A Method of Combined SHPB Technique and BP Neural Network to Study Impact Response of Materials

Abstract:  A new method combining the split Hopkinson pressure bar (SHPB) technique with the back-propagation (BP) neural network program is proposed. By this method, the treated strain wave signals become smooth with less noises induced by the transverse inertia. Moreover, the dynamic rate-dependent constitutive behaviour of materials can be identified, without any pre-assumption of a constitutive model. It is found that by taking the experimentally measured data of strain, strain rate and time as 'input' and the corresponding data of stress as 'output' of the BP neural network, the dynamic constitutive behaviour with internal damage or phase transformation evolution is easy to be identified, where the time could represent either the internal damage evolution or phase transformation process accompanied with the deformation process. It is emphasised that the data learnt by the BP neural network must include both loading and unloading processes, if the whole loading and unloading response is to be correctly predicted. The comparisons between the predictions and experimental results are in good agreement for both polyamide (PA) polymer (as an example of nonlinear viscoelastic materials) and Ti–Ni alloy (as an example of superelastic materials with stress-induced phase transformation).     

硬粒子对聚合物材料表面的冲击摩擦研究

冲击摩擦为接触面切向与法向冲量之比。用聚合物材料试件与弹簧钢片组成振动测试系统 ,硬粒子以一定入射角冲击试件水平面引起系统振动 ,接触冲量通过振幅反映出来 ,记录初始振幅值而计算出一个冲击过程的冲击摩擦因数。结果表明 ,当入射角α在 2 5°-60°变化时 ,冲击摩擦因数f从 0 .5降至 0 .2左右。文章同时分析讨论了摩擦对聚合物表面接触强度的影响。     

电视节目制作中的压缩格式

电视节目制作系统中存在着几种格式的压缩问题 ,对几种压缩方式及其优缺点进行了分析。     

Hypervelocity impact computations with finite elements and meshfree particles

The ability of computations to model characteristics of hypervelocity impact is demonstrated using an algorithm for the automatic conversion of distorted finite elements to meshfree particles. The Lagrangian formulation tracks material boundaries and properties without the errors typical in an Eulerian formulation as the material traverses large distances. A computation of a sphere impacting a bumper is shown to reproduce three regions in the debris cloud that are observed in tests: a front region composed of droplets of melted projectile and target, a middle region of fragmented projectile, and a back region of spalled projectile. Additional computations reproduce the observed traits that result from variations in the projectile shape and obliquity. The computation of a projectile impacting spaced plates demonstrates the ability of the method to model the damage to the rear plate of a Whipple shield for spacecraft protection.     

刚性圆板自由落体在水面上的冲击压力

本文对一个刚性圆板自由落体在水面上的冲击过程建立了比较精确的理论模型,并建立了相应的数值格式。计算结果与已有的实验结果符合良好,通过计算进一步给出了下落质量和下落高度对冲击压力峰值的影响。     

Flow stress of high density polyethylene and nylon 66 at high rates of strain

Measurement of the flow stress of high density polyethylene (HDPE) and nylon 66 at strain rates of 10³ s^?1 using a split Hopkinson pressure bar technique is discussed. The flow stress at a strain of 10% has been determined for both polymers at 20°C. The intrinsic errors involved in this technique are briefly reviewed. The results indicate that the flow stress of HDPE and nylon 66 were 50MPa and 150MPa, respectively, at strain rates of about 10³s^?1.     

Flame retardant and the degradation mechanism of high impact polystyrene/Fe-montmorillonite nanocomposites

High impact polystyrene/Fe-montmorillonite (HIPS/Fe-MMT) nanocomposites were successfully prepared by melting intercalation. The nanostructures of HIPS/Fe-MMT were testified by X-ray diffraction (XRD) and transmission electron microscope (TEM). Corresponding to pure HIPS, the thermal stability of HIPS/Fe-MMT nanocomposites was notably improved. The peaks of heat release rate (PHRR) and the mass loss rate (MLR) were significantly reduced after the formation of the HIPS/Fe-MMT nanocomposites from cone calorimetry. And nanocomposites PHRR was further lower with the increase of Fe-MMT content in the range of 1 to 5 wt%. The degradation mechanism of HIPS and HIPS/Fe-MMT nanocomposites was conducted by pyrolysis gas chromatography mass spectrometry (Py-GC-MS). And the reason of the enhancement of thermal stability maybe is that structural iron is the operative site for radical trapping in the Fe-MMT and the nanostructure enhances the interaction of the chains of the HIPS.     

14 kV single-phase superconducting fault current limiter based on YBCO films

We present the fabrication and short circuit test results of a 14 kV single-phase resistive superconducting fault current limiter (SFCL) based on YBa₂Cu₃O₇ (YBCO) films. Individual components were processed using the 4″ YBCO films and have the rated voltage and current of 600 V and 35 A at 77 K, respectively. Twenty four components, eight components in series and three lines in parallel, make a module having the rated voltage and current of 4.8 kV and 105 A, respectively. Three modules were assembled in series to produce the SFCL working at 77 K, a 14 kV single-phase machine for the 22.9 kV Y-Y grid. short circuit tests were successfully conducted in an accredited test facility with the maximum fault currents up to 14.1 kA_P. All components quenched together upon faults and shared the rated voltage evenly without any supplementary device between the modules. This proves that the SFCL based on YBCO films may not only work reliably at 22.9 kV, but also provide technical feasibility for higher voltage application including the transmission grids.     

Second-order Wagner theory of wave impact

The paper deals with the two-dimensional unsteady problem of the impact of a liquid parabola onto a rigid flat plate at a constant velocity. The liquid is assumed ideal and incompressible and its flow potential. The initial stage of the impact is the main concern in this study. The non-dimensional half-width of the contact region between the impacting liquid and the plate plays the role of a small parameter in this problem. The flow region is subdivided into four parts: (i) the main flow region, the dimension of which is of the order of the contact-region width, (ii) the jet-root region, where the curvature of the free surface is very high and the flow is strongly nonlinear, (iii) the jet region, where the flow is approximately one-dimensional, (iv) the far-field region, where the flow is approximately uniform at the initial stage of impact. A second-order solution in the main flow region has been derived and matched to the first-order inner solution in the jet-root region. The matching conditions provide an estimate of the dimension of the contact region for small time. Pressure distributions in both the main flow region and the inner region are derived. The accuracy of the obtained asymptotic formulae is estimated. The second-order hydrodynamic force acting on the plate is obtained and compared with available experimental data. A fairly good agreement is reported.     

起落架落震试验飞机机轮三向冲击位移的动态测量

为了研究飞机起落架受冲击载荷时的功量吸收状况，须在起落架落震试验中，对飞机着陆瞬间起落架机轮在空间三个方向上冲击位移的动态变化量进行测量。为此，本文设计了四自由度空间定位装置，并使其与计算机连接，实现了落震试验过程中，起落架在高速冲击状态下机轮三向位移的有效测量与记录。