I. Residual Stresses and Their Measurement
Residual (locked-in) stresses in a structural material or
component are those stresses that exist in the object without
(and usually prior to) the application of any service or
other external loads. Manufacturing processes are the
most common causes of residual stress. Virtually all
manufacturing and fabricating processes — casting, welding,
machining, molding, heat treatment, etc. — introduce
residual stresses into the manufactured object. Another
common cause of residual stress is in-service repair or
modification. In some instances, stress may also be induced
later in the life of the structure by installation or assembly
procedures, by occasional overloads, by ground settlement
effects on underground structures, or by dead loads which
may ultimately become an integral part of the structure.
The effects of residual stress may be either beneficial
or detrimental, depending upon the magnitude, sign,
and distribution of the stress with respect to the loadinduced
stresses. Very commonly, the residual stresses are
detrimental, and there are many documented cases in which
these stresses were the predominant factor contributing
to fatigue and other structural failures when the service
stresses were superimposed on the already present residual
stresses. The particularly insidious aspect of residual stress
is that its presence generally goes unrecognized until after
malfunction or failure occurs.