2.6. SLITTING (CRACK COMPLIANCE) METHOD 25
like wood whose physical properties are highly variable and typically are not accurately known
without further testing.
Figure 2.3c shows another geometrical variant of the Splitting Method, commonly used
to assess the circumferential residual stresses in thin-walled heat exchanger tubes. Diameter
increase caused by the opening of the cut indicates tensile circumferential stresses around the
exterior surface of the tube, balanced by compressive stresses around the interior. Diameter de-
crease indicates the opposite. e experimental method is well-established and is specified by
ASTM Standard Test Procedure E1928. e approximate sizes of the surface circumferential
stresses can be estimated using:
D ˙
Et
1
2
D D
0
DD
0
; (2.1)
where
are the circumferential bending stresses at the outer (C) and inner () surfaces, D
0
and D, respectively, are the diameters before and after making the cut, t is the wall thickness,
E is Young’s modulus and is Poisson’s ratio. e calculated results are approximate because
Equation (2.1) is based on an assumption of linearly varying bending stresses through the tube
wall thickness. In practice, the residual stresses do not vary exactly linearly.
2.6 SLITTING (CRACK COMPLIANCE) METHOD
From a conceptual point of view, the Slitting Method, schematically illustrated in Figure 2.4, is
a further variant of the Two-Groove Method. In this case, relieved strain measurements are
sequentially made as slot cutting proceeds in a series of small incremental steps. e set of
strain measurements provides sufficient data for the evaluation of the stress profile within the
slot depth. is process differs from the typical Two-Groove measurement, where just a single
before-and-after strain measurement is made while cutting the slots in just one step directly to
the steady-state depth. is latter measurement gives only the weighted average stress within
the cut depth, with no within-depth profile information.
e purpose of using two deep slots in the Two-Groove Method is to create full strain
relief in the enclosed material, thereby providing a simple residual stress evaluation. Since a
stress profile measurement using intermediate slot depths involves measurement and analysis
of partial strain reliefs, full strain relief never occurs except perhaps at the very end. us, the
second slot does not provide any computational advantage and so is omitted to simplify the
required experimental procedure. In addition, as shown in Figure 2.4, the strain gauge position
is not limited to being on the specimen top surface. Other locations are also useful, notably on
the opposite surface of the material specimen. As a general rule-of-thumb, strain measurements
are most sensitive to nearby stresses. us, the top and bottom surface strain gauges shown in
Figure 2.4 are useful for determining stresses near their respective locations, thereby achieving
better spatial coverage.