Fatigue damage of unalloyed steels in the high cycle regime is
governed by localized cyclic plastic deformations and subsequent
crack initiation. The extent of early microplastic deformations
depends on the applied stress level, stress concentration at
macroscopic notches, surface treatment, residual stresses etc. The
onset of a nonlinear material response can be regarded as an early
indicator of fatigue damage. During fatigue loading thermoelastic
coupling and thermoplastic dissipation cause characteristic
temperature variations in tested specimens which have been assessed
by a highly sensitive infrared camera. A specialized data processing
method in the time domain has been developed which allows to
separate the different contributions to the measured temperature
signal. In contrast to other methods - as e.g. measuring the rise of
mean temperature during fatigue loading - the proposed methodology
is based on measurements during the stabilized temperature regimen
and offers very high spatial resolution of localized phenomena.
Investigations have been made on mildly notched cylindrical and also
on welded specimens. The results confirm the close relation between
the local temperature signal and typical fatigue phenomena. The new
methodology allows for a much better localization and quantification
of effects as cyclic plasticity, crack initiation, crack growth etc.
The following paper presents considerations and experimental results
of an application of thermography to the local assessment of fatigue
damage.
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