[en] It is shown that the deviation from the ideal Hollomon relation in describing the stress-strain behaviour is characteristic of all materials at low strains. The Ludwigson relation describing the deviation from the Hollomon relation at low strains is critically analysed and it is shown that the deviation at low strains is a consequence of some unknown 'plastic strain equivalent' present in the material. Stress strain curves obeying an ideal Hollomon relation as well as that of a structurally modified (prior cold worked) material were simulated and compared. The results show that the yield strength and the flow strength of a material at constant strain rate and temperature are dictated by the magnitude of the 'plastic strain equivalent' term. It is shown that this component need not necessarily mean a prior plastic strain present in the material due to prior cold work alone and that prior cold work strain will add to this. If this component is identified, the stress-strain behaviour can be adequately described by the Swift relation. It is shown that in both formalisms, the strain hardening index is a function of the yield strength of the material

[en] The effect of dynamic strain aging on the cyclic stress and the fatigue life of ASTM A508 C1.3 steel for nuclear reactor pressure vessel was studied at temperatures ranging from room temperature to 300 deg C. Total strain range and strain rate were varied from 0.7 to 2.0% and 4 x 10^-4 to 4 x 10^-3 s^-1, respectively. In was found that the dynamic strain aging enhanced the fatigue life, and caused cyclic hardening such as primary and secondary hardening. This cyclic hardening was dependent on the strain rates. Cyclic softening occurred at room temperature and 150 deg C. In comparison with the design fatigue curves in the ASME Boiler and Pressure Vessel Code, it was concluded that the SA-508 steel possesses safety margins for the integrity. (Author)

[en] A series of uniaxial tension and relaxation tests were completed at temperatures from 20 degree C to 700 degree C. The results show that, the tensile constitutive curves of the alloy present obvious yield breaks, subsequent yield hardening in the temperature range from 20 degree C to 500 degree C and softening in the range from 500 degree C to 700 degree C. The elastic modulus, strength and percentage elongation of the alloy in the rolling direction are closed to those of the alloy in the transverse direction, but percentage reduction of area of the uniaxial specimen in the transverse direction is somewhat large. With increase of temperature, elastic modulus and strength of the alloy decrease, and percentage elongation and percentage reduction of area increase. It is seen that the percentage elongation has a sudden reduction in the temperature range from 350 degree C to 450 degree C. The elevated temperature makes properties of the alloy to have strong sensitivity of strain rate under strain loading. The sensitivity of strain rate is slightly affected by the variety of temperature below 300 degree C and the alloy behaves least sensitivity to strain rate at 350 degree C. Subsequently, the sensitivity of strain rate enhances with increase of temperature. The influence of temperature on stress relaxation effect of N18 alloy is outstanding. The influence of the variety of temperature on the maximum of stress relaxation is unobvious below 300 degree C, and with increase of strain level the maximum of stress relaxation reduces obviously. Conversely, the strain level is higher, the extent of the maximum of stress relaxation is larger in the temperature range from 350 degree C to 550 degree C. About 350 degree C, the maximum of stress relaxation reaches minimum value. In the range from 350 degree C to 450 degree C, N18 alloy presents characteristic of dynamic strain aging obviously. (authors)

[en] It has been reported that SA508 class 3 RPV steel exhibits Dynamic Strain Aging phenomenon(DSA) at the operating temperature, 288 .deg. C.Therefore, DSA effect on the low cycle fatigue behaviors must be necessarily considered for precise life prediction. In this work, the influence of temperature and strain rate on tensile and low cycle fatigue properties of two kinds of steels which was produced by the same steel making method has been investigated. Test temperatures and strain amplitudes at strain rate of 4x10^-3/s and 4x10^-4/s were varied from room temperature to 400 .deg. C and 0.6 to 1.8%, respectively. Steel with smaller grain and fine carbide size and homogeneous distribution exhibited longer fatigue life and higher mechanical properties. DSA region was considered as the temperature range which appeared inverse strain rate dependence of tensile stress and stress amplitude. Marked secondary strain hardening took place on cyclic stress response curve at DSA temperature, whereas cyclic softening remarkably occurred at different temperatures. As increase of temperature and decrease of strain rate, fatigue life decreased. But DSA resulted in the increase of fatigue life. It was believed that occurrence of secondary crack and surface multi crack improved fatigue crack growth resistance

[en] In this paper, the motion of dislocations in the field of solute atoms which also move in the lattice is analysed. Solute atoms influence the dislocation velocity and the friction stress. The proposed model enables to explain the dynamic strain aging phenomena such as the occurrence of the plateau stress, the appearance of plastic instabilities and anomalies in the stress dependence of the strain rate sensitivity of the flow stress. (author)

[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] The tensile data involving austenitic Alloy C-276 suggest that this alloy is capable of maintaining appreciable structural strength at temperatures relevant to the sulfuric acid decomposition process related to the nuclear hydrogen initiative. Reduced failure strain and formation of serrations, characteristics of dynamic strain ageing (DSA), were noted within susceptible temperature regimes. An average activation energy of 55 kJ/mol, and work hardening index ranging from 0.68 to 0.75 were determined as functions of different testing temperature and strain rates. The occurrence of DSA was also associated with enhanced dislocation density. Depending on the testing temperature, a combination of ductile and intergranular brittle failures was observed with the tested specimens

[en] Deformation behaviors of the Ti_5_0Zr_2_0Nb_1_2Cu_5Be_1_3 bulk metallic glass composite in the dendrite softening region have been investigated using uniaxial compressive and tensile tests. All the compressive stress–strain curves show work-hardening phenomenon with the same tendency, the tensile stress–strain curves show work-softening phenomenon. The strain rate sensitivity exponent ranges from 0.061 to 0.072 in the compressive tests, and 0.33 to 0.38 in the tensile tests. The activation volume of the Ti_5_0Zr_2_0Nb_1_2Cu_5Be_1_3 bulk metallic glass composite at 710 K ranges from 1.6 nm"3 to 2.5 nm"3 under compressive tests and 1.0 nm"3 to 2.7 nm"3 under tensile tests, indicating that there is a migration of atomic group (about 170–180 atoms) during the deformation. The dendrite of the Ti_5_0Zr_2_0Nb_1_2Cu_5Be_1_3 BMGC has been well spheroidized and undergoes severe plasticity deformation after the deformation in the dendrite softening region.

[en] The formation and evolution of the adiabatic shear bands in the zirconium alloy impacted by split Hopkinson pressure bar were investigated by means of the experimental design. The different strain levels in the zirconium alloy subjected to high strain rate loading were first designed by two times impact at almost same strain rate. Only deformed bands were formed in the zirconium alloy impacted once, and two types of adiabatic shear bands, namely deformed bands and transformed bands, were distinguished in the zirconium alloy impacted twice. Microstructural observation shows that the deformed bands showed the severe strain localization, and the transformed bands composed the ultrafine and equiaxed grains. The formation of the ultrafine and equiaxed grains in the transformed bands could be attributed to the rotational dynamic recrystallization mechanism. Based on the characterization of the ASBs formed at different strain stages, the evolution process of the ASBs is proposed. The deformed bands are suggested as the initial stage of the transformed bands formation, and the transformed bands are considered as the further development of the deformed bands. The microhardness measurements show that the microhardness values in the transformed bands were higher than that of the deformed bands and original sample because of strain hardening and grain refining

[en] A combined approach including experimental investigation and constitutive modelling was followed in this work to study the stress–strain behaviour of rubber-toughened glassy polymers. The large inelastic deformation response of rubber-toughened poly(methyl methacrylate) (RT-PMMA) was experimentally studied under uniaxial compression tests at different strain rates and temperatures. The studied composite system consists of spherical core–shell (PMMA hard shell and soft rubber core) particles embedded in a PMMA matrix. The influence of particle concentration (ranging from 0% to 45%) on the macroscopic behaviour was also investigated from small to large strain. The physically based hyperelastic–viscoplastic constitutive model of Boyce–Socrate–Llana was extended to describe the stress–strain behaviour of rubber-toughened glassy polymers. The model accounts for the effective contribution of the two polymeric phases to the overall composite macroscopic behaviour, by including in the original model the hyperelastic deformation of rubber particles. The capabilities of the model to describe the rate-dependent yield and post-yield behaviour of PMMA over a wide range of temperatures and strain rates are pointed out. The model is able to successfully capture the significant features of the stress–strain behaviour including the initial linear elasticity, the gradual rollover to yield, the strain softening after yield (when it exists) followed by the strain hardening. Its predictive capabilities are further tested by comparison with compression data on RT-PMMA for different rubber contents