found that a lower stress level was required to reach a specified number of cumulative AE events for specimens fatigued to higher percentage of the fatigue life. This stress level is called stress delay. Approximately a linear relation was found between stress delay and fatigue damage. Using the procedure defined in this study, the residual fatigue life can be predicted by testing the specimen in tension and monitoring the AE events. The number of the cumulative AE events increased exponentially with the increase of strain during tensile tests. This exponential increase occurred when the material was in the plastic regime and was attributed mainly to SiC particulate/matrix interface decohesion and linkage of voids. In high cycle fatigue, it was observed that the residual tensile strengths of the material did not change with prior cyclic loading damages since the high cycle fatigue life was dominated by the crack initiation phase.     

Characterization of the aramid/epoxy interfacial properties by means of pull-out test and influence of water absorption

Single fiber pull-out tests were carried out to investigate the influence of water absorption on the interfacial properties of aramid/epoxy composite. The fiber/matrix interfacial strength was severely decreased between 4 and 7 week immersion time in deionized water at 80 °C, and thereafter showed a plateau. This change with immersion time did not correspond with that of the water gain of the pull-out specimens, because the water gain did not reflect the one in the fiber/matrix interface. As a result of the degradation of the fiber/matrix interfacial strength, the pulled-out fiber surfaces of 7, 10 and 13 week wet specimen were smooth. In situ observations of interfacial crack propagation by a video microscope and an analysis of acoustic emission (AE) signals showed that AE signals obtained during the pull-out process were classified into four types according to fracture modes. AE signals detected at final unstable crack propagation and fiber breakage had high amplitude and long duration.     

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。      

Identification of failure mechanisms of metallised glass fibre reinforced composites under tensile loading using acoustic emission analysis

In this work, failure mechanisms of metallised glass fibre reinforced epoxy composites under tensile loading were investigated using acoustic emission analysis. Sandblasting with Al₂O₃ was used to pre-treat the composite surface prior to metallisation, and therefore to improve adhesion. The sandblasting time was varied from 2 s to 6 s. A two-step metallisation process consisting of electroless and subsequent electroplating was used for depositing the copper coating on the pre-treated composite surface. The mechanical pre-treatment had no significant negative effect on the mechanical properties of the composite laminate. The acoustic emission (AE) from the metallised composite was recorded during tensile testing in order to investigate the failure mechanisms. AE-Signals were analysed using pattern recognition and frequency analysis techniques. A correlation between the cumulative absolute AE-energy and the mechanical behaviour of uncoated and coated specimens during tensile testing was successfully observed. It was shown that a stronger adhesion between substrate and coating leads to a lower release of mechanical elastic energy, which could be recorded by means of AE analysis. Furthermore, differences in peak frequency, frequency distribution and the use of pattern recognition techniques allowed classifying the signal into three failure mechanisms for the uncoated samples and four failure mechanisms for the coated samples, namely matrix cracking, fibre-matrix interface failure, fibre breakage and substrate-coating interface failure. Waveform and frequency analysis of the classified signals supported the identification of the failure mechanisms. Furthermore, optical investigation and SEM images of the tested samples and fracture surfaces confirmed the identified mechanisms evaluated by acoustic emission analysis.     

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.     

Acoustic emission from crack growth in an advanced zirconia refractory under thermal shock

Crack initiation and growth during the thermal shock tests of a partially stabilized zirconia advanced refractory were investigated by the analysis of acoustic emission (AE) amplitudes. The growth of cracks that were detected by AE was systematically monitored by SEM observations as increasingly severe thermal shocks were applied. The measurements of strength loss after thermal cycling in the ribbon test with various applied temperature differentials correlated with continuous monitoring by acoustic emission and confirmed the effects of microcrack growth on the resistance to thermal shock damage.     

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。     

基于声发射检测技术的PE/PE自增强复合材料破损机理分析

用声发射(AE)技术研究了聚乙烯自增强复合材料的拉伸损伤与断裂行为.宽带传感器记录了不同角度纤维铺层的复合材料试样在拉伸破坏过程中的声发射信号,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.声发射检测能有效提取热塑性复合材料损伤破坏信息,在材料的结构损伤主动监测中有良好的应用潜力.     

界面结构对SiCf／Al复合材料性能和声发射行为的影响

运用高分辨场发射扫描电镜（ＦＥ－ＳＥＭ）和声发射（ＡＥ）测试研究了ＳｉＣ纤维铝基复合材料的不同界面组成和界面产物及其对复合材料性能和ＡＥ行为的影响发现富碳自理的ＳｉＣｆ／Ａｌ生成Ａｌ４Ｃ３脆性界面,在伸过程中界面脆断产生许多中幅ＡＥ信号,而富ＳｉＯ２处理的ＳｉＣｆ／Ａｌ生成韧工较高的富氧产物,界面强度较高,在形变过程中不易发生界面断裂,不产生中幅ＡＥ信号。     

Mechanical,thermal and morphological properties of a bio-based composite derived from banana plant source

This paper focuses on the synthesis and testing of a novel bio-based composite structure in which banana fibres was infused with resin made from banana sap. The mechanical, thermal, morphological and biodegradation properties of the bio-composite were characterized and it was found that the material was suitable for general non-functional components. Mechanical tests indicated 15% increase in tensile strength, 12% improvement in tensile modulus and a 25% improvement in flexural modulus when compared to structures produced without banana sap. At elevated temperatures a decrease in the moduli was observed. The thermal stability of the biocomposite composite improved and this corresponded with an increase in the glass transition temperature. Morphological studies using scanning electron microscopy revealed improved compatibility between the fibre and banana sap matrix. This resulted in improved dynamic modulus values and low damping values. Finally, degradation tests revealed increased microbial activity on the banana sap composite. This was indicative of improved biodegradation rates.     

Feasibility and limitations of damage identification in composite materials using acoustic emission

One of the current challenges in health monitoring of composite materials is the use of acoustic emission to identify damage modes. Many classification procedures have been reported in the literature but none of them clearly state limitations to their applicability, making it difficult to quantify them in different testing conditions. In the present paper, a method is described to characterize energy attenuation and how it affects AE signals features based solely on AE signals recorded during mechanical tests. Limitations to damage identification based on AE signals features can therefore be defined. The method is demonstrated on AE signals recorded during tensile tests on four different layups of carbon fiber reinforced polymer composites using signals frequency centroids to describe AE sources.     

Fracture behavior and acoustic emission in bending tests on single-fiber composites

The bending fracture mechanisms and interfacial behavior of single-fiber composites (s.f.c.) with different fiber surface treatments and embedded fiber positions were investigated in three-point bending with simultaneous acoustic emission monitoring. Microfractures occurring at fiber breakages were examined by AE parameters and observations by a polarized microscope. As a result, it was found that AE signals in a bulk resin specimen were almost not detected, while many AE events were monitored in the s.f.c. bending specimens. The number of AE events was in good agreement with the number of fiber breakages, except for specimens with an embedded fiber near the compressive surface. Using AE parameters, especially the peak frequency and its power energy obtained by a power spectrum analysis, failure modes can be identified. A transition of failure mode from fiber break accompanied by a matrix crack and debonding to buckling is observed when the stress in the embedded fiber changes from tension to compression. The debond length is very long near the loading point for the specimens with a fiber near the tensile surface, but it decreases with increasing distance from the loading point. The debond length is small for the specimen with an embedded fiber near the neutral plane since the strain in the fiber decreases. Furthermore, a model for debonding failure is proposed and the maximum interfacial shear stress is derived. It is confirmed that fiber fragment lengths for the specimens with a fiber near the tensile side can be also expressed by the Weibull distribution as done in s.f.c. tensile tests.     

Quasi-Isotropic Triaxially Braided Cellulose-Reinforced Composites

In this article, we investigate experimentally and analytically the mechanical properties of a natural fiber quasi-isotropic triaxially braided composite. The composite is prepared from triaxially braided regenerated cellulose fibers and a high-bio-content epoxy resin system using a resin infusion process. Simultaneous mechanical loading, digital image correlation, and acoustic emission tests were performed on notched and unnotched specimens to understand the tensile behavior of the composites and the initiation and propagation of damage. Experimental results were compared with the effective tensile properties determined using an analytical model. The model is a discrete three-layer analytical representation based on a mechanics transformation-based representation of the quasi-isotropic braided layers. The model is used to determine the elastic stiffness and Poisson effects based on the constituent properties such as the fiber volume fractions, the waviness of the bias tows, and the relative thickness of the braided preform. The experimental results show the analytical model's ability in predicting the composite's elastic properties. The unique fabric architecture is found to have a large influence on the strength properties across the different specimen geometries investigated.     

Damage analysis of concrete members containing expansive agent by mechanical and acoustic methods

Shrinkage compensating concrete (SCC) and Self-stressing concrete (SSC) technique have been employed for reducing early-age cracking and leakage while the addition of expansive agent would have a negative impact on mechanical properties and durability. The objective of the current research was to quantitatively assess the damage development in cementitious materials with expansive agent by both the strength tests and nondestructive acoustic tests including ultrasonic measurements and acoustic emission (AE) tests. The damage degree was defined based on strength as well as ultrasonic properties and a significant linear relationship was observed between the damage degree and autogenous strains. AE parameters such as AE amplitude, AE counts and AE energy were related to AE activity of the cement-expansive agent system. Crack mode identification was performed based on the relationship between average frequency and RA value (rise time/amplitude). A decreasing ratio of tensile cracks and an increasing ratio of shear cracks were observed which could be an indication of aggravated damage inside the materials.     

织物结构对复合材料力学性能影响的试验研究

为探讨不同结构形式织物对复合材料力学性能的影响及其损伤破坏机制之间的差异,通过宏观拉压试验,研究了经编及平纹碳纤维织物增强树脂基复合材料的拉伸及压缩力学性能,并利用声发射对试验过程进行实时监测,对破坏后的断口进行显微镜观察分析,分别给出了两种材料的拉伸和压缩破坏机制.研究结果表明:织物结构形式对复合材料的力学性能有较大影响,与经编织物复合材料相比,平纹织物复合材料的拉伸、压缩强度均较低,且其拉伸、压缩破坏试样的断口相对齐平,分层现象不明显;根据声发射监测结果可以判定两种复合材料损伤过程中的损伤类型,与经编织物相比,平纹织物复合材料拉/压过程中的声发射电压信号相对稳定且整体强度较低.