Strength of Materials最新文献

This study aimed to perform friction stir welding experiments using a 2-mm-thick copper plate. The tool’s rotating speed (ω) and transversing speed were 800–1200 rpm and 200 mm/min, respectively. The impact of the tool’s transversing speed on the properties of welded copper was analyzed while keeping the transversing speed constant. Analyzing the experimental results, it is evident that the metallographic microstructure of the welding zone undergoes similar changes at different tool rotation speeds. Specifically, the grain in the welding core zone becomes refined, while the grain near the return side appears larger. The region is the thermomechanically affected zone and heat affected zone, which experiences less agitation from the tool but undergoes compression from the matrix metal, resulting in the grain size being larger than that of the welding zone but smaller than that of the matrix metal. The same was true for the advancing side. The tensile strength of the sample, which was 266.2 MPa, exhibited a high degree of consistency with that of the base metal when the tool’s ω value reached 800 rpm. The hardness of each sample exceeded that of the base metal. The hardness of the sample increased by 133.2% to 185.9 HV0.2 when the tool’s ω value was set at 1000 rpm, surpassing that of the base metal. The friction coefficient of each sample was lower than that of the base metal. The friction coefficient in the welding zone, which is merely 0.21, represents less than half of that in the base material, which is 0.55. The friction stir welding technique has significantly enhanced copper’s mechanical properties, facilitating its widespread application.

Recent studies have shown that nozzle zones are one of the most dangerous elements of the reactor vessel. High stresses in such nodes can lead to the appearance of angular cracks. At the same time, the issue of choosing the critical dimensions and direction of crack location from the point of view of calculations for resistance to brittle fracture remains open. The paper presents the results of numerical modeling of the stress-strain state of the nozzle zone of the reactor vessel by the classical finite element method (FEM) and the extended finite element method (XFEM) using the submodeling technique. The results of numerical modeling by the classical FEM for the mode of hydraulic testing of the reactor vessel pressure vessel nozzle zone with three types of cracks are presented: surface, subweld, and a crack with 1 mm penetration into the weld. For twelve types of cracks with variations in their size and direction of location in the reactor vessel pressure vessel nozzle zone, the results of calculations of resistance to brittle fracture by the XFEM method for one of the characteristic modes of thermal shock are presented. The calculation results proved that axial cracks are more dangerous than circular cracks of the same dimensions. Cracks with a semi-axis ratios a/c = 0.3 and a/c = 0.7 are more dangerous for the axial and circumferential directions, respectively. At the same time, cracks with a/c = 0.3 are more sensitive to the direction of location than cracks with a/c = 0.7. It was shown that the use of the XFEM method makes it possible to conduct a rapid assessment of the resistance to brittle fracture with the possibility of varying the shape, size, and location of the crack, which allows one to effectively determine its critical size and the most dangerous location in the structural element.

The paper analyzes the peculiarities of forming honeycomb structures by additive methods (in particular, from plastic filament (Fused Deposition Method, FDM) and shows that the engineering application of products with honeycomb structures should take into account the peculiarities of structural damage development under the influence of working loads. For typified products in the form of closed multilayer shells (tanks for various technological purposes), these are thermobaric effects that lead to a loss of the long-term strength of the product and to the development of damages on the initial defects. It is proposed to divide the latter into three groups: defects in the form of cavities of various volumes and shapes, as well as contact patch defects at the adhesion boundary of the laid-down layers under the conditions of stable technological process; defects formed by random factors of a technological nature; defects arising from heterogeneities and initial differences in the material used (plastic filament). It is concluded that the loss of the bearing capacity of shells made of structural plastics (except for highly elastic ones) can be described by the development of cracks in the interlayer areas of the product in accordance with Griffiths’ theory. It is in these areas that the presence of defects is greatest. Since there is a significant difference between the mechanical properties of plastic filament and the product made by extrusion, it is proposed to use the equivalent properties of the components of a cellular system in the form of a three-layer connected shell in analyzing the stress-strain state of the product model.

The effect of cold isostatic pressing of WC-15Co powder blanks on the sintered alloy strength in threepoint bending was evaluated. The specimens treated or nontreated by cold isostatic pressing (CIP) before sintering were analyzed by scanning electron microscopy (SEM), energy dispersive X-ray spectrometry (EDS), X-ray diffraction (XRD), strength and Vickers and Rockwell hardness test equipment. The strength of CIP-treated specimens in three-point bending was found to be 2410 MPa, which is 13% higher than that of the CIP-nontreated ones and 30% higher than the standardized strength of a VK15 hard alloy of a similar composition. The hardness was shown to be the highest for the specimens CIP-treated before sintering, while average WC grain sizes remained unchanged compared to the CIP-nontreated ones. The predominant crystal modification of cobalt in the CIPtreated specimens was revealed to be HCP-Co, in contrast to the CIP-nontreated ones, where cobalt was mainly in the metastable FCC modification. It is shown that in the specimens CIP-treated before sintering, the cobalt content on the fracture surface is 3-4 times higher than for the nontreated ones, and the fracture crack propagates mainly along the WC matrix phase–Co binder interface. In the CIP-nontreated specimens, the crack propagates in the Co volume, and WC grains on the fracture surface are covered with cobalt interlayers of different thicknesses. Cold isostatic pressing has been proposed as a requisite additional operation of standard hard alloy technology since already at the pressing stage, it provides essential conditions for enhancing the strength of sintered specimens.

The present study assessed the effect of the active application of self-etch adhesives with phototherapy on enamel bond strength. Sixty bovine incisors, individually embedded in acrylic blocks, were divided into two groups according to the adhesive system used: Group 1 (AEO) Adper Easy One and Group 2 (CSEP) Clearfil SE Protect. According to the application technique of self-etch adhesives, each main group was further divided into three subgroups (n =10): (PA) passive application, (AP) active application, and (PAA) phototherapy active application. Shear bond strength (SBS) tests were conducted using a universal testing machine. The data were analyzed via two-way ANOVA and posthoc multiple comparisons using the Tukey HSD test. Furthermore, statistical analysis of the distribution of failure modes was carried out using the χ² test (p < 0.05). Two-way ANOVA indicated no significant differences in either adhesive systems (p = 0.797) or application technique (p = 0.869). Similarly, a χ² test showed no significant difference in the distribution of failure modes concerning surface treatments (p = 0.905). The use of phototherapy as a diode laser for the active application of the tested self-etch adhesives in the present study exhibited similar initial enamel bonding performance to conventional application methods.

The most common defect in the axles of wheelsets of railroad cars of the RU1Sh type is damage or wear of M20 threaded holes for bolts for fixing the locking strips of roller bearings. It is proposed that such threaded holes be repaired by cladding them with a restoring sleeve and then cutting a new thread. Fatigue tests of the repaired threaded holes showed that their durability practically corresponds to the basic values for new axles. The repair technology was tested on two full-scale axles mounted on a railcar bogie and subjected to route tests. The railcar with repaired axles traveled 35,000 km without damaging or wearing the restored threaded holes.

To deal with the safety problems with regard to the failure of reinforced concrete (RC) foot-bridge, it is necessary to know the ultimate deformations, that the failure calculation based on the principles of limit analysis does not allow taking into account. In this study, we develop a method of nonlinear analysis of plane RC frames based on the displacement method. Nonlinearity is considered under its two aspects, material and geometric. The foot-bridge is discretized into one-dimensional finite elements connecting two nodes and the loading is applied, step by step, until the structural failure. For the equilibrium state of the structure, an iterative procedure is used at each step of increase in loading where the nonlinear system is solved by using the method of successive substitutions based on the stiffness matrix connecting the increases of applied forces to the increases of nodal displacement. The influence of the tensile concrete between the cracks is taken into account in order to correctly assess the displacements under external loads and the redistribution of stresses. An automatic calculation program is thus developed. The developed method is applied to a practical example of a foot-bridge realized in Bordj Bouararidj (Algeria), whose dimensioning is realistic. The comparison of the numerical results obtained with the mathematical model and those obtained with Abaqus^© software is satisfactory and shows a very good agreement.

The mixed formulation of the finite element method for the problems of the Toupin–Mindlin gradient theory of elasticity is justified. This theory permits accounting for scale effects stemming from the material microstructure dimensions, particularly in problems with the limitations of classical elasticity. The variational formulation of the boundary problem is examined where strains, stresses, and their gradients enter into the variational equations along with displacements as equivalent arguments. The key feature of these equations is that they involve only the first-order partial derivatives of displacements, in contrast to the differential equations of the classical problem formulation that involve the derivatives of displacements to the fourth order inclusive, and the Lagrange variational equation in displacements, which incorporates their double differentiation. Their solution based on the mixed finite element method greatly simplifies the choice of approximation functions since there is no need to use finite elements that ensure the continuity of the first displacement derivatives at the element boundaries. This formulation based on a separate approximation of displacements, strains, stresses, and their gradients, is applied to solving the boundary problems of elasticity theory, where the strain gradient is included. For the mixed-method variational equations, the condition is formulated so as to ensure the uniqueness of the solution and stability of the mixed approximation for gradient elasticity theory problems. This condition is defined with the orthogonal projection operator that establishes a one-to-one correspondence between the classical and mixed approximation of strain distributions. A well-suited formulation for the practical application of variational equations for displacements and strains is proposed with the weakest requirements for mixed approximation stability.

As an important hub for crossing river cliffs in road transportation, the protective effect of bridges has always been the focus of research. In this paper, LS-DYNA software is used to simulate the process of explosion shock wave damage to large single-span T-beam bridge, and the influence of explosive type, detonation position and bridge reinforcement on the damage degree of bridge deck is studied. The results indicate that WY-1 explosive has the most severe impact on bridge damage among GOL-2 and WY-1 explosives. When only reinforcing steel is laid on the flange of the T-beam, the blast resistance performance of the bridge is significantly improved compared to without reinforcing steel. When explosives detonate at different positions on the bridge deck, there is no substantial damage to the entire bridge. But when the equivalent explosive is placed on the ground near the bridge pier, the pier completely fractures, the bridge collapses, and loses its load-bearing function. The research results provide a reference for the protection of bridges under explosive loads.

The formation of sheet metal products is now widely utilized for multi-purposes in the automotive, aerospace and in industrial sectors. In this study, the phenomenon of plastic strain, von Mises stress, shear train by the V-bending method and to analyze the results theoretically, by using a special program called ANSYS. The multi-layer sheet metal in the rectangular plate of Al and Cu with three different thicknesses (1.0, 1.25, and 1.5 mm) is carried out by the Explicit solver. These parameters have been investigated such as effect sheet setting condition (Al/Cu/Al and Cu/Al/Cu), sheet thickness, and traveling of punch. In the explicit analysis, the position of Al/Cu/Al achieved maximum plastic strain in maximum thickness and punch travel is improved to save computation duration at cost of solution accuracy. Also, maximum shear stress obtained in larger punch travel in position Al/Cu/Al than Cu/Al/Cu. As the thickness is increased, the shear stress and von Mises stress becomes increases in Al/Cu/Al, and position of Cu/Al/Cu produced decreasing shear stress and von Mises stress in increasing sheet thickness with different punch travel.