Keynote Lectures ICCSE2

Renewable energy has developed rapidly in Italy over the past decade and provided the country a means of diversifying from its historical dependency on imported fuels. Marine renewable energy is going to give a huge contribution in the next decades in the diversification of sources. Italian Regions on the Sea will contribute to a rapid development of new installation of devices for marine renewable energy the ecological transition to reach the challenge of a carbon free country in 2050. The speech will present a review of the expected sea climate conditions by 2100 in the Italian Seas and the recent strategy foreseen to generate clean energy from the Sea.

Derivatives are massively traded on the everyday financial markets. Their pricing involves two main steps: the construction of appropriate stochastic models for the dynamics of the underlying asset(s) and the application of analytical techniques or computational methods for computing a “fair” price based on the chosen models. These steps must be done with care if the goal is to obtain accurate derivative prices. In particular, if either the model selected or the derivative itself is particularly complex, the second step requires some numerical approximation. In this talk, we will present high-order numerical methods for pricing derivatives based on both single- and multi-factor models. We will propose very efficient approaches, based on the so-called repeated Richardson extrapolation in both space and time, which allow us to obtain accurate and fast approximations of option prices. Exotic options, such as options with a double barrier, and commodity derivatives will also be considered.

In recent years one of the most essential duties for civil engineers is to develop rehabilitation solutions both for historic and modern structures. Reinforced concrete (RC) is a relatively modern construction material, and it can be often damaged both with cracking due to loading or corrosion of reinforcement. The Near Surface Mounted (NSM) technique of inserting Fiber Reinforced Polymer (FRP) rods into grooves has been shown to be a suitable method for repairing damaged RC elements. This work elaborates considerations on NSM Carbon-FRP and Glass-FRP rod strengthening based on results of an experimental investigation using bending tests on non-strengthened beams and strengthened with NSM C-GFRP rods and, lastly, through vibration tests on the beams mentioned above. The experimental bending tests were performed on the RC beams for defining static behaviour until failure. Given that damage in concrete elements leads to a change in dynamic response, vibration tests were carried out to evaluate the experimental vibration both for non-strengthened and strengthened beams with NSM C-GFRP rods. Envelope of Frequency Response Functions (FRFs) is obtained, and frequency values variations are related to the damage degree of beam models subjected to static tests. Finally, a comparison between data obtained by finite element analysis and experimental results leads to discussing the actual behaviour of RC beams strengthened using NSM strengthening technique and assessing the availability of CFRP and GFRP rods.

Dynamic Soil-Structure Interaction (SSI) is known to be a factor that can significantly affect the seismic response of bridges. When this is the case, the structural model used in the analyses should be able to take into account, as rigorously as possible, the complex dynamic response of foundations, which is characterized by a frequency-dependent behaviour. For time domain applications, which are generally more familiar to engineers and are mandatory in case of nonlinear structural behaviour, the sub-structure approach can be suitably adopted to perform SSI analyses, by supposing the foundation to behave linearly, consistently with hierarchy principles of modern codes. The frequency-dependent of the soil-foundation systems are simulated through suitable Lumped Parameter Models (LPMs) in a sub-structuring scheme, able to simulate the frequency-dependent nature of the soil-foundation system dynamic behaviour. The presentation aims to address the methodology for including SSI effects in the linear and nonlinear seismic analysis of bridges founded on piles, exploiting common software for structural analysis and suggesting simple and efficient LPMs. In addition, an overview of the SSI effects on the dynamic response of bridges is presented, starting from phenomenological investigations based on simplified models, and passing through applications to real case studies representative of multi-span bridges, arch bridges, and isolated overcrossings.

With a length of over 100 m, composite wind turbine blades are among the largest single components in the world made of fiber composite materials. Working in harsh environments and under complex loading, such large composite structures must maintain their structural integrity and reliability. The wind energy industry urgently needs more accurate and much faster modeling and simulation techniques to evaluate various structural designs toward more damage tolerant blade structures. In this lecture, the latest research and innovation in high-performance modeling techniques of large-scale blade structures and their components will be presented. Particu-lar focus will be placed on advanced finite element modeling techniques that can accurately predict complex fracture and damage phenomena in full-scale wind turbine blade structures and their critical structural details. With equal importance, simulation efficiency and calculation speed are greatly escalated to an unparalleled level by several recently developed key enabling technologies which will be introduced and demonstrated in this lecture.

Damage sustained during past earthquakes has highlighted that the seismic behaviour of structures and infrastructures is highly influenced by the response of the soil-foundation system. For this reason, modern seismic codes have started, at least in particular cases, to address the design and the verification of the structure, its foundation and the local deposit as a whole system. The substructure method plays a significant role for performing soil-structure interaction analysis allowing the analysis of the soil-foundation and superstructure subsystems separately and a more easily identification of their dynamic behaviours through the use of dedicated software. In the framework of the substructure method, the dynamic response of soil-foundation systems in the case of deep foundations can be studied with different approaches. The goal of this presentation is to present a general overview of analytical and numerical procedures for the dynamic analysis of soil-pile foundation systems subjected to the propagation of seismic waves in the soil. Both single piles and pile groups with general layouts (including pile inclination) embedded in homogeneous as well as in inhomogeneous soil profiles are addressed. The procedures presented herein may be used in practice or in research to obtain realistic estimation of the dynamic impedances, the foundation input motion and stress resultants along piles. Finally, the results of different in situ experimental campaigns conducted on deep foundations are shown and compared to those obtained by some of the above mentioned numerical procedures.

In this paper, a completely new collocation type numerical method, Finte Line Method (FLM), is proposed for solving general engineering problems governed by second or high order partial differential equations with proper boundary conditions. The method is based on the use of finite number of lines crossing each collocation point, and the Lagrange interpolation formu-lation is used to construct the shape functions for each line. When the number of the finite lines exceeds the dimension of the considered problem, the Least-Square technique is applied to all lines’ equations to form an equation set to solve the first and high orders of partial derivatives of any physical variables with respect to the global coordinates. Then, the derived spatial partial derivatives are directly substituted into the governing partial differential equations and related boundary conditions to set up the system of equations with physical variables, temperatures and displacements, at all collocation nodes as unknowns. As an application, the proposed method is used to perform the thermal stress analysis of layered composite structures. A few numerical examples will be given to verify the correctness and stabil-ity of the proposed method.

Nowadays the design of better lightweight structures is becoming more and more importance especially in automotive, aeronautical and aerospace fields where weight reduction is a must. A solution towards this end is represented by variable stiffness composites where the reinforcing long fibres are not necessarily straight but their path can be steered according to the specific design needs and requirements. Variable stiffness composites are, therefore, unconventional solutions that can result in a considerable weight-saving and enhanced mechanical properties when compared to classical ones. On the other side, they comport a considerable increase in the complexity of modelling and design since anisotropy is now a field. In this regard, a hierarchical higher-order modelling approach based on Carrera’s unified formulation is used for deriving a class of plate finite elements having an enriched kinematical description along the thickness and a generic number of in-plane nodes. Results present a static analysis aiming at comparing the structural performance in terms of displacements and stress of variable stiffness composite plates versus classical long fibres laminated solutions as well as at assessing the accuracy and efficiency of the proposed hierarchical finite elements.

This study aims to investigate the flutter of hybrid nanocomposite panels under supersonic flow. The hybrid composite panel incorporates layers made of the polymer matrix with different nanofillers such as carbon nanotubes (CNTs) and graphene nanoplatelets (GPLs). The effective Young’s and shear moduli of each CNTs and GPLs layers are determined by Mori-Tanaka scheme and modified Halpin-Tsai homogenization technique, respectively. Poisson’s ratios and mass densities are determined according to the rule of mixtures. The displacement gradients of the panel are assumed to be small so that the components of the Green-Lagrange strain tensor are linear and infinitesimal. Additionally, the first-order shear deformation theory and the first-order piston theory are applied to formulate the aeroelastic model. The element-free IMLS-Ritz method with meshless features is employed to discretize dynamic equations and define multi-parametric characteristic equations functionally dependent on material and geometric parameters of the panel. The accuracy of the obtained solution and results is examined by comparing determined natural frequency and critical aerodynamic pressure with those from simpler panel models presented in the literature. Then, a comprehensive parametric study is carried out to show effects of hybridization of different distribution patterns of CNTs and GPLs, weight fractions, total layer number, and aspect ratio of hybrid composite panels on their flutter bound.

In the mechanics of heterogeneous materials – in particular, in the effective media theories – quantitative modeling focuses, mostly, on inhomogeneities (inclusions, pores, cracks) of the ellipsoidal shapes and the classic results of Eshelby are utilized. However, inhomogeneities in real materials (ceramics, geo-materials, metals) tend to have highly “irregular” shapes. The question arises, of quantitative characterization of such microgeometries and on extending the effective media theories to them. This challenge can be rephrased as bridging the gap between mechanics (quantitative methods) and materials science (largely observational). The progress that has been achieved recently in this direction will be reviewed.

Despite the development of composite application technologies in recent decades, the manufacturing process that greatly affects the performance of composite materials still depends on the empirical approach, and the understanding of the physical phenomena occurring during the manufacturing process is insufficient. The key in the composite material manufacturing process is to minimize the pores inside the resin by uniformly impregnating it between the fiber bundles. Since most of the previous studies analyzed the flow of resin during the process on a single scale, there was a limit to accurately predicting the effect of the process on the properties of the composite materials. In this study, through multi-scale analysis, from the fiber unit distribution to the resin flow in the composite laminate, the optimum curing cycle was suggested to reduce the pores and enhance the physical properties of the composite material.

The main focus is on high-performance metamaterials featuring a periodic distribution of highly tunable infinite-dimensional resonators. The resonators are first treated as linear systems exhibiting eigenspectra with distinct features. Then these resonators are enriched by geometric and hysteresis nonlinearities. The band gap structure is investigated showing its sensitivity with respect to the design parameters. The nonlinear wavefrequencies and waveforms away from internal resonances obtained via a perturbation approach are shown to exhibit a high nonlinear tunability which is a key for advanced applications. Various metamaterial samples are 3D printed and tested experimentally using a 3D laser scanning vibrometer system to show very interesting stop band behaviors.

New advanced composite materials such as Fiber-Reinforced Polymer (FRP) or Fiber Reinforced Cementitious Matrix (FRCM - where the organic matrix of FRP is substituted by an inorganic matrix) improved the industry of masonry and concrete confinement by means of easily and effectively wrapping layers of fibers around the columns. Several studies showed that circular cross sections gain a meaningful increase in strength and ductility when wrapped with high performance materials. Available analytical models for the assessment of the stress–strain behaviour of confined columns are easily derived for cylindrical plain solids. However square- and rectangular-cross section columns are typically found in real structures and it was clearly observed that they suffer reduced increases in terms of strength and ductility compared to their circular counterparts. Reason for lower performance of square and rectangular cross sections is the non-uniform distribution of lateral confining pressure, having a peak at the corners of cross sections along the diagonals and much lower pressure between them. A practical solution for the evaluation of confinement effectiveness for such non-uniform pressure field was found involving unconfined parabolic regions of the cross section and counterparts fully confined. At the same time transmuting a quadrilateral section into an equivalent circular section (e.g. inscribed, circumscribed, or with same area). This simple method showed to have significant drawbacks, hence a more refined method is proposed. The proposed method accounts for the pointwise variable principal stress state in the cross-section by means of a detailed analysis of the stress field through Mohr's circles. The final goal is to provide the mean integral value of the stress field in a simplified closed form solution of an effective equivalent pressure. This equivalent pressure inserted in triaxial confinement models allows to describe the particularities of non-axisymmetric cross sections, namely square and rectangular. This simplified model is the basis for a more refined iterative confinement model to evaluate the entire stress-strain relationship of confined solids. Models aimed at directly evaluate the confined strength depend on a supposed ultimate strain value for the wraps. It is commonly supposed that wrap fails at the same tensile strain of confining material, as it can be reached in a flat coupon test. In reality, experimental tests demonstrated that in most cases, peak confined strength is not reached experimentally at ultimate tensile strain of the jacket. The discrepancies may include the shape and sharp cracks of substrate as well as stress concentrations or defects in the wraps (namely, the multiaxial stress state in the jacket wrapping or wavy initial configuration of fibers). The average absolute error of confinement models usually had notable reductions when effective hoop strain (less than ultimate value) is inserted in direct models, e.g. by means of strain efficiency factors (i.e. strain ratio between the effective hoop strain at failure and the flat coupon test results). Accounting for the 3D evolution of stresses and strains in confined solids and wrapping jackets allows to simulate satisfactorily all this peculiarities in the stress-strain relationships for confined non-circular solids.

Supersonic panel flutter is an aeroelastic instability of plates and shells that has regained the attention of researchers during middle 1990s, particularly with the expansion of advanced composites applications. The use of advanced materials in aircraft may lead to improvements in the control of static and dynamic defor- mations, thereby allowing aeroelasticity tailoring design of aerospace components. Real aerospace structures operating in external flows are typically made of thin-walled panels restrained to spars and stringers that act as stiffeners. These stringers are usually distributed along the inner surfaces of panels to provide structural stability, especially against shell buckling. Various approaches exist for idealizing the kinematic boundary conditions that the stringers impose to the panel. The most common approach, and also the one used herein, is to consider large panels with simply supported lines uniformly distributed along the surface. A panel delimited by externally supported boundaries is denoted a single bay or single panel. For aeroelastic analysis, much of the modeling addresses the flutter problem of single bays with either simply supported or clamped boundary conditions to represent the structural constraints along the edges. Nonetheless, adjacent panels within a multibay arrangement interact under external loading. The interaction between adjacent bays was investigated by few researchers, mostly in the 1960s. Earlier models for multibay supersonic flutter analysis were based on the linear structural behavior of isotropic materials. The finite element method was widely employed, and the flutter prediction was entirely based on eigenvalue solutions. There was a very long gap before the multibay supersonic flutter problem was revisited when the interest in composite materials grew for aerospace applications. Very few investigations on the supersonic flutter onset on multibay composite panels were performed in the early 1990s, mostly using a linear finite element model. Since then, no studies have been found in the literature addressing the multibay panel flutter problem. Furthermore, all the works published to this date on the matter have focused solely on predicting the flutter boundary, always employing linear aeroelastic models for mode coalescence analysis. Little or no attention has been given to the nonlinear post-flutter regime, which is critical for the structural design and fatigue life estimation. The post-flutter response of nonlinear multibay panels under supersonic flow is discussed in this keynote. Modeling using nonlinear finite elements theory and Rayleigh-Ritz approaches are compared for the multibay case. Geometrical nonlinearity is based on the von K ́arm ́an strain-displacement relations. The aeroelastic model is achieved by including the first-order piston theory to account for supersonic aerodynamics and solved in the time domain using traditional integration schemes. The multibay panel flutter analysis has revealed complex jumps in the Hopf bifurcation behavior. This kind of phenomena has been observed in flat panels or in shallow shells panels. The mechanism of jump occurrence is originated by the interaction of adjacent bays and influenced by the intermediate boundary conditions between bays. Moreover, the jump has also been encountered in laminate composite panels, and the fiber orientation plays an important role in postponing either the flutter onset or the jump phenomenon. The effect of temperature and panel curvature are also presented. The influence of having irregular bay size arrangements to the jump occurrence is also discussed. Important engineering design guidelines can be established from the present analysis, as the multi- bay and single-panel models are shown to be conservative regarding linear behavior, but potentially unsafe in the post-flutter regime.

Advanced structural applications call for the development of numerical methods able to offer modelling flexibility as well as high computational performances. Among various computational approaches, the discontinuous Galerkin (DG) method is emerging as a powerful and versatile numerical technique for a wide class of problems. In DG methods the unknown fields are expressed by discontinuous basis functions, whose coefficients become the unknowns of the discrete problem. The continuity of the solution and the fulfilment of the boundary conditions are then retrieved in weak sense via suitably defined penalization boundary integrals. Despite the possible involvement of a large number of degrees of freedom and the computational effort needed for the evaluation of the mentioned boundary integrals, the discontinuous nature of the basis functions used in DG methods comes with some desirable features for computational modelling. Indeed, DG methods: i) offer high-order accuracy for conventional and non-conventional meshes, ii) simplify the implementation of hp adaptive mesh refinement schemes, iii) allow a straightforward coupling between different models in multi-physics simulations, and iv ) allow massive parallelizations. These features justify the success of DG methods in several research areas of computational science. Composite laminated shells gained a principal role in lightweight structural technology as they allow, when optimized, significant weight savings and performance enhancements. Additionally, they can be used effectively in building geometrically very complex structures whose behaviour is significantly affected by the interplay among the inherent heterogeneity, the shell curvatures and the possible presence of cut-outs. On the other hand, the accurate appraisal of the laminated shells structural response appears crucial to fully attain their potential. Thus, due to the high number of involved design possibilities and the inherent complexity of the multilayered shell problem the development of accurate numerical models is then mandatory. In view of the above presented framework, in this work, a novel DG-based formulation for the analysis of composite multilayered shells is presented. The shell geometry is represented via either analytical functions or NURBS parametrizations, while generally shaped cut-outs are defined implicitly within the shell modelling domain via a level set function. Models are presented for different order shell theories introduced via the Carrera Unified Formulation, which is used to describe the covariant components of the displacement field throughout the thickness. The shell governing equations are then derived from the Principle of Virtual Displacements and solved via an Interior Penalty discontinuous Galerkin method over a discretization of the shell modelling domain that is obtained by intersecting a background structured grid with the level set function defining the cut-outs. High-order accurate quadrature rules for implicitly-defined regions are employed to compute the integrals of the method. The combined use of these features provides a high-fidelity approach to the analysis of multilayered shells with cut-outs. Numerical results are present tor static, buckling and dynamic shell analysis to show the approach capabilities.

Fibre-reinforced composites (FRCs) are advanced materials widely used in the aerospace, automotive, and naval industries. Advancements in additive manufacturing (AM) technologies such as continuous fibre fused filament fabrication (CF4) allow the layer-by-layer printing of FRCs with a spatial in-plane variation of the fibre orientation and fibre volume fraction, thus expanding the design space for variable and constant stiffness laminates. With these offered degrees of freedom, computational design tools such as topology optimization (TO) are essential to optimized layout designs that take advantage of FRC materials' anisotropic properties. However, most design approaches either do not take complete advantage of this extended design freedom or fail to consider CF4 constraints. We present a simultaneous topology and stacking sequence optimization for variable composite laminate structures. The proposed method integrates the geometry projection (GP) and the solid isotropic material with penalization (SIMP) to effectively perform the arrangement of FRCs material in each lamina, enabling spatial in-plane variation of the fibre orientation and fibre and matrix volume fraction. In addition, lamination parameters (LPs) are used as design variables to extend the approach to design variable composite laminate, considering only in-plane stiffness. In the outcomes, various benchmark problems are considered to demonstrate the applicability of the proposed method. Lastly, to illustrate the practical relevance, the attained designs are fabricated employing CF4.

3D printers and automated fibre placement machines have allowed the introduction of Variable Angle Tow (VAT) composites that, allowing the fibres to be relaxed along curvilinear patterns, offer greater tailoring capabilities than classic CFRP laminates. Nevertheless, the steering of brittle fibres is not flaw-exempt and, in fact, is greatly affected by the printer signature. In this work, we explore the use of multi-scale, high-order finite elements based on the Carrera Unified Formulation (CUF) to demonstrate the importance of manufacturing defects on the mechanical response and the reliability-based design of VAT composite laminates. In detail, the effect of fibre waviness, gaps, overlaps and micro-scale defects variation on the buckling and vibration response as well as on failure onset are investigated by using Monte Carlo analysis and stochastic random fields. Eventually, the use of metamodels and evolutionary algorithms are proposed and validated for uncertainty quantification and robust design. One of the main scope of the work is to show that high-order, multi-scale structural modelling based on component-wise kinematics can be helpful to broaden the design space that classical structural theories may have shrunk.

We provide the state of the art of Reduced Order Methods (ROM) for parametric Partial Differential Equations (PDEs), and we focus on some perspectives in their current trends and developments, with a special interest in parametric problems arising in offline-online Computational Fluid Dynamics (CFD). Efficient parametrisations (random inputs, geometry, physics) are very important to be able to properly address an offline-online decoupling of the computational procedures and to allow competitive computational performances. Current ROM developments in CFD include: (i) a better use of stable high fidelity methods, considering also spectral element method and finite volume discretisations, to enhance the quality of the reduced model too, and allowing to incorporate some turbulent patterns and increasing the Reynolds number; (ii) more efficient sampling techniques to reduce the number of the basis functions, retained as snapshots, as well as the dimension of online systems; (iii) the improvements of the certification of accuracy based on residual based error bounds and of the stability factors, as well as the guarantee of the stability of the approximation with proper space enrichments. For nonlinear systems, also the investigation on bifurcations of parametric solutions are crucial and they may be obtained thanks to a reduced eigenvalue analysis of the linearised operator. All the previous aspects are quite relevant in CFD problems to focus on real time simulations for complex parametric industrial, environmental and biomedical flow problems, or even in a control flow setting with data assimilation and uncertainty quantification. Model flow problems will focus on few benchmarks, as well as on simple fluid-structure interaction problems and shape optimisation applied to some industrial problems of interest.

Adhesive bonding is increasingly being considered by engineers for load-bearing joints to complement, or supplement, traditional mechanical fasteners for a large variety of structural materials, including fibre-reinforced polymers, timber, and steel. However, practitioners are still largely applying dimensioning methods that rely heavily on fudge factors. This is not due to the relative novelty of the joining technique, and the complexity of the associated mechanics. This paper guides the reader through some of the most important aspects of adhesively bonded joints, and the reasons why classical mechanics can only be applied in conjunction with fudge factors. Subsequently, the principles of fracture mechanics (FM) are presented; it is shown that, despite its conceptual strengths, FM is rather difficult to implement, as it introduces mechanical concepts practitioners are not familiar with. Then probabilistic methods (PM) are introduced; it is shown that PM can be considered as an extension of classical mechanics, as it can be easily implemented as a post-processing routine of the latter. Additionally it is shown that PM, unlike FM, neither require specific tests procedures for characterisation, nor complicated numerical analyses. Lastly, the paper offers a step-by-step guide for the implementation of PM for the dimensioning of adhesively bonded joints. The second part of this paper illustrates the latter on a series of examples involving FRP, timber, and steel.

Fracture mechanics experimental tests are assuming an increasingly important role in the context of methodologies aimed at characterizing the mechanical behaviour of materials and structures. As a matter of fact, the knowledge of the parameters that characterise the ductility or brittleness of advanced materials is becoming a fundamental element for safety problems. Consequently, methods that seemed limited to highly specialized sectors of aeronautical and nuclear engineering are rapidly spreading to a more traditional sector, such as that of mechanical and civil engineering. These results, for example, in extension to concrete and rocks regulations that were initially limited to metallic materials. In this context, the present lecture is focused on two types of advanced materials, that is: cement-based materials and nanomaterials, proposing for both a computational method for fracture toughness. More in detail, in the context of cement-based materials (with and without reinforcement) a modified formulation of the Two-Parameter Model (TPM) is proposed. On the other hand, in the context of nanomaterials the non-local Stress-Driven integral Model (SDM) is proposed.

Fatigue is one of the most encountered problems with dynamically loaded engineering structures. Initially, to ensure the fatigue damage evolution during operation under perfect control, accurate lifing models by abundant experiments and full understanding of fatigue mechanism have been developed, ensuring that the designed structures with sufficient fatigue strength under predefined service condition. Unfortunately, it was later found that the fatigue performance is affected by multi-source uncertainties, including those arising from material property, load spectrum, geometrical dimension etc. In particular, sometimes, the computation is very sensitive to tiny changes in the inputs, varying quantities over reasonable ranges can even lead to outputs with 1000 times difference. Under this circumstance, traditional deterministic models no longer work, and methods developed from probabilistic perspective with well description on uncertain inputs are highly expected. To boost the development on probabilistic fatigue modeling and emphasize its crucial significance in fatigue reliability design and optimization, this study systematically recalls the theoretical basis and recent progress in this field, and ends with discussions as well as conclusions and future prospect.