Fatigue Analysis with Multiple Load Cases

[PSteven]

Hello,

I can't seem to use multiple load cases in a fatigue analysis.  I want to apply two forces (loads) to one model and apply different load curves to these forces (loads) while running a fatigue analysis.  I tried several approaches using Static Stress with Linear Materials type analysis. 

1. Apply two forces in one Design Scenario and perform analysis.   Run Fatigue Wizard only one load case shows.

2. In two Design Scenarios, run one force in each and perform analysis.

a. Combine the load cases.  Run Fatigue Wizard only one load case shows.

b. Comibne the load cases and select "Use SRSS Combination Mode" option.  Run Fatigue Wizard only one load case shows.

How can I get the two forces (loads) to show up in Fatigue Wizard?

Regards,

PSteven

[S.LI]

I guess you should do it in one DS:

1.) simulate two load cases in LSS;

2.) As shown in the attachment, you can input the combination relations in the highlighted cells.

[PSteven]

Thanks S.Li for your reply. 

I was applying surface forces instead of nodal forces, and there is no load case assignment option for surface forces.  I switched to nodal forces instead and was able to assign load case numbers to my forces.

Why can`t you assign a load case # to surface forces?

Regards,

PSteven

[S.LI]

I'm not sure if we are talking about the same thing on loadcases.

As shown in the attachment, there are two loadcases for pressure loads, namely surface loads.

[PSteven]

I mean Surface Force vs Nodal Force when defining Loads and Constraints for LSS.  There is no load case assignment for a Surface Force but there is for Nodal Force, as illustrated in the attachments.   Multiple surface forces show up as one load case in the fatigue wizard.

[S.LI]

I'm not sure why the loadcase definitions for nodal and surface forces are not consistent.

My guess is no loadcase number input for surface force means surface forces always appear in all loadcases, and they could be controlled by the multiplier in parameter dialog, as shown in my previous attachment. For example, "0" means no surface load in the specific loadcase.

Hope this is helpful in your problem.

I'll check this with other people later.

[PipePakPat]

A nodal force is its own object, which also has a field that indicates for which load case it will apply.  A nodal force can only be create to act within its specified load case.

A surface force is actually a surface pressure.  It is converted to a surface pressure during the decoding process prior to the analysis.  Surface pressures can be scaled from one load case to the next, but cannot change proportions from one surface to another between different load cases of the same analysis.  For this reason, the rules associated with the surface pressure also apply to surface forces.