Reliability assessments of vibration-induced tube fatigue failure

This paper devotes to the fatigue damage analysis of a solenoid tube under vibration-induced fatigue stresses and is concerned with the estimation of tube reliability from variable amplitude stress histories in the time domain. The complete procedure of reliability assessments for the tube fatigue failure has been presented in this paper. The fatigue strength of the tube material was represented by the S-N curve with variable slope when plotted on paper with logarithmic scales and with variable statistical spread. The maximum fatigue stresses at the stress raiser were obtained from the measured nominal stresses with strain gauges and the stress concentration factor determined by finite element method. Rainflow cycle counting method was used to reduce complex variable amplitude stress history into blocks of constant amplitude loads. The fatigue damage accumulated from these constant amplitude blocks was calculated individually and summed using Miner´s rule to obtain the total accumulative fatigue damage. The reliability was estimated as a function of field service usages based on the estimated total cumulative fatigue damage. Iterative numerical calculation processes were developed to solve nonlinear equations. The success of any fatigue analysis procedure depends largely on the availability of reasonably reliable material fatigue properties. The estimation of the material fatigue strength is usually a very important and difficult part of reliability analysis for fatigue failures. If sufficient fatigue test data is available, the S-N curves modeled in this paper can be mathematically determined by maximum likelihood algorithm. However, in reality, the fatigue data often are inadequate, and this is the case in the analysis presented in this paper. This paper presents the techniques that could be useful to estimate the fatigue strength with all available sources of information when no sufficient fatigue data is available.