Bearing hole strength

[pcomp]

I am planning to do bearing hole strength of carbon fiber laminate coupons as per ASTM D5961.

The standard recomends to use coupons having 0.25 inch hole and same diameter pin, with Width/Dia ratio 6 and edge/dia ratio 3.

Now my big doubt is that, for a given thickness of a laminate, Is the bearing hole strength is a constant? I other words, Is it independent of the diameter of the hole? If I test the same laminate thickess with big size hole (0.5 dia) will I get same bearing strength from a coupon with 0.25 dia hole (provided if I keep the W/d ratio and E/d ratio same)?

Appreciate, if composite experts thro some light on this.

 

[GBor]

I believe these ratios were set to help avoid edge effects and other fiber discontinuities issues. For instance, a smaller hole may provide the same strength, but a larger hole may result in small delaminations that are imperceptable, but dramatically reduce the strength with no additional cross-section to help recover from the fabrication flaw. Also, consider the asymtotic shear stress at the edge. If there is less "meat" between the hole and the edge, this asymtotic curve doesn't have any room to "level off" and your still at an abnormally high shear stress when you get to the edge of the hole.

I think the intent with these minimum ratios is to allow you to actually test what you THINK you are trying to test. Violate these guidlines and you need to make sure you are actually seeing the proper failure mode for the strength number that you are trying to derive.

I don't think I've explained this very well, but hopefully you get my meaning...

 

[SWComposites]

pcomp - bearing strength is not constant with diameter, nor thickness, layup, fastener head style, fastener clamp-up, etc. Bearing strength tends to decrease with increasing hole size with everything else held constant. You should test the specific joint configuration(s) in your structure.