Large deformation rate-dependent stress-strain behavior of polyurea and polyurethanes

The thermoplastic elastomer polyurethane and the elastomeric thermoset polyurea are finding new applications in increasing the survivability of structures under impact loading, including those encountered in blast and ballistic events. However, the mechanical behavior of polyurea and polyurethane materials under these high rate conditions is relatively unknown. Here, the rate-dependent stress-strain behavior of one polyurea and three representative polyurethane materials is studied by dynamic mechanical analysis, quasi-static compression testing and split Hopkinson pressure bar (SHPB) testing. The polyurethane chemistries were chosen to probe the influence of the hard segment content on the mechanical behavior, where the volume fraction and the amorphous vs. crystalline structure of the hard segment domains were varied. The large strain stress-strain behavior of polyurea and polyurethane shows strong hysteresis, cyclic softening, and strong rate-dependence. The polyurethane with a non-crystalline well-dispersed hard segment morphology did not exhibit cyclic softening. The materials are observed to transition from a rubbery-like behavior under low strain rate (∼10⁻³-10⁰ s⁻¹) loading conditions to either a leathery or glassy-like behavior under high strain rate (∼10⁻³ s⁻¹) loading conditions.     

梅钢微合金化钢B510L的高温变形动态再结晶研究

利用热模拟机进行单道次压缩实验，获得了梅钢B510L钢的应力-应变曲线变化规律，利用最小二乘法对数据进行回归分析，获得梅钢微合金化钢B510L钢发生再结晶的条件，建立了B510L的动态再结晶图。     

Properties of lightweight expanded polystyrene aggregate concretes containing fly ash

Lightweight concretes can be produced by replacing the normal aggregates in concrete or mortar either partially or fully, depending upon the requirements of density and strength levels. The present study covers the use of expanded polystyrene (EPS) beads as lightweight aggregate, both in concrete and mortar. The main aim of this programme is to study the mechanical properties of EPS concretes containing fly ash and compare the results with these in literature on concretes containing OPC alone as the binder. The effects of EPS aggregate on the green and hardened state characteristics of concretes containing fly ash were evaluated. The compressive strength of the EPS concretes containing fly ash show a continuous gain even up to 90 days, unlike that reported for OPC in literature. It was also found that the failure of these concretes both in compression and split tension was gradual as was observed earlier for the concretes containing plastic shredded aggregates. The stress-strain relations and the corresponding elastic modulus were also investigated.     

Finite Element Modeling of Compressive Deformation of Super-long Vertically Aligned Carbon Nanotubes

The super-long, vertically aligned carbon nanotubes (SL-VACNTs) are novel carbon nanomaterial produced from template-free synthesis. The mechanical responses of such material have been investigated by continuum finite element modeling and compared with experimental observations. The crushable foam model has been adequate in modeling the stress-strain curve and deformation of the SL-VACNTs under compression. SL-VACNTs are seen to exhibit transient elastic deformation at small displacement and then plastic deformation at large displacement. The deformation mostly occur at the position immediately beneath the compression platen (indenter face) due to the high stress/strain concentrations.     

对钢管材料在液压膨胀试验中的应力-应变关系的测定

基于塑性膜理论和力平衡方程等,对试验数据进行曲线拟合,来确定在液压成形过程(THF)中薄壁管的应力-应变关系。由此提出一种简单实用的液压膨胀试验方法,并对不锈钢和低碳钢管进行自由膨胀试验,以得到所需的试验变形数据。而自由膨胀的有限元模拟同时也验证了该方法的有效性。结果表明:目前的做法是正确的,可用于对钢管材料的应力-应变性能进行界定,此外,由此法也可得到扩展的大应变流动应力曲线。     

轴心受压FRP约束混凝土柱应力-应变关系曲线计算

本文采用多轴应力下Ottosen混凝土应力 应变关系及Ottosen破坏准则 ,以核心混凝土与FRP约束层变形协调为边界条件 ,通过计算机编制程序对FRP约束混凝土轴压短柱应力 应变关系曲线进行了计算 ,并与试验结果进行了对比。以往国内外大部分关于FRP约束混凝土的计算方法都主要以试验为基础 ,将试验得出的应力 应变关系曲线用数学公式进行拟合 ,然后通过试验数据来确定式中的参数。本文方法主要是通过理论推导得出的 ,这不同于以往的半经验性计算方法     

Properties of near- and sub-micrometre Al matrix composites strengthened with nano-scale in-situ Al2O3 aimed for low temperature applications

Mechanical and electrical properties of in-situ Al–Al₂O₃ metal matrix composites (MMCs) fabricated by powder metallurgy approach using different aluminium powders of particle size 0.8–21 µm and purity 99.8–99.996% were examined. Hot working powder consolidation by vacuum hot pressing at 270 °C and direct extrusion at 425 °C and reduction ratio of 7:1 were applied. Subsequently, extruded composite rods with the diameter of 7.5 mm were cold worked by groove rolling and rotary swaging to the wires with the diameter of 1.1 mm. Detailed microstructural characterization of composite materials was carried out. Stress-strain characteristics of composite wires were measured at 77 and 300 K. In addition, resistivity of all wires were measured by four-probe method between 25 and 300 K and eddy current losses at frequency 72 Hz and temperatures between 18 and 77 K. Obtained results clearly showed that properly designed Al–Al₂O₃ MMC materials can be utilized at low temperatures e.g., for the thermal stabilization of superconducting wires.     

Structural strength analysis and optimization of portable hydrogen storage vessel made of fiberglass tube

Compared to steel, glass has higher strength and lower density, which makes it stand out as a pressure resistant vessel for hydrogen storage. However, glass easily breaks when it encounters locally concentrated stress. For high pressure hydrogen storage, the stress distribution of a glass vessel during pressure loading needs to be homogeneous without local stress concentration. Herein, the stress-strain behavior of portable hydrogen storage vessels made of glass fiber tubes is investigated theoretically and experimentally, respectively. The effects of different glass materials, wall thickness and pressure on the strength of the microtubes are investigated. Meanwhile, the method of filling the triangular gaps and adding solid fiberglass border was proposed to reduce the stress concentration. The result reveals that glass materials have little effect on the strength of microtubes. The optimal wall thickness is 12.5 μm for microtubes with an inner diameter of 100 μm. Filling the triangular gaps can reduce stress and expansion. The arrayed microtubes with solid fiberglass border are less deformed during fiber drawing technique. The stress increases at the tangent point of the arrayed microtubes with solid fiberglass border, but the stress of the outermost microtubes is significantly decreases.     

应变片位置的应变水平对旋转弯曲疲劳试验的影响

旋转弯曲疲劳试验是检测航空导管合格率的重要手段之一。在无扩口导管旋转弯曲疲劳试验中,通常将应变片粘贴在靠近导管承受最大应力处,但二者之间难以避免地存在误差,影响试验结果。针对此问题,基于应力-应变的理论分析,采用Abaqus软件对7种常见规格的TA18无扩口导管进行有限元仿真模拟,得到其应变片位置处的应变值及导管最大应变值,并进行试验验证。结果表明：导管实际产生的最大应变与应变片位置处所测得的应变值的相对误差在7%左右。参照航标,采用以应变片读数低于理论应变值5%的修正方法来进行试验,对确保试验结果的准确性具有一定的指导意义。