BRITTLENESS, TOUGHNESS, STIFFNESS, CREEP

BRITTLENESS is that property of a material that permits it to be only slightly deformed
without rupture. Brittleness is relative, no material being perfectly brittle, that is, capable
of no deformation before rupture. Many materials are brittle to a greater or less degree,
glass being one of the most brittle of materials. Brittle materials have relatively short
stress–strain curves. Of the common structural materials, cast iron, brick, and stone are
brittle in comparison with steel.
TOUGHNESS is the ability of the material to withstand high unit stress together with great
unit strain without complete fracture. The area OAGH, or OJK, under the curve of the
stress–strain diagram (Fig. 7), is a measure of the toughness of the material. The distinction
between ductility and toughness is that ductility deals only with the ability to deform,
whereas toughness considers both the ability to deform and the stress developed during
deformation.
STIFFNESS is the ability to resist deformation under stress. The modulus of elasticity is the
criterion of the stiffness of a material.
HARDNESS is the ability to resist very small indentations, abrasion, and plastic deformation.
There is no single measure of hardness, as it is not a single property but a combination
of several properties.
CREEP, or flow of metals, is a phase of plastic or inelastic action. Some solids, as asphalt
or paraffin, flow appreciably at room temperatures under extremely small stresses; zinc,
plastics, fiber-reinforced plastics, lead, and tin show signs of creep at room temperature
under moderate stresses. At sufficiently high temperatures, practically all metals creep
under stresses that vary with temperature; the higher the temperature, the lower the stress
at which creep takes place. The deformation due to creep continues to increase indefinitely
and becomes of extreme importance in members subjected to high temperatures, as parts
in turbines, boilers, superheaters, etc.


Mechanical Engineers’ Handbook: Materials and Mechanical Design, Volume 1, Third Edition.
Edited by Myer Kutz
2006 by John Wiley & Sons, Inc.

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