Key Engineering Materials Vols. 353-358

Abstract: Equal channel angular pressing (ECAP) was performed on extruded Mg-Zn-Y-Zr (Mg-5.0wt%Zn-0.9wt%Y-0.2wt%Zr) alloy at 300 oC. After 8 ECAP passes, average grain size of the alloy was reduced to about 1.4 μm, and the quasicrystalline phases were broken and dispersed in the matrix. In addition, nano- quasicrystallines were precipitated from the matrix during ECAP processing. After ECAP, the elongation to failure of the extruded material was significantly improved. Only after 2 ECAP passes, the elongation to failure was 29%, and after 8 ECAP passes, it reached 35%, which was three times larger than that of the as-extruded alloy. However, both yield strength and ultimate tensile strength were decreased with the increasing ECAP passes, which was considered to be resulted from the {0002} basal plane texture modification during ECAP.

Abstract: It is well-known that the mechanical properties of nanocomposites are better than those of conventional composites. One of problems in fabricating nanocomposites is a dispersion of nanoparticles in the composites. In this study, the effect of MMT content on the tensile characteristics of trifunctional silane-treated MMT/epoxy nanocomposite was investigated. It was found that the tensile strength and the elastic modulus increased by over 24% and 83%, respectively, as the content of MMT increases from 2wt% to 10wt%. The improvement of tensile strength and modulus with increasing content of MMT occurs because the reinforcing effect of MMT becomes greater than the deteriorating effect due to the interfacial debonding between MMT and epoxy as the MMT content increases.

Abstract: The viscous pressure bulge (VPB) testes of the aluminum Al1060 sheet were carried out with stainless steel overlapped outside without lubricant. The influence of the compressive normal stress on the failure behavior of aluminum sheet was investigated by scanning electron microscope observations. The results indicate that the forming limit of overlapped aluminum sheet is increased significantly compared with the single aluminum sheet. Fractography shows that the void growth and conjunction in the thickness direction is restrained by normal pressure and the fracture mode of overlapped sheet transits from the dimple fracture to shear fracture due to the reduction of stress triaxiality.

Abstract: The present paper investigates relationships between the macroscopic viscoplasticity and the surface morphological changes at room temperature for commercially pure titanium (CP-Ti) and austenitic stainless steel (SUS316L). Pure tensile test and tension-intermittent creep test are conducted. Both CP-Ti and SUS316L are deformed up to 16% of inelastic strain with a few unloading, and surface conditions are observed during pure tension test and tension-intermittent creep test. Qualitative surface observations and quantitative surface roughness measurements are made for the unloaded specimens. The surface roughness measurement shows that the curves plotted between surface roughness and inelastic strains are almost linear for all the present experiments. The slopes of curves depend, however, on material and type of tests, and this tendency agrees well with the qualitative surface observations by an optical microscope. The experimental results for CP-Ti suggest that different deformation mechanisms during tensile loading and creep contribute to different surface morphological changes.

Abstract: A constitutive equation was developed for an infinite body in plane wave impact problem, and implemented using the finite element software ABAQUS user subroutine. Bilinear material behavior under monotonically increasing loading was considered for the constitutive equation. The finite element governed by this type of material behavior can be used as an infinite body transmitting longitudinal and shear plane waves, so that the number of finite elements can be reduced in impact simulations. To test the developed method, results of a plate impact experiment were examined. The numerical results show the accuracy of the developed constitutive equation.

Abstract: The mechanical strength and failure behavior of two photonic crystal silica optical fibers with different diameters were investigated using tensile test. The effect of polymer coating on the failure behavior was also studied. The results indicated that all fibers failed in a brittle manner and the failure normally initiated from fiber surfaces. The failure loads observed in the coated fibers are higher than that in bare fibers and the reason is explained by the apparent delamination between the fiber and the polymer coating when loaded on the fiber surfaces. The relationship between a characteristic parameter measured on the fracture surfaces and the failure stress was examined.

Abstract: The Split Hopkinson Pressure Bar (SHPB) technique with some special experimental apparatus can be used to obtain the dynamic material behavior under high strain rate loading conditions. An experimental technique that modifies the conventional SHPB has been developed for measuring the compressive stress strain responses of materials with low mechanical impedance and low compressive strengths such as rubber. This paper uses PEEK (Poly-ether-ether-ketone plastic) bars to achieve a closer impedance match between the pressure bar and the specimen materials. In addition, a pulse shaper is utilized to lengthen the rise time of the incident pulse to ensure stress equilibrium and homogeneous deformation of the rubber specimen. It is confirmed that the modified technique is useful to record the dynamic deformation behavior of rubbers under various conditions such as high strain rate with various temperature effect. Furthermore, the dynamic deformation behaviors of heat-aged rubber material under compressive high strain rate are evaluated using the modified SHPB technique.

Abstract: In order to improve the formability of magnesium alloys at low temperatures, a thorough understanding of the twinning behaviour is required. Cylindrical samples were compressed uniaxially from room temperature to 250oC, with the compression axis parallel to the transverse direction (TD) of a hot-rolled Mg-3Al-1Zn sheet. Twinning was investigated by use of orientation maps constructed from electron backscattered diffraction (EBSD) data. Tensile twinning was observed at all temperatures. In some grains two twin variants were observed. For each twin the specific twinning variant among the 6 possible twinning systems was identified by consideration of the twinning rotation. The twinning was then investigated by comparing the specific twin variant with the calculated Schmid factors for all 6 possible twin variants. In cases of grains that are completely reoriented by two twin variants it is possible to back-calculate the initial grain orientation by considering all possible twin rotations for both of the twin variants. It was found that the operative twin variants where those with the highest Schmid factors.

Abstract: The high temperature compressive tests of squeeze casting ZK60 magnesium alloy with temperatures of 573-723K and strain rate in the range of 0.001-1s-1 were performed on Gleeble-1500D thermal simulator testing machine. Optical microscopy was performed to elaborate on the dynamic recrystallization (DRX) grain growth. TEM findings indicate that mechanical twinning, dislocation slip, and dynamic recrystallization are the materials typical deformation features. Variations of flow behavior with deformation temperature as well as strain rate were analyzed. Analysis of the flow behavior and microstructure observations indicated that flow localization was observed at lower temperature and higher strain rates, which should be avoided during mechanical processing. Dynamic recrystallization occurred at higher temperature and moderate strain rates, which improved the ductility of the material. The optimum hot working conditions for ZK60 alloy were suggested.

Abstract: Numerical simulation can be successfully applied to evaluate product manufacturability and predict, while material wrinkling and splitting are associated with entry angle and pressure-pad force during sheet metal stamping process. In this paper, the influence of entry angle and pressure-pad force on deep-drawing thermal forming of TC1 titanium alloy was analyzed by using explicit dynamic finite element software STAMPACK, two kinds of main invalidation, including wrinkling and splitting, and obtained that the entry angle is an important factor for deep-draw thermal forming. By analyzing the forming operation and optimizing the entry angle and the pressure-pad force of the die, the best range of the entry angle and the suitable pressure-pad force without wrinkling were obtained and splitting phenomena were controlled effectively. The researched results of numerical simulation provided some parameters for die design.