nondestructive characterization of fatigue processes in cyclically ...

2 nd International Workshop on Structural Health Monitoring, Stanford University, 1999 2
control of machining, heat treatment processes and material characteristics within
the production line [1, 2].
An especially complex task is the identification of fatigue damage in welded joints
with the aim of a fatigue life estimation for that part. The reason for this is that
welded joints consist of different zones with different material properties, e. g. yield
strength, ductility, hardness, grain structure, chemical composition etc., and residual
stresses [3]. In general, these zones of a weld can be divided into the formerly molten
weld bead, the heat affected zone (HAZ) which was not molten and the unaffected
base material. Besides the material properties, the fatigue life of a weld is
influenced by the size and sharpness of notches especially at the weld toe where
certain welding processes can produce undercutting or high reinforcement angles
reducing the fatigue strength. Additionally, so-called metallurgical notches at the
intersections of two microstructures with significantly different material properties,
can lead to local stress concentrations reducing the fatigue life, too.
To assess the fatigue behaviour of the entire welded joint it is essential to characterize
the microstructural processes and their influence on the residual stresses in each
zone of the weld during cyclic loading. Changes in microstructure and residual
stresses can be observed both in the period before crack initiation and during the
micro- and macrocrack growth period [4]. Additionally, it is well known that residual
stresses can influence the fatigue life of welded joints significantly [5]. Therefore,
an observed change in microstructural parameters and residual stresses could
be used to characterize the actual stage of fatigue of a loaded weldment. A major
drawback of nearly all nondestructive techniques for characterising microstructural
changes and residual stresses is the limitation to near-surface regions. The analysing
depths vary from a few microns using common X-ray diffraction to approximately 1
mm with micromagnetic or eddy current devices. Only neutron diffraction enables a
nondestructive investigation of residual stresses up to a few centimeters below the
surface. But since a nuclear reactor and extensive shielding is necessary for this
method, it is limited to small parts on a laboratory scale. For a further interpretation
of microstructural changes in near-surface regions, a study of the deformation behaviour
and the relaxation or build-up of residual stresses in the entire weld volume
by a FEM-simulation is essential. The present paper presents investigations on the
fatigue of cyclically loaded welds by the nondestructive testing methods using micromagnetic
Barkhausen noise analysis and X-ray diffraction with the further aim of
a fatigue life prediction.
2 EXPERIMENTAL SETUP AND EVALUATION PROCEDURE
The investigations were carried out using flat specimens of the structural steel
S355J2G3 with transverse butt welds according to Fig. 1. The pulsed gas-tungsten-
F m + F a·sin(t)
Strain Gauges
BNA and X-Ray
Measurements
thickness 10 mm
Fig. 1
Specimen geometry and location of strain gauges