Impact damage resistance of CFRP with nanoclay-filled epoxy matrix

The influence of nanoclay on the impact damage resistance of carbon fibre–epoxy composites has been investigated using the low-velocity impact and compression after impact (CAI) tests. The load–energy vs. time relations were analyzed to gain insight into the damage behaviours of the materials. The CFRPs containing organoclay brought about significant improvement in impact damage resistance and damage tolerance in the form of smaller damage area, higher residual strength and higher threshold energy level. The presence of nanoclay in the epoxy matrix induced the transition of failure mechanisms of CFRP laminates during the CAI test, from the brittle buckling mode to more ductile, multi-layer delamination mode. Addition of 3 wt% clay was shown to be an optimal content for the highest damage resistance.     

Low-NOx combustors for hydrogen fueled aero engine

Against the background of increasing environmental concerns, analytical modelling and experimental tests on low-NO_x hydrogen combustors for aero engines have been conducted within the Euro-Québec Hydro-Hydrogen Pilot Project (EQHHPP). This project is jointly supported by the European Union and the Government of Québec. This is a four year program started in 1992; the objectives are to establish the NO_x reduction potential and to provide design guidelines for later development of real aero engine combustors. The paper discusses the findings from phase III.0–2 (demonstration phase) of this program (DASA Airbus, PWC, DASA, DASA MTU, DLR Final Report on the NO_x Reduction Concept EQHHPP Phase III.0–2 “Potential Use of Hydrogen in Air Propulsion”, December 1996).Various candidate fuel preparation concepts for lean hydrogen combustion with and without fuel/air premixing have been evaluated in a generic can type combustor at Pratt and Whitney Canada (PWC). Significant reduction in NO_x emissions at simulated representative engine conditions has been achieved. Two concepts were then selected for further investigation in a research type combustion chamber which represents a typical 60 ° sector of a modern annular combustor. A test rig has been built by MTU especially for these tests. Steady state testing is being performed at Daimler-Benz Aerospace. Further tests to study the transient behaviour have been conducted by United Technologies Research Centre (UTRC).In addition, kinetic modelling was performed at Deutsche Forschungsanstalt für Luft- und Raumfahrt (DLR) and PWC to provide a useful design tool for low-NO_x hydrogen combustors.     

Damage mechanisms under tensile and fatigue loading of continuous fibre-reinforced metal-matrix composites

Metal-matrix composites are gaining increasing attention for structural applications. However, the database relating to their mechanical properties and microstructural characterization remains limited. In the present study, aluminium-matrix composites reinforced with SiC fibres, α-Al₂O₃ fibres and carbon fibres have been investigated. Continuous fibre reinforcements, unidirectional in the 0° and 90° directions, were used. Tensile and compression tests were performed using specially designed test equipment for metal-matrix composites. The best results were achieved when cylindrical hour-glass shaped specimens were used. Fatigue testing of the composites showed that a pronounced improvement in the fatigue behaviour can be achieved upon the addition of fibre reinforcement.     

Synergistic reinforcement effects of ZrB2 on the ultra-high temperature stability of Cf/SiC composite fabricated by liquid silicon infiltration

A carbon fiber-reinforced silicon carbide (C_f/SiC) composite was fabricated with ZrB₂ via the liquid silicon infiltration (LSI) method. A prepreg was prepared by impregnating the phenolic resin with the ZrB₂ powder. The as-LSIed composites were tested for 5 min with an oxyacetylene torch to evaluate their ablation and oxidation properties under an ultra-high temperature environment. The ZrB₂ powders and SiC matrix between carbon fiber bundles generated a dense ZrO₂-SiO₂ layer, which inhibited further oxygen diffusion into the composite and minimized the ablation and oxidation of the carbon fibers. Weight loss and linear ablation rate were further reduced with the addition of ZrB₂ to the C_f/SiC composite; moreover, the synergistic effect of ZrB₂ and SiC reinforced the ablation properties with increased ZrB₂ content. ZrB₂ also reduced the amount of residual silicon, which was detrimental to the mechanical properties of C_f/SiC composite.     

Preparation and properties of SiC/SiC-SiYC with excellent water-oxygen corrosion resistance

In this study, the SiC/SiC-SiYC composites were fabricated via chemical vapor infiltration (CVI) combined with the reactive melt infiltration (RMI) process. The excellent infiltration of Si-Y alloy assisted in fabricating composites with a density of 2.94 g/cm³ and a porosity of only 2.0%. After 20 h of corrosion at 1300 °C in the water-oxygen environment, the generated oxide layer, consisting of a glass layer and a diffusion layer, effectively protected the composites, and the flexural strength retention is 114.2%. This study highlights the significant potential of Si-Y alloy as a modification phase that is resistant to water and oxygen. It also presents a novel approach for developing high-density ceramic matrix composites that are resistant to water-oxygen corrosion.     

不同分散剂对复掺GO/CNFs水泥基复合材料力学和导电性能的影响

新型碳纳米材料氧化石墨烯(GO)和纳米碳纤维(CNFs)在分散性良好的前提下可用于改善传统水泥基材料的性能。采用聚羧酸减水剂(PCs)、十二烷基硫酸钠(SDS)、十二烷基苯磺酸钠(SDBS)3种不同分散剂对复合GO和CNFs在水泥基材料中进行分散,研究分散剂种类对复掺GO/CNFs水泥基复合材料的力学及导电性能的影响,并通过扫描电子显微镜(SEM)对不同分散剂制备的复掺GO/CNFs水泥基试件的微观结构进行分析。结果表明:当单独使用PCs作为分散剂时,在质量分数0.05%GO和0.5%CNFs掺量下,试件的抗压强度达到最大(70.1 MPa);在0.05%GO和0.3%CNFs掺量下,试件的电阻率最小(112.65 Ω·m),且在加载条件下表现出良好的电阻率-应力变化响应。而采用SDS、SDBS两种离子型分散剂时,在GO/CNFs混合分散液的配制和试件制备过程中均会产生大量绵密且难以排出的气泡,使得水泥基复合材料的内部结构疏松,抗压强度降低,电阻率变大,导电性能下降。使用PCs单独分散的GO/CNFs水泥基试件表面水化产物结构致密,而采用SDS分散时水泥基试件微观结构疏松,且仅在100倍下即可观察到表面存在大量孔隙,因此使用PCs分散GO/CNFs对水泥基复合材料性能改善的效果最好。     

钢渣沥青混合料薄层罩面层间剪切性能研究

为了探究钢渣沥青混合料薄层罩面层间剪切破坏机理,设计一种间断级配钢渣沥青混合料和两种构造深度的表面层,研究表面层构造深度、温度、SBR改性乳化沥青用量和应力比对钢渣薄层罩面层间剪切性能的影响。通过马歇尔稳定度试验、冻融劈裂试验和动稳定度试验评价薄层罩面及表面层沥青混合料的基本路用性能,直剪试验和剪切疲劳试验评价薄层罩面层间抗剪切性能。研究结果表明:钢渣集料的压碎值和磨耗值明显低于石灰岩集料,且与沥青的粘附性好,使得钢渣沥青混合料的路用性能优异;层间抗剪强度主要受温度控制,在相同测试温度下,随着黏结材料用量增加,表面层与薄层罩面的层间接触状态得到改善,从而提高了抗剪强度,但过量的黏结材料在表面层与薄层罩面之间形成滑动层,导致构造深度对层间剪切强度的作用减弱;薄层罩面的层间剪切疲劳寿命随应力比的升高而降低,通过剪切疲劳试验得到了疲劳寿命方程。     

A numerical study on combustion and emission characteristics of premixed hydrogen air flames

Main challenges for micro power generators that utilize combustion process for energy production are inadequate residence time, destructive radical wall interactions and intensified heat loss which are mainly rooted from size limitation of such devices. To achieve high and uniform energy output, and bring in a solution to these challenges in an environment friendly manner without any kind of fundamental modification, effect of equivalence ratio on combustion and emission behavior of premixed hydrogen/air flames is numerically investigated in this study. For this purpose, an experimentally tested micro cylindrical combustor model is constructed and premixed hydrogen/air combustion in this model is simulated by varying equivalence ratio between 0.5 and 1.2 to find an optimal equivalence ratio with respect to drawbacks of micro power generators. Combustion and turbulence models implemented in this study are Eddy Dissipation Concept and Standard k-ε models, respectively. A detailed hydrogen/air reaction mechanism which consists of 9 species and 19 steps is employed to accurately gain insight into combustion process. Simulation results show that as the equivalence ratio decreases; centerline temperature distribution gets a lower value and the place where chemical reactions take place moves downstream. The most uniform temperature distribution is achieved between 0.8 and 1.0 equivalence ratios. The highest NO_x formation is at 0.9 equivalence ratio and its mass fraction decreases sharply when the equivalence ratio reduces from 0.9 to 0.5.