Strain rate

About: Strain rate is a research topic. Over the lifetime, 29156 publications have been published within this topic receiving 667347 citations.

Supra-Ductile and High-Strength Manganese-TRIP/TWIP Steels for High Energy Absorption Purposes

Abstract: The microstructural properties of advanced high strength and supra-ductile TRIP and TWIP steels with high-manganese concentrations (15 to 25 mass%) and additions of aluminum and silicon (2 to 4mass%) were investigated as a function of temperature (−196 to 400°C) and strain rate (10−4≤e≤103 s−1). Multiple martensitic γfcc (austenie)→ehcpMs (hcp-martensite)→αbccMs (bcc-martensite)-transformations occurred in the TRIP steel when deformed at higher strain rates and ambient temperatures. This mechanism leads to a pronounced strain hardening and high tensile strength (>1 000 MPa) with improved elongations to failure of >50%. The austenitic TWIP steel reveals extensive twin formation when deformed below 150°C at low and high strain rates. Under these conditions extremely high tensile ductility (>80%) and energy absorption is achieved and no brittle fracture transition temperature occurs. The governing microstructural parameter is the stacking fault energy Γfcc of the fcc austenite and the phase stability determined by the Gibbs free energy ΔGγ→e. These factors are strongly influenced by the manganese content and additions of aluminum and silicon.The stacking fault energy Γfcc and the Gibbs free energy G were calculated using the regular solution model. The results show that aluminum increases Γfcc and suppresses the γfcc→ehcpMs transformation, whereas silicon sustains the γfcc→ehcpMs transformation and decreases the stacking fault energy. At the critical value of Γfcc≈25 mJ/mol and for ΔGγ→e>0, the twinning mechanism is favored. At lower stacking fault energy of (Γfcc 0, martensitic phase transformation will be the governing deformation mechanism.The excellent ductility and the enhanced impact properties enable complex deep drawing or stretch forming operations of sheets and the fabrication of crash absorbing frame structures.

Constitutive analysis in hot working

Abstract: Constitutive equations including an Arrhenius term have been commonly applied to steels with the objective of calculating hot rolling and forging forces. The function relating stress and strain rate is generally the hyperbolic-sine since the power and exponential laws lose linearity at high and low stresses, respectively. In austenitic steels, the equations have been used primarily for the peak stress (strain) associated with dynamic recrystallization (DRX) but also for the critical and steady state stresses (strains) for nucleation and first wave completion of DRX. Since the peak strain is raised by the presence of solutes and fine particles, the stress is raised more than by simple strain hardening increase, thus causing a marked rise in activation energy in alloy steels. In contrast, large carbides, inclusions or segregates, if hard, may lower the peak strain as a result of particle stimulated nucleation. Due to the linear relation between stress and strain at the peak, flow curves can be calculated from the constitutive data with only one additional constant. Maximum pass stresses can also be calculated from a sinh constitutive equation determined in multistage torsion simulations of rolling schedules. Comparison is made between carbon, micro-alloyed, tool and stainless steels and to ferritic steels which usually do not exhibit DRX. Parallels to the effects of impurities and dispersoids on the constitutive equations for Al alloys are briefly discussed.

Real-time strain rate imaging of the left ventricle by ultrasound

Abstract: The regional function of the left ventricle can be visualized in real-time using the new strain rate imaging method. Deformation or strain of a tissue segment occurs over time during the cardiac cycle. The rate of this deformation, the strain rate, is equivalent to the velocity gradient, and can be estimated using the tissue Doppler technique. We present the strain rate as color-coded 2-dimensional cine-loops and color M-modes showing the strain rate component along the ultrasound beam axis. We tested the method in 6 healthy subjects and 6 patients with myocardial infarction. In the healthy hearts, a spatially homogeneous distribution of the strain rate was found. In the infarcted hearts, all the infarcted areas in this study showed up as hypokinetic or akinetic, demonstrating that this method may be used for imaging of regional dysfunction. Shortcomings of the method are discussed, as are some possible future applications of the method. (J Am Soc Echocardiogr 1998;11:1013-9.)