Hi All,

My Ph.D. dissertation is heavily involved in using Abaqus FEA to build models that can predict the fit/performance of space suit components. My specific project involves modeling the joint torque required to bend a pressurized elbow joint. Over the past 6 months, I have been trying to resolve element distortion errors that have prevented me from getting useful data. I could really use some help from someone that has had experience with this type of error or has modeled inflatable structures.

Some model details:

The elbow joint is a two-layer garment, with an inner bladder layer and an outer restraint layer. I'm using the *FABRIC subroutine to define the material properties of these materials. A rigid body arm scan is implemented which is donned into the elbow joint during analysis. The arm scan is broken into an upper and lower portion, with the lower portion being everything from the elbow to the fingertips. This lower arm portion rotates 120 degrees relative to the upper arm to simulate elbow flexion.

To preserve the stress state of a stitched (or heat sealed) garment made up of flat patterns, the first step of the analysis involves stitching elbow joint gores to the outer side of the joint that would expand during bending. In the model, this is accomplished by assigning stitch points between seams I want to stitch, creating wires between appropriate stitch points, and using connector assignments to pull stitch points to one another.

The next step is pressurizing, which simply involves applying a uniform pressure load to the inside of the bladder layer.

Last, the elbow bend is executed by having the lower arm rotate 120 degrees at a point defined as the elbow joint.

For the past 6 months, I have not been able to get an analysis to solve to completion due to the deformation/wave speed ratio exceeding 1 for at least one element. I assume that this error code is in tandem with excessive element distortion. The error code reports that this often has to do with the model definition. I have tried multiple revisions to the model definition, including the contact model, material properties of the fabric, and mass scaling to no avail. Fabric elements are typically M3D4R, but I have also played around with M3D4 and M3D4R with non-default hourglass control. For all troubleshooting cases I have attempted, the same error arises at different locations of the joint and at different points in time of the analysis. I can't seem to uncover the root cause of this error. 

I look forward to hearing your feedback. I'm more than happy to expand on some of the details of the model and what I have tried to date if it helps. Even general ideas for things to consider/test are helpful. I'll try anything at this point. Thanks!