PROCESS TECHNOLOGIES
FATIGUE FAILURE REDUCTION
POLIGRAT
HIGHER FATIGUE RESISTANCE
DUE TO POLIGRAT-PROCESSES
Fatigue failures, also called fatigue fractures,
mostly start small crack initiating structures
and defects. Cracks grow mostly undetected
until they finally cause a sudden forced
rupture of the reduced remaining cross
section.
The fatigue resistance i.e. the resistance
to fatigue cracking of dynamically loaded
components is largely determined by shape
and structure of near surface layers of
material.
More than 80% of fatigue cracks start at
local surface related structures. In many
cases the application of POLIGRAT-Processes
significantly can reduce the risk of fatigue
cracking and correspondingly increase
lifetime by several times.
POLIGRAT Processes eliminate or reduce
most of the near surface crack initiating
structures, which can be divided into two
groups:
• Cracks initiated by micro notches.
Essentially these are cracks at burrs and
edges and sharp structures of the profile
or crevices at grain boundaries. Stress
concentration at the base of notches cause
local concentration of tensile stresses with
the risk, that there will start a fatigue crack
with the consequence of later rupture.
• Crack initiating structures within a surface
related material layer with a thickness of
several micro meters, which are caused
by mechanical, thermal or chemical
influence from fabrication. These layers,
also called Beilby-layers, significantly
differ from the original base material with
respect to composition, structure and
stresses. They often contain local tensile
stresses, embedded foreign matter, oxides
coarse grains and precipitations at grain
boundaries, which are capable to initiate
cracks.
POLIGRAT-Processes controlled and stress
free remove all damaged material layers from
the surface with all the inherent stresses,
foreign matter and other defects. They bring
the undisturbed base metal with its best
properties to the surface. At the same time
edges and surfaces are deburred and the
micro structures smoothened and rounded.
Existing micro cracks and sharp structures
are removed or if they are larger in size
opened and rounded to a degree, that
they are no more notch factors. Revealing
of cracks also provides a reliable quality
control.
POLIGRAT-Processes are applicable widely
independent of shape and size of parts
and hardness of material. For most of
the commonly used metals and alloys
industrially proven processes are available.
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PECM Issue 36
Long time experience show, that on
dynamically heavily loaded components
an increase of fatigue resistance by the
factor 4 to 10 or more will result from the
application of POLIGRAT-Processes. Typical
applications are springs, turbine blades,
hydraulic parts and nozzles.
POLIGRAT provides comprehensive advice for
all applications and verification of achievable
results by processing samples of original
parts in their own laboratory.
Valentin-Linhof-Straße 19 D-81829
München
Tel. +49 (0)89 42778-0 / 01252 336337
Fax +49 (0)89 42778-309
email: [email protected]
or [email protected]
www.poligratuk.co.uk