Not surprisingly over my career…I’ve seen it once and I continue to see it again and again and again…a failure to check a fastened joint for pull-through. What is seemingly a simple stress check is often overlooked. This tip provides a quick method to check for pull-through failure anytime...but hopefully you check during the preliminary phase of the design before a failure or a messy redesign is required. A check that readily ascertains whether a composite joint is thick enough, or possesses the minimum inter-laminar shear strength needed to prevent a pull-through failure is discussed in this article.
Performing a simple preliminary check for fastener pull-through is good practice. The equation below can be used to calculate the pull-through stress for either countersunk or protruding head fasteners.
Why is checking pull-through important? Mainly because if pull-through should occur, there is no residual load carrying capability near the failed fastener. Also, we all (or at least we should) know that the composite strength in the “third” direction is considerably less than its respective in-plane strength properties; consequently, failure can occur even if the axial load is much lower than the in-plane loading.
Given that fastened joints are typically the Achilles heel of a composite design…checking pull-through regardless of how benign one might believe the out-of-plane load to be is...well, a prudent practice—always!
Bonus Info: In a tension fitting design, the tension load is transferred over a reduced bearing area equal to about half of the total bearing area; consequently, the allowable pull-through load should be reduced by a factor of 2. This is especially important given tension fittings typically have a prying load associated with them that adds to the axial loading.
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