泡沫镍力学性能的实验研究

本研究在室温下控制位移,先以5mm/min的位移速度对泡沫镍进行了单轴拉伸、压缩实验,然后在不同应变率情况下进行了一系列单轴拉伸实验,得到了相应的应力-应变曲线,讨论了材料的应变率相关性.结果表明在普通拉伸试验范围内(准静态),改变变形速度会影响应力-应变曲线,屈服应力、强度极限随变形速度增大而下降;单轴拉伸时,应力应变关系明显分为线弹性变形、塑性变形、线性硬化和破坏4个阶段;单轴压缩时,具备其他泡沫材料受压典型应力-应变曲线的3阶段特征,即明显的弹性变形段、屈服平台段和紧实段.     

A plastic shakedown constitutive curve based on uniaxial ratcheting behavior of 304 stainless steel at room temperature

On the basis of the Bauschinger effect, a relationship between the elastic space defined in this study and the accumulated plastic strain is measured in uniaxial ratcheting tests of 304 stainless steel at room temperature. According to this relationship, a new model of uniaxial ratcheting is established and used to simulate uniaxial ratcheting behavior. The results of simulation agree well with the experimental results. These results demonstrate that the relationship between the elastic space and the accumulated plastic strain plays an important role in uniaxial ratcheting simulation. Furthermore, by taking into account the interaction of ratcheting and viscoplasticity, the relationships among elastic space, accumulated plastic strain and loading cases are discussed. It can be seen that, when the stress ratio R of valley stress versus peak stress is not less than zero, the accumulated plastic strain is a function of the peak stress. So, a constitutive curve is obtained to describe the stable states of plastic shakedown for 304 stainless steel material under the stress ratio R ≥ 0. It can be used to determine the accumulated plastic strain of engineering structures under cyclic loading only by an elastic–plastic analysis.     

岩样单轴压缩峰后泊松比理论研究

研究了单轴压缩岩样应变软化阶段侧向应变与轴向应变的比值(峰后泊松比)的变化规律。岩样的塑性变形假设根源于塑性应变局部化。岩样的轴向及侧向变形被分别分为两部分:弹性变形(由虎克定律描述)及由局部化引起的塑性变形(由梯度塑性理论及几何关系确定)。应变软化阶段的轴向应变-侧向应变曲线、轴向应力-轴向应变曲线及轴向应力-侧向应变曲线都得到了实验验证。在峰值强度时,峰后泊松比等于峰前泊松比。当压缩应力降至零时,峰后泊松比达到临界值。该临界值可能比峰前泊松比大,也可能比它小。峰后泊松比还和试件尺寸有关,这与峰前泊松不同。峰后泊松比与轴向压应力之间的关系可能是一条直线,也可能是上凸的,或上凹的。这取决于岩石的本构参数(弹性模量、剪切及软化模量、剪切带宽度及峰前泊松比)、试件的结构尺寸(试件宽度及高度)及剪切带倾角之间的关系。     

Modelling of concrete behaviour in uniaxial compression and tension with DEM

The paper focuses on the DEM modelling of the behaviour of plain concrete during uniaxial compression and uniaxial tension using the discrete element method. The model takes into account the concrete heterogeneity at the meso-scale level. The effects of concrete density, size of aggregate grains and specimen size on the stress–strain curve, volume changes and fracture process are studied. In addition, the evolution of contact forces, grain rotations, displacement fluctuations and strain localization during deformation is investigated. The elastic, kinetic, plastic and numerical dissipated energy is calculated and analysed at a different stress–strain stage. Concrete is described as a 1-phase or 3-phase material. The macroscopic 2D and 3D results are compared with the corresponding experiments. A satisfactory agreement between experiments and calculations is achieved.

     

泡沫陶瓷压缩性能的实验研究

研究了两种不同工艺(11#自然硬化法和22#烧结法)生产的泡沫陶瓷分别在1mm／min和5mm／min的变形速率和不同方向受到单向压缩条件下的应力—应变曲线或力—位移曲线。结果表明：泡沫陶瓷受压缩的应力—应变曲线并不具备泡沫材料受压缩的典型应力—应变曲线的三阶段特征，它没有明显的弹性变形段，只有屈服平台段和紧实段；在压缩过程中，11#泡沫陶瓷的骨架变形以弯曲为主，22#泡沫陶瓷的骨架变形主要是由局部断裂产生；泡沫陶瓷结构为各向同性。     

金字塔形栅格材料与胞状铝合金的准静态压缩性能

在金字塔形栅格材料、胞状铝合金压缩试验的基础上研究其力学性能、吸能能力和吸能效率,结果表明,金字塔形栅格材料的单轴压缩应力-应变曲线呈现线弹性变形、弹塑性、软化、致密化等4个阶段,与胞状铝合金的压缩性能相比,其压缩强度更高,吸能能力更好。     

The effect of different forms of strain energy functions in hyperelasticity‐based crystal plasticity models on texture evolution and mechanical response of face‐centered cubic crystals

Micro‐mechanical and macro‐mechanical behavior of face‐centered cubic (FCC) crystals is investigated by using different forms of strain energy functions in hyperelastic material models in crystal plasticity finite element framework. A quadratic strain energy function with anisotropic elastic constants, a polyconvex strain energy function with invariants associated with the cubic symmetry, and a strain energy function from an inter‐atomic potential are considered in hyperelastic material models to describe the elastic deformation of FCC crystals. In our numerical experiments, the trajectories of {111} poles in the pole figure and the accumulated plastic slips of FCC coppers under uniaxial tension and simple shear depend on the choice of strain energy functions when the slip resistance of the slip systems is high. The ability of strain energy functions in this study to represent elastic lattice distortions in crystals varies with the amount of elastic deformation and the shape of deformed lattice. However, numerical results show that the change of macroscopic mechanical behavior of FCC coppers is not significant for the choice of strain energy functions, compared with the change of crystallographic texture evolution. Copyright © 2014 John Wiley & Sons, Ltd.     

Deformation mechanisms and the yield surface of low-density,closed-cell polymer foams

The geometry of low-density, closed-cell, polyethylene and polystyrene foams was modelled with a Kelvin foam having uniform-thickness cell faces; finite element analysis (FEA) considered interactions between cell pressures and face deformation. Periodic boundary conditions were applied to a small representative volume element. In uniaxial, biaxial and triaxial tensile stress states, the dominant high-strain deformation mechanism was predicted to be tensile yield across nearly flat faces. In uniaxial and biaxial compression stress states, pairs of parallel plastic hinges were predicted to form across some faces, allowing them to concertina. In hydrostatic compression, face bowing was predicted. The rate of post-yield hardening changed if new deformation mechanisms became active as the foam strain increased. The effects of foam density and polymer type on the foam yield surface were investigated. Improvements were suggested for foam material models in the FEA package ABAQUS.     

Static indentation and unloading response of sandwich beams

This paper deals with analysis of foam core sandwich beams subject to static indentation and subsequent unloading (removal of load). Sandwich beams are assumed continuously supported by a rigid platen to eliminate global bending. An analytical model is presented assuming an elastic-perfectly plastic compressive behaviour of the foam core. An elastic part of indentation response is described using the Winkler foundation model. Upon removal of the load, an elastic unloading response of the foam core is assumed. Also, finite element (FE) analysis of static indentation and unloading of sandwich beams is performed using the FE code ABAQUS. The foam core is modelled using the crushable foam material model. To obtain input data for the analytical model and to calibrate the crushable foam model in FE analysis, the response of the foam core is experimentally characterized in uniaxial compression, up to densification, with subsequent unloading and tension until tensile fracture. Both models can predict load–displacement response of sandwich beams under static indentation and a residual dent magnitude in the face sheet after unloading along with residual strain levels in the foam core at the unloaded equilibrium state. The analytical and FE analyses are experimentally verified through static indentation tests of composite sandwich beams with two different foam cores. The load–displacement response, size of a crushed core zone and the depth of a residual dent are measured in the testing. A digital speckle photography technique is also used in the indentation tests in order to measure the strain levels in the crushed core zone. The experimental results are in good agreement with the analytical and FE analyses.     

一种新的多轴高周疲劳模型

将疲劳强度以上加载等效为塑性应变,建立了塑性应变与加载应力呈线性关系的表达式,由此得到循环加载的塑性应变能。该塑性应变能使材料微观组织结构发生不可逆变化而引起等效宏观应力。假定该应力符合一种特定的分布函数,导出其最大应力与外加应力叠加达到材料本征断裂应力时的裂纹成核寿命,从而并由微裂纹引起上述两部分应力变化,得到继续加载直至宏观裂纹出现的疲劳寿命。所建立的多轴疲劳寿命公式由3个材料参数表达,并通过单轴疲劳试验数据确定。初步研究表明：该模型对所引用的多轴疲劳试验数据有很好的预测能力。     

K-cor增强泡沫夹层结构制备与力学性能

选取NHZP-1型双马树脂拉挤Z-pin, 并结合差示扫描量热法(DSC)测定及工艺参数优化来调控其固化度, 将Z-pin按70°角(Z-pin植入方向与水平方向夹角)植入Rohacell-51WF泡沫、 采用5429/HT7双马单向预浸料作为蒙皮, 成功制备K-cor夹层结构, 并展开了相应的力学性能测试。根据Z-pin在K-cor与X-cor夹层中与蒙皮结合方式差异建立微观拉伸结构简图, 并借助欧拉杆屈曲模型来估算其临界失稳载荷, 定性分析了平面压缩过程中Z-pin的破坏模式与增强机制。结果表明: Z-pin固化度为62.74%时, K-cor夹层结构的平面拉伸强度和模量分别为1.55 MPa与88.56 MPa, 平面压缩强度和模量高达3.61 MPa与128.84 MPa, 均比空白泡沫试样和具有相同Z-pin参数的X-cor夹层结构有所提高。     

The mechanical response of Rohacell foams at different length scales

The quasi-static mechanical response of polymethacrylimide (PMI) foams of density ranging from 50 to 200 kg m⁻³ is investigated in order to provide experimental data to inspire and validate numerical constitutive models for the response of polymer foams. The macroscopic mechanical response is characterised by conducting quasi-static compression, tension, shear and indentation experiments, whereas microscopic deformation mechanisms are identified by conducting in situ SEM observations during static compression and tension tests; it is shown that foams of low density collapse by cell wall buckling while foams of high density undergo plastic cell-wall bending. As a result, both the elastic and plastic macroscopic response of the foam display a tension/compression asymmetry.     

Polymeric foam behavior under dynamic compressive loading

Polymeric foams are commonly used in many impact-absorbing applications and thermal-acoustic insulated devices. To improve their mechanical performances, these structures have to be modeled. Constitutive equations (for their macroscopic behavior) have to be identified and then determined by appropriate tests.Tests were carried out on polypropylene foams under high strain rate compression. In this work, the material behaviour has been determined as a function of two parameters, density and strain rate. Foams (at several densities) were tested on a uniaxial compression for initial strain rates equal to 0.34 s⁻¹ and on a new device installed on a flywheel for higher strain rates. This apparatus was designed in order to do stopped dynamic compression tests on foam. With this testing equipment, the dynamic compressive behaviour of the polymeric foam has been identified in the strain rate range [6.7.10⁻⁴s⁻¹, 100s⁻¹].Furthermore, the sample compression was filmed with a high speed camera monitored by the fly wheel software. To complete this work, picture-analysis techniques were used to obtain displacement and strain fields of the sample during its compression. Comparisons between these results and stress-strain responses of polypropylene foam allow a better understanding of its behaviour. The multiscale damage mechanism, by buckling of the foam structure, was emphasised from the image analysis.     

环氧树脂复合泡沫材料的压缩力学性能

对空心玻璃微珠填充环氧树脂复合泡沫材料进行了准静态压缩实验, 研究了材料的宏观压缩力学性能, 并提出了弹性模量和屈服强度的预测公式。此外, 对压缩试件的断口进行了宏、细观观察, 研究了材料的压缩破坏机理。结果表明, 复合泡沫材料在压缩过程中, 具有普通泡沫材料的应力-应变曲线的典型特征, 在应变为2 %左右时材料发生屈服, 在应变大于30 %后发生破坏。此外, 材料的杨氏模量和强度均随密度的减小而下降, 预测公式给出的结果与实验值基本一致。压缩试件断口的宏、细观观察表明, 复合泡沫材料主要的破坏形式为剪切引起的弹塑性破坏。      

Optimisation methods for the determination of cyclic material parameters from strain controlled uniaxial fatigue tests

Four new methods are presented for the determination of cyclic strain-life curves according to Manson/Coffin/Basquin/Morrow and of cyclic stress–strain curves according to Ramberg/Osgood from results of strain controlled uniaxial fatigue tests. The material parameters are assessed by combined linear regressions of linearised parts of these curves with the method of least squares either in strain–stress-life space or in stress–strain and strain-life planes. The compatibility terms are applied as constraints. Experimental data from fourteen test series of magnesium die-casting alloys AE42, AM50, and AZ91 at different elevated temperature levels between 30 °C and 140 °C is used exemplarily. The four new methods and two methods known from literature, which lack combined linear regression, are valuated with six different measures, which are based on the remaining absolute deviation. The comparison shows that the method that combines the linear regressions of the plastic part of the cyclic stress–strain curve and the elastic and plastic part of the strain-life curve in an optimisation leads to the best results.     

Lattice strain evolution in IMI 834 under applied stress

The effect of elastic and plastic anisotropy on the evolution of lattice strains in the titanium alloy IMI834 has been examined during a uniaxial tensile test, by in situ monitoring on the Engin instrument at the ISIS pulsed neutron source. Measurements were made at load during an incremental loading test. The data is analysed in the light of the requirements for engineering residual stress scanning measurements performed at polychromatic neutron and synchrotron diffraction sources. Comparisons between the measured strains from different lattice families and the predictions from an elasto-plastic self-consistent model are made. Agreement is good in the elastic regime and for most diffraction planes in the plastic regime.     

Description of fatigue damage in carbon black filled natural rubber

The present paper describes macroscopic fatigue damage in carbon black‐filled natural rubber (CB‐NR) under uniaxial loading conditions. Uniaxial tension‐compression, fully relaxing uniaxial tension and non‐relaxing uniaxial tension loading conditions were applied until sample failure. Results, summarized in a Haigh‐like diagram, show that only one type of fatigue damage is observed for uniaxial tension‐compression and fully relaxing uniaxial tension loading conditions, and that several different types of fatigue damage take place in non‐relaxing uniaxial tension loading conditions. The different damage types observed under non‐relaxing uniaxial tension, loading conditions are closely related to the improvement of rubber fatigue life. Therefore, as fatigue life improvement is classically supposed to be due to strain‐induced crystallization (SIC), a similar conclusion can be drawn for the occurrence of different types of fatigue damage.     

Transient Current Behaviour for Iron in 3.5%NaCl and 3.5%NaCl+ 1%NaNO_2 Solutions during Corrosion Fatigue

The transient current behaviour for Iron in 3.5%NaCl and 3.5%NaCl +1%NaNO2 solutions during corrosion fatigue (CF) process has been investigated at different given strain amplitudes and strain rates. The results show that elastic strain has little contribution to material dissolution. The elastic tension strain results in the decrease in the transient current, while the elastic compression strain increases the transient current. Compared to the elastic deformation, plastic deformation affects material dissolution evidently For iron in 3.5%NaCl solution, the strain amplitude plays a dominant role in the dissolution process accelerated by the plastic strain, while in 3.5%NaCl+1%NaNO2 solution, both the strain amplitude and strain rate play an important role in this process. In this paper, the effect of the elastic deformation on the material dissolution and the relation between the tension and compression current peak values under the plastic cycle deformation are discussed