Page 85 - Manufacturing Engineering and Technology - Kalpakjian, Serope : Schmid, Steven R.

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

The true area at the onset of necking is obtained from P = 0-Aneck = 0-A050-5,

Aneck = n = ()_5_ where cr is the true ultimate tensile strength. Hence,
.. g
Thus, P == 488 0.606 A ..> = 2900A k .
><
><
<
A ck 2 A e-0.5, Since UTS = P/Ao.
DE O
UTS = 296 MPa.
and the maximum load, P, is

200 leo
Siren#iii;
;_E/astic m
-‘ Od I
600- Tensile ‘\`\ .5 A
V -40 $
'ni ~ -150 Q5
Q. U9 "’
2 400 - -§ ‘5
jg Y -100 <3 sg
Q leld strength E §>
5 ,g -20 _
200- _50 3 U1
E\or\Qa‘\0“ Lu

0 _J O _I 0
0 200 400 600
Temperature (°C)

FIGURE 2.1 Effect of temperature on mechanical properties of a carbon steel. Most
materials display similar temperature sensitivity for elastic modulus, yield strength, ultimate
strength, and ductility.

2.2.6 Temperature Effects

Increasing the temperature generally has the following effects on stress-strain curves
(Fig. 2.7>¢
a. The ductility and toughness incease, and
b. The yield stress and the modulus of elasticity decrease.

Temperature also affects the strain-hardening exponent of most metals, in that n
decreases with increasing temperature. The influence of temperature is, however,
best described in conjunction with the rate of deformation.

2.2.7 Rate-of-deformation Effects
just as we can blow up a balloon or stretch a rubber band at different rates, we can
shape a piece of material in a manufacturing process at different speeds. Some
machines, such as hydraulic presses, form materials at low speeds; others, such as
mechanical presses, form materials at high speeds. To incorporate such effects, it is
common practice to strain a specimen at a rate corresponding to that which will be
experienced in the actual manufacturing process.
The deformation rate is defined as the speed at which a tension test is being
carried out, in units of, say, mfs. The strain rate, on the other hand, is a function of