Page 96 - Manufacturing Engineering and Technology - Kalpakjian, Serope : Schmid, Steven R.

Section 2.9 Impact 75

2.8 Creep 0.8
§, 0_7 -
Creep is the permanent elongation of a component under a static é 06- Titanium
load maintained for a period of time. This phenomenon occurs in U)
metals and certain nonmetallic materials, such as thermoplastics 2 Steels
17, 0.5 -
and rubbers, and it can occur at any temperature; lead, for exam-
E Cast irons
ple, creeps under a constant tensile load at room temperature.
E OA _ Copper alloys
However, for metals and their alloys, creep of any significance : 0.3-
occurs at elevated temperatures, beginning at about 200°C for cu
é 0 2 _ Cast Aluminum alloys
aluminum alloys and at about 1500°C for refractory alloys. The
5 magnesium
mechanism of creep at elevated temperature in metals is general-
E 0_1 _ alloys Wrought
ly attributed to grain-boundary sliding (Section 1.4).
magnesium alloys
Creep is especially important in high-temperature applica-
0
tions, such as gas-turbine blades and similar components in jet 0 200 400 600 80010001200
engines and rocket motors; high-pressure steam lines, nuclear- Tensile strength (MPa)
fuel elements, and furnace components are also subject to creep.
Creep can also occur in tools and dies that are subjected to high
stresses at elevated temperatures during hot-working operations
FIGURE 2.| 1 Ratio of endurance limit to tensile
such as forging and extrusion. strength for various metals, as a function of tensile
The creep test typically consists of subjecting a specimen to strength. Because aluminum does not have an
a constant tensile load (hence, constant engineering stress) at endurance limit, the correlations for aluminum
elevated temperature and measuring the changes in length at are based on a specific number of cycles, as is seen
various time increments. A typical creep curve usually consists of in Fig. 2.16.
primary, secondary, and tertiary stages (Fig. 2.18). The specimen
eventually fails by necking and fracture, called rupture or creep rup- Rupture
ture. As expected, the creep rate increases with specimen temperature
and applied load.
Design against creep usually involves a knowledge of the second- C
ary (linear) range and its slope, because the creep rate can be determined `§ Primary TGVUHVY
reliably only when the curve has a constant slope. Generally, resistance 5
to creep increases with the melting temperature of a material. Stainless l‘_ Secondary
steels, superalloys, and refractory metals and alloys are thus commonly
used in applications where resistance to creep is required. lnstamafleous
deformation
Stress Relaxation. Stress relaxation is closely related to creep. In Time
stress relaxation, the stresses resulting from loading of a structural
component decrease in magnitude over a period of time, even though FIGURE 2.l8 Schematic illustration of a
the dimensions of the component remain constant. An example is the typical creep curve. The linear segment of
decrease in tensile stress of a wire in tension between two fixed ends the curve (secondary) is used in designing
(as in the wires in a piano); other examples include stress relaxation components for a specific creep life.
in rivets, bolts, guy wires, and similar parts under either tension, compression, or
flexure. Stress relaxation is particularly common and important in thermoplastics
(Section 7.3).

2.9 Impact

In many manufacturing operations and machinery components, materials are sub-
jected to impact, or dynamic loading-for example, in high-speed metalworking
operations such as heading to make bolt heads, and in drop forging (Section 14.9).
A typical impact test consists of placing a notched specimen in an impact tester and
breaking the specimen with a swinging pendulum.