safety margins for composites

[groveri]

Do aircraft companies that use damage tolerant strain allowables (eg. open hole) for composites use maximum limit loads to calculate safety margins or do the loads have safety factors included in them. Like MS = Damage tolerant strain allowable for the laminate/max limit load strain -1.

 

[dcascap]

I've always seen structural substantiation in aerospace for failure in terms of ultimate loads (which include a safety factor), and limit load mostly for yield deflections and special considerations.

 

[SWComposites]

No.

Typically MS = Damage tolerant strain allowable for the laminate/max ultimate load strain -1. Assuming you are using a maximum strain failure criteria.

And, open hole allowables may or may not cover for all damage types required for Category 1 damage, depending on the material, layup, structure, environment, BVID dent criteria, manufacturing flaw criteria, etc, etc. In some cases BVID residual strength strains are lower then OHC strains.

 

[Ng2020]

Sw: in the context of part 25 composite primary structures, aren't ohc design values used for static strength in order to envelope the effect of bvid, and therefore the static strength check supports the overall 'no detrimental growth' dt philosophy?

 

[SWComposites]

in the context of part 25 composite primary structures, aren't ohc design values used for static strength in order to envelope the effect of bvid, and therefore the static strength check supports the overall 'no detrimental growth' dt philosophy?

In general, no.
You have to prove that OH static strength is less than BVID static residual strength for your material and structural configuration. In some but not all cases OH strength is less than BVID strength.
And for "no detrimental growth" you need to have fatigue test data demonstrating no-growth of damages (such as BVID and VID) account for fatigue scatter with a life or load enhancement factor. No-growth of BVID needs to be shown for full life, whereas no-growth of VID needs to be shown for typically 2 inspection intervals.

 

[Sparweb]

Is there something like a FAA AC or EASA CM that spells that out for aircraft certification? I can think of some references like AC 20-107A regarding substantiation of composite structure, but they don't discuss this issue, specifically. I'd really appreciate an authoritative source to use, rather than a college textbook (even if it was written by Tsai and Hill).

 

[SWComposites]

The ACs unfortunately don’t get into that detail. The closest thing is the Composite Material Handbook, CMH-17, specifically Volume 3 Chapter 12, Damage Tolerance. (purchase at sae.org). V3 Rev H should be published by the end of this year.

Brian Esp’s text might discuss it (I don’t remember and am too lazy right now to walk upstairs and look). https://www.amazon.com/Practical-Analysis-Aircraft-Composites-Brian/dp/0983245398

 

[rb1957]

there's an FAA DER paper about radomes (like above if I could be bothered enough I'd get the title ... back on Monday)