Mechanical properties of metals Stress and Strain in metals Engineering Materials Mechanical properties of metals Stress and Strain in metals Elastic and Plastic Deformation Elastic Deformation : If a metal deformed by a force returns to its original dimensions after the force is removed, the metal is said to be elastically deformed. During elastic deformation the metal atoms are displaced from their original positions but not to the extent that they take up new positions. Thus, when the force removed, the metal atoms return to their original positions and the metal takes back its original shape.

Elastic Deformation :

Mechanical properties of metals Stress and Strain in metals Engineering Materials Mechanical properties of metals Stress and Strain in metals Elastic and Plastic Deformation Plastic Deformation : If a metal deformed by a force dose not returns to its original dimensions after the force is removed, the metal is said to be plastically deformed. During plastic deformation the metal atoms are permanently displaced from their original positions and take up new positions.

Plastic Deformation :

Plastic Deformation : The ability of some metals to b extensively plastically deformed without fracture is one of the most useful engineering properties of metals. Example; the extensive plastic deformability of steel enables automobile parts to be stamped out mechanically without the metal fracturing.

Mechanical properties of metals Stress and Strain in metals Engineering Materials Mechanical properties of metals Stress and Strain in metals lo Ao Engineering Stress and Engineering Strain Engineering Stress( σ segma): Average uniaxial force divided by the original cross-sectional Area. Units: U.S system SI systemm

Mechanical properties of metals Stress and Strain in metals Engineering Materials Mechanical properties of metals Stress and Strain in metals Engineering Stress and Engineering Strain Engineering Stress( σ segma):. Example 1: A 0.5 in diameter aluminum bar is subjected to a force of 2500 lbf. Calculate the engineering stress in pounds per square inch (psi) on the bar. Example 2: A 1.5 cm diameter bar is subjected to a load of 2500 kg. Calculate the engineering stress in megapascal (MPa) on the bar.

Mechanical properties of metals Stress and Strain in metals Engineering Materials Mechanical properties of metals Stress and Strain in metals Engineering Stress and Engineering Strain Engineering Stress( σ segma):. F Δl lo l lo Ao Δl=l-lo

Mechanical properties of metals Stress and Strain in metals Engineering Materials Mechanical properties of metals F Δl lo l Stress and Strain in metals Engineering Stress and Engineering Strain Engineering Strain ( ε epsilon): Changes in length of sample divided by the original length of sample. Units: U.S system inches per inch (in./in.) SI system : meters per meter (m/m) It is common to convert eng. Strain into percent strain or percent elongation: % engineering strain= eng. Strain *100%=% elongation

Mechanical properties of metals Stress and Strain in metals Engineering Materials Mechanical properties of metals Stress and Strain in metals Engineering Stress and Engineering Strain Engineering Strain ( ε epsilon): In most cases engineering strain is determined by using a small length, usually 2 in., called the gage lengthy, within a much longer, for example, 8 In.: 2 in. 2.65 in. Gage marking 8 in. F Unstressed sample Stressed

Mechanical properties of metals Stress and Strain in metals Engineering Materials Mechanical properties of metals Stress and Strain in metals Engineering Stress and Engineering Strain Engineering Strain ( ε epsilon): Example 3: A sample of commercially pure aluminum 0.500 in. wide, 0.040 in. thick, and 8 in. long that has gage markings 2.00 in. apart in the middle of the sample is strained so that the gage marking are 2.65 in. apart (Fig.) . Calculate the engineering strain and the percent engineering strain that the sample undergoes. 2 in. 2.65 in. Gage marking 8 in. F Unstressed sample Stressed

Mechanical properties of metals Stress and Strain in metals Engineering Materials Mechanical properties of metals Stress and Strain in metals Engineering Stress and Engineering Strain Poisson’s Ratio( ν nu): A longitudinal elastic deformation of a metal produces an accompanying lateral dimensional change. A tensile stress segma produces an axial strain and lateral contraction of and For ideal material, nu=0.5, for real materials nu =0.25 - 0.4 (average 0.3)

Mechanical properties of metals Stress and Strain in metals Engineering Materials Mechanical properties of metals Stress and Strain in metals Shear Stress and Shear Strain Shear stress ( τ tau): Shear force S divided by the area A over which the shear force acts

Mechanical properties of metals Stress and Strain in metals Engineering Materials Mechanical properties of metals Stress and Strain in metals Shear Stress and Shear Strain Shear strain ( γ gamma): Shear displacement (D) divided by the distance (H) over which the shear act; The relationship between shear stress and shear strain is G=elastic modulus