Strength of Materials最新文献

With the development of various multi-composite materials, there are several methods to study the mechanical properties of materials with complex microstructures. As a typical multi-composite material, foam concrete’s mechanical properties and research methods are also widely concerned. To predict the mechanical properties of foam concrete more quickly, we use Python software to realize the random generation of foam pores to establish a random three-dimensional geometric model of foam for many foam concrete materials. Based on the random foam three-dimensional geometric model, the equivalent parameters of foam concrete are calculated by the method of calculating the equivalent parameters of materials in mesomechanics. The relationship of equivalent material parameters between porosity and foam pore diameter is sought. Finally, the tensile calculation of a foam concrete cube with a cohesive unit is carried out to study its stress-strain distribution and the initiation and development of cracks.

A mechanophysical model for crack growth kinetics computation on stress corrosion fracture of modified 06G2BA and 08KhMCHA pipe steels is adequately expressed through the plane stress-strain state dα/dt and dJ/dt ratios that are dependent on the strain crack tip rate. The crack growth accelerated by an aggressive environment occurs under static and cyclic loading due to transient dissolution and repassivation processes at the crack tip. Such accelerations are divided into three categories, determined by the strain rate: mechanical cracking (fatigue crack and stationary plastic crack), corrosion-accelerated mechanical cracking (corrosion fatigue and corrosion-accelerated plastic crack), and sulfide stress corrosion fracture. Metallographic studies revealed the change in the crack nucleation and propagation mechanisms, from transcrystalline to intercrystalline, related to the viscoplastic and brittle structure of steel specimens cyclically loaded and simultaneously affected by a corrosive environment.

To explore the effect of head shape on the penetration capability of a fragment simulation projectile (FSP), single-stage kinetic energy penetration FSP with different head shapes was taken as the research object. The ballistic limit velocity (BLV) is used as the evaluation index in the study of experiments and numerical simulations. The nonlinear dynamic analysis finite element software LS-DYNA with the Lagrange algorithm is used in the simulation for FSP from 35CrMnSi steel penetrating target plates from 10CrNi3MoV (921A) steel of various thicknesses (3, 5, 8, and 10 mm). The ballistic experiment corresponding to the numerical simulation has been designed and carried out to verify the effectiveness of the numerical results. The results show that the values of BLV obtained through numerical simulation and ballistic experiment coincide well, and the average relative error in both is 7.14%. The research results provide a reference for designing FSPs and engineering applications of fragmentation kill warheads.

This study proposes a new numerical-and-analytical method for solving geometrically nonlinear problems of bending of complex-shaped plates made of functionally graded materials developed. The problem was formulated within the framework of a refined higher-order theory considering the quadratic law of distribution of transverse tangential stresses along the plate thickness. To linearize the nonlinear problem, we used the method of continuous continuation in the parameter associated with the external load. For the variational formulation of the linearized problem, a Lagrange functional was constructed, defined at kinematically possible displacement velocities. To find the main unknowns of the problem of nonlinear plate bending (displacements, strains, and stresses), the Cauchy problem for a system of ordinary differential equations is formulated. The Cauchy problem was solved by the Runge-Kutta–Merson method with automatic step selection. The initial conditions are found from the solution of the problem of geometrically linear deformation. The right-hand sides of the differential equations, at fixed values of the load parameter corresponding to the Runge-Kutta–Merson scheme, were obtained from the solution of the variational problem for the Lagrange functional. The variational problems were solved by the Ritz method in combination with the R-function method. The latter makes it possible to present an approximate solution as a formula. This solution structure exactly satisfies all (general structure) or part (partial structure) of the boundary conditions. Test problems are solved for a homogeneous rigidly fixed and functionally graded hinged square plate subjected to a uniformly distributed load of varying intensity. The results for deflections and stresses obtained by the developed method are compared with the solutions obtained by radial basis functions. The problem of bending of a functionally graded plate of complex shape is solved. The influence of the gradient properties of the material and geometric shape on the stress-strain state is investigated.

The galvanic corrosion behavior of anodized 7B04 aluminum/TC16 titanium alloy joints in different corrosion time and its effect on fatigue life were studied, providing a basis for structural corrosion and protection of typical joints of aircraft. The results show that corrosion begins near the coupling parts in the early stages of corrosion. A high concentration of Cl^– can damage the anodic oxide film on the surface of the 7B04 aluminum alloy, resulting cracking and pitting corrosion. The fatigue life of joints with corrosion was longest for 40 h, after which the fatigue life shortened rapidly. And sulfuric acid anodizing treatment of the 7B04 aluminum alloy plate was not conducive to the corrosion fatigue life of the specimen at the later stages of corrosion. The crack initiation of the specimen was located at the edge of the bolt hole.

Calculation methods for assessing the strength and durability of critical structural elements for various purposes generally assume the isotropy hypothesis of structural materials. At the same time, their technological and operational anisotropy significantly affects the bearing capacity of the entire product. Introducing damageability parameters into the system of governing equations makes it possible to increase the reliability of stress-strain state calculations and assess the durability of structural bearing elements. The use of anisotropic structural materials requires specification of the damageability tensor. The laws governing the influence of anisotropy of mechanical characteristics of metal materials on the kinetics of damage accumulation (scattered fractures) have been established. The results of experimental studies and kinetic damageability diagrams for anisotropic metallic structural materials are presented. A generalized damage accumulation model with consideration of anisotropy parameters has been developed. It allows determining the components of the damageability tensor based on the results of a single basic experiment to measure the degradation of the elastic modulus of a material. The dependence of the limit values of scattered fracture on anisotropy coefficients is shown.

Three cemented carbide samples, WC-12Co, WC-12Co-0.5NbC and WC-12Co-0.5NbC-0.5TiC were prepared by vacuum liquid phase sintering. Combining SEM (scanning electron microscope), XRD (X-ray diffraction), and mechanical property testing, the effects of different combinations of grain growth inhibitors on the properties and microstructure of cemented carbide were analyzed and studied. The results show that there are significant differences in the microstructure and mechanical properties of cemented carbide samples with the addition of NbC and TiC. Both NbC and TiC can effectively suppress the abnormal growth of WC grains in the alloy, resulting in a uniformly distributed and fine grained hard alloy. Further comparative analysis shows that adding appropriate amount of NbC can effectively improve the fracture toughness of the sample; adding an appropriate amount of NbC and TiC simultaneously can further improve the surface hardness and wear resistance of the sample. Compared to the alloy sample without any inhibitor, the Vickers hardness value of the sample with NbC and TiC added simultaneously reached 1952.4HV30, increasing by about 10%, and the wear loss decreased by about 30%, compared to the alloy with single addition of NbC, the wear loss of the sample with both NbC and TiC decreased by about 27.5%. In general, when NbC and TiC are added simultaneously, the comprehensive properties of the sample alloy are more excellent.

Radar stealth was a remarkable technical feature of the fourth-generation fighter aircraft, the damage of the stealth coating would seriously affect the stealth performance of the aircraft. This paper focuses on the membrane treatment of coating in the process of stealth coating damage repair, the effect of laser cleaning process on the cleaning effect of stealth coating on the surface of aircraft metal construction was studied. The results show that laser pulse width has the greatest influence on the laser cleaning of stealth coating, so pulse width should be prioritized when selecting the process parameters. And during the process experiment, the relationship between the effect of laser cleaning and process parameters were respectively negative correlation with scanning speed, positive correlated first and then weakly correlation with laser power, weakly correlated first and then positive correlation with pulse width, positive correlation with scan number. Finally, the optimal laser cleaning process parameters for the stealth coating of 500 μm on the surface of the metal structures were obtained by comparing the experiments between the preferred and the solution groups.

A procedure for acquiring and analyzing digital fatigue crack propagation images is advanced, which provides automatic crack size detection and the deviation from the axis perpendicular to the loading direction. The procedure is based on comparing the original image with the subsequent ones and eliminating all elements present, only getting the changes in the fatigue crack extension. A slight shift of the images concerning each other was fixed with their cross-correlation. Choosing an optimum image segmentation level permits rejecting the elements that appear due to the digital matrix noise. The four-dimensional pixel assembly was used to get the geometric elements. As a result, a set of geometric figures characterizing the fatigue crack propagation was obtained. For analyzing the length and direction of crack components, the ellipses were applied whose second moments coincide with those of the components. The total change in the crack propagation direction and length is determined from the parameters of the vector connecting the centers of mass of the first and last components. The advanced procedure can be employed in crack resistance tests.