Papers by Keyword: Endurance Limit

Abstract: In large-sized metal structures, various stress concentrators are often present, which affects the operation of the material. These are intermittent bonds, holes, welded joints, material defects, etc. As a result of overloads under the action of an external cyclic load on structures in the area of stress raisers, the cycle asymmetry, the level of maximum stresses and deformations increase. In this case, the determination of the limit values of the stress cycles can be performed using a diagram of the limit stress amplitudes. The paper presents an engineering method for calculating the limiting stress amplitudes and constructing Hay’s diagrams. It is based on the use of mathematical models of classical linear and structural-mechanical fracture mechanics. Analytically and by calculation, the validity of the method is shown, which consists in determining the endurance limits and limiting stress amplitudes under high-cycle loading in a wide range of variation of the cycle asymmetry coefficient for ferrite-pearlite steels with a yield strength of up to 400 MPa. Thus, a generalized calculation method has been developed for determining the endurance limits for high values of cycle asymmetry and cycle stresses. The error of the method is estimated in the area of low-cycle load. The influence of constant and average load in a cycle on the endurance limit has been investigated both for high-cycle and low-cycle loading. The proposed approach allows to construct Smith and Hay’s fatigue diagrams for the tensile region, taking into account the structural characteristics of the material and the error allowed for engineering calculations.

Abstract: In this study, a structural-deformation analysis of the process of crack development is performed, on the basis of which an engineering methodology is developed for assessing the endurance limit and resource of large-sized structures. A simple analytical dependence (correlation аналитическая зависимость) was obtained, which allows one to determine the critical size of macrocracks for ferritic-pearlitic steels without using the well-known Griffith formula. The results of calculating the cracks critical lengths of various steels depending on their yield strength are presented. The analytical dependence of the calculation of the fatigue limit for the most dangerous symmetric loading cycle according to the standard set of mechanical characteristics of ferrite-pearlite steel is presented. The obtained results make it possible to calculate the endurance limit of structural elements of marine equipment and other structures subject to cyclic loads

Abstract: In the constructions and weld seals microdefects while repeated stress lead to metal structure damage, low-cycle and high-cycle fatigue, and cracks dissemination. The calculation method of operating life estimate presupposes a microcrack development hypothetical speed application. This speed in many cases is unknown. Initial speed determination suggested in this work is based on endurance limit values design analysis when considering a combined problem for the first two sections of fatigue failure kinetic diagram.

Abstract: The paper considers the first developed algorithm of processing the data of specimen life tests based on the kinetic theory of mechanical fatigue and methods of nonparametric statistics. It makes it possible to determine the distribution density function of the material endurance limit. Implementation of the algorithm is illustrated on example of processing the data obtained in fatigue tests of steel 50 specimens.

Abstract: The paper contains results of experimental programme aimed at evaluation of fatigue resistance of laser welded 6 mm and 10 mm thick sheets of a low alloy structural S355 steel. High cycle fatigue tests were performed to verify fatigue strength of the sheets containing laser welds performed with optimised laser welding parameters. Surface of the sheets was in as-received conditions simulating real welding applications. The results are compared to each other and to fatigue S-N curve of the S355 material evaluated on quite small standard specimens. Endurance limit of the 10 mm thick laser welded sheet was by more than by 30% higher than that of the 6 mm thick sheet. In connection with this effect, all fatigue cracks in the 6 mm thick sheets initiated in the weld area unlike the 10 mm sheet, where cracks initiated both in the weld area and outside it indicating no or just minor unfavourable effect of the welding. However, fatigue strength and endurance limit of small material specimens were considerably higher even in comparison with the 10 mm welded sheets. The results are discussed considering, results of microstructure analyses, different values and distribution of residual stresses and also differences between surface conditions of the sheets and the standard specimens.

Abstract: Analysis of fracture toughness of high-strength low alloyed steel, Nionikral-70, welded joint constituencies, subjected to variable load is given in this paper, using two parameters. Crack initiation resistance is determined by testing of endurance limit and by designing of Wöhler curves. Crack propagation resistance is determined by testing of crack growth rate, and accordingly determination of Paris equation parameters. Analysis of fracture toughness is given based on testing results, comparing obtained values for base metal, weld metal and heat affected zone.

Abstract: Review of results in anodic plasma electrolytic saturation of structural steels with nitrogen and carbon in the aqueous electrolytes containing ammonia chloride and required additional constituents is presented. Proposed nitrohardening technology of the medium-carbon steels includes the short-time nitrogen saturation with the following hardening in the electrolyte. Short-time anodic carburizing possibility of the low-carbon steels with following hardening is shown. Treatment samples microhardness is 630±30 HV, their surface roughness decreases from 1.2 to 0.22 μm.

Abstract: One of functional parameters which have a big influence on the reliability of machine is the surface quality of mobile and immobile parts. The quality working ability of the surface in contact is depending of their surfaces strength. In this regard, we used vibro-mechanical treatment to increase the hardness of the contact surfaces which is considered as formed surface of plastic deformation. The formed plastic deformation surface is also a kind of micro relief surface, characterized by multitude closed and superimposed on each other traces of contact with the particles working environment cultivated surface. The quality of the surface resulting from the vibro-impact treatment, provide a sufficiently high wear parts. This paper presents experimental results of using the vibro-impact treatment on surface fatigue strength using samples of steels, aluminums and titans alloys. The objective of this study is to improve performance functional surfaces in contact by choosing optimal hardening parameters.

Abstract: The significant gear fatigue bending life variation between the single directional versus bi-directional loading had been observed over the decades, but very limit information has been revealed. To gain the comprehend understanding of this subject for practical usages, the author thus first tried to apply the conventional popular fatigue theories to evaluate the difference, but in vein. This report is to describe the course of assessment and also propose a new method to minimize the discrepancy between the theoretical and test results.

Abstract: Fatigue properties of mild steel are investigated under cyclic tension-compression and cyclic torsion loading using ultrasonic fatigue testing equipment and cycling frequency of approx. 20 kHz. Both S-N curves show a distinct change of slope at about 107 cycles, and endurance limits determined at 107 and 109 cycles differ by less than their respective standard deviations. Endurance limit shear stress determined for cyclic torsion loading is about 60% of the tension-compression endurance limit stress, and the slopes of the S-N curves are comparable. Non-propagating cracks could be found in specimens, which did not fail within 109 cycles in torsion loading endurance tests. The endurance limit can be understood as maximum stress amplitude, where possibly formed small cracks do not propagate to failure.