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.     

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.     

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.     

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.     

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 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.     

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.     

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.     

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%。     

Fracture of fiber reinforced composites

Characteristics of the fracture of fiber reinforced plastic composites are described in terms of the elastic stress distribution at the crack tip, the mechanism of crack tip damage, and the modes and conditions of final fracture. The three-dimensional, stress field at the tip of a sharp crack in a laminate is presented and contrasted to traditional two-dimensional models. The response of the material in the form of inter- and intraply damage formation and growth under increasing load is characterized, and its effect in blunting the main crack is examined. The final fracture conditions, which may range from quasi-brittle to notch insensitive, are discussed and related to the damage zone extension. Observed and anticipated effects of various material and geometric parameters are also discussed.     

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.     

聚氨酯/氧化锌复合控释材料对包膜尿素控释性能的影响

采用折光率法测定包膜控释尿素在水中养分释放特性,以普通聚氨酯包膜尿素为对照,通过研究聚氨酯/氧化锌复合材料包膜尿素的控释性能,探究聚氨酯/氧化锌复合控释材料应用于控释肥料的效果及其可行性。结果表明:聚氨酯/氧化锌复合材料包膜尿素控释性能优于普通聚氨酯包膜尿素,且以氧化锌质量分数为5%复合材料包膜尿素控释性能最佳,适合于作为控释肥料的包膜材料。     

Flash joining of CVD-SiC coated Cf/SiC composites with a Ti interlayer

Flash joining of CVD-SiC coated C_f/SiC samples with a Ti interlayer was achieved using a Spark Plasma Sintering machine. The influence of different heating powers and discharge times were investigated. The sample flash joined at a maximum heating power of 2.2 kW (peak electric current of 370 A) within 7 s showed the highest apparent shear strength of 31.4 MPa, which corresponds to the interlaminar shear strength of the composites. A maximum joining temperature of ∼1237 °C was reached during the flash joining. An extremely rapid heating rate of 9600 °C/min combined with a very short processing time hindered any reaction between the CVD-SiC coating and the Ti interlayer. The formation of a metallic joint (Ti based) in the absence of any detectable reaction phase is proposed as a new joining mechanism. For a conventionally joined SPS sample, the formation of titanium silicide phases inhibited the formation of a bond.     

Preparation and crystallization behavior of UHMWPE/LLDPE composites

UHMWPE fiber reinforced LLDPE composites were prepared to develop the impact resistant materials. The crystallization kinetics of LLDPE with UHMWPE fiber was investigated to understand the interfacial adhesion and composite performance. The crystallization behavior of LLDPE depends on the crystallization temperature and existence of UHMWPE fiber. LLDPE matrix crystallization was affected by the inclusion of UHMWPE fiber via preceded transcrystallization on the fiber surface. The interfacial adhesion of composites was changed by cooling rate control with different crystallization behavior. Received: 29 August 1997/Revised version: 7 November 1997/Accepted: 13 November 1997     

Thermoelastic behavior of carbon fiber/polycarbonate model composites

Model composites of polycarbonate (PC) containing single, multiple and chopped carbon fibers (CF) with and without and epoxy sizing were prepared by hot pressing. The thermoelastic behavior of model CF/PC composites was characterized by stretching calorimetry at room temperature. For small strains ? (i.e., ? ≈ 0.01) the specific mechanical work, specific heat effects and specific internal energy changes ΔU were completely reversible in stretching/contraction cycles and quantitatively obeyed the standard relationships for elastic solids. Young's moduli E and ΔU were significantly higher, whereas the linear thermal expansivities α_L were lower for model CF/PC composites compared to those for the neat PC. Smaller values of the above parameters for composites reinforced with sized CF suggested weaker CF/PC interfacial interactions. Current theoretical models of thermoelastic properties of composite materials suggest the existence of unusually stiff, highly oriented PC structures in fairly thick boundary layers around CF. The onset of inelastic deformation, as well as mechanical failure in CF/PC model composites at significantly smaller strains compared to the neat PC were tentatively explained by the yield and subsequent plastic flow of the matrix polymer initiated by heat effects of fiber fragmentation processes, and by higher concentration of microvoids generated in fiber fragmentation/debonding events, respectively.