Hi everyone, I hope someone got an idea to help me solving a principal problem in Abaqus/Explicit VUELs. I got a ductile damage material model, that i already implemented successfully in Abaqus/Standard. In a staggered approach, I decoupled the problem into a pure displacement problem, where the stress algorithm for the plasticity equations was implemented in an UMAT and a pure damage problem, where the governing equations for the damage (and its driving force) are solved with FEM and the weak form is implemented in an UEL. For a plane stress problem I use the DOFs 4 and 5 for the damage and its driving force in the user element and CPS4 elements for the displacement problem.

To speed up runtimes for large problems I integrated the equations explicitly and want to use mass scaling to solve this static problem with Abaqus/Explicit. The displacement problem and the plasticity equations are implemented in a VUMAT and (for constant damage) yield the same results as the implicit calculation with the UMAT. The implementation of the VUEL is quite challanging. The damage and its driving force are given as evolution equations and therefore I could easily provide a "capacity" matrix and rhs for the system of equations C*dot(u)=F in exactly the same way, how Abaqus/Explicit solves heat transfer equations. Now the funny things start:

1. Abaqus/Explicit does not allow you to use multiple temperature degrees of freedom in a "Fully Coupled Thermal-Stress Analysis", an error occures if you try to use DOFs 11 and 12.
2. When trying to use DOFs 4 and 5 an error occurs, telling me the mass matrix is unsymmetric. When i print the assembled "mass" matrix inside the VUEL its clearly a diagonal (and therefore symmetric) matrix. I then tried to use DOFs 4 and 11 and Abaqus finally runs but the results are not correct. Im not sure if the false results are coming from errors in the VUEL or from a "false" use of DOFs.
3. In my "capacity" matrix i got a non-constant term coming from the staggered approach that changes every increment and depends on the results of the VUMAT. Because the mass and capacity matrices are only calculated once at the start of a job I tried to move this term to the right hand side and added an additional state variable.

Its really hard to debug my own errors in the VUEL code when the setting Abaqus provides is not quite fitting for this problem. Can anyone provide tips on how to possibly overcome these difficulties?