1. DEPARTMENT OF MECHANICAL AND MANUFACTURING ENGINEERING
UNIVERSITY OF NAIROBI
DEPARTMENT OF MECHANICAL AND MANUFACTURING ENGINEERING
ENGINEERING DESIGN II
FME 461
PART 2
GO NYANGASI
November 2008

2. MATERIALS IN DESIGN OF MACHINE PARTS
PROPERTIES OF ENGINEERING MATERIALS
SUMMARY

3. SELECTION OF MATERIAL FOR MACHINE PARTS
MATERIALS FROM WHICH MACHINE OR STRUCTURAL PARTS ARE MADE ARE:
Cast iron;
Steel;
Copper and its alloys;
Aluminium and its alloys;
Plastics.
FACTORS CONSIDERED IN SELECTING THE MATERIAL ARE
Mechanical properties;
Manufacturing considerations;
Availability;
Cost.

4. MECHANICAL PROPERTIES OF ENGINEERING MATERIALS
STATIC TESTS
Tension Test
Items reported in a tension test are:
Tensile strength;
Yield point;
Limit of proportionality;
Modulus of elasticity;
Percent elongation, and;
Percent reduction in area.

5. STATIC TESTS Tension Test
Engineering stress-strain diagram depicts the variation of stress with strain based on the original gauge length and diameter of the specimen
The stress is then computed as, while the strain is computed as

6. STATIC TESTS
TENSION TEST

7. STATIC TESTS Tension Test
DUCTILE MATERIALS
Proportional limit identifies region of linear relationship between stress and strain where stress-strain curve is linear and Hooke’s Law applies
Elastic limit identifies where permanent set starts
Yield point identifies where large deformation occurs without increase in stress. Some materials do not show the yield point
Point of ultimate tensile strength is the highest stress in the diagram
Point of fracture is the stress and strain at which fracture of the specimen occurs in the diagram
For materials where yield point is not obvious, the offset method is used to identify a yield stress. A value of 0.2 % of gauge length is often used, implying a strain of
BRITTLE MATERIALS
Brittle material such as cast iron and some high strength steels fracture while the stress strain curve is still rising
The fracture strength and the ultimate tensile strength therefore coincide
The plastic deformation represented by percentage elongation and reduction in area at fracture is much lower for brittle materials

8. MECHANICAL PROPERTIES OF ENGINEERING MATERIALS
TESTS FOR MECHANICAL PROPERTIES
Compression test
This is run in the same general way as a tension test.
It is usually limited to those materials which are primarily loaded in compression, such as concrete and cast iron.

9. MECHANICAL PROPERTIES OF ENGINEERING MATERIALS
TESTS FOR MECHANICAL PROPERTIES
Torsion test
One end of the specimen is held in a fixed grip, and the other end is rotated about its own axis.
The measured variables are torque and angular displacement.
Stress-strain curves plotted from these tests are similar to those from tensile tests.

10. SELECTION OF MATERIAL FOR MACHINE PARTS
MECHANICAL PROPERTIES
Table 6.1 Mechanical properties of metals criteria measured as indicators
MECHANICAL PROPERTY
MEASURED BY
Strength (under static load)
Ultimate tensile strength or tensile yield strength
Strength (repeated load)
Endurance strength
Rigidity
Modulus of elasticity
Ductility
Percentage elongation
Hardness
Brinell or Rockwell hardness number
Toughness
Charpy or Izod impact value
Frictional properties
Co-efficient of friction

11. SELECTION OF MATERIAL FOR MACHINE PARTS
MANUFACTURING FACTORS
Available methods for producing metal parts are:
Cutting;
Machining;
Welding;
Casting;
Forging;
Heat treatment;
Rolling;
Extrusion, e.t.c.
The choice of material for part must consider the intended manufacturing method.
AVAILABILITY
In practice, the factor of availability should be considered first.
COST
The cost of a machine part is made up of the direct cost of input (raw) material, and the cost of processing it.
The choice of manufacturing process must therefore consider this cost of manufacturing.