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SUMMER 2020

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Distributor's Link Magazine Summer 2020 / Vol 43 No3

104 THE DISTRIBUTOR’S

104 THE DISTRIBUTOR’S LINK ROB LaPOINTE FASTENER SCIENCE - CHARPY IMPACT TESTING from page 14 The stress riser is controlled by the angle of the V-notch and its radius at the bottom of the V (Figure 3). Since the bottom of the V is radiused, the material is forced to tear apart initially rather than to cleave if the bottom of the V where sharp. The resistance to cleaving provided by the smooth transition enables the material to withstand greater tension forces without fracturing. This is the same principle used in designing the transition between the body and head of a fastener known as the fillet radius. Without a fillet radius, tensile force would be focused on the transition between the body and head of the fastener, causing it to fracture in the body-head transition rather than in the thread area. The distance from the bottom of the V-notch to the opposite side of the specimen, known as the ligament, measures 8mm, thus with a 10 mm wide specimen, the notch depth is 2mm. FIGURE 3 CHARPY V-NOTCH DIAGRAM The test itself is very simple and involves a system where energy is conserved. That is, a system where energy can be transferred from one form to another but not lost to the environment during the transfer. The energy must always be contained in the system and be able to be accounted for, thus it is conserved. Common sources of energy such as a compressed spring or chemical energy, as in combustion or an explosion, might be exciting but are generally part of nonconservative systems. A system is nonconservative when energy leaks out and is unaccounted for. Friction is perhaps the most notable thief in any nonconservative system. Friction is a force that transfers mechanical work (a form of energy) into thermal energy. This transfer of work into thermal energy is known has heat. When heat is involved in the system, the energy is dissipated into the environment by heating up the surrounding area, never to be captured again. Once it leaves the system, it is very hard to keep a good accounting of the total energy. One force that is particularly good as an agent of work for conservative systems is gravity. Gravity is known as a conservative force and the work it does within a system is also conservative. There is no friction or heat involved in gravitation. Therefore, it is a perfect source of energy for the Charpy impact test. The objective of the test is to determine how much energy is consumed in the process of rupturing the Charpy specimen. To do that, we need to start with a known amount of energy. That energy is used to break the specimen. What is left over after breaking the Charpy specimen is measured. The energy consumed in the rupture of the specimen is nonconservative and therefore not easy to measure. The beauty of this test is that we do not have to measure it to know precisely what it is. We only have to measure how much energy we have at the beginning of the test and how much energy we have at the end the test. As long as our mechanism of energy transfer is conservative, the energy lost will have been consumed by rupturing the specimen. It is like solving the algebraic equation 6 – x = 4. It is pretty easy to see that x must equal 2. There is a deep beauty and simplicity in using the conservation of energy to solve complicated problems. This is why physicists have been taking advantage of these principles for hundreds of years to solve some of the most complicated problems in nature. “Simple,” you say to yourself in jest. Let’s use an example of a basic system that is very relatable to the Charpy impact test. A rollercoaster. A rollercoaster gains all the energy needed for the entire ride on the first hill. At the top of that hill, all its energy is in the form of gravitation (point A, Figure 4). FIGURE 4. A ROLLERCOASTER EXCHANGES GRAVITATIONAL ENERGY FOR KINETIC ENERGY. CONTINUED ON PAGE 130

THE DISTRIBUTOR’S LINK 105

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