Allowable Damage by FEM

[SandwitchPanel]

Dear Experts, from an SRM standpoint, is there a good way to present allowable damages using FEM for interiors such as galleys, cargo linings etc.? Also, are there any FEM based methods to present allowable flight cycles for unrepaired areas? Would it be reasonable to assume the damage as a through hole in the FEM and present the minimum MS to be greater than a threshold, such as 0.1 incuding the hole, to substantiate those areas?

 

[SWComposites]

When you say "present" do you mean "calculate"?

In order to determine an ADL, you first need to specify in detail the damage type: scratch, gouge, hole, impact, delamination, etc, etc. Some damage types can be conservatively treated as a hole.

In general you will need test data for residual strength with the ADL damages, in order to validate the analytical predictions.

Typically for composite structure, which I presume you are using for the interior structures mentioned, the cert approach is to demonstrate "no growth" of damage; therefore if the damage meets ultimate load requirements, there is no need to calculate the allowable flight cycles (and there is typically no way to calculate damage growth rates and determine inspection intervals for composites).

Please provide more details on your situation.

 

[gwolf2]

> is there a good way to present allowable damages using FEM

Yes and no, mostly No.

In terms of damage, FEA is used to stress components on the basis of reduced material properties associated with "Barely Visible Impact Damage" BVID or similar. The damage implied is very small.

If you have significant visible damage in something composite then the boundary of the damaged area is too complex to model with FE. You just fix anything which you spot - to stop water getting in and wicking the layers apart as much as anything else.

With composites the often more important issue is the damage which you can't see which may extend invisibly via delamination a long way from the damaged area. You need to use ultrasound or more modern thermal techniques to find the true extent of the damage and cut back vigorously to 'good composite' before repair.

Very tricky.

gwolf