timeout for tech
METAL FRACTURE The fatigue cycle that really counts.
BY GARY T. FRY, PH.D., P.E., VICE PRESIDENT, FRY TECHNICAL SERVICES, INC.
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elcome to “Timeout for Tech with Gary T. Fry, Ph.D., P.E.” Each month, we examine a technology topic that professionals in the railway industry have asked to learn more about. This month our topic is fracture of steel. I am often asked, “Why did this break?” Typically, the inquirer is pointing to a piece of steel that has suffered an obvious fracture. Structural fractures can occur even when a component is only subjected to the normal service loading that it was designed to withstand with a significant margin of safety. In fact, the component may have supported millions of applications of the loading for many years before the fracture suddenly occurred. So what was special about that last load application that caused the fracture? The simple answer is: probably nothing special at all. A more vexing answer might be this: The last load did not cause the fracture; it was the fracture which resulted in that becoming the last load. Let’s consider generally what comprises a fracture event using a broadly familiar object: a pistachio. Figure 1 contains a sequence of four photographs of pistachio shells: intact, slightly cracked, very railwayage.com
cracked, and fractured. This sequence is representative of how steel components might fail in fatigue. They start off intact, develop small fatigue cracks which grow into larger fatigue cracks, and then fracture. If offered one of the first three pista-
There are fascinating microscopic things happening inside solid material at the tip of a crack. chios, I would certainly choose the one that contains a larger crack, since little force is needed to pry it open. In general, the larger the crack, the easier it is to fracture a solid, and we should always distinguish between a crack and a fracture. A crack is a defect in a component; a fracture is a failure of a component, often into two or more separate pieces.
There are fascinating microscopic things happening inside solid material at the tip of a crack just prior to a fracture event. These things comprise the fracture process and are best represented mathematically as a tipping point in a balance among three energies. In the context of opening the pistachio shell, these energies are: 1) the energy created as the forces in our fingers spread apart; 2) the strain energy inside the solid shell caused by our fingers prying the shell open; and 3) the energy required to extend the length of the crack within the solid shell material. That third energy, the energy required to extend the crack, involves a material property that we call toughness (also referred to as critical strain energy release rate). It has units of energy per unit area of crack formed. All solid materials have toughness. The toughness of metals usually varies with temperature: generally lower at cold temperatures and higher at warm temperatures. Here are some toughness values of a few materials at room temperature. For our pistachio shells, it is around 10,000 Joules per square meter. For glass, it is very low: around 10 Joules per square meter. For copper, it is very high: around one million Joules per square meter. For the low-carbon steels used in bridges and February 2022 // Railway Age 41