Split Hopkinson Torsional Bar Research Articles

Gradient hierarchical grain structures of Al0.1CoCrFeNi high-entropy alloys through dynamic torsion

Al0.1CoCrFeNi high-entropy alloys are firstly processed by cyclic dynamic torsion (DT) using Split-Hopkinson Torsional Bar. Gradient hierarchical microstructures produced by DT are investigated using electron backscattered diffraction, transmission electron microscopy, and nanoindentation tests. Based on the gradient torsional strain, significant grain refinement, remarkable work hardening, and hierarchical deformation substructures are introduced into the current DT-HEAs.

Evolution of texture and microstructure during high strain rate torsion of aluminium zinc magnesium copper alloy

Deformation behaviour of aluminium-zinc-magnesium-copper alloy AA 7075 was investigated at high strain rate using Split Hopkinson Torsional Bar (SHTB). Cylindrical AA 7075 specimens were deformed at different angles of twist, giving rise to strain rate ranging from 700s−1 to 2000s−1. Comprehensive microstructural and textural investigation was carried out using electron back scatter diffraction (EBSD), X-ray line profile analysis and bulk texture measurement using X-rays for all the deformed samples. The high strain rate torsion tested samples showed a characteristic torsion texture for face centre cubic materials with a dominant A {11̅1̅}<110>/{1̅11}<1̅1̅0> component and the overall texture weakened with increase in strain rate and strain. Intragranular misorientation parameters like local average misorientation and grain orientation spread indicated occurrence of dynamic recovery in the sample deformed at the highest strain rate which was characterized by the maximum fraction of low angle grain boundaries. Microstructural parameters determined from X-ray diffraction using modified Williamson-Hall and Variance method further corroborated these results indicating the onset of dynamic recovery at the highest strain rate.

A modified split Hopkinson torsional bar system for correlated study of τ – γ relations, shear localization and microstructural evolution

The conventional split Hopkinson torsional bar (SHTB) system consists of two bars, which can successfully produce the data for the construction of dynamic torsional shear stress and strain relationships. However, the system cannot provide reliable information on the progression of the deformed micro-structure during the test. The reverberation of waves in the bars and the tested specimen can spoil the microstructural pattern formed during the effective loading. This paper briefly reviews a modified version of the SHTB system consisting of four bars that has been developed. This modified system can eliminate the reverberation of waves in the specimen and provide only a single rectangular torsional stress pulse, thus it can properly freeze the microstructure formed during the effective period of loading in the specimen. By using the advantage of the modified SHTB system, together with a new design of specimen, it is possible to perform a correlated study of the dynamic stress-strain response, shear localization and the evolution of the microstructure at a fixed view-field (position) on a given specimen during the sequence of the loading time. The principles, experimental set-up and procedure, calibration and some preliminary results of the correlated study are reported in this paper.

Dynamic Shear Characteristic and Fracture Feature of Inconel 690 Alloy under Different High Strain Rates and Temperatures

The high strain shear rate behaviour of Inconel 690 alloy was investigated by using the split Hopkinson torsional bar. The shear strain rates were tested at 900 s−1, 1900 s−1, and 2600 s−1and at temperatures of −100°C, 25°C, and 300°C, respectively. It was found that the dynamic shear behaviour of Inconel 690 alloy was sensitive to strain rate and temperature. The fracture shear strain increased with increasing strain rate and temperature. In addition, the strain rate sensitivity was increased with increasing strain and strain rate but decreased with increasing temperature. Finally, the fracture surfaces were found to contain dimple-like features, and the dimple density increased with increasing strain rate and temperature.

Dynamic Shear Properties of Alloy 718 over Wide Temperature Range

In this study, a split Hopkinson torsional bar was utilised to study the dynamic shear deformation behaviour of Alloy 718 at shear strain rates of 1000, 1500 and 3200s ¹1 and temperatures of ¹150, 25 and 300°C. It was found that the mechanical behaviour of Alloy 718 is sensitive to both the strain rate and the temperature. The yield shear stress and work hardening coefficient increased with increasing strain rate, but decreased with increasing temperature. In addition, the fracture shear strain increased with increasing strain rate and temperature. The strain rate sensitivity and temperature sensitivity increased with increasing strain and strain rate, but decreased with increasing temperature. It was found that the high strain rate plastic deformation of Alloy 718 alloy can be adequately described using the Kobayashi and Dodd constitutive equation. Finally, the fracture surfaces were found to contain multiple dimple-like features. The dimple density increased with increasing strain rate and temperature. Thus, it was inferred that the ductility of Alloy 718 increases at higher strain rates and temperatures. [doi:10.2320/matertrans.M2012073]

Effects of Strain Rate and Temperature on Shear Properties and Fracture Characteristics of 316L Stainless Steel

The dynamic shear deformation behaviour and fracture characteristics of 316L stainless steel are investigated using a split-Hopkinson torsional bar system at temperatures of ¹150, 25 and 300°C and strain rates ranging from 1000 to 3000s ¹1 . The results show that the flow stress, shear fracture strain, work hardening rate, and strain rate sensitivity all increase with increasing strain rate for a given temperature, but decrease with increasing temperature given a constant strain rate. The activation energy decreases with increasing shear stress for a constant shear strain, but increases with increasing shear strain given a constant shear stress. Optical microscopy observations reveal that localized plastic flows occur in the shear deformation region. Moreover, the flow angle increases with increasing strain rate and temperature. Scanning electron microscopy observations show that the fracture surfaces are characterized by a dimple-like structure, which indicates a ductile failure mode. The morphology and density of the dimple-like structures are highly sensitive to the strain rate and temperature conditions. Overall, the microstructural observations show that the shear response of 316L stainless steel is directly related to the effects of the strain rate and temperature on the evolution of the sheared microstructure. [doi:10.2320/matertrans.M2011274]

Effect of Specimen Dimensions in Dynamic Torsion Testing

The effect of specimen’s mean radius to average wall-thickness ratio rs/t, has been studied using the Split Hopkinson Torsional Bar. The rs/t ratio was varied between 6.5 and 39 keeping the outer diameter and the gauge length of the specimen constant. Higher strain rates and strains are observed for specimens with larger specimen radius to wall-thickness ratio rs/t. However no monotonic trend of the shear stress with rs/t variation is found. A sharp decrease in the torsional stiffness of the specimen is observed with increasing the specimen’s rs/t ratio. The rs/t ratio can also be used as a useful dimension to enhance the strain rate in tubular specimens.

Dynamic deformation and failure behaviour of Al-Sc alloy under shear loading

The dynamic shear deformation behaviour and fracture characteristics of Al-Sc alloy are studied using a split-Hopkinson torsional bar at shear strain rates of 800 s -1 , 1800 s -1 and 2800 s -1 and temperatures of -150°C, 25°C and 300°C. The experimental results indicate that the shear strain, shear strain rate and temperature all have a significant influence on the mechanical properties of the alloy. The fracture surfaces of the gauge length section of the deformed specimen are twisted into band-like features as a result of intensive localized shear deformation. The width and microhardness of the shear band changes with strain rate and temperature as a result of different work hardening effects. Specimen fracture initiates at the Al 3 Sc precipitate/matrix interface. The presence of precipitates accelerates the dynamic shear fracture formation via microcracking.

Strain rate and temperature dependence of dynamic shear properties of Al–Sc alloy

The dynamic shear deformation behaviour and fracture characteristics of Al–Sc alloy are studied using a split Hopkinson torsional bar at shear strain rates of 800, 1800 and 2800 s−1 and at temperatures of −150, 25 and 300°C. The experimental results indicate that the shear strain, shear strain rate and temperature all have a significant influence on the mechanical properties of the alloy. At constant temperature, the flow shear stress, fracture shear strain, work hardening rate, yield shear strength, work hardening coefficient, strain rate sensitivity and temperature sensitivity increase with increasing strain rate, while the activation energy decreases. Meanwhile, under a constant strain rate, the flow shear stress, work hardening rate, yielding shear strength, work hardening coefficient, strain rate sensitivity and temperature sensitivity decrease with increasing temperature while the fracture shear strain and activation energy increase. The Kobayashi and Dodd constitutive equation accurately describes the high strain rate shear plastic behaviour of Al–Sc alloy under the current test conditions. The fracture surfaces are characterised by transgranular dimpled features, which are indicative of ductile fracture. The appearance and density of these dimples are significantly dependent on the strain rate and temperature. The fracture surfaces of the gauge length section of the deformed specimen are twisted into band like features as a result of intensive localised shear deformation. This provides clear evidence of 'deformed' type shear bands. The width and microhardness of the shear band change with strain rate and temperature as a result of different work hardening effects. Specimen fracture initiates at the Al3Sc precipitate/matrix interface. The presence of precipitates accelerates the dynamic shear fracture formation through microcracking.

Experimental Investigation of the Shear Strength of a Unidirectional Carbon/Aluminum Composite under Dynamic Torsional Loading

In this paper, the dynamic shear strength of a unidirectional C/A356·0 composite and A356·0 alloy, respectively, are measured with a split Hopkinson torsional bar (SHTB) technique. The results indicate that the carbon fibers make very little contribution to the enhancement of the shear strength of the matrix material. The microscopic inspections on the fracture surface of the composite show a multi-scale zigzag feature. This implies that there is a complicated shear failure mechanism in the unidirectional carbon/aluminum composite.

Direct Evaluation of Strain and Strain Distribution Using Grid Method during Shear Deformation of Metal Matrix Composites.

A grid method is applied to measure the specimen's strain during dynamic deformation in the split-Hopkinson torsional bar(SHTB) experiments. A streak camera is used to record the motion of an inclined grid pattern(5 lines/mm)previously formed on the outer surface of the specimen. The average shear strain of the metal matrix composites(MMC) is successfully obtained in a large strain range by analyzing frequency of the streak image. The nominal shear strain evaluated by the conventional SHTB method agrees well with the directly measured strain for these materials. Microscopic strain distribution in MMC is also measured by observing a diffraction grating(600 lines/mm)on the specimen's surface in quasi-static testings. Localization of the shear strain becomes strong with increasing volume fraction of the reinforcement, and would reduce ductility of the composite materials.

Material Response and Failure Mechanism of Unidirectional Metal Matrix Composites under Impulsive Shear Loading

The material response and failure mechanism of unidirectional metal matrix composite under impulsive shear loading are investigated in this paper. Both experimental and analytical studies were performed. The shear strength of unidirectional C-f/A356.0 composite and A356.0 aluminum alloy at high strain rate were measured with a modified split Hopkinson torsional bar technique. The results indicated that the carbon fibers did not improve the shear strength of aluminum matrix if the fiber orientation aligned with the shear loading axis. The microscopic inspection of the fractured surface showed a multi-scale zigzag feature which implied a complicated shear failure mechanism in the composite. In addition to testing, the micromechanical stress field in the composite was analyzed by the generalized Eshelby equivalent method (GEEM). The influence of cracking in matrix on the micromechanical stress field was investigated as well. The results showed that the stress distribution in the composite is quite nonhomogeneous and very high shear stress concentrations are found in some regions in the matrix. The high shear stress concentration in the matrix induces tensile cracking at 45 degrees to the shear direction. This in turn aggravates the stress concentration at the fiber/matrix interface and finally leads to a catastrophic failure in the composite. From the correlation between the analysis and experimental results, the shear failure mechanism of unidirectional C-f/A356.0 composite can be elucidated qualitatively.

Elimination of loading reverberation in the split Hopkinson torsional bar

The loading reverberation is a multiple wave effect on the specimen in the split Hopkinson torsional bar (SHTB). Its existence intensively destroys the microstructure pattern in the tested material, and therefore, interferes with the study correlating the deformed microstructure to the macroscopic stress–strain response. This paper discusses the problem of the loading reverberation and its effects on the post-mortem observations in the SHTB experiment. The cause of the loading reverberation is illustrated by a stress wave analysis. The modification of the standard SHTB is introduced, which involves attaching two unloading bars at the two ends of the original main bar system and adopting a new loading head and a couple of specially designed clutches. The clutches are placed between the main bar system and the unloading bars in order to lead the secondary loading wave out of the main bar system and to cut off the connection in a timely manner. The loading head of the standard torsional bar was redesigned by using a tube-type loading device associated with a ratchet system to ensure the exclusion of the reflected wave. Thus, the secondary loading waves were wholly trapped in the two unloading bars. The wave recording results and the contrasting experiments for examining the post-mortem microstructure during shear banding both before and after the modification highly support the effectiveness of the modified version. The modified SHTB realizes a single wave pulse loading process and will become a useful tool for investigating the relation between the deformed microstructure and the macroscopic stress–strain response. It will play an important role especially in the study of the evolution of the microstructure during the shear banding process.

A modified split Hopkinson torsional bar in studying shear localization

A modified split Hopkinson torsional bar (SHTB) is introduced to eliminate the effect of the loading reverberation of the standard SHTB on the study of evolution of shear localization. The effect, the cause and the method by which to eliminate loading wave reverberation are carefully analysed and discussed. By means of the modified apparatus, the post-mortem observation of tested specimens can provide data on actual evolution of micro-structure and micro-damage during shear localization. Some test results of shear banding conducted with this apparatus support the use of the modified design. Moreover, the modification makes possible the correlation of evolving micro-structures to the transient shear stress-strain recording.

Characteristics and microstructure in the evolution of shear localization in Ti-6A1-4V alloy

A new interrupting method was proposed and the split Hopkinson torsional bar (SHTB) was modified in order to eliminate the effect of loading reverberation on post-mortem observations. This makes the comparative study of macro- and microscopic observations on tested materials and relevant transient measurement of τ  γ curve possible. The experimental results of the evolution of shear localization in Ti-6A1-4V alloy studied with the modified SHTB are reported in the paper. The collapse of shear stress seems to be closely related to the appearance of a certain critical coalescence of microcracks. The voids may form within the localized shear zone at a quite early stage. Finally, void coalescence results in elongated cavities and their extension leads to fracture along the shear band.

On post-instability processes in adiabatic shear in hot rolled steel

Experiments carried out using a split Hopkinson torsional bar have show that only one shear band develops in specimens of hot rolled steel which break during testing. We observed, however, that in specimens which were not deformed to failure, several fine shear bands appeared. We believe that these formed during the loading cycle before the appearance of the shear band and were not due to the effect of unloading. So we developed a numerical model to study the evolution of shear banding from several finite amplitude disturbances (FADs) in both temperature and strain rate. This numerical model reveals the detailed processes by which the FADs evolve into a fully developed shear band and suggests that beyond instability, the so-called shear banding process consists of two stages: inhomogeneous shearing and true shear-banding. The latter is characterized by the collapse of the stress and an abrupt increase of the local shear strain rate.