The ballistic properties of the aluminium alloy AA6070 in different tempers are studied, using target plates of 20 mm thickness in tempers O (annealed), T4 (naturally aged), T6 (peak strength) and T7 (overaged). The stressstrain behaviour of the different tempers was characterised by quasi-static tension tests and was found to vary considerably with temper in regards to strength, strain hardening and ductility. Ballistic impact tests using 7.62 mm APM2 bullets were then carried out, and the ballistic limit velocities were obtained for all tempers. In the material tests it was shown that the O-temper was most ductile and almost no fragmentation took place during the ballistic impact tests. The T6-temper proved to be least ductile, and fragmentation was commonly seen. The experiments show that despite fragmentation, strength is a more important feature than ductility in ballistic impact for this alloy, at least for the given projectile and within the velocity range investigated. A thermoelasticthermoviscoplastic constitutive relation and a ductile fracture criterion were determined for each temper, and finite element analyses were performed using the IMPETUS Afea Solver with fully integrated 3 rd -order 64-node hexahedrons. The numerical simulations predicted the same variation in ballistic limit velocity with respect to temper condition as found in the experiments, but the results were consistently to the conservative side. In addition, analytical calculations using the cylindrical cavity expansion theory (CCET) were carried out. The ballistic limit velocities resulting from these calculations were found to be in good agreement with the experimental data.
In the present work, we investigate the effects of strain rate (ė = 0.01 s −1 , 0.1 s −1 , and 1.0 s −1 ) and low temperature (T = −30 • C, −15 • C, 0 • C, and 25 • C) on the mechanical behaviour in tension and compression of two materials: a rubber-modified polypropylene copolymer (PP) and a cross-linked low-density polyethylene (XLPE). Local stress-strain data for large deformations are obtained using digital image correlation (DIC) in the uniaxial tension tests and point tracking in the compression tests. Since both materials exhibit slight transverse anisotropy, two digital cameras are used to capture the strains on two perpendicular surfaces. Self-heating resulting from the elevated strain rates is monitored using an infrared (IR) camera.To enable the application of multiple digital cameras and an IR camera, a purpose-built transparent polycarbonate temperature chamber is used to create a cold environment for the tests. The mechanical behaviour of both materials, including the true stress-strain response and the volume change, is shown to be dependent on the temperature and strain rate. The dependence of the yield stress on the temperature and strain rate follows the Ree-Eyring flow theory for both materials, whereas Young's modulus increases with decreasing temperature for PP and XLPE and with increasing strain rate for XLPE. Furthermore, a scanning electron microscope (SEM) study was performed on both materials to get a qualitative understanding of the volumetric strains.
In this study, a thermo-elasto-viscoplastic model is developed for a low density cross-linked polyethylene (XLPE) in an attempt to describe the combined effects of temperature and strain rate on the stress-strain response and the self-heating of the material at elevated strain rates. The proposed model consists of two parts. On the one side, Part A models the thermo-elastic and thermo-viscoplastic response, and incorporates an elastic Hencky spring in series with two Ree-Eyring dashpots. The two Ree-Eyring dashpots represent the effects of the main α relaxation and the secondary β relaxation processes on the plastic flow. Part B, on the other side, consists of an eight chain spring capturing the entropic strain hardening due to alignment of the polymer chains during deformation. The constitutive model was implemented in a nonlinear finite element (FE) code using a semi-implicit stress update algorithm combined with sub-stepping and a numerical scheme to calculate the consistent tangent operator. After calibration to available experimental data, FE simulations with the constitutive model are shown to successfully describe the stress-strain curves, the volumetric strain, the local strain rate and the self-heating observed in the tensile tests. In addition, the FE simulations adequately predict the global response of the tensile tests, such as the force-displacement curves and the deformed shape of the tensile specimen.
In this study, we present a method to determine the large-strain tensile behaviour of polymers at low temperatures using a purpose-built temperature chamber made of polycarbonate (PC). This chamber allows for several cameras during testing. In our case, two digital cameras were utilized to monitor the two perpendicular surfaces of the test sample. Subsequently, the pictures were analysed with digital image correlation (DIC) software to determine the strain field on the surface of the specimen. In addition, a thermal camera was used to monitor self-heating during loading. It is demonstrated that the PC chamber does not influence the stress-strain curve as determined by DIC. Applying this set-up, a semi-crystalline cross-linked low-density polyethylene (XLPE) under quasi-static tensile loading has been successfully analysed using DIC at four different temperatures (25 • C, 0 • C, −15 • C, −30 • C). At the lower temperatures, the conventional method of applying a spray-paint speckle failed due to embrittlement and cracking of the spray-paint speckle when the tensile specimen deformed. An alternative method was developed utilising white grease with a black powder added as contrast. The results show a strong increase in both the Young's modulus and the flow stress for decreasing temperatures within the experimental range. We also observe that although the XLPE material is practically incompressible at room temperature, the volumetric strains reach a value of about 0.1 at the lower temperatures.
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