Failure mechanisms in mission-cycled carbon/carbon composites under flexural load at room and elevated temperatures

The response of quasi-isotropic laminates of SiC coated carbon/carbon (C/C) composites under flexural load was studied. Mission-cycled as well as virgin specimens were tested to compare the thermal- and pressure-cycling effects. Variation of flexural strength and stiffness with temperature was investigated to study the load-deflection behaviour and the thermal stability of C/C composites up to 1371 °C. Increase in flexural strength and stiffness were observed with the rise in temperature. A distinct shift in failure modes from compressive to tensile was found with the mission-cycled specimens with the increase in test temperatures, while the failure mode for virgin material was found always on the tensile side. Change in the load-deflection behaviour was examined and increase in non-linearity of the stress-strain behaviour with the mission cycling was observed. Although the number of test specimens was few, Weibull characterization on the flexure data was performed to study the variation of Weibull modulii and the characteristic lives. Failed, as well as untested, specimens were C-scanned to identify the location and the extent of the damaged zone. Post-failure analyses through optical microscopy and scanning electron microscopy were performed to study the damage growth and failure mechanisms. Degradation and separation of the porous matrix structure, localized damage of the reinforcing fibres in the transverse direction, complete fibre bundle failure in the mission-cycled specimens, and delamination near the loading zone were observed.     

Rheological behaviour of SiC (particulate)-borosilicate composites at elevated temperatures

Journal of Materials Science Letters -     

Influence of grain size on wear behavior of SiC coating for carbon/carbon composites at elevated temperatures

To improve the wear performance of SiC coating for C/C composites at elevated temperatures, the grain was refined by adding small amounts of titanium, in the raw powders for preparing this coating. The related microstructure and mechanical characteristics were investigated by scanning electron microscopy, X-ray diffraction, energy dispersive spectroscopy and nano-indention. The results show that the grain size of SiC coating decreased from ∼30 μm to ∼5 μm due to the addition of grain refiner. TiC formed by reacting titanium with graphite, can act as perfect heterogeneous nucleus for the nucleation and growth of β-SiC. The wear resistance and fracture toughness of SiC coating was improved by grain refinement. However, the increasing interfaces increased the friction resistance and resulted in the high friction coefficient of fine-grained coating at room temperature. As the temperature rose, oxides layer formed on the surface of fine-grained coating, which can reduce the adhesive wear and decrease the friction coefficient. The fine-grained coating exhibited relative low friction coefficient of ∼0.41 owing to a compact silica film formed on the worn surface at 600 °C, and the wear was dominated by plastic deformation and shear of silica film. The wear of coarse-grained coating was controlled by the fracture of SiC at high temperature.     

Continuous fibre composites with a nanocomposite matrix: Improvement of flexural and compressive strength at elevated temperatures

Polymer layered silicate nanocomposites can improve the flexural and compressive strength of continuous fibre reinforced composites by means of increasing the matrix modulus. A three-phase thermoplastic composite consisting of a main reinforcing phase of woven glass fibres and a polyamide 6 (PA6) nanocomposite matrix was fabricated. Flexural testing of a conventional PA6 fibre composite has shown a decrease of the flexural strength upon increasing temperature. This behaviour is associated with the decrease of the matrix modulus, especially above T_g. The nanocomposite used in this study has a modulus that is much higher than unfilled PA6, even above T_g and after moisture conditioning. The results showed that the fibre composites with a nanocomposite matrix have a more than 40% increased flexural and compressive strength at elevated temperatures. This also means that the temperature at which the materials can be used is increased by 40–50 °C. Therefore, by using a nanocomposite matrix the high temperature performance of fibre composites can be improved without any change in processing conditions. The combination with other advantages of nanocomposites in areas such as barrier properties, flammability and creep makes this a very attractive approach.     

Fracture behaviour at elevated temperatures of alumina matrix composites reinforced with silicon carbide whiskers

Creep tests were performed on alumina matrix composites containing 9.3,18 and 30 vol % of SiC whiskers. Careful examination after failure showed that the fracture characteristics were dependent upon the microstructure of the material. Whisker pullouts were visible at the fracture surfaces of the composites with 30 and 18 vol % of SiC whiskers due to preferential debonding at the whisker-matrix interfaces, and these composites also developed crack networks due to the propagation of cracks along whisker-free channels between whisker agglomerates. No whisker pullouts or crack networks were visible in the composite with 9.3 vol % of SiC whiskers where the whisker distribution was reasonably uniform.     

Tensile creep behavior of a SiC-fiber/SiC composite at elevated temperatures

The tensile creep behavior of a SiC-fiber-reinforced SiC composite has been investigated in argon at temperatures of 1000–1300°C. The apparent stress exponents for creep of the composite and the apparent activation energies for creep increase with decrease in stress. The threshold stress approach can be used to treat the data. Creep of the CVI–SiC matrix controls the creep of the composite. The relationship between creep rate and the time to rupture can be described by the Monkman–Grant equation which provides a method of life prediction. The Larson–Miller parameter can also be used for creep-life prediction of the composite when the appropriate constant is selected.     

Deformability of a SiCw/6061Al composite during high strain rate compression at elevated temperatures

The deformability of SiCw/6061Al composite during high strain rate compression has been investigated at elevated temperatures around the solidus of the matrix alloy. The results show that the maximum deformability was obtained at 580°C which is near the solidus of the matrix. Analysis of the results indicates that the composites deformed at 580°C have the largest strain rate sensitivity (m value) and the lowest threshold stress, both of which lead to the maximum deformability. Microstructure observation shows that microcracks were formed at the interfaces in the composites deformed at 540°C and 620°C, whereas, in the composite deformed at 580°C, microcracks were rarely found because of the low stress concentration at the interfaces due to the presence of a small amount of liquid. It is suggested that the presence of an adequate amount of liquid phase gives rise to the effective accommodation required for grain boundary sliding for the composite, and thus directly affects the deformability of SiCw/6061Al composite.     

Compressive and flexural behavior of carbon fiber-reinforced PPS composites at elevated temperature

ABSTRACT

The effect of temperature on the mechanical behavior of carbon fiber reinforced polyphenylenesulfide (PPS) composites was investigated by compressive and flexural tests from ambient temperature up to 150°C. The failure morphologies of the C/PPS composites were analyzed to identify the variation of failure modes. Related results showed that the mechanical behavior of C/PPS composites decreased severely with the increase of temperature due to the softening of matrix. The PPS resin film tensile test was carried out and the PPS matrix behavior was recognized as the main factor to dominate the mechanical behavior of composites under compressive/flexural loading at elevated temperatures. It can be found that there was an approximate linear relationship between the compression properties of C/PPS composites and the PPS matrix. The dependence of failure modes of composites on temperatures was closely related to the mechanical behavior of PPS matrix.     

Microstructure and properties of SiCw/6061 aluminium alloy composites after compression at temperatures around solidus of matrix

Abstract

The effect of compressive deformation at temperatures around the solidus of the matrix on the microstructure and properties of SiC_w/6061 aluminium alloy composites was investigated. It was found that the temperature, strain rate, and amount of deformation affect whisker distribution and breakage, densification and uniformity of composites, and SiC_w/matrix alloy interfacial bonding. The microstructural evolution due to compression affects the properties of the composites, which is considered to be the most important aspect for evaluating high temperature plastic forming of the composites. The optimum parameters for compressive deformation were determined by analysing the microstructure and the properties of the composites.     

增强体含量对SiC_W/SiC层状陶瓷复合材料强韧性的影响

采用流延-化学气相渗透(TC-CVI)工艺制备SiC晶须(SiC_W)/SiC层状陶瓷复合材料,研究了SiC_W含量对层状陶瓷复合材料力学性能和微观结构的影响,探讨了SiC_W/SiC层状陶瓷复合材料的强韧化机制。结果表明:TC-CVI工艺能够有效提高复合材料中晶须含量(40vol%),减少制备过程对晶须损伤,所制备的SiC_W/SiC层状陶瓷复合材料具有合适的层内及层间界面结合强度。随着SiC_W含量增加,层状陶瓷复合材料的密度和力学性能均有明显提高。含40vol%晶须的SiC_W/SiC层状陶瓷复合材料的密度、弯曲强度和断裂韧性均比含25vol%晶须的分别提高了8.4%、30.8%和26.7%。断口形貌中能够观察到层间及层内的裂纹偏转,层内的裂纹桥接和晶须拔出等,这些为主要的增韧机制。高含量SiC_W及合适的层间和层内界面结合强度,对提高SiC_W/SiC层状陶瓷复合材料强韧性有明显作用。     

SiC含量对SiCw/Cu复合材料组织与力学性能的影响

为了研究SiCw/Cu复合材料的制备工艺、形成机理,进一步研究SiC含量对材料的组织结构、力学性能的影响.采用热压法和热等静压法制备了不同SiCw含量的SiCw/Cu复合材料,并对复合材料的致密度、显微组织和物相组成、维氏硬度、拉伸和压缩性能进行了研究,对拉伸断口进行分析.结果表明:SiCw有效阻碍Cu基体晶粒的长大,随着SiCw含量的增加,热压制备的SiCw/Cu复合材料的致密度、断后伸长率、拉伸屈服强度下降,而气孔率、维氏硬度与压缩屈服强度显著增加,抗拉强度先增加后降低.热压制备得到的1wt%SiCw/Cu复合材料,具有相对最优的综合力学性能:抗拉强度为156.9 MPa,拉伸屈服强度为112.5 MPa.采用热等静压法制备的3wt%SiCw/Cu复合材料,各方面性能都要优于相同组分的热压材料,抗拉强度达到175.6 Mpa,拉伸屈服强度达到123.2 MPa,维氏硬度达到101.8 HV.复合材料的强度是SiCw的增强作用与孔隙的弱化作用共同作用的结果,SiCw/Cu复合材料的断裂行为既表现出一定的韧性特征,又表现出一定的脆性特征.     

Mechanical characterization of steel/CFRP double strap joints at elevated temperatures

This paper examines the mechanical performance of steel/CFRP adhesively-bonded double strap joints at elevated temperatures around the glass transition temperature (T_g, 42 °C) of the adhesive. A series of joints with different bond lengths were tested to failure at temperatures between 20 °C and 60 °C. It was found that the joint failure mode changed from adherend failure to debonding failure as the temperature approached T_g. In addition, the ultimate load and joint stiffness decreased significantly at temperatures near to and greater than T_g, while the effective bond length increased with temperature. Based on the ultimate load prediction model developed by Hart-Smith for double lap joints and kinetic modelling of the mechanical degradation of the adhesive, a mechanism-based model is proposed to describe the change of effective bond length, stiffness and strength degradation for steel/CFRP double strap joints at elevated temperatures. The modelling results were validated by the corresponding experimental measurements.     

Failure and strength of 2D-C/SiC composite under in-plane shear loading at elevated temperatures

In-plane shear strength (IPSS) of a two-dimensional carbon fiber reinforced silicon carbide composite (C/SiC) was measured by compression of double-notched specimens (DNS) from room temperature to 1873 K. The result indicates that the compression of DNS is an effective method to measure IPSS of C/SiC. A significant dependence of IPSS on temperature was found for C/SiC. IPSS increases with increasing temperature up to 1273 K, and then decreases when the temperature was higher. The main failure modes under the shear loading contain matrix cracking, delamination and pullout of fibers and fiber bundles.     

Effects of microstructure on flexural strength of biomorphic C/SiC composites

Biomorphic C/SiC composites were fabricated from different kinds of wood by liquid silicon infiltration (LSI) following a two-step process. In the first-step, the wood is converted into carbon preforms by pyrolysis in a nitrogen atmosphere. The carbon preforms are then infiltrated by silicon melt at 1,560°C under vacuum to fabricate C/SiC composites. The mechanical properties of the C/SiC composites were characterized by flexural tests at ambient temperature, 1,000, and 1,300°C, and the relationship between mechanical properties and microstructure was analyzed. The flexural strength of the biomorphic composites was strongly dependent on the properties of the carbon preforms and the degree of silicon infiltration. The flexural strength increased with increasing SiC content and bulk density of composite, and with decreasing porosity in the C/SiC composite. An analysis of fractographs of fractured C/SiC composites showed a cleavage type fracture, indicating brittle fracture behavior.     

Fracture behaviours of epoxy nanocomposites with nano-silica at low and elevated temperatures

An investigation was conducted to characterize fracture behaviours of nano-silica modified epoxies at low and elevated temperatures. A nano-silica dispersed epoxy (Nanopox XP 22/0516, Hanse-Chemie, Germany) with 40 wt% silica nano-particles was used as modifier to toughen an epoxy resin, Araldite F (Bisphenol A based, Ciba-Geigy). Fracture toughness and other mechanical properties were measured using standard compact tension (CT), tensile and flexural specimens to elaborate the effects of nano-silica particles on fracture behaviours of epoxy nanocomposites at different temperatures, −50, 0, 23, 50 and 70 °C. Dynamic mechanical analysis (DMA) was utilized to define the glass transition temperature (T _g) upon the addition of different amounts of nano-silica particles. Fracture toughness of the nano-silica modified epoxies was clearly increased at 23 °C and 50 °C, but the role of nano-silica particles in enhancing the fracture toughness became less pronounced at 0 °C and −50 °C and disappeared at 70 °C.