Browsing by Author "Gallo, P."

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    Creep behavior of v-notched components
    (2017) Gallo, P.; Razavi, S.M.J.; Peron, M.; Torgersen, J.; Berto, F.; Department of Mechanical Engineering; Norwegian University of Science and Technology
    Geometrical discontinues such as notches play a significant rule in structural integrity of the components, especially when the component is subjected to very severe conditions, such as the high temperature fatigue or creep. In this paper, a generalized form of the existing notch tip creep stress-strain analysis method developed by Nuñez and Glinka, is developed and extended to a wide variety of blunt V-notches. Assuming the generalized Lazzarin-Tovo solution that allows a unified approach to the evaluation of linear elastic stress fields in the vicinity of both cracks and notches is the key in getting the extension to blunt V-notches. Numerous cases have been analysed and the stress fields obtained according to the proposed method were compared with proper finite element data, showing a very good agreement.
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    Fatigue behavior of innovative alloys at elevated temperature
    (2017) Berto, F.; Gallo, P.; Razavi, S.M.J.; Ayatollahi, M. R.; Norwegian University of Science and Technology; Department of Mechanical Engineering; Iran University of Science and Technology
    The present paper summarizes the results from uniaxial-tension stress-controlled fatigue tests performed at different temperatures up to 650 degrees C on Cu-Be and 40CrMoV13.9 specimens. Two geometries are considered: hourglass shaped (both materials), plates weakened by a central hole (Cu-Be alloy). The motivation of the present work is that, at the best of authors' knowledge, only a limited number of works on these alloys under high-temperature fatigue are available in the literature and no results deal with notched components. In the present contribution, after a brief review of the recent papers, material properties and experimental procedure are described. The new data from un-notched and notched specimens are summarized in the corresponding fatigue curves. The Cu-Be specimens fatigue data are re-analysed in terms of the mean value of the Strain Energy Density (SED). The approach, successfully used by the same authors to summarise fatigue data from notched specimens made of different materials tested at room temperature, is extended here for the first time to high-temperature fatigue. In the plates with central holes the SED is evaluated over a finite size control volume surrounding the highly stressed zone at the hole edge. A value of the radius equal to 0.6 mm seems to be appropriate to summarize all fatigue data in a quite narrow scatter-band Thanks to the SED approach it is possible to summarise in a single scatter-band all the fatigue data, independent of the specimen geometry. Copyright (C) 2017 The Authors. Published by Elsevier B.V.
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    Influence of crack tip plasticity on fatigue behaviour of laser stake-welded T-joints made of thin plates
    (Elsevier Limited, 2018-02-01) Gallo, P.; Guglielmo, M.; Romanoff, J.; Remes, H.; Department of Mechanical Engineering; University of Padova
    The paper investigates the crack tip plasticity of laser stake-welded T-joints when the thickness is varied. For thin joints, mixed-mode condition is dominant, and the Minimum Plastic Zone Radius (MPZR) criterion is employed to estimate the crack initiation direction. Subsequently, the same first-order plastic zone size ry as defined by Irwin is kept equal for the selected load levels in the MPZR direction, and finite element (FE) analyses are carried out to investigate the stress gradient over ry comparing different thicknesses. The results allow us to explain why the fatigue slope m is different for thin and thick joints. Moreover, a new method is proposed that permits the number of cycles to failure of thin joints to be derived directly from the fatigue curve of thick joints, employing an effective J-integral. The results prove that the method is able to give an estimation of the number of cycles to failure for thin joints with acceptable error.
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    Non-localized creep assessment of V-notched components
    (2017) Gallo, P.; Berto, F.; Razavi, S. M. J.; Ayatollahi, M. R.; Department of Mechanical Engineering; Norwegian University of Science and Technology; Iran University of Science and Technology
    Geometrical discontinues such as notches need to be carefully analysed by engineers because of the stress concentration generated by them. Notches become even more important when the component is subjected, in service, to very severe conditions, such as the high temperature fatigue and imposed visco-plastic behaviour such as creep. The aim of the paper is to present an improvement and extension of the existing notch tip creep stress-strain analysis method developed by Nunez and Glinka, validated for U-notches only, to a wide variety of blunt V-notches. The key in getting the extension to blunt V-notches is the assumption of the generalized Lazzarin-Tovo solution that allows a unified approach to the evaluation of linear elastic stress fields in the neighbourhood of both cracks and notches. Numerous examples have been analysed up to date, and the stress fields obtained according to the proposed method were compared with appropriate finite element data, showing a very good agreement. In view of the promising results, authors are considering possible further extension of the method to sharp V-notches and cracks introducing the concept of the Strain Energy Density (SED). Copyright (C) 2017 The Authors. Published by Elsevier B.V.
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    Numerical Evaluation of T-stress under Mixed Mode Loading Through the Use of Coarse Meshes
    (Springer Science + Business Media, 2018-03-01) Acanfora, M.; Gallo, P.; Razavi, S. M.J.; Ayatollahi, M. R.; Berto, F.; Department of Mechanical Engineering; Marine Technology; Norwegian University of Science and Technology; Iran University of Science and Technology
    The present paper investigates the employment of coarse meshes in evaluating the T-stress with the displacement method. Several finite element analyses have been carried out with different mesh refinements and accuracies. Mode I and mixed mode I/II loadings have been considered in finite element analyses. Under mode I loading, single and double edge notched geometries have been considered, while plate with central crack has been considerd for mixed mode loading condition. The analyses are compared with the results by the well-nown stress based approach, and showed that the displacement method permits the evaluation of the T-stress with the employment of coarse meshes. By the way, several precautions must be taken when dealing with coarse and very coarse meshes.