Advancing the damage tolerance of laser beam welded steels using crenellation technique
Author(s)
Eren, Sefika Elvin
Type
Thesis
Abstract
Fatigue life extension, via the retardation of a growing crack under cyclic loading conditions, has
been an important issue for designers and operators to reduce the cost and to improve the structural
safety of welded structures. The methods to be employed for retarding fatigue crack propagation,
needless to say, should ideally be knowledge based and practical (i.e. easy to implement, cost
effective and should not lead to weight increase etc.).
In this study, a novel approach based on modifying the distribution of the stress intensity factor (SIF)
in plates using thickness variations is presented. By varying thickness via the introduction of ribs,
“crenellations”, on one side or surface of the welded or non-welded plates, it was shown that this
novel technique can contribute to the fatigue crack propagation life of plates substantially.
Furthermore, crenellations can potentially lead to weight reduction for a given target fatigue crack
propagation life as well.
This research programme aimed to gain a fundamental understanding of the mechanisms involved in
the damage tolerance behaviour of crenellated parent material and laser beam welded steel plates
which operate, in principle, under cyclic loading. Particular emphasis was given to describe the
crack growth retardation mechanism using the crenellation technique.
The “crenellations” were introduced on the surface of the steel sheets and plates of medium grade
and high-strength steels with and without laser beam welds. Experiments were conducted to study
and investigate the fracture toughness, fatigue response and fatigue life of crenellated plates. In
parallel, numerical analyses were carried out to compute the actual SIF for the combinations of
crenellations in order to correlate the fatigue crack growth data.
Investigations focused on fatigue crack propagation (FCP) under constant amplitude loadings, with
cracks growing within (in butt joints) and towards (in fillet joints) the fusion zone. A number of
crenellation configurations was investigated and the fatigue crack growth data were correlated using
SIF. It was shown that the edge regions -where the thickness sharply changes from thin to thick or
vice versa- and crack length have a substantial effect on the retardation and acceleration of the
fatigue cracks. Factors such as sudden thickness variation and crack lengths -where plasticity can be
substantial depending on the load level- affect crack retardation. However, it was shown that the
evaluation of the correct SIF with respect to thickness and crack length conveniently describes
fatigue cracking in these crenellated plates and allows the behaviour of these plates under cyclic
loads to be compared with the behaviour of the reference, as-received plates subject to the same
conditions.
Based on the present tests and analyses, it is expected that an improvement in the fatigue crack
propagation life of crenellated plates can be as much as 50 - 120 % depending on the configuration machined on the parent material and fillet welded plates, and crack arrest is very likely to occur
depending on the crenellation configuration machined on the butt welded plates.
The roles of local microstructural properties, residual stress profiles and K-distribution variations
(due to crenellations) were discussed. The governing micro-mechanism(s) of failure of the plain and
stiffened panels were identified and compared. The regions -where there is a rapid thickness change-exhibit
complex crack fronts which contribute to the retardation of the crack under cyclic loading
conditions. However, the overall cracking mechanism is the same as the mechanism observed in the
un-crenellated, reference plates.
Additionally, a numerical damage model based on strain energy was used to predict fatigue crack
growth in the crenellated plates. In general, the results were conservative. The comparisons
presented are used to identify the future trends in the development of numerical techniques that
should be developed to complement testing. This is imminent especially for the cases, where large
components with various features need to be tested. Given the difficulty and expense of running such
industrial scale test programmes, numerical methods will be crucial for the optimization of the
number of tests that may be necessary.
This work and the conclusions presented are novel, and it was shown that no investigations exists in
the open literature and on-going publicly funded projects, that considers both experimental and
FE-methods to study the effect of crenellations. Furthermore, no previous work -using both
extensive experimental and numerical programmes- has been reported on the role of crenellations on
steels and their weldments.
The results of this thesis are expected to have a significant impact on the existing knowledge and
design as well as methods to extend fatigue life of the welded structures where crack propagation
plays an important role.
been an important issue for designers and operators to reduce the cost and to improve the structural
safety of welded structures. The methods to be employed for retarding fatigue crack propagation,
needless to say, should ideally be knowledge based and practical (i.e. easy to implement, cost
effective and should not lead to weight increase etc.).
In this study, a novel approach based on modifying the distribution of the stress intensity factor (SIF)
in plates using thickness variations is presented. By varying thickness via the introduction of ribs,
“crenellations”, on one side or surface of the welded or non-welded plates, it was shown that this
novel technique can contribute to the fatigue crack propagation life of plates substantially.
Furthermore, crenellations can potentially lead to weight reduction for a given target fatigue crack
propagation life as well.
This research programme aimed to gain a fundamental understanding of the mechanisms involved in
the damage tolerance behaviour of crenellated parent material and laser beam welded steel plates
which operate, in principle, under cyclic loading. Particular emphasis was given to describe the
crack growth retardation mechanism using the crenellation technique.
The “crenellations” were introduced on the surface of the steel sheets and plates of medium grade
and high-strength steels with and without laser beam welds. Experiments were conducted to study
and investigate the fracture toughness, fatigue response and fatigue life of crenellated plates. In
parallel, numerical analyses were carried out to compute the actual SIF for the combinations of
crenellations in order to correlate the fatigue crack growth data.
Investigations focused on fatigue crack propagation (FCP) under constant amplitude loadings, with
cracks growing within (in butt joints) and towards (in fillet joints) the fusion zone. A number of
crenellation configurations was investigated and the fatigue crack growth data were correlated using
SIF. It was shown that the edge regions -where the thickness sharply changes from thin to thick or
vice versa- and crack length have a substantial effect on the retardation and acceleration of the
fatigue cracks. Factors such as sudden thickness variation and crack lengths -where plasticity can be
substantial depending on the load level- affect crack retardation. However, it was shown that the
evaluation of the correct SIF with respect to thickness and crack length conveniently describes
fatigue cracking in these crenellated plates and allows the behaviour of these plates under cyclic
loads to be compared with the behaviour of the reference, as-received plates subject to the same
conditions.
Based on the present tests and analyses, it is expected that an improvement in the fatigue crack
propagation life of crenellated plates can be as much as 50 - 120 % depending on the configuration machined on the parent material and fillet welded plates, and crack arrest is very likely to occur
depending on the crenellation configuration machined on the butt welded plates.
The roles of local microstructural properties, residual stress profiles and K-distribution variations
(due to crenellations) were discussed. The governing micro-mechanism(s) of failure of the plain and
stiffened panels were identified and compared. The regions -where there is a rapid thickness change-exhibit
complex crack fronts which contribute to the retardation of the crack under cyclic loading
conditions. However, the overall cracking mechanism is the same as the mechanism observed in the
un-crenellated, reference plates.
Additionally, a numerical damage model based on strain energy was used to predict fatigue crack
growth in the crenellated plates. In general, the results were conservative. The comparisons
presented are used to identify the future trends in the development of numerical techniques that
should be developed to complement testing. This is imminent especially for the cases, where large
components with various features need to be tested. Given the difficulty and expense of running such
industrial scale test programmes, numerical methods will be crucial for the optimization of the
number of tests that may be necessary.
This work and the conclusions presented are novel, and it was shown that no investigations exists in
the open literature and on-going publicly funded projects, that considers both experimental and
FE-methods to study the effect of crenellations. Furthermore, no previous work -using both
extensive experimental and numerical programmes- has been reported on the role of crenellations on
steels and their weldments.
The results of this thesis are expected to have a significant impact on the existing knowledge and
design as well as methods to extend fatigue life of the welded structures where crack propagation
plays an important role.
Date Issued
2012-07
Date Awarded
2012-11
Copyright Statement
Attribution NoDerivatives 4.0 International Licence (CC BY-ND)
Advisor
Nikbin, Kamran
Garwood, Stephen
Davies, Catrin
Publisher Department
Mechanical Engineering
Publisher Institution
Imperial College London
Qualification Level
Doctoral
Qualification Name
Doctor of Philosophy (PhD)