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# Rough draft of - Ch 7 Mechanical Properties and Behavior Part 2 Objectives Introduction to topics of Fracture Mechanics Fatigue Creep Calculate

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Ch. 7 Mechanical Properties and Behavior Part 2 Objectives: Introduction to topics of: Fracture Mechanics Fatigue Creep Calculate Critical defect Sizes Fatigue Life Creep Life Look at fractography as a means to understand failure Processes Fracture Mechanics The study of the behavior of materials containing flaws. May have unexpected brittle failure. Flaws can sometimes be avoided by proper design. Depending on the depth of the crack and the crack-tip radius you may see a dramatic reduction in the load the sample can support. When a flaw exists on develops during use. The material can fracture through the flaw. You get a concentrating of stress at the flaw. Typical sequences of events. Typical sequences of events. 1. A small flaw during fabrication or operation 2. 2. Flaw propagates during use (may be difficult to detect it) 3. 3. Fracture occurs when the crack reaches a critical size 4. Misc. Aluminum is bad about cracking over time. 5. What is safe? 6. Looking at Brittle materials:

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Remember stress around cracks, notches, sharp corners, will be higher then the applied stress. .. Griffith Cracked Model (Brittle only More stress for a longer crack (deeper crack) and for a sharper crack. Also for brittle materials, look at the energy When you have fractures for 2 surfaces (each have a surface energy.) So you can equate the points where the elastic strength, contains enough energy to create two surfaces Ductile materials are more forgiving because energy is consumer by plastic deformation at the crack tip. Crack tip can increase Larger radius -> lower critical stress Fracture Mechanics A= short glass fiber B= long glass fiber A=B (diameter_ Same batch Which one is stronger? A Why? If you increase the length you decrease the strength. There is a whole group of materials called whiskers. They are short and thing, they are so short; they have no flaws, and can be used to strengthen other materials. So the longer something gets the more flaws it has. Metals Work better, can survive, extreme conditions. Welded ships broke in half in cold waters. Low strength before ductile materials doesn’t need to worry much about future mechanics. Higher strength-lighter product
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## This note was uploaded on 06/28/2011 for the course MSE 280 taught by Professor Dr.barryandrews during the Fall '10 term at University of Alabama at Birmingham.

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Rough draft of - Ch 7 Mechanical Properties and Behavior Part 2 Objectives Introduction to topics of Fracture Mechanics Fatigue Creep Calculate

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