• Assessment of structural integrity of subsea wellhead system: analytical and numerical study

      Maligno, Angelo; Citarella, Roberto; Silberschmidt, Vadim V.; Soutis, Constantinos; University of Derby; University of Salerno; Loughborough University; University of Manchester (Italian Group of Fracture, 2015-01)
      Subsea wellhead systems exposed to severe fatigue loading are becoming increasingly a significant problem in offshore drilling operations due to their applications in wells with higher levels of pressure and temperature, situated at larger depths and in harsher environments. This has led to a substantial increase in the weight and size of offshore equipment, which, in combination with different loading conditions related to the environmental factors acting on the vessel and riser, has greatly increased the loads acting on subsea well systems. In particular, severe fatigue loading acting on the subsea wellhead system was detected. For this reason, a combined analytical and numerical study investigating the critical effect of crack depth on the overall structural integrity of subsea wellhead systems under cyclic loading was carried out. The study is based on a Linear Elastic Fracture Mechanics (LEFM) approach.
    • FEM simulation of a crack propagation in a round bar under combined tension and torsion fatigue loading

      Citarella, Roberto; Maligno, Angelo; Shlyannikov, Valery; University of Salerno; University of Derby; Russian Academy of Sciences (Italian Group of Fracture, 2015-01)
      An edge crack propagation in a steel bar of circular cross-section undergoing multiaxial fatigue loads is simulated by Finite Element Method (FEM). The variation of crack growth behaviour is studied under axial and combined in phase axial+torsional fatigue loading. Results show that the cyclic Mode III loading superimposed on the cyclic Mode I leads to a fatigue life reduction. Numerical calculations are performed using the FEM software ZENCRACK to determine the crack path and fatigue life. The FEM numerical predictions have been compared against corresponding experimental and numerical data, available from literature, getting satisfactory consistency
    • Retardation effects due to overloads in aluminium-alloy aeronautical components

      Maligno, Angelo; Citarella, Roberto; Silberschmidt, Vadim V.; University of Derby; University of Salerno; University of Loughborough; Institute for Innovation in Sustainable Engineering; University of Derby; Quaker Way, Derby DE1 3HD UK; Department of Industrial Engineering; University of Salerno; via Giovanni Paolo II, Fisciano SA Italy; Wolfson School of Mechanical and Manufacturing Engineering; Loughborough University; Loughborough UK (Wiley, 2017-02-08)
      Fatigue data are generally derived under constant-amplitude loading conditions, but aircraft components are subjected to variable-amplitude loading. Without interaction effects, caused by overloads and underloads intermingled in a loading sequence, it could be relatively easy to establish a crack growth curve by means of a cycle-by-cycle integration. However, load-spectrum effects largely complicate a crack growth under variable-amplitude cycling. In this paper, fatigue crack growth behaviour of aeronautical aluminium alloy 2024-T3 was studied. Effects of various loading conditions such as stress ratio and amplitude loadings were investigated. In particular, the effect of different overloads on the fatigue crack growth was simulated using Zencrack code. Preliminary analyses on Compact Tension (CT) specimens proved that the numerical results generated were in agreement with the results provided by an afgrow code for the same conditions. A case study was carried out on a helicopter component, undergoing repeated overloads, to compare numerical results obtained implementing yield zone models in Zencrack.
    • Simplified and accurate stiffness of a prismatic anisotropic thin-walled box.

      Canale, Giacomo; Rubino, Felice; Weaver, Paul M.; Citarella, Roberto; Maligno, Angelo; Rolls-Royce Plc; University of Salerno; University of Bristol; University of Derby (Bentham Open, 2018-02-14)
      Background: Beam models have been proven effective in the preliminary analysis and design of aerospace structures. Accurate cross sectional stiffness constants are however needed, especially when dealing with bending, torsion and bend-twist coupling deformations. Several models have been proposed in the literature, even recently, but a lack of precision may be found when dealing with a high level of anisotropy and different lay-ups. Objective: A simplified analytical model is proposed to evaluate bending and torsional stiffness of a prismatic, anisotropic, thin-walled box. The proposed model is an extension of the model proposed by Lemanski and Weaver for the evaluation of the bend-twist coupling constant. Methods: Bending and torsional stiffness are derived analytically by using physical reasoning and by applying bending and torsional stiffness mathematic definition. Unitary deformations have been applied when evaluation forces and moments arising on the cross section. Results: Good accuracy has been obtained for structures with different geometries and lay-ups. The model has been validated with respect to finite element analysis. Numerical results are commented upon and compared with other models presented in literature. Conclusion: For cross sections with a high level of anisotropy, the accuracy of the proposed formulation is within 2% for bending stiffness and 6% for torsional stiffness. The percentage of error is further reduced for more realistic geometries and lay-ups. The proposed formulation gives accurate results for different dimensions and length rations of horizontal and vertical walls.