1. Materials Engineering
Lecture 7:
Mechanical Properties

2. 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.

3. Stress load
פיתול גזירה מתיחה דחיסה

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

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

6. 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

7. 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.

8. 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)

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

10. 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

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

12. 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.

13. 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

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

15. 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.