Elizabeth Sharp

Matrix Singular or Indefinite Stiffness Matrix Negative Diagonal

Discussion created by Elizabeth Sharp on Mar 9, 2020
Latest reply on Mar 12, 2020 by Ryan Dark



I am using Solidworks to model an assembly comprised of two parts, an inner and outer can. The inner can is concentric to the outer can, the inner can base is coincident with the inside base (face) of the outer can. 


I am applying a pressure of 50PSI to the inside walls of the outer can, the outer can is fixed at the top edge and base edge, the inner can is fixed at the base edge. I am using approximately a 4mm mesh in draft quality, however I am aware the outer can wall thickness is 1mm thick. I hope to adjust my meshing more appropriately when the model works. The inside can is mesh controlled by a rather large mesh size as I am not interested in the inside can. I have further defined a contact point of no penetration between the inside and outer can, at the base of the inner can. 


When I run the analysis, I seem to get an error of *matrix is singular or indefinite* and * Stiffness matrix has a negative diagonal* I have no idea why this is happening! 


I have adjusted the mesh of the model, I have applied a shell mesh to the outer can cylinder wall, I have excluded the inner can completely, I have even fixed and bonded the inner can to the outer can (A condition that isn't true to real life) and I still get the above errors. I have also noticed I do not reach the yield point at all for the simulation even when increasing the pressure, I still maintain the error. 

My objective is to show the deformation of the outer can at different pressures until it undergoes plastic deformation. 


Please could someone help, I do not know what else to try. 


Thank you!