Strength of Materials最新文献

The direct strength method (DSM) has become an alternative to the effective width method (EWM) to compute the bending moment capacity of cold-formed channel sections. The traditional EWM examines the nominal moment capacity by the PURLIN program. DSM uses a signature curve from tests on many Australian channel sections to obtain design moment capacity for local and distortional buckling modes. Moreover, the non-linear analysis performed by Strand7 will be compared with the current DSM curve. In general, these comparisons of results are in good agreement: the THIN-WALL program results and the linear static analysis results in Strand7; the moment capacity calculated by the PURLIN program (EWM) and the DSM curve in the local buckling mode; the non-linear analysis results in Strand7 and the DSM curve for each buckling mode. The outcome of this study will benefit structural engineers by providing them with design methods analysis.

A high competition level in modern space-rocket technology requires continuous improvement of structural elements and enhancement of their reliability, on the other hand, reduction in production costs and lead times. One of the pressing problems of national rocket engineering is to hold down the number of physical tests (especially destructive) of the samples and replace them with computational methods. The first consideration in efficiently designing space-rocket power structures, such as propellant tanks, high-pressure cylinders, etc., is to increase the net volume of the structure and cut down its materials consumption without losing strength properties. Various engineering designs are employed to enhance the reliability and strength of such structures: end and intermediate rib stiffening, variable shell thickness, etc. The new model of a bimetallic waffle-skin shell of a launch vehicle propellant tank, made of an aluminum alloy and strengthened with a titanium skin, is advanced. The finite element method-based software was used to perform its 3D stress-strain state computations. The results for bimetallic shell computations showed that a titanium skin was liable to elastic strains that do not exceed 0.54 %, and maximum equivalent strains of an aluminum alloy reached about 0.7 %, while equivalent elastic strains were approximately half as much. Computational studies confirmed that the bimetallic shell of a lower weight exhibited insignificant plastic strains compared to the conventional waffle-skin design. Moreover, the thickness of an aluminum alloy sheet for shell fabrication is reduced by more than half; thus, the shell alternative as a double-layer structure can be employed to advantage. The computational results can be used to design new space-rocket structural elements and assess their stress-strain state.

The possibility of increasing the survivability of the system of refueling combat vehicles during combat operations through the development of a new method that reduces refueling time and the use of new technical means to implement it is substantiated. The essence of the method of accelerated refueling of combat vehicles is to place an elastic fuel reservoir (EFR)under the combat vehicle wheel and run it over at low motion velocity, resulting in the fuel displacement from EFR to the fuel tank of the vehicle being refueled. The study results indicate the feasibility of accelerated refueling of combat vehicles by this method. A mathematical model for determining the pressure in the EFR run over a vehicle wheel was refined in this study. A mathematical model for determining the stress-strain state of an EFR loaded with internal excessive alternating pressure was proposed, allowing one to assess stresses arising in the EFR wall when refueling combat vehicles and optimize the EFR design. Experimental results on the of accelerated refueling of vehicles confirmed the efficiency of the proposed refueling method and the EFR design.

In the recent years, composite polymers have been widely involved in a large scale of sensitive applications such as biomedical, remote healthcare, electronic devices, smart industry 4.0, and so on. This is due to their numerous advantages in terms of high precision measurement, mechanical and electrical strengths, and affordable cost, witnessed by various recent researches. Moreover, and to meet their environment installation conditions, polymers are usually reinforced by a protective layer, thoroughly produced by various resistant materials such as carbon fibers (CF). In that context, our present work aims to analyze and compare the temperature effect and the mechanical stress on the fiber-matrix interface damage for the three reinforced polymers: CF/poly ether ether keton (PEEK), CF/poly methyl methacrylate (PMMA), and CF/poly phenylene sulfide (PPS). The obtained results by a genetic approach and a non-linear acoustic technique obviously depict that for the three composite materials, and above 137°C, the interface damage increases very swiftly. In addition, the fiber-matrix interface adhesion of CF/PEEK shows better response compared to the other interfaces CF/PMMA and CF/PPS, this conclusion was confirmed by Aucher where he found in his study that the ductility of the resin and the strong fibre/matrix adhesion, especially in the case of CF/PEEK, is better compared to CF/PMMA and CF/PPS. Results obtained by our present study are intended to complete those already found in the field of the reinforced polymers designed for the aforementioned engineering applications.

Along with the classical finite element method (FEM), other calculation methods for assessing crack resistance characteristics are currently being actively developed. This is due to the existing shortcomings of the FEM caused by the dependence of the calculation results on the density of the finite element mesh. One of the promising methods being developed in world practice is the extended finite element method (XFEM), which allows obtaining satisfactory calculation results while simplifying the crack modeling procedure and saving calculation time. In this paper, three problems are numerically modeled using the classical FEM and XFEM methods: calculation of a disc crack in a cube under uniaxial tension, calculation of the off-center tension of a compact CT specimen, and calculation of a cylindrical part of an NPP reactor vessel with a semi-elliptical crack under thermal shock. The obtained results showed that the extended finite element method gives sufficiently accurate results compared to analytical solutions and the classical FEM. At the same time, using the XFEM method does not require considering the singularity of stresses at the crack tip when building an FE model. Therefore, the minimum size of the FE can be increased by almost five times while maintaining the accuracy of the results. This greatly simplifies the procedure for constructing the FE mesh, reduces the total number of FEs in the model, and saves computational time. Thus, the XFEM method can be used to calculate the crack resistance characteristics and improve the efficiency of assessing the resistance to brittle fracture of structural elements.

The instrumented indentation method, based on the digital recording of the parameters of the entire process of continuous local deformation of the material by an indenter, provides more complete and accurate information about the material’s behavior under load. Obtaining such information without compromising the integrity of a critical structure during operation increases its value. Therefore, more and more attention has recently been paid to portable installations and devices that use the instrumented indentation method. To date, no such equipment is manufactured in Ukraine. Therefore, there is a need to develop portable equipment for conducting such tests in the field under various modes of static and cyclic loading in the macro range of forces. The paper describes an updated portable installation PIIT-02M for determining the strength characteristics of structural elements in service by instrumented indentation, which was developed following the requirements of ISO 14577 and is intended for testing at the macro level. The system of fastening the portable installation on the test object was updated from mechanical (hooks and chains) to electromagnetic. This made it possible to conduct tests on pipes and flat surfaces, including sheet materials and structures made of them, significantly increasing the test efficiency and expanding the range of applications. Comparative tests on a 12 mm thick structural carbon 20 steel plate using the previous mechanical fastening system and the new electromagnetic one showed that the indentation diagrams obtained overlap well. Based on the results of tests of high-strength steels, the portable installation PIIT-02M allows one under both laboratory and operating conditions to record the process of indentation of the indenter into specimens of structural materials with high accuracy and, accordingly, to determine their mechanical characteristics by instrumented indentation.

The laws governing the influence of a hydrogen atmosphere at a pressure of up to 15 MPa with controlled total oxygen and water vapor content of up to 0.004 g/m³ and pre-absorbed hydrogen on the strength and ductility characteristics, short-term and long-term static crack resistance of 05Kh13N8M3 cast martensitic steel (in wt.%: 0.045 C, 0.38 Si, 12.9 Cr, 2.69 Mo, 0.43 Mn, 7.8 Ni, 0.057 La) at room temperature were studied. After hydrogen pre-charging for 4 h at a temperature of 773 K and a pressure of 5, 10, and 15 MPa, the hydrogen content of the specimens determined with a LECO TCH 600 device by infrared adsorption with melting was 3.3, 4.9, and 7.6 ppm, respectively. It was found that under short-term loading, the intensity of the effect of hydrogen on the fracture toughness of steel increases with an increase in the absorbed hydrogen content, crack sharpness, and a decrease in the loading rate. At the maximum hydrogen concentration of 7.6 ppm and a tensile rate of 0.1 mm/min, the relative elongation, lateral contraction ratio of smooth 25 mm long cylindrical specimens with a test portion diameter of 5 mm and the critical stress intensity factor of beam specimens measuring 20×10×100 mm with a relative length of the pre-induced crack of (upvarepsilon =) 0.53 decrease almost twofold. At the same concentration, hydrogen does not affect the stress intensity factor of cracked specimens at a rate of 10 mm/min and specimens with a stress concentrator in the form of a notch with a tip radius of 0.065 mm at a rate of 0.1 mm/min. Under long-term static loading with a test duration of 300 h of double-cantilever beam specimens in the form of a rectangular plate 10 mm thick with milled grooves 3 mm deep and a tip angle of 60°, the threshold value of the stress intensity factor decreases. The rate of subcritical crack growth in the second portion of the hydrogen cracking diagram increases in proportion to the logarithm of hydrogen concentration.