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Mechanical Properties of Materials
Chapter 3 Mechanical Properties of Materials
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3.6 Poisson’s Ratio When compressed, the bar is shortened longitudinally, but expands in lateral direction.
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Under tension force, the bar increases its length by , and the radius reduces by . The strains in longitudinal and lateral directions are
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S.D. Poisson found (1880s) that , within the elastic range and for a homogeneous and isotropic material , * Since long and lat have opposite signs, is positive. * is a non-dimensional quantity. * For most nonporous solids, is in the range of 1/4~1/3. * It was proved theoretically max =1/2. Thus 0 1/2
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3.7 The Shear Stress-Strain Diagram
Tests can be conducted for pure shear case, and draw - curve.
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* Hooke’s law for shear:
* G – shear modulus modulus of rigidity * G, E, , - same unit force/area * Relationship:
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Chapter 5 Torsion
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5.1 Torsional Deformation of a Circular Shaft
Concept of Torsion * Torque – a moment tending to twist a member about its longitudinal axis. * Shaft or axis – transmit torque from motor to drive machine * Torsion – deformation due to torque
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Observation * Every cross-section remains planar and circular. * Radial lines remain straight, and rotate through the same angle. * Longitudinal lines are twisted. * The length and radius remain unchanged.
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Angle of Twist * Radial line on a cross-section rotate through an angle (x). * The angle of twist is a function of x. = 0 at a fixed end (x = 0) * Since the angle is changed, there is shear strain. * Shear strain is the largest at outer surface.
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5.2 The Torsion Formula Torsion Formula
* Shear strain and shear stress are proportional to radius. * At the center, they are zero, and At outer surface they are maximum.
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J – polar moment of inertia
of the cross-section
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Solid Shaft
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Tabular Shaft
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5.3 Power Transmission
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Concepts of Work and Power
Unit - SI system: J, FPS: lbft, IPS: lb in * Power – work done per unit time Unit - SI system: W, FPS: ftlb/s, IPS: inlb/s hp = 550 ftlb/s
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Power Transmission – angular velocity, circular frequency, radians/second f – f = 2, frequency, Hz, cycles/second Note: If the speed is given as n rpm (revolutions per minute), then
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Shaft Design allow – from material, c, J – from geometry T – from working requirement * Given material (allow), to determine the size. * Give size, to determine the material.
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5.4 Angle of Twist Angle of Twist
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- angle of twist of one end (x = L) with respect to the
other end (x = 0), measured in radians T(x) – internal torques at location x J(x) – polar moment of inertia at x G – shear modulus of the shaft material
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Constant Torque and Cross-Section Area
G can be determined from pure shear tests.
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Multiple Torques Sign Convention Use right-hand rule: Thumb is directed outward. Finger curl gives positive T and
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* Divide the shaft into several segments.
* Cut each segment in middle, and find internal torque. * Draw torque diagram.
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