Hopkinson Tensile Bar Research Articles

Effects of different initial states on dynamic tensile properties and microstructure of 7075 aluminum alloy

In the present work, the effects of solution quenching (SQ), under-aging (UA), peak-aging (PA), and over-aging (OA) on the dynamic tensile properties of 7075 aluminum alloy have been systematically studied by Hopkinson tensile bar. The results show that the dynamic deformation behavior of the alloy is remarkably affected by the initial aging state. As the aging time increases, the tensile strength of the material first increases and then decreases. Among them, the alloy in the UA state has the highest tensile strength and strain hardening rate. Besides, the evolution of microstructures such as grains, dislocations and precipitation phases has been quantitatively analyzed by electron backscattered diffraction (EBSD), transmission electron microscopy (TEM) and scanning electron microscopy (SEM). It has been found that with the increase of aging time, the grain size gradually becomes refined, the density of geometrically necessary dislocations gradually decreases, and the alloy deformation grain distribution is the most uniform in the UA state. Besides, dynamic stretching promotes the growth of precipitated phases, and a large number of fine η′ phases are distributed in the matrix in the UA state, and the interaction of the fine η′ phases with the dislocations greatly improves the strain hardening rate and tensile strength of the material.

Dynamic tensile and compressive behaviour of AA7475-T7351 alloy under different strain rates and temperatures

The mechanical behaviour of the aluminium alloy AA7475-T7351 under different loading rates and temperatures is investigated in this article. The high strength AA7475-T7351 alloy under consideration, with its quasi-static tensile strength of 560 MPa at room temperature (25 °C), is stronger than most other aluminium alloys. At the beginning, the specimens are evaluated under quasi-static tension and compression along the rolling direction at five different temperatures (25 °C, 100 °C, 150 °C, 200 °C and 250 °C) on a universal testing machine. Thereafter, an extensive experimental study is conducted on the Split Hopkinson pressure bar set up (SHPB) and the tensile Hopkinson bar (SHTB) to study the constitutive behaviour of AA7475-T7351 specimens at higher strain rates (up to +3000 s−1 for tensile and −4400 s−1 for compression) and elevated temperatures. The strength and ductility of AA7475-T7351 are observed to depend on the loading direction (tension/compression), strain rate and temperature. The efficiencies of the two widely used constitutive models (Johnson-Cook and KHL) for predicting the mechanical behaviour of AA7475-T7351 are also examined.

How sensitivity of metals to strain, strain rate and temperature affects necking onset and hardening in dynamic tests

The material characterization at high strain rates still has room for improvements, especially regarding the possible interactions between strain, strain rate and temperature effects. Some known approximations are still common practice, such as the use of simplified stress–strain calculations at post-necking strains, based on the specimen length, instead of the formulation based on the specimen cross-section. In this work, the behaviour of A2-70 stainless steel at different strain rates and temperature is analyzed, with particular attention to how strain rate and temperature sensitivities affect necking onset and hardening. At the strain rates of interest for this work, the inertia effect is shown to play a secondary role in anticipating or delaying the necking onset. Therefore, the instability onset is firstly analytically derived by just explicitly accounting for the coupling between strain, strain rate and temperature variables within a general multiplicative hardening function, without the need of recalling bifurcation or perturbation analyses. The strains at necking onset, obtained from experiments at different temperatures and strain rates (by hydraulic machine and by tensile Hopkinson bar – SHTB), allow to check the validity of the proposed relationships expressing the combined effect of the relevant variables on the instability condition. Video recordings and image analysis measurements of the deforming round specimens allow comparing the diameter-based effective true stresses, true strains and true strain rates to their nominal length-based counterparts, enabling to quantify the approximation intrinsic in the simplified calculations. The fracture strains and the shapes of necked specimens at incipient failure finally highlighted that the elongation-based approach spoils the accuracy of stress–strain measurements in remarkably different ways for same material, depending on the temperature – strain rate history at hand, thus preventing the use of the latter approach also for merely comparative purposes.

Influence of the fastening between thread–test samples in the stress–strain curves in tensile dynamic tests

A Hopkinson tensile bar has been numerically simulated by means of the commercial software ABAQUS, with different gaps between the sample and the incident and the transmitted bars. This simulated a poor adjustment in the preparation of the tests and observed the effect the measurements of the stress of the incident, reflected and transmitted bars might have in the results of the stress–time and strain–time curves, as well as in the stress–strain curve of an aluminum sample. It is verified that the gap may significantly affect the elastic limit to be obtained from the dynamic tests and the expected results of the ultimate stress and deformation, and also affect the sample stress–strain curve to a large extent. We also concluded in the importance that is necessary in the testing equipment for a dynamic characterization of materials, in this case, the equipment for dynamic tensile testing.

Dynamic tensile behavior of twinning-induced plasticity/low-carbon (TWIP/LC) steel clad sheets bonded by hot rolling

Commercialization of TWinning Induced Plasticity (TWIP) steels has been delayed because of deteriorated surface quality, delayed fracture, and liquid metal embrittlement in spite of their excellent tensile properties. In order to complement these drawbacks, TWIP/low-carbon (LC) steel clad sheets were fabricated by hot rolling, and their dynamic tensile properties were measured by a Hopkinson tensile bar. In all TWIP/LC clad sheets, TWIP/LC interfaces were well bonded without pores or voids by mechanical and metallurgical bonding. Thanks to well-activated dynamic Hall-Petch effect in the TWIP steel region of the clad sheets, the dynamic strength and ductility of the clad sheets were higher than quasi-static ones because the twin formation was more activated under the dynamic loading. They were also considerably higher than those calculated by a rule of mixtures because the interfacial area took most of dynamic applied tensile stress and strain. The present TWIP/LC clad sheets covered wide ranges of excellent dynamic tensile strength and ductility (e.g., 792–1076MPa and 50–57%, respectively) by controlling the volume fraction of TWIP region, which can readily discover new applications for automotive structural steel sheets.

Experimental Study of In-plane Mechanical Properties of Carbon Fibre Woven Composite at Different Strain Rates

Carbon fibre woven composite has been increasingly employed in engineering applications undergoing complex loading conditions. For effective use of composite material in dynamic applications, it is essential to fully understand the mechanical behaviour of composite at different strain rates. In the present study, both in-plane tensile and compressive experiments loaded at 0 degree axial direction and 45 degree off-axial direction of a TC33 carbon fibre woven composite were investigated over the strain rate range from 0.001 to 1000 s−1. High strain rate tests were carried out using Split Hopkinson Pressure and Tensile Bar apparatus respectively. The results indicated that the in-plane mechanical properties and failure patterns were strain rate sensitive under both tensile and compressive loadings. The mechanical properties, failure patterns and strain rate effect also showed highly direction dependent and tension/compression asymmetric characteristic within the considered strain rate range. For higher strain rate sensitivity under compression than that under tension, the asymmetry of mechanical properties was less obvious with the increase of strain rate. Finally, two phenomenal models were proposed to quantitatively fit the relationship between strength property and strain rate.

Experimental issues in tensile Hopkinson bar testing and a model of dynamic hardening

This paper is firstly aimed at the quantitative evaluation of the error levels achievable in the dynamic characterization of ductile metals via Hopkinson bar tensile tests, ran according to the classical strain-gauge-based experimental procedure and to the enhanced high-speed-camera-assisted procedure.Also the effect of the specimen slenderness is investigated for checking the sensitivity of both the above techniques to different specimen geometries.Secondly, a material model is proposed here for the dynamic hardening, based on the experimental true stress and on a dynamic postnecking correction derived from the MLR function, originally developed for the quasistatic necking.Finally, the necking onset is demonstrated to freeze the sensitivity of the stress-strain curve to the strain rate; this feature, supported here by both experiments and finite elements analyses, is a novelty according to the authors knowledge, and might have a considerable impact on the whole dynamic characterization of ductile metals undergoing large postnecking strains before failure.

Anisotropic properties of Ti6Al4V titanium sheet under quasi-static and dynamic loading conditions

This paper presents analysis of an anisotropic mechanical properties of Ti6Al4V Titanium alloy determined under quasi-static and dynamic loading conditions using servo-hydraulic testing machines and tensile Hopkinson bar, respectively. The material was examined at three loading angles with respect to the rolling direction, i.e. 0o, 45o and 90o. It was founded that strain rate sensitivity is independent on the loading direction. Fracture mechanism is orientation depended and as a consequence various strain distributions on the specimens surfaces were observed. Moreover, at the loading angle transverse to the rolling direction significant lowering of flow stress and fracture strain was obtained.

High rate test method for fiber-matrix interface characterization

A new test method to directly characterize fiber-matrix interface properties under high rate of loading has been developed. A tensile Hopkinson bar with a modified incident bar is used to load a microdroplet test specimen. Numerical simulations were carried out to design the test specimen geometry and validate data reduction procedures for the dynamic interface experiments. Stress wave propagation in an S-2 Glass/Epoxy microdroplet specimen was studied with different droplet sizes (100 μm–200 μm) and fiber gage lengths (2 mm–6 mm). Simulation results indicate that dynamic equilibrium can be maintained up to a displacement rate of 10 m/s. Dynamic microdroplet experiments were conducted at a displacement rate of 1 m/s on S-2 glass/epoxy interface. Experimental results and post-failure inspection of the fiber matrix interface showed that the new test method is effective in measuring high rate interface properties of composites.

Static and dynamic response of titanium alloy produced by electron beam melting

The suitability of Titanium alloys for many specialized applications requiring excellent performances at both static and dynamic strain rates, benefits of modern manufacturing technologies like the additive manufacturing, oriented toward the obtainment of complicated component shapes.The EBM methodology for the production of Ti6Al4V components is based on the localized melting of alloy powders by way of guided electron beams scanning the powder volume by successive planar trajectories; for this reason, the whole production process may confer a certain degree of anisotropy to the components. The material behavior of the EBM alloy may be orientation-dependent in terms of stress-strain elastoplastic response as well as in terms of damage sensitivity and ductile fracture under given triaxiality histories.The static and dynamic behavior of a sintered Ti6Al4V alloy is investigated here by way of quasistatic tension-torsion tests and dynamic tensile Hopkinson bar (SHTB) tests.The outcome of the latter experiments, compared to similar tests results from the literature concerning Ti alloy obtained by classical metallurgical techniques, gives some indications about how the technological process may affect the final performance of the material and the component.

Mechanical response of dual phase steel at quasi-static and dynamic tensile loadings after initial fatigue loading

This paper presents an analysis of the mechanical behavior of DP500 steel previously exposed to cyclic loadings with constant stress amplitude. Stress–strain curves were determined for a two strain rates using servo-hydraulic testing machine and tensile Hopkinson bar. Tests were accompanied by digital image correlation, which enabled us to estimate strain localization at various initial fatigue loading conditions. Independently of the pre-fatigue parameters, initial cyclic loadings induce increase of yield stress. Similar behavior is observed for the ultimate tensile strength (UTS) estimated during the Hopkinson bar test. For low strain rates, UTS remains almost constant. On the contrary, increasing the number of cycles lowers the value of elongation at fracture and decreases the strain corresponding to UTS. In every case discussed, higher initial cyclic loading stress induces stronger evolution of mechanical parameters of DP500 steel. Macroscopically estimated strain measured at quasi-static loading conditions is higher than dynamic ones. The opposite behavior was found during analysis of local strain, i.e. strain obtained during the Hopkinson bar experiment was higher than that obtained by using the servo-hydraulic testing machine.

A constitutive model for hardness considering the effects of strain, strain rate and temperature

The relation between hardness, strain and temperature has been widely investigated over the past decades. However, less attention has been paid to the effect of strain rate on hardness induced in high rate deformations. In this investigation, the relation between strain, temperature and strain rate is studied and a new model is proposed. The investigation is performed by experiment and numerical simulation. The simulations are used to predict the distribution of the strain and strain rate within the specimen. The high rate experiments are conducted using Taylor test, compressive and tensile Hopkinson bar. Quasi-static tests are also carried out using Instron testing machine. The results show a quadratic relation between the hardness and strain rate. The results also indicate that the relation between hardness, strain and temperature is the same for the compressive and tensile loading but the relation between hardness and strain rate is different for the compression and tension. The hardness increases with the increase of the strain and strain rate but decreases with the temperature. The reason is believed to be due to dislocation pile up which is denser in compression than that in tension.

Strain localization during tensile Hopkinson bar testing of commercially pure titanium and Ti6Al4V titanium alloy

The goal of the analysis was to determine the strain localization for various specimen shapes (type A and type B according to PN-EN ISO 26203-1 standard) and different loading conditions, i.e. quasi- static and dynamic. Commercially pure titanium (Grade 2) and titanium alloy Ti6Al4V (Grade 5) were selected for the tests. Tensile loadings were applied out using servo-hydraulic testing machine and tensile Hopkinson bar with pre-tension. The results were recorded using ARAMIS system cameras and fast camera Phantom V1210, respectively at quasi-static and dynamic loading conditions. Further, specimens outline was determined on the basis of video data using TEMA MOTION software. The strain distribution on the specimen surface was estimated using digital image correlation method. The larger radius present in the specimen of type B in comparison to specimen of type A, results in slight increase of the elongation for commercially pure titanium at both quasi-static and dynamic loading conditions. However this effect disappears for Ti6Al4V alloy. The increase of the elongation corresponds to the stronger necking effect. Material softening due to increase of temperature induced by plastic work was observed at dynamic loading conditions. Moreover lower elongation at fracture point was found at high strain rates for both materials.

Specimen Inertia in high strain rate tensile testing

It is well established that measurements of the dynamic mechanical response of materials are susceptible to errors due to the inertia of the specimen, which causes stresses in addition to the intrinsic material strength. A number of authors have derived equations for these stresses in compression experiments; these equations can be used as a guideline for good specimen design. However, no such equations have been presented in the literature for the equivalent effects in tensile experiments. This paper begins by considering and rationalising the equations available for compression, before producing a set of equations which can be used in design of specimens for, e.g., tensile Hopkinson bar experiments.

Dynamic Crack Tip Opening Displacement (DCTOD) as governing parameters for material fragmentation

Fragmentation in metals can be approached either by Mott statistical or by Energy-based fragmentation theory. Recently, Grady showed that the two theories can be reconciled showing that the material parameter that drives tendency to fragmentation and fragment size is the dynamic fracture toughness. Experimental data do not completely agree with these conclusions. In this paper, the dynamic crack tip opening displacement (CTOD) is proposed as fracture parameter which can account for plastic deformation occurring prior fracture. Here, an experimental procedure for determining the critical dynamic CTOD is presented. The circumferential crack bar tension (CCB(T)) was investigated for its use with tensile Hopkinson bar testing equipment. The calibration function in the dynamic range was determined via finite element analysis (FEA). The critical dynamic CTOD was measured using both high speed video recording, with digital image correlation (DIC) technique, and clip gauge at the crack mouth. The proposed procedure has been used to investigate dynamic fracture resistance of high purity copper (99.98%) and to correlate it with available fragmentation data.

Experimental Characterisation of Aluminium 6082 at Varying Temperature and Strain Rate

Abstract: This article describes the experimental methodology used in overcoming the challenges of performing tests and recording results on specimens, which are suitable for such a wide range of test conditions. Uniaxial tensile tests were conducted on aluminium alloy 6082‐T352 at varying temperatures and strain rates to validate testing techniques and to determine the effect of these parameters upon this material. The applied strain rate varied over several orders of magnitude – using a screw‐driven tensometer for quasi‐static loading (6.9 × 10−4 s−1), a hydraulic piston rig for moderate strain rate (4.0 × 101 s−1) and a tensile Hopkinson bar for high strain rate (1.5 × 103 s−1). Temperature was varied using a heat gun, and the air temperature was measured using a thermocouple in the hot air stream. Specimen temperature is determined by finite element modelling, and this correlates well with other work. Although it would have been possible to improve the design of individual tests for specific test conditions, an important objective was to conduct the entire series of tests in as consistent a manner as possible. The procedure for characterising the stress–strain behaviour for this material under these different loading conditions is also considered in some detail, as the real material behaviour deviates from simplified elasto‐plastic material models. Results presented for Al 6082 samples show a slight increase in yield stress with increasing strain rate, and a decrease in yield stress with increasing temperature.

Effect of fatigue damage on the dynamic tensile behavior of 6061-T6 aluminum alloy and AISI 4140T steel

Usually material properties are determined from damage free materials, but it is not well known how these properties vary with respect to previous fatigue damage. In the present work the dynamic response of fatigue damaged 6061-T6 aluminum alloy and AISI 4140T steel specimens subjected to impact loading was investigated. Samples subjected to previous damage under high cycle fatigue and low cycle fatigue were tested. Different fatigue damage levels were considered. In addition, the effect of previous fatigue damage on the quasi-static behavior, ductility and fracture mechanism was also evaluated for both materials. A tensile Hopkinson bar apparatus was used in this work to investigate the dynamic response of the pre-fatigued specimens. Projectile speeds ranged from 18 m/s to 30 m/s giving strains rates from 550 to 2850 s −1. The quasi-static mechanical properties of aluminum are not affected by the way the fatigue damage is induced. The dynamic properties, however, are sensitive to the previous fatigue damage, but are not affected by the strain rate. In the steel case, and when damage is induced by strain control, the mechanical properties are influenced by the previous fatigue damage. The dynamic properties are sensitive to the previous fatigue damage and depend on the strain rates. The analysis results show an increase in the ductility of the aluminum alloy when increasing the fatigue damage level; the steel exhibits an opposite behavior, a decrease in the ductility when increasing the damage level. The results show how the previous fatigue damage can modify the quasi-static and dynamic mechanical properties of the tested materials.

Numerical analysis of interruption process of dynamic tensile tests using a Hopkinson bar

In this study, it was modified a tensile Hopkinson bar with which it can be carried out tests up to a predetermined strain. During dynamic test using a Hopkinson bar, the final strain reached by the specimen was controlled by a stopping elongation system; this system include a snap ring to attenuate the effects of the compressive waves produced as result of the stopping system. To establish the characteristics of stopping system and to determine the suitable material for snap ring, simulation models had been performed using finite element method. Different mateials were considered for the snap ring, such as lead and polypropylene. The incident, reflected and transmitted waves obtained on the numerical simulation of the trapped dynamic test were compared with the unmodified one.

A technique for rapid two-stage dynamic tensile loading of polymers

A method for rapid two-stage dynamic-dynamic tensile loading of polymers, based on a tensile Hopkinson bar apparatus, is established. In this technique, the initial incident wave and its reflection are used to load a specimen in quick succession. Consequently, the specimen is stressed, momentarily unloaded, then reloaded until fracture. By adopting appropriate assumptions, a procedure to obtain the associated stress-strain curves for such double-stage loading is formulated. These assumptions are examined experimentally and analytically to substantiate their validity. To verify the proposed approach, a relatively rate-insensitive material, LEXAN 141 polycarbonate, was subjected to two-stage dynamic tension. The stress-strain curves obtained via the procedure established were compared with results from static loading. Favorable correlation between the two indicates that the proposed technique can be applied to the study of load history effects on the dynamic behavior of polymeric materials.

An evaluation of constitutive relations for high-rate material behaviour using the tensile Hopkinson-bar

Tensile tests have been performed on specimens of Remco iron at strain rates from quasi-static to ∼2500/s and high-speed photography has been used to monitor the deformation of the specimen in tests at the highest rates. The DYNA 2D finite element code and the Johnson-Cook and Zerilli-Armstrong constitutive relations have been used to model the specimen behaviour in the impact test and the numerical predictions are compared with experimental measurements