Carbon-fiber aluminum-foam sandwich with short aramid-fiber interfacial toughening

Interfacial toughness and toughening mechanisms of a sandwich beam, consisting of an aluminum foam covered with two carbon-fiber/epoxy composite surface layers, are investigated in this study. To improve the interfacial toughness of the sandwich beam, short aramid fiber of different lengths and densities were inserted at the interface during the sandwich fabrication process. The interfacial toughness between face-sheet and core was then measured through the Double Cantilever Beam test for various sandwich designs. Improvements of varying degrees in the interfacial toughness were observed for all specimens toughened with short aramid fibers of different lengths. The interfacial toughening performance and underlying mechanisms for energy absorption were discussed and analyzed using scanning electron microscopy.     

Transfer film growth of continuous carbon fiber reinforced thermoplastic poly(ether ether ketone) facilitated by surface texture during dry sliding

Journal of Materials Science - To show the effect of surface texture on transfer film growth of continuous carbon fiber-reinforced thermoplastic poly(ether ether ketone) (CCF-PEEK), tribological...     

Mg空位缺陷对MgH2解氢性能影响的赝势平面波法研究

采用基于密度泛函理论的第一性原理赝势平面波方法.考察了Mg空位缺陷对MgH<,2>体系解氢性能的影响及微观机理.结果表明:Mg空位缺陷的出现使MgH<,2>晶胞发生收缩,晶格产生畸变,且极大地降低了体系的结构稳定性,显著提高了体系的解氢动力学;电子结构分析表明:Mg空位缺陷改善MgH<,2>体系解氢性能的微观机理在于体...     

Impact characteristics of a vehicle population in low speed front to rear collisions

Rear impact collisions are mostly low severity, but carry a very high societal cost due to reported symptoms of whiplash and related soft tissue injuries. Given the difficulty in physiological measurement of damage in whiplash patients, there is a significant need to assess rear impact severity on the basis of vehicle damage. This paper presents fundamental impact equations on the basis of an equivalent single vehicle to rigid barrier collision in order to predict relationships between impact speed, maximum dynamic crush, mean and peak acceleration, time to common velocity and vehicle stiffness. These are then applied in regression analysis of published staged low speed rear impact tests. The equivalent mean and peak accelerations are linear functions of the collision closing speed, while the time to common velocity is independent of the collision closing speed. Furthermore, the time to common velocity can be used as a surrogate measure of the normalized vehicle stiffness, which provides opportunity for future accident reconstruction.     

Influence of the modification with MWCNT on the interlaminar fracture properties of long carbon fiber composites

This paper reports the processing of a carbon fiber reinforced epoxy composite with carbon nanotube integration and examines the potential improvement on the interlaminar toughness. Carbon nanotube enhanced composites were fabricated by spreading a solution of reinforcing nanoparticles between prepreg layers with the aid of an airbrush. The influence of MWCNTs incorporation has been studied by the End Notched Flexure (ENF) test by means of a new test methodology named “Beam Theory including Bending Rotation effects” (BTBR) proposed recently. This method allows the determination of the critical energy release rate at each point of the test when stable crack advance occurs, obtaining the R-curve. A maximum increase of 22% in initiation fracture toughness was observed in the samples with functionalized CNTs. Moreover, the propagation fracture toughness increased 14%.     

Unified nonlinear quasistatic and dynamic analysis of RF-MEMS switches

In this paper, the modified couple stress-based strain gradient theory is used to provide a unified nonlinear model of the quasistatic and dynamic behavior of an electrostatic microelectromechanical systems microbeam capacitive switch of the Euler–Bernoulli type. Our model not only accounts for the contact between the microbeam and the dielectric substrate using nonlinear springs and dampers, but also accounts for the system size by introducing an internal material length scale parameter. In view of the size of the microbeam and electrostatic gaps involved, Casimir and Van der Waals forces, damping force due to the squeeze membrane effect and electrostatic force with first-order fringing field effects were accounted for in our model. The resulting nonlinear system of PDEs was expanded into a coupled system using series expansion and integrated into ODEs using weighted residuals of the Galerkin type. To overcome the difficulties associated with the determination of the contact length, the Heaviside function for deflection was replaced with a Heaviside function for the contact length, and an iterative procedure was adopted to determine the contact length. To obtain the time variation of the microbeam, the dynamic system of equations was solved using Newmark’s integration scheme. The outcome of our work shows the dependence of the pull-in voltage upon the inertia force, slenderness ratios of the microbeam, the electrostatic gap and the initial boundary conditions of the switch. In addition, we were also able to provide the full history of the microbeam past the pull-in threshold.     

Tailoring polysulfide trapping and kinetics by engineering hollow carbon bubble nanoreactors for high-energy Li-S pouch cells

Despite great progress of lithium-sulfur(Li-S)battery performance at the laboratory-level,both key parameters and challenges at cell scales to achieve practical high energy density require high-sulfur-loading cathodes and lean electrolytes.Herein,a novel carbon foam integrated by hollow carbon bubble nanoreactors with ultrahigh pore volume of 6.9 cm3·g?1 is meticulously designed for ultrahigh sulfur content up to 96 wt.%.Tailoring polysulfide trapping and ion/electron transport kinetics during the charge-discharge process can be achieved by adjusting the wall thickness of hollow carbon bubbles.And a further in-depth understanding of electrochemical reaction mechanism for the cathode is impelled by the in-situ Raman spectroscopy.As a result,the as-prepared cathode delivers high specific capacitances of 1,269 and 695 mAh·g?1 at 0.1 and 5 C,respectively.Furthermore,Li-S pouch cells with high areal sulfur loading of 6.9 mg·cm?2 yield exceptional practical energy density of 382 Wh·kg?1 under lean electrolyte of 3.5μL·mg?1,which demonstrates the great potential for realistic high-energy Li-S batteries.     

Determination of temperature difference in squeeze casting hot work tool steel

The effects of process parameters on temperature difference (??T_RECS) of the squeeze casting part were studied by employing artificial neural network (ANN) and Procast software when cooling up to recrystallization temperature, such as interface heat transfer coefficient of metal/cavity die (h1), applied pressure (Ap), interface heat transfer coefficient of metal/male die (h2), die pre-heat temperature (Dt) and pouring temperature (Pt). An ANN model on the relationship between the processing parameters and ??T_RECS was constructed. The test results on performance of the trained network show that the ANN model can predict ??T_RECS with reasonable accuracy. Employing the ANN model the following conclusions could be made from this work: as far as the influence of process parameters on ??T_RECS are concerned, the most important and the secondary parameter are Dt and Ap. H1 and Ap increase within a certain range, ??T_RECS are found to increase. When Ap and h1 are above their respective critical point, the ??T_RECS decreased rapidly. Pt, h2 and Dp increase within a certain range, ??T_RECS are found to decrease.     

A non-probabilistic structural reliability analysis method based on a multidimensional parallelepiped convex model

Compared with a probability model, a non-probabilistic convex model only requires a small number of experimental samples to discern the uncertainty parameter bounds instead of the exact probability distribution. Therefore, it can be used for uncertainty analysis of many complex structures lacking experimental samples. Based on the multidimensional parallelepiped convex model, we propose a new method for non-probabilistic structural reliability analysis in which marginal intervals are used to express scattering levels for the parameters, and relevant angles are used to express the correlations between uncertain variables. Using an affine coordinate transformation, the multidimensional parallelepiped uncertainty domain and the limit-state function are transformed to a standard parameter space, and a non-probabilistic reliability index is used to measure the structural reliability. Finally, the method proposed herein was applied to several numerical examples.     

Three-dimensional transient heat conduction analysis by Laplace transformation and multiple reciprocity boundary face method

In this paper, a new multiple reciprocity formulation is developed to solve the transient heat conduction problem. The time dependence of the problem is removed temporarily from the equations by the Laplace transform. The new formulation is derived from the modified Helmholtz equation in Laplace space (LS), in which the higher order fundamental solutions of this equation are firstly derived and used in multiple reciprocity method (MRM). Using the new formulation, the domain integrals can be converted into boundary integrals and several non-integral terms. Thus the main advantage of the boundary integral equations (BIE) method, avoiding the domain discretization, is fully preserved. The convergence speed of these higher order fundamental solutions is high, thus the infinite series of boundary integrals can be truncated by a small number of terms. To get accurate results in the real space with better efficiency, the Gaver-Wynn-Rho method is employed. And to integrate the geometrical modeling and the thermal analysis into a uniform platform, our method is implemented based on the framework of the boundary face method (BFM). Numerical examples show that our method is very efficient for transient heat conduction computation. The obtained results are accurate at both internal and boundary points. Our method outperforms most existing methods, especially concerning the results at early time steps.