Cosmos Xpress factor of safety

Capability/Reality=FOS

In other words you divide the stress that the part is capable of handling by the stress that you really experience and if the number is greater than 1 you can be somewhat certain that the part can withstand that load. The following assumptions need to be made to calculate the factor of safety:

1. You know the loads on the part

1a. And you have applied them to the FEA model correctly which isn't always possible with Cosmos Xpress. 1b. The part will see the same loads that you put on the FEA model

1. The geometry you are modeling accurately reflects the geometry of the real part.

2a. Things like manufacturing tolerances or errors can reduce or thicken a part and change its behavior. 2b. When in service the part may change due to wear and tear, corrosion, consumer modification, etc.

1. You know the capabilities of the materials comprising the part.

3a. Plastics can only be roughly approximated in CosmosXpress (and I do mean roughly) 3b. The material properties will stay the same during use.

1. The part will fail due to a single applicaion of the load. CosmosXpress does not handle fatigue.

2. The part will not collapse due to buckling or compression instability.

Given all these uncertainties and a few more that I haven't mentioned you might want to have a factor of safety of 2 to 10 in order to be safe.

Roark and Young has a good synopsis of the various failure modes of metallic structures and these can to some degree be applied to plastics. Roark and Young also has numerous hand calculations that can be used to check and approximate the answer from FEA methods.

If this answer scares you a little bit that is OK. After all when you start saying a part is some percentage of the way from failure you are putting people's lives or economic benefit in danger. This is not to be taken lightly or done in an uniformed manner.

Knotfreak wrote: