Strength of Materials最新文献

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.