Sviluppo di un modello non lineare di danneggiamento a taglio per laminati compositi

Questa tesi illustra le attività di studio, sviluppo e prova, in ambito numerico, di un modello non lineare di danneggiamento a taglio per laminati compositi. Per simulare la parte non lineare del comportamento costitutivo tensione-deformazione il modello sfrutta una curva esponenziale, mentre viene usato un legame costitutivo lineare per il danneggiamento a taglio fino a completa rottura del materiale. Il modello è stato implementato in un codice di danneggiamento progressivo, già esistente, e utilizzato (mediante routine utente) in abbinamento con un software FEM commerciale. E’ stata quindi predisposta una vasta campagna di test-case per verificarne il corretto funzionamento numerico e per poi effettuare un confronto con i risultati di un modello di danneggiamento a taglio di letteratura. Sono state effettuate, preliminarmente, prove numeriche di indentazione quasi-statica per verificare la capacità del modello di simulare l’indentazione residua. Il modello è stato poi utilizzato per simulare una prova d’impatto a bassa velocità confrontando i risultati ottenuti con i dati sperimentali, valutando l’entità del danneggiamento intralaminare, mettendo a confronto le zone delaminate e rilevando l’indentazione residua. Parallelamente è stata portata avanti una campagna di analisi numeriche preliminari su un pannello irrigidito in composito, con alcuni difetti pre-esistenti, sottoposto a compressione. Tali analisi rappresentano un primo passo per la valutazione del codice sviluppato nell’ambito di analisi di tipo Compression After Impact per il pannello irrigidito caratterizzato da danneggiamenti da impatto

Damage initialization techniques for non-sequential FE propagation analysis of delaminations in composite aerospace structures

The experimental effort required to develop, damage tolerant, aerospace composite structures could be significantly reduced if reliable numerical simulations were used to perform engineering studies of complex damaged structures. Finite element (FE) simulations of impact damaged structures typically follow a sequential approach that require large computational resources to reproduce complex damage scenarios. A numerical tool capable to reconstruct such scenarios using data from previous impact simulations or NDI could noticeably improve the simulation workflow for damaged composite structures. The paper proposes a method to inizialize the damage variables in numerical analyses aimed at assessing damage propagation, and that are potentially able to evaluate the residual strength of damaged structures. The approach is developed within FE software ABAQUS, and uses SDVINI subroutine to initialize damage variables defined by a user-material-subroutine (UMAT), that provides the constitutive models of the lamina and of the interlaminar layers. Albeit the proposed technique might deal with both inter-laminar and intra-laminar damage, the paper is focused on delaminations. A user defined traction-separation law is coded in an UMAT that endows ABAQUS cohesive elements with damage initialization capabilities. Then, results of test cases, of increasing complexity, are presented in order to assess the damage initialization procedure and verify the performances of its different operating modes. Two test-cases are based on plate-like specimens for which literature data exist: the first is relevant to a circular artificial delamination while the second presents multiple delaminations caused by an impact and measured via NDI techniques. The last test-case is a stiffened panel which incorporates the typical complexities of aerospace structures, but is still tractable with the sequential simulation approach whose results are used as a term of comparison

Topology optimisation of architected cellular materials from additive manufacturing: Analysis, design, and experiments

This work deals with an experimental/numerical validation of the optimised topologies found through a special density-based topology optimisation (TO) method wherein the topological descriptor, i.e., the pseudo-density field, is represented through a non-uniform rational basis spline (NURBS) hyper-surface. The framework is that of multi-scale TO methods to design architected cellular materials (ACMs). Specifically, in the most general case, the topological variables are defined at the scale of the representative volume element (RVE) of the ACM and at the macroscopic scale of the structure. The transition among scales is performed via a numerical homogenisation scheme based on the strain energy of elements. The proposed formulation exploits the properties of NURBS entities to determine the relationships occurring among the topological variables defined at different scales to correctly state the optimisation problem and to satisfy the hypotheses at the basis of the homogenisation method. Three design cases are considered: in the first one, TO is performed only at the macroscopic scale; in the second one, TO is performed only at the RVE scale; in the last one, TO is performed simultaneously at both scales. Multiple design requirements related to lightness, scale separation condition (to ensure the validity of the results of the homogenisation method) and minimum printable size are included in the problem formulation. Particularly, the last two requirements are implicitly satisfied by controlling the integer parameters of the NURBS entity (describing the pseudo-density field at each scale) without introducing explicit optimisation constraints. The multi-scale TO strategy is applied to a structure made of ACM subject to three-point bending test-like boundary conditions: for each design case, the optimised topology is manufactured through stereo-lithography and a comparison between experimental and numerical results (obtained through non-linear analysis conducted a posteriori on the optimised topology) is performed to assess the effectiveness of the approach

Composites Part C: Open Access

Recently, the necessity of reducing the probability of spread of viruses has fostered the creativity of engineers to develop tools that would allow actions of every-day life to be executed differently. Moreover, the maturity of the Fused Filament Fabrication (FFF) technology and the associated low costs has allowed creative solutions to be produced and used in real-life applications. A distinctive example is represented by the common action of opening a door. Since hands are a typical vector of contamination for viruses such as Coronavirus, hands-free devices aim at making use of the existing structure and kinematic to complete the same action in a different fashion. Typically, the mechanical and manufacturing requirements of these devices include a suitable stiffness-to-mass ratio, a reduced printing time as well as the minimization of supports which need to be removed in a post-printing phase. To tackle all these requirements a dedicated topology optimization (TO) method can be used since the preliminary design phase. Several design requirements of different nature can be included in the problem formulation: mechanical ones, like mass and stiffness, and manufacturing ones, like drawing direction or minimum member size. In this paper, a feasibility study on a hands-free 3D printed door opener has been carried out by means of the Solid Isotropic Material with Penalization (SIMP) method for TO and its CAD-compatible variant, i.e., the SIMP approach reformulated in the framework of non-uniform rational basis spline hyper-surfaces. The aim of the study is to identify optimal solutions to be adapted to a real-case scenario wherein different loading cases and manufacturing constraints are evaluated. Different optimal solutions are obtained, reconstructed to be compatible with CAD environment and the optimized geometry numerically assessed. Finally, the optimal solutions are also evaluated with respect to indicators such as printing time, total filament mass and mass of the supports required by the printing process.Conception et Optimisation de Forme pour la Fabrication Additiv

Low-velocity impact tests on carbon/epoxy composite laminates: A benchmark study

Low-velocity impacts (LVI) on composite laminates pose significant safety issues since they are able to generate extended damage within the structure, mostly delaminations and matrix cracking, while being hardly detectable in visual inspections. The role of LVI tests at the coupon level is to evaluate quantities that can be useful both in the design process, such as the delamination threshold load, and in dealing with safety issues, that is correlating the internal damage with the indentation depth. This paper aims at providing a benchmark of LVIs on quasi-isotropic carbon/epoxy laminates; 2 laminates are tested, 16 and 24 plies and a total of 8 impact energies have been selected ranging from very low energy impacts up to around 30 J. Delamination threshold loads, shape and extension of délaminations as well as post-impact 3D measurements of the impacted surface have been carried out in order to characterize the behavior of the considered material system in LVIs. The analysis of test results relevant to the lowest energies pointed out that large contact force fluctuations, typically associated to delamination onset, occurred but ultrasonic scans did not reveal any significant internal damage. Due to these unexpected results, such tests were further investigated through a detailed FE model. The results of this investigation highlights the detrimental effects of the dissipative mechanisms of the impactor. A combined numericale-experimental approach is thus proposed to evaluate the effective impact energies

Thin-Walled Structures

This work presents a multi-scale design methodology for the deterministic optimisation of thin-walled composite structures integrating a global–local approach for the assessment of the buckling strength and a dedicated strategy to recover blended stacking sequences. The methodology is based on the multi-scale two-level optimisation strategy for anisotropic materials and structures. In the first step, focused on the macroscopic scale, several design requirements are included in the problem formulation: lightness, feasibility, manufacturing, blending, buckling failure, static failure and stiffness. The second step, which focuses on the laminate mesoscopic scale, deals with the recovery of blended stacking sequences, for the structure at hand, matching the optimal geometric and elastic properties determined in the first step. As a case study, the unconventional PrandtlPlane box-wing system is used to show the effectiveness of the proposed design methodology

How future surgery will benefit from SARS-COV-2-related measures: a SPIGC survey conveying the perspective of Italian surgeons

COVID-19 negatively affected surgical activity, but the potential benefits resulting from adopted measures remain unclear. The aim of this study was to evaluate the change in surgical activity and potential benefit from COVID-19 measures in perspective of Italian surgeons on behalf of SPIGC. A nationwide online survey on surgical practice before, during, and after COVID-19 pandemic was conducted in March-April 2022 (NCT:05323851). Effects of COVID-19 hospital-related measures on surgical patients' management and personal professional development across surgical specialties were explored. Data on demographics, pre-operative/peri-operative/post-operative management, and professional development were collected. Outcomes were matched with the corresponding volume. Four hundred and seventy-three respondents were included in final analysis across 14 surgical specialties. Since SARS-CoV-2 pandemic, application of telematic consultations (4.1% vs. 21.6%; p < 0.0001) and diagnostic evaluations (16.4% vs. 42.2%; p < 0.0001) increased. Elective surgical activities significantly reduced and surgeons opted more frequently for conservative management with a possible indication for elective (26.3% vs. 35.7%; p < 0.0001) or urgent (20.4% vs. 38.5%; p < 0.0001) surgery. All new COVID-related measures are perceived to be maintained in the future. Surgeons' personal education online increased from 12.6% (pre-COVID) to 86.6% (post-COVID; p < 0.0001). Online educational activities are considered a beneficial effect from COVID pandemic (56.4%). COVID-19 had a great impact on surgical specialties, with significant reduction of operation volume. However, some forced changes turned out to be benefits. Isolation measures pushed the use of telemedicine and telemetric devices for outpatient practice and favored communication for educational purposes and surgeon-patient/family communication. From the Italian surgeons' perspective, COVID-related measures will continue to influence future surgical clinical practice

Numerical simulation of low-velocity impacts on composite laminates

Composite materials are assuming a predominant role in structural applications, in particular in aeronautics where high specific strength and stiffness, advancements in manufacturing and assembly technologies and the possibility of adapting mechanical properties to structural requirements have shown how significant weight savings, lower maintenance costs and innovative design philosophies are already within reach. In aeronautics, safety is a primary concern, thus detrimental conditions and events to which composites are particularly sensitive must be carefully taken into account. Among these, low-velocity impacts, due to a combination of difficulties in damage detection and presence of significant damage within the structure, can reduce strength and stiffness of the structure. To satisfy the structural requirements, safety factors are introduced to cope with detrimental effects and, as a consequence, the real structural potential of composites is not completely exploited. This thesis deals with numerical simulations of low-velocity impacts for which, in particular, the mechanisms that generate the permanent indentation left after the impacts and the onset and propagation of delaminations are numerically investigated in the context of traditional composite laminates. Intra-laminar and inter-laminar numerical damage models have been implemented into user-defined material routines to be used within the Finite Element software Abaqus in which meso-scale composite laminate models are created and analyzed. An heuristic non-linear shear damage model has been developed, in the context of the Continuum Damage Mechanics, to characterize the out-of-plane shear lamina constitutive behavior which, once tuned on the considered material system, is able to reproduce the shape and depth of the dent caused by the impactor. Due to the fact that the experimental characterization of the lamina out-of-plane shear behavior is quite challenging, sensitivity studies of the main parameters that define the constitutive law are performed. Inter-laminar damage (delaminations), approached through the Cohesive Zone Model, is modeled via a bi-linear traction-separation constitutive law which characterizes pure and mixed-mode behavior of Abaqus cohesive elements. Both the accuracy and the computational performances of the simulations results obtained for different combinations of cohesive parameters are investigated and compared with experimental references. Since delaminations are, typically, the most common, extended and threatening type of damage in low-velocity impacts on composite laminates, procedures were developed to create damage scenarios (inter-laminar damage is here considered but also intra-laminar damage can be used) on composite structures in order to evaluate their residual mechanical properties. The damage to be initialized can be user-designed, extracted by other numerical simulations or based on real data obtained, for example, through non-destructive inspections. An initialization technique is, then, used to investigate the mechanical response of damaged composite laminates in simulations of compression after impact tests. In these simulations, the delaminations scenario is captured from previously performed low-velocity impact simulations and injected into the new Finite Element model. Thanks to this procedure, the boundary conditions and the laminate discretization can be modified as well as the cohesive parameters for which a sensitivity analysis regarding their influence on the results is performed. Eventually, an experimental low-velocity impact campaign on composite laminates followed by ultrasonic inspections, to evaluate the damage extension, is presented. In this campaign, impacts are performed at different energy levels on thick and thin multi-directional quasi-isotropic laminates. Unexpected results have been obtained for the smallest energy levels, in which damage is essentially absent or negligible. To investigate these results, Finite Element analyses, where a detailed impactor model is used, are carried out to evaluate the role played by internal dissipation mechanisms, related to the impactor assembly, in modifying the expected force and displacement time histories. In fact, only by suitably tuning the impactor model, impact simulations to reproduce internal damage can be correctly performed. This thesis, contributes in different ways to enhance the progressive failure analysis of composite laminates in low-velocity impact events. The permanent indentation prediction, the comprehension of the relationship between accuracy and computational costs associated to the reproduction of delaminations and the numerical capability of design damage scenarios on larger composite structures can increase the confidence in structural applications of composite materials and, at the same time, reduce the costs associated with experiments if a smart synergy bewteen numerical analysis and experimental activities is devised

A NON-LINEAR MODEL FOR IN-PLANE SHEAR DAMAGE AND FAILURE OF COMPOSITE LAMINATES

Composite material characterization is typically carried out by time-consuming and expensive experimental tests aimed at establishing strengths at both lamina and laminate levels. In this scenario, numerical analyses are valuable tools in order to reduce the number of tests and to gather, at the same time, knowledge about the complex interactions of the composite damage mechanisms. The paper presents a new constitutive model for the non-linear shear behavior of composite laminates, which has been implemented in an user-defined Fortran routine (UMAT) to be used within ABAQUS non-linear FE code. A numerical model of the ASTM Standard V-notch specimen shear test has been developed in order to identify the key parameters of the non-linear shear constitutive model. This has been achieved by means of a systematic comparison of the numerical results with experimental data. Material anisotropy and the geometry of the notch have been found to cause the shear strain field to be non-uniform in the notch section. This prevents a direct measure of the shear constitutive law parameters, which must be alternatively evaluated through an indirect procedure. A modified notch geometry, which mitigates strain non-uniformities, has been evaluated and assessed through numerical simulations

Delaminations growth in compression after impact test simulations: influence of cohesive elements parameters on numerical results

The compressive strength of impact damaged composite laminates may be reduced by ply damage and delaminations, which may grow under load. At coupon level, the residual strength of impacted laminates is assessed through Compression After Impact compression after impact (CAI) tests; as a complement to the experiments, numerical tools are sought that predict the behavior of delaminated composites. In this context, the paper investigates the influence of the cohesive elements parameters -– the inter-facial strengths and the cohesive mesh size -– in CAI test simulations. The study is carried out using a recently developed initialization technique that allows CAI simulations be run independently from the impact simulations (used to calculate induced delaminations). The first part of the study uses a conventional sequential approach (CAI analyses follows the impact simulations which share the same parameters) to asses the sensitivity of force time histories and in-plane delaminations growths to parameters variation. Then, for a selected initial damage scenario, the inter-facial strengths sensitivity is performed also evaluating the computational efficiency. The study shows how the cohesive parameters affect both the accuracy and the computational cost of the analyses and contributes to establish guidelines for an effective use of cohesive elements in CAI tests simulations