Thermomechanical surface instability at the origin of surface fissure patterns on heated circular MDF samples

When a flat sample of medium density fibreboard (MDF) is exposed to radiant heat in an inert atmosphere, primary crack patterns suddenly start to appear over the entire surface before pyrolysis and any charring occurs. Contrary to common belief that crack formation is due to drying and shrinkage, it was demonstrated for square samples that this results from thermomechanical instability. In the present paper, new experimental data are presented for circular samples of the same MDF material. The sample was exposed to radiant heating at 20 or 50 kW/m2, and completely different crack patterns with independent Eigenmodes were observed at the two heat fluxes. We show that the two patterns can be reproduced with a full 3-D thermomechanical surface instability model of a hot layer adhered to an elastic colder foundation in an axisymmetric domain. Analytical and numerical solutions of a simplified 2-D formulation of the same problem provide excellent qualitative agreement between observed and calculated patterns. Previous data for square samples together with the results reported in the present paper for circular samples confirm the validity of the model for qualitative predictions, and indicate that further refinements can be made to improve its quantitative predictive capability.Comment: 9 pages, 13 figures. New title and abstract, added experimental and simulation details and figures, conclusions unchanged. Matches the version published in Fire And Material

Thermomechanical surface instability at the origin of surface fissure patterns on heated circular MDF samples

When a flat sample of medium density fibreboard (MDF) is exposed to radiant heat in an inert atmosphere, primary crack patterns suddenly start to appear over the entire surface before pyrolysis and any charring occurs. Contrary to common belief that crack formation is due to drying and shrinkage, it was demonstrated for square samples that this results from thermomechanical instability. In the present paper, new experimental data are presented for circular samples of the same MDF material. The sample was exposed to radiant heating at 20 or 50 kW/m2, and completely different crack patterns with independent Eigenmodes were observed at the two heat fluxes. We show that the two patterns can be reproduced with a full 3-D thermomechanical surface instability model of a hot layer adhered to an elastic colder foundation in an axisymmetric domain. Analytical and numerical solutions of a simplified 2-D formulation of the same problem provide excellent qualitative agreement between observed and calculated patterns. Previous data for square samples together with the results reported in the present paper for circular samples confirm the validity of the model for qualitative predictions, and indicate that further refinements can be made to improve its quantitative predictive capability.Comment: 9 pages, 13 figures. New title and abstract, added experimental and simulation details and figures, conclusions unchanged. Matches the version published in Fire And Material

Static and Dynamic Thermomechanical Buckling Loads of Functionally Graded Plates.

In the paper the buckling phenomenon for static and dynamic loading (pulse of finite duration) of FGM plates subjected to simultaneous action of one directional compression and thermal field is presented. Thin, rectangular plates simply supported along all edges are considered. The investigations are conducted for different values of volume fraction exponent and uniform temperature rise in conjunction with mechanical dynamic pulse loading of finite duration

Thermomechanical Buckling Analysis of the E&P-FGM Beams Integrated by Nanocomposite Supports Immersed in a Hygrothermal Environment

none4siDue to the widespread use of sandwich structures in many industries and the importance of understanding their mechanical behavior, this paper studies the thermomechanical buckling behavior of sandwich beams with a functionally graded material (FGM) middle layer and two composite external layers. Both composite skins are made of Poly(methyl methacrylate) (PMMA) reinforced by carbon-nano-tubes (CNTs). The properties of the FGM core are predicted through an exponential-law and power-law theory (E&P), whereas an Eshelby-Mori-Tanaka (EMT) formulation is applied to capture the mechanical properties of the external layers. Moreover, different high-order displacement fields are combined with a virtual displacement approach to derive the governing equations of the problem, here solved analytically based on a Navier-type approximation. A parametric study is performed to check for the impact of different core materials and CNT concentrations inside the PMMA on the overall response of beams resting on a Pasternak substrate and subjected to a hygrothermal loading. This means that the sensitivity analysis accounts for different displacement fields, hygrothermal environments, and FGM theories, as a novel aspect of the present work. Our results could be replicated in a computational sense, and could be useful for design purposes in aerospace industries to increase the tolerance of target productions, such as aircraft bodies.Khorasani, Mohammad; Lampani, Luca; Dimitri, Rossana; Tornabene, FrancescoKhorasani, Mohammad; Lampani, Luca; Dimitri, Rossana; Tornabene, Francesc

Thermomechanical buckling analysis of the e&p-fgm beams integrated by nanocomposite supports immersed in a hygrothermal environment

Due to the widespread use of sandwich structures in many industries and the importance of understanding their mechanical behavior, this paper studies the thermomechanical buckling behavior of sandwich beams with a functionally graded material (FGM) middle layer and two composite external layers. Both composite skins are made of Poly(methyl methacrylate) (PMMA) reinforced by carbon-nano-tubes (CNTs). The properties of the FGM core are predicted through an exponential-law and power-law theory (E&P), whereas an Eshelby–Mori–Tanaka (EMT) formulation is applied to capture the mechanical properties of the external layers. Moreover, different high-order displacement fields are combined with a virtual displacement approach to derive the governing equations of the problem, here solved analytically based on a Navier-type approximation. A parametric study is performed to check for the impact of different core materials and CNT concentrations inside the PMMA on the overall response of beams resting on a Pasternak substrate and subjected to a hygrothermal loading. This means that the sensitivity analysis accounts for different displacement fields, hygrothermal environments, and FGM theories, as a novel aspect of the present work. Our results could be replicated in a computational sense, and could be useful for design purposes in aerospace industries to increase the tolerance of target productions, such as aircraft bodies

Multi-fidelity probabilistic optimisation of composite structures

In this thesis, novel multi-fidelity modelling-based probabilistic optimisation methods are presented to address the computational challenge of stochastic design philosophies applied to complex aircraft composite structures. Novel multi-fidelity formulations developed in this thesis, blending High-Fidelity Model (HFM) and Low-Fidelity Model (LFM), are shown to significantly improve computational efficiency by making use of machine learning techniques, such as Artificial Neural Networks (ANN) and Non-linear Auto-Regressive Gaussian Process (NARGP). To further improve the computational efficiency compared to the conventional probabilistic optimisation methods, a multi-level optimisation approach and a new sampling strategy to collect training data points are incorporated into the multi- fidelity formulations for the first time. In the developed optimisation methods, the HFM covers part of the design space whilst the LFM explores the whole design space to fill the lack of high-fidelity information. This improvement enables the multi-fidelity formulations to request a much smaller number of high-fidelity information causing considerable computational costs. Several engineering examples such as aircraft mono-stringer composite panels are used to demonstrate the accuracy and computational efficiency of the developed methods when used with different reliability and robustness analysis techniques, including Monte Carlo Simulation (MCS), the First-Order Reliability Method (FORM) and the Second-Order Reliability Method (SORM). The composite panels are subjected to mechanical and thermomechanical loads to show the broad range of potential applications. It is shown that the newly developed multi-fidelity probabilistic optimisation methods offer substantial computational time savings ranging from 50 % to 70 % and levels of error typically less than 1 % when compared with traditional probabilistic optimisation methods. Results demonstrate that the newly developed multi-fidelity probabilistic optimisation methods herein provide significant computational benefits and accurately predict the influence of uncertainties associated with design and manufacturing stages. As a result, the presented methods confidently carry out reliability-based and robust design optimisation of large-scale and complex aircraft composite structures.Open Acces

On the thermal buckling behaviour of laminated composite plates with cut-outs

In this work, the thermal buckling behaviour of laminated plates with rectangular cut-outs is studied using the finite element method. Based upon the classical plate theory, the used finite element is a combination of a linear isoparametric membrane element and a high precision rectangular Hermitian element. After validating the results obtained by the finite element, a parametric study is made using three types of materials commonly used in the industry, namely: the T300/5208 Graphite/Epoxy, the AS4/3501-6 Graphite/Epoxy and the E-glass/Epoxy. The study was about the effect of the size of the cut-outs, the boundary conditions, the stacking sequence and the stress resultants distribution on the critical buckling temperature. The study showed that the critical buckling temperature is strongly affected by the discussed parameters

Thermo-Mechanical Buckling and Non-Linear Free Oscillation of Functionally Graded Fiber-Reinforced Composite Laminated (FG-FRCL) Beams

We investigated the thermal buckling temperature and nonlinear free vibration of functionally graded fiber-reinforced composite laminated (FG-FRCL) beams. The governing nonlinear partial differential equations were derived from the Euler-Bernoulli beam theory, accounting for the von Karman geometrical nonlinearity. Such equations were then reduced to a single equation by neglecting the axial inertia. Thus, the Galerkin method was applied to discretize the governing nonlinear partial differential equation in the form of a nonlinear ordinary differential equation, which was then solved analytically according to the He's variational method. Three different boundary conditions were selected, namely simply, clamped and clamped-free supports. We also investigated the effect of power-index, lay-ups, and uniform temperature rise on the nonlinear natural frequency, phase trajectory and thermal buckling of FG-FRCL beams. The results showed that FG-FRCL beams featured the highest fundamental frequency, whereas composite laminated beams were characterized by the lowest fundamental frequency. Such nonlinear frequencies increase for an increased power index and a decreased temperature. Finally, it was found that FG-FRCL beams with [0/0/0] lay-ups featured the highest nonlinear natural frequency and the highest thermal buckling temperature, followed by [0/90/0] and [90/0/90] lay-ups, while a [90/90/90] lay-up featured the lowest nonlinear natural frequency and critical buckling temperature