Discussion Forum

Hello everyone,

I am having trouble simulating the bending of a flexible hollow tube when internal pressure is applied. A drawing of my system can be found in the image attached. The geometry consists of two bodies: 1) the flexible tube which is bent at an angle of 180 degrees, 2) caps attached to the two ends of the tube and 3) a thin band of inextensible layer around the cirucumference. When implemented into COMSOL Multiphysics, I form a union between the three bodies rather than an assembly because I believe they are a single body with two different materials.

The physics applied here is Solid Mechanics with the inextensible layer modelled as a linear elastic material and the flexible tube with the caps as a hyperelastic material, using Neo-Hookean as the material model. A boundary load in the form of internal pressure is applied on the inner walls of the hollow tube and the caps. One end is fixed in space. The study preformed is stationary using Fully Coupled Solver settings. The boundary load applied is 0.006[bar]

My understanding is that when the flexible tube is interally pressurized it will balloon around the inner circumference. This expansion will result in the clockwise bending of the tube, in other words, the tube will begin to straighten up. This behaviour is visible up to an applied pressure of 0.004[bar]. Internal pressure higher than that results in the error:

Attempt to evaluate negative power of zero - Function: ^ Failed to evaluate expression. - Variable: comp1.solid.J@VDN$comp1.solid.Cl23 - Defined as: (0.5(((((((comp1.solid.Cl11comp1.solid.Cl22)comp1.solid.Cl33)+(((2comp1.solid.Cl12)comp1.solid.Cl23)comp1.solid.Cl13))-(comp1.solid.Cl11(comp1.solid.Cl23^2)))-(comp1.solid.Cl33(comp1.solid.Cl12^2)))-(comp1.solid.Cl22(comp1.solid.Cl13^2)))^(-0.5)))(((2comp1.solid.Cl12)comp1.solid.Cl13)-(comp1.solid.Cl11(2comp1.solid.Cl23)))

Based on the research I've done on issues others have had, I believe that this error is due to the Cauchy-Green deformation tensor which is occuring due to the user defined hyperelastic material. As a beginner on COMSOL, I have been unable to solve this issue. I believe that I should be able to simulate the inflation and extension of the flexible tube by applying an internal pressure of 0.1 bar. However, COMSOL seems to run into issues at very low pressures and I can't see how its deforming. What am I missing and how can I simulate full bending and extension?

Thank you for taking time and helping me.

Kind regards,

Nehal