Creep (elongation under load) is defined as the time-dependent
strain that takes place under a given load. There are generally,
three distinct stages of zinc creep: - Primary - the creep
strain that occurs at a diminishing rate - Secondary - the creep
strain that show a minimum and almost constant rate - Tertiary -
the creep strain that exhibits an accelerated rate, usually
leading to rupture. The strain rate behaviour of zinc alloys
under stress is non-linear and time-temperature dependent, i.e.,
the ratio of stress to strain is not constant, even at low
stresses, and varies with time, resulting in continuous plastic
flow under applied load (creep). As a result, zinc alloys do not
have a single value for elastic modulus. Yield strength
(allowable stress) under sustained loading is dependent on the
allowable design strain (% elongation) over the required service
life. Working values for the elastic modulus and yield strength
are important for design purposes. These can be generated from
the experimental curve