| dc.description.abstract | The aim of the study is to contribute towards a better understanding of the effect of
residual stress and shakedown on fatigue strength of welded joints. Residual stress
may have a significant effect on the fatigue strength of welded joints. As a nonfluctuating
stress, it has an effect similar to that of the mean stress. Recently the
International Association of Ship Classification Societies (IACS) has issued Common
Structural Rules (CSR) for respectively tankers and bulk carriers. The effect of mean
stress in fatigue design is taken into account in both sets of rules. However, the
treatment is quite different, in particular with regard to residual stress and
shakedown effects. A comparative study of fatigue design procedures of several
rules/standards is reported, with emphasis on residual stress effects.
This work involves of several tasks, firstly the experimental study to investigate
behaviour of welding residual stress. Measurements of welding residual stress on
totally 26 of welded joint specimens have been performed using the sectioning
method. Various load cases comprising of static preloads and cyclic loads have been
applied to simulate loading history in marine structures. The initial magnitude of
residual stress was determined in as welded condition. The static preloads and cyclic
loads had effects in reducing or relaxing residual stress. The magnitude of residual
stress relaxation was determined by the highest tensile stress in the load history. The
compressive preloads had only a minor effect on residual stress relaxation. The cyclic
loads had effect on residual stress relaxation only in the first cycle. The empirical
formulae showing relationship between residual stress magnitudes after relaxation
and applied loads had been established that has aim to be incorporated in fatigue
assessment procedure. Hence, a fatigue assessment procedure which considers
residual stress in more realistic manner can be established.
Secondly, stress categories have to be determined in fatigue analysis. Structural
discontinuities lead in general to stress concentrations that significantly affect fatigue
strength. The effect of stress concentrations may be accounted by the hot-spot stress
approach using linear extrapolation of surface stresses. A longitudinal attachment
weld is studied experimentally and by finite element analyses (FEA). The detailed
stress measurements are used to evaluate various modelling techniques in FEA. The solid element and the shell element models are studied. The different models for
inclusion of weld stiffness are studied, with solid elements and shell elements. Based
on the results, recommendations are given for choice of element model and mesh
size.
Lastly, fatigue tests have been carried out using totally 59 of the small scale
specimens of welded joints applying stress ratio, R=0 and R=-1, as well as various
pre-load conditions. S-N curves have been plotted with different fatigue assessment
procedure, namely IACS CSR bulk-carrier (2006a), IACS CSR tanker (2006b), DNV
CN30.7 (2008), and IIW. Zhang-Moan proposed procedure based on Yamamoto &
Matsuoka’s work (2001) was also included. It has been found that by applying the
different procedures, scatter on S-N data were somewhat higher than the standard
design curves indicating that residual stress is a hidden parameter causing increased
scatter. Hence, the new improvement procedures based on modification of the IACS
CSR bulk-carrier procedure have been proposed. The empirical formulae of residual
stress relaxation established from previous study had been explicitly incorporated in
the mean stress correction factor led to reducing scatter of S-N data. | nb_NO |
