About residual stresses:
During the
manufacture and processing of a component, the residual stresses arise and
change to a considerable extent. Mechanical processing (grinding, turning,
milling, shot peening, drawing, rolling, forging, ...), thermal (structural
transformation, annealing processes) and chemical processes (coating,
oxidation, corrosion, ...) as well as combined processes (nitriding,
nitrocarburizing) lead to changes in the residual stress state.
Fig. 2. Gear segment with grinding burns
Residual
stresses can reach values in the order of magnitude of the yield or elongation
limit of the current material condition. External loads acting on the component
during operation add to the existing residual stresses - in unfavorable cases,
exceeding the local strength can lead to cracks or complete failure of the
component.
The
following 3 cases of residual stresses can be distinguished:
1. tensile
stress
2.
compressive stress
3. shear
stress
The service
life of a component that is subjected to alternating stress under operating
load depends, among other things, on its residual stress state. A viable solution for
increasing the load-bearing capacity of the component or its design service life can be
the targeted incorporation of residual compressive stresses in the functionally
relevant contact surface zone. This requires a controlled
approach to heat treatment and material processing with knowledge of the level,
direction and distribution of the residual stresses generated in the edge zone.
On the other hand, residual tensile stresses should always be avoided as they promote the
formation of cracks and reduce the service life and load-bearing capacity of
the component.
X-ray
residual stress analysis in accordance with DIN EN 15305 is a precise and
proven method for determining residual stresses. X-ray diffractometers are used
for this purpose, which make it possible to determine the crystal lattice
spacing using X-rays with a defined wavelength and with the help of Bragg's law:
\begin{multline}
\shoveleft \lambda = 2d \cdot \sin \theta,
\end{multline}
where \(\lambda\) is the wavelength of the X-rays, \(d\) is the lattice plane and \(\theta\) is the
measured diffraction angle
As a rule,
the sin²psi method is used for this purpose, in which the direction-dependent
grating strain is determined at different angles between the surface normal and
the normal of the diffraction grating plane.
In addition
to the value of the residual stress, the X-ray measurement also provides the
value of the half-width. The half-width is a sensitive indicator of the
previous thermal load on the component, e.g., from a grinding process.
By
measuring and recording the residual stress and the half-width as a curve over
the process-sensitive thickness of the edge zone, a characteristic fingerprint
can be created that is highly informative about the quality of the hard fine
machining.
Fig. 3. Residual stress at and below the material surface due to grinding burns
Measurement method to determine the residual stresses:
Measurement of residual stress on the component surface according to the d-sin²psi method in
accordance with EN15305:2009 using an X-ray diffractometer with Cr-K radiation in the agreed
measurement directions parallel to the component surface. Creation of depth profiles by additional
preparation of the measuring point using an electrolytic polishing process to generate a largely
residual stress-neutral removal on the workpiece surface. The residual stress is then determined again
at the same point.
The
material removal achieved in each step is determined on a depth gage and
assigned to the respective residual stress measured value. The residual stress
depth profile is displayed graphically as a plot of the residual stress values
over the depth steps.
The X-ray
half-width is also determined for each individual residual stress measurement
and displayed accordingly. The values are listed in tabular form in the written
results report.
About the author
Dapprich Ingenieurbüro works in cooperation with RD Motion to provide a complete service package
around the measurement of residual stresses by X-ray diffraction. The bureau has broad expertise
in the field of non-destructive X-ray diffraction testing, who will be able to advise you on which
materials are suitable and support you in the selection and determination of reasonable measuring
positions on your component and how you can specifically change the residual stresses and their
relevance for your component. Preorder the testing service (id: 08-009) or contact us directly to get more info and
reserve a slot for performing your required X-ray diffraction tests.
Back to blog
Go to next
post
