How do you define the allowable limits of a polymer? One suggestion I heard recently was to take the yield stress at a 10 % proof stess point and then make the allowable limit a quarter of that. I want to stay in the elastic region and avoid problems with creep. I'm doing some FEA on a plastic platform that has to support in excess of 13 tonne (metric) for an extended period of time. The polymers are EPS and HDPE. How about limits on strain. I understand this is important with composites. How about with polymers? I had a long search on the net and could not find a scrap. Any assistance here would be helpful. Oh and if you know of any good plastics design books that would be helpful also. Thanks in advance.