Bogdan Ligaj

Comparative analysis of fatigue life calculation methods of C45 steel in conditions of variable amplitude loads in the low- and high-cycle fatigue ranges

ABSTRACT In the paper [1] assumptions for fatigue life calculations in the low- and high- cycle fatigue ranges of metal alloys, have been formulated. Three calculation methods: that expressed in stresses, strains and a mixed (stress-strain) one, have been there presented. In this paper results of fatigue life calculations of C45 steel performed in line with the above mentioned methods have been described, and their comparative analysis has been made. The calculations have been carried out for two-level and multi-level load programs of the varying parameters: maximum load within a load program and different values of spectrum filling factor. The comparative analysis of the results of fatigue life calculations in compliance with the above mentioned methods makes it possible to assess differences in the results and depenendence of the differences on values of load program parameters.

Experimental verification of two-parametic models of fatigue characteristics by using the tests of S55J0 steel as an example

Experimental verification of two-parametic models of fatigue characteristics by using the tests of S55J0 steel as an example This paper presents experimental verification of models of two-parametric fatigue characteristics N(σa, σm) on the basis of results of fatigue tests of specimens made of S355J0 steel. On the basis of comparative analysis of results of computations and fatigue life tests of the specimens, performed under constant amplitude sinusoidal loads of different cycle asymmetry ratios, the most versatile models were distinguished. The tests in question were carried out in the high-cycle fatigue range.

Effect of the Exponent in the Description of Wöhler Fatigue Diagram on the Results of Calculations of Fatigue Life

The paper presents issues concerning the calculation of fatigue life under programmable load. Aim of this study is to analyze the sensitivity of the calculation method based on the variation of the exponent m used in the description of Wöhler fatigue chart. Arbitrary adoption of the recommendations contained in the FITNET procedures in fatigue module (the path 2b for normal stress) for the value of the exponent m = 5 can lead to significant differences in the calculation of fatigue life. This is due to the fact that the value of the exponent m obtained by experimental test of constructional steels and alloy is in within wide limits of variation from 3 to 30 with mean value in excess of 5. Based on the results of the fatigue life calculations for steel C45 and X5CrNi18-10 (1.4301) is noticeable a significant impact of exponent m on the fatigue life. This effect depends on the strain value in the load spectrum and increases with the increase of the maximum amplitude of stresses in the program Sa max.

Description of Cyclic Properties of Steel in Variability Conditions of Mean Values and Amplitudes of Loading Cycles

The work includes description of determination of two-parametric fatigue characteristics (TFC) on the base of experimental test results and description of mathematical models of these characteristics. Mathematical models were verified in C45 and 41Cr4 tests that are essentially different from the point of view of mechanical properties. On the base of analysis of verification test results there were determined ranges of applications of TFC models described in the work.

Problems of Equivalent Load Amplitude in Fatigue Life Calculations

Abstract This paper presents issues of fatigue life calculations in operational load conditions. The assumed runs were so processed as to get a set of sinusoidal cycles by using the following methods: full cycles counting method and rainflow counting method. On the basis of such sets of cycles of the varying parameters Sm i and Sa i were prepared block load spectra of equivalent amplitudes Saz , obtained with the use of an original method of these authors, in which two-parameter fatigue characteristics were applied. The work resulted in comparison of fatigue life results for load spectra determined by using the assumed cycles counting methods and the assumed two-parameter fatigue characteristics: the model IM, model II, model III and model IV, respectively.

Analysis of a Simplified Method for Determining Fatigue Charts ΔS-N on the Example of Welded and Soldered Connectors

Abstract The paper describes the method of determining fatigue charts ΔS-N, which is particularly useful in the description of fatigue properties of welded and soldered joints. This method is based on IIW (International Institute of Welding) guidelines and FITNET procedures. This method makes it possible to design ΔS-N charts in a probabilistic approach, which is important in the reliability analysis of structural elements. The work also contains examples of charts resulting from the development of test results for selected welded and soldered joints.

Cumulative Energy of Fatigue Cracking under Variable Amplitude Loading on the Example of C45 Steel

The article presents the results of low and high cycle fatigue tests of C45 steel for constant and variable amplitude loading in the stress and energy based approach. Values of the cumulated energy of plastic strain ΣΔWpl for programmed loading (at selected levels of stress amplitude Sa) were found to meet reliability criteria for results obtained in the condition of constant amplitude loading. (R= -1). It was also found that the results obtained in the energy based approach are characterised by a smaller range of relative difference changes (δ(N) = 0,77 to 1,17) than the results obtained in the stress based approach (δ(N) = 0,15 to 1,94). At stress level Sa = 428 MPa, (corresponding to yield point of C45 steel) value δ(N) is similar for the stress based approach and energy based approach.

Effect of Stress Ratio on the Cumulative Value of Energy Dissipation

Aim of this work is to analyze the stress-strain loops recorded during the study c45 steel under programmable stress for constant amplitude loads of the stress ratio: R = -2.0, R = -1.0 and R = -0.5. The results presented in the paper of cumulative plastic strain energy show that the nature of the relationship ΣΔWpl-N for loads of different values of stress ratio R is similar. Course of changes of relationship ΣΔWpl-N for loads with R = -0.5 and R = -2.0 shows a similar nature and range of changes in the value ΣΔWpl, despite significantly different parameters of load.

The Comparison of Cyclic Properties of X5CrNi18-10 Steel in the Range of Low-Cycle Fatigue in Conditions of Stress and Strain Control

Service loading of constructional elements are caused by mass forces (dynamic loading) or displacement (kinematic loading). There are methods of estimation of cyclic properties of material based on stress or strain range control corresponding to the mentioned loading types. The paper includes an analysis of differences between diagrams determined in controlled stress and strain conditions as well as relations among: Wöhler curves (W), Manson-Coffin curves (M-C) and Ramberg-Osgood curves (R-O). Basing on the performed analysis there have been formulated conclusions that generally are on connecting loading character with conditions of determination of atigue curves.

Fatigue Life Calculation in Conditions of Wide Spectrum Random Loadings – The Experimental Verification of a Calculation Algorithm on the Example of 41Cr4 Steel

Precision of fatigue life calculations of structural elements in programmed loading conditions is connected with proper elaboration of loading spectrum and assumption of a proper fatigue characteristic. On the base of literature data and own research there has been elaborated an algorithm for fatigue life calculations in random loading conditions with wide spectrum. Calculations were performed with the usage of chosen mathematical models of two-parametric fatigue characteristics. Results calculated with accordance to the described procedure were validated with experimental test results of specimens made of 41Cr4 steel with a method of programmed fatigue life tests.