# chapter 6 - PART III Mechanical Properties Chapters 6,7,8 1...

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1 PART III: Mechanical Properties Chapters: 6,7,8.

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2 There are actually 3 chapters devoted to this subject. In chapter 6 we start with an overview of mechanical properties at the macroscopic level. In chapter 7 we look at mechanisms which determine the strength of a material. This involves microscopic characteristics. In chapter 8 we study fracture behavior.
3 What you should understand by the end of this chapter How engineering stress and strain are defined. The main states of stress and strain. Elastic deformation of materials. Relationship between elastic response to atomic bonding . Plastic behaviour and ductility in metals What parameters are measured in a tensile test. The concepts of true stress and true strain

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4 Introduction The vast majority of materials are used to bear and sustain loads . buildings bridges cars space shuttle We need to understand… … how much force a material will withstand without: deflecting too much. deforming permanently. cracking or breaking. … how the mechanical behaviour changes over time due to: high temperature exposure. fatigue. combinations of the above.
5 Basic Concepts What is a “ mechanical response ”? If you apply forces to an object it can: Move Rotate Deform To eliminate the first two you must apply balanced forces. You can measure deflection and distortion of the material under a load. The actual response depends on the: material. structure and geometry. A material’s response is correlated by: stress ( normalized force ). strain ( normalized deflection ).

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6 ALS What Parameter Correlates A Material’s Behaviour? If it requires 2 baby elephants to break the lecture table how many elephants would it take to break two tables joined together ? Why and what is the principle? A material’s response is correlated by: stress ( normalized force). strain ( normalized deflection). If a 20 m long bungee cord stretches 6 m when you jump how long will a 30 m cord stretch ?
7 Tensile and Compressive Stresses A material’s response is determined by the load per unit area stress . In tension or compression: = F / A 0 This is called the engineering stress . The units of stress are N / m 2 = Pa . Typical stresses are expressed in MPa. Canyon Bridge, Los Alamos, NM Ski lift (photo courtesy P.M. Anderson) Cable in tension: &>±0 Strut in compression: &<±0

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8 M M A o 2R F s A c Other States of Stress: Shear Shear stress, = F / A 0 Example: a drive shaft
9 Biaxial Tension Hydrostatic compression Fish under water z > 0 > 0 σ h < 0 (Courtesy P. M. Anderson) (Courtesy P. M. Anderson) Pressurized tank Other States of Stress: Pressure

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