# Contact doesn't converge with no load **URL:** \ **Category:** Uncategorized\ **Created:** [September 22, 2020, 10:35am UTC](https://calculix.discourse.group/t/contact-doesnt-converge-with-no-load/200 "2020-09-22T10:35:40Z")\ **Posts on this page:** 13\ **Page:** 1
**Author:** ![vicmw](https://avatars.discourse-cdn.com/v4/letter/v/6de8d8/32.png) [@vicmw](https://calculix.discourse.group/u/vicmw)\ **Post date:** [September 22, 2020, 10:35am UTC](https://calculix.discourse.group/t/contact-doesnt-converge-with-no-load/200/1 "2020-09-22T10:35:40Z")
Contact with no load and all parts fully constrained sometimes doesn’t converge and ends with \*_ERROR: increment size smaller than minimum_. The true solution might be all zero but nonetheless, is there a way to make that work without changing the meaning of the model? [https://mecway.com/zero.inp](https://mecway.com/zero.inp) I’m wondering if CCX uses a convergence criterion that requires some force or displacement to exist or change. I can’t see anything in \*CONTROLS that looks helpful. ---
**Author:** ![konstantin](https://avatars.discourse-cdn.com/v4/letter/k/858c86/32.png) [@konstantin](https://calculix.discourse.group/u/konstantin)\ **Post date:** [September 22, 2020, 2:54pm UTC](https://calculix.discourse.group/t/contact-doesnt-converge-with-no-load/200/2 "2020-09-22T14:54:00Z")
Hello, Could you clarify - how many iterations(calculation time) to the error. Also, not clear this moment: ```auto *STATIC 1,1,0,0 ``` please try ```auto *STATIC 1,1 ``` ---
**Author:** ![vicmw](https://avatars.discourse-cdn.com/v4/letter/v/6de8d8/32.png) [@vicmw](https://calculix.discourse.group/u/vicmw)\ **Post date:** [September 22, 2020, 7:31pm UTC](https://calculix.discourse.group/t/contact-doesnt-converge-with-no-load/200/3 "2020-09-22T19:31:37Z")
It seems to have taken about 1000 iterations or a couple of minutes to eventually fail. Same happens with 1,1 instead of 1,1,0,0. ---
**Author:** ![konstantin](https://avatars.discourse-cdn.com/v4/letter/k/858c86/32.png) [@konstantin](https://calculix.discourse.group/u/konstantin)\ **Post date:** [September 22, 2020, 9:15pm UTC](https://calculix.discourse.group/t/contact-doesnt-converge-with-no-load/200/4 "2020-09-22T21:15:00Z")
Excuse me but I have not enough experience for that task. ![contact](https://global.discourse-cdn.com/free1/uploads/calculix/original/1X/c06372ab0b105b91031e627eb1f6517374d10bda.png) ---
**Author:** ![dhondt](https://avatars.discourse-cdn.com/v4/letter/d/7bcc69/32.png) [@dhondt](https://calculix.discourse.group/u/dhondt)\ **Post date:** [September 23, 2020, 6:57pm UTC](https://calculix.discourse.group/t/contact-doesnt-converge-with-no-load/200/5 "2020-09-23T18:57:01Z")
Hi, without load the problem may not be well defined. Looking at the \*.sta, \*.cvg and the displacements may help to understand the problem. Guido ---
**Author:** ![vicmw](https://avatars.discourse-cdn.com/v4/letter/v/6de8d8/32.png) [@vicmw](https://calculix.discourse.group/u/vicmw)\ **Post date:** [September 24, 2020, 10:15pm UTC](https://calculix.discourse.group/t/contact-doesnt-converge-with-no-load/200/6 "2020-09-24T22:15:42Z")
Thanks Guido. Physically, it seems to be valid. Just 2 cubes touching each other with contact and both fixed in place. It also solves OK if you change one of the \*BOUNDARY nodes from 0 to 0.01 (and move it to the \*STEP section). So it feels like this should be able to solve but perhaps mathematically it’s not well defined. Actually, this is just a workaround for another issue which maybe you can shed some light on. Contact stiffness changes throughout the first step and that can give unexpected results in models where the stiffness value is important during the step, not just at the end. I’m not sure if that’s a bug or intentional? I’m trying to work around it by putting this kind of “do nothing” first step before the real step that has loads. ---
**Author:** ![dhondt](https://avatars.discourse-cdn.com/v4/letter/d/7bcc69/32.png) [@dhondt](https://calculix.discourse.group/u/dhondt)\ **Post date:** [October 16, 2020, 11:00am UTC](https://calculix.discourse.group/t/contact-doesnt-converge-with-no-load/200/7 "2020-10-16T11:00:51Z")
Hi, the spring stiffness constants are only changed (decreased) if there are problems with convergence. As far as I remember, this is only done at the start of the iterations, i.e. in the final iterations the user-defined value is restored and the result at the end of the increment should be fine. A step consists of one or more increments, so you can can get correct results within a static step by using more increments. Best Greetings, Guido ---
**Author:** ![vicmw](https://avatars.discourse-cdn.com/v4/letter/v/6de8d8/32.png) [@vicmw](https://calculix.discourse.group/u/vicmw)\ **Post date:** [October 16, 2020, 7:18pm UTC](https://calculix.discourse.group/t/contact-doesnt-converge-with-no-load/200/8 "2020-10-16T19:18:58Z")
It doesn’t seem to behave that way. Here’s an example where the result is correct at the first and last increment of a step but, in the middle, it behaves as if the contact stiffness is reduced. **Inp file:** ``` *NODE 1,0,0,0 2,1,0,0 3,1,1,0 4,0,1,0 5,0,0,1 6,1,0,1 7,1,1,1 8,0,1,1 9,0,1,0 10,1,1,0 11,1,2,0 12,0,2,0 13,0,1,1 14,1,1,1 15,1,2,1 16,0,2,1 *ELEMENT,TYPE=C3D8,ELSET=Default 1,1,2,3,4,5,6,7,8 2,9,10,11,12,13,14,15,16 *MATERIAL,NAME=Material *ELASTIC,TYPE=ISOTROPIC 200000000000,0 *SOLID SECTION,ELSET=Default,MATERIAL=Material *BOUNDARY 1,1,3,0 2,1,3,0 5,1,3,0 6,1,3,0 11,1,,0 11,3,,0 12,1,,0 12,3,,0 15,1,,0 15,3,,0 16,1,,0 16,3,,0 *SURFACE,NAME=contact_faces(2) 2,S3 *SURFACE,NAME=contact_faces 1,S5 *CONTACT PAIR,INTERACTION=SI_1,TYPE=SURFACE TO SURFACE contact_faces(2),contact_faces *SURFACE INTERACTION,NAME=SI_1 *SURFACE BEHAVIOR,PRESSURE-OVERCLOSURE=LINEAR 100000 *STEP,NLGEOM=YES,AMPLITUDE=STEP *STATIC,DIRECT 0.5,2 *CLOAD 11,2,-2500 12,2,-2500 15,2,-2500 16,2,-2500 *NODE FILE U *END STEP ``` **Solution:** Time, displacement in y at node 9: 0.5, -0.1 (correct) 1.0, -0.167 1.5, -0.183 2.0, -0.1 (correct) ---
**Author:** ![dhondt](https://avatars.discourse-cdn.com/v4/letter/d/7bcc69/32.png) [@dhondt](https://calculix.discourse.group/u/dhondt)\ **Post date:** [November 1, 2020, 5:42pm UTC](https://calculix.discourse.group/t/contact-doesnt-converge-with-no-load/200/9 "2020-11-01T17:42:28Z")
Hi, I could reproduce the problem. I will have a look at it. Best Greeting and thanks, Guido ---
**Author:** ![vicmw](https://avatars.discourse-cdn.com/v4/letter/v/6de8d8/32.png) [@vicmw](https://calculix.discourse.group/u/vicmw)\ **Post date:** [November 1, 2020, 8:01pm UTC](https://calculix.discourse.group/t/contact-doesnt-converge-with-no-load/200/10 "2020-11-01T20:01:48Z")
Thanks a lot Guido! This has been a real tricky one. ---
**Author:** ![dhondt](https://avatars.discourse-cdn.com/v4/letter/d/7bcc69/32.png) [@dhondt](https://calculix.discourse.group/u/dhondt)\ **Post date:** [December 25, 2020, 12:54pm UTC](https://calculix.discourse.group/t/contact-doesnt-converge-with-no-load/200/11 "2020-12-25T12:54:45Z")
Hi, now I know the exact line causing the effect: the line running springarea(2,igauss)=clear/(1.d0-theta) in gencontelem\_f2f.f (about line 578). I am not sure, whether I want to limit its execution to iinc=1 (the first increment). It seems to help convergence in some examples. I will have a closer look at the start of the new year. best greetings, Guido ---
**Author:** ![vicmw](https://avatars.discourse-cdn.com/v4/letter/v/6de8d8/32.png) [@vicmw](https://calculix.discourse.group/u/vicmw)\ **Post date:** [December 26, 2020, 4:34am UTC](https://calculix.discourse.group/t/contact-doesnt-converge-with-no-load/200/12 "2020-12-26T04:34:18Z")
Fantastic. Thanks for figuring this out! Maybe it can at least be an option? ---
**Author:** ![oxygenliu](https://yyz2.discourse-cdn.com/free1/user_avatar/calculix.discourse.group/oxygenliu/32/834_2.png) [@oxygenliu](https://calculix.discourse.group/u/oxygenliu)\ **Post date:** [May 31, 2022, 11:15am UTC](https://calculix.discourse.group/t/contact-doesnt-converge-with-no-load/200/13 "2022-05-31T11:15:31Z")
Similar issue still exists in 2.19 release.