Abstract: Effects of laminate misalignment on the thermoelastoviscoplastic properties of ultrafine plate-fin structures are investigated using a homogenization theory for thermoelastoviscoplasticity. For this, a homogenization theory for time-dependent materials is combined with a homogenization theory for thermoelasticity. Moreover, the substructure method is introduced into the theory to deal with the random laminate misalignment in ultrafine plate-fin structures. The present method is then applied to the analysis of thermoelastoviscoplastic behavior of ultrafine plate-fin structures made of a Ni-based alloy subjected to a macroscopic temperature increment from 20 to 200. The number of fin layers in a unit cell is five kinds, i.e. N = 10, 20, 30, 40 and 50, for each of which, twenty patterns of random laminate misalignment are considered. In addition, five cases of periodic laminate misalignment are also considered for comparison. The results reveal the effects of the laminate misalignment on the macroscopic and microscopic thermoelastoviscoplastic properties of ultrafine plate-fin structures.
Abstract: Elasto-plastic tensile deformations in pearlite lamellar and two-colony structures are studied by finite element analyses to investigate the effects of lamellar thickness ratio and difference of lamellae orientation of two colonies in pearlite microstructure. The results obtained from plastic strain distributions in lamellar and colony structures show that plastic deformation in cementite lamellar stabilized when ferrite lamellar is thicker than cementite lamellar thickness and plastic strain concentrates when the difference between cementite lamellar orientation in two colonies are larger than 45°.
Abstract: Concrete is a brittle material, especially under tension. Intensive researches have been reported to add various types of fibres into concrete mix to increase its ductility. Recently, the authors proposed a new type of steel fibre with spiral shape to reinforce concrete material. Laboratory tests on concrete cylinder specimens demonstrated that compared to other fibre types such as the hooked-end, deformed and corrugated fibres the new fibres have larger displacement capacity and provide better bonding with the concrete. This study performs drop-weight impact tests to investigate the behaviour of concrete beams reinforced by different types of steel fibres. The quasi-static compressive and split tensile tests were also conducted to obtain the static properties of plain concrete and steel fibre reinforced concrete (FRC) materials. The quasi-static tests were carried out using hydraulic testing machine and the impact tests were conducted using an instrumented drop-weight testing system. Plain concrete and concrete reinforced by the commonly used hooked-end steel fibres and the proposed spiral-shaped steel fibres were tested in this study. The volume dosage of 1% fibre was used to prepare all FRC specimens. Repeated drop-weight impacts were applied to the beam specimens until total collapse. A 15.2 kg hard steel was used as the drop-weight impactor. A drop height of 0.5 m was considered in performing the impact tests. The force-displacement relations and the energy absorption capabilities of plain concrete and FRC beams were obtained, compared and discussed. The advantage and effectiveness of the newly proposed spiral-shaped steel fibres in increasing the performance of FRC beam elements under impact loads were examined.
Abstract: Traditionally, NdFeB magnets with high remanent flux density or high energy product could only be manufactured through altering the material compounds. In recent years, studies indicated that the magnet properties of NdFeB magnets could be improved through plastic deformation. These studies pointed out that the degree of plastic deformation is a key factor to improve magnetic properties. However, there are still many other process parameters that could affect the magnetic properties either positively or negatively. In this paper, process parameters such as strain, strain rate, and temperature are studied to illustrate their influences on the magnetic properties of NdFeB magnets. The magnetic property could be greatly improved when the preferred orientation appears on the microstructure of deformed NdFeB magnets. One of the experimental results showed that the energy product value had been increased by 76.7% when the effective strain value had reached 0.65. Experimental results also showed that strain rate is a dominating factor with regard to the flow stress of material. Through a proper combination of these parameters, one can obtain NdFeB magnets with their magnetic properties greatly improved.
Abstract: In this paper, the uniaxial dynamic compressive response and rheological properties of a newly developed commercially available polymer based shear stiffening (PSS) composite is experimentally studied at different crushing velocities. The results showed that the compressive stress of PSS composites increases with the rising strain rates. Comparing the stress-strain curves of PSS composites and neoprene at the same strain rate, it was found that the compressive stress of PSS composite increased gradually with strain, while the compressive stress of neoprene increased sharply with strain. The uniaxial dynamic mechanical analyses of PSS composites showed that storage modulus of PSS composite increased with the increase of sweep frequency. The rheological study of PSS composites showed that the storage modulus of PSS composite significantly increased when the angular frequency was higher than a critical value, e.g., 100 rad/s, demonstrating evident shear stiffening properties.
Abstract: Single point diamond tools are commonly used for ultraprecision machining. At high cutting speeds, frictional contact and local heat may cause material damage to the diamond tool. The diamond crystal is softened and its mechanical strength decreases with the increase in temperature. Plastic deformation of diamonds was recently reported in some experimental studies. In this work, a molecular dynamics (MD) simulation was implemented to predict the deformation of single crystal diamond at various temperatures. Diamond is brittle at room temperature, however, it starts to exhibit plastic dislocation at a temperature above 1200 K under a confining pressure. The condition in ultraprecision machining is indeed a temperature gradient distribution at the tool tip, between the maximum temperature at the tool-workpiece interface and the average temperature at the core. The simulation results predicted that diamond deformed plastically under the gradient between 1500K and 860K. It is surprising that secondary cracks were resulted from the gradient, as comparing to a single slip obtained in an evenly distributed temperature. Bond dissociation nucleated the fractures along the (111) shuffle planes, perfect dislocation merely occurred in the hot zone and sp3-to-sp2 disorder at the cool zone. The temperature gradient created a lattice mismatch and nucleated the secondary cracks. The results give an insight that a catastrophic fracture and local material damage can occur at a diamond tool tip at the cutting temperature above 1200 K, due to softening and graphitization.
Abstract: The friction holding effect and the friction reducing effect occurring during Hydraulic Deep Drawing and the pre-bulging resulting in more plastic deformation on products are applied on sheet hydro-forming. For Hydraulic Deep Drawing of a square cup, the thickness distribution and the relation between the height and the pressure of pre-bulging are simulated with SPCC steels as the specimen by the finite element method. An experimental apparatus of sheet hydro-forming has been constructed to carry out the hydraulic deep drawing experiments of square cups. Experimental thickness distribution and punch load are compared with simulation results. Good agreement was found. The flow patterns of the circular and square blanks with the condition of being firmly pressed against the punch observed from the experiments are in agreement with the predicted results.Keywords:Hydraulic Deep Drawing, sheet hydro-forming, finite element method
Abstract: In the last few decades, energy absorption of materials becomes an critical issue in a design process of a vehicle because risks of primary and secondary accidents against pedestrians, other road users and structures can be reduced by a performance of absorbing energy in its support structures. Among various materials used for the structures, TRIP steel with favorable mechanical properties such as excellent formability and higher impact energy absorption is attractive to automotive industries. Huge numbers of research works have been carried out to investigate deformation behavior of TRIP steel. However, just few studies can be found on the performance in TRIP steel, especially, at higher deformation rate during the crash of the vehicle. Kinetic energy by higher speed of the vehicle will be consumed by inelastic bending deformation of components. Thus, a consideration of bending deformation at high impact velocity is required for the evaluation of the performance. In this study, the performance in TRIP steel at high deformation rate is clarified by conducting both quasi-static and impact three-point bending tests for pre-cracked specimen.
Abstract: This paper is concerned with the prediction of fracture strains for DP980 steel sheets using a modified Lou–Huh ductile fracture criterion. The usage of DP980 steel is significantly increasing in the automotive industry for weight reduction, enhancement of crashworthiness and safety of car body. The material behavior of AHSS show unpredictable and sudden fracture during sheet metal forming process. A modified Lou–Huh ductile fracture criterion is utilized to predict the formability of AHSS because the conventional FLD constructed based on necking is unable to evaluate the formability of AHSS. Fracture loci were extracted from 3D fracture envelopes by assuming the plane stress condition to evaluate equivalent plastic strain up to the point of fracture at a wide range of loading paths. Three different types of specimens such as pure shear, dog-bone and plane strain grooved specimens were used for tensile tests to construct 3D fracture envelopes of DP980. Fracture strain of each loading path was evaluated to show that there is little deviation between predicted fracture strains and experimentally acquired ones. From the comparison, it is concluded that the 3D fracture envelopes can accurately predict the onset of the fracture of DP980 steel sheets in complicated loading conditions including the pure shear condition.
Abstract: The tension/compression hardening behavior is important in sheet metal forming processes because of complicated loading paths. Experimental methods to measure the tension/ compression behavior have not considered the effect of the strain rate although the strain rate is related to the hardening behavior of sheet metal. The tension/compression tests need to be conducted considering the strain rate to acquire accurate hardening behavior.This paper deals with an experimental technique to measure the tension/compression behavior of sheet metal at various strain rates. A new clamping device was developed to prevent a sheet specimen from buckling under compression loading condition. Compared to previous clamping devices, the clamping device was devised to uniformly impose a clamping force and easily measure the strain from side of a specimen. Tension/compression tests have been conducted at various strain rates for SPCC and DP590 with displacement of 10%. Hardening curves under the tension or compression loading condition were obtained and analyzed with respect to the strain rate.