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.     

Water absorption and hygrothermal ageing of ultraviolet cured glass‐fiber reinforced acrylate composites

In this article, an ultraviolet curable glass fiber reinforced polymer (UV‐GFRP) composite developed for fast repair or strengthening of concrete structures, was investigated on its water absorption and hygrothermal ageing behaviors. Cured UV‐GFRP coupons were subjected to immersion in distilled water or concrete pore solution (pH value around 13) for 4–8 months at room temperature and elevated temperatures (40 and 60°C), respectively. Water absorption and thermomechanical properties of the samples were tested as a function of immersion time. Water uptake curves of UV‐GFRP exposed to elevated temperatures and/or alkaline solutions show serious mass loss. Debonding of fibers from resin matrix brought in increased coefficient of diffusion along fiber directions, due to the capillary effect. After 4 months of immersion in both media, the tensile strength of UV‐GFRP was deteriorated remarkably, while the tensile modulus was less affected. According to Arrhenius equation, the tensile strength of UV‐GFRP is predicted to remain 77.6% of its original value after50 years when immersed in water at 20°C, but only 24.5% left in the case of alkaline solution. This suggests that the present UV‐GFRP system does not suit for the application in strong alkaline environments. POLYM. COMPOS., 2012. © 2012 Society of Plastics Engineers     

碳纤维复合材料压力容器的透声性能研究

以环氧树脂为基体材料,分别以S2高强玻璃纤维和T700碳纤维为增强材料,采用缠绕成型工艺,制备了玻璃纤维复合材料（GFRP）透声试件和碳纤维复合材料（CFRP）透声试件;测试了试件的透声性能,为成功研制碳纤维复合材料透声压力容器提供依据。结果表明：CFRP的透声性能和力学性能远远优于GFRP;内衬层对容器的透声性能影响不大;研制的碳纤维透声压力容器达到了满意的透声效果。     

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.     

Effect of compaction treatment on laminated CFRP composites fabricated by vacuum‐assisted resin‐transfer molding

Carbon fiber‐reinforced polymer (CFRP) composites were fabricated using ordinary and compaction setups (OS and CS, respectively) in the vacuum‐assisted resin‐transfer molding (VARTM) process. The mechanical properties and acoustic emission (AE) spectra of the CFRP composites were compared among fabricated samples. The CFRP plates with sequences of [+30/−30]₆ were sectioned to make specimens for Mode I interlaminar fracture tests and three‐point bending tests. The difference between the material properties and AE characteristics of the OS and CS specimens were statistically compared using one‐way analysis of variance. The OS specimens had a thicker resin layer, a higher resin fraction, larger average fracture toughness, and AE energy corresponding to the Mode I fracture, whereas the CS specimens had more macro‐scale voids and higher bending strength. AE analysis showed that frequency bands in the interlaminar fracture tests corresponding to matrix‐related fracture were dominant for the OS specimens, whereas those corresponding to the mixed fracture mode of the fiber and matrix fracture were dominant for the CS specimens. In the bending tests, mixed fiber‐matrix fractures were dominant for the OS specimens, and fiber‐related fractures were dominant for the CS specimens. In conclusion, the compaction treatment diminished interlaminar fracture toughness, due to the enhanced formation of macro‐scale voids around the fiber bundles during the resin impregnation stage. However, the bending strength improved with an increased fiber volume fraction. POLYM. COMPOS., 38:217–226, 2017. © 2015 Society of Plastics Engineers     

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.     

Comparison of the failure mode in short and long glass fiber-reinforced injection-molded polypropylene composites by acoustic emission

The failure mode in injection-molded short (SGF) and long glass fiber (LGF) reinforced polypropylene (PP) was studied on compact tension (CT) specimens simultaneously by acoustic emission (AE) and transmitted light microscopy. A significant difference was revealed in the failure manner characterized by the cumulative run, amplitude and energy distribution curves between the SGF- and LGF-PP both in the crack initiation and propagation stage. It was established that the failure of SGF-PP did not alter with the loading; this composite failed mostly by matrix deformation along with fiber/matrix debonding and some fiber pull-out. The failure mode of the LGF-PP differed from that scenario, since fiber fracture was resolved in every stage of the loading. On the contrary to SGF-PP, the failure of this composite was governed by fiber-related events (fracture, pull-out, debonding). The amplitude and energy of the AE signals were assigned to individual failure events and thus the failure sequence concluded.     

Flexural behavior of methyl methacrylate modified unsaturated polyester polymer concrete beams reinforced with glass‐fiber‐reinforced polymer sheets

This study represents the behavior of flexural test of methyl methacrylate modified unsaturated polyester polymer concrete beam reinforced with glass‐fiber‐reinforced polymer (GFRP) sheets. The failure mode, load–deflection, ductility index, and separation load predictions according to the GFRP reinforcement thickness were tested and analyzed. The failure mode was found to occur at the bonded surface of the specimen with 10 layers of GFRP reinforcement. For the load–deflection curve, as the reinforcement thickness of the GFRP sheet increased, the crack load and ultimate load greatly increased, and the ductility index was found to be the highest for the beam with the thickness of the GFRP sheet at 10 layers (6 mm) or 13 layers (7.3 mm). The calculated results of separation load were found to match only the experimental results of the specimens where debonding occurred. The reinforcement effect was found to be most excellent in the polymer concrete with 10 layers of GFRP sheet reinforcement. The appropriate reinforcement ratio for the GFRP concrete beam suggested by this study was a fiber‐reinforced‐plastic cross‐sectional ratio of 0.007–0.008 for a polymer concrete cross‐sectional ratio of 1 (width) : 1.5 (depth). © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Degradation behavior of mechanically fastened joints of glass-reinforced crosslinked polyester immersed in hot water

This study deals with the degradation of mechanically-fastened GFRP joints immersed in hot water (80°C). The material used was randomly oriented E-glass fiber continuous strand mat with a crosslinked polyester. Three kinds of joint geometries were adopted; thickness was 3 mm, hole diameter was 6 mm, the distance from hole center to top-edge was 18 mm (3e), and specimen widths were 18 (3w), 30 (5w), and 42 mm (7w). Failure modes of original dry specimens were a function of joint geometry, The dominant failure mode of 3w3e joints was net-tension, whereas 5w3e and 7w3e joints displayed bearing failure. As degradation progressed, the dominant failure mode gradually shifted from net-tension to bearing failure. Strength reduction was marked in 5w3e and 7w3e joints, in which the dominant failure mode was bearing. Joint strength and failure mode were predicted from the combination of a macroscopic failure criterion and characteristic curves obtained from tensile testing of rectangular specimens with holes, bearing tests, and finite element analysis. Predictions agreed with experiment.     

Nanoindentation of dry and aged pultruded composites containing fillers and low profile additives

In this article, the distribution through the thickness of both the matrix's modulus and the interfacial shear strength (IFSS) in dry and aged pultruded polyester/glass fiber composites containing fillers and low profile additives are experimentally evaluated, using the nanoindentation test. The obtained results indicate that for the dry composite, both the matrix's modulus and the IFSS are constant across the thickness of that material. As a consequence of the immersion in distilled water at high temperature (65°C), the IFSS was found to exhibit a parabolic trend through the thickness of the aged specimen, with the lowest value at the external surfaces. Such minimum was reached after a short time of immersion. At the saturation time, the IFSS at all layers of the aged specimen reaches lowest value. This was not the case for the matrix's modulus, since the later was found to be not affected by the amount of the absorbed humidity. Similar results were obtained after immersion in sea water at the same temperature. However, the only difference noted was at the external surface of the aged specimen where additional degradation had taken place in the matrix and at the fiber/matrix interface. This was attributed to the accumulation of a large amount of salt molecules at the external surface of the aged specimen during the exposure process. Additional mechanical tests show that after 120 days immersion in hot water the interlaminar shear strength of the material as measured according to the ASTM standard D2344, is reduced by 35%. POLYM. COMPOS., 2008. © 2008 Society of Plastics Engineers     

Effects of hygrothermal aging on the fracture and failure behavior in short glass fiber-reinforced,toughened poly(butylene terephthalate) composites

The fracture response of injection molded short glass fiber (GF) reinforced and rubber-toughened poly(butylene terephthalate) (PBT) composites has been characterized by the fracture toughness (K_c) and energy (G_c), measured on static-loaded compact tension (CT) specimens. The related failure of the composites with 30 wt% GF reinforcement in as-received (AR), hygrothermally aged (HA) and re-dried (RD) states, respectively, was studied by acoustic emission (AE) and fractography. Tougheners were functionalized ethylene/acrylate (EAF), crosslinked acrylate (XAR) and core-shell type (CSR) rubbers, at 20 wt% in the composites. It was shown that both K_c and G_c decrease with hygrothermal aging at 90°C, and their values cannot be restored by subsequent drying. This is attributed to severe hydrolysis degradation of the PBT matrix. Deterioration in the fracture parameters was affected by the composition of the rubbery toughener: The toughness retention by EAF was superior to the other modifiers. The difference in the failure mode of the GF-PBT composites before and after hygrothermal aging was revealed by viewing the fracture surface of the CT-specimens in scanning electron microscope (SEM). Based on the fractographic results, changes in the AE amplitude envelopes are interpreted and discussed.     

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     

Unsaturated polyester resin modified with poly(organosiloxanes). II. Acoustic emission study on glass-fiber-reinforced resin

This work aims to study the adhesion of an isophthalic acid based unsaturated polyester resin chemically modified by grafting copolymerization of a poly(organosiloxane) to the glass fiber. The failure mode of the single-edge notched tensile specimen, cut from pressed plates containing 50 wt % of nontreated and silane-treated milled glass fiber, was studied by acoustic emission (AE) technique. It was found that the aforementioned resin modification enhanced the adhesion between the fiber and the resin. This was suggested by a shift in the AE amplitude and energy toward higher values. On the other hand, the matrix modification had no significant effect if glass fiber with suitable unsaturated polyester resin (UP) sizing was incorporated. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 81: 3280–3289, 2001     

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.     

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.     

Effects of alkaline conditioning and temperature on the properties of glass fiber polymer composite rebar

This article investigated long term alkaline conditioning and temperature on the physical and mechanical properties of glass fiber‐reinforced polymer (GFRP) composite rebar for structural applications. The GFRP rebar was immersed in alkaline solution (pH ≈ 13) for 23 months at 23°C, and for 24 months at 60°C. The moisture absorption was found to be 0.34% at 23°C after 23 months, and 0.76% at 60°C after 24 months. At both temperatures, moisture absorption did not reach equilibrium which was attributed to two stages non‐Fickian behavior. Glass transition temperature (T_g) of the polymer matrix of rebar that conditioned at 23°C was found to be decreased because of plasticization, whereas T_g of the rebar that conditioned at 60°C was remained greater than the T_g of control rebar due to nonplasticization effect. Shear strength was retained by 83.5% at 23°C and 80.5% at 60°C, flexural strength was retained by 81% at 23°C and 69% at 60°C, and tensile strength was retained by 91.2% at 23°C and 74.3% at 60°C. It was revealed that durability of GFRP rebar in alkaline environment was controlled by the absorbed moisture; this was because the load transfer efficiency of fiber/matrix interface is vulnerable to moisture. POLYM. COMPOS., 37:3181–3190, 2016. © 2015 Society of Plastics Engineers     

Hydrothermal effects on mechanically fastened glass/polypropylene composite joints

This study deals with the hydrothermal effects on mechanically fastened glass fiber mat/polypropylene composite joints with different joint geometry. Three kinds of joint geometries were adopted; hole diameter was 6 mm, the length from top edge to the hole center was 18 mm (3e) and the specimen width was 12 (2 w), 18 (3w), and 30 mm (5w). The joints were immersed in hot water at 60 and 80°C. The effect of water temperature on the failure load appeared clearly in 2w3e joints. The higher water temperature induced more strength reduction. The dominant failure mode changed from the net-tension to the bearing for the original dry joints for wider specimens. The failure mode changed from multiple to net-tension only for 3w3e joints, because of a remarkable tensile strength reduction. The failure strength and the failure mode were predicted by the macroscopic failure criterion, and the characteristic experimental curves, and the FEM analysis. The predicted results agreed with experiment.     

Characterization of interfacial properties of composite materials by acoustic emission

The global mechanical properties of composite structures in service depend on fiber/matrix interface and interlaminar strength. The paper proposes to use Acoustic Emission (AE) and advanced signal processing to evaluate the interlaminar performance of polymeric composites. A delaminating process simulated with a Double Cantilever Beam (DCB) in opening mode (Mode I) coupled with an Acoustic Emission (AE) technique has been employed. Different samples were analyzed to observe the damage evolution and to evaluate the interlaminar decohesion processes. The resistance to delamination growth is expressed in terms of the interlaminar DCB mode (mode I) fracture toughness, measured by strain energy release rate, G_I, dissipated per unit area of delamination growth in composite. Three categories of samples were used: two unidirectional carbon fiber/epoxy resins with one degraded by heat and one with a commercially used resin. It was found that sample that was exposed at a temperature greater than the glass transition temperature Tg of the epoxy had a higher cumulative energy release rate than the two other samples types. The original type having the lowest release rate. Acoustic emission parameters have been found to be powerful indicators of the intensity of the damage. Multivariate analysis of up to 49 parameters was performed in order to group classes of AE signals with matching characteristics. A correlation was established between the energy release rate and the acoustic emission energy.     

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。     

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.