Damage accumulation

Classification of stress collective
Classification of stress collective

Regardless of the fatigue concept comes with the respective specific WÖHLER lines used, the fatigue analysis must be performed in conjunction with appropriate damage accumulation hypotheses. For this, the actual stress range is simplistic in a manageable set of n blocks, each with a constant stress range is classified (see figure at right). The lifetime is then predicted computationally by means of accumulation of damage components from the individual load cycles each voltage collective. A common hypothesis for the damage accumulation is the rule according to Palmgren and MINER

  

 

 

 

This rule is based on the assumption that an oscillating stress each closed hysteresis loop caused a partial degradation of the material in the considered notch root. With the number of cycles ni at a stress horizon Part the damage is ni / Ni determined, the linear in the sum of the total damage D is received. Here Ni is that of the stress range associated with fatigue life for the WÖHLER line of the relevant construction details. The failure due to fatigue stress occurs by definition a simplified under the assumption that if the sum of the partial damage ni / Ni reaches the value D = 1:

Should a failure be prevented before a certain useful life, the loss amount must be D<1. The partial safety factors on the resistance side and other influences (e.g. plate thickness effect) are to be considered. At non-periodic stress can swing widths that are smaller than the threshold value of the fatigue strength, cause damage. In this case the horizontal threshold value is omitted.