A shear-compression specimen for large strain testing

A new specimen geometry, the shear-compression specimen (SCS), has been developed for large strain testing of materials. The specimen consists of a cylinder in which two diametrically opposed slots are machined at 45° with respect to the longitudinal axis, thus forming the test gage section. The specimen was analyzed numerically for two representative material models, and various gage geometries. This study shows that the stress (strain) state in the gage, is three-dimensional rather than simple shear as would be commonly assumed. Yet, the dominant deformation mode in the gage section is shear, and the stresses and strains are rather uniform. Simple relations were developed and assessed to relate the equivalent true stress and equivalent true plastic strain to the applied loads and displacements. The specimen was further validated through experiments carried out on OFHC copper, by comparing results obtained with the SCS to those obtained with compression cylinders. The SCS allows to investigate a large range of strain rates, from the quasi-static regime, through intermediate strain rates (1–100 s⁻¹), up to very high strain rates (2×10⁴s⁻¹ in the present case).     

A New Shear-Compression-Specimen for Determining Quasistatic and Dynamic Polymer Properties

A new design of the shear compression specimen (SCS) for investigating the viscoelastic shear response of polymers is presented. The specimen consists of a polymer gage section with two metal ends that remain essentially rigid during deformation. Two closed-form analytic models are developed to predict the average stress and strain in the gage section from the deformation-load histories. This new SCS design and its analytic models are thoroughly evaluated via laboratory measurements and numerical simulations. These simulations show that the deformations in the gage section are more uniform than in the original design, and the distribution of the average shear stress and strain are highly homogenous. The simulation results yield good agreement with those of closed-form analytic results and the experiments demonstrate that the new SCS geometry and its analytic models are as reliable as other commonly employed specimens. It can also generate higher strain rates under usual loading conditions because of its smaller specimen gage length. The need for care in specimen preparation is also discussed in detail as illuminated by the experimental and simulation results.

A New Experimental Technique for the Multi-axial Testing of Advanced High Strength Steel Sheets

This paper deals with the development of a new experimental technique for the multi-axial testing of flat sheets and its application to advanced high strength steels. In close analogy with the traditional tension-torsion test for bulk materials, the sheet material is subject to combined tension and shear loading. Using a custom-made dual actuator hydraulic testing machine, combinations of normal and tangential loading are applied to the boundaries of a flat sheet metal specimen. The specimen shape is optimized to provide uniform stress and strain fields within its gage section. Finite element simulations are carried out to verify the approximate formulas for the shear and normal stress components at the specimen center. The corresponding strain fields are determined from digital image correlation. Two test series are performed on a TRIP-assisted steel sheet. The experimental results demonstrate that this new experimental technique can be used to investigate the large deformation behavior of advanced high strength steel sheets. The evolution of the yield surface of the TRIP700 steel is determined for both radial and non-proportional loading paths.     

Analysis of Slot Orientation in Shear-Compression Specimen (SCS)

This paper presents an analytical treatment as well as experimental measurement of the plastic deformation field in shear-compression specimen (SCS) by using digital image processing (DIC) technique. The results provide a set of useful expressions that relates externally applied displacement and load quantities to the equivalent stress and equivalent plastic strain within the gage section. Based on the analysis, we propose modifying the slot angle of SCS geometry from its original value of 45º to 35.26º in order to enhance the uniformity of stress and strain fields in gage section. It is shown by analysis that this enhancement is essentially because the compatibility and boundary conditions that yield a homogeneous deformation field is naturally satisfied for the particular slot orientation of ???=?35.26°. This conclusion is also supported by experimental evidence that comparatively shows the edge effects for varying slot angles.     

试样形状对轴承钢绝热剪切带微观组织的影响

绝热剪切带(ASB)的微观组织受试样几何形状的影响。对圆柱、帽形和剪切压缩型三种不同形状的试样进行分离式霍普金森压杆高速冲击试验,研究试样形状对轴承钢绝热剪切带的形成和微观组织的影响。结果表明,在应变率为1 800～3 100 s^-1的范围内,材料对应变率的敏感性很低。圆柱试样呈现明显的应变硬化,而帽形试样和剪切压缩型试样(SCS)在不同应变率下分别出现应变硬化和无应变硬化的特征,但流变应力并未因应变硬化而提高。试样形状对ASB的微观形貌和组织有很大影响。圆柱试样上产生了窄且细长的ASB,只发生了应变诱发的晶粒细化,属于形变ASB;帽形试样和SCS则形成大片状的ASB,由等轴晶组成,且发生了体心立方体(BCC)马氏体转变为面心立方体(FCC)奥氏体的相变,属于相变ASB。尤其是SCS中ASB的等轴晶,有非常清晰的晶界,是典型的动态再结晶晶粒。温升计算结果显示,圆柱试样ASB的温升远低于奥氏体相变温度,而帽形试样和SCS的温升高于马氏体的熔点,导致局部熔融。     

Sensitivity of the shear gage to normal strains

This paper documents an experimental study that was conducted to demonstrate the sensitivity of the shear gage to the presence of normal strains. The shear gage is a specially designed strain gage rosette that measures the average shear strain in the test section of notched specimens such as the losipescu, Arcan and compact shear specimens. These specimens can have complicated stress states with high shear and normal strain gradients. To evaluate the sensitivity of the shear gage to normal strains, shear gages were tested on an Arcan specimen. The Arcan specimen is a notched specimen that can be loaded in pure shear (90 deg), pure tension (0 deg) and at intermediate 15- deg increments. The shear modulus for an aluminum specimen was determined at each of these loading angles. It was found that the gages display nearly zero sensitivity to normal strains ( _x, _y). Moiré interferometry was used to document the shear and normal strain distributions in the test section and to provide an independent method for determining the average shear strain. These results reinforce the robust nature of testing with the shear gage.     

Numerical validation of the shear compression specimen. Part II: Dynamic large strain testing

Part I of this work addressed quasi-static loading of the shear compression specimen (SCS), which has been especially developed to investigate the shear dominant response of materials at various strain rates. The stress and strain states were characterized numerically. Approximations were presented to reduce the measured load,P, and displacement,d, into equivalent stress and strain . This paper addresses dynamic loading of the SCS. Several simulations were made for representative materials, whose stress-strain behavior is assumed to be rate-independent. The results show that stress wave loading induces strong oscillations in theP-d curve. However, the curve remains smooth in the gage section. The oscillations are about the quasistatic load values, so that with suitable filtering of the dynamicP-d curves, the quasi-static ones are readily recovered. Consequently, the approach that was developed for quasi-static loading of the SCS is now extended to dynamic loading situations. The average strain rate is rather constant and scales linearly with the prescribed velocity. As the plastic modulus becomes smaller, the strain rate reaches higher values. Friction at the end pieces of the specimen is also investigated, and shown to have a small overall influence on the determined mechanical characteristics. This paper thereby confirms the potential of the SCS for large strain testing of materials, using a unified approach, over a large range of strain rates in a seamless fashion.     

Calibration of Stress-triaxiality Dependent Crack Formation Criteria: A New Hybrid Experimental–Numerical Method

A new experimental technique has been developed to investigate the onset of fracture in metals at low and intermediate stress triaxialities. The gage section of a flat specimen has been designed such that cracks are most likely to initiate within the specimen center, remote from the specimen boundaries. Along with the specimen, a biaxial testing device has been built to apply a well-defined displacement field to the specimen shoulders. The stress state within the specimen is adjusted by changing the biaxial loading angle. Using this new experimental technique, the crack initiation in metals can be studied experimentally for stress triaxialities ranging from 0.0 to 0.6. The stress and strain fields within the specimen gage section are determined from finite element analysis. The reliability of the computational model of the test set-up has been verified by comparing the simulation results with laser speckle-interferometric displacement measurements during testing. Sample experiments have been performed on the Al-7Si-Mg gravity die casting alloy. A three-step hybrid experimental–numerical calibration procedure has been proposed and applied to determine a phenomenological crack formation criterion for the Al-7Si-Mg alloy.

Determination of thermal-expansion characteristics of metals using strain gages

A method has been developed to measure thermal-expansion characteristics of metals using bonded resistance strain gages. The method allows rapid and accurate determination of expansion properties at similar or lower cost (depending on the particular application) than conventional dilatometric techniques. Other advantages include elimination of a need for perfectly flat sample material, elimination of specimen machining, and applicability to structures and components. To utilize this technique, the ‘apparent strain’ of the gage is determined by attaching it to a ‘standard’ material for which the thermal-expansion characteristics are accurately known and subtracting the known thermal response of the material from the total gage output. ‘Apparent strain’ is therefore the temperature-induced output of the gage when bonded to a material having a thermal-expansion coefficient of zero. When the gage is then attached to a test material and cycled through the same temperature range, this ‘apparent strain’ is subtracted from the total gage output to obtain the actual unit-length change of the test material. Using this technique, mean-expansion coefficients of experimental alloys were determined over the temperature range ?320°F (?196°C) to room temperature.     

Dynamic strain measurement with the interferometric strain gage

The interferometric strain gage has a short gage length, high-frequency response and the capability of measuring large plastic strains. Furthermore, the gage is easily ruled directly onto the specimen, and no mechanical or electrical contact needs to be made during the measurement. These features make the interferometric strain gage particularly suitable for dynamic plastic-strain measurement. In this paper, the details of an experimental setup for generating and measuring dynamic plastic strain are given. The photometric techniques of measuring the fringe motion of the interference patterns are describle as well as the data-reduction procedure. A typical result is presented, and the validity of the method is established.     

Embedded optical fiber Bragg grating sensor in a nonuniform strain field: Measurements and simulations

This paper investigates the use of embedded optical fiber Bragg gratings to measure strain near a stress concentration within a solid structure. Due to the nature of a stress concentration (i.e., the strong nonuniformity of the strain field), the assumption that the grating spectrum in reflection remains a single peak with a constant bandwidth is not valid. Compact tension specimens including a controlled notch shape are fabricated, and optical fiber Bragg gratings with different gage lengths are embedded near the notch tip. The form of the spectra in transmission varies between gages that are at different distances from the notch tip under given loading conditions. This variation is shown to be due to the difference in the distribution of strain along the gage length. By using the strain field measured using electronic speckle pattern interferometry on the specimen surface and a discretized model of the grating, the spectra in transmission are then calculated analytically. For a known strain distribution, it is then shown that one can determine the magnitude of the applied force on the specimen. Thus, by considering the nonuniformity of the strain field, the optical fiber Bragg gage functions well as an embedded strain gage near the stress concentration.     

Pseudo Stress Response in Kolsky Tension Bar Experiments

An abnormal stress peak was measured in the specimen response in a Kolsky tension bar experiment. Efforts in changing specimen gage length and applying Teflon tape on the threads of both the specimen and the adapters have been conducted to address this stress peak which was found to be pseudo. The pseudo stress peak was caused by the interfacial impact of the threads on the specimen and the bar ends and must be removed from the intrinsic stress response of the specimen material. Higher impact speeds result in higher amplitudes in the peak stress. The peak stress can be significantly reduced by applying Teflon tape on the threads. At a certain strain rate, it becomes more efficient to minimize the peak stress by simultaneously using a specimen with a shorter gage length and applying Teflon tape on the threads.     

An experimental study of inhomogeneous cyclic plastic deformation of 1045 steel under multiaxial cyclic loading

An experimental study was conducted on the inhomogeneous cyclic plastic deformation of 1045 steel under multiaxial cyclic loading. Thin-walled tubular specimens were used and small strain gages were bonded on the specimen surface to characterize the local deformation. The controlled loading paths included cyclic tension–compression, cyclic torsion, proportional axial-torsion, 90°-out-of-phase axial-torsion, and fully reversed torsion with a constant axial stress. The maximum stress in each experiment was lower than the lower yield stress of the material. It was found that the cyclic plastic deformation within the gage section of the specimen under multiaxial stress state followed the three-stage process that was observed from uniaxial loading, namely, incubation, propagation, and saturation. The plastic deformation was significantly inhomogeneous during the propagation stage, and the inhomogeneity continued through the saturation stage. The duration of each stage and the saturated strains were dependent on the cyclic stress amplitude and the loading path. Multiaxial stress state reduced the incubation stage. With identical equivalent stress magnitude, the nonproportional loading path resulted in the shortest incubation and propagation stages, and the saturated equivalent plastic strain magnitude was the smallest. Although the deformation over the gage section was inhomogeneous, the plastic deformation in a given local area was found to be practically isotropic.     

平面应变下紧凑拉伸试样的动态断裂韧性的实验研究

材料的动态断裂韧性是衡量材料在动载荷作用下抵杭裂纹扩展能力的重要指标,以往的材料动态断裂韧性测试多采用三点弯曲试样,而针对紧凑拉伸试样的动态断裂韧性研究很少.本文将紧凑拉伸试样(即CT试样)简化成等效弹簧质量模型,得到了CT试样动态应力强度因子的近似表达式.对Hopkinson压杆装置进行了改进,利用改进后的实验装置进...     

Torsion test of aluminum in the large strain range

A series of experiments was conducted on cast and extruded high purity aluminum material under monotonic large strain torsion condition. Both free-end and fixed-end torsions were studied using tubular specimens of different gage lengths (long, medium and short). The experiments used an axial–torsional extensometer. A procedure of calibration for elevated temperature test was determined. The torque versus angle of twist curves were recorded and converted into true shear stress–strain curves by use of the modified Nadai method developed previously by the authors. The axial extension for free-end torsion and the axial stress developed during fixed-end torsion were recorded. The hoop strain was also measured and was found to be approximately 0.8–0.9 times the axial strain when the shear strain is 150%. The effect of specimen geometry was studied. It was found that the long, thick-walled tubular specimen is suitable for torsion test in the large strain range.     

The Effect of Machining the Gage Section on Biaxial Tension/Shear Plasticity Experiments of DP780 Sheet Steel

An experimental approach for determining the effect of machining the gage section of specimens for quasi-static, biaxial tension/shear testing of sheet steels is described. This method is demonstrated by comparing the results found by an existing testing method with a reduced thickness (Mohr and Oswald Exp Mech 48:65–77, 2008) to that of full-thickness specimens. Finally, the same results are compared to a specimen in which half of the thickness is removed. These tests are performed on DP780, a first generation advanced high-strength steel. Analysis of the full-thickness and one of the reduced thickness specimens shows that some second-order effects in measuring stress occur from these procedures, but these effects are small. Direct comparison of the engineering stress versus strain curves for the tested conditions shows that for the steel under consideration, there is little dependence on full-thickness, partial thickness, or through-thickness sampling location for continuum-level anisotropic plasticity properties in multi-axial loading for the material investigated and the methods used in manufacturing the specimens.     

The shear gage: For reliable shear modulus measurements of composite materials

A special strain gage called the shear gage was developed for composite materials testing with notched shear specimens. The shear-gage records the average shear strain across the entire test section between the notches of the losipescu and compact shear specimens rather than just sampling the shear strain over a small region in the center of the test section. Hence, the shear stress/strain response is obtained by dividing the average shear stress (load divided by the cross-sectional area between the notches) by the average shear strain. By placing gages on both faces of the specimen, accurate and repeatable shear-modulus measurements can be made without prior knowledge of the shear strain or stress distributions. This scheme essentially integrates the shear strain through the entire test section. Knowledge of other material properties is not required to accurately determine shear modulus values. The shear gage was tested on a variety of composite and isotropic materials resulting in more reliable shear modulus determination and less scatter than previously possible.     

In situ determination of adhesive shear moduli using strain gages

A new, convenient and cost-effective method of determining in situ adhesive shear moduli using strain gages is proposed and evaluated. Thick-adherend lap shear specimens with stacked gage rosettes at the center of the bond line are loaded in tension for adhesive shear strain measurement. Experimental and numerical results indicate that the test specimen has a nonuniform adhesive shear stress (or strain) distribution in the test section and that this distribution (except at the center point of the bond line) is greatly affected by load eccentricity. In addition to the nonuniformity in the shear stress distribution, the issue of material nonhomogeneity in the gage-covered region affects the strain gage measurement. By taking into account these two issues and assuming linear-elastic behavior, two approaches for converting the gage-measured shear strain into the adhesive shear strain are developed and verified by experiment. It is shown that the strain gage measurement associated with either of two conversion techniques can determine the in situ adhesive shear moduli, which are comparable with moiré experiment and KRG-1 extensometer measurements.     

Load-Inversion Device for the High Strain Rate Tensile Testing of Sheet Materials with Hopkinson Pressure Bars

A high strain rate tensile testing technique for sheet materials is presented which makes use of a split Hopkinson pressure bar system in conjunction with a load inversion device. With compressive loads applied to its boundaries, the load inversion device introduces tension into a sheet specimen. Two output bars are used to minimize the effect of bending waves on the output force measurement. A Digital Image Correlation (DIC) algorithm is used to determine the strain history in the specimen gage section based on high speed video imaging. Detailed finite element analysis of the experimental set-up is performed to validate the design of the load inversion device. It is shown that under the assumption of perfect alignment and slip-free attachment of the specimen, the measured stress–strain curve is free from spurious oscillations at a strain rate of 1,000 s^?1. Validation experiments are carried out using tensile specimens extracted from 1.4 thick TRIP780 steel sheets. The experimental results for uniaxial tension at strain rates ranging from 200 s^?1 to 1,000 s^?1 confirm the oscillation-free numerical results in an approximate manner. Dynamic tension experiments are also performed on notched specimens to illustrate the validity of the proposed experimental technique for characterizing the effect of strain rate on the onset of ductile fracture in sheet materials.     

Dynamic Tensile Characterization of a 4330-V Steel with Kolsky Bar Techniques

Dynamic tensile experimental techniques of high-strength alloys using a Kolsky tension bar implemented with pulse shaping and advanced analytical and diagnostic techniques have been developed. The issues that include minimizing abnormal stress peak, determining strain in specimen gage section, evaluating uniform deformation, as well as developing pulse shaping for constant strain rate and stress equilibrium have been addressed in this study to ensure valid experimental conditions and obtainment of reliable high-rate tensile stress–strain response of alloys with a Kolsky tension bar. The techniques were applied to characterize the tensile stress–strain response of a 4330-V steel at two high strain rates. Comparing these high-rate results with quasi-static data, the strain rate effect on the tensile stress–strain response of the 4330-V steel was determined. The 4330-V steel exhibits slight work-hardening behavior in tension and the tensile flow stress is significantly sensitive to strain rate.