Papers by Keyword: High Strain Rate

Abstract: Ti-6Al-4V alloy is one of the most important engineering alloys, combining attractive properties with inherent workability. The aim of this study is to investigate the effect of strain rate on the compressive mechanical properties of Ti6Al4V alloy manufactured by a selective laser melting process. The mechanical tests were performed by means of a compression split Hopkinson pressure bar apparatus under high strain rate ranging from 1400 s^-1 to 4500 s^-1. The true stress-strain curves obtained from static and dynamic compressive tests show strain rate sensitivity from quasi-static (peak strength 1300MPa) to high strain rate (peak 1500 MPa). Within the high strain rate range tested, the strain rate sensitivity is not remarkable. The fractographic analysis shows a relatively smooth and smeared fractured surface along with a dimple like structure. The observation of elongated dimples confirms the operation of a dynamic shear failure mechanism for the additively manufactured Ti-6Al-4V parts.

Abstract: This paper is concerned with the characterization of the OFHC copper flow stress at strain rates ranging from 10⁻³ s⁻¹ to 10⁶ s⁻¹ considering the large strain and high temperature effects. Several uniaxial material tests with OFHC copper are performed at a wide range of strain rates from 10⁻³ s⁻¹ to 10³ s⁻¹ by using a INSTRON 5583, a High Speed Material Testing Machine (HSMTM), and a tension split Hopkinson pressure bar. In order to consider the thermal softening effect, tensile tests at 25°C and 200°C are performed at strain rates of 10⁻³ s⁻¹,10¹ s⁻¹, and 10² s⁻¹. A modified thermal softening model is considered for the accurate application of the thermal softening effect at high strain rates. The large strain behavior is challenged by using the swift power law model. The high strain rates behavior is fitted with the Lim–Huh model. The hardening curves are evaluated by comparing the final shape of the projectile from numerical simulation results with the Taylor impact tests.

Abstract: This paper introduces some representative measurement methods of friction coefficient for plastic deformation of metals under high strain rate both in our country and abroad in recent years, and mainly includes several measurement methods of friction coefficient based on the upsetting, forging, extrusion and tube hydroforming. Furthermore, the working principles, applicable occasions and technical characteristics of these methods are explained, and the development of these methods in the future are presented.

Abstract: In the fields of explosive forming, aerospace industry, military industry etc. metallic materials will withstand a high strain rate in the process of deformation. It is very instructive to put forward an accurate strength model which can be used to describe the process of deformation reasonably. In this paper, several strength models are introduced which are mainly based on the macroscopic mechanical behavior of materials and the mechanisms of deformation of material, and their sphere of application, shortcomings and advantages were introduced and reviewed.

Abstract: The performance of reinforced concrete structures under combined effects of blast and fire is growing in interest of the research and engineering communities specially after the recent terrorist attacks as well as severe accidents (i.e. Gotthard tunnel, etc.). The mechanical behaviour of concrete and reinforcing steel when are subjected to extreme temperatures, impacts or blast has still many aspects open to investigation. In this paper the behaviour of AISI304, B500B and B500A reinforcing steel at high strain rate (500 s^-1) and at three levels of temperature (200, 400 and 600°C) is presented. The results were obtained by using a Split Hopkinson Tensile Bar (SHTB) equipped with a heating system.

Abstract: Aluminium sheet metal is nowadays used to fabricate lighter, crashworthy, fuel efficient and environment friendly vehicles. Ductile damage of sheet metals affects significantly the crashworthiness, as it naturally exhibits anisotropic behavior due to the grain orientation. Johnson-Cook (J-C) damage model is widely used in numerical simulation for assessing the failure modeling of crash component in particular at high strain rate. The Johnson-Cook material model available in literature is meant for isotropic material behavior which cannot be used directly for anisotropic behavior of materials. To characterize the plastic anisotropy of the rolled sheet, the modified Johnson-Cook material model should be developed. In this research the combination of experimental work and numerical analysis with clear and simpler calibration strategy for damage model is demonstrated. It aims to reduce laboratory tests using advanced numerical analysis to predict failure in order to save overall cost and development time.

Abstract: The brittle to ductile failure mode transition and the formation of adiabatic shear bands (ASB) of polypropylene were observed at high strain rate impact loading. The dynamic experiments were conducted using a split Hopkinson pressure bar (SHPB) set-up with hat-shaped specimens. The post-test observations of the recovered specimens were performed by polarized light microscopy. The mechanical behaviors of specimens are strongly influenced by temperature, the strength of specimen decreases with increasing temperature. Furthermore, the specimen fractures as a brittle solid at room temperature (20°C), while at a high temperature (100°C), the specimen fractures on a ductile model. At high temperature impact tests, shear bands are observed in the shear zone of the hat-shaped specimen, and the cracks formed at the corners propagate along the shear bands.

Abstract: Deformation and fracture behaviors of Ti-6Al-4V-0.1B alloy with Widmanstätten, equiaxed and bimodal microstructures were investigated by Split Hopkinson Pressure Bar (SHPB) under high strain rates of 2100-3200 s^-1. The results showed that the equiaxed and bimodal structures had a higher bearing capacity at high strain rates than that of the Widmanstätten structure. With the same microstructure, the increase of strain rate gave rise to an improved uniform plastic deformation. According to an observation on the deformed microstructure, it was found that adiabatic shear behavior was the main reason for failure and fracture of the alloy. The formation and propagation of adiabatic shear bands (ASBs) was the precursor for the failure and fracture of the material. Cavities at the interface between TiB phase and the matrix readily formed due to the uncoordinated deformation, which are not the dominate reason for the failure and fracture.

Abstract: In the paper, results of impact bending tests of a high-manganese steel of X30MnAlSi26-4-3 grade are presented. The tests were carried out using a flywheel machine, suitable for dynamic tensile tests and impact bending tests in the range of linear velocity of the forcing element from 5 ÷ 40 m/s. The obtained test results were compared with the results of impact resistance of the studied steel determined using Charpy machine. Structural investigations were carried out using light microscope and scanning transmission electron microscopy. Creating a mechanical twins at different strain rates was analyzed. The surfaces of fractures formed in the break point during bending tests were analyzed, and they indicate a presence of mixed transcrystalline fractures with a predominance of plastic fractures. Substructure studies revealed the presence of mechanical twinning induced in a high strain rate for the X30MnAlSi26-4-3 steel.

Abstract: Friction stirring is a fundamental process in the friction stir welding (FSW), and moreover, high strain rate deformation in elevated temperature to lead to extremely high ductility and fine grain size. In the present study, friction stirring process has been successfully modelled as a high temperature deformation depending on strain rate and temperature, assuming shear deformation of material in stir zone and generation of frictional heat by rotating tool. Axial load and torque during the process were estimated based on the model, and compared with the experimental data at two kinds of combination ratio in FSW of aluminum and Al-Mg alloy. It was, consequently, confirmed that the model could evaluate flow stress and strain rate from the experimental load and torque.