68 research outputs found
Data for: Probing athletics tracks degradation using a microscratch technique
No full textAverage scratch data curves used to evaluate hardness of the model coloured rubber compounds investigated before and after natural/artificial agein
A visco-hyperelastic numerical model for the dynamic behaviour of rubbers
No full textIn this work, a visco-hyperelastic numerical model is proposed, based on the decoupling of the strain and time dependent contributions. Four different rubber blends, used for the production of athletics tracks, have been experimentally characterized in compression under varying loading histories. A robust identification procedure provided reliable constitutive parameters to be implemented in the numerical simulations. Model predictions have been validated against the outcome of impact tests performed on the different materials using an Artificial Athlete. Results demonstrate that the presence of a viscoelastic component grants a more accurate description of the energy return characteristics of rubbers under dynamic conditions
Environmental stress cracking of high-density polyethylene under plane stress conditions
No full textHigh-Density Polyethylene is prone to Environmental Stress Cracking if mechanically stressed in the presence of solutions containing surfactants. Even if this polymer is widely used to produce containers for industrial and household detergents, its Environmental Stress Cracking Resistance is generally evaluated under plane strain conditions irrespective of the actual stress state experienced during service life. In this work the Slow Crack Growth of thin specimens, under plane stress conditions, was studied in air and in the presence of an “active” environment. The J-integral approach was adopted to account for the extensive plastic deformations thereby occurring and the obtained results were compared to those describing the plane strain behaviour of the same polyethylene, reported in previous works. The effect of the production process was also assessed by comparing the behaviour of compression moulded and blow moulded specimens, the latter having a lower degree of crystallinity. Despite the difference in fracture resistance expected in air, the behaviour in presence of the active environment was very similar, suggesting that the production process has only negligible influence on the Environmental Stress Cracking resistance of the considered polyethylene
PROPRIETÀ MECCANICHE DI ELETTROFORMATI NANOCRISTALLINI IN LEGA Ni-Co
No full textL’elettroformatura di componenti per impieghi strutturali è un processo di crescente importanza industriale e richiede lo sviluppo di materiali con un insieme calibrato di caratteristiche fisico-meccaniche di non facile conseguimento. In questo lavoro si è messo a punto un processo di elettrodeposizione di lega Ni-Co di composizione ottimizzata rispetto ad una possibile specifica di progetto. Morfologia e microstruttura sono state caratterizzate mediante analisi di microscopia elettronica e diffrazione di raggi X; la caratterizzazione meccanica è stata condotta attraverso prove di trazione, indentazione strumentata, microscratch e misura degli sforzi residui. I risultati forniti da prove di trazione e di microdurezza sono stati analizzati allo scopo di definire una correlazione precisa e riproducibile tra limite elastico e durezza Vickers per il materiale in studio. La struttura nanocristallina della lega ha offerto lo spunto per studiare la dipendenza delle proprietà meccaniche dalla velocità di deformazione, sia attraverso prove di trazione sia attraverso prove di microscratch, in modo da potere sondare un intervallo di velocità di deformazione il più ampio possibile
Polymeric foams 3D numerical mechanical modelling
No full textOne 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
Environmental Stress Cracking of Polymers
No full textSlow crack growth (SCG) is usually the most dominant failure mechanism in polymer products, typically occurring over extended periods under sustained low-stress conditions. This phenomenon is particularly critical in applications requiring long-term durability, such as gas pipes, water distribution systems, and storage tanks.
A phenomenon closely related to SCG is Environmental Stress Cracking (ESC), in which the progression of slow crack growth is accelerated when a polymer is exposed to surface-active agents under stress. These agents do not alter the polymer's chemical structure but drastically reduce the time to failure, by accelerating crack initiation and propagation. In ESC, the same crack growth mechanisms observed in SCG occur, but at a significantly accelerated rate due to environmental factors, making it a critical consideration in the design of polymer products intended for long-term use under stress.
The resistance to ESC is typically evaluated using standard methods, which unfortunately are quite limited in terms of both the amount of information provided (typically a single ranking parameter) and their accuracy, hindering the possibility of establishing clear structure-properties relationships for the materials under investigation. Over the years we developed an alternative approach, based on fracture mechanics (FM), which provides a much richer picture about the active ESC mechanisms and allows the identification of clear structure-properties relationships. These insights enable the optimization of the long-term performance of polymers under various stress conditions, at the same time providing reliable quantitative predictions of product lifetime, which can be of high interest for the industry
An image-based approach for structure investigation and 3D numerical modelling of polymeric foams
No full textPolymeric 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
Experimental tests and numerical modeling of a sandwich panel
No full textThe aim of this paper is to investigate the main characteristics that a “good enough” FE model of a sandwich plate must have in order to predict with sufficient accuracy its dynamic characteristics up to medium-frequency. Results obtained using different models are discussed in terms of accuracy and inclusion of material losses, and they are compared with results obtained experimentally. A suitable experimental setup and a test plan are designed in order to evaluate the dynamic behaviour of the panel. Material properties, and in particular material losses, of both the foam core and the adhesive bond are obtained as a function of frequency by dynamo-mechanical tests. These are subsequently used for numerical modelling, and the influence of material damping in the frequency range of interest is discussed. The panel is tested on a wide frequency range, and a sensitivity analysis is carried out to explore the effects of different sensor number/position and excitation position
Elastic modelling of polymeric foams using a Representative Volume Element approach
No full textNon-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
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