Transcript Slide 1

CREEP
CASE STUDIES
THE CREEP OF LEAD WATER PIPES
Lead pipes on a 75-year-old building in
southern New England. The creepinduced curvature of these pipes is
typical of Victorian lead water piping.
The two examples of sagging lead pipes
analysed in this case study. All dimensions
are in mm.
A map for antimonial lead with
a grain size of 50 µm, showing
the conditions of operation of
the pipes. Both deform by
diffusional flow.
Antimonial lead with a grain size
of 1 mm. If the pipes had this
grain size they would deform
much more slowly than they do.
CREEP OF A SUPERALLOY TURBINE BLADE
The approximate distribution of axial
stress and temperature along a turbine
blade operating in the first sage of a
typical turbine of the 1960s.
When a turbine is running at a steady speed,
centrifugal forces subject each rotor blade to
an axial tension. If the blade has a constant
cross section, the tensile stress rises linearly
from zero at its tip to a maximum at its
root. As an example, a rotor of radius r of 0.3
m rotating at an angular velocity of 1000
radians/s (11,000 r.p.m.) induces an axial
stress of order 10-3 µ.
Summary of average steady running conditions on blade
Range of temperature
Range of Stress
σs/µ
Maximum acceptable
strain rate
0.45–0.58 TM
0→2.3 x 10-3
~10-8 /s
Improvements in blade microstructure
Equiaxed
Crystal Structure
Directionally
Solidified Structure
Single Crystal
From Cervenka, Rolls Royce, 2000
A map for pure nickel with a grain size
of 100 µm, showing the conditions of
operation of the blade
A map for MAR–M200, with the
same grain size as that for the
nickel of Fig. 19.10 (100 µm). The
shaded box shows the conditions
of operation of the blade.
A map for MAR–M200 with a large grain size (10 mm)
approximating the creep behaviour of directionally
solidified or single crystal blades. The shaded box
shows the conditions of operation of the blade.
“Superalloys as a class constitute the currently reigning
aristocrats of the metallurgical world. They are the alloys which
have made jet flight possible, and they show what can be
achieved by drawing together and exploiting all the resources of
modern physical and process metallurgy in the pursuit of a very
challenging objective.”
from R.W. Cahn The coming of materials science, 2001.
THE CREEP OF TUNGSTEN LAMP FILAMENTS
Typical light bulb specifications
Typical dimensions of a
40 Watt, 110 Volt,
tungsten filament lamp.
The filament is a simple
coil of doped tungsten
110 volt single-coiled
lamps
25 Watt
40 Watt
Burning temperature (°C)
Design life (s)
Turns/metre (m-1)
Spacing of turns, S (mm)
Wire diameter, d (mm)
Coil diameter, 2R (mm)
Total length of wire (mm)
Total length of coil (mm)
Total mass of coil (mg)
Number of intermediate
supports
2250–2350
3.6 x 106
2.6 x 104
0.038
0.030
0.15
660
41
9.0
3
2400–2500
3.6 x 106
2.4 x 104
0.043
0.036
0.14
430
41
7.2
3
Summary of conditions under which filaments operate
Power of lamp
Range of σs/μ
Range of T/TM
25 Watt
40 Watt
0→1.0 x 10-4
0→6.2 x 10-5
0.68→0.71
0.72→0.75
Maximum
3.0 x 10-9
3.6 x 10-9
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