Statistical Estimation of Resistance to Cyclic Deformation of Structural Steels and Aluminum Alloy

Resistance to cyclic loading is a key property of the material that determines the operational reliability of the structures. When selecting a material for structures operating under low-cycle loading conditions, it is essential to know the cyclic deformation characteristics of the material. Low-cycle strain diagrams are very sensitive to variations in chemical composition, thermal processing technologies, surface hardening, loading conditions, and other factors of the material. The application of probability methods enables the increase in the life characteristics of the structures and the confirmation of the cycle load values at the design phase. Most research papers dealing with statistical descriptions of low-cycle strain properties do not look into the distribution of low-cycle diagram characteristics. The purpose of our paper is to provide a probability assessment of the low-cycle properties of materials extensively used in the automotive and aviation industries, taking into account the statistical assessment of the cyclic elastoplastic strain diagrams or of the parameters of the diagrams. Materials with contrasting cyclic properties were investigated in the paper. The findings of the research allow for a review of durability and life of the structural elements of service facilities subjected to elastoplastic loading by assessing the distribution of low-cycle strain parameters, as well as the allowed distribution limits.

On the fatigue strength calculation of the welded shell structures from high-strength steels under low-cycle loading. Part 1: Estimation at the initial stage of fatigue failure

The normative methods for calculating the fatigue strength of welded joints are of limited use for low-cycle loads, as they do not take into account the possible variation in the asymmetry of the operating stress cycle, differences in the expected level of residual stresses, and the possible variety of joint geometry. Estimation procedures have been developed for shell structures made of high-strength steels subjected to external and internal pressure. They were based on experimental data on the resistance to fatigue fracture, physical modeling of individual stages of fatigue damage, and generalization of the results of numerical studies of the FEM of the stress-strain state.

Study of the regularities of resistance to deformation and damage accumulation under irregular low cycle loading

The results of computation and experimental studies of changes in the regularities of the strain resistance and damage accumulation in conditions of the irregular low cycle loading are presented and compared with similar data for a regular cyclic elastoplastic deformation at the same loading. The irregular mode of low cycle loading implemented in the study is adopted in the form of an equiprobable distribution of changes in the stress amplitudes within a given range between maximum and minimum levels at the symmetric form of the cycles. This mode was reproduced on test equipment through introduction of the corresponding functional dependence of changes in the stress amplitude in the cycles into the control program. The data on a cycle-by-cycle kinetics of both cyclic and unilaterally accumulated strains obtained under irregular mode of loading were recorded in a databank and then compared with the data for a regular loading. This provided the possibility of their analytical description by the corresponding equations of state with the correction of the parameters of the diagrams of cyclic elastoplastic deformation taking into account the conditions of irregularity of loading modes. The results of the experiments are presented in the form of diagrams of the tests modes, curves of a low cycle fatigue of the studied material at the soft and hard loading modes, diagrams of a cycle-by-cycle kinetics of the cyclic and accumulated strains at the regular and irregular modes, and also as kinetic diagrams of damages accumulation for these conditions. Using the summation criteria expressed through the deformation characteristics of accumulated damage, it is shown that taking into account change in the character of the strain development under irregular low-cycle loading, the criterion dependences can be used to assess the durability and compare it with the similar data under regular modes when accepting the condition of attaining the limiting state.

Dynamic Measurements of Tank Wall Deformations as a New Method for Assessing its Technical Condition

The paper proposes a new approach to assessing the residual resource of the wall of a vertical steel tank by the criterion of low-cycle loading, taking into account its actual deformations. The method is based on the principle of measuring the difference in the power of the luminous flux at the ends of the light tube (light guide) when it is bent. An experiment was carried out using an optical pair (LED and photodiode) and an optical fiber. For this purpose, templates with known radii of curvature were prepared, a prototype of an Arduino-based device was developed and an optical fiber was manufactured, which was subsequently fixed on the wall of a two-hundred-liter metal barrel. As a result of the experiments, a formula was obtained for determining the curvature of the wall depending on the readings of the sensor. The sensitivity of the proposed method is estimated. An algorithm for determining the number of cycles before the formation of fatigue cracks is proposed, taking into account the actual values of the wall deflection.

Thermal and Mechanical Characterization of Drawn Polypropylene Fibres

The main objective of this work was to investigate the effect of different cellulose (CEL) content and the draw ratio on the thermal and mechanical properties of drawn polypropylene (PP) fibres. The modification of PP fibres during their production can helps to prepare PP fibres with improved properties, guarantees new opportunities for the expansion of an assortment of PP fibres in the clothing and domestic textile industries. The modified PP/CEL fibres were prepared from PP pellets and PP/cellulose masterbatch via the melt spinning technique at 260 °C followed by drawing for various draw ratios in the company Chemosvit, Fibrochem a. s. (Svit, Slovakia). Differential scanning calorimetry (DSC) was used to evaluate the thermal properties of PP fibres. The mechanical properties (tenacity and elongation at break and modulus of elasticity) and low cycle loading of modified PP fibres were also studied. The obtained experimental results of drawn PP/CEL fibres were compared with neat PP fibre prepared under the same technological conditions. Cellulose had a minimal effect on the melting temperatures of fibres and increased of the PP crystallization temperatures in comparison with the neat drawn PP fibre. The limited decrease of mechanical properties of prepared fibres were observed, but the decreases do not influence on the fibres commercial use.

Rating of the limiting state of 20K steel under low-cycle loading using the using the method of LM-hardness

In the express diagnostics of the condition of the material of the elements of the structures in operation and the materials for the manufacture of the responsible elements of the equipment, it is necessary to take into account the current damage. At the same time, non-destructive methods of control are more relevant, which allow to assess the level of scattered damage in the material and the degree of degradation of its structure of the metal. Another important factor in controlling the current state of the material is the assessment of the residual life of the structural element under operating loads. This paper presents the results of research on the application of the non-destructive method of LM-hardness in assessing the characteristics of the limiting state of the material of real structural elements operating under low-cycle load.

Planning an Experiment for Low-Cycle Fatigue under Conditions Deep Cooling

In the elements of structures with a limited resource during operation, significant cyclic stresses can occur, reaching and exceeding the yield strength; the results of an experimental study of the effect of the magnitude of preliminary plastic deformations on the strength and durability of structural alloys under low-cycle loading can be of undoubted interest for practice. The use of experimental planning methods in the study of the influence of the maximum cycle stresses and the magnitude of the preliminary permanent deformation on the low-cycle fatigue of 03Kh13AG19 chromium-manganese steel at T = 4.2 K under pulsating tension showed that these methods can be successfully used.

DEFORMABILITY OF REINFORCED CONCRETE BEAMS UNDER THE ACTION OF CYCLIC LOADING

The aim of the article is an experimental research of the influence of low-cycle sign-constant loading, as well as the most significant design factors on the deformability of reinforced concrete beam elements. In this regard, for experimental research, the authors developed a four-factor three-level Boxing plan B4. The experimental factors of the plan were varied according to the literature review, which showed that the most significant factors are the following: the value of the relative shear span a/h0, the concrete class C, the value (amount) of transverse reinforcement on the beams support sections ρsw, the level of sign-constant loading η. The samples were tested according to the scheme of a single-span beam, alternately loaded with two centre-point forces. The number of cycles of sign-constant loading was accepted as 10. According to the results of the experiment, using the COMPEX program, adequate mathematical models of the basic parameters of reinforced concrete specimens-beams deformability under the action of low-cycle sign-constant loading were derived, that reflect the influence of these factors both individually and in interaction with each other. Analyzing these models, the features of the development of tensile reinforcement and compressed concrete deformations, as well as beams deflections in the specified conditions, were established. In particular, the factors that have the greatest influence on deformations and deflections are the relative shear span and the level of low-cycle loading. Thus, with their increase, the relative deformations of tensile reinforcement increase by 51% and 52%, the relative deformations of compressed concrete by 40% and 37%, accordingly, by series. The increase of deflections is 43% and 40% with an increase of relative shear span and 38% and 12% with an increase of loading level, accordingly, by series.