SUMMER 2021

10 THE DISTRIBUTOR’S

10 THE DISTRIBUTOR’S LINK Rob LaPointe AIM TESTING LABORATORY Rob LaPointe is a noted authority in materials and fastener technology. With extensive experience in the management and science of materials testing laboratories combined with master’s degrees in physics and education, he excels at bringing solutions to the client. Working specifically in the fastener testing industry, he has developed expertise in mechanical, nondestructive, metallurgical and chemical testing. With a background of 20 years in physics education, Rob is effective at communicating complex ideas in a simple and understandable manner, communicating well with clients enabling them to make informed decisions about their products and business. AIM is located at 1920 Cordell Court #101, El Cajon, CA, 92020. Tel: 909-254-1278, email: sales@aimtestlab.com or online at www.aimtestlab.com FASTENER SCIENCE: SHEAR TESTING - SINGLE AND DOUBLE SHEAR FOR FASTENERS Although not as common as tensile testing and nowhere near as ubiquitous as hardness testing, shear testing for fasteners is considered a common test and is required by many specifications. Shear testing measures the strength of the material perpendicular to the metal’s predominate grain direction, which is typically in the same direction as the longitudinal (long) axis of the fastener. In other words, shear testing loads the fastener sideways to the normal loading direction that is achieved by tightening the fastener. It is common for metal to have a shear strength in the range of 50-70% of the material’s ultimate tensile strength (UTS). Just like wood, metal is stronger in the direction of the grain than it is perpendicular (crossgrain) to the direction of the grain. For example, an SAE J429, Grade 8 fastener has a minimum UTS of 120,000 psi. That would mean that the expected shear strength would be about (120,000 psi x 0.60) 72,000 psi. Fasteners can be used in two principal ways to hold a joint together and keep it from slipping of failing. The predominate use of a fastener is to create sufficient clamp-load to keep a joint ridged and keep it from slipping. Most of the time fasteners are used in tension and experience all forces in the longitudinal direction (along the long axis) of the fastener. These longitudinal forces are created by threading the fastener into a hole against the head of the fastener or by putting a nut on the end of the fastener and tightening it against the head and assembly. When a fastener is properly tensioned (about 60 % of UTS) it creates an enormous amount TECHNICAL ARTICLE FIGURE 1 TENSION LOADS ARE IN THE LONGITUDINAL DIRECTION OF THE FASTENER AND SHEAR LOADS ARE IN THE TRANSVERSE DIRECTION (PERPENDICULAR TO THE LONGITUDINAL DIRECTION) OF THE FASTENER. of friction between the components of the assembly. It is this friction that holds the joint tight and keeps the components of the assembly from slipping due to loads in the transverse (perpendicular to longitudinal) direction. Friction is created by holding the pieces of the assembly tight together. The tighter the hold, the greater the frictional force available to resist movement of the assembly. Friction is a product of the clamp-load of the fastener and the quality of the surfaces in contact with one another. From a physics perspective, Frictional force is equal to the Normal force times the Coefficient of friction, where the Normal force originates from the clamp-load created by tensioning the fastener and the Coefficient of friction is a value that characterizes the smoothness or roughness of the surfaces in contact with one another. Figure 2 illustrates the frictional force as a product of the Normal force (N) and the Coefficient of friction, which is given by the symbol mu (µ), Ff = Nµ. CONTINUED ON PAGE 102