Materials Engineering Lecture 7: Mechanical Properties

Stress and Strain Concept Engineering stress (מאמץ הנדסי) is a measure of the average amount of force exerted per unit area of the surface on which internal forces act within a deformable body. Where s is the stress, Pa (MPa); F is the force (N); A is the area (m2). Engineering strain (מעוות הנדסי) is expressed as the ratio of total deformation to the initial dimension of the material body in which the forces are being applied. Where e is the strain (dimensionless), Dl is the deformation (elongation), l0 is the original length.

Stress load פיתול גזירה מתיחה דחיסה

Tension / Compression and Torsion Test Apparatuses Tension test machine Torsion test machine

Stress-Strain Curves UTS: Ultimate tensile strength חוזק מתיחה מרבי Yield: limit of elastic deformation כניעה

Elastic deformation Hooke’s law (חוק הוק): s = E e, where E is modulus of elasticity or Young’s modulus (מודול ינג, GPa, psi; 1 psi ≈ 6.89 kPa) E(∙106 psi) E (GPa) Metal / Alloy 10 69 Al 14 97 Brass 16 110 Cu 6.5 45 Mg 30 207 Ni Steel 15.5 107 Ti 59 407 W TºC= (5/9∙ TºF) - 32

Poisson’s Coefficient Poisson’s ratio (יחס פואסון): n = - (ex / ez) For anisotropic materials theoretically n = 0.25. Maximal n for isotropic materials = 0.5. Real metals: n = 0.25 – 0.35. E = 2G (1 + n), where G is shear modulus Polycrystalline metals and ceramics are anisotropic.

Elastic properties Elasticity (אלסטיות): elastic properties do not depend on time, full recovery upon release of the load; Anelasticity (אי-אלסטיות): elastic properties are time-depending; partial recovery only after some time. Metals: practically, elastic Polymers: viscosoelastic (ויסקוסואלסטי) (high anelasticity)

Plastic deformation Yield (כניעה) is the beginning of plastic deformation. Some steels demonstrate YPP (Yield Point Phenomenon)

Ductility Ductility (רקיעות): how easily the fracture is achieved after the beginning of plastic deformation. Ductility is expressed: %EL (elongation), %RA (reduction area). Brittle materials (חומרים פריכים): ~ fracture strain (מעוות השבר) < 5% 40 Ni %EL Al 25 Steel (1020) 45 Cu Ti 68 Brass 35 Mo Fe

Toughness Toughness (חוזק התנגדות לשבר) is the measure of a material to withstand fracture. High toughness requires both ductility and strength

Hardness HM: Mohs: Talc =1…Diamond=10 (If a tested material is scratched by a standard mineral, the Moh’s hardness of the former is lower than that of the latter.

Hardness Rockwell hardness test Hardness (קשיות) is the measure of a material to withstand localized plastic deformations (e.g. scratch or dent). Rockwell hardness test

Hardness scales HM: Mohs: Talc =1…Diamond=10 HV: Vickers (microhardness); HRA, HRB, HRC etc.: Rockwell

Musts of this lecture Stress; strain – concept, definitions, classification; Tension test: principle; Stress-strain curves; UTS; Elastic deformation: Young’s modulus, Poisson’s ratio, elestic properties; Plastic deformation: yielding, YPP; ductility; toughness; Hardness: concept, measurements, main scales.