{"id":461857,"date":"2026-07-25T23:14:51","date_gmt":"2026-07-25T23:14:51","guid":{"rendered":"https:\/\/innovationspace.ansys.com\/forum\/forums\/topic\/spg-background-mesh-inversion-at-twist-extrusion-die-exit-elform-47-mat_cscm\/"},"modified":"2026-07-25T23:14:51","modified_gmt":"2026-07-25T23:14:51","slug":"spg-background-mesh-inversion-at-twist-extrusion-die-exit-elform-47-mat_cscm","status":"publish","type":"topic","link":"https:\/\/innovationspace.ansys.com\/forum\/forums\/topic\/spg-background-mesh-inversion-at-twist-extrusion-die-exit-elform-47-mat_cscm\/","title":{"rendered":"SPG background mesh inversion at twist extrusion die exit (ELFORM 47, MAT_CSCM)"},"content":{"rendered":"

<div>Hello,<\/div><div> <\/div><div>I am simulating twist extrusion (helical\/ECAP-type die) of Ti powder using SPG<\/div><div>(ELFORM 47) with LS-DYNA R16.1.1 MPP double precision, 12 cores.<\/div><div> <\/div><div>MODEL<\/div><div>- Billet: SPG solid, MAT_CSCM (cap model), relative density 0.70,<\/div><div>  rate effects disabled, softening disabled.<\/div><div>- Die: rigid shells; contact via *DEFINE_SPG_TO_SURFACE_COUPLING (SPGP=powder,<\/div><div>  SURF=die segment set).<\/div><div>- Helical section: cross-section rotates ~64 deg over 18 mm of channel length.<\/div><div>- Loading (both tried): (a) pressure on null-shell skins on the billet end faces,<\/div><div>  (b) prescribed velocity through a soft elastic spacer merged to the SPG nodes.<\/div><div>- Back pressure 200 MPa on the exit face in both cases.<\/div><div> <\/div><div>PROBLEM<\/div><div>The run is perfectly healthy for a long time: time step stays constant<\/div><div>(~1.2e-7 s), KE\/IE ~ 0, internal energy grows smoothly, no erosion at all.<\/div><div>Then, within less than 1 ms, an entire transverse layer of background elements<\/div><div>inverts (negative volume), and the job dies with either<\/div><div> <\/div><div>    “gt ispgnumextn”<\/div><div> <\/div><div>or a hard crash:<\/div><div> <\/div><div>    forrtl: severe (157): Program Exception – access violation<\/div><div>    SPGBSORT (spgbsort.f) \/ SPGMGBUCKET_SMP \/ SPGFINOD3 \/ SPGFINOD \/ SPGSMOOTH<\/div><div> <\/div><div>Checking the deleted nodes, the first failures are all from one original element<\/div><div>row, located at the EXIT of the helical section (where the cross-section stops<\/div><div>rotating and the material “unwinds”), and they are spread across the whole<\/div><div>cross-section (r = 2…11 mm, channel radius 14.7 mm) – i.e. in the core, NOT at<\/div><div>the die wall.<\/div><div> <\/div><div>SETTINGS ALREADY TRIED (none of them removes the failure, they only delay it)<\/div><div>- DX = DY = DZ: 1.5 \/ 1.6 \/ 1.8 \/ 2.0<\/div><div>- KERNEL = 1 (Eulerian); ISPLINE = 0<\/div><div>- SMSTEP: 15 \/ 5 \/ 3<\/div><div>- MSC = 1 together with ITB = 3<\/div><div>- IDAM = 1, STRETCH = 1e10 and 6<\/div><div>- PDAMP = -0.005 \/ -0.01, BOXID active<\/div><div>- Coupling SFP = 0.5 \/ 0.7, THK = 0.25 \/ 0.5, friction = 0 \/ 0.05 \/ 0.1<\/div><div>- CONTROL_TIMESTEP: TSSFAC 0.5, DT2MS = 0, ERODE = 1; DTMIN = 0.1<\/div><div>- Mesh size of the billet: 2.8 mm \/ 1.24 mm \/ 1.0 mm<\/div><div>- Drive speed 100 mm\/s and 50 mm\/s<\/div><div> <\/div><div>QUESTIONS<\/div><div>1. Are there recommended SPG settings for large TORSIONAL\/shear deformation of<\/div><div>   this kind (background mesh must follow a ~64 deg rotation)? Any guidance on<\/div><div>   DX and SMSTEP for this class of problem?<\/div><div>2. What controls the internal array dimension behind “gt ispgnumextn”, and can<\/div><div>   it be increased (keyword\/parameter\/memory)?<\/div><div>3. For a helical rigid wall, is *DEFINE_SPG_TO_SURFACE_COUPLING the recommended<\/div><div>   approach, or is *CONTACT_AUTOMATIC_NODES_TO_SURFACE preferred?<\/div><div>4. Is *CONTROL_SPG required or recommended here? I have not used it so far.<\/div><div>5. For powder consolidation I do NOT want material separation. What are the<\/div><div>   recommended IDAM \/ STRETCH \/ FS settings for a “no bond failure” setup?<\/div><div> <\/div><div>Any advice would be greatly appreciated.<\/div><div>\"Full<\/div><div>\"Full<\/div><div>\"Full<\/div><\/p>\n","protected":false},"template":"","class_list":["post-461857","topic","type-topic","status-publish","hentry"],"aioseo_notices":[],"aioseo_head":"\n\t\t\n\t\n\t\n\t\n\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t