Strength of Materials最新文献

The application and comparison of different fracture mechanics concepts are discussed to be used in computing stress intensity factors (SIF) from the numerical solutions of model crack theory problems with a mixed scheme of the finite element method. Approximation of displacements rests on piecewise-linear interpolation functions with triangular elements, and strain and stress distributions are approximated by the linear combination that includes the piecewise-linear interpolation and interior bell function. The latter ensures the stability and convergence of the approximate discrete problem solution. The solution results for linear elastic and elastoplastic model plane central mode I crack-strip tension problems under different loading and plane strain state conditions are presented. Elastoplastic calculations were made with an ideal elastoplastic material model. The application of the energy balance and G-integral concepts to the calculation of the specific work of fracture at the stationary crack tip is substantiated. It is shown that on condition of uniform plate partition in the vicinity of the crack tip, the application of those concepts to SIF calculation for one loading stage is consistent with the Irwin plastic zone correction, maintaining this approach in further mesh thickening. Elastoplastic calculations on repeated loading demonstrated that tensile stresses ahead of the crack tip were about the same as on the initial one, but the opening at the crack tip on the former was larger than on the latter, and this effect was most pronounced for the first half of active loading values. Several aspects of SIF calculations on repeated loading are presented.

Firstly, this paper used a laser particle size analyzer to test the particle size distribution characteristics of four specific surface areas of phosphate slag powder particles. Then, it used the powder fractal theory to determine the fractal dimension of the different specific surface areas of the phosphate slag powder particle group. Finally, the effect of the fractal dimension of the phosphate slag powder particle group on the mechanical strength and activity index of the phosphate slag powder-cement cementitious system at 3 d, 28 d, and 180 d ages were studied. The results showed that the phosphate slag powder with different specific surface areas has self-similar fractal characteristics; the larger the specific surface area of phosphate slag powder, the higher the fractal dimension, and the fractal dimension of phosphate slag powder particle can characterize the properties of phosphate slag powder’s fineness and particle gradation in the process of grinding and refining. The phosphorus slag powder particle fractal dimension, the mechanical strength, and activity index of the phosphorus slag powder-cement cementitious system have linear correlation characteristics; the larger the fractal dimension, the higher the mechanical strength and activity index of the cementitious system with phosphate slag powder.

To date, most WWER-1000 nuclear power units in Ukraine have reached the end of their design life (30 years). In view of this, work is under way to extend the service life of critical equipment elements beyond the design life (to up to 60 years). In addition to reactor vessels, these critical elements include vessel internals (VI). One of the key approaches in such works is a predictive assessment of the structural integrity of VI structures using mathematical modeling of physical processes typical for power unit operation under intense radiation exposure of structural material. The service life prediction methodology constantly requires refining and taking into account more factors affecting structural integrity. The paper considers the kinetics of changes in the stress intensity factor for postulated cracks in the reflection shield of WWER-1000 VIs during long-term operation and at the time of reactor emergency during the rupture of primary coolant circuit pipelines with a nominal diameter of 100 to 850 mm in the most dangerous areas in terms of brittle fracture resistance. A significant impact of taking into account the residual stresses formed as a result of welding and heat treatment of the reflection shield during manufacturing on the results of calculating the brittle fracture resistance was revealed, which may affect the conservatism of assessment methods when justifying long-term operation.

Due to critical vibration and excess weight in the frame structure of the automobile, the automobile acceleration effects are the primary problems. This paper describes the material selection based on its weight, vibration reduction, and increasing vehicle performance. The front cabins heavy-duty van frame was considered for this research work and designed according to the frame dimension. Generally, the frame is made of mild steel and replaced by high-strength extrusion magnesium alloy. The frame structures crash and impact analysis was investigated using nonlinear buckling and transient analysis. The stress, vibration and nonlinear buckling analysis was obtained under different loading conditions. The bending performance of the frame was analytically investigated and verified with the FEA code (ANSYS 15.0).

This paper systematically analyzed and discussed the microstructure and mechanical properties of as-cast AZ61 magnesium alloy with rare-earth content of 1.0~4.0 wt.% by optical microscopy, SEM, EDS, and XRD. The results indicate that in addition to the obvious distribution of skeletal β-Mg₁₇Al₁₂ phase, a small amount of Mg₃₂(Al, Zn)₄₉ quasicrystal phase is distributed in as-cast AZ61 alloy. With the addition of rare-earth content, the skeletal β-Mg₁₇Al₁₂ phase dendrites in the as-cast AZ61 magnesium alloy matrix are decomposed obviously. The dendrite decomposition is more serious with increased Rare-earth content, and the quasicrystal Mg₃₂(Al, Zn)₄₉ disappears. When the rare-earth content is 1%Ce~2%Ce, the alloy mainly consists of needle-like, dot-like, or clustered Al₄Ce and Al₈Mn₄Ce phases with a small amount of Al₄La. When 2.8%Ce+1.2%Nd mixed rare-earth was added, the Al₈Mn₄Ce phase was not found in the alloy but mainly composed of Al₃Nd and Al₄Ce rare-earth phases. These rare-earth phases were radiative needle-like, willow leaf-like, rod-like, spot-like, layer-like, and a little irregular block. The as-cast AEZ641(2.8 wt.% Ce+1.2 wt.% Nd) magnesium alloy has the best comprehensive performance, and the yield strength is 2% higher than the as-cast AZ61 alloy. Its tensile strength, hardness, and elongation are similar to as-cast AZ61 alloy. The fracture mechanism of as-cast AZ61 + xRE is mainly a cleavage-type brittle fracture.

The paper considers the use of hydrogen as a component of the technological process to solve an applied problem - the control of material defects, which consisted of controlled fragmentation of steel under dynamic loading to facilitate the cracking of the metal volume. Medium-carbon pearlite-ferrite steel of steel 60 types under electrochemical hydrogenation is studied. The criterion parameter of the material state was the energy of impact loading by three-point bending of smooth beam specimens. By selecting the electrochemical hydrogenation modes in terms of intensity (current) and duration of the process, as well as the composition of the electrolyte, a significant reduction in the energy intensity of steel fracture was obtained, which correlates with the residual hydrogen concentration in the metal. Metallographic analysis of the surface of the specimens confirmed the hydrogenation effect on the initiation of defects, which can be used for controlled steel fragmentation.

The paper presents the results of an experimental study of the deformation properties of 10GN2MFA and 15Kh2MFA steels at different ratios of principal stresses under cyclic loading conditions. Applying the proposed phenomenological approach of an improved elastic-viscoplastic model allows one to qualitatively describe the effects of the influence of structure degradation under the action of temperature and force factors that accompany the operation of actual structural elements significantly affecting the regularities of deformation and damageability and the strength of structural materials under combined stress conditions. An evaluation of the variation rate in the homogeneity of the studied steels at different ratios of principal cyclic stresses under cyclic creep conditions is presented. Based on the experimental results, correlation dependences between the relative values of the homogeneity coefficient and the function of the deformation properties of the material and the type of stress state were obtained, which significantly simplifies the determination of the parameters characterizing the limiting state of the structural element.

Member stiffness in bolted connections has been widely studied for homogenous and isotropic materials contrary to composite sandwich ones. Therefore, a numerical simulation based on the augmented Lagrangian algorithm to solve contacts problems is conducted in ANSYS software to investigate various preloaded sandwich bolted joints with carbon and glass laminate skins and two types of foam cores. Tested samples show a rise of the ratio of maximum shear stress to maximum principal stress when the applied preload increases considerably which improves the risk of core failure, additionally, an analytical approximation of stiffness at preload is proposed, for that, a search algorithm is used to deduce the expression of the equivalent elastic modulus of tested joint members and a distance correlation method is applied to determine the theoretical formulation of the introduced stiffness model whose accuracy is optimized through analyzing results of root mean square error (RMSE) of its mathematical approximations , the convergence of the chosen model is ensured for all tested samples except the ones having the most rigid skins with the greatest elastic modulus (209 GPa). Furthermore, the equation introduced by Zhang and Poirier concerning the tension load causing a bolted joint separation is adapted and validated with a percent error less than 13%.