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Table 7.2.
Mechanical properties of common metals and alloys (typical values at room temperature)
Tensile CHEMICAL ENGINEERING
0.1 per cent
Modulus of
strength proof stress elasticity Hardness Specific
2
2
2
(N/mm ) (N/mm ) (kN/mm ) Brinell gravity
Mild steel 430 220 210 100 200 7.9
Low alloy steel 420 660 230 460 210 130 200 7.9
Cast iron 140 170 140 150 250 7.2
Stainless steel
(18Cr, 8Ni) >540 200 210 160 8.0
Nickel
(>99 per cent Ni) 500 130 210 80 150 8.9
Monel 650 170 170 120 250 8.8
Copper
(deoxidised) 200 60 110 30 100 8.9
Brass
(Admiralty) 400 600 130 115 100 200 8.6
Aluminium
(>99 per cent) 80 150 70 30 2.7
Dural 400 150 70 100 2.7
Lead 30 15 5 11.3
Titanium 500 350 110 150 4.5
7.3.3. Toughness
Toughness is associated with tensile strength, and is a measure of the material’s resistance
to crack propagation. The crystal structure of ductile materials, such as steel, aluminium
and copper, is such that they stop the propagation of a crack by local yielding at the crack
tip. In other materials, such as the cast irons and glass, the structure is such that local
yielding does not occur and the materials are brittle. Brittle materials are weak in tension
but strong in compression. Under compression any incipient cracks present are closed up.
Various techniques have been developed to allow the use of brittle materials in situations
where tensile stress would normally occur. For example, the use of prestressed concrete,
and glass-fibre-reinforced plastics in pressure vessels construction.
A detailed discussion of the factors that determine the fracture toughness of materials
can be found in the books by Institute of Metallurgists (1960) and Boyd (1970). Gordon
(1976) gives an elementary, but very readable, account of the strength of materials in
terms of their macroscopic and microscopic structure.

7.3.4. Hardness
The surface hardness, as measured in a standard test, is an indication of a material’s ability
to resist wear. This will be an important property if the equipment is being designed to
handle abrasive solids, or liquids containing suspended solids which are likely to cause
erosion.

7.3.5. Fatigue
Fatigue failure is likely to occur in equipment subject to cyclic loading; for example,
rotating equipment, such as pumps and compressors, and equipment subjected to pressure
cycling. A comprehensive treatment of this subject is given by Harris (1976).