Fracture behavior of glass-cloth-reinforced composites

The fracture behavior of glass-cloth-reinforced composites has been studied. Fracture of these composites proceeds by tensile failure of fibers rather than by the shear failure of the matrix or the interface. Although the spread of damage in these composites is restricted to small distances away from the crack path due to the interweaving of the fibers, this distance is found to be appreciably larger for the samples with smaller initial crack lengths. Characteristic distances associated with the Whitney-Nuismer criteria are, in turn, found to be smaller for these composites than for the angle ply laminates or randomly oriented short-fiber composites reported in literature in which the spread of the damage is much greater. Analysis through the crack growth resistance also supports this correlation with the extent of damage spread and indicates that the critical crack length for these composites may be equal to the best fit value of the characteristic distance of the average stress criterion.     

Wetting behavior of polymer melts on coated and uncoated tool steel surfaces

The wettability of steel and coatings used for tools and screws in polymer processing is often determined at room temperature. However, it has to be taken into account that polymeric materials are processed at higher temperatures. Contact angle measurements of melted PP, HDPE, PMMA, and PA 6.6 on steel and on TiN, TiAlN, CrN, DLC, and PTFE were performed in this work to investigate the wetting behavior under closer‐to‐processing conditions. The contact angle is dependent on time and the ambient atmosphere. Oxidation and degradation of the polymer melts influence wetting significantly. TiN, TiAlN, CrN, and DLC exhibit a rather good wettability, whereas the highest contact angle of the polymer melts was observed with PTFE. Higher roughnesses of the surfaces lead to an increase in the contact angle. It was also shown that a higher temperature causes a better wetting of the solid surfaces. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43469.     

Fracture and toughening behavior of aramid fiber/epoxy composites

The effect of short Aramid fibers on the fracture and toughening behavior of epoxy with high glass transition temperature has been studied. Fine dispersion of the fibers throughout the matrix is evidenced by optical microscopy. Compared with neat epoxy resin, the fracture toughness (K_IC) of the composites steadily increases with increasing fiber loading, indicating that addition of Aramid fibers has an effective toughening effect to the intrinsically brittle epoxy matrix. Scanning electron microscopy (SEM) indicates that formation of numerous step structures for fiber‐filled epoxy systems is responsible for the significant toughness improvement. SEM and transmitted optical microscopy show that fiber pullout and fiber breakage are the main toughening mechanisms for the Aramid fiber/epoxy composites. POLYM. COMPOS. 26:333–342, 2005. © 2005 Society of Plastics Engineers.     

Fracture behavior of PVC/Blendex/nano‐CaCO3 composites

PVC/Blendex/Nano‐CaCO₃ composites were prepared by melt‐blending method. The Blendex (BLENDEX® 338) (GE Specialty Chemicals Co., Ltd., Shanghai, China) was an acrylonitrile‐butadiene‐styrene copolymer with high butadiene content. The fracture behavior of PVC/Blendex/nano‐CaCO₃ composites was studied using a modified essential work of fracture model, U/A = u₀ + u_dl, where u₀ is the limiting specific fracture energy and u_d is the dissipative energy density. The u₀ of PVC/Blendex blend could be greatly increased by the addition of nano‐CaCO₃, while the u_d was decreased. Nano‐CaCO₃ with particle size of 38 nm increased the u₀ of PVC/Blendex blend more effectively than that with particle size of 64 nm, when nano‐CaCO₃ content was below 10 phr. Both the u₀ and u_d of PVC/Blendex/nano‐CaCO₃ composites were not much affected by increasing specimen thickness from 3 mm to 5 mm, while the two fracture parameters were increased with increasing loading rate from 2 mm/min to 10 mm/min, and u_d was found to be more sensitive to the loading rate than u₀. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 95: 953–961, 2005     

Interfacial shear strength and failure modes of interphase-modified graphite-epoxy composites

Electrocopolymerization of acrylonitrile/methyl acrylate/acrylic acid, acrylonitrile/methylacrylate, and glycidyl-acrylate/acrylonitrile interlayers onto Hercules AS4 graphite fibers was used to improve simultaneously the effective interfacial shear strength, τ_e, and the fracture toughness of graphite-epoxy composite materials. With a single-fiber fracture test, τ_e for these coated fibers (embedded in a diglycidyl ether of bisphenol A - 4,4′-methylenedianiline matrix) was determined at various temperatures and under various hygrothermal treatments. At room temperature, the coated samples showed slightly improved shear strength over the uncoated sample. At elevated temperatures, a plot of τ_e vs. temperature for the uncoated sample showed two distinctive regions: an interface-controlled plateau region at low temperatures, and a matrix-controlled region at high (>80°C) temperatures. Only one region, which was controlled by the matrix and the interlayer, was observed for the GA/AN coated sample. The τ_e values determined were slightly higher than the shear strength of the bulk matrix, possibly because of stronger matrix properties at the interface. Optical micrographs of the coated sample tested at temperatures less than 100°C showed no matrix crack perpendicular to the fiber axis, indicating that the interlayer has effectively blunted the crack tip and restricted its propagation. The effect of moisture in the sample was to reduce τ_e for the coated sample. Upon dehydration, the strength was partially recovered. The treatments did not affect the uncoated sample, however. The fragmentation length data were fitted well by both Gaussian and Weibull distributions.     

Correction of the secondary flow effect for binary diffusion coefficients using coated/uncoated diffusion columns

To determine binary diffusion coefficients from tracer response measurements with a coiled capillary column (curved tube) at relatively high flow rates, a conversion formula to correct for the secondary flow effect was derived from the second central moment of the impulse response curve. Its practical expression was obtained analytically by the perturbation method up to λ¹⁰, where λ is the ratio of the tube radius to the coil radius. The reliability of the correction was evaluated by an alternative series of λ. As a result, the effective range of DeSc^1/2<8-9.5 without correction widened to DeSc^1/2<18-19 to determine the diffusion coefficient within 1% error. Applicability of the correction was demonstrated for phenol and ubiquinone CoQ10 using a coated column, and for benzene using an uncoated column (the Taylor dispersion), in supercritical carbon dioxide at 16- and .     

Fracture behavior of hybrid composites containing both short and continuous fibers

A method for improving the fracture resistance of brittle polymer composites was explored. This method involved the incorporation of short fibers in a thermosetting resin before being used for impregnating the continuous fibers or fabrics. The impact fracture energy and the maximum load experienced by the hybrid can be associated with the parameters of short fibers (e.g., volume fraction and mechanical properties). Electron micrographs of the fracture surfaces of the specimen loaded under mode I and mode II conditions indicate that the short fiber modified composites have a significantly greater fracture surface area and higher level of plastic deformation than the unmodified composites. The data from all the tests demonstrate that adding a small amount of short fibers can significantly improve the interlaminar fracture toughness and impact resistance of graphite/epoxy composites. However, a high volume fraction of added short fibers could make it difficult to out-gas during compression-molding, leading to a high void content and reduced mechanical properties of composites.     

Thermally grown oxide in water vapor on coated and uncoated SiC

Coupons of Hexoloy^®, CVD SiC, and SiO₂ were thermally cycled in a flowing steam atmosphere of 90%/10% H₂O/O₂ at 1426°C in order to simulate the water vapor partial pressure of a turbine environment. The paralinear model for oxidation and volatilization is examined and a condensed version is provided that allows for extraction of the oxidation (k_p) and volatilization (k_l) rates from only a few measured data points across a small time window. Due to high Si(OH)₄ (g) volatility rates, SiO₂ scale thickness approached nearly invariant paralinear limiting values (~3-6 μm) for all conditions, including cycle frequency or material. However, a disparity still exists between weight changes measured and the thickness of the resulting oxide as well as the contribution from the material properties to the oxidation and volatilization rates. Comparisons are made for the oxidation and volatilization rates, specific weight changes, and oxide thicknesses for a number of cycle times for both uncoated and environmental barrier coated SiC.     

Ablation behavior and thermal protection performance of TaSi2 coating for SiC coated carbon/carbon composites

For extending application of TaSi₂ in complex coating system, the ablation behavior and thermal protection performance of TaSi₂ coating is studied to evaluate its potential applications for anti-ablation protection of C/C composites. TaSi₂ coating is prepared by supersonic atmospheric plasma spraying (SAPS) on the surface of SiC coated carbon/carbon (C/C) composites. Phase variation and microstructure are characterized by XRD and SEM, respectively. During the ablation process, the coating is quickly oxidized to SiO₂ and Ta₂O₅ accompanied by a lot of heat consumption. The linear and mass ablation rates are 0.9?µm?s^?1 and ??0.4?mg?s^?1 after ablation for 80?s, respectively Results show that the prepared coating possesses optimal ablation performance under the heat flux of 2.4?MW/m². Moreover, the TaSi₂ coating and SiC inner coating have good chemical and physical compatibility during the ablation process. Therefore, the excellent performance of TaSi₂ coating during the ablation process makes it a candidate for anti-ablation protection for C/C composites.     

Fracture characteristics of SiC/graphene platelet composites

Silicon carbide/graphene platelet (SiC/GPLs) composites were prepared using different weight percent of GPLs filler by hot pressing (HP) technology at 2100 °C in argon. The influence of the GPLs addition on bending strength, fracture toughness and related fracture characteristics was investigated. Both the bending strength and fracture toughness increased with increasing GPLs additives. The main fracture origins – strength degrading defects were pores at the low content of platelets and combination of pores and GPLs or clusters of GPLs particles in systems with a higher content of platelets. The fracture toughness increased due to the activated toughening mechanisms mainly in the form of crack bridging and crack branching, while the crack deflection was limited. The highest fracture toughness of 4.4 MPa m^1/2 was achieved at 6 wt.% of GPLs addition, which was ∼30% higher than the K_IC value of the reference material.     

Fracture of ultrafine calcium carbonate/polypropylene composites

The strength and fracture properties of a polypropylene filled with ultrafine calcium carbonate (0.07 μm) have been studied in the composition range of 0 to 40 percent by volume. Untreated and surface treated (with stearic acid and a titanate coupling agent) grades have been considered. The untreated filler caused a decrease of toughness whereas a maximum, at ∼10 percent, was observed for the treated filler. The fracture energy was analyzed in terms of the crack pinning model. Due to the very small size of particles the pinning contribution proved to be negligible.     

PS/MH复合材料的燃烧性能研究

将聚苯乙烯（PS）树脂与改性后的氢氧化镁（MH）进行熔融共混制备PS／MH复合材料。通过X射线衍射分析了复合材料的微观结构，采用水平燃烧实验、氧指数实验和高温处理实验研究了复合材料的燃烧性能。结果表明：在制备和加工过程中，复合材料中的MH的微观结构并没有发生变化。因此，复合材料中MH的阻燃机理不变。随着MH含量增加，复合材料的发烟和滴落减少，水平燃烧速率从34．08mm/min下降至22．84mm／min．MH用量达到80phr时复合材料能够自熄，材料的氧指数达24．O％。MH的引入使复合材料在高温下的残留率增大，热稳定性和阻燃性能增强．     

Particulate composites from coated powders

A method is described for coating SiC whiskers with a thick layer of alumina to form homogeneous alumina matrix/silicon carbide whisker particulate composites. The method uses controlled heterogeneous precipitation in a relatively concentrated suspension of whiskers. The suspension-coated whiskers can be converted to a free-flowing powder after calcination. The processing parameters and the results of consolidation of the composite powder are reported. Nearly fully dense composites, containing over 40 vol.% whiskers, could be prepared.     

Fracture behavior of Ce-TZP/alumina/aluminate composites with different amounts of transformation toughening. Influence of the testing methods

The fracture behavior of four Ce-TZP zirconia composites containing 8 vol% alumina and 8 vol% strontium hexa-aluminate was investigated. The composites exhibited different degrees of transformation toughening obtained by varying the amount of the CeO₂ stabilizer and the sintering temperature. The strength was measured by 4-point bending (4PB) and piston-on-three balls (POB) methods Toughness and crack growth resistance (R-curve) were determined from Single Edge V-Notched Beam (SEVNB) and double torsion (DT) samples, and slow crack growth (SCG) curves were determined by DT method.Increasing the transformability of the composites enhanced their crack growth resistance and consequently, increased their resistance to SCG, which was completely inhibited for the most transformable composites. Simultaneously, flaw tolerance was also improved although a decrease in strength was observed. Under all configurations, the composites exhibited a plastic behavior and it was shown that their properties are correlated to the crack shielding due to autocatalytic phase transformation that not only depend on the material transformability, but is also strongly influenced by the testing method.     

Fracture behavior of sisal fiber–reinforced starch‐based composites

The fracture behavior of biodegradable fiber–reinforced composites as a function of fiber content under different loading conditions was investigated. Composites with different fiber content, ranging from 5 to 20 wt%, were prepared using commercial starch‐based polymer and short sisal fibers. Quasistatic fracture studies as well as instrumented falling weight impact tests were performed on the composites and the plain matrix. Results showed a significant increase in the crack initiation resistance under quasistatic loading. This was caused by the incorporation of sisal fibers to the matrix and the development of failure mechanisms induced by the presence of the fibers. On the other hand, a modest increasing trend of the resistance to crack initiation with fiber loading was detected. An improved fracture behavior was also observed when the impact loading was parallel to the thickness direction. Under these experimental conditions, the composites exhibited higher values of ductility index, energy at initiation and total fracture energy than the plain matrix. Furthermore, an increasing trend of these parameters with fiber content was detected in the biocomposites. Overall, the addition of sisal fibers to the biodegradable matrix appears to be an efficient mean of improving fracture behavior under both quasistatic and impact loading conditions. POLYM. COMPOS. 26:316–323, 2005. © 2005 Society of Plastics Engineers     

Fracture behavior of thermoplastic polyolefin/clay nanocomposites

A thermoplastic polyolefin (TPO) containing 70 wt % styrene–ethylene–butadiene‐styrene‐g‐maleic anhydride and 30 wt % polypropylene and its nanocomposites reinforced with 0.3–1.5 wt % organoclay were prepared by melt mixing followed by injection molding. The mechanical and fracture behaviors of the TPO/clay nanocomposites were investigated. The essential work of fracture (EWF) approach was used to evaluate the tensile fracture behavior of the nanocomposites toughened with elastomer. Tensile tests showed that the stiffness and tensile strength of TPO was enhanced by the addition of low loading levels of organically modified montmorillonite. EWF measurements revealed that the fracture toughness of the TPO/clay nanocomposites increased with increasing clay content. The organoclay toughened the TPO matrix of the nanocomposites effectively. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

包覆型MCA/PA6复合材料的制备及其阻燃性研究

以水为反应介质、无水乙醇为溶剂,将正硅酸四乙酯(TEOS)制备成二氧化硅(SiO_2)溶胶,利用溶胶的网络结构对三聚氰胺氰尿酸盐(MCA)进行表面包覆,制备出包覆型MCA阻燃剂;通过熔融共混方式,将包覆前后MCA与聚己内酰胺(PA 6)切片混合制备成不同阻燃剂含量的阻燃PA 6复合材料;采用红外光谱仪X射线光电子能谱仪、差示扫描量热仪、热重分析仪、垂直燃烧法和极限氧指数法等研究了阻燃PA 6复合材料的结构、热性能及阻燃性能。结果表明:SiO_2溶胶成功接枝在MCA表面,且主要分子结构没有发生改变;随着阻燃剂含量的增加,PA 6复合材料的熔点均有降低,但下降幅度较小;包覆型MCA在材料燃烧过程中能够有效参与成炭,在材料表面形成致密的保护层,增强PA 6复合材料的凝聚相阻燃效果,提高其阻燃性能;随着阻燃剂含量增加,PA 6复合材料的阻燃性逐步提高,添加包覆型MCA质量分数为8%时,PA 6复合材料阻燃性可达到UL-94 V-0等级,极限氧指数为28%。     

Rheological behavior of cycloolefin copolymer/graphite composites

In this study, morphological and rheological properties of cycloolefin copolymer (COC)/graphite composites prepared in a twin screw extruder by using various amounts of graphite (G) and expanded graphite (EG) were investigated in detail. Rheological behaviors of the samples were measured in a dynamic oscillatory rheometer in the melt state. Rheology data were analyzed in different ways in order to quantify the microstructural features which indicate the solid‐state physical properties of the composite materials. In the linear viscoelastic region, increasing of storage modulus (G′) with the filler amount and the van Gurp‐Palmen plots were used to determine the percolation threshold which is the critical filler amount for the physical network formation by the G sheets. Percolation threshold values were found to be about 21.5 phr and 3.8 phr for the G‐ and EG‐loaded samples, respectively. Microstructures of the samples which include quite higher amount of filler than the percolation were observed in a scanning electron microscopy. It was found that the sheets of pristine G maintained their original stack form while the EG was successfully dispersed in the COC phase and formed three dimensional house‐of‐card structures without a compatibilizer. POLYM. ENG. SCI., 2012. © 2012 Society of Plastics Engineers     

Experimental fatigue lifetime of coated and uncoated aluminum alloy under isothermal and thermo-mechanical loadings

This paper presents the fatigue lifetime of an aluminum–silicon–magnesium alloy, widely used in diesel engine cylinder heads, both with and without a thermal barrier coating (TBC) system. The coating system in this study consists of two layers including a 150 μm thick metallic bond coat and a zirconium oxide top coat 350 μm thick. These coating layers were applied on the substrate of A356.0 alloy by air plasma thermal spraying. The isothermal fatigue tests were conducted in low cycle fatigue (LCF) regime at various temperatures. Out-of-phase thermo-mechanical fatigue (OP-TMF) tests were also performed at different maximum temperatures and constraint factors. Experimental results demonstrate that at high temperature the coating increases the LCF lifetime of A356.0 alloy at higher strain amplitude and decreases it at lower strain amplitude. At LCF situation, thicker coating has more detrimental effect. However, the coating increases significantly the OP-TMF lifetime. To analyze the failure mechanisms, the fracture surfaces of specimens were investigated by scanning electron microscopy. The fractography of surfaces showed that the failure mechanism of TBC system in coated A356.0 alloy was separations of coating layers in the interface of the substrate and the bond coat layer. This was observed on both high temperature LCF and OP-TMF conditions.