Material Variability Factor for Composites

[jetmaker]

Hi all,

Wondering if someone can provide a little insight on the material variability factor that is used for composite materials under static test.

Per 25.307(d) - Proof of Structures, "... appropriate material correction factors must be applied to the test results...".

The standard that seems to be used for the testing of composite materials is 1.15, but no where can I seem to find this number in the FARs, MIL-HDBK-17, or other related documents. Can someone shed some light on this factor and actually provide a reference for this value (1.15) as it relates specifically to static testing of composite materials for the purpose of static ultimate testing.

Regards,

Trevor

 

[rb1957]

i don't know where 1.15 comes from ... it looks pretty low, but it might be applicable to very good quality manufacture (is there any other kind ?) of composite.

i thought you pretty much had to come up with your own factor, and realize it to whoever is approving the certificate.

where i worked, where they didn't have a clean room, we used a factor of 1.4 which was appected without much rationalisation (urban legend has that that this factor was proposed by the certifying agency).

not much help i'm afraid !

 

[SWComposites]

There is no "standard" factor for composite material variability for static ultimate tests. The factor must be derived for each material/structure from coupon/element test data for the same material, process, environment, and failure mode. Usually coupon allowables data which includes mulitple material batches is used; the variability factor is usually the ratio of the B-basis value to the mean strength. Mil-Handbook-17 has some allowables data, and if your material happens to be included in there, you may be able to persuade your certification agency to accept that data to derive a factor. Otherwise, you are likely going to have to obtain your own data.

There is a 1.15 "standard" load enhancement factor that has been used in the industry to scale up fatigue test loads in order to shorten the required fatigue test lifes for composite structures. But the FAA is now requiring fatigue data for the particular material used in a fatigue test article to justify any LEF that is used.

Steve