Variable Thickness Composite FEA of a Type-4 CNG Cylinder
Composite FEA of a filament-wound Type-4 CNG cylinder under proof pressure, with continuous ply thickness so the dome transitions show no false stress peaks.
Project brief
A maker of Type-4 CNG cylinders needed a structural check of its composite wrap under proof pressure. A Type-4 cylinder has a non-metallic liner, and all of the pressure load is carried by the filament-wound composite around it.
In a wound vessel the laminate is never uniform. Fibre builds up at the domes, where the helical layers turn around, and the hoop layers stop short along the body. Shirsh Technosolutions built the laminate in ANSYS Composite PrepPost so that the model follows the real winding, and then checked the vessel in ANSYS Mechanical.
What was at risk
The first versions of the model described the thickness in separate zones, each with a constant ply stack. Where one zone met the next, the shell thickness jumped within a single row of elements.
The solver reads each jump as a sudden change in stiffness. The result was a band of high stress at the dome transitions, created by the way the model was built. Those peaks looked like failures and hid the real margin. The task was to describe the laminate the way it is actually wound, with thickness changing smoothly along the axis and plies ending with a taper, and to do it without upsetting the shell mesh.
Solution offered
We divided the shell profile at reference stations along the meridian and used spatial interpolation fields in ANSYS Composite PrepPost to vary each ply's thickness continuously along the tank axis. The heavy fibre build-up at the domes comes out of the interpolation instead of being forced in as a block.
The hoop plies near the dome tangent lines were ended with edge-taper rules, so they ramp down over a length instead of stopping dead. The helical layers were kept as balanced plus and minus angle pairs. The interpolation fields were tuned until the thickness mapped cleanly onto the mesh without distorting the elements.
The finished layup went into ANSYS Mechanical, where the proof pressure was applied. We then checked fibre strain and the ply failure criteria over the whole shell, paying particular attention to the dome to cylinder transitions.
Common questions
Why do stepped thickness zones give false stress peaks? A sudden change in shell thickness is a sudden change in stiffness. The load has to cross it within one element, so the result shows a local peak that depends on the mesh, not on the design. Making the thickness vary smoothly, the way the winding does, removes the jump.
Why check fibre strain instead of an equivalent stress? A composite ply fails along the fibres, across them and in shear, at very different levels. Fibre strain against its allowable, together with a ply-level failure criterion, shows which of these governs. A single equivalent stress cannot.
Challenges we overcame
False stress peaks at the dome transitions caused by stepped thickness zones
Fibre build-up at the domes where the helical layers turn around
Hoop plies ending along the body that had to taper off rather than stop
Mapping a smoothly varying thickness without distorting the shell elements
Keeping the helical plus and minus angle pairs balanced through the layup
How we executed
- 1
Divided the shell profile at reference stations along the meridian
- 2
Set up spatial interpolation so each ply's thickness varies continuously along the tank axis
- 3
Applied edge-taper rules to end the hoop plies gradually near the dome tangent lines
- 4
Kept the helical layers as balanced plus and minus angle pairs
- 5
Checked the mapped thickness and element quality before solving
- 6
Transferred the layup to ANSYS Mechanical and applied the proof pressure
- 7
Evaluated fibre strain and ply failure criteria over the whole shell
Project outcome
The false stress peaks at the dome to cylinder transitions disappeared, leaving smooth stress contours that reflect the real vessel.
Fibre strain under proof pressure stayed within the allowable limit.
The client received a laminate model that follows the real winding and can be reused for design changes and certification work.
Project gallery
Where this lives
Software used
Related solutions
At a glance
We apply the same rigour to every engagement
Let us show you what's possible for your product.