Strength of Materials最新文献

The methodological foundations of assessing the fracture toughness of elastic-plastic materials are considered. A methodology is proposed based on the results of assessing the fracture toughness of a material based on the structure state in the region of the crack tip. It is shown that the state of the material structure in the fracture zone of a uniaxially loaded specimen under the action of a stress close to the material’s ultimate strength is adequate to the structure state in the region of the crack tip before crack initiation and is a structural characteristic of this material. Based on the above correlation of material states, a structural parameter is proposed for assessing the fracture toughness of elastic-plastic materials with a crack. A statistical parameter, determined by the LM-hardness method from the Weibull homogeneity coefficient m, which characterizes the degree of scattering of hardness characteristics obtained by indenting the test portion of the loaded specimen, was taken as the fracture toughness index. This characteristic is invariant to the type of stress state and the loading method, i.e., the state of the material structure in the region of the crack tip does not depend on the loading method, which affects only the rate of reaching the ultimate structural damage in the region of the crack tip and the direction of crack propagation. It has been experimentally proved that the structural criterion of fracture toughness can be the same material characteristic as the force or strain characteristic of material fracture toughness. The difference between materials in their ability to resist crack initiation and propagation is determined by their initial structure. It depends only on the value of the thermal and force load parameters at which the material reaches a level of damage (loosening) of the structure at the crack tip sufficient for crack initiation. For comparison, we consider possible options for assessing the fracture toughness of elastic-plastic materials about the damageability of the structure: based on the scattering of the hardness values of the material of the specimen with a crack, on the state of the material structure in the test portion after the failure of the loaded specimen or after its reloading, as well as based on the state of the material structure of the specimen under loading to the ultimate strength of the material.

One of the most common ways to assess the activity of acoustic emission (AE) sources and their hazard under mechanical loading of materials and structures is to use the local-dynamic criterion. To verify the latter, specimens of various materials, namely fiberglass, corundum refractory, and steel, were tested under static loading during three-point bending. A special software AE-Criterion was developed to determine the AE parameters, including the current parameters of source activity, based on the local dynamic criterion using both the force factor and the time since the beginning of the test. Taking into account the multiplicity and stochastic nature of AE events during the loading of materials, it is proposed to determine the average values of M_S, M_T, and the values of the indices I_AS and I_AT of the activity degree of AE sources when applying the force parameter and time, respectively. The analysis of the data obtained during the bending tests of the above-mentioned specimens and, accordingly, the calculation of the parameters of source activity showed that the value of the index I_AS, when using the force factor, satisfactorily reflects the deformation and fracture processes of these materials. The study’s results indicate the suitability of the local dynamic criterion for diagnosing the deformation and fracture processes of the above materials.

The mechanical properties of the crack tip are vital for material safety evaluation. The weld material of welded joints is prone to work hardening during processing and installation, which changes the material mechanical properties and seriously affects the safety assessment of nuclear power welded joints. At the same time, nuclear power pipelines will creep under high temperatures and cause changes in the mechanical field at the crack tip. This paper obtains the mechanical properties of nickel-based alloy 182 in a nuclear power water environment. With the help of ABAQUS software, the change rules of the crack tip stress field and creep field under different hardening degrees are analyzed, and then the influence on crack tip creep and crack propagation rate is analyzed. Results show that increasing pre-hardening will increase the Mises stress around the crack tip, and the creep around the crack tip is expressed under pre-deformation, indicating that pre-hardening can accelerate the cracking of stress corrosion to a certain extent. Research has shown that the work hardening induced by the pre-deformation has an important influence on the creep rate at the crack tip, and the high creep zone is mainly concentrated in the direction of crack propagation. Under a certain work hardening rate, the crack growth will be accumulated.

Austenite steel, Nb-bearing alloy steel, and high-Cr alloy cast iron surfacing layers (SL) were grown on 45 steel base metal (SBM) by using the arc surfacing method with self-shielded flux-cored welding wire and fusion electrode, and the composition and microstructure of these SLs were analyzed. The criss-cross push-off (CCPO) specimens for each type were processed, and the bonding property between SLs and 45 SBM was measured using a customized CCPO test. The fracture path and fractograph characteristics were also investigated using a metallographic and scanning electron microscope. The experimental results indicated that the bonding strength (BS) of austenite steel SLs and 45 SBM was the highest (465 MPa); the fracturing sites were near the fusion line, and the fracture exhibited cleavage fracturing features. The BS of niobium-bearing alloy steel and 45 SBM was 310 MPa, and the fracture was at the bottom of the SLs and exhibited quasi-cleavage fracturing features. The BS of the alloy cast iron SLs and 45 SBM was 175 MPa, and the fracture was at the bottom of the SLs and showed brittle fracturing features.

Ti-Al-based multifunctional composite materials are widely analyzed for their outstanding properties—ultra-lightweight, high strength, high temperature, and corrosion resistance. Due to these special and unique properties, the new composite materials are widely used in various applications such as water filters, medical equipment, high-performance gas turbine engines, automobile and aerospace industries, and elsewhere. This work aims to determine the formation of MAX phases involving different systems for samples of TI-Al-C composites of a defined composition based on the research conducted by scientists.

The 3D problem became the basis for modeling the critical states and shapes of bifurcation buckling of longitudinal force and torque-stressed nanotubes. The homogeneous system of ordinary differential equations built on the theory of rectilinear elastic rods was formulated. Its nontrivial solutions result in critical torque or longitudinal force levels at preset values of one of those entities. The closed-form solutions under chosen boundary conditions indicate that only spatial curves as variable diameter spirals with the left or right orientation consistent with that of the torque can be stability loss modes. The spiral is single-wound without longitudinal force, regardless of the tube length. If the tube is also prestressed with axial compression or tension, the bifurcation spiral is multi-wound; its number is determined by the eigenvalue of the equations, which increases with growing forces and tube length.

A numerical procedure for determining the parameters of the quadratic strength criterion for composite materials is presented, which considers the difference in tensile and compressive ultimate stresses. In addition to the reinforcement geometry, the elastic properties and ultimate stresses of fibers and the matrix are used as input data. The developed approach is based on the finite element modeling of a representative element of the volume of a three-dimensionally reinforced composite material. The boundary conditions are formulated, which makes it possible to reproduce the stress state within this volume under different types of homogeneous (on average) loads of the composite material. The local maximum equivalent stresses were determined separately for fibers and the matrix, which were used to determine the calculated ultimate stresses of the composite. The numerical results for flat specimens under tension in two directions and shear in the specimen plane were experimentally verified. The maximum difference between the calculated and experimental values of ultimate stress is 6.13%, which makes it possible to use the proposed procedure in design works.

A friction stir joining technology was used in preparing a performance gradient aluminum alloy sheet through friction stirring. The sheet was treated by solution aging, and the metallographic structure and hardness of the sheet before and after heat treatment were analyzed. Results showed that the sheet with good hardness gradient distribution can be obtained by selecting appropriate process parameters. When the rotation speed of the stirring tool is 300 rpm, the feed speed of the stirring tool is 250 mm/min, the downward pressure of the stirring tool is 6.6 mm, the hardness distribution of the upper and lower surfaces and sections of the specimens with or without heat treatment improves, and the best gradient hardness distribution of the sections is obtained. The grains in the stirred region of the prepared gradient sheet obtained by each parameter are refined to a certain extent, especially after heat treatment. Grain changes are obvious, but the grains grow after heat treatment. The second phase precipitated after heat treatment is evenly distributed, and this is effect improves the hardness of the sheet. The aluminum alloy gradient material prepared in this paper can meet the requirements of the automobile industry and other molding plate performance requirements. It is also found that the friction stir joining technology can better prepare aluminum alloy gradient material, and the fabricated material has fewer defects than the continuous casting technology.

In this paper, a low power fully acoustic non-destructive testing (NDT) of glass fiber reinforced composite plate has been presented. Input acoustic power in microwatt range has been used for local defect resonance (LDR) based delamination activation. Both numerical and experimental results are compared here. Numerical simulation has been carried out in ABAQUS platform. An oblique incident wave interaction with 3D composite plate has been created and the sound radiation pattern over the plate has been studied with an air layer over the plate. In experiment, the sound has been generated by a piezo-speaker and a MEMS microphone has been used for reception over the plate. Moreover, a laser Doppler vibrometer has been used for LDR frequency and corresponding mode shape validation. Frequency spectrum, mode shape at LDR frequency, best wave impinging angle, efficiency and sound directivity patterns are studied. θ, ϕ = 10°, 30° has been found to the best direction for sonic excitation. Moreover, fully acoustic system is found to be more efficient than partial acoustic system, i.e., contacts excitation. This method can be used for inspection of large structures due to long distance non-contact excitation.

The present work aims to perform an extensive numerical study based on genetic algorithms and Weibull probabilistic approach to highlight the effect of date palm fiber (DPF), doum palm and alfa fibers on the fiber–matrix interface damage of date palm/epoxy, doum palm/epoxy, and alfa/epoxy biocomposite materials. The results obtained in this study coincide perfectly and in good agreement with very recent theoretical and experimental studies. Mehdi Jonoobi et al. found through experimental trials that date palm fibers can be used to improve the ductility of epoxy matrix, and the use of date palm fibers in an epoxy-based composite can improve epoxy strength according to the weight ratio of fiber used, in addition, Noora Al-Qahtani et al. found that the morphology of the composites was improved which present superior adhesion among the palm fiber and the polymer matrix. Thus, it is useful now to study the opportunity to promote date palm fiber in different development sectors to encourage industries to manufacture ecological products from the date palm which will be a candidate promoter for new technological applications.