Crack detection via strain measurements in fatigue testing

Fatigue 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

Fatigue life extension of existing welded structures via high frequency mechanical impact (HFMI) treatment

High-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

Interactions between irregular wave fields and sea ice: A physical model for wave attenuation and ice breakup in an ice tank

Irregular, 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

HYDRODYNAMIC AND MECHANIC RESPONSE OF A FLOATING FLEXIBLE ICE FLOE IN REGULAR WAVES WITH THE ICFD METHOD

Climate 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

NEW FINDINGS ON THE IMPACT OF THE IDEALIZATION OF CORROSION ON THE BRITTLE FAILURE OF STEEL

Corrosion is mostly considered as homogeneous thickness reduction in construction guidelines of the maritime industry. Several studies concluded that besides a homogeneous thickness reduction the corroded non-uniform surface morphology is affecting the strength and strain behavior of steel. However, the effects of non-uniform corrosion on the behavior of steel is still not clearly assessed. The effects of corrosion on the local behavior of steel structures under tensile loading were investigated by conducting tensile tests on naturally corroded steel specimens and analyzing the surface morphology based on surface scans. Influences of geometrical parameters on the strength and strains were determined by carrying out a correlation analysis combined with a Particle Swarm Optimization. Then a non-linear finite element analysis was performed to compare a homogeneous idealization with finite element models considering the naturally corroded surface morphology. Both independent approaches conclude that non-uniform corrosion leads to a more brittle stress-strain behavior of steel compared to a homogeneous corrosion approach, including higher stresses and decreased total breaking strain. In addition we can confirm that pittings and surface roughness are leading to a decrease of the total breaking strain. In contrast to many other studies we highlighted that instead of the pitting corrosion in general, the volume or thickness loss are the primary driver for the ultimate tensile strength reductio

Experimental and numerical investigations of the ultimate strength of two subsea power-transmission cables

Integration of renewable energies into existing grids must be safe and reliable including the subsea power-transmission cables. Therefore, this paper determines the purely mechanical ultimate strength of a DC and an AC power transmission cable under axial load. Aspects concerning the electrical functionality of these cables are outside the scope of this paper and failure of the cable is defined as rupture of the cable following ultimate strength. At first, the individual components of the cable are tested to obtain the engineering stress strain relationships to derive material model data for the numerical simulation of the components and cables. The latter is compared to full-scale experimental results of the power transmission cables. Very good comparison of the numerical and experimental ultimate strength of the cables was achieved with the presented straightforward procedure