Results 1 - 10 of 5075
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[en] Flow behaviors of 22MnB5 ultra-high strength steel (UHSS) at different temperatures and strain rates were investigated using uniaxial tension tests. The variables of the Arrhenius model, which involved n, Q, α, and lnA, was calculated using a fifth order polynomial fitting method. At a strain of 0.1, the values of n, Q, α, and lnA were respectively 8.24579, 177.7588 kJ·mol−1, 0.00618 MPa−1, and 25.77352 s−1. The results indicate that the change in rheological stress was inversely proportional to the change in temperature but was positively correlated with the change in strain rate. The new model reproduced the flow behaviors of this novel UHSS sheet well for a wide range of strain rates and temperature conditions. (paper)
[en] The mechanism of the critical strain of serrated yielding is studied via tension tests at various strain rates. Before the critical strain, it is deduced that dislocations are not pinned at high strain rates, and dislocations at low strain rates are pinned but cannot escape. The critical strain depends on the first pinning process at high strain rates and on the first unpinning process at low strain rates. The calculated results based on the two criteria are in good consistency with the experiment. (paper)
[en] Wafer warpage is becoming a serious problem when adopting wafer level package (WLP), because of the diversity of materials used in redistribution layer and the complicacy of constitutive model during deformation. Most studies focus on the organic material used in WLP, such as PI and epoxy resin, and trying to reduce the wafer warpage by numerical simulation or adjusting the manufacturing process. However, the effect of copper trace layer on wafer warpage, which is proved to be significant, is often neglected. In this paper, the microstructures of four sets of copper films electroplated using different parameters are studied, and the wafer warpage introduced by copper layer is in situ measured. Then, the mathematical relation among plastic strain rate, film stress, and temperature is established. In the end, the detailed relationship between the microstructure and warpage evolution is revealed, and methods to reduce the wafer warpage introduced by copper layer are provided.
[en] The hot compressive deformation behavior of 20CrMnTiH steel was studied using a Gleeble-3500 thermomechanical simulator at temperatures ranging from 600 °C to 650 °C at a strain rate of 5 s−1 and in a strain range of 30% to 70%. The effects of the ausforming temperature, strain rate and strain on the martensite microstructure were studied. The results show that in the absence of recrystallization, when the strain rate is large, the deformation temperature of the supercooled austenite is lowered, the deformation amount is increased, the martensite structure is refined, and the microhardness is increased. The deformation of supercooled austenite can create nanostructured lath martensite with high microhardness. (paper)
[en] The split Hopkinson pressure bar (SHPB) technique is extensively used to characterize material deformation behavior under high strain rate condition. In this study, the dynamic deformation behavior of aluminum 7075-T6 under a high strain rate and at a high temperature is investigated by using a modified SHPB set-up with the pulse shaper technique. The parameters used in the Johnson-Cook constitutive equation are determined by using the SHPB experimental results including the data on the effects of strain rate, temperature, strain hardening, and thermal softening of the material
[en] Based on dislocation theory, we investigate the mechanism of strain rate effect. Strain rate effect and dislocation motion are bridged by Orowan's relationship, and the stress dependence of dislocation velocity is considered as the dynamics relationship of dislocation motion. The mechanism of strain rate effect is then investigated qualitatively by using these two relationships although the kinematics relationship of dislocation motion is absent due to complicated styles of dislocation motion. The process of strain rate effect is interpreted and some details of strain rate effect are adequately discussed. The present analyses agree with the existing experimental results. Based on the analyses, we propose that strain rate criteria rather than stress criteria should be satisfied when a metal is fully yielded at a given strain rate. (condensed matter: structure, mechanical and thermal properties)
[en] The strain amplitude analysis of low cycle fatigue (LCF) on nodular cast iron processed with TSA aimed to observe the fatigue behavior and fatigue cracking phenomenon until the failure. The LCF test followed the ASTM A606 standard and it was performed at room temperature with strain ratio = −0.1, strain rate 2.5 × 10–3s−1, various strain amplitudes of 0.002, 0.0025, 0.00275 and 0.003 mm mm−1, and TSA holding time of 5, 10, and 15 min. LCF test results were shown in the hysteresis loops curve (s versus e) and peak stress versus number of cycle curve. LCF test showed that the 0.0025 mm mm−1 strain amplitude at 5- and 10-minutes TSA generated the highest cycles compared to 0.002, 0.00275 and 0.003 strain amplitudes. The 0.0025 strain amplitude and 5 min TSA had 10845 cycles and 10 min TSA had 7414 cycles. Therefore, 0.002, 0.00275 and 0.003 strain amplitudes had total cycles which were below the above cycles. (paper)
[en] Highlights: • Heart deformation analysis (HDA) can quantify global and regional cardiac function. • HDA works based on cine CMR images without the needs of operator interaction. • HDA-derived cardiac motion indices are reproducible. - Abstract: Objective: To test the performance of HDA in characterizing left ventricular (LV) function and regional myocardial motion patterns in the context of cardiomyopathy based on cine cardiovascular magnetic resonance (CMR). Materials and methods: Following the approval of the institutional review board (IRB), standard cine images of 45 subjects, including 15 healthy volunteers, 15 patients with hypertrophic cardiomyopathy (HCM) and 15 patients with dilated cardiomyopathy (DCM) were retrospectively analyzed using HDA. The variations of LV ejection fraction (LVEF), LV mass (LVM), and regional myocardial motion indices, including radial (Drr), circumferential (Dcc) displacement, radial (Vrr) and circumferential (Vcc) velocity, radial (Err), circumferential (Ecc) and shear (Ess) strain and radial (SRr) and circumferential (SRc) strain rate, were calculated and compared among subject groups. Inter-study reproducibility of HDA-derived myocardial motion indices were tested on 15 volunteers by using intra-class correlation coefficient (ICC) and coefficient of variation (CoV). Results: HDA identified significant differences in cardiac function and motion indices between subject groups. DCM patients had significantly lower LVEF (33.5 ± 9.65%), LVM (105.88 ± 21.93 g), peak Drr (0.29 ± 0.11 cm), Vrr-sys (2.14 ± 0.72 cm/s), Err (0.17 ± 0.08), Ecc (−0.08 ± 0.03), SRr-sys (0.91 ± 0.44s−1) and SRc-sys (−0.64 ± 0.27s−1) compared to the other two groups. HCM patients demonstrated increased LVM (171.69 ± 34.19) and lower peak Vcc-dia (0.78 ± 0.30 cm/s) than other subjects. Good inter-study reproducibility was found for all HDA-derived myocardial indices in healthy volunteers (ICC = 0.664–0.942, CoV = 15.1%–37.1%). Conclusion: Without the need for operator interaction, HDA is a reproducible method for the automated characterization of global and regional LV function in the context of cardiomyopathy.