section%209.6%20-%20Fracture - 9.6 Fracture Objectives...

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Objectives Describe types of fracture. Calculate critical stress and crack length for failure. Cracks are almost always present in materials, yet quite often the material can remain in service for years without breaking. On the other hand, materials that appear to have no cracks may suddenly fail with no warning, often with catastrophic consequences. Behavior like this can make failure seem like a random event, but in fact we can predict the point at which failure occurs. The best example of the difference is described by J.E. Gordon ( in Structures, or Why Things Don’t Fall Down , Plenum, New York, 2003). He tells the story of a cook who noticed a crack in the floor of his ship’s galley. The crack was so small that it was ignored, but the cook decided to mark how long the crack was each day. The crack steadily grew larger, until one day the ship suddenly broke in two without warning. We know about this story because the piece of deck with the cook’s markings was recovered. This story illustrates the important difference between stable crack growth and unstable crack growth . We can summarize these two types of crack growth as follows: Stable crack growth: When the crack is below a certain critical length the crack will only grow if the force applied increases. This means that if the force is constant, the crack will just stay the same length. Unstable crack growth: When the crack gets to a certain critical length it will grow spontaneously, at supersonic speeds, leading to failure. Obviously it is important to be able to determine when the transition from stable to unstable growth occurs. This is the realm of fracture mechanics . There are many different ways to approach fracture mechanics. We are going to stick with a simple approach, called the Griffith approach . In 1920 A.A. Griffith developed an approach to a simple approach to fracture mechanics based on energy considerations. In Figure 9.6.1 we see an elliptical crack in a material. As the crack grows two things happen: 1. New surface is created. 2.
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This note was uploaded on 03/30/2010 for the course EMA 3010 taught by Professor Unknown during the Spring '08 term at University of Florida.

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section%209.6%20-%20Fracture - 9.6 Fracture Objectives...

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