Results 1 - 10 of 1928
Results 1 - 10 of 1928. Search took: 0.027 seconds
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[en] The Bauschinger effect in Zircaloy-2 has been investigated at room temperature using (a) specimens cut from the L and ST directions of a thick, rolled slab, and (b) specimens machined from swaged rod. The blocks of material, from which the slab specimens were cut, received compressive prestrains of up to 3.8%, in either the L or ST direction, prior to the tensile specimens being machined from them. The slab specimens were deformed in tension using a computerized Instron, while the swaged rod specimens were deformed in tension/compression using a prototype, MTS Alpha System. A large Bauschinger effect, manifested by a reduction in the yield stress and by permanent softening, was observed. The magnitude of the reduction in the yield stress depended on the magnitude and direction of the prestrain, and on crystallographic texture, and is clearly large enough to influence design and fabrication of reactor-core components. The data from (b) have been analyzed by four different techniques to obtain estimates of the mean back stress which is responsible for the Bauschinger effect in Zircaloy-2. From a comparison of those values with a theoretical estimate based on the analysis of Brown and Clarke, it is concluded that the second phase (Ni,Cr,Fe) particles in Zircaloy-2 are not responsible for the effects observed. It is suggested that the Bauschinger effect is a direct consequence of crystallographic texture and the limited number of deformation modes which can occur in hcp zirconium alloys. (orig.)
[en] Branch pipes (segments of fuel claddings of a full cross section) and ring specimens of two sizes were subjected to tensile tests. The experimental results are used to plot hardening curves, which are true stress–strain curves. The set of these curves is employed to plot the desired flow surface of EK-181 steel.
[en] In recent times hardness measurement for assessing remaining life of high temperature components undergoing microstructural degradation has become a popular method. However, most of the reported techniques work on the basis of a linear relationship presumed to exist between hardness degradation as a function of Larson-Miller parameter (LMP) in which time and temperature of service are combined. This is based on empirical correlationship obtained through data fittings of the results of controlled laboratory experiments and/or field data collections. However, if the degradation of hardness is based on simple but well-established physical models relating hardness-via-flow stress to the various microstructural parameters, and extending these models to include the kinetics of microstructural changes under high temperature and applied stress, a different form of functional relationship is obtained. This functional form resembles the nature of hardness change as a function of LMP, as reported in the literature, much better than a linear representation. It is also able to explain the effect of applied stress on this correlation. The model also highlights the need to develop a better functional correlationship between hardness and LMP, which is more representative of the actual observations and not based on the presumption of a linear relationship
[en] The purpose of this paper is to examine the microscopic nature of the contact angles that dissociated boundary segments make with undissociated boundary segments in grain boundaries which have undergone DIGM. Ig local equilibrium exists at these grain boundary intersections, this study will reveal information about the nature of the driving force for grain boundary dissociation during DIGM. Evidence will then be presented which suggests that an applied stress can result in grain boundary migration and dissociation in a manner analogous to that observed during DIGM
[en] A detailed investigation of martensite islands in ultra-high strength dual phase (DP) steels using TEM EELS carbon measurements, nano-indentation studies and micro-mechanical modelling has been carried out. EELS analysis showed that the dispersion in the martensite island-to-island carbon content increases at lower intercritical annealing temperatures due to the influence of undissolved cementite. In a coarse-grained DP alloy, the median martensite island nano-hardness values and those calculated from EELS carbon data were in excellent agreement. However, in a fine-grained (microalloyed) DP alloy significant and unexplained softening occurred that is not consistent with the measured martensite carbon content. In both steels, the dispersion in martensite nano-hardness was greater than that expected from the measured carbon variations. Micro-mechanical modelling using the continuous composite approach (CCA) method was employed to calculate the martensite flow stress distribution required to fit the bulk tensile response of the two materials. The median martensite nano-hardness values derived from the fitted CCA stress spectra were in good agreement with those measured by nano-indentation, corroborating the observed martensite softening. These results provide experimental support for the CCA approach and suggest that the physical origins of the martensite stress spectrum can be strongly influenced by mechanisms other than carbon segregation. Finally, these data explain why the beneficial effect of reducing the α'/α phase strength ratio (PSR) on DP damage properties is highly asymmetrical, depending on whether the ferrite is strengthened or the martensite is softened (by tempering).
[en] Using digital computer-based methods, models for dispersion strengthening can now be developed which take into account many of the important effects that have been neglected in the past. In particular, the self interaction of a dislocation can be treated, and a computer simulation method to determine the flow stress of a random distribution of circular, impenetrable obstacles has been developed taking into account all such interactions. The flow stress values depend on the obstacle sizes and spacings, over and above the usual 1/L dependence, where L is the average obstacle spacing. From an analysis of the results, it was found that the main effects of the self-interactions can be captured in a line tension analog in which the obstacles appear to be penetrable
[en] This paper presents a study on the material behaviors and constitutive models of lead at high strain rates. Quasi-static compressive tests and split Hopkinson pressure bar (SHPB) tests were conducted at room temperature. The results of the SHPB tests were verified by high-speed photography, and the error of strain is less than 3% at high strain rate (5000/s). Our results show that the yield stress and flow stress increase at high strain rates. This result indicates that lead is sensitive to the strain rate at high strain rates, but the dependence is not linear during 3000-5000/s. The strain-rate dependence of lead was fitted by a quadratic polynomial curve. To describe the nonlinear strain-rate relationship of lead, modified Johnson–Cook and Cowper–Symonds material models were used to fit the experimental stress–strain curves. The modified Cowper–Symonds model agrees better with the experimental results and can better describe the dynamic mechanical behavior of lead under high strain rates.
[en] Highlights: • Dynamic transformation is shown to take place in three titanium alloys. • The driving forces are calculated using the concept of transformation softening. • A thermodynamic explanation for the downward shift of the beta transus temperature is proposed. The high temperature flow stress data of Koike et al. (2000) determined on a Ti-5.5 wt%Al-1.5 wt%Fe alloy are reanalyzed in terms of transformation softening. They observed that the harder alpha transforms dynamically into the softer beta phase when deformation is being carried out below the beta transus temperature. These observations are interpreted here as being driven by the thermodynamic requirement to do the least possible work during deformation. Tests were carried out on a near-alpha Ti alloy (IMI-834) in order to test the generality of these results. Here the driving force for transformation is taken as the flow stress difference between the work hardened alpha and the yield stress of the fresh beta that takes its place. This type of analysis is then applied to the experimental results of Xu and Zhu (2010) on C.P. Ti grade 2, which also display marked sub-transus softening. Such behavior is shown to be consistent with the occurrence of transformation softening.
[en] Highlights: → FIV of a foursquare fix-supported flexible plate exposed to axial flow was studied. → Special designed test section and advanced measuring equipments were adopted. → The narrow-band vibration phenomenon with large amplitude was observed. → Line of plate's vibration amplitude and flow rate was investigated. → The phenomenon and the measurement error were analyzed. - Abstract: An experiment was performed to analyze the flow-induced vibration behavior of a foursquare fix-supported flexible plate exposed to the axial flow within a rigid narrow channel. The large-amplitude and narrow-band vibration phenomenon was observed in the experiment when the flow velocity varied with the range of 0-5 m/s. The occurring condition and some characteristics of the large-amplitude and narrow-band vibrations were investigated.
[en] In order to clarify the competition between work hardening (WH) caused by dislocation movements and the dynamic softening result from dynamic recovery (DRV) and dynamic recrystallization (DRX), a new two-stage flow stress model of X12CrMoWVNbN10-1-1 (X12) ferrite heat-resistant steel was established to describe the whole hot deformation behavior. And the parameters were determined by the experimental data operated on a Gleeble-3800 thermo- mechanical simulation. In this constitutive model, a single internal variable dislocation density evolution model is used to describe the influence of WH and DRV to flow stress. The DRX kinetic dynamic model can express accurately the contribution of DRX to the decline of flow stress, which was established on the Avrami equation. Furthermore, The established new model was compared with Fields-Bachofen (F-B) model and experimental data. The results indicate the new two-stage flow stress model can more accurately represent the hot deformation behavior of X12 ferrite heat-resistant steel, and the average error is only 0.0995. (paper)