36 2. RELAXATION TYPE RESIDUAL STRESS MEASUREMENT METHODS
finite element calculations by determining the stresses required to return the deformed surface
shape to a flat plane. e surface deformations are very small, so the EDM cutting and the sur-
face height map measurement must be done to very high precision. In addition, to avoid any
asymmetry effects, it is desirable to measure the surfaces on both sides of the cut and to use
the average surface height map. It is also possible to make further cuts on perpendicular planes
to get maps of the normal residual stresses on those planes. Figure 2.10d shows an example
measurement of the axial residual stress profile within the cross-section of a railway rail.
(a)
(b) (d)
(c)
Original
Stresses
Stress-free
After
Cutting
Stresses
to Restore
Flat Surface
Wheel Profile
Tensile
residual
stresses
from plastic
flow
Compressive
residual
stresses
from wear
44
35
26
17
9
0
-9
-17
-26
-35
-44
-52
-61
300
240
180
120
60
0
-60
-120
-180
-240
-300
-360
-420
σ
z
ksi
MPa
-60 -40 -20 0 20 40 60
tension compression
x(mm)
Figure 2.10: Contour Method. (a) Original stresses, (b) stress-free after cutting, (c) stresses to
restore flat surface, and (d) measured stress profile of a railway rail (diagrams courtesy of Michael
Prime, Los Alamos National Labs, USA).
2.12 SECTIONING METHOD
e previously described measurement methods each have a particular geometrical arrangement
designed to achieve a specific type of measurement. e Sectioning Method is not so much a
“method” as a generic approach that can be highly customized to fit specific individual specimens
and measurement needs. e approach is to choose a measurement procedure that combines
one or more of the previously described measurement methods and to use them sequentially
to reach the desired measurement needs. e chosen measurement procedure must be selected
with care to fit and perhaps also to exploit particular features of the specimen geometry. e
Sectioning Method involves making deformation measurements at multiple steps within the
sectioning process and at multiple locations. Any suitable deformation measurement technique
can be used, although most reported Sectioning Method examples tend to be dominated by
strain gauges.
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