WINTER 2023

12 THE DISTRIBUTOR’S

12 THE DISTRIBUTOR’S LINK Guy Avellon Guy Avellon has been in MRO and Fastener Distribution for over 30 years, in such positions Sales Engineer, Chief Engineer, Manager of Product Marketing, Product Engineering & Quality and Director of Quality & Engineering. He founded GT Technical Consultants where he performs failure analysis, lectures on fastener safety, works for law firms and designs/audits Quality systems. He is a member of SAE, is Vice Chairman of the ASTM F16 Fastener Committee, Chairman of the F16.01 Test Methods Committee and received the ASTM Award of Merit in 2005. Guy can be contacted at 847- 477-5057, Email: ExpertBoltGuy@gmail.com or visit www.BoltFailure.com. WHAT DISTRIBUTORS NEED TO KNOW ABOUT JOINT STRENGTH Many have probably seen a Force Diagram and thought it too complicated. This article will help explain the mechanisms involved because if a customer continues to have failures from his fasteners, maybe the fasteners he is using isn’t the correct type for his joint and loads. All metal components behave like springs. The relative response to stress under loading is governed by the comparative stiffness of each component in the joint. If the spring rate of the bolt is equal to the combined spring rate of the joint, then half the deformation will occur in the bolt and the rest will be absorbed by the joint. The modulus of elasticity determines the relative stiffness of the joint members and the bolts used to produce the clamp load. The stiffness of each will determine how much clamp force is necessary, what size of bolts to use and how the clamp loads will change when the service loads act upon the joint. Clamping force exerted by the bolt on the joint is produced by an equal and opposite force by the joint onto the bolt. All things being elastic, the bolt wants to return to its original length while the joint also wants to return to its original thickness. To illustrate this, a Force Diagram (Figure 1) is used which is a stress-strain diagram combined with elastic curves for both the bolt and joint member. CONTRIBUTOR ARTICLE FIGURE 1 As the bolt is tightened, the bolt elongates (B). Due to the natural internal forces resisting the elongation, a tension force or preload is produced (Fp). The applied force and elongation are linear. Since any action requires a reaction, the deformation of the joint is linear and is represented by J, the amount of joint compression. Regardless of how soft the joint or bolt are, the bolt becomes longer and the joint becomes compressed under load when the bolt is tightened. Typically, the spring rate, or stiffness of the bolt is only 1/3-1/5 that of the joint. The following diagrams illustrate the importance of the stiffness ratio between the bolt and the joint in determining how much of the applied external load is absorbed by the fastener and how much is absorbed by the joint. CONTINUED ON PAGE 98