Originally Posted by
dragartist
What a great post Genghis, may I please ask a supplementary question about the use of composites in the modern era. I do understand that higher reserve factors were applied in the early days of composites because not much was known about failures. Also the inability to inspect for defects. (I am guessing during manufacture or damage/ overstress in use)
With 787 and A350 are the RFs still increased. I would have thought that with all the experience gained and modern NDT 1.1 would have been sufficient.
Thanks, glad I could help.
Yes, SFs are still increased for modern composite aeroplanes, but not by as much as they used to be because they're now using high temperature / pre-fatigued structural data.
There's more variability in composites: with temperature, moisture absorption, invisible stress / impact damage, manufacturing quality and ageing, than there is with metals. So, the larger safety factors are being applied, but they depend very much upon how you do it. The more work to demonstrate exactly what the safety factors are due to all of these factors, the more you can bring them down.
(SF = margin between limit and ultimate, whilst RF = margin between ultimate and failure)
I am with you all the way about deflection being used to absorb energy. Local stiffening just moved the problem. When I was a student I recall a book entitled the New Science of Strong Materials. It was littered with cartoons and phrases such as "why wings bend" and the likes.
I've read that a few times, first when I was doing my A-levels IIRC; it was recommended for the Physics course, then during my degree, and once or twice since. He wrote a sequel as well: "Structures, or why things don't fall down". JE Gordon is the author.
Had I still been a student I would have asked my Dad to buy me the Guy Gratton book for Christmas or my Birthday.
I wrote it off against tax! You can read a few sample bits for free on Springer's website if you take a look.
G