Abstract

Fatigue calculations require complex stress states to be transformed into an equivalent variable, like von-Mises stress, Principal stress, or some other stress variable. von-Mises is often proposed because it has the ability to account for stress biaxiality (or triaxiality) within the material, but it cannot account for rotation of stress tensor directions in time (or frequency). Principal stresses can be used as long as it is acceptable to ?follow? the movement of the principal stress direction, thus ignoring the movement with time. If one is working in the frequency domain, the calculation of Principal stress can only be done if the phase variation for each frequency component is removed by making the assumption that the worst phase angle will be used. In other words, we pick the highest stress value as a function of phase, for each frequency. Von-Mises is also a complicated calculation in the frequency domain because the classic algorithm (for von-Mises) was developed for non-complex stresses. For complex stress tensors a method proposed by Segalman has become widely adopted. All of these methods are well established in both time and frequency domain and acceptable as long as stresses are not rotating too much in space. One modern alternative for highly fluctuating stress vector conditions, which indirectly creates an equivalent normal stress, is the so-called Critical Plane Approach. It is based on scanning all the layers (slices) within the material, transforming the stress state to obtain the normal stress and damage on that layer, rotating the plane (in 3 dimensions) and then picking the slice (plane) that gives the highest damage. This can become a laborious computational process when implemented in the time domain and until recently the time domain was the only option. However, modern software?s such as CAEfatigue have made this a practical option for the time domain. Further, the approach has now been extended to work in the frequency domain and this paper will provide examples of its use for frequency domain fatigue analysis on automotive components. There are extensions of the Critical Plane Approach that have been proposed for elastomers (rubbers) and this paper will also examine these approaches.