dc.description.abstract |
Steel structures such as steel bridges greatly contribute to the socio economic development of
the world. The current traffic demand has exhausted the service life of steel bridges paving the
way for failures without prior warning due to fatigue. In fact, fatigue contributes to change the
microstructure of a material which fails below the yield point. Therefore, fatigue could be
considered as an issue related to materials, even though it is linked to the area of engineering.
Interestingly, several unavoidable stress types on structures occur on steel bridges due to
various reasons. As a result, avoiding fatigue on structures has become impossible during their
service life. The result of stress fluctuation has caused crack initiation on steel structures while
the initial stage is at a micro scale level and not visible to the naked eye. Thus, it should be
controlled at the initial stage avoiding adverse effects later. Although the conventional crack
repair techniques have extended service lives of structures they have led to numerous
drawbacks too.
The crack stop hole technique could be considered as an emergency repairing technique to
extend the fatigue life of a cracked steel structures that is quick, simple and economic. This
technique was successfully applied in the aerospace industry primarily, however there had
been irregularities due to the size of the hole with re-cracking appearing due to continuous
service loads. Carbon fiber reinforce polymer (CFRP) materials have become popular as it has
potential to replace the conventional repairing techniques with recent research focused on
CFRP materials due to its light weight, corrosion resistivity, damping characteristics, fatigue
resistivity and high tensile features. Therefore, this study proposes a crack stop hole (CSH)
technique combined with a CFRP strengthening method to acquire the lost capacity due to
fatigue in old structures with delaying re-cracking by further continue their services by steel
bridges in the road and railway network operate at present.
An experimental test program carried out to determine the behavior of strengthened and nonstrengthened
CSH
in
steel
members
subjected
to
low
cycle
flexural
fatigue. Overall, the test
program was focused on estimating yield strength losses and yield strength gained by CFRP.
Interestingly, various types of fatigue testing apparatus are available in the open market for a
relatively high cost which is not affordable in a university laboratory, thus a hydro-electric
controlling fatigue loading apparatus was designed and fabricated as an initiation to this
research study to fulfill this vacuum. In this development process, machine operation, and
development technique with finite element analysis on the test frame was investigated.
In the next phase of this research, a numerical model was developed using an advanced finite
element model (FEM) and results were validated using the laboratory test results. The
proposed numerical model was based on the cyclic J-integral method under the detect cyclic
mode. The test results agreed with the model results consisting nine key parameters affecting
the final results. This CFRP strengthened CSH technique is significantly enhanced fatigue life
of the structural members. This investigation reported the yield strength losses; which are in
the range of 13.4 % to 25.2 % compared to the non-conditioned and yield strength gains with
CFRP; which is in the range of 32.2 % to 45.3 % compared to the non-strengthened CSH with
the diameter varies from 4 mm to 25 mm. A considerable amount of strain controlled were
recorded by CFRP with respect to non-strengthened CSH. When considering the critical
parameter effects, the test results recorded a yield strength gain with respect to off-set distance;
which was in the range of 36 % to 131 % compared to the CSH at the midpoint. The yield
strength variation recorded due to the length of CFRP layer was in the range of 89 % to 223
% compared to the least length considered. This investigation recommended by CFRP
strengthened technique has significantly enhanced fatigue bearing capacity of structural
members with CSH. Design guidelines are developed for practical implementations. |
en_US |