Reduction of residual stresses in thick-walled composite cylinders by smart cure cycle with cooling and reheating

The nozzle parts of solid rocket motors must endure both the internal pressure generated by high temperature exhaust gas and the mechanical load generated by steering operation. Therefore, the nozzle parts of solid rocket motors are fabricated with thick carbon fiber phenolic resin composites. When the thick-walled phenolic composite cylinder is cooled down from the curing temperature of about 155 °C to the room temperature, thermal residual stresses are created due to the anisotropic thermal deformation of the composite structure.

In this paper, a smart cure method with cooling and reheating was developed to reduce residual stresses in thick-wound composite cylinders made of carbon phenolic woven composite. The optimal cure cycle was obtained to reduce the residual stresses without increasing processing time and applied to fabrication of the thick-walled composite cylinder. From the residual stresses measured by the radial-cut-cylinder-bending method, it was found that the residual stresses were reduced 30% by using the smart cure method.     

Effect of aluminum closed-cell foam filling on the quasi-static axial crush performance of glass fiber reinforced polyester composite and aluminum/composite hybrid tubes

The effect of Al closed-cell foam filling on the quasi-static crushing behavior of an E-glass woven fabric polyester composite tube and thin-walled Al/polyester composite hybrid tube was experimentally investigated. For comparison, empty Al, empty composite and empty hybrid tubes were also tested. Empty composite and empty hybrid tubes crushed predominantly in progressive crushing mode, without applying any triggering mechanism. Foam filling was found to be ineffective in increasing the crushing loads of the composite tubes over the sum of the crushing loads of empty composite tube and foam. However, foam filling stabilized the composite progressive crushing mode. In empty hybrid tubes, the deformation mode of the inner Al tube was found to be a more complex form of the diamond mode of deformation of empty Al tube, leading to higher crushing load values than the sum of the crushing load values of empty composite tube and empty metal tube. The foam filling of hybrid tubes however resulted in axial splitting of the outer composite tube due to the resistance imposed by the foam filler to Al tube inward folding and hence it was ineffective in increasing crushing load and SAE values over those of empty hybrid tubes.     

Effect of postcuring immersed in water under hydraulic pressure on fatigue performance of large‐diameter pultruded carbon/glass hybrid rod

A pultruded carbon fibre core (CFC) and glass fibre shell (GFS) hybrid fiber reinforced polymer (HFRP) rod with the diameter of 19 mm was developed. It was kept immersed in water in a self‐designed pressure chamber device with adjustable hydraulic pressure (20 MPa); after immersion, the tension‐tension fatigue performance was measured at stress levels of 41.7%, 33.4%, and 25.0%. Significant postcuring of resin was observed, resulting in the increase in T_gs for the core and shell layers. There was no significant decrease in the tensile strength of the hybrid rod. The fatigue failure of the hybrid rod was accompanied by debonding of CFC/GFS interface, redistribution of cyclic load, and catastrophic splitting or bursting of GFS. The immersion in water under hydraulic pressure led to a significant increase in fatigue life. The increase in the fatigue life was because of the improvement in interface bonding strength and toughness of the resin owing to the postcuring of the resin. After fatigue, significant degradation in the residual interface bonding strength was observed for the hybrid rods.     

锂离子电池负极材料Sn-SnOx/C复合体的制备及充放电性能

用炭热还原方法制备了Sn-SnOx/Carbon 复合材料,二氧化锡(SnO2)和羧甲基纤维素钠(CMCNa)的混合物在450℃炭化作用下同时生成了炭和SnSnO2纳米颗粒.应用XRD、SEM/EDS、BET和电化学方法对获得的复合材料进行了表征和性能研究.SEM观测结果证明Sn-SnO2纳米颗粒很好地分散在生成的炭复...     

An experimental investigation of the static and fatigue fracture behaviour of hybrid composite/metal joints for a tilting car body

The hybrid composite joint structures considered in this work, for application in a tilting railroad car body, are subjected to shear and bending loads. Two types of the joint specimens were fabricated and tested under both static and fatigue conditions: a hybrid bolted-joint specimen subjected to a shear loads, and a hybrid beam-joint specimen for the bending tests. The fracture behaviours of these specimens under static loads were different from those under cyclic loads. For the hybrid bolted-joint specimens, static shear loads caused a pure shear fracture in the bolt pin body itself. However, cyclic fatigue shear loads brought about an opening-mode fracture at the local site of the bolt which was the valley of the screwed region of the bolt pin and/or the perpendicularly angled region between the bolt head and the pin body. On the other hand, for the hybrid beam-joint specimens, static bending loads caused shear deformation and fracture in the honeycomb core region, while fatigue cyclic bend loading caused delamination along the interface between the composite skin and the honeycomb core, and/or caused a fracture in the welded part jointed with the steel under-frame. These fracture behaviours could arise in other industrial hybrid joints with similar sub-structures, and were used in developing a design parameter to improve a hybrid joint structure.     

Effect of fibre/matrix adhesion on residual strength of notched composite laminates

Effects of fibre/matrix adhesion and residual strength of notched polymer matrix composite laminates (PMCLs) and fibre reinforced metal laminates (FRMLs) were investigated. Two different levels of adhesion between fibre and matrix were achieved by using the same carbon fibres with or without surface treatments. After conducting short-beam shear and transverse tension tests for fibre/matrix interface characterisation, residual strength tests were performed for PMCLs and FRMLs containing a circular hole/sharp notch for the two composite systems. It was found that laminates with poor interfacial adhesion between fibre and matrix exhibit higher residual strength than those with strong fibre/matrix adhesion. Major failure mechanisms and modes in two composite systems were studied using SEM fractography. The effective crack growth model (ECGM) was also applied to simulate the residual strength and damage growth of notched composite laminates with different fibre/matrix adhesion. Predictions from the ECGM were well correlated with experimental data.     

海水环境下老化周期对T800碳纤维/环氧树脂复合材料的影响EI北大核心CSCD

研究T800碳纤维/环氧树脂基复合材料在海水环境中进行湿热腐蚀老化,将制备好的试件放置在人工制备70℃,3.5%NaCl溶液中腐蚀30,60,90 d,通过质量变化、老化前后表面形貌、红外光谱、动态力学性能、压缩实验和层间剪切实验分析材料的力学性能变化。结果表明:T800碳纤维/环氧树脂复合材料在3.5%NaCl溶液中吸湿率分别为0.39%,0.47%,0.53%;未老化试样纤维与基体之间黏结良好,在3.5%NaCl溶液老化后纤维与基体界面破坏随时间的增加老化更加严重;玻璃化转变温度T_(g)下降,分别在老化30,60,90 d后从189.16℃下降到177.54,171.88,168.06℃;经3.5%NaCl溶液老化后,老化30,60,90 d试样的最大破坏载荷分别降低3.2%,8.4%,15.3%,压缩强度分别降低3.0%,8.2%,15.9%;层间剪切最大破坏载荷分别降低3.0%,9.2%,14.9%,剪切强度分别降低3.0%,9.7%,16.4%。     

气氛与应力对3D C/SiC复合材料热震行为的影响

为了确定薄界面 3D C/ SiC复合材料,即热解炭界面( PyC界面) 厚度低于标准厚度(200 nm) 的复合材料,在应力下和氧化性气氛中的抗热震性 , 利用感应加热环境箱在 700～1200℃氧化性气氛中进行了热震试验,基于试验后的强度保持率变化、拉伸应力2位移曲线变化、微结构变化和试验过程中的长度变化等研究了气氛和应力对其热震损伤行为的影响。研究发现,对于薄界面3D C/SiC复合材料,应力增加了裂纹开度,促进了C相的氧化,加快了热震损伤饱和速度,且蠕变应力对热震损伤的加速作用高于疲劳应力。氧化性气氛对界面的适度氧化和应力导致的界面脱粘能提高了薄界面3D C/SiC复合材料的强度保持率,说明其在应力条件下具有较好的抗氧化和抗热震性能。      

Evaluation of the ultimate stresses in a cycle for welded structures with high residual stresses based on test results for small specimens without residual stresses

We propose an equation for the evaluation of the fatigue limits of welded joints of low-carbon and low-alloy steels with high residual stresses based on the results of testing of small specimens without residual stresses. The independence of the amplitude of stresses on the mean stress in a cycle is proved for welded joints with high residual stresses. __________ Translated from Problemy Prochnosti, No. 2, pp. 66–81, March–April, 2008.     

Analysis of the influence of the level of residual stresses on the ultimate stresses in a cycle for welded structures according to the results of testing of small-size specimens

We propose a procedure for the determination of the diagrams of ultimate stresses in a cycle for welded joints with preliminarily induced steady-state residual stresses according to the results of testing of small-size specimens without residual stresses. Translated from Problemy Prochnosti, No. 3, pp. 107–115, May–June, 2009.     

Effect of microwave irradiation on carbon fiber/epoxy resin composite fabricated by vacuum assisted resin transfer molding

The effects of microwave irradiation for resin-curing of carbon fiber/epoxy resin composite (CFRTS), which was fabricated by vacuum-assisted resin transfer molding (VARTM) method, were investigated at 2.45 GHz frequency. The mechanical properties of CFRTS cured by microwave irradiation for 20 min at 120 °C were similar as compared to the conventional oven for 300 min at 120 °C. Moreover, the CFRTS irradiated by microwave had better adherence property between fiber and resin as compared to conventional oven at same resin-curing time. From the relation between resin-curing and mechanical property, it was found that the curing rate of microwave-irradiated CFRTS was 15 times faster as compared to conventional heating. Furthermore, the activation energies for resin-curing reaction on conventional- and microwave-cured CFRTS were estimated to be 2.7 and 1.3 × 10⁴ J/mol, respectively. The resin-curing reaction in CFRTS prepared by VARTM method was significantly promoted by microwave irradiation at short time.     

石墨表面TiC涂层对高定向石墨/Cu复合材料热导率和抗弯强度的影响

以高温盐浴法对天然鳞片石墨粉体(GF)进行表面TiC镀层处理,然后采用真空热压烧结法制备TiCGF/Cu复合材料,研究了粉体表面涂层和GF体积分数对复合材料微观结构、热导率及抗弯强度的影响。系列测试结果表明:随着GF体积分数的降低以及粉体表面TiC镀层的形成,TiC-GF/Cu复合材料平行于GF片层方向的热导率有所降低,抗弯强度有所提升。其中在GF的体积分数占TiC-GF/Cu复合材料70%时,这种变化最为明显,平行于GF片层方向的TiC-GF/Cu复合材料热导率下降幅度最大,从676W/(m·K)下降到526 W/(m·K)。同时,TiC-GF/Cu复合材料的微观结构进一步说明,GF表面的TiC涂层对GF/Cu复合材料的断裂模型起着重要的作用。