Improved fracture toughness and integrated damage sensing capability by spray coated CNTs on carbon fibre prepreg

The development of a hierarchically engineered micro-nano hybrid composite system is described. A spray coating technique has been utilized as an effective way to deposit carbon nanotubes (CNTs) onto carbon fibre prepregs with good control of network formation and the potential for localization. Compared to more traditional approaches of introducing CNTs into epoxy matrices for enhancing composite properties, this technique has benefits in terms of its simplicity and versatility, as well as the potential for industrial scale-up. The effectiveness of the technique is demonstrated by the extremely low CNT loading (0.047 wt.%) needed to significantly increase the Mode-I fracture toughness of the carbon fibre laminates by about 50%, which is so far the largest reported improvement for such extremely low concentrations of non-functionalized CNTs. In-situ damage sensing has also been presented for the monitoring of structural health of these nano-engineered composite laminates upon loading, and a systematic analysis of sensing signals is performed.     

流变岩体中椭圆洞室断面开挖过程的力学分析

给出了黏弹性流变岩体中深埋椭圆隧洞断面开挖过程中力学响应的解析解答。通过设定长短半轴随时间变化的函数可模拟施工过程。首先,结合变边界黏弹性问题对应关系和平面弹性复变方法,求得岩体应力及开挖引起位移的统一表达。在推导中,通过建立逆映射函数,分离出参与Laplace变换的时间。然后,对模拟为Boltzmann黏弹模型的岩石材料,给出了积分形式的位移表达式。最后,针对不同类型岩石进行不同开挖方式和速度下无量纲位移及应力的参数分析。分析表明,施工方式对位移与应力有明显影响。给出的解析解答可用于地下椭圆孔型隧道在开挖过程中的力学分析,并可作为实际工程初步设计的手段。     

单片低辐射玻璃谱带模型及其数值模拟

采用谱带模型,考虑了各个波段透射率、反射率对传热的影响,对单片低辐射玻璃的传热过程进行了研究,提出了描述此传热过程的非线性方程组;考虑对流、太阳辐射及外界和室内辐射等边界条件,并采用了牛顿法解方程组.计算结果表明,单片低辐射玻璃镀膜层向室内放置比较合理;另外,讨论了玻璃厚度、室内温度、室外风速对玻璃传热的影响.     

Modelling complex progressive failure in notched composite laminates with varying sizes and stacking sequences

The emergence of advanced computational methods and theoretical models for damage progression in composites has heralded the promise of virtual testing of composite structures with orthotropic lay-ups, complex geometries and multiple material systems. Recent studies have revealed that specimen size and material orthotropy has a major effect on the open hole tension (OHT) strength of composite laminates. The aim of this investigation is develop a progressive failure model for orthotropic composite laminates, employing stepwise discretization of the traction–separation relationship, to predict the effect of specimen size and laminate orthotropy on the OHT strength. The results show that a significant interaction exists between delamination and in-plane damage, so that models without considering delamination would over-predict strength. Furthermore, it is found that the increase in fracture toughness of blocked plies must be incorporated in the model to achieve good correlation with experimental results.     

Effect of interfacial imperfection on bending behavior of composites and sandwich laminates by an efficient C0 FE model

The bending behavior of composites and sandwich plates having imperfections at the layer interfaces is investigated by a refined higher order shear deformation plate theory (RHSDT) and a Least Square Error (LSE) method. In this theory, the in-plane displacement field is obtained by superposing a globally varying cubic displacement field on a zig-zag linearly varying displacement field. This plate theory represents parabolic through thickness variation of transverse shear stresses which satisfy the inter-laminar continuity condition at the layer interfaces and zero transverse shear stress condition at the top and bottom of the plate. In this plate model, the interfacial imperfection is represented by a liner spring-layer model. Finite element method is adopted and an efficient C⁰ continuous 2D finite element (FE) model is developed based on the above mentioned plate theory for the static analysis of composites and sandwich laminates having imperfections at the layer interfaces. In this model, the first derivatives of transverse displacement have been treated as independent variables to circumvent the problem of C¹ continuity associated with the above plate theory (RHSDT). The LSE method is applied to the 3D equilibrium equations of the plate problem at the post-processing stage, after in-plane stresses are calculated by using the above FE model based on RHSDT. The proposed model is implemented to analyze the laminated composites and sandwich plates having interfacial imperfection. Many new results are also presented which should be useful for the future research.     

Heat transfer characteristics of impinging jets: The influence of unsteadiness with different waveforms

A series of experiments are conducted in which specially designed periodic air jets are applied to a heated surface for the purpose of enhancing heat transfer relative to the corresponding steady air jet. The periodic jets are produced by a mass flow rate controller. The experiments are performed for steady jets and for specially designed periodic jets, including a combination of sinusoidal, triangular and rectangular jets at frequencies from 1.25 to 20 Hz, the triangular signal having a different symmetry (representing the ratio of time to increasing velocity in one cycle to total cycle time) and the rectangular jets having a variable duty cycle (representing the ratio of the pulse cycle on-time to off-time) at frequencies of 10 Hz and 20 Hz. Various unsteady jets show distinguishing frequency dependences and the step change, i.e. the sudden increase or decrease of signal, shows some advantageous influence on heat transfer improvement, especially negative step change. Therefore, the enhancement of combined signals lies between the performances of the individual signals. The enhancement for triangular (or sinusoidal) plus rectangular signals shows some improved performance compared with that of rectangular plus triangular (or sinusoidal) signals. The heat transfers are enhanced as Strouhal number increases from 0.004 to 0.068 on the whole for impinging jet with such combined signals. The signals of triangular jet with symmetry parameter of 0 and 1 have improved heat transfer, and the latter has a slightly better result than the former. The instantaneous changing rate of velocity also has an influence on heat transfer improvements. Thus the duty cycle of 1:2 has the best performance in terms of heat transfer enhancement in this study.     

3D thermo-electro-chemo-mechanical coupled modeling of solid oxide fuel cell with double-sided cathodes

A solid oxide fuel cell based on double-sided cathodes is developed in our group, showing special properties and many advantages under some harsh conditions. To optimize the cell further, a thermo-electro-chemo-mechanical coupled 3D model is developed to simulate the distributions of temperature, current density, fuel gas and thermal stress under different voltages. The numerical results indicate that the temperature distribution, current, fuel gases and thermal stress is non-uniform in the cell at different voltages. The distribution of thermal stress in the electrolyte is also non-uniform because of the un-even electrochemical reaction and convective heat transfer. Furthermore, the result shows that about 47%~54% of maximum 1st principal stress in SOFC is caused by the mismatch of coefficients of thermal expansion (CTEs) among materials, while the other part of the maximum 1st principal stress is mainly caused by temperature gradient.     

A Total Order Heuristic-Based Convex Hull Algorithm for Points in the Plane

Computing the convex hull of a set of points is a fundamental operation in many research fields, including geometric computing, computer graphics, computer vision, robotics, and so forth. This problem is particularly challenging when the number of points goes beyond some millions. In this article, we describe a very fast algorithm that copes with millions of points in a short period of time without using any kind of parallel computing. This has been made possible because the algorithm reduces to a sorting problem of the input point set, what dramatically minimizes the geometric computations (e.g., angles, distances, and so forth) that are typical in other algorithms. When compared with popular convex hull algorithms (namely, Graham’s scan, Andrew’s monotone chain, Jarvis’ gift wrapping, Chan’s, and Quickhull), our algorithm is capable of generating the convex hull of a point set in the plane much faster than those five algorithms without penalties in memory space.     

Carbon black reinforced C8 ether linked bismaleimide toughened electrically conducting epoxy nanocomposites

The present work deals with the toughening of brittle epoxy matrix with C8 ether linked bismaleimide (C8 e-BMI) and then study the reinforcing effect of carbon black (CB) in enhancing the conducting properties of insulating epoxy matrix. The Fourier transform infrared spectroscopy (FTIR) and Raman analysis indicate the formation of strong covalent bonds between CB and C8 e-BMI/epoxy matrix. The X-ray diffraction (XRD) and Field Emission Scanning Electron Microscope (FESEM) analysis indicate the event of phase separation in 5 wt% CB loaded epoxy C8 e-BMI nanocomposites. The impact strength increased up to 5 wt% of CB loading with particle pull and crack deflection to be driving mechanism for enhancing the toughness of the nanocomposite and beyond 5 wt% the impact strength started to decrease due to aggregation of CB. The dynamic mechanical analysis (DMA) also indicates the toughness of the nanocomposites was improved with 5 wt% of CB loading due to the phase segregation between epoxy and C8 e-BMI in the presence of CB. The electrical conductivity was also increased with 5 wt% of CB due to classical conduction by ohmic chain contact.     

Study on short ramie fiber/poly(lactic acid) composites compatibilized by maleic anhydride

Short ramie fiber reinforced poly(lactic acid) (PLA) composites without and with maleic anhydride (MA) were developed. The influence of PLA-g-MA as a compatibilizer on the properties of the composites was studied. The tensile, flexural and impact strength of the composites have improvements with the addition of PLA-g-MA. The morphology of fracture surface evaluated by SEM indicates that the composites with the addition of PLA-g-MA can get better adhesion between the fiber and the matrix. And the Vicat softening temperature and the degradation temperature of the composites are increased with the addition of PLA-g-MA. However, PLA-g-MA leads the glass transition temperature (T_g) decrease according to the DSC results.