预加载路径对低碳钢强化规律的影响

本文报导了２０＾＃低碳钢薄壁圆管试件在比例加载时Ｐ－ｐ复杂应力状态的实验结果。主要研究了：（１）初始屈服面；（２）经简单拉伸预加载和比例预加载情况下的强化规律；（３）折线预加载情况下的强化规律；（４）预施塑性变形量相等路径不同时的强化规律。     

Acoustic Emission Memory Effect in Coal Samples under Uniaxial Cyclic Loading

This paper gives results from experimental studies of the acoustic emission and strain memory effects in anthracite samples under cyclic loading. Trends and regularities were found in the formation and manifestation of the acoustic emission memory effect of coal in nonmonotonic uniaxial tests. __________ Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 47, No. 2, pp. 103–109, March–April, 2006.     

Large deformation extensional rheology of bread dough

The stress–strain curves of bread dough were derived under uniaxial compression, uniaxial tension and equi-biaxial tension loading conditions. In uniaxial compression, a lubricant was used to eliminate frictional effects between the loading platens and the sample. In uniaxial tension, cylindrical samples with thin flat discs at both ends (‘I’ samples) were tested. The discs at both ends were allowed to air-dry and were subsequently glued onto the loading platens. In equi-biaxial tension, a thin disc of dough was inflated into a bubble using pressurised air. The thickness at the top of the bubble was measured by shining a light through the walls of the bubble and recording the change in light intensity as the wall becomes thinner. All methods ensured that uniform deformation was obtained. Stress and strain were accurately evaluated using image analysis techniques. The tests were performed at various strain rates and speeds that defined the time dependence of the material. A non-linear viscoelastic model based on the Prony series and Van der Waals hyperelasticity was used to predict all test data. The model had a total of five material parameters and two time constants, which were set to represent the actual time scales of the experiments. A reasonable agreement between the experimental data and the chosen material model was observed.     

Mesoscale measurement of strains by analyzing optical images of the surface of loaded solids

An algorithm for estimating the strains in solids at the mesoscale level, which is based on constructing fields of displacement vectors and calculating strain components, is studied and tested. Verification of the method is performed by comparing the analytically calculated strain components with experimental data obtained on the basis of model images and images recorded by a TOMSC optical TV measurement system during tension of polymer samples. The estimates of the strain components obtained by a correlation analysis of images are shown to be in good agreement with the results of analytical calculations for known loading parameters and sizes of the images under analysis. The results of the study confirm that the TOMSC system can be used to estimate local strains by processing images of the surface of loaded samples of materials. __________ Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 47, No. 6, pp. 158–164, November–December, 2006.     

Differences between stress and strain control in the non-linear behavior of complex fluids

Various techniques have been proposed to characterize the behavior in the non-linear regime. A new theoretical framework, as proposed recently by Ewoldt et al. (J Rheol 52(6):1427–1458, 2008), provides a quantitative analysis of Lissajous figures during large-amplitude oscillatory shear (LAOS). Intra- and intercycle non-linearities, strain stiffening and softening, and shear thinning and thickening are described and can be distinguished. The new LAOS framework from Ewoldt et al. has been extended to a sinusoidal stress input. Measurements on two different samples reveal significant different results for sinusoidal strain or sinusoidal stress input. For both sinusoidal inputs, the results have been verified by cyclic stress and strain loading tests. The sinusoidal input tests are analyzed as an oscillatory test by the rheometer software and firmware, whereas the cyclic loading tests are purely rotational tests. Since both types of testing give the same results, any instrumental artifacts can be excluded. This implies that complex fluids can behave differently whether periodic stress or strain input functions outside the linear visco-elastic range are applied. All tests in controlled strain and stress in rotational and oscillatory modes have been performed with the same rheometer based on an air bearing-supported electrically commutated synchronous motor.     

M-shaped Specimen for the High-strain Rate Tensile Testing Using a Split Hopkinson Pressure Bar Apparatus

An experimental technique is proposed to determine the tensile stress–strain curve of metals at high strain rates. An M-shaped specimen is designed which transforms a compressive loading at its boundaries into tensile loading of its gage section. The specimen can be used in a conventional split Hopkinson pressure bar apparatus, thereby circumventing experimental problems associated with the gripping of tensile specimens under dynamic loading. The M-specimen geometry provides plane strain conditions within its gage section. This feature retards necking and allows for very short gage sections. This new technique is validated both experimentally and numerically for true equivalent plastic strain rates of up to 4,250/s.     

PMMA材料的动态压缩力学特性及应变率相关本构模型研究

利用INSTRON万能试验机和分离式Hopkinson压杆(SHPB)对PMMA试件在较宽应变率范围内进行了单轴压缩实验,研究加载应变率对PMMA材料力学性能的影响.利用扫描电子显微镜对回收的试样进行了显微观察,重点分析不同加载应变率下PMMA的微观损伤破坏模式.结果表明:随着应变率的增大,PMMA的流动应力显著地增加,且冲击加载条件下,峰值应力的应变率敏感性明显高于准静态;在准静态加载条件下,PMMA试样呈现明显的延性破坏特征,在动态加载条件下则表现为脆性破坏.最后,对PMMA材料的ZWT粘弹性本构模型参数进行了拟合,拟合结果与实验结果吻合较好,表明该本构模型能够较好地描述较宽应变率范围内PMMA材料的应力应变关系.     

纯铝在单轴应力循环作用下棘轮行为的试验研究

对纯铝进行了单轴应变控制和应力控制下的系统循环试验。对纯铝应变循环下的循环应变幅值、应变幅值历史、平均应变对循环特性的影响进行了揭示，对纯铝在非对称应力循环下的应力幅值、平均应力及其历史对循环蠕变〈即棘轮〉的影响进行了分析，得到了纯铝单轴循环行为的一些有意义的结果。     

Erratum to: Experimental Investigation on Mechanical Behavior of Coarse Marble Under Six Different Loading Paths

A series of triaxial compression experiments were preformed for the coarse marble samples under different loading paths by the rock mechanics servo-controlled testing system. Based on the experimental results of complete stress-strain curves, the influence of loading path on the strength and deformation failure behavior of coarse marble is made a detailed analysis. Three loading paths (Paths I–III) are put forward to confirm the strength parameters (cohesion and internal friction angle) of coarse marble in accordance with linear Mohr-Coulomb criterion. Compared among the strength parameters, two loading paths (i.e. Path II by stepping up the confining pressure and Path III by reducing the confining pressure after peak strength) are suggested to confirm the triaxial strengths of rock under different confining pressures by only one sample, which is very applicable for a kind of rock that has obvious plastic and ductile deformation behavior (e.g. marble, chalk, mudstone, etc.). In order to investigate re-fracture mechanical behavior of rock material, three loading paths (Paths IV–VI) are also put forward for flawed coarse marble. The peak strength and deformation failure mode of flawed coarse marble are found depending on the loading paths (Paths IV–VI). Under lower confining pressures, the peak strength and Young’s modulus of damage sample (compressed until post-peak stress under higher confining pressure) are all lower compared with that of flawed sample; moreover mechanical parameter of damage sample is lower for the larger compressed post-peak plastic deformation of coarse marble. However under higher confining pressures (e.g. σ ₃ = 30 MPa), the axial supporting capacity and elastic modulus of damage coarse marble (compressed until post-peak stress under lower confining pressure) is not related to the loading path, while the deformation modulus and peak strain of damage sample depend on the difference of initial confining pressure and post-peak plastic deformation. The friction among crystal grains determines the strength behavior of flawed coarse marble under various loading paths. In the end, the effect of loading path on failure mode of intact and flawed coarse marble is also investigated. The present research provides increased understanding of the fundamental nature of rock failure under different loading paths.     

On determination of elastic limits by dissipative heating of materials

This paper considers a method for determining the conventional elastic limit by dissipative heating of materials and the results of experimental investigation of these characteristics during periodic asymmetric loading of steel and alloy.     

An experimental study on the ratcheting behavior of pure aluminium under uniaxial cyclic stressing

An experimental study was carried out for the cyclic properties of pure aluminium subjected to uniaxial cyclic straining and stressing. For a material of pure aluminium the effects of the cyclic strain amplitude history and mean strain on the cyclic deformation behavior were investigated, and the influences of stress amplitude, mean stress and their histories on cyclic creep (i. e., ratcheting) were analyzed. It is shown that either uniaxial cyclic property under cyclic straining or ratcheting behavior under asymmetric uniaxial loading depends not only on the current loading, but also on the previous loading history. Some significant results were obtained.Financial support from NFSC is acknowledged.     

Process zone in the Single Cantilever Beam under transverse loading - Part II: Experimental

This paper describes an experimental arrangement to evaluate stress/strain fields in the process zone of asymmetric adhesively bonded joints. A transparent polycarbonate flexible beam was bonded to an aluminium alloy rigid block with an epoxy adhesive in a Single Cantilever Beam (SCB) configuration. The flexible adherend was loaded in the direction parallel to the initial crack front at constant rate. To monitor strains induced by bending and shear along the beam, electric strain gauges were attached to the upper surface of the flexible adherend. Thus strain distribution was measured above the bonded surface, which could be used to monitor crack propagation and investigate stress redistribution in the process zone. A Timoshenko beam lying on a Pasternak elastic foundation model was used for the analysis of experimental findings. Subsequently, the Digital Image Correlation technique was used to measure the flexible substrate in-plane displacement field in the vicinity of the crack front and to assess the specimen kinematics. We found that strain gauge instrumentation of the fracture mechanics specimen was a very sensitive technique for experimental analysis of crack propagation under complex loading, offering fine investigation of stress distribution in the cohesive zone.     

A Tensile Split Hopkinson Bar for Testing Particulate Polymer Composites Under Elevated Rates of Loading

A tensile split Hopkinson bar apparatus is developed for testing high strain rate behavior of glass-filled epoxy. The apparatus uses a specimen gripping configuration which does not require fastening and/or gluing and can be readily used for castable materials. Details of the experimental setup, design of grips and specimen, specimen preparation method, benchmark experiments, and tensile responses are reported. Also, the effects of filler volume fraction (0–30%) and particle size (11–42 μm) are examined under high rates of loading and the results are compared with the ones obtained from quasi-static loading conditions. The results indicate that the increase in the loading rate contributes to a stiffer and brittle material response. In the dynamic case lower ultimate stresses are seen with higher volume fractions of filler whereas in the corresponding quasi-static cases an opposite trend exists. However, the absorbed specific energy values show a decreasing trend in both situations. The results are also evaluated relative to the existing micromechanical models. The tensile response for different filler sizes at a constant volume fraction (10%) is also reported. Larger size filler particles cause a reduction in specimen failure stress and specific energy absorbed under elevated rates of loading. In the quasi-static case, however, the ultimate stress is minimally affected by the filler size.     

悬臂梁角点的应力非对称问题的实验研究

采用网格法研究弹性悬臂梁自由端角点在端部竖向均布荷载下的大变形,利用现代图像采集技术获得实验图像,借助MATLAB图像处理技术对图像数据进行处理,以应变与转动的S-R分解定理为理论基础,计算整旋角并绘制整旋角等高线分布图。等高线分布图显示越靠近角点处,整旋角的等值线越密集,即角点处的整旋角的梯度增大,从而证明体力矩的存在是引起悬臂梁自由端角点应力非对称现象的原因。     

Damage accumulation in specimens with edge crack in the prefracture region under nonstationary few-cycle loading

We present the results of experimental studies of strain and fracture of the aluminum alloy D16-T and titanium alloy VT5-1 under nonstationary few-cycle loading by a symmetric three-point bending and consider several loading conditions under which the load is gradually increased or decreased. The parameter determining the damage accumulation process is found. Special attention is paid to the distinction between the damage accumulation mechanisms in titanium and aluminum alloys. It is shown that, as a rule, the damage accumulation is of nonlinear character and significantly depends on the type of the stepwise variation in the load (increase or decrease). In experiments, we also succeeded in relating the characteristic point on the “deflection-force” graph in the case of single loading to a similar point on the graph constructed for a few-cycle loading, which permits predicting the structure survivability in a few-cycle loading from a single loading.     

Response of a “Plastic hinge” to low-cycle alternating deflections

The paper describes an experimental technique for obtaining the response of a “plastic hinge” to low-cycle alternating loads in which a deflection (strain) range is prescribed. The cycle of loading is confined to equal amplitudes of positive and negative curvature. This response is expressed in terms of a nonlinear moment-curvature relationship which may subsequently be employed in “pseudo-static” structural analysis to derive the load-deflection characteristics of elastic-plastic beams at various stages of cyclic history. A brief account is presented of a fatigue machine designed and constructed for testing mild steel at low endurance levels. A fixture for pure bending is designed to adapt the machine for testing beams and also to provide a measuring system for the applied alternating moments. A meter to measure the curvature in this environment is also described in detail. Similar fixtures are also described for testing cantilever-beam specimens. Moment-curvature models are generated for beams under terminal alternating moments in which ambient strain conditions are controlled. Paper was presented at 1967 SESA Annual Meeting held in Chicago on October 31–November 3.     

Shear Failure of Inconel 718 under Dynamic Loads

Kinetics of deformation and fracture of nickel–iron alloy Inconel 718 under dynamic shear loading was measured using a split torsional Hopkinson bar facility and high-speed photography. Tubular specimens with a reduced gage length and a starter notch were sheared at strain rates up to 6 × 10³ s⁻¹. High-speed photographs of fiducial lines scribed on the specimen surface showed the development of local strains and cracking. This paper describes the experimental and analytical procedures, illustrates average and local plastic strain evolution, and presents shear crack initiation times and propagation speeds.     

Experimental study of the cyclic visco-elasto-plastic behaviour of a polyamide fibre strap

Experimental tensile tests were performed on polyamide-based (PA66) woven strap samples. A strain measuring device was used to measure the strain in the middle and effective part of the woven tensile sample. The tests were performed, on the one hand under monotonous tension at different strain rates and on the other hand under sophisticated cyclic loading histories, including relaxation and creep sequences. The analysis of experimental results was made through a visco-elasto-hysteresis model, based on the superimposition of three stress components. The proposed method allows for characterizing the steady state viscous stress as a function of strain and strain rate, the time-independent irreversible behaviour and the instantaneous modulus increasing with the strain. Based on the visco-elasto-hysteresis model, an analysis enabled us to understand and predict the change in relaxation and creep orientations during complex loading histories.