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As only a few studies have been reported so far, mechanical properties of stress fibers (SF) have poorly been understood, mainly due to difficulties with experimental technique. A previous study in our laboratory has described development of a micro-tensile tester to measure tensile properties of single SF chemically isolated from vascular smooth muscle cells (VSMC). Tensile tests were performed at a single strain rate of 0.02 s^-1. In this study, the strain rate dependency of tensile properties of SF was investigated. The results showed that the initial stiffness of SF at strain rate of 0.01 s^-1, 0.05 s^-1 and 0.1 s^-1 were 0.010 ± 0.004 N/m, 0.006 ± 0.006 N/m, 0.028 ± 0.011 N/m, respectively. Averaged force-strain relationships were found to be almost linear in the physiological strain range of 0.0-0.4 for 0.01 s^-1 and 0.05 s^-1. These linear relationships were consistent with a previous report at a strain rate of 0.02 s^-1. In contrast, the force-strain relationship was nonlinear for the 0.1 s^-1 strain rate. The reason for this difference is unclear, possibly due to a threshold in strain rate between 0.01 s^-1 and 0.1 s^-1. Mechanical properties of SF may be strain rate dependent, particularly having a threshold in the linearity of the force-strain relationships.

A nonlinear rate dependent constitutive model for fiber-reinforced composites is presented. Through the use of a plastic potential function, effective stress and effective plastic strains are introduced, with which a single equation is established for the orthotropic and nonlinear rate dependent behavior of composites. This viscoplasticity constitutive model is employed together with a microbuckling model to predict compressive strength of composites for different strain rates. Results of qasi-static and high strain rate compression tests using off-axis composite specimens of S2/8552 glass/epoxy are reported and compared with model predictions. Excellent agreement between experimental data and predictions is found. From the experimental results as well as model predictions, it is noted that the longitudinal compressive strength of a composite is significantly influenced by the presence of shear stresses.

Dynamic compressive mechanical behaviors of an S-2/SC15 glass/epoxy composite along two perpendicular directions have been determined with a pulse-shaped split Hopkinson pressure bar (SHPB). The composite specimen deforms at a nearly constant strain rate under dynamically equilibrated stress during SHPB tests. The compressive stress-strain responses along both directions were found to be strain-rate sensitive, but with different rate sensitivities.

A dynamic tensile experiment was performed at various strain rates from 10^-4 to 10³ s^-1 on a dual phase steel, and the strain rate sensitivity on their tensile properties was studied. Results showed that the dual phase steel exhibited an obvious strain rate effect on strength. There were two stages in the effect of strain rate on strength. In the first stage, there was small sensitivity of strength to strain rate. In the second stage, there was obvious strain rate sensitivity, and the strength increased rapidly with the increase in strain rate. The strain rate sensitivity of the dual phase steel is considered to be originated from thermal activated mechanism. There is enriched thermal activated region in the first stage, and a lack thermal activated region in the second stage.

The response characteristics of high strength steel for automotive under high strain rates were comprehensively reviewed, the relationship of vehicle lightweight with improvement of vehicle safety and the application of high strength steel were discussed, and the demand of response characteristics of materials for crash safety simulation was proposed. The developments of highspeed tensile testing equipment, test methods, data processing and constitutive equations were further discussed, and finally presented the test results, response characteristics, the related constitutive equation and typical application of various types of automotive high strength steel under high strain rates. Perfection of tests for response characteristics of high-strength steel under high strain rates and deep understanding of its deformation mechanisms will contribute to the development and applications of high-strength steel.

This paper is to understand and model the thermomechanical response of the rotary forged WHA, uniaxial compression and tension tests are performed on cylindrical samples, using a material testing machines and the split Hopkinson bar technique. True strains exceeding 40% are achieved in these tests over the range of strain rates from 0.001/s to about 7,000/s, and at initial temperatures from 77K to 1,073K. The results show: 1) the WHA displays a pronounced changing orientation due to mechanical processing, that is, the material is inhomogeneous along the section; 2) the dynamic strain aging occurs at temperatures over 700K and in a strain rate of 10^-3 1/s; 3) failure strains decrease with increasing strain rate under uniaxial tension, it is about 1.2% at a strain rate of 1,000 1/s; and 4) flow stress of WHA strongly depends on temperatures and strain rates. Finally, based on the mechanism of dislocation motion, the parameters of a physically-based model are estimated by the experimental results. A good agreement between the modeling prediction and experiments was obtained.

In order to research the strain rate effect on reinforced concrete members during earthquake loading, based on Timoshenko beam theory and fiber model theory, considering strain rate effect of materials, a new beam element of reinforced concrete member is presented. Dynamic properties of reinforced concrete members subjected to monotonic loading at different loading rates that might be experienced during earthquakes are simulated by the nonlinear finite element program. The influences of loading rate on loading capability and stiffness of reinforced concrete member and the distribution of strain rate at high loading rate are investigated. The loading capability increases with increasing loading rate, the stiffness is not significant influenced by loading rate, the distribution of strain rate changes with time. The simulation results agree well with the test results.

The response for the reinforced concrete frame columns under quasi-static load and dynamic load with high strain rate are analyzed by finite element software ABAQUS. The influence of loading rate on loading capability, failure modes of reinforced concrete column are investigated, and the simulation results are compared with the test results. The results of high strain rate are compared with those of quasi-static to study the effects of strain rate on dynamic behavior of reinforced concrete columns. Results show that the dynamic behavior of reinforced concrete columns changed greatly when the strain rate effect were taken into account, and the effect of strain rate were different for different axial compression ratio of reinforced concrete columns. When reinforced concrete columns subjected to dynamic load, there was an increase in load carrying capacity. Therefore, the strain rate effect should be considered properly in the seismic analysis of reinforced concrete structures.

To research the uniaxial tension characteristics of the four-component HTPB propellant, we tested the propellant specimens by the stretch rates of 2 mm/min, 25 mm/min, 50mm/min, 100 mm/min, 500 mm/min with a Instron Stretch Machine, and then simulated the stress-strain curves by assuming a hyperbolic equation. The results show that the mechanical property of the four-component HTPB propellant is associated with the stretch rate, which is the larger value is the stretch rate, the greater is the modulus of the propellant. The stress-strain curves show obviously modulus characteristic of hyperbolic types, and the failure strains are closed to the maximum strains with no common “yield” process. The simulated curves obtained by the assumed hyperbolic equations coincide well with the test results, which verifies that the uniaxial tension curves are accord with hyperbolic type modulus.