MEAN STRESS VALUE IN SPECTRAL METHOD FOR THE DETERMINATION OF FATIGUE LIFE

Abstract: The paper presents a proposal of account of mean stress value in the process of the determination of the fatigue life, using the spectral method. The existing approaches have been described and some chosen stress models used to take into account the influ-ence of the mean value in the process of the determination of fatigue life have been introduced. Those models, referring to their linear character, have been used to determine the power spectral density function (PSD) of the transformed stress taking into account the mean value. The method introduced by the authors allows a wide usage of many formulas used to predict the fatigue life by means of the spec-tral method. Key words: Mean Stress, Fatigue Life, Spectral Method 1. INTRODUCTION Structures and machine components being subjected to vari-able loads require constant monitoring during operation due to the emerging phenomenon of material fatigue. Also, when designing new constructions or modification of nodes of machine elements, it is required to check their load capacity and fatigue life before finally being put into operation. The verifications of this type are performed in laboratories carrying out the strength of materials fatigue tests, or if it is not possible because of e.g. the size and cost, then calculations are made only with a view to the best possible estimate of fatigue life. The way of calculations depends of the character of the load. In the case of load-amplitude with no mean value, the expected number of cycles to fatigue crack initiation can be read directly from graphs of fatigue, for example, Wohler curve. If there are evident mean values in the course, then their effect must be taken into account on fatigue. For this purpose you can use the charts to take account of the influence of the mean load, for example, Smith chart or Wohler graphs drawn up for various cycle asymmetry coefficients