FEA on Non Metalic Materials

[Robanik]

I have recently been faced with an issue where a manufacturer has made a tri-clamp flange connection out of a CPVC material. A tri-clamp flange consists of a ferrule (flange portion) and clamp. The flange portion was made out of the CPVC materials based on dimensions for a typical CPVC fitting wall thickness and a standard stainless steel flange ferrule. This is the best way to describe it. The design conditions required are 75 psig @ 140F. At these conditions CPVC is normally derated by ~50% of the room temperature strength.

My issue is no engineering was done to ensure the design could meet the pressure requirements at an elevated temperature. I would like to do an FEA on the design to determine the stresses. I have experience using FEA in a DIV 2 analysis on metalic materials, but none on non metalic materials. Does anybody know a good reference for an FEA application on non metalic materials (CPVC) at elevated temperatures?

I have an FEA completed based on room temperature values but I am not sure how to eveluate the results. Any help would be appreciated. B31.3 provides allowable hydrostatic design stress values for simple pipe wall thickness calcs, but no rules for other types of analysis.

Any help would be appreciated. Also I did think of doing a burst test but the requirement in Ontario is the MAWP is 1/10 of the bursting pressure corrected for temperature. The test would have to exceed 1500 psi which the manufacturer has stated that the flange would not hold.

Thanks for any help in advance.

 

[Robanik]

All,

After reading my post I found that I did not spell metallic correctly. Sorry about that.

 

[stanier]

I am not an FEA experienced engineer. I do however work with thermoplastic materials for piping systems. My approach is to consider the same model using Algor's Pipepak using the extremes of properties ( Instant and 50 year).

The challenge with non metallic materialsapart from the change in properties over a moderate temperature range, is the non linear behaviour. The stresses are load & time dependent and rate of time dependent. The shorter time in applying a load yields apparent higher properties. Or put another way a component will fail at a lower stress applied over a long time period compared to an impact load.

The FEA package at hand may not have the non linear ability to analyze such materials. ALgor have a non linear package as I am sure the others do.

One reference that is a little dated and is not specific to FEA but is worthwhile is Applied Stress Analysis of Plastics is by Krishnamachari. It contains some FEAa examples.

I would recommend sub contracting the analysis to a specialist plastics designer for this case and learn as much from him/her as possible about the approach.