108 research outputs found
Sea ice thickness, ice loads, and navigability during the North Pole cruise CC110823 of Le Commandant Charcot in August 2023. Cruise report.
Get PDFFatigue life extension of existing welded structures via high frequency mechanical impact (HFMI) treatment
Get PDFHigh-Frequency Mechanical Impact (HFMI) is one of the post-weld treatment methods. In this study, comparative axial fatigue tests were conducted on as-welded and HFMI-treated welded transverse attachment details. The test results demonstrated the efficiency of HFMI-treatment in fatigue life extension of cracked welded structures, providing that the existing crack size is less than 1.2 mm. Cracks were created in some specimens through fatigue testing before HFMI-treatment, while other specimens were not subjected to any fatigue loading prior to treatment. Many of the treated specimens ran out after 10 million cycles of loading when tested at a stress range of 150 MPa. Therefore, the stress range was increased to 180 MPa or 210 MPa. No remarkable difference was found between the fatigue strength of the crack-free and the cracked treated specimens. It was found that the induced compressive residual stress can exceed the material yield limit, and reach a depth larger than 1.5 mm in most cases. The induced compressive residual stress, the local material hardening, the increase in weld toe radius, the change in crack orientation and the shallowness of the crack size were the causatives of the obtained long fatigue lives of the HFMI-treated specimens. Besides, linear elastic fracture mechanics calculations were conducted to predict the fatigue lives of as-welded and HFMI-treated details. The results were in agreement with the experiment. Moreover, the calculations showed that the initial crack size, the clamping stress and the induced compressive residual stress were the main factors behind the scatter in fatigue lives
Crack detection via strain measurements in fatigue testing
Get PDFFatigue cracks have appeared as a significant issue for joints and connections in existing steel structures in the last decades. Therefore, those are a major inspection and maintenance matter for any steel structure\u27s operator. This emphasises the importance of using a reliable detection method to determine the crack size and assessing the severity of such a crack on the structural integrity of a structure. In this article, the effectiveness of strain measurement in detecting fatigue cracks in transversal non‐load carrying welded attachment subjected to out of plane axial loading is studied. Numerical analysis and experimental investigations allowed to correlate the decrease in strain measured by attached gauges to the crack depth at the weld toe. In addition, different strain evolution patterns were found during fatigue testing, and the fracture surfaces of the specimens were observed to interpret these patterns. Moreover, the crack position with respect to the weld toe surface was predicted via strain measurements
Ultra-low cycle fatigue of ship hull structure-an alternately-cyclically loaded four-point bending test of a large box girder.
Get PDFUltra-low cycle fatigue (ULCF) refers to material failure at small number of loading cycles. For large complex structures like ships, the damage from ULCF can bring hazardous consequences. In this study, an alternately-cyclically loaded four-point bending test of a large box girder is introduced as the specimen to represent the ULCF of ship hull structure. In every load during the test, large deformation is applied to the specimen even after reaching its ultimate hull girder strength (UHGS), thus extensive plastic deformation and obvious fracture can occur in the specimen. The severely damaged specimen is further tested until 1.5 cycles of bending are finished, thus the test of post-damage box girder is realized. Moreover, the box girder is divided into 3 sub-sections, which show different but still interacting structural behavior. The result of the test shows the structural behavior of a large complex structure suffering severe damage during alternate hogging and sagging after reaching its UHGS, which corresponds to the consequence of ULCF. The presented ULCF test also provides experiences for investigations of large complex structures with existing damages or after accidental loads. Considering the number of cycles in the test, this study can bridge the gap between monotonic overload and ultra-low cycle fatigue
The non-linear behavior of aqueous model ice in downward flexure
Full text linkAs aqueous model ice is used extensively in ice tanks tests on the
performance of ship hulls in sheet ice, it is imperative that such model ice
replicate the main flexural strength behavior of sheets of sea ice and
freshwater ice. Ice tanks use various types of aqueous model ice types, each of
which contain brine dopants to scale-reduce ice-sheet strength. Dopants,
though, introduce non-linear trends in the scaled flexural behavior of model
ice sheets, and can affect ice loads and ice-rubble at ship-hulls and
structures. This paper analyzes the non-linear behavior of model ices, and
shows that all types behave non-linearly in flexure independent from crystal
structure or chemical dopant. Such behavior is attributable to plasticity and
vertical variations in stiffness and strength through sheets of model ice.
Additionally, the problematic formation of a top layer in model ice sheets is
shown to have a greater impact of sheet behavior than the literature reports
heretofore. There remains a significant knowledge gap regarding the freezing
and movement of brine dopants within ice sheets and their impact on the
non-linear behavior. Additionally, it is found that the Hertz method for
estimating the Cauchy number of model ice does not reflect the actual
deformation behavior of model ice and should be revised.Comment: 45 pages, 20 figure
Interactions between irregular wave fields and sea ice: A physical model for wave attenuation and ice breakup in an ice tank
Get PDFIrregular, unidirectional surface water waves incident on model ice in an ice tank are used as a physical model of ocean surface wave interactions with sea ice. Results are given for an experiment consisting of three tests, starting with a continuous ice cover and in which the incident wave steepness increases between tests. The incident waves range from causing no breakup of the ice cover to breakup of the full length of ice cover. Temporal evolution of the ice edge, breaking front, and mean floe sizes are reported. Floe size distributions in the different tests are analyzed. The evolution of the wave spectrum with distance into the ice-covered water is analyzed in terms of changes of energy content, mean wave period, and spectral bandwidth relative to their incident counterparts, and pronounced differences are found between the tests. Further, an empirical attenuation coefficient is derived from the measurements and shown to have a power-law dependence on frequency comparable to that found in field measurements. Links between wave properties and ice breakup are discussed
Condition Monitoring of Ship Propulsion Systems: State-of-the-Art, Development Trend and Role of Digital Twin
Get PDFThis paper describes the current implementations and development trends of condition monitoring as it pertains to ship propulsion systems. In terms of total incidents in the shipping industry in the last five years, failures relating to the propulsion system represent the majority. Condition monitoring offers effective early detection of failure which translates to increased reliability and decreased maintenance costs. Current industrial practices are often limited to performance monitoring rather than condition monitoring. Special focus is afforded to how condition monitoring is implemented on board ships, which regulatory codes are relevant and the summary of state-of-the-art research in marine machinery. Moreover, operation and monitoring in extreme environmental conditions, such as the Arctic and Antarctic with ice impact on the propulsion has been discussed. The new developments, in particular, digital twin approaches in health and condition monitoring have been highlighted, considering its pros and cons and potential challenges.acceptedVersio
HYDRODYNAMIC AND MECHANIC RESPONSE OF A FLOATING FLEXIBLE ICE FLOE IN REGULAR WAVES WITH THE ICFD METHOD
No full textClimate change provides for a stronger wave climate and
the open water area in this region will increase. The increase in
open water area allows for an increase in higher energy surface
waves than previously. In addition, human activities are much
more complex in this zone, called Marginal Ice Zone, compared
to the open ocean, so that there is an increased research interest in
the physical processes of wave-ice interaction and the resulting
statements on the influence on maritime technology. This work
deals with the development of an efficient simulation environment for the 2D wave-ice interaction on the finite platform LSDYNA. The numerical model based on the coupled ICFD and
implicit structural solvers of LS-DYNA. The motion behavior
and the mechanical behavior of the ice floe due to the interaction with different waves are investigated. The stronger wave
climate in the Marginal Ice Zone intensifies, making the waves
more energetic and increasing the wave steepness. The results of
the simulation clearly show that larger wave steepnesses have a
large influence on the mechanical stress in the ice floe and the
deflection increases. These mechanical stresses lead in the critical cases to a fracture of the ice floe into several smaller ice
floes. Consideration of the ratio of ice floe length to wave length
showed that the smaller the ratio, the larger the heave and surge
movements of the ice floe induced by the waves
Corrigendum to “Numerical analysis of the bending strength of model-scale ice” Cold Reg Sci Technol 118 (2015) 91–104
No full textOn the scalability of model- scale ice experiments
No full textIce model tests are a frequently used mean to assess and predict the performance of ships and structures in ice. The model ice composition is adjusted to comply with Froude and Cauchy similitude. Recent research indicates that the internal mechanics of Aalto modelscale ice and sea ice differ significantly. This consequently limits the scalability and challenges state-of-the-art scaling procedures. This paper presents a qualitative assessment on selected topics to assess the differences between model-scale ice and sea ice and the influence of related experiments on determined mechanical properties. Furthermore, existing scaling approaches are discussed in context of recent research findings
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