In the present work, an investigation of the fatigue life benefits emerging from
fillet weld geometries optimization has been carried out.
At first, an introduction to ageing mechanisms, corrosion and especially fatigue,
acting on operating marine structures has been made. Residual stresses at
weld toes, stress modes, and types, geometrical factors (weld angle, toe radius,
leg length), welding techniques selected, post-welding treatment and plate‟s
material are some of the principal factors affecting the fatigue life of a fillet weld
joint.
Especially, the accuracy of various approaches in fatigue life estimation of
specific geometries under pre-set types and levels of stress is studied. It is
evident so far that even the notch stress concept is the most accurate method
based on S-N curves, the Fracture Mechanics approach can offer more
accurate solutions of a crack development through the material. Towards this, a
literature review on crack evolution aspects in welded and non-welded plates
under bending and tension was performed; substantial parameters were
determined and finally implemented in the LEFM model which was used for the
simulation purposes of Chapter 6.
As far as the crack aspect ratio evolution is concerned, an extensive reference
is available in literature since many researchers have investigated its
contribution to the determination of geometrical paths, commonly known as
“Preferred Propagation Paths”. Their significance is related with our ability to
determine accurate SIF solutions leading to precise fatigue life estimations.
A typical fillet weld joint 2-D model has been developed in CAE Abaqus
software and a Finite Element Analysis of subject T-profile has been carried out.
Through this analysis, the fillet weld angle, the weld leg length, the weld toe
curvature radio ρ and the carrying load plate thickness are examined for their
impacts on the maximum surface stress. Finally, a number of stress mitigating
measures are proposed and their effects are analyzed.
Undoubtedly, the notch stress concept today is gradually gaining more and
more acceptance among other fatigue analysis practices, hence the need for an
estimation of the actual surface stresses along fillet weld toes, has become
imperative. Towards this, different 2-D geometries are tested against stress
concentration factors developed at weld toes, which are calculated on the basis
of maximum in-plane principal stresses over nominal stresses in mode I pure
bending and pure tension respectively. Moreover, validation with corresponding
results from literature is provided. Finally, three different concepts for reducing
the maximum surface stresses are presented. The first one proposes grinding
of the weld toe area and formulation of an artificial U-notch or a part- circular
profile. The second one applies to non-penetrating welds and assumes the
existence of a root gap of a specific geometry which is related to the fatigue life
and stress concentration factor of the fillet weld joint. Last but not least, the
relatively recent concept of the variable radius notch is discussed, even though
it is applicable mostly to notched bodies, not weld joints.
Afterwards, a Linear Elastic Fracture Mechanics analysis of reference 2D fillet
weld model is demonstrated. A number of geometrical parameters considered
at previous stage for their impact on surface Stress Concentration levels at the
weld toe region, have been correlated to fatigue life benefits in terms of
increased number of stress cycles till failure.
An extensive analysis of 9 different T-butt weld joint geometries has been
provided in order to investigate how positively a possible SCF reduction can
affect the fatigue life of a weld joint. Essential geometric variations (weld angle,
length, toe radius, root slot) were considered in the 2D model. All calculated
benefits both in pure bending and pure tension cases have been reported
accordingly.
Based on a linear interpolation of the points scatter (SCF, N-cycles) both in
banding and tension, it was observed that a surface stress mitigation of 1%
could lead to 1,33 up to 2,5% fatigue life benefit in the range of SCF=2 – 2,5. It
is evident so far that the geometrical optimization of a weld joint in respect of
notch stress mitigation can be a powerful tool both in shipbuilding and
maintenance practice in the future. However, technically wise their application
may incur high initial costs of improved tools of welding and post welding
treatment and robots even though it would consist a cost effective solution in a
medium/long term basis.
Finally, the above process is followed by a reliability analysis of the most critical
geometrical parameters affecting the fatigue life of a fillet weld joint. Reliability
assessment results concerning medium, high and low cycle fatigue are provided
and a comparative analysis of each factor‟s impact on fatigue life has been
carried out.
| Identifer | oai:union.ndltd.org:CRANFIELD1/oai:dspace.lib.cranfield.ac.uk:1826/8470 |
| Date | 09 1900 |
| Creators | Louvros, Dimitrios |
| Contributors | Brennan, Feargal, Kolios, Athanasios |
| Publisher | Cranfield University |
| Source Sets | CRANFIELD1 |
| Language | English |
| Detected Language | English |
| Type | Thesis or dissertation, Masters, MSc by Research |
| Rights | © Cranfield University 2013. All rights reserved. No part of this publication may be reproduced without the written permission of the copyright owner. |
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