MECHANICAL PROPERTIES OF MATERIALS Department of Physics KL
- Slides: 32
MECHANICAL PROPERTIES OF MATERIALS Department of Physics KL University
OBJECTIVES Identification of elastic and plastic behaviour of materials through molecular level Behaviour of ductile and brittle materials with external load Measurement of various properties like hardness and toughness of materials
BASIC TERMS Stress Plasticity Strain Ductility Types Brittleness of Stress Types of Strain Hooke’s law Elasticity Hardness Toughness Fatigue Creep Fracture
ELASTICITY Regains to original shape PLASTICITY Deforms permanently
ELASTIC DEFORMATION MOLECULAR APPROACH 1. Initial 2. Small load 3. Unload bonds stretch return to initial d F Elastic means reversible
PLASTIC DEFORMATION MOLECULAR APPROACH 1. Initial 2. Small load 3. Unload bonds stretch & planes shear d elastic + plastic F planes still sheared d plastic Plastic means permanent ØProcess of plastic deformation in crystals is by slip process (motion of dislocation) and in non-crystalline solids, the plastic deformation is by viscous flow mechanism.
Stress: The internally developed forces per unit area of a material due to the application of external force. Its SI unit is Pascal (or) N/m 2
Strain Fractional change in the dimensions of a material due to the application of external force. It has no unit
TYPES OF STRESS &STRAIN 1. Tensile stress – Longitudinal Strain 2. Compressive 3. Shear Stress – Volume Strain stress – Shear Strain
TENSILE STRESS –LONGITUDINAL STRAIN F W Tension Equal and opposite forces directed away from each other
COMPRESSIVE STRESS LONGITUDINAL STRAIN W F Compression Equal and opposite forces directed towards each other
SHEAR STRESS –SHEAR STRAIN Tangential force
HOOKE’S LAW Within elastic limit, Stress α Strain Stress = E x Strain E - Modulus of elasticity, Unit : N/m 2
DIFFERENT MODULLI OF ELASTICITY
ØModulus of elasticity of materials depends on bond strength between atoms, stronger the bond, larger will be the modulus of elasticity. ØValues of the modulus of elasticity for ceramic materials are about the same as for metals; for polymers they are lower. These differences are a direct consequence of the different types of atomic bonding in the three materials types.
ØIncrease in temperature of material, decreases the modulus of elasticity.
Poisson’s Ratio: unloaded Ø If lateral strain along x- and y- directions is same (ε x = ε y), material may be isotropic.
DUCTILITY Material – thin wires – withstands plastic deformation
BRITTLENESS Fractures without deformation
Hardness: - withstand plastic deformation or indentation produced in the material. Creep: - time dependent deformation at constant load
Strength : Ability to withstand loads Types of strength 1. Yield strength : Strength beyond which it exhibits plasticity 2. Tensile strength (or) Ultimate strength : Strength at which the material breaks or fractures
Tensile Strength • Maximum stress on engineering stress-strain curve. TS F = fracture or ultimate strength stress engineering y Typical response of a metal strain engineering strain Necking acts as stress concentrator • Metals: TS occurs when noticeable necking starts. • Polymers: TS occurs when polymer chains are aligned and about to break.
Toughness : Energy absorbed up to fracture. Fatigue: Failure under cyclic (or) repeated stress Fracture: Breakage of a material into separate parts under the action of stress
ultimate tensile strength =E 3 Slope Strain Hardening yield strength necking Fracture 5 Stress (F/A) 2 Plastic Region Elastic Region 1 4 Elastic region slope =Young’s (elastic) modulus yield strength Plastic region ultimate tensile strength strain hardening fracture Strain ( ) (DL/Lo)
STRESS – STRAIN CURVE – BRITTLE AND DUCTILITY
ENGINEERING AND TRUE STRESS-STRAIN Engineering stress (σn ) = F/A 0 – original area of cross section Engineering strain εn = (L-L 0)/L 0
ENGINEERING AND TRUE STRESS-STRAIN CURVE
HARDNESS TESTS INDENTATION METHOD 1) 2) 3) Brinell Hardness Test Rockwell Hardness Test Vicker Test
VH = 1. 854 F/d 2 , d = (d 1+d 2)/2
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