Characterization of Composites Manufactured Through Reshaping of EoL Thermoplastic Polymers Reinforced with Recycled Carbon Fibers

This article investigates if and at what extent a recycling process based on grinding, melting and re-shaping of recycled carbon fibers reinforced thermoplastic polymers (rCFRPs) can affect their physical, mechanical and thermal properties. The aim is to establish if they can be taken into consideration in the manufacturing of new composite materials in different sectors: automotive, marine, sporting goods, etc. Composites materials were submitted to the measurement of the fibers length they are composed of, and then analyzed by means of tensile and impact tests and a dynamic mechanical analysis (DMA). All the characterizations were performed to both initial and recycled composites and, in some cases, they were replied also after the intermediate accelerated aging. Characterization performed confirmed that, as expected, the recycling process affects the properties of the composites, but in different manners and to a different extent when different polymers are involved. Tensile and impact tests pointed out that the polypropylene based composites showed a less stiff and a more brittle behaviour after the recycling process and the DMA confirmed this evidence, highlighting in addition a more viscous behavior of the polymer after the recycling. Conversely, the polyamide 6 based composites increased their stiffness and ductility after the recycling. For all the composites the tensile strength dropped, confirming the weakening of the materials

Polymeric foams 3D numerical mechanical modelling

One of the main open issues in the field of polymeric foam materials is the lack of a relationship between the foam geometrical characteristics and its constituent material properties, on one side, and the macroscopic mechanical behavior. This link is an essential ingredient for the development of a predictive numerical model able to fully describe the mechanical behavior of polymeric foams under different loading conditions, which is the ultimate goal of the present work. In order to build up a systematic and methodological approach to this problem, polymeric structural closed cell foams having different nominal densities (ranging from 60 to 120 kg/m3) were considered. The internal foam structure was investigated throughout micro-Computed Tomography; the acquired stack of images were processed with a home-made algorithm in which Mean Intercept Length method was implemented to compute material volume distribution and the degree of structural anisotropy. The algorithm also allowed the reconstruction of the real geometry using a voxel-based scheme, to perform Finite Element Analysis. With the aim of reducing geometric discontinuities, inherent in the reconstructed voxel mesh, Taubin’s smoothing algorithm was employed to obtain more accurate results. Numerical simulations mimicking experimental quasi-static uniaxial compression test were performed to obtain nominal stress vs. strain curves. To this purpose, suitable mechanical properties were identified for the (equivalent solid) constituent material: the resulting constitutive law highlights the contribution of the material to the macroscopic foam properties. Relevant mechanical parameters such as elastic moduli, buckling strain and plateau stress were then evaluated and related to geometrical features of the real foam

An image-based approach for structure investigation and 3D numerical modelling of polymeric foams

Polymeric expanded materials are of great importance in many engineering applications. Despite this, as of today the development of models able to describe the mechanical behaviour of these material as a function of their microstructure is still an open challenge. In this study an image-based approach is proposed for both microstructure characterisation and 3D numerical mechanical simulations. Microstructure is investigated through different algorithms, such as Mean Intercept Length and Autocorrelation function, to determine synthetic parameters able to describe the internal structure. A novel algorithm has been developed to convert the images obtained from computed tomography into a finite element mesh with an optimized number of elements: this method preserves the original structure and can also be used to generate other fictitious structures that can be analysed. The investigation led to the identification of general relationships between foam microstructure and relevant macroscopic physical and mechanical properties. These relationships can serve as a tool to optimize foam morphology or product final properties for several different engineering applications

Uniformly dispersed Pr+3 doped silica glass by the sol-gel process

Praseodymia-silica glass samples with high praseodymia content (5 wt%) were prepared by the sol-gel process. Amino alkoxy silane was used to immobilize the Pr+3 cation in the silica (derived from tetraethyl orthosilicate) matrix. The gel to glass transformation was examined by ultraviolet, visible, near infrared and Fourier transform infrared spectroscopy at different stages of drying/densification up to 1000°C. The X-ray microanalysis for praseodymium throughout the bulk glass samples confirmed the homogeneous distribution of the Pr+3 in the matrix

Uniformly dispersed Pr+3 doped silica glass by the sol-gel process

Praseodymia-silica glass samples with high praseodymia content (5 wt%) were prepared by the sol-gel process. Amino alkoxy silane was used to immobilize the Pr -3 cation in the silica (derived from tetraethyl orthosilicate) matrix. The gel to glass transformation was examined by ultraviolet, visible, near infrared and Fourier transform infrared spectroscopy at different stages of drying/densification up to IO00°C. The X-ray microanalysis for praseodymium throughout the bulk glass samples confirmed the homogeneous distribution of the Pr +3 in the matrix

Elastic modelling of polymeric foams using a Representative Volume Element approach

Non-destructive, high-precision imaging techniques provide a wealth of information on the internal structure of materials used for a variety of applications, ranging from structural composites to biomedical devices. The main issue to deal with is the large amount of generated data, and the numerical resources required to process it. In this work high-resolution X-ray computed tomography is chosen to investigate a closed-cell polyethylene terephthalate (PET) foam available in four different densities, typically used for composite sandwiches. The resulting set of images is used for both morpho-structural and finite element analyses. The material spatial distribution is computed by exploiting the Mean Intercept Length (MIL) algorithm, as proposed by Moreno [1]. Other macroscopic structural parameters are extracted, such as solid volume fraction and mean structure thickness, with the aim of identifying a relationship with macroscopic mechanical properties. Since the reconstruction of the entire inspected volume would result into a prohibitive number of finite elements, a 2D statistical approach is developed. The sets of images are divided into smaller subdomains for which individual morphostructural properties are computed. The density and the material spatial distribution are represented by a synthetic parameter called degree of anisotropy (DA): a 2D frequency statistics is derived and for each sample the most frequent domain is detected and then converted into a finite element mesh, by exploiting the marching cube algorithm. For each domain finite element analyses are run under elementary loading conditions [2] and the macroscopic stress and strain tensors evaluated through Gurson’s homogenization algorithm; the homogenized compliance matrix is assembled to obtain a set of orthotropic elastic constants. The approach is validated with uniaxial compression data; then, all the sub-domains are considered as valid samples to be reconstructed and simulated to broaden the range of investigated structural and mechanical properties. This larger dataset allows the identification of global macroscopic relationships between structural parameters and elastic constants. REFERENCES [1] R. Moreno, M. Borga and O. Smedby, Medical physics 39(7):4599-4612, 2012. [2] V.Kouznetsova, M.G.D. Geers and W.A.M. Brekelmans, International Journal for Numerical Methods in Engineering 54(8):1235-1260, 2002

On the resolution of semiconductor multilayers with a scanning electron microscope

The main factors affecting the spatial resolution of the scanning electron microscope have been analysed using a suitable inhomogeneous specimen consisting of semiconductor multilayers. Operating in secondary electron as well as in backscattered electron imaging mode, with the support of Monte Carlo simulations of beam-specimen interaction, it has been possible to achieve the following conclusions. All the backscattered or secondary electrons positively contribute to the image formation independently of their trajectories and specimen exit points. The resolution of the backscattered electron images depends only on the beam size and the signal to noise ratio. The generation volume of backscattered electrons affects the contrast but does not represent in itself a limit to the resolution. The contrast of secondary electron images of the semiconductor multilayer has essentially a compositional nature, being linked to the collection of secondary electrons produced by the backscattered ones. Moreover, using a converter of backscattered electrons in secondary electrons located underneath the pole pieces of the microscope objective lens, it has been demonstrated an improvement of the signal that allows a resolution equal to the electron beam size. This resolution cannot be obtained, operating in the same electron optical conditions, without the converter, due to the lower signal level