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In large vessels, the thickness of deck plating has increased significantly during the last decade. Hence, it is not possible to ignore the effect of plate thickness on fatigue strength for these structures. To complement existing data and to establish the effect of plate thickness on fatigue strength, constant amplitude fatigue tests were carried out under tensile and bending loads for base material and butt welded specimens of various thicknesses made from high-tensile EH40 steel. Based on the fatigue test results and available literature, the thickness exponent was derived.
“Container ships are increasing in size all the time, with the 10,000 TEU barrier finally having been broken. Nearly half of the container ship order book is comprised of ships with a capacity of greater than 6,000 TEU. With containerships of this size the owner and the builder will be particularly focussed on the reliability and performance of the hull structure over the course of the vessel’s transpacific trading life’’ - these are quotations from the article “Towards the ultra-large container ship” published in Lloyd’s Register technical news magazine “Horizons”, March 2005 issue. Increase in container ship capacity inevitably leads to increase in size and thicknesses of hull structures. Hence it is not possible to ignore effect of plate thickness on fatigue strength of deck structures such as deck (hatch) opening corners. To estimate fatigue life of these structures, fatigue resistance data (S-N curves) for base material and butt welded specimens of various thicknesses are required. The effect of plate thickness on fatigue strength has been studied by a number of investigators who have primarily considered T and cruciform non–load-carrying joints. However experimental data for base material and butt welded specimens are very scarce. To complement existing data constant amplitude fatigue tests were carried out under tensile and bending loads.
Sitek, L. (Institute of Geonics, Academy of Sciences of the Czech Republic) | Foldyna, J. (Institute of Geonics, Academy of Sciences of the Czech Republic) | Souc;Ek, K. (Institute of Geonics, Academy of Sciences of the Czech Republic)
Fu, Dongsheng (Kyushu University) | Toda, Hiroyuki (Kyushu University) | Su, Hang (Kyushu University) | Hirayama, Kyosuke (Kyushu University) | Uesugi, Kentaro (Japan Synchrotron Radiation Research Institute) | Takeuchi, Akihisa (Japan Synchrotron Radiation Research Institute)
Stress corrosion cracking (SCC) behavior of Al-10Mg aluminium alloys is studied with the help of the high resolution X-ray tomography in the present research. The SCC-induced crack is formed from the corrosion pit near the surface, and gradually propagates and coalescence with the voids due to the fracture of β phase ahead of the crack tip at low applied strain levels. In addition, hydrogen concentration in the ligament between the SCC-induced crack tip and voids accelerates the propagation of the crack, indicating that hydrogen concentration and partitioning among various trap sites dominates the SCC and entire fracture of Al-10Mg aluminium alloys.
Al-Mg aluminium alloys are widely used in the fields of marine and automobile due to its high strength and strength-to-weight ratio. Al-10Mg aluminium alloys that increase the content of Mg to 10 % are designed to satisfy the industrial requirement in recent years. However, the application of Al-10Mg aluminium alloys is undermined by stress corrosion cracking (SCC) arising from the high susceptibility of Al-10Mg aluminium alloys to degradation in aggressive environments. According to the previous research, it is reported that SCC-induced cracks lead to unexpected failure under applied strain levels far lower than the strain that materials can sustain, indicating that SCC is a formidable problem for the application of Al-10Mg aluminium alloys, especially in the fields of marine science and nuclear power.
The resistance to SCC is determined by the chemical composition, heat treatment technology and related microstructural features of materials. According to Goswami, Spanos, Pao, et al. (2010), β-phase (Al3Mg2) is mainly precipitated on the grain boundaries of Al-Mg aluminium alloys during a sensitization heat treatment process. Due to the initiation and growth of β-phase during the sensitzation heat treatment, Crane and Gangloff, (2016) proposed that the growth rate of SCC-induced intergranular crack increases with an increase in the sensitization time and sensitization temperatures during the heat treatment process, revealing that the content of β-phase plays a significant role in the initiation and propagation of SCC-induced intergranular cracks in AA 5083-H131 aluminium alloys. Jones, Vetrano, and Windisch, (2004) proposed that the open circuit potential (OCP) of β-phase and aluminium matrix is −1.120 Vsce and −0.800 Vsce, respectively, leading to the anodic dissolution of β-phase and accelerating the propagation of SCC-induced intergranular cracks in AA5083 aluminium alloys that exposed in 3.5 % NaCl+chromate solution. In addition, Tanguy, Bayle, Dif, et al. (2002) studied the effects of hydrogen to the propagation of SCC-induced intergranular cracks in Al-5Mg aluminium alloys, revealing that the β-phase dissolution triggers the dissolution of the Al-Mg solid solution, providing a high over potential for the production and uptake of the external hydrogen ahead of the SCC-induced intergranular crack tip. The authors also proposed that the external hydrogen is mainly trapped at the interface of β-phase/ matrix and constrained β-free ligament due to a high hydrostatic strain concentration, leading to the propagation of SCC-induced crack along grain boundaries in terms of hydrogen enhanced decohesion (HEDE) models. Furthermore, Burnett, Holroyd, Scamans, et al. (2015), reported that the applied stress is shared by a number of corrosion-induced cracks due to the crack branching behavior, leading to the limited SCC-induced intergranular crack propagation in aluminium alloys.