Rosario Borrelli

Numerical/Experimental Correlation of a Plasma Wind Tunnel Test on a UHTC-Made Nose Cap of a Reentry Vehicle

The 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...

Application to plate components of a kinematic global‐local approach for non‐matching finite element meshes

Purpose – The purpose of this paper is to investigate and to assess the capabilities of the most common finite element (FE)‐based tools to deal with global‐local analysis. Two kinds of coupling were investigated: shell to shell and shell to solid.Design/methodology/approach – The issue of connecting non‐matching FE global and local models, characterized by different mesh refinements and/or different element types, was addressed by introducing appropriate kinematic constraints on the nodes at the interfaces. The coupling techniques available in the three FE‐based codes (ABAQUS®, NASTRAN® and ANSYS®), were assessed by applying them on a common numerical test case (non‐linear buckling analysis of a square plate). Results of the global‐local simulations were compared to the results obtained for relevant reference solutions.Findings – The continuity of displacements and stresses across the interface between global and local models and the influence of the presence of the local model on the global model solutio...

Application of the Mesh Superposition Technique to the Study of Delaminations in Composites Thin Plates

Laminated composite structures are increasingly finding more applications in various fields thanks to their lower weight if compared with other materials of the same strength. Nevertheless, composites thin plates show a critical behavior in terms of damage propagation mechanisms when subjected to (low velocity) impact. Indeed they tend to produce delaminations which can be hardly detected by optical inspections and can affect the global load carrying capability, leading to a premature structural collapse. The aim of this paper is to assess the capabilities of the Davies-Zhang approach (introduced in 1994 and aimed to the estimation of both the delamination initiation impact load and the size of the impact induced delaminations) by using a multiscale FE model based on the mesh superposition technique. Indeed the impact area has been modeled layer-wise with an element per layer while the rest of the structure has been modeled at laminate level by layered elements by means of a homogenization approach for the determination of the equivalent laminate material properties. The impact induced delamination area has been determined by adopting stress-based criteria. The results (in terms of delamination initiation impact force and delamination size) have been compared to the ones obtained by adopting the Davies-Zhang approach.

Influence of Powder Characteristics on Formation of Porosity in Additive Manufacturing of Ti-6Al-4V Components

This paper aims to study the genesis of defects in titanium components made through two different additive manufacturing technologies: selective laser melting and electron beam melting. In particular, we focussed on the influence of the powders used on the formation of porosities and cavities in the manufactured components. A detailed experimental campaign was carried out to characterize the components made through the two additive manufacturing techniques aforementioned and the powders used in the process. It was found that some defects of the final components can be attributed to internal porosities of the powders used in the manufacturing process. These internal porosities are a consequence of the gas atomization process used for the production of the powders themselves. Therefore, the importance of using tailored powders, free from porosities, in order to manufacture components with high mechanical properties is highlighted.

Study on the Factors Affecting the Mechanical Behavior of Electron Beam Melted Ti6Al4V

In this study, a mechanical characterization has been performed on EBM built Ti-6Al-4V tensile samples. The results of tensile tests have shown a different behavior between two sets of specimens: as built and machined ones. Supporting investigations have been carried out in order to physically explain the statistical difference of mechanical performances. Cylindrical samples which represent the tensile specimens geometry have been EBM manufactured and then investigated in their as built conditions from macrostructural and microstructural point of view. In order to make robust this study, cylindrical samples have been EBM manufactured with different size and at different height from build plate. The reason of this choice was arisen from the need of understanding if other factors as the massivity and specific location could affect the microstructure and defects generations consequently influencing the mechanical behavior of the EBMed components. The results of this study have proved that the irregularity of external circular surfaces of examined cylinders, reducing significantly the true cross section withstanding the applied load, has given a comprehensive physical explanation of the different tensile behavior of the two sets of tensile specimens.

Assessment of progressive failure analysis capabilities of commercial FE codes

Purpose – The main objective of this work is to assess the current capabilities of different commercial finite element (FE) codes in simulating the progressive damage of composite structures under quasi-static loading condition in post-buckling regime. Design/methodology/approach – Progressive failure analysis (PFA) methodologies, available in the investigated FE codes, were applied to a simple test case extracted from literature consisting in a holed composite plate loaded in compression. Findings – Results of the simulations are significantly affected by the characteristic parameters needed to feed the degradation models implemented in each code. Such parameters, which often do not have a physical meaning, have to be necessarily set upon fitting activity with an experimental database at coupon level. Concerning the test case, all the codes were found able to capture the buckling load and the failure load with a good accuracy. Originality/value – This paper would to give an insight into the PFA capabilit...

3D Global-Local Analysis Using Mesh Superposition Method

One of the main issue in a FEM analysis is to determine and minimizing discretization errors. This kind of errors assume a critical importance especially where the solution, in terms of displacement and stress, quickly changes inside the finite element. This issue can be overcame adopting a very refined element discretization in those regions.Hence, for this kind of simulations, it is common practice to use global-local methods rather than adopt a refined discretization over the entire domain. Indeed, global-local methods allow to define very refined elements distributions in some regions of interest, which can be coupled with coarser element distributions in the rest of the domain.A global-local approach based on the superposition technique is presented in this work. This approach allows the coupling of two different meshed domain by superimposing the refined local mesh on the global mesh for the region of interest. The coupling takes place without introducing multi-point constraints or transition regions; the mesh continuity and the well-conditioning of the stiffness matrix are satisfied by appropriate boundary conditions. This approach allows to obtain accurate solutions in the areas of interest while keeping the computational time within satisfactory limits. Several numerical applications are presented which allow to assess the effectiveness of the proposed approach for 3D linear static simulations.

Water impact tests and simulations of a steel structure

Purpose – The purpose of this paper is to improve knowledge of the water impact phenomenon from both the experimental and numerical points of view.Design/methodology/approach – A drop test campaign on water was carried out on semi‐cylindrical steel structures. Therefore, an experimental database for validation purpose was generated. Subsequently, a finite element model was developed in LS‐DYNA in order to reproduce the tests. The behaviour of water was modeled by using the smoothed particle hydrodynamics (SPH) methods. Numerical simulations were compared to experimental data and the influence of some numerical parameters on the simulations was investigated.Findings – The FE model was found to be able to reproduce the tests, at least in terms of acceleration peak and distribution of plastic deformation. Acceptable prediction was also found for the pressure peak in soft areas.Research limitations/implications – In case of low velocity impact, the water model was found to be too rigid and the acceleration pe...

Microstructural and micromechanical study of a Ti6Al4V component made by electron beam melting

Additive Layer Manufacturing is one of the most promising and investigated manufacturing system due to its advantages to produces near net shape components, also with a very complex shape, in a single shot. Among the different techniques now available, the Electron Beam Melting (EBM) is of particular interest in the production of metal components. Particularly the application of this technique to titanium alloys allows to produces components with a very low buy to fly ratio. In the present paper the microstructure attained is accurately described and mini tensile tests performed allowed to understand the fracture behavior of specimen with the specific microstructure realized under static load.

Buckling analysis of a delaminated panel by using a kinematic global-local coupling approach

Purpose – The purpose of this paper is to investigate on the behaviour of a delaminated stiffened panel; the delamination growth is simulated via fracture elements implemented in B2000++® code based on the Modified Virtual Crack Closure Technique (MVCCT), matrix cracking and fibre failure have been also taken into account. Design/methodology/approach – In order to correctly apply the MVCCT on the delamination front a very fine three-dimensional (3D) mesh is required very close to the delaminated area, while a 2D-shell model has been employed for the areas of minor interest. In order to couple the shell domain to the solid one, shell-to-solid coupling elements based on kinematic constraints have been used. Findings – Results obtained with the global/local approach are in good correlation with those obtained with experimental results. Originality/value – The global/local approach based on kinematic coupling elements in conjunction with fracture elements allows to investigate and predict the behaviour of a s...