In this present work, dynamic tests have been performed on hardened SKD11 steel (62 Rockwell C hardness) specimens by means of a high temperature split Hopkinson pressure bar (SHPB) test system. Effects of temperature as well as those of strain and strain rate for the hardened steel are taken into account by using two ellipsoidal radiant heating reflectors with two halogen lamps and magnetic valve. The result obtained at high stain rates were compared with those at low strain rates under the different temperature. It was seen that the flow stress curves are found to include a work hardening region and a work softening region and the mechanical behavior of the hardened steel is highly sensitive to both the strain rate and the temperature. To determine the true flow stress-true strain、temperature relationship, specimens are tested from room temperature to 1073K at a strain rate form 0.01 s -1 to 104 s -1 : The parameters for a Johnson-Cook constitutive equation and a modified Johnson-Cook constitutive equation are determined from the test results by fitting the data from both quasi-static compression and high temperature-dynamic compression tests. The modified Johnson-Cook constitutive equation is more suitable for expressing the dynamic behavior of the hardened SKD11 steel above the vicinity of the recrystallization temperature. Keyword: constitutive model, high temperature, high strain rate, flow stress, hardened SKD11 steel IntroductionMaterial properties generally differ between dynamic and quasi-static states. The dynamic mechanical properties of a material are known to be dependent on the strain, strain-rate, and temperature [1]. Hardened SKD11mould steel (Pre-hardened 40 Rockwell C hardness, 40 HRC) is an important class of industry engineering materials due to its excellent combination of strength, good resistance to thermal softening, high toughness and high hardness as well as high wear-resistance [2]. However, it is not widely used because the deformation behaviors of this material at temperature and high strain rate are unknown. In particular, it is regarded as difficult to machine because of its high strength, high hardness and big brittle in the high-speed machining [3][4]. Moreover, the high strength and hardness inhibits material plastic deformation producing larger cutting forces and higher temperature at the cutting edge and generating for higher cutting speed a rapid chipping at the cutting edge which leads to catastrophic failure [5]. Many investigators have studies the mechanism and machinability including tool wear, residual stresses, cutting force and vibration on high speed milling process [6][7][8][9][10]. But the mechanical behavior of materials subjected to high temperature and high strain rates is of great interest in a wide range of engineering applications as well as in fundamental studies of material behavior. In some of these applications such as high-strength and high wearable structures, characterizing and modeling the high strain rate response of the hardened steel are necessary f...
Geopolymer concrete (GC) has been gaining attention in research and engineering circles; however, it is a brittle material with poor tensile performance and crack resistance. To address these problems, we introduced fibers into GC. In this study, axial compression and scanning electron microscope (SEM) tests were carried out on polyvinyl alcohol (PVA) short fiber reinforced low-calcium fly ash-slag-based geopolymer concrete (PFRGC). The ratio of PVA short fibers and low-calcium fly ash on the compression behavior of fiber reinforced geopolymer concrete (FRGC) were investigated and discussed. The test results show that PVA fibers play a bridging role in the cracks of the specimen and bear the load together with the matrix, so the addition of PVA fibers delayed the crack propagation of GC under axial compression. However, with the increase of low-calcium fly ash/PVA fibers, the number of unreacted fly ash particles in PFRGCs increases. Too many unreacted fly ash particles make GC more prone to micro-cracks during loading, adversely affecting compressive properties. Therefore, the axial compressive strength, elastic modulus, and Poisson’s ratio of GC decrease with the increasing low-calcium fly ash/PVA fibers.
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