Cosmos Xpress factor of safety

Hi,

I have a question...

I read about the factor of safety in the tutorial and help file but for some
reason I have trouble understanding what it represents. Could someone
explain?

Sorry if this is a stupid question.

Thanks,

Lucas

 

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

2. 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.

3. 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.

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

5. 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.