Papers by Dimitrios Pavlou
Loading Sequence Effects on Fatigue Damage Accumulation of Offshore Structures: A Deterministic Approach
Offshore structures are subjected to irregular loading spectra due to their exposure to waves and... more Offshore structures are subjected to irregular loading spectra due to their exposure to waves and wind. The environmental loads cause variable amplitude stress histories on critical spots of the structures. The existing engineering methodology (adopted by most of the national standards) to estimate the accumulated fatigue damage is based on Miner’s rule for crack initiation. Paris rule and its modifications are used for crack propagation prediction. However, Miner’s rule is a linear model and does not take into account the sequence effect of loading blocks with different stress amplitude. On the other hand, the widely used Paris rule does not take into account the load interaction effects (e.g. overload-induced crack growth retardations). The prediction of the crack growth rate and the crack growth direction of mixed mode cracks is an important issue as well. Aim of the present paper is the analysis of the weaknesses of the engineering tools for fatigue analysis, and the demonstration of the advantages of non-linear damage functions and crack propagation models. A review of models for fatigue crack initiation and growth (for mode I or mixed mode loading) developed by the author is presented. Representative results are discussed and commented.
WIT transactions on engineering sciences, Mar 9, 2004
The estimation of the axial and radial component distributions of the blood velocity during its f... more The estimation of the axial and radial component distributions of the blood velocity during its flow in a diverging artery is attempted. To this scope part of the Navier-Stokes equations and the continuity equation were treated analytically. Several flow characteristics, such as the back flow near the internal artery wall surfaces are investigated. Analytical results obtained by the proposed methodology are also included.
Maritime engineering, Apr 1, 2023
Although the endeavour of placing offshore structures dates back to the late nineteenth century (... more Although the endeavour of placing offshore structures dates back to the late nineteenth century (see e.g. Fazeres-Ferradosa et al., 2019, 2020), the last three decades have seen amazing growth in the number and complexity of offshore structures and subsea technologies. While many remain related to the exploitation of oil and gas, a remarkably large and increasing proportion are now a result of rising sectors of the Blue Economy. In particular, the offshore wind sector is playing a key role in this picture. Additionally, the growing investments in demonstration projects of wave and tidal energy, coupled with investments going into offshore aquaculture represent good prospective for the upcoming scenarios concerning these fields of engineering.
THE INTERNATIONAL JOURNAL OF COMPUTATIONAL METHODS AND EXPERIMENTAL MEASUREMENTS, Nov 30, 2016
A numerical formulation of flow-induced instability modelling of laminated anisotropic pipelines ... more A numerical formulation of flow-induced instability modelling of laminated anisotropic pipelines is derived. The analysis is based on fluid-structure interaction equations and FEA. Taking into account the flow parameters and the material properties, critical flow velocities causing instability are calculated for fibre-reinforced polymeric (FRP) pipelines resting on elastic supports. A parametric study of the effect of fibre orientation, stiffness of elastic supports and span length between supports is carried out. The results are commented and discussed.
Nonlinear fatigue life prediction model based on the theory of the S‐N fatigue damage envelope
Fatigue & Fracture of Engineering Materials & Structures, Mar 8, 2022
A nonlinear model is proposed in light of the theory of isodamage curves, to assess the cumulativ... more A nonlinear model is proposed in light of the theory of isodamage curves, to assess the cumulative fatigue damage under multistage loading. The isodamage curves were developed through the theory of the S‐N fatigue damage envelope, proposed by Pavlou. The adopted functional form of the fatigue damage is a commonly accepted general form. In the present work, a stress function for the fatigue life exponent is derived with the aid of the S‐N curve only. In the proposed nonlinear model, no adjusting parameters are necessary for fatigue damage estimation. The fatigue life prediction of the derived model is compared with experimental results of various alloys and existing models. The comparison is made for C35, C45, Al‐2024‐T42, 15HM, A336 GR 5, and A387 GR22 for two‐stage loading high‐low and low‐high, and Al 6082‐T6 for multistage loading.
Bending-induced local buckling during offshore installation of multi-layered FRP pipelines
IOP conference series, Nov 1, 2021
In the last two decades FRP pipelines have attracted the attention of the oil industry because of... more In the last two decades FRP pipelines have attracted the attention of the oil industry because of their high strength, excellent fatigue performance and low specific weight. On the other hand, the final cost of installation of FRP pipelines is comparable to the cost of carbon steel ones. Therefore, their implementation in offshore applications seems to be advantageous. During offshore installation, the curvatures of the pipes during the S-lay or J-lay installation processes cause high bending stresses and risk for bending-induced local buckling. Since the pipe wall is multi-layered and the laminae are anisotropic, the calculation of critical bending moments is difficult. In the present work, an analytical solution of critical bending moments for bending-induced local buckling is provided. The proposed method uses the classical lamination theory of multi-layered anisotropic materials and Flügge’s assumption for local buckling analysis of pipelines. Results for E-Glass fiber reinforced polymeric pipelines are provided and discussed.
Essentials of the Finite Element Method: For Mechanical and Structural Engineers
Fundamental coverage, simplified mathematics, and up-to-date software applications are hard to fi... more Fundamental coverage, simplified mathematics, and up-to-date software applications are hard to find in a single text on the finite element method (FEM). This book makes the search easier by providing a comprehensive but concise text for those new to FEM, or just in need of a refresher on the essentials. The text explains the basics of FEM, then relates these basics to a number of practical engineering applications. Specific topics covered include linear spring elements, bar elements, trusses, beams and frames, heat transfer, and structural dynamics. Throughout the text, readers are shown step-by-step detailed analyses for finite element equations development. The text also demonstrates how FEM is programmed, with examples in MATLAB, CALFEM, and ANSYS allowing readers to learn how to develop their own computer code. Suitable for everyone from first-time BSc/MSc students to practicing mechanical/structural engineers, the text presents a complete reference text for the modern engineer.
Flow-Riser Interaction in Deep-Sea Mining: An Analytic Approach for Multi-Layered FRP Risers
Fiber reinforced polymeric laminated materials are suitable for risers in deep-sea applications d... more Fiber reinforced polymeric laminated materials are suitable for risers in deep-sea applications due to their superior strength, corrosion and fatigue resistance, light weight, low maintenance cost, low transportation cost, and ability for continuous manufacturing. However, due to their anisotropic material properties, the modeling of the dynamic response due to interaction with the internal flow and the sea water is more complicated. In the present work a model for flow induced instability analysis of long, multi-layered, fiber reinforced risers is performed. The motion equations take into account the elastic flexural restoring force of the anisotropic material, the centrifugal force of the fluid flowing in curved portions of the pipe, the Corriolis force, the inertia force of the mass of pump, pipe, and fluid, and the effect of the surrounding water. Combination of the motion equations yields a fourth order partial differential equation in terms of flexural displacements. The transfer matrix method is implemented to the above equation for the critical flow velocities calculation. The “global stiffness matrix” of the pipe-pump system containing the boundary conditions, the anisotropic material properties and the flow parameters, is derived. The condition for non-trivial solution is solved numerically yielding the values of the critical flow velocity, i.e. the internal flow velocity causing flow induced pipeline instability. The results are affected by the anisotropic properties of the material, the mass of the hanged pump, the drag coefficient, and the flow parameters. The results are commented and discussed.
A Computer Code for Wave Propagation in Flexible Pipelines Under Fluid Hammer Conditions
International Review of Mechanical Engineering-IREME, Feb 28, 2022
Mechanical Behavior of Sandwich Materials Under Concentrated Loads
International Review of Mechanical Engineering-IREME, Feb 28, 2018
Automotive and aerospace industry have benefited from advanced materials to improve the stiffness... more Automotive and aerospace industry have benefited from advanced materials to improve the stiffness of the structures and reduce the weight. Sandwich materials are widely used for designing advanced structures due to their excellent capacity to carry bending moments. However, special techniques are required to improve the performance of sandwich plates under concentrated loads. Inserts are used to distribute concentrated loads in a sandwich plate. Shear stresses are taking place on the interfaces between the insert and the adhesive, as well as on the interface between the adhesive and the core. In the present work an analytical model for calculating these stresses, stress concentrations and maximum load capacity is presented. The model is based on equilibrium equations of the insert and the adhesive in addition to material properties of the mentioned elements. Implementation of the model in a representative example is provided, and the results are commented and discussed.
WIT transactions on engineering sciences, May 5, 2000
In this paper the problem of cumulative creep damage is considered. A general non-linear creep da... more In this paper the problem of cumulative creep damage is considered. A general non-linear creep damage accumulation model, suitable for variable loading and/or temperature, is presented. The proposed damage accumulation function takes into account the previous damage history as well as the loading order effect. This damage function is specified with respect to damage parameters derived by the stress versus Larson-Miller parameter curve. It is shown that some of the existing models can be obtained as a particular case of the model developed here in. Theoretical results are correlated well with experimental data for austenitic steel X-8-CrNiMoNb-16-16 at 700 °C subjected to a square block loading.
State-of-Technology and Barriers for Adoption of Fuel Cell Powered Multirotor Drones
2020 International Conference on Unmanned Aircraft Systems (ICUAS), 2020
Industrial use of multirotor drones is gaining traction, and fuel cell based power sources have b... more Industrial use of multirotor drones is gaining traction, and fuel cell based power sources have been identified as a way of improving the flight endurance from what is possible to achieve with current lithium-based battery options. The state-of-technology and barriers for further adoption are presented. It is found that there are lightweight options commercially available and that the viability of powering multirotor drones for long-range and high endurance missions is demonstrated. The barriers mainly relate to the future required level of certification, technical improvements, and operational aspects. For advancing the state-of-technology, liquid-cooled fuel cells are identified as an attractive alternative that can expand the environmental flight envelope. However, a high system mass of these fuel cells remains a constraint. Hydrogen storage is a central challenge, and storage alternatives are investigated. To further improve the adoption of fuel cell based power sources for multirotor drones, operational and financial rewards must be well proven for realistic operations and relevant operating conditions.
Derivation and Optimization of the Material Cost Function for Natural Gas Pipe-Lines
International Conference of Computational Methods in Sciences and Engineering 2004 (ICCMSE 2004), 2019
Chapter 10 – The Principle of Minimum Potential Energy for Two-Dimensional and Three-Dimensional Elements
This chapter covers the implementation of the principle of minimum potential energy on two-dimens... more This chapter covers the implementation of the principle of minimum potential energy on two-dimensional (2D) and three-dimensional (3D) elements. Interpolation functions of nodal displacements are derived for linear triangular elements, quadratic triangular elements, bilinear rectangular elements, tetrahedral solid elements, eight-node rectangular solid elements, and plate bending elements. The next topic is dedicated to isoparametric elements and Lagrange polynomials. Using interpolation functions, strain-displacements formulae, Hooke’s law, and principle of the minimum potential energy, the derivation of the stiffness matrices for widely used 2D and 3D elements is described. Using such stiffness matrices, analytic procedures for the solution of plane stress problems in Cartesian and in polar coordinates are provided and the methodology of how to compose the matrix equation consisting of the global stiffness matrix and the boundary conditions submatrix is described. Finally, step-by...
Fatigue design challenges: Recent linear and nonlinear models
IOP Conference Series: Materials Science and Engineering, 2019
Reliable fatigue design rules affect the proactive identification of safety parameters in enginee... more Reliable fatigue design rules affect the proactive identification of safety parameters in engineering structures. Numerous fatigue crack initiation and propagation models, linear and nonlinear, have been developed for designing purposes or estimation of the remaining life of aging structures. Depending on the adopted assumptions, the accuracy varies for different loading histories, loading types, and materials. Semi empirical models are simple but yield significant inaccuracies. Models with better theoretical basis provide better accuracy, but implementation in real conditions is problematic. In the present work, a review of author’s recent fatigue crack initiation and propagation models based on physical mechanisms is presented and improvements are proposed. Verification of the models on test results is provided and discussed.
Mechanics Research Communications, 2015
Highlights The motion equation and boundary conditions for dynamic loading of cracks yield DIE... more Highlights The motion equation and boundary conditions for dynamic loading of cracks yield DIE Solution of DIE by double Laplace-Hankel transforms and properties of Bessel functions Far from the loading source the wave's amplitude decreases while its period increases
Structural Dynamics
Essentials of the Finite Element Method, 2015
This chapter is dedicated to the analysis of framed structures subjected to dynamic loads. Mass m... more This chapter is dedicated to the analysis of framed structures subjected to dynamic loads. Mass matrices are developed for two-dimensional (2D) and three-dimensional (3D) truss and frame elements as well as for the linear triangular element. The above mass matrices along with the stiffness matrices are used to compose the dynamic equation. The next topic presents the numerical procedure for the time-domain discretization of the structure and the finite element method (FEM) solution of the dynamic equation using the central difference method and the Newmark-Beta method. Using the above methodologies, the chapter proceeds with the derivation of the natural frequencies and the associated modes under free vibration. Detailed steps for the solution of the dynamic response of structures under dynamic loading is provided, and the development of a CALFEM/MATLAB computer code is described for the solution of a plane frame subjected to time-varying loading.
Trusses
Essentials of the Finite Element Method, 2015
In this chapter, the derivation of the stiffness matrix of inclined bar elements is exhibited. Us... more In this chapter, the derivation of the stiffness matrix of inclined bar elements is exhibited. Using an analytic procedure for coordinate system transformation, the stiffness matrix is correlated to the direction cosines of a 3D bar element. Step-by-step FEM detailed solutions of integrated examples for 2D and 3D trusses are presented to help the reader to achieve an indepth understanding of the methodology. Special attention is given to explain how the structural matrix equation incorporating the global stiffness matrix and boundary conditions submatrix can be composed. Furthermore, a jacket-type offshore structure is selected to present: (a) the FEM modeling procedure, (b) the computer code development in MATLAB/CALFEM platform, and (c) the ANSYS implementation.
Computational Materials Science, 2006
Vl la ac ch ha ak ki is s, , C Ch h.. T Ts si it to ou ur ra as s, , M M. .G G.. P Pa av vl lo ou... more Vl la ac ch ha ak ki is s, , C Ch h.. T Ts si it to ou ur ra as s, , M M. .G G.. P Pa av vl lo ou u a an nd d A A.
Crack Arresting With Crack Deflecting Holes in Steel Plates
Volume 3: Materials Technology, 2020
Experimental and numerical studies of the effect of crack deflecting holes in steel plates under ... more Experimental and numerical studies of the effect of crack deflecting holes in steel plates under high cycle fatigue are presented in this paper. The experimental studies show that with the careful location of the holes, crack propagation can be arrested. A numerical model is provided and validated against the experimental work. The numerically predicted crack propagation direction and crack growth rate were in good agreement with the crack propagation obtained in the experimental work.
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Papers by Dimitrios Pavlou