cbk14
Mechanical
- Jun 13, 2014
- 17
Suppose I have a FEM with two bodies. One body represents the washer underneath a bolt head; it is circular and small relative to the other body, which is a large, thick, flat plate. The washer is bonded to the plate in a circular region with compatible meshes. The edge of the bonded contact is a discontinuity, and of course the stresses peak at this discontinuity.
In one example, the peak stress observed at the discontinuity exceeds the material allowable. Looking at the stress plot, it appears to me that the peak stress is artificial, due to the discontinuity caused by the boundary condition. I would also argue that if this stress were real, localized yielding would occur and would redistribute the stress until the system was in equilibrium. I don't think "localized yielding will occur" is going to fly with my customer, regardless of whether that is accurate.
One method proposed to resolve this is to use a more realistic contact, such as a spring element with friction between the bodies and no penetration. This of course results in a non-linear model, which takes far longer to solve. Suppose for this discussion that non-linear analyses are prohibitively expensive to run.
A simpler method to resolve the peak stress that has been proposed in our group is to average the stress values in a small area around the peak stress. For example, if the observed peak is at a node, we take the stress values from the surrounding nodes at a distance of 3 element edges away. I would argue this is fairly well supported by the Saint-Venant principle, where stresses are equivalent at sufficient distances from the contact, as long as the load is statically equivalent. This raises the question how far away is sufficient? How far away is too far? Or am I completely misunderstanding the Saint-Venant principle and applying it inappropriately?
Now my core questions for you all are:
1) is it reasonable to average von Mises stresses at nodes to reduce the artificially high stresses observed at boundary condition discontinuities?
2) if the above is reasonable, is an arithmetic mean the correct measure? or would RMS, geometric mean, or something else be more appropriate?
3) are there alternative methods for overcoming artificially high stresses at boundary condition discontinuities (aside from running non-linear studies)?
Thank you.
In one example, the peak stress observed at the discontinuity exceeds the material allowable. Looking at the stress plot, it appears to me that the peak stress is artificial, due to the discontinuity caused by the boundary condition. I would also argue that if this stress were real, localized yielding would occur and would redistribute the stress until the system was in equilibrium. I don't think "localized yielding will occur" is going to fly with my customer, regardless of whether that is accurate.
One method proposed to resolve this is to use a more realistic contact, such as a spring element with friction between the bodies and no penetration. This of course results in a non-linear model, which takes far longer to solve. Suppose for this discussion that non-linear analyses are prohibitively expensive to run.
A simpler method to resolve the peak stress that has been proposed in our group is to average the stress values in a small area around the peak stress. For example, if the observed peak is at a node, we take the stress values from the surrounding nodes at a distance of 3 element edges away. I would argue this is fairly well supported by the Saint-Venant principle, where stresses are equivalent at sufficient distances from the contact, as long as the load is statically equivalent. This raises the question how far away is sufficient? How far away is too far? Or am I completely misunderstanding the Saint-Venant principle and applying it inappropriately?
Now my core questions for you all are:
1) is it reasonable to average von Mises stresses at nodes to reduce the artificially high stresses observed at boundary condition discontinuities?
2) if the above is reasonable, is an arithmetic mean the correct measure? or would RMS, geometric mean, or something else be more appropriate?
3) are there alternative methods for overcoming artificially high stresses at boundary condition discontinuities (aside from running non-linear studies)?
Thank you.