Vorrichtung zur Prüfung von Feinblech unter Druckbeanspruchung

Jig for testing thin sheet under compression load The experimental determination of the stress strain relation of thin sheet under uniaxial compression load is difficult, because the specimen are thin‐walled. A jig for testing thin sheet under uniaxial compression load and reversed tension‐compression load as well is presented in this article. It is shown, that in the relevant cross section an uniaxial stress state is dominating, which is not effected by the devices for lateral support. This jig gives the possibility to determine material properties, which are important for the manufacturing process and for the design of thin‐walled building components and verify valid assumptions.     

Correlation of tensile and flexural responses of strain softening and strain hardening cement composites

Closed form equations for generating moment–curvature response of a rectangular beam of fiber reinforced concrete are presented. These equations can be used in conjunction with crack localization rules to predict flexural response of a beam under four point bending test. Parametric studies simulated the behavior of two classes of fiber reinforced concrete: strain softening and strain hardening materials. The simulation revealed that the direct use of uniaxial tension and compression responses under-predicted the flexural response for strain softening material while a good prediction for strain hardening material was obtained. The importance of strain softening range on the flexural response is discussed using non-dimensional post-peak parameters. Results imply that the brittleness and size effect are more pronounced in the flexural response of brittle materials, while more accurate predictions are obtained with ductile materials. It is also demonstrated that correlations of tensile and flexural results can be established using normalized uniaxial tension and compression models with a single scaling factor.     

Über die Verschiebung der neutralen Linie und ihr Zusammenhang mit den Randfaserspannungen bei biegebeanspruchten Plastomeren

The displacement of the neutral axis as a function of the stress level in a deflected cantilever beam of two polymeric materials Designing of stress - bearing polymeric products is usually done by employment of the Hookean formula. This procedure whide is sufficient for classic material will give only approximations when employed for polymeric materials. The support of stress is usually better (less deformation) in practice as calculated. This report tries to explain the observed deviations. By the means of several measuring methods the different response for load in tension than in compression is shown. Hints for design considerations are given.     

Formability of a high-strain-rate superplastic Al–4.4Cu–1.5Mg/21SiCW composite under biaxial tension

The cavitation behavior and forming limits of a high-strain-rate superplastic 21 vol.% SiC whisker-reinforced Al–4.4Cu–1.5Mg (Al–4.4Cu–1.5Mg/21SiC_W) under biaxial stress states were investigated in this paper. The composite sheet was bulged using dies with aspect ratios of 1:1, 4:3 and 2:1 at the constant applied stress of 4 MPa and at the optimal temperature of 793 K determined from superplastic tensile tests. The thickness distributions of bulged diaphragms were measured at different strain levels. For diaphragms deformed equibiaxially, a good agreement between experimental thickness distributions and the theoretical predictions of Cornfield and Johnson (Int. J. Mech. Sci. 12 (1970) 479) was observed at fractional heights of the deformed diaphragms ranging from 0.4 to 1.0. The cavitation behavior of the composite under biaxial tension was compared with that of uniaxial tension. It was found that at a similar effective strain, the amount of cavities obtained under equibiaxial tension is slightly greater than that under uniaxial tension, and the cavity growth rate parameter under uniaxial tension was also slightly larger than that of uniaxial tension. The influence of stress state on cavity growth rate was discussed. Limit strains of Al–4.4Cu–1.5Mg/21SiC_W at different stress ratios were predicted based on a plastic damage model recently developed for superplastic materials (Chan and Chow, Int. J. Mech. Sci., submitted). The trend of the prediction was in good agreement with the experimental findings.     

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.     

Fatigue life evaluation of tension‐compression asymmetric material using local stress–strain method

In this paper, the low‐cycle fatigue characteristics of cold‐drawn steel were investigated under strain‐controlled uniaxial fatigue load. Cyclic softening was observed throughout fatigue life except for the initial relatively short period which exhibited cyclic hardening. Positive mean stress was found under fully reversed strain loading, indicating that there was a significant cyclic asymmetry. A modified local stress–strain method was proposed to estimate fatigue life of notched tension‐compression asymmetric material. In order to verify this method, fatigue experiments on two kinds of notched specimens with different notch radius were carried out under constant and block load spectrum. It was found that the modified local stress–strain method was more accurate than the traditional ones, the maximum relative error between predicted and experimental fatigue life was less than 6%.     

含孔天然纤维织物复合材料力学性能

研究了含孔天然苎麻纤维织物/异氰酸酯复合板在双轴向拉伸载荷下的力学行为。对0.5、1.0、2.0、4.0mm 4种孔径板进行了单向和双轴向载荷拉伸试验, 同时采用数字散斑相关方法对全场位移及孔径大小对应变的影响进行了表征。结果表明: 当载荷线性变化时孔周围的位移场分布较为均匀; 随着载荷接近破坏值, 位移场呈非线性分布并出现高应变值点, 破坏以极快的速度沿孔边在这些点首先发生。随着孔径的增大, 在1000~2000N双轴向载荷下孔周围相同面积内x、y方向正应变的平均值减小, 应变值波动小但范围增大。材料在单向和双轴向拉伸时表现出不同的力学特征: 双轴向载荷下失效强度要比单向拉伸时低, 降低比例为14%～27%, 且随着孔径的增加而增加。      

Energy dissipation capacity of fibre reinforced concrete under biaxial tension–compression load. Part I: Test equipment and work of fracture

The objective of this research was to analyse the differences in the dissipated energy under uniaxial tension and biaxial tension–compression load of fibre reinforced concretes using the Wedge Splitting Test. Under biaxial load the specimens were subjected to compressive stress ratios from 10% to 50% of the concrete compressive strength perpendicular to the direction of the tensile load.Under biaxial tension–compression load the energy dissipation capacity of the specimens decreases compared to the uniaxial tension load case on average 20–30%. It is believed that the decrease is a result of the damage mechanism of the concrete matrix and deterioration of the fibre–matrix and/or aggregate–cement paste interfaces in case the section is additionally loaded with compression stresses. This indicates that dimensioning of concrete elements under biaxial stress states using material parameters obtained from tests conducted on specimens under uniaxial tensile load is unsafe and could potentially lead to a non-conservative design.In the second part of this paper the extent of the fracture process zone under uniaxial tension and biaxial tension–compression load will be examined with the Acoustic Emission technique and the reasons for decrease of the energy dissipation capacity under biaxial load will be further discussed.     

Compressional behaviour of carbon fibres

Spectroscopic-mechanical studies have been conducted on a range of carbon fibres by bonding single filaments on the top surface of a cantilever beam. Such a loading configuration allows the acquisition of the Raman spectrum of carbon fibres and the derivation of the Raman frequency strain dependence in tension and compression. Strain hardening phenomena in tension and strain softening phenomena in compression were closely observed. The differences in the slopes of the Raman frequency versus applied strain curves in tension and compression respectively, have been used to obtain good estimates of the compression moduli. A method of converting the fibre Raman frequency versus strain data into stress-strain curves in both tension and compression, is demonstrated. Values of fibre stress and fibre modulus at failure in compression compare exceptionally well with corresponding estimates deduced from full composite data. The mode of failure in compression has been found to depend upon the carbon fibre structure. It is demonstrated that certain modifications in the manufacturing technology of PAN-based fibres can lead to fibres which show resistance to catastrophic compressive failure without significant losses in the fibre compressive modulus.     

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.     

Strain rate dependence of HPFRCC cylinders in monotonic tension

High-Performance Fiber-Reinforced Cementitious Composite (HPFRCC) materials exhibit strain hardening in uniaxial, monotonic tension accompanied by multiple cracking. The durability of HPFRCC materials under repeated loading makes them potentially suitable for seismic design applications. In this paper, the strain rate dependence of tensile properties of two HPFRCC materials in cylindrical specimens is reported from a larger study on strain rate effects in tension, compression and cyclic tension–compression loading. The cylindrical specimens were loaded in monotonic tension at strain rates ranging from quasi-static to 0.2 s⁻¹. To evaluate the impact of specimen geometry on tensile response, coupon specimens loaded in monotonic tension under a quasi-static strain rate were compared to corresponding cylindrical specimens made from the same batch of material. Tensile strength and ductility of the HPFRCC materials were significantly reduced with increasing strain rate. Multiple cracking, strain hardening, strain capacity, and the shape of the stress–strain response were found to be dependent on specimen geometry. SEM images taken of the fracture plane of several specimens indicated that pullout and fracture of the fibers occurred for both HPFRCC materials studied here.     

Tensile and compressive deformation behavior of the Al–Si–Cu–Mg cast alloy with additions of Zr,V and Ti

The deformation behavior of the Al–Si–Cu–Mg cast alloy with micro-additions of Zr, V, and Ti was investigated under uniaxial tension and compression. It was found that after T6 heat treatment the change of the load from tension to compression caused an increase in strength from 348 MPa to 417 MPa and in fracture strain from 1.3% to 37.0%. As calculated based on Mott’s theory of strain hardening, the dislocation slip distance in compression was twice of that in tension. The observed differences in alloy fracture strain were explained by changes in re-orientation and fracturing of the eutectic silicon particles. Due to deformation, fracturing of the silicon particles occurred with major cracks being parallel to the compression axis but perpendicular to the tensile load axis. An influence of deformation mode on change in orientation of the silicon particles was revealed. While for tensile load, the silicon particles were stationary during deformation and exhibited an orientation practically the same as in unstrained structure, for compression there was a substantial change in the particle orientation, especially for an angle between the load axis and the particle axis in the range from 0° to 30°.     

Design,manufacture, mechanical testing and numerical modelling of an asymmetric composite crossbow limb

This paper considers the design, manufacture, mechanical testing and numerical analysis of a crossbow beam (limb). The limb should be lightweight and permit a high deflection of the beam’s tip in order to achieve a good ballistic performance. Consequently, fibre-reinforced polymer matrix composites are suitable candidate materials. However, carbon fibres were considered too brittle for this application. Aramid fibres combine low density and high stiffness but are weak in compression. E-glass fibres are relatively flexible but are of high density. The optimised design developed here uses aramid fibres on the tension face with E-glass fibres on the compression side. This component was manufactured using resin infusion, modelled using a commercial finite element code (Abaqus^®) and the model was validated by mechanical testing. A good correlation was found between the experimentally measured deflections and the numerical results.     

Einfache Stoffgleichungen für mehrachsiges Kriechen nichtlinearviskoelastischer Werkstoffe

Simple Constitutive Equations for Multiaxial Creep of Non-linear Viscoelastic Materials Isochronous creep data in uniaxial tension, compression and shear were used to formulate and test constitutive equations for nonlinear viscoelastic materials with different creep response in simple tension and compression.     

Compressive deformation of an Mg-Al-Si-RE alloy between 120 and 180 °C

The deformation behaviour of an Mg-Al-Si-RE (ASE210) alloy between 120 and 180 °C was investigated by means of uniaxial compression tests to identify possible differences in the deformation response compared with uniaxial tensile data. Early fracture was observed in the low-temperature/high strain rate regime, fracture occurring by crack propagation at 45° with respect to the compression axis. In the high-temperature/low strain rate regime, the flow curves exhibited the typical shape that is usually observed in materials where deformation is controlled by recovery of substructure. The peak flow stresses obtained in this regime of temperature and strain rate were compared with other data obtained by testing the same alloy in tension. The strength of the alloy was found to be slightly greater in compression than in tension, this difference gradually disappearing as strain rate decreased.     

Study on coexistence of brittle and ductile fractures in nano reinforcement composites under different loading conditions

Experimental study on high volume fraction of metallic matrix nano composites (MMNCs) was conducted, including uniaxial tension, uniaxial compression, and three-point bending. The example materials were two magnesium matrix composites reinforced with 10 and 15% vol. SiC particles (50 nm size). Brittle fracture mode was exhibited under uniaxial tension and three-point bending, while shear dominated ductile fracture mode (up to 12% fracture strain) was observed under uniaxial compression. The original Modified Mohr–Coulomb (MMC) fracture model (Bai and Wierzbicki in Int J Fract 161:1–20, 2010; in a mixed space of stress invariants and equivalent strain) was transferred into a stress based MMC (sMMC) model. This model was demonstrated to be capable of predicting the coexistence of brittle and ductile fracture modes under different loading conditions for MMNCs. A material post-failure softening model was postulated along the damage accumulation to capture the above two different failure modes. This model was implemented to the Abaqus/Explicit as a material subroutine. Numerical simulations using finite element method well duplicated the material strength, fracture initiation sites and crack propagation modes of the Mg/SiC nano composites with a good accuracy. The proposed model has a good potential to predict fracture for a wide range of material with strength asymmetry and coexistence of brittle and ductile fractures modes.     

Mechanical characterization and modeling of the deformation and failure of the highly crosslinked RTM6 epoxy resin

The nonlinear deformation and fracture of RTM6 epoxy resin is characterized as a function of strain rate and temperature under various loading conditions involving uniaxial tension, notched tension, uniaxial compression, torsion, and shear. The parameters of the hardening law depend on the strain-rate and temperature. The pressure-dependency and hardening law, as well as four different phenomenological failure criteria, are identified using a subset of the experimental results. Detailed fractography analysis provides insight into the competition between shear yielding and maximum principal stress driven brittle failure. The constitutive model and a stress-triaxiality dependent effective plastic strain based failure criterion are readily introduced in the standard version of Abaqus, without the need for coding user subroutines, and can thus be directly used as an input in multi-scale modeling of fibre-reinforced composite material. The model is successfully validated against data not used for the identification and through the full simulation of the crack propagation process in the V-notched beam shear test.     

In-plane and through-thickness properties,failure modes,damage and delamination in 3D woven carbon fibre composites subjected to impact loading

Two noncrimp 3D woven carbon fibre composites (through thickness angle interlock) of binder volume fractions 3% and 6% were characterised for their response to applied deformation. Experiments were performed at quasi static, medium and high strain rates under a large variety of load cases (tension in warp/weft direction, interlaminar/intralaminar shear, through thickness tension/compression, 3-point bending and plate bending). During the study, novel experimental methods were developed in order to address several challenges specific to 3D composite materials. The results show that, while the different binder volume fractions of 3% and 6% have only a small effect on the in-plane stiffness (warp and weft direction), its effect on the delamination resistance in plate bending experiments is considerable. This is a very important result for the use of these materials in the future. The availability, in previous publications, of complementary data for the matrix and the interface between matrix pockets and fibre bundles makes the comprehensive data set a generically useful reference for hierarchical numerical modelling strategies.     

Effect of hot water immersion on the performance of carbon reinforced unidirectional poly(ether ether ketone) (PEEK) composites: Stress rupture under end-loaded bending

This paper describes the behaviour of AS4 and T700SC reinforced PEEK composites (SUPreM™ and ACP-2) under applied compressive bending strain. The effect of an increased molecular weight of the polymer matrix on the residual time under endloaded compression bending conditions is studied. Generally for a given composite material, the higher the testing temperature and the applied strain the faster the failure occurs. At test temperatures exceeding the glass transition temperature or at high strain ratios the time-to-failure for CF/PEEK composites follows a master curve. The residual times under endloaded compression bending conditions increase with increasing toughness of the PEEK matrix but decrease with increasing tensile strength of the reinforcing fibres. It seems that the better the fibre/matrix adhesion the lower is the time to failure of an endloaded composite, because more load is transferred from the matrix into the fibres.In order to simulate composite applications under ‘harsh’ conditions the CF/PEEK composites have been exposed to boiling water. PEEK is known to be highly resistant to environmental effects, but water uptake significantly influences the overall performance of CF/PEEK composites under endloaded compression bending conditions. The tensile properties of the composites have been measured as function of exposure time in boiling water. The fibre dominated uniaxial tensile strength is not/or only slightly affected by the boiling water conditioning even after extended exposure times but the transverse tensile strength decreases significantly after exposure to boiling water. The performance of SUPreM™ CF/PEEK-150 and 450 composites under endloaded compression bending conditions are positively affected by water conditioning whereas APC-2 fails at shorter residual times. The fracture behaviour under endloaded conditions is also affected by the ingress of water into the composite.The obtained results show clearly that applications of thermoplastic composites leading to large out of plane deformations can only be ‘safe’ if the maximum service temperatures of the finished part will be well below the glass transition temperature of the polymer matrix otherwise even at low bending radii a dramatic failure of the material cannot be excluded.     

Introduction of imperfections in the cubic mesh of the truss‐like discrete element method

In the present version of the truss‐like discrete element method (DEM), masses are considered lumped at nodal points and interconnected by means of unidimensional elements with arbitrary constitutive relations. In previous studies of non‐homogeneous concrete cubic samples subjected to nominally uniaxial tension, it was verified that numerical predictions of fracture using DEM models are feasible and yield results that are consistent with the experimental evidence so far available, including the prediction of size and strain rate effects. In the DEM formulation, material failure under compression is assumed to occur by indirect tension. In previous simulations, it was verified that the response is satisfactorily modelled up to the peak load, when a sudden collapse usually occurs, characteristic of fragile behaviour. On the other hand, experimental stress versus displacement curves observed in small specimens subjected to compression typically present a softening branch, in part due to sliding with friction of the fractured parts of the specimens. A second deficiency of DEM models with a perfectly cubic mesh is that the best correlations with experimental results are obtained with material parameters that differ in tension and compression. This paper examines another cause of the excessively fragile behaviour of DEM predictions of the response of concrete elements subjected to nominally uniaxial compression, which is due to the regularity of the perfect cubic mesh, unable to capture nonlinear stability effects in the material. It is shown herein that the introduction of small perturbations of the DEM regular mesh significantly improves the predicting capability of the model and in addition allows adopting a unique set of material properties, which are independent of the nature of the loading.