SPRING 2018

10 THE DISTRIBUTOR’S

10 THE DISTRIBUTOR’S LINK Laurence Claus Laurence Claus is the President of NNi Training and Consulting, Inc. He has 25 years of experience with a medium sized automotive fastener manufacturer, holding positions including Vice President of Engineering, General Manager, Director of Quality, Director of New Business Development and Applications Engineer. In 2012 he formed NNi offering technical and business training courses as well as technical consulting, expert witness and consultation work. He can be reached at 847-867-7363 or by email: Lclaus@NNiTraining.com. You can learn more about NNi at www.NNiTraining.com. WHY FASTENERS FAIL PART 4 - MANUFACTURING DEFECTS In Part’s One through Three of this series we have looked at how fasteners fail once they have been placed into service. This has included looking at immediate failures by overload and delayed failures as a result of progressive mechanisms like fatigue, creep and corrosion. In this final installment we will investigate the role that manufacturing defects play in fastener failure. In this segment we will not only consider failures that result in product failure but also those that prevent a part from being assembled. Although failures of this type can be categorized primarily as nuisances, they are still failures and should be considered as such. In this segment we will consider manufacturing defects that fall into three primary categories; 1. Manufacturing defects that fail on installation or may lead to progressive failures (such as fatigue), 2. Manufacturing defects which prevent assembly, and, 3. Manufacturing defects that are simply a nuisance. Although these categories seem to suggest a sliding scale relative to defectiveness, the reality is that any problem with renders the part incapable of being used for its intended purpose should be considered equally bad. Manufacturing Defects Which May Lead to Immediate or Progressive Failures Cracks Cracks are perhaps the biggest nemesis of manufacturers and users alike. They can be completely benign but unsightly or nearly invisible yet a lurking, TECHNICAL ARTICLE malevolent presence. Once discovered it is rare for any crack to be accepted by the end user, often for good reasons, not the least of which is the fact that cracks are potential sources of disaster in applications exposed to fatigue loading. To appreciate the genesis of cracks, one must first consider how fastener parts are made. Whether cold, warm, or hot formed, the process of transforming a part from a simple cylindrical tube into a complicated series of coaxially connected shapes, often with internally formed holes or complex geometric recesses, involves extreme forming forces at exceptionally high strain rates. (Strain rate refers to the speed at which the deformation occurs.) These conditions place exceptional instantaneous stresses on the subject parts. If the raw material is incapable of accepting such stress or has inherent weaknesses present, the material will seek to find a way to relieve itself, normally in the form of a crack. Cracks may be generated from an inherent weakness or flaw present in the raw material. These cracks are usually relatively easy to spot and diagnose because the contributing defect will be evident along the entire length of the part rather than localized to just the cracked area. These sorts of cracks are almost always longitudinal, running along the axis of the part and result from either a seam, lap, or scratch in the raw material. Figure 1 shows a part exhibiting this type of crack. Note that the defect is evident running the entire length of the part, although it only opened up in the form of a crack in the highly stressed area of the head. CONTINUED ON PAGE 102