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AER507_c2_handout

# AER507_c2_handout - Chapter 2 Mechanical Properties The...

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1 Materials and Manufacturing (AER507), F. Xi Chapter 2 Mechanical Properties The objective for this chapter is to understand the following topics: 2.1 Introduction to mechanical properties 2.2 Stress-strain relationships Tensile, compression, bending, shear 2.3 Hardness Hardness vs. strength 2.4 Effect of Temperature 2.5 Other properties: fatigue, impact and creep. Textbook: Chapter 3; Reference 2: Chapter 6 and Chapter 8 Materials and Manufacturing (AER507), F. Xi 2.1 Introduction to Mechanical Properties Mechanical properties are concerned about material behavior when subject to mechanical stress (force), including - strength, modulus of elasticity, ductility, hardness. On one hand, design objective is to produce the products that can withstand high force without significant change in geometry and surface, meaning high strength, modulus and hardness . On the other hand, manufacturing objective is to apply sufficient force so that the material can be cut or deformed to alter its shape. Usually, high strength materials are difficult and expensive to manufacture. Hence, mechanical properties are an important factor for both design and manufacturing.

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2 Materials and Manufacturing (AER507), F. Xi 2.2 Stress-Strain Relationships Stress (unit area force) = Force / Area Strain (unit deformation) = Deformation / Length Stress-strain relationship indicates how much a material will deform under a given force ! independent of size F L o L F L o L Materials and Manufacturing (AER507), F. Xi Stress and Strain Calculation (Engineering) stress-strain (theoretical) Stress: σ e = F/A o MPa (psi) where F - applied force in N (Ib); A o - original area in mm 2 (in 2 ). 1 psi (lb/in 2 ) = 6895 Pa (N/m 2 ) Strain: e = (L – L o ) / L o (dimensionless) where L o - original length in mm (in); L - length at any point. Hooke’s Law: σ e = Ee (in elastic region) where E - modulus of elasticity (Young’s modulus) in Pa (psi). ! inherent material stiffness.