Distributor's Link Magazine Fall 2024 / Vol 47 No 4
96 THE DISTRIBUTOR’S
96 THE DISTRIBUTOR’S LINK GUY AVELLON WHAT FASTENER DISTRIBUTORS NEED TO KNOW ABOUT HOW TO INVESTIGATE FASTENER FAILURES from page 26 of a wheel stud. There are several initiation points in the fracture zone which suggests the wheel stud experienced a rotational load and a bending load in a loose condition. Typically, fatigue fractures initiate either at the last thread run-out or at the first unengaged thread protruding from the nut. Ductile Fracture A brittle fracture may be flat, similar to metal fatigue but is more shiny, exposing grain boundary cracking, or grain boundary cleaving. Brittle fractures will not show visible signs of distortion. Some fractures may have the characteristic markings of chevron or waterfall patterns, which point to origin of stress initiation. Metal fatigue will display the conchoidal or ‘beach marks’ striations across the fracture surface. These will be a series of bands which may be dark, light or both. The darker bands signify low frequency impacts or vibration, while the lighter bands are indicative of a higher frequency or more rapid impact loads. When the failure is at the head, this indicates that the joint may not be parallel, the hole too small, burrs around the hole, etc. What happens is the radiused fillet that smoothly connects the head to the shank has been nicked by some external source that created a stress raiser and subsequent crack propagation through the body of the bolt. Ancillary Components Used This basically means to check the grade strength or property class of the bolt and nut to be sure the nut is compatible with the strength of the bolt. If not, there will be stripped threads and / or fatigue marks on the fracture surface because clamp load was lost. Clamp load is also lost if a wrought flat washer was used with a Grade 5 (8.8 metric) bolt or higher. If the washers have indentations, clamp load was lost. The washers in the following photograph have caused a massive loss of clamp load. Metal fatigue from this lost clamp load will cause a rapid bolt failure. Metal Fatigue The following photograph is of a metal fatigue fracture CONTINUED ON PAGE 134
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