Percent loss of strength when the fibers aren't aligned perfectly?

General Composites Discussions Engineering Talk
#1

Hi guys,

Does anyone have any charts describing much strength is lost when a typical carbon fiber/epoxy laminate has its fibers oriented at varying degrees from the load? Like, if a UD laminate’s fibers were 5 degrees off axis, or 45 degrees off. Or a woven laminate that’s 45 degrees off? In my own testing, a 2x2 twill weave was about 63% as stiff in the 45 degree angle than at 90 degrees, but I didn’t exactly use a very sophisticated setup :stuck_out_tongue:

Also, what’s the theory on the failure of these laminates? Will the fibers reach their ultimate strengths, or will the epoxy experience shear failure before that happens, or will the fibers themselves experience micro-shear failure due to carbon fiber’s extremely low shear strength to tensile strength ratio?

I have read somewhere that a UD laminate that’s only 5 degrees off axis will lose 50% of its stiffness and strength. But wouldn’t it depend on the aspect ratio of the laminate too - assuming the epoxy would fail in interlaminar shear? Or are the carbon fibers themselves inherently 50% weaker when they’re 5 degrees off axis?

Thanks.

-Gene

#2

Gene,

There are a multitude of charts out there showing strength & stiffness differences at various fiber alignments (one reference that comes to mind is 'Engineering Mechanics of Composite Materials" by Daniel and Ishai). Classical lamination theory can be used to estimate differences is stiffness at various fiber alignments by inputting the 0 and 90 degree properties of a uni lamina (great spreadsheet exercise).

As for failure theories, there are many, but most of the ones used deal with macromechanics (max strain theory, max stress theory, Tsai-Hill, etc.). A good reference I have seen lately describing failure theories is from Firehole Composites (www.firehole.com). As for what typically breaks first, for a fiber dominant property the composite should be designed to have the stiffer fiber take most of the load and fracture first. Of course, for off-fiber axis properties the first failure is typically at the fiber-resin bond.

5% off of fiber axis reducing 50% of the stiffness and strength seems a bit high to me. Again, please look into CLT in a good composites reference book if you want to get some good estimates. Note that the composite material properties will be independent of geometry (modulus and strength) and vary with Vf and fiber orientation.