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.
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3 Materials and Manufacturing (AER507), F. Xi Stress-Strain Curve slope = E σ =Ee σ = K ε n TS Y Materials and Manufacturing (AER507), F. Xi • The Elastic Limit (E.L.) is the limit of elastic deformation, below which the material will not be permanently deformed. Since it is difficult to determine this limit, the yield strength is used instead. • Yield strength (Y or YS) is defined as the stress at which a material deforms from the elastic region to the plastic region. • Y is determined as the stress at which a 0.2% strain offset from the straight line has occurred. Yield point 0.2% Material Strength elastic plastic
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4 Materials and Manufacturing (AER507), F. Xi Material Strength • (Ultimate) Tensile Strength (TS) is defined as the maximum stress. After this point, a localized elongation, known as necking, occurs. TS = F
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This note was uploaded on 09/12/2010 for the course AER AER312 taught by Professor Kobe during the Spring '10 term at Rupanuga.

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AER507_c2_handout - Chapter 2 Mechanical Properties The...

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