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mechanical

# mechanical - Force stress and strain Mass in kilograms kg...

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Force, stress and strain Mass in kilograms, kg often called weight Produces, in gravity, Force in Newtons. 1kg produces a force of 9.8 Newtons A force pulling (or pushing) on a rod of a cross-sectional area A Produces a stress, measured in Pascals (Newtons per square meter) Stress = Force/(Area of cross section) For a weight of 10 kg hanging from a rod that is 1cm (0.01meters) in diameter Force = 98 N Area = πr 2 = 78.5x10 -6 m 2 Stress = 1.2x10 6 Pa, 1.2 MegaPascals This will cause a strain (change in length/original length) in rod.

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Mechanical Properties Response to Stress Elastic: reversible strain (e.g a spring) Plastic: permanent strain (e.g. a bent paperclip) Fracture: propagation of a crack (e.g. breaking glass) Viscous: flow of a liquid Visco-elastic: slow elastic strain and recovery (e.g. toffee) Fatigue: slowly growing crack in cyclic loading Creep: slow permanent strain due to vacancy motion (e.g. old lead pipe)

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Elastic modulus in tension Linear elasticity Hooke’s Law, 1660 Deformation Load Young’s modulus 1807 Stress = Modulus x Strain, σ = E . ε Engineering Stress = Load/Original Area True Stress = Load/ Actual Area Engineering Strain = Extension/Length True strain = log (L/L o )

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Stiffness is good for: Stiff: low elastic deflection under load Aircraft: stiff & light: aluminum alloy, carbon fiber composite Columns prone to elastic compressive buckling: pillars, legs, bicycle frames Radio masts Stiffness is bad for: Trees Buildings in earthquakes Bungee cords Structures subject to impacts: bicycle frames, auto suspensions, bodies
A column under a centric axial load exhibiting the characteristic deformation of buckling.

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