Material Engineering - Lecture - Mechanical Properties, Lecture notes of Material Engineering

Download Material Engineering - Lecture - Mechanical Properties and more Lecture notes Material Engineering in PDF only on Docsity!

1

MECHANICAL

PROPERTIES

Chapter Outline

 Terminology for Mechanical Properties

 The Tensile Test: Stress-Strain Diagram

 Properties Obtained from a Tensile Test

 True Stress and True Strain

 The Bend Test for Brittle Materials

 Hardness of Materials

4 Stress-Strain Test specimen machine

5 Tensile Test

Terminology

 (^) Load - The force applied to a material during testing.  (^) Strain gage or Extensometer - A device used for measuring change in length (strain).  (^) Engineering stress - The applied load, or force, divided by the original cross-sectional area of the material.  (^) Engineering strain - The amount that a material deforms per unit length in a tensile test.

8

F

bonds stretch return to initial

1. Initial 2. Small load 3. Unload Elastic means reversible.

F

Linear- elastic Non-Linear- elastic Elastic Deformation

10

Typical stress-strain

behavior for a metal

showing elastic and

plastic deformations,

the proportional limit P

and the yield strength

y

, as determined

using the 0.002 strain

offset method (where there

is noticeable plastic deformation).

P is the gradual

elastic to plastic

transition.

11 Plastic Deformation (permanent)

• (^) From an atomic perspective, plastic

deformation corresponds to the breaking of

bonds with original atom neighbors and

then reforming bonds with new neighbors.

• (^) After removal of the stress, the large

number of atoms that have relocated, do

not return to original position.

• (^) Yield strength is a measure of resistance

to plastic deformation.

(c)2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning™ is a trademark used herein under license.

• (^) Localized deformation of a ductile material during a tensile test produces a necked region.
• (^) The image shows necked region in a fractured sample

14 Permanent Deformation

• (^) Permanent deformation for metals is

accomplished by means of a process called

slip, which involves the motion of

dislocations.

• (^) Most structures are designed to ensure that

only elastic deformation results when stress

is applied.

• (^) A structure that has plastically deformed, or

experienced a permanent change in shape,

may not be capable of functioning as

intended.

Stress-Strain Diagram Strain ( ) (e/Lo) 4 1 2 3 5 Stress (F/A) Elastic Region Plastic Region Strain Hardening Fracture ultimate tensile strength Slope= E Elastic region slope=Young’s(elastic) modulus yield strength Plastic region ultimate tensile strength strain hardening fracture necking yield strength UTS  y  σ  E ε ε σ E   2 1 y ε ε σ E  

Stress-Strain Diagram (cont)

• (^) Elastic Region (Point 1 –2) - The material will return to its original shape after the material is unloaded( like a rubber band). - The stress is linearly proportional to the strain in this region. σ  E ε : Stress(psi) E : Elastic modulus ( Young’s Modulus ) (psi) : Strain (in/in)

• (^) Point 2 : Yield Strength : a point where permanent deformation occurs. ( If it is passed, the material will no longer return to its original length.) ε σ or^ E^ 

• (^) Tensile Strength (Point 3) - The largest value of stress on the diagram is called Tensile Strength(TS) or Ultimate Tensile Strength (UTS) - It is the maximum stress which the material can support without breaking.
• (^) Fracture (Point 5) - If the material is stretched beyond Point 3, the stress decreases as necking and non-uniform deformation **occur.
• Fracture will finally occur at Point 5.** Stress-Strain Diagram (cont)

(c)2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning™ is a trademark used herein under license. The stress-strain curve for an aluminum alloy.