1,720,974 research outputs found
Stochastic Approach to Damage Resistance Analysis of Stiffened Composite Panels
No full textComposite materials exhibit complex phenomena associated with damage onset, and are characterized by significant uncertainties in their material properties. The development of damage resistance design methodology, especially for damage induced by low velocity impacts in aircraft structures, have allowed a more efficient management of the damage in composites. In this paper a probabilistic approach has been adopted in order to evaluate the impact of the material properties scattering on the damage resistance of a composite panel. Furthermore two different composite panels have been investigated and compared in order to detect, based on a probabilistic approach, the one exhibiting the best performances in terms of damage resistance
Thermo Mechanical Design Methodologies
No full textDuring the Preliminary Design Phase, a simplified model has been developed in order to make fast the optimization process of thermal protection systems thicknesses.
It is adopted to minimize the weight of the thermal protection systems by choosing the best configuration that fits a given set of requirements.
It is mainly suitable for axisymmetric geometries in which axial conduction/diffusion phenomena can be neglected
A fast procedure for optimizing thermal protection systems of re-entry vehicles
No full textThe aim of the present work is to introduce a fast procedure to optimize thermal protection systems for re-entry vehicles subjected to high thermal loads. A simplified one-dimensional optimization process, performed in order to find the optimum design variables (lengths, sections etc.), is the first step of the proposed design procedure. Simultaneously, the most suitable materials able to sustain high temperatures and meeting the weight requirements are selected and positioned within the design layout. In this stage of the design procedure, simplified (generalized plane strain) FEM models are used when boundary and geometrical conditions allow the reduction of the degrees of freedom. Those simplified local FEM models can be useful because they are time-saving and very simple to build; they are essentially one dimensional and can be used for optimization processes in order to determine the optimum configuration with regard to weight, temperature and stresses. A triple-layer and a double-layer body, subjected to the same aero-thermal loads, have been optimized to minimize the overall weight. Full two and three-dimensional analyses are performed in order to validate those simplified models. Thermal-structural analyses and optimizations are executed by adopting the Ansys FEM code
“Thermo-Structural Behaviour of an UHTC Made Nose Cap of a Re-entry Vehicle
No full textIn the frame of the technology project sharp hot structures (SHS), focused on the assessment of the applicability of ultra-high temperature ceramics (UHTCs) to the fabrication of high performance and SHS for reusable launch vehicles, the nose cap demonstrator named Nose_2 has been tested in the plasma wind tunnel (PWT) facility. In this paper, the FEM based thermo-structural analyses, carried out for the rebuilding of this PWT test are presented. Comparisons with experimental data measured in the PWT have been introduced to validate the FEM model and to help in interpreting the experimental test itself. Synergies between numerical and experimental activities have been finalized to the improvement of knowledge on the physical phenomenon under investigation. The effects on the thermal response due to the assumption of the catalytic condition of the wall, due to the uncertainties related to heat flux and pressure measurements on the probe (which influence the heat flux computation) and due to uncertainties in the determination of some UHTC thermal properties, have been investigated. The experimental temperatures curves falls very close to the numerical envelope (taking in account several sources of error) for all the test duration and the NCW model was found more reliable in reproducing thermal behaviour of the nose cap
Numerical and Experimental Study of Defects Evolution in a Composites Wing Box Under Compressive Loads.
No full textDue to their high specific strength and stiffness, Carbon Fibres Reinforced Plastics (CFRP) are commonly considered suitable for aerospace structural applications. However, their failure mechanisms are not completely predictable and this is the main reason why the CFRP integration in the aerospace industry has been generally slow in the last twenty years. Indeed, the lack of robust numerical tools, able to take into account the damage tolerance of composite structures, has led to over-conservative designs, not fully realising the promised economic benefits of composites materials. The Project MACMES, funded by the General Defence Secretariat/National Armaments Directorate, of the Italian Ministry of Defence, in the framework of the National Military Research Plan (PNRM), addresses this issue by suggesting an integrated approach for the damage management of aircraft composite structures monitored by embedded optic fibres. Within the MACMES project, such integrated approach was applied to a composite wing-box in order to monitor the buckling and the internal damage evolution under compressive loads. The experimental compressive test was performed by ALENIA under displacement control. Two defects were artificially included into the wing box: a skin/stringer debonding and an embedded circular bay delamination. Position and size of the delamination and skin-stringer debonding were chosen as a result of a sensitivity analysis performed during the preliminary design phase and aimed at determining the best configuration (in terms of defects size and position) which guarantees a satisfactory experimental measurement of the damage growth from initiation to the global buckling load and well before the failure load of the wing box.
The non-linear post-buckling behaviour of the damaged composite structure was simulated by developing appropriate FE numerical models. The adopted numerical models use the Virtual Crack Closure Technique to simulate the inter-laminar damage evolution and the numerical analyses have been performed by means of the FEM code ABAQUS and B2000++. The obtained numerical results have been assessed and compared each other in terms of delaminated area evolution, delamination growth initiation load and strain distributions in order to investigate the effectiveness of the adopted numerical platforms in predicting the evolution of inter-laminar damages. Comparisons with experimental data, in terms of load displacement curves and strains in the dedonding area, are presented to assess the accuracy of the numerical simulations
Numerical/Experimental Correlation of a Plasma Wind Tunnel Test on a UHTC-Made Nose Cap of a Reentry Vehicle
No full textThe nose cap demonstrator named Nose-2 has been tested for the second time in the plasma wind tunnel (PWT) facility which is part of the sharp hot structure (SHS) technology project, focused on the assessment of the applicability of ultrahigh temperature ceramics (UHTC) to the fabrication of high performance vehicles and SHS for reusable launch vehicles. In this paper the FEM based thermal analyses, carried out for the rebuilding of this PWT test, are presented. Experimental data measured in the PWT have been compared with numerical ones in order to validate the FEM model and to help in interpreting the experimental test itself. The knowledge on the physical phenomenon under investigation has been greatly improved, thanks to the synergy between numerical and experimental activities. In particular, a qualitative study of the modeling of the tip-dome interface has been performed in order to estimate the thermal contact resistance that heat flux encounters in passing through the demonstrator. The correlation between numerical and experimental temperature curves has been found to be satisfactory for both internal and surface temperature distribution, and the FEM model was found reliable in reproducing the thermal behavior of the nose cap
Hot Structure Design Modelling of Reusable Re-entry Vehicles
No full textNext generation reusable re-entry vehicles must be capable of sustaining consistent repeated aero-thermal loads without damage or deterioration. This means that such structures must tolerate the high temperatures engendered by aero-thermal re-entry fluxes but also resist the internal stresses related to such temperature fields. The TPS concepts - successfully tested on the Space Shuttle for over thirty years - have been proved to be quite ineffective with respect to the reusability requirement: in fact, the TPS tiles are subjected to a complex after flight inspection/reparation management, which is not very efficient from a time/cost point of view. To overcome these limitations, for highly aero-thermally stressed structures the TPS concepts may be substituted by "hot structure" concepts, in which the structure itself is conceived to resist the aero-thermal fluxes without employing TPS tiles. In order to achieve such a goal, "hot structure" parts must employ high temperature materials and provide a good management of the temperature distribution in order to guarantee safe interfaces with cold parts. Moreover the total weight of the structure should be kept low in order to limit costs. In the present paper we present a simple model for the design and optimisation of "hot structures" for reusable re-entry vehicles. The model makes use of 2D finite elements but, through the imposition of temperature/structural constraints, performs one-dimensional analyses, therefore it is well adapted for optimisation purposes. Issued from the model are some hot structure configurations optimised for re-entry flight
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