Page 103 - Manufacturing Engineering and Technology - Kalpakjian, Serope : Schmid, Steven R.

Chapter 2 Mechanical Behavior, Testing, and Manufacturing Properties of Materials

(H) (D) (C)

FIGURE 2.3! Distortion of parts with residual stresses after cutting or slitting: (a) flat sheet
or plate; (b) solid round rod; (c) thin-walled tubing or pipe.

Tensile residual stresses on the surface of a part are generally undesirable, be-
cause they lower the fatigue life and fracture strength of the part. This is due to the
fact that a surface with tensile residual stresses cannot sustain additional tensile
stresses from external forces as high as those that a surface free from residual stres-
ses can. This reduction in strength is particularly characteristic of brittle or less duc-
tile materials, in which fracture takes place with little or no plastic deformation
preceding fracture. Tensile residual stresses can also lead, over a period of time, to
stress cracking or to stress-corrosion cracking of manufactured products (Section
2.1O.2). Compressive residual stresses on a surface, on the other hand, are generally
desirable. In fact, in order to increase the fatigue life of components, compressive
residual stresses can be imparted to surfaces by techniques such as shot peening and
surface rolling (Section 342).

Reduction and Elimination of Residual Stresses. Residual stresses can be reduced
or eliminated either by stress-relief annealing or by a further deformation of the
part, such as stretching it. Given sufficient time, residual stresses may also diminish
at room temperature (by relaxation of residual stresses). The time required for relax-
ation can be greatly reduced by raising the temperature of the workpiece.

2.12 Work, Heat, and Temperature

Almost all the mechanical work in plastic deformation is converted into heat. This
conversion is not complete, because a portion of this work is stored within the
deformed material as elastic energy. Known as stored energy (Section 1.7), it is gen-
erally 5 to 10% of the total energy input; in some alloys, however, it may be as high
as 30%.
In a simple frictionless deformation process, and assuming that work is
completely converted into heat, the theoretical (adiabatic) temperature rise, AT, is
AT =
given by

P (2.15)
where u is the specific energy (work of deformation per unit volume), p is the density,
and c is the specific heat of the material. It can be seen that higher temperatures are
associated with large areas under the stress-strain curve and with smaller values of
specific heat. However, such physical properties (Chapter 3) as specific heat and ther-
mal conductivity can also depend on temperature; thus, they must be taken into
account in the calculations.
The temperature rise for a true strain of 1 (such as occurs in a 27 mm-high spec-
imen when it is compressed down to 10 mm) can be calculated to be as follows; alu-
minum, 75°C; copper, 14()°C; low-carbon steel, 280°C; and titanium 57O°C. In