Strength of Materials最新文献

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.

The method of linear conjugation of analytical functions of complex variable was used to solve the problem of circular transversally isotropic plate bending hinged on the edge and loaded over the outer surface by the force distributed along the concentric curve. The complex potentials employed for registering the stress and deformation characteristics of the problem can possess the specific features at the concentrated force loading points, their nature was investigated and applied to the existing loading as conditionally concentrated. For getting the solution, the equation for the refined transtropic plate bending model was used that includes transverse shear strains and cross-sectional reductions, and, unlike other refined theories, the formulas with those refinements are advanced. The constants in the complex potentials were established with the boundary conditions and conjugation conditions for the moments and generalized angles of cross-section rotation along the loading line. With the approach by Timoshenko and Woinowsky-Krieger, from the circular loading solution, as a particular case, the solution for the centered concentrated force-loaded plate was obtained. For both cases, the refined normal radial and circumferential stresses were calculated in the center and on the edge of the plate. The data are summarized in tables and graphs. The model and numerical results show that an increase in the transverse plate anisotropy can radically change stress distribution patterns in its transverse cross-sections, up to the change in the radial stress signs on the outer surfaces. The classical model of plate bending and refined models such as by Timoshenko and Reissner are inapplicable in this case.

To investigate the relationship between service temperature ranging from –40~250℃, and size effect on tensile properties of thin ASS-304 sheets with nine different thicknesses (40~500 μm), uniaxial tensile tests were performed on thin ASS-304 sheets of the same average grain size in the present study. Within the thickness range of 40 to 300 μm, corresponding to η = t/d values from 1.1 to 8.1. The ultimate tensile strength (UTS), yield strength (YS), and elongation (EL) of ASS-304 exhibits a dimensional effect of “the thinner, the stronger”. For example, as the η increases from 1.1 to 8.1, the UTS rapidly decreased from 1798.8 to 839.0 MPa at 20℃, from 1703.1 to 526.9 MPa at 150℃, and from 1661.2 to 478.9 MPa at 250℃, attenuation of 53.36, 69.06, and 71.17%, respectively. Meanwhile, the YS at 20℃ are separately 1768.9 to 418.7 MPa with 1695.2 to 343.3 MPa at 150℃ as well as 1645.7 to 330.1 MPa at 250°C, decrease the proportion of 76.33%, 79.75% and 79.94% respectively. Notably, the UTS, YS, and EL at 150 and 250℃ are lower than those at 20℃. The true stress value of ASS-304 was enhanced at –20 and –40℃, and the true strain increases first and then weakens as the thickness increases, the reason is the transformation- induced-plasticity (TRIP) effect of ASS-304 in stretching. The asymptotic function describes the relationship between strength and the values of η, while the Chapman function represents the relationship between elongation and the η. A linear variation exists between service temperature and tensile properties. And relevant empirical equations including T-η- and T-η-tensile properties were established, which can predict the UTS, YS, and EL of thin ASS-304 sheets under different service temperatures.

During the operation of a steam turbine, its structural elements are subjected to significant thermal and mechanical loads. The consequence of the long-term effect of such a load is the accumulation of scattered fatigue damage to the material of the structural elements of the steam turbine, which is localized over time in the form of fatigue cracks of various types. Evidence of this is several accidents and catastrophic failures of steam turbines due to significant fatigue damage to the shafting. The localization of damage in turbine rotors is facilitated by stress concentration in the gouges and fillets, as well as damage to the surface layer of the rotors during the thermomechanical treatment stage since all metal processing operations (forging, turning and milling, heat treatment) are accompanied by plastic deformation of the material. One of the reasons for the long-term accumulation of fatigue damage in the structural elements of steam turbines is thermal stresses, which can reach dangerous values during turbine startup operations. In certain parts of the rotors, these stresses are sufficient to cause scattered fatigue damage to the material (the so-called thermoplasticity), especially when starting the turbine from a cold state. In the case of crack initiation, the thermal stresses are all the more sufficient for its further intensive development even when starting the turbine from uncooled and hot states, which are less damaging than starting from the cold state. To study the intensity of crack growth in the turbine rotor due to thermal stresses arising during turbine startup, a computational model based on using a finite element model of the shaft of the K-200-130 steam turbine and fracture mechanics approaches is proposed. Studies based on the proposed computational model have demonstrated the ability to predict the process of crack growth in the rotor due to turbine startup from different thermal states and assess its danger to structural integrity. The initial size of the internal annular crack was determined, which has the potential for further intensive growth under the influence of thermal stresses.