Fastener Bearing and Pull-thru Strengths

[ctan67]

In regards to a bolted quasi-isotropic composite joint there are two failure modes that I am having questions about;

1)Bearing strength and
2)Fastener Pull-thru strength.


In MIL-HBK-17, I have come across text referring to bearing and pull-thru test procedures being used to determine the bearing and pull-thru responses of a composite.

Is there any type of micromechanics approach or method I can use to calculate such strengths knowing just the typical lamina tensile, comp, shear, moduli, strengths, etc?

The bearing load seems just like a compressive load on the interface, and my assumption was to simply calculate margins using the in-plane compressive strength of the laminate. How incorrect is this assumption?

The pull-thru strength is something I am a lot less familiar
with. My only grasp of this loading is that capacity seems to be dependent on thru-the-thickness properties. Can anyone please elaborate on what is going on mechanically and if there is at least a preliminary step I can take to figure out pull-thru strength before performing actual experimental tests?

Sidenote: Is there a hand calc equation for estimating pull-out stress?

 

[SWComposites]

No, there is no reliable micro-mechanics or other approach for predicting bearing strength, and no, you cannot use the laminate compression strength. If I don'thave any data for a material I typically use 60 ksi for bearing for a quasi CFRP laminate (assuming the temp is not too high, tight hole tolerances, a fastener with reasonable clamp-up, countersink depth not more than 2/3's the thickness, etc.)

Pull-thru strength is even more complicated. There are several competing failure modes, including interlaminar shear and local bending. It is highly dependent on the type of fastener head and the laminate thickness. I have seen attempts to use interlaminar strength to predict pull-thru strength, and if you make a bunch of conservative assumptions then you should get a conservative pull-thru value, but it won't correlate to test data.