1. CHAPTER 4 : MECHANICAL PROPERTIES OF METALS

2. TOPIC 1) TYPES OF MECHANICAL PROPERTIES 2) MECHANICAL TESTS OF METALS
3) HARDNESS TESTS
4) IMPACT TEST
Beginning course details and/or books/materials needed for a class/project.

3. TYPES OF MECHANICAL PROPERTIES
Defined as those properties which completely define its behavior under action of external load or forces.
Those properties which associated with :
Its ability to resist failure
Its behavior under action of external forces
Knowledge of mechanical properties :
Essential for engineers in selecting suitable materials for various applications.
Objectives for instruction and expected results and/or skills developed from learning.

4. Ability to undergo appreciable plastic deformation before rupture.
Mechanical properties
Strength
Ability to withstand or support external forces or load without rupture
Hardness
Ability to resist deformation by abrasion, indentation or penetration and scratching by harder bodies.
Ductility
Ability to undergo appreciable plastic deformation before rupture.
Brittleness
Ability to fracture when deformed . Opposite to ductility
Toughness
Ability to absorb maximum energy up to fracture. Must be strong & ductile to be tough. Shows ability to withstand impact. Value increase when temperature increases.
Elasticity
Ability to retain its original shape & size after removal of load
Plasticity
Ability to experience permanent deformation without fracture when subjected to external forces
Resilience
Ability to absorb energy when it is elastically deformed
Malleability
Ability to be deformed into thin sheets by rolling or hammering without fracture
Machinability
Ability to be cut or removed by cutting tools in various ,machining operations.
Weldability
Ability of 2 similar or dissimilar metals to be joined by fusion& with or without filler
Castability
Ability to be formed into different shapes & sizes from its liquid state

5. MECHANICAL TESTS OF METALS
All mechanical properties of metal- established by conducting tests on various testing machines.
Types of mechanical test :
Tensile test
Hardness test
Impact test
Compressive test
Fatigue test
Creep test

6. Tensile test understand mechanical behavior by stress-strain test.
3 ways of load application –tension, shear, compression
Most common mechanical stress-strain performed in tension
Performed to determined ;
Elastic limit
Yield point
Ultimate tensile strength
% of elongation & reduction of area

7. Standard tensile specimen
Circular cross section
Reduced section diameter = 12.8 mm
Reduce section length = 60mm
Gauge length = 50mm
Procedure :
Specimen held in holding grips of apparatus
Load applied gradually at a constant rate
Specimen will be elongated until fracture
Elongation – measured by extensometer

8. Data recorded – load vs. elongation Engineering stress :
Normalized to engineering stress & engineering strain
Engineering stress :
where : A0 = original cross sectional area
before any load applied (m2)
F = instantaneous load (N)
Engineering strain :
where : li = original length
l0 = instantaneous length

9. Plot a graph – stress vs. strain
OA is a straight line
Stress & strain proportional (Hooke’s Law)
where E = modulus of elasticity
Slope corresponds to E
Called elastic deformation
When applied load is released, specimen returns to its original shape.

10. AB is a small curve
A is a point where elastic deformation end and plastic deformation begin
Phenomenon of yielding occurs
Called proportional limit
Initial departure from linearity
To determine yielding point precisely
Construct straight line parallel to elastic portion of curve at a specified strain offset (0.002)
B is the intersection of parallel line with curve
The stress defined as a yield strength,

11. behavior for some steels
Elastic-plastic transition
Well defined
Occurs abruptly
Yield point phenomenon
Upper yield point – plastic deformation initiated. decrease
Lower yield point – deformation fluctuated at constant
Yield strength , average of lower yield point

12. BC is an upward curve Stress increase to maximum limit (point C)
Called tensile strength , TS
Corresponds to maximum that can be sustained by a structure in tension .
Necking begin to form- decrease of cross-sectional area

13. CD is a downward curve True stress True Strain
Specimen continues to elongate
Requires lesser load to continue deformation. decrease.
D is a point of fracture or rupture
True stress
where : Ai = instantaneous cross sectional area
True Strain
where : li = instantaneous length

14. Ductility % elongation (% E)
Measure degree of plastic deformation that has been sustained at fracture
Can be expressed as :
% elongation
% area reduction
% elongation (% E)
where : lf = length after fracture
lo = original length

15. where : Af = cross sectional area at
% area reduction (%AR)
where : Af = cross sectional area at
fracture
Brittle material
Has little or no plastic deformation upon fracture
Has less than 5% elongation

16. Measure hardness by forcing an indenter into materials surface.
Hardness test
Measure hardness by forcing an indenter into materials surface.
Indenter – made of harder material
- usually in form of ball. pyramid or
cone
Early hardness test
Done by comparing with 10 standard mineral
Increasing hardness on Moh’s scale
talc 6) orthoclase
Gypsum 7) quartz
Calcite 8) topaz
Fluorite 9) corundum
Apatite 10) diamond

17. Important test Brinell Hardness Test Rockwell Hardness Test
Viekers Hardness Test
Knoop Hardness Test

18. Brinell Hardness Test By Dr Johan August Brinell in 1900
Performed by pressing steel ball into surface of test pieces using appropriate force.
Formula :
Brinell Hardness Number , HB (or BHN)
Where D = diameter of steel ball (mm)
d = diameter of indentation (mm)
P = applied load (kg)

19. Rockwell Hardness Test
Devised in the USA
The most common method:
Simple to perform
Require no special skills
Quick & direct reading
Performed when hardness is beyond range of Brinell’s
load is smaller than Brinell’s

20. Viekers Hardness Test The most accurate test Indenter Load
Has continuous scale of hardness (10 to 1000)
Indenter
Diamond square based pyramid with 136ᵒ angle between opposite faces.
Load
Smaller than Rockwell & Brinell
Between 1 and 1000 g
Suitable for:
Small, thin selected specimen region

21. Where P = applied load (kg)
d = length of diagonal (mm)

22. Where l = longest diagonal length
Knoop Hardness Test
Very much similar to Vickers
Indenter :
Diamond pyramid with short depth and diagonal in ratio in ratio of 7:1
Measure diagonal length under microscope.
Knoop hardness number designated by HK
Where l = longest diagonal length

23. Hardness scale for Knoop & Vickers
Approximately equivalent
Both are referred as micro hardness testing
Basis of reload & indenter size
Suitable for testing
Brittle materials (ceramics)
Extremely thin metal
Exceptionally hard, very shallow carburized or nitride surface

24. IMPACT TEST
Many machine parts are subjected to sudden applied loads – impact loads
Important engineering wise to have :
Material that can withstand impact load without fracturing
A hard, strong may not be suitable when subjected to sharp sudden load.
Capacity of metals to withstands impact without fracture
Impact resistance or impact strength
Indication of toughness

25. Method of measuring toughness
Impact- testing apparatus
Types of impact-testing apparatus
Charpy
Izod

26. Question
A cylindrical specimen of metal having a diameter of mm and a gauge length of mm is tested using a tensile testing machine. The elongation measurement are recorded in Table 3.
Plot the stress-strain curve on the graph paper provided based on data in Table 3
Based on the stress-strain curved plotted in (i) :
Compute the modulus of elasticity
Determine the yield strength at a strain offset of
Determine the tensile strength
Determine the ductility in percent elongation and percent area reduction