Modal Transient & Superposition Fatigue Setup - SPCD, TLOAD, DLOAD

DS-10

I'm running two simulations: (1) Modal transient fatigue in OptiStruct and (2) Modal superposition fatigue using OptiStruct + HyperLife/nCode. Loading consists of four Z-axis acceleration time histories applied at front and rear suspension mounting plates (two each) using SPCD (DOF3 = 9810). I’ve defined 4 SPCDs, 4 SPCs (fully fixed/DOF3 fixed), and grouped the SPCs via SPCADD. For loading, I’ve created 4 TLOAD1 entries (TYPE=ACCE, EXCITEID=SPCD), all combined into a single DLOAD. The modal transient subcase uses SPC: SPCADD, METHOD: EIGRL/EIGRA, TSTEP, SDAMPING, and DLOAD. For fatigue, I use FATPARAM, PFAT, etc., while for modal superposition fatigue, I extract SDISP via MTRAN and modal stresses via a MODES subcase (outputting STRESS and DISPLACEMENT).
My questions:

1. Is it valid to combine all 4 TLOAD1 into a single DLOAD, or should I run 4 separate MTRAN load steps?
2. SPCs to be fully fixed or only DOF3 fixed?
3. Will using SPCADD (with all 4 SPCs) over-constrain the model in MTRAN or MODES subcases? If so, should I use inertia relief, and how to properly define for MTRAN with 4 different loading points?

solon

• Hello,

1) It depends on the use case, both are correct approaches. If you want to have more versatility, scale loadings, or make certain combinations for fatigue loading then you could run 4 separate transient analysis.

2)If you're only interested in excitation along the Z-axis, then it makes sense to constrain all degrees of freedom to isolate that effect.

3)You can also try with inertia relief and compare it with fully constrained model, to determine which results make more sense to your use case. You must at least constraint the DOF that you apply the SPCD on.