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

MATERIALS IN DESIGN OF MACHINE PARTS PROPERTIES OF ENGINEERING MATERIALS SUMMARY

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.

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.

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 .

STATIC TESTS TENSION TEST

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 0.002. 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

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.

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.

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

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.