Learning Forum

[nm0770]

I would like to perform a transient simulation of carbon fiber components. The two sandwich plates with attached stiffening Rips are connected using bonded MPC contacts.

The boundary conditions are as follows: on the front side, a remote displacement is fully constrained in all directions and on the rear side, it is constrained in such a way that expansion in the length direction is still possible.The plates rest on 8 points on the ground. 
These points are constrained in the Z direction using remote boundary conditions. (Also tried with fixed fixed configuration same result.)

I applied measured accelerations to each of the 8 boundary conditions on the ground (6000 steps measured at 10 000 Hz -> as I understand, this results in a time step of 1/10 000 (Mesurement Time was 0.6 seconds)).
The mesh used has already been applied in static simulations and has converged in those cases.I am outputting the equivalent stresses and obtaining a maximum of approximately 6000 MPa, which I consider unrealistic. 
The maximum stresses occur in the area of the support points on the ground.

My main idea is to perform a static analysis that includes gravitational acceleration, followed by a indeoendent transient simulation.
I would then sum the maximum stresses from both analyses (at a critical Element of the geometry). If the sum of these stresses is below the fatigue limit (~450 MPa), can I assume that the structure is durable and will not experience fatigue failure? Is this approach valid for a preliminary  assesment, or are there other factors I should consider?

Can anyone provide me with tips on how to proceed, or possible Problems?

[danielshaw]

If you are sure that the alternating stress is below the endurance limit, you can assume infinite life. Based on the S-N curve that you showed, this material appears to have a well-defined endurance limit at about 450 MPa, but in general 

1. Not all materials have a well-defined endurance limit.  So, if the part experiences very high cycle fatigue, you may not be able to assume infinite life,
2. 450 MPa seems like a relatively high endurance limit.

[dlooman]

The gravity step can be solved with time-integration off in the Analysis Settings details to avoid any dynamic effect from gravity.

[dlooman]

The gravity step can be solved with time-integration off in the Analysis Settings details to avoid any dynamic effect from gravity.

[nm0770]

@danielshaw: Thank you for your feedback. The Wöhler diagram is from the materials I actually use.

@dlooman: Even if i would place a Point Mass on the Panels?

Is it possible that this behaviour can be caused by a sensor offset. When integrating the offset the values are increasing over time?