# Constraints Not Detected and Analysis Fails with Missing Section / Nonpositive Jacobian in PrePoMax **URL:** \ **Category:** General Questions\ **Created:** [April 8, 2026, 12:14am UTC](https://prepomax.discourse.group/t/constraints-not-detected-and-analysis-fails-with-missing-section-nonpositive-jacobian-in-prepomax/3395 "2026-04-08T00:14:08Z")\ **Posts on this page:** 6\ **Page:** 2
**Author:** ![shohei](https://avatars.discourse-cdn.com/v4/letter/s/73ab20/32.png) [@shohei](https://prepomax.discourse.group/u/shohei)\ **Post date:** [April 10, 2026, 2:46pm UTC](https://prepomax.discourse.group/t/constraints-not-detected-and-analysis-fails-with-missing-section-nonpositive-jacobian-in-prepomax/3395/21 "2026-04-10T14:46:39Z")
Thank you for your advice. I created the shell model in Rhino and exported it as a STEP file, but it is not working properly. Could you please help me identify what might be the problem? [shell.pmx](https://prepomax.discourse.group/uploads/short-url/AhQuFzZqC1I3hPClJmKCQ5EGsI0.pmx) (263.9 KB) ---
**Author:** ![FEAnalyst](https://yyz2.discourse-cdn.com/free1/user_avatar/prepomax.discourse.group/feanalyst/32/688_2.png) [@FEAnalyst](https://prepomax.discourse.group/u/FEAnalyst)\ **Post date:** [April 10, 2026, 4:02pm UTC](https://prepomax.discourse.group/t/constraints-not-detected-and-analysis-fails-with-missing-section-nonpositive-jacobian-in-prepomax/3395/22 "2026-04-10T16:02:20Z")
There are some flaws here: ![image](https://global.discourse-cdn.com/free1/uploads/prepomax/original/2X/6/6fcaa4090015518f42e7ee1ba0fb26fe9e8bdc09.png) ![image](https://global.discourse-cdn.com/free1/uploads/prepomax/original/2X/8/817347639175d5f73fb8f8524c9e881519a24918.png) You can still try meshing - it should work. Just watch out for elements with poor quality (use visual inspection + Model → Element → Element Quality). ---
**Author:** ![fatmac](https://avatars.discourse-cdn.com/v4/letter/f/b782af/32.png) [@fatmac](https://prepomax.discourse.group/u/fatmac)\ **Post date:** [April 10, 2026, 5:24pm UTC](https://prepomax.discourse.group/t/constraints-not-detected-and-analysis-fails-with-missing-section-nonpositive-jacobian-in-prepomax/3395/23 "2026-04-10T17:24:24Z")
I cleaned up ur surfaces and just meshed with gmesh and re combined with quasi structured quad. Lots of potential improvement to the mesh but it may give some ideas on one way forward ![cleansurfaces](https://global.discourse-cdn.com/free1/uploads/prepomax/original/2X/8/87470ce6ae8737a3d328f91ae1068e65efde25db.jpeg) [shellcleansurfaces.pmx](https://prepomax.discourse.group/uploads/short-url/wKuPDPRvvRzOO5qPMwZMQZG2WGr.pmx) (672.4 KB) ---
**Author:** ![shohei](https://avatars.discourse-cdn.com/v4/letter/s/73ab20/32.png) [@shohei](https://prepomax.discourse.group/u/shohei)\ **Post date:** [April 10, 2026, 5:29pm UTC](https://prepomax.discourse.group/t/constraints-not-detected-and-analysis-fails-with-missing-section-nonpositive-jacobian-in-prepomax/3395/24 "2026-04-10T17:29:03Z")
Thank you for your advice. Sorry to bother you again. [shell.ver2pmx.pmx](https://prepomax.discourse.group/uploads/short-url/hf2Zy9MAIrds1VfI3WnIqLkFt3O.pmx) (1.4 MB) I’m trying to add thickness using _Thicken Shell Mesh_, but for some reason it doesn’t work. The mesh quality is excellent, so I feel like I’m very close to solving the problem. ---
**Author:** ![FEAnalyst](https://yyz2.discourse-cdn.com/free1/user_avatar/prepomax.discourse.group/feanalyst/32/688_2.png) [@FEAnalyst](https://prepomax.discourse.group/u/FEAnalyst)\ **Post date:** [April 10, 2026, 5:39pm UTC](https://prepomax.discourse.group/t/constraints-not-detected-and-analysis-fails-with-missing-section-nonpositive-jacobian-in-prepomax/3395/25 "2026-04-10T17:39:50Z")
Even in Abaqus offset (thickening) fails in the problematic region discussed at the beginning: ![image](https://global.discourse-cdn.com/free1/uploads/prepomax/original/2X/b/bbfc4c2a5091ef503de055d9e140ae1e22600220.png) Negative jacobians still there: ![image](https://global.discourse-cdn.com/free1/uploads/prepomax/original/2X/6/65430d990520d697c4a6a09e52f963d0c1626a7d.png) You should eliminate them first (by correcting the issues with the geometry and changing the meshing algorithm if needed). ---
**Author:** ![FEAnalyst](https://yyz2.discourse-cdn.com/free1/user_avatar/prepomax.discourse.group/feanalyst/32/688_2.png) [@FEAnalyst](https://prepomax.discourse.group/u/FEAnalyst)\ **Post date:** [April 10, 2026, 5:43pm UTC](https://prepomax.discourse.group/t/constraints-not-detected-and-analysis-fails-with-missing-section-nonpositive-jacobian-in-prepomax/3395/26 "2026-04-10T17:43:09Z")
Try with the cleaned (simplified) model: [shellcleansurfaces mod.pmx](https://prepomax.discourse.group/uploads/short-url/t8kmQmGsDnqxgCf0ntMWr2RqwB7.pmx) (2.0 MB) [Previous page](https://prepomax.discourse.group/t/constraints-not-detected-and-analysis-fails-with-missing-section-nonpositive-jacobian-in-prepomax/3395.md?page=1)