FALL 2024

134 THE DISTRIBUTOR’S

134 THE DISTRIBUTOR’S LINK GUY AVELLON WHAT FASTENER DISTRIBUTORS NEED TO KNOW ABOUT HOW TO INVESTIGATE FASTENER FAILURES from page 96 In addition to being too soft to sustain the service loads of the joint, the washer on the left has an inside diameter that is too large to fully support the loads under the head of the cap screw. This washer is typically called a USS washer. The washer on the right is an SAE flat washer whose inside diameter will fully support the loads under the head of the cap screw. Washers without indentations are made from hardened steel and will sustain fluctuating loads. Coatings Coatings can make a difference in environments with elevated temperatures. Photos A and B are of an all-metal lock nut that came off a turbocharger. Similar examples have been used in exhaust manifolds or boiler vessels. The inside will look like Photo B. The gold colored nuts are cadmium plated and will fail from Liquid Metal Embrittlement at temperatures exceeding 400ºF or 204ºC. LME is time dependent so failure will take longer at lower temperatures than higher. Zinc plating will also cause LME but at much higher temperatures. PHOTO B vibration and impact loads will cause metal fatigue if the bolts are not properly tightened and maintained. There should also be enough bolts in the connection to carry the load and reduce the individual bolt stress. PHOTO A Types Of External Loads And Amount Of Load Proper bolt selection depends on knowing what the service loads are. If the bolt is bent, it was not the correct strength or diameter for the application. Heavy Environment Some corrosive environments can lead to stresscorrosion-cracking, where the grain boundaries become under chemical attack. Standing water will set up an electrolytic cell and cause hydrogen embrittlement. Other times, when there is rust in the fracture zone, it can signify that the failure was not new and had been propagating for some time. This could also cause other stress initiation sites. CONTINUED ON PAGE 154

THE DISTRIBUTOR’S LINK 135