I was curious to see if VUMATs written in the format instructed in Abaqus users manual return the same results as native models for a specific loading/unloading scenario. So I developed a simple 2D elastic beam model with uniaxial compressive loading/unloading and fixed end. I used a simple isotropic elastic VUMAT to define the material behaviour. But the element started rotating/swinging during the unloading despite having a fixed end. Then I used the native elastic material model in Abaqus and the simulation was completed as expected without rotation. I have attached the inp file and the vumat. There are two vumats. One of them is for 2D beam and the other one is general for all types of elements. Both caused the element rotation. Any thoughts on what could be wrong in the vumat?

I have also attached a screenshot of the axial stress which shows two peaks in the unloading step instead of remaining constant.

​​​​​​​Abaqus ​​​​​​​