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According to linear elastic fracture mechanics the stress intensity factor and the energy release rate are two fracture parameters linked by the elastic modulus and Poisson’s ratio of the considered material. This concept has been extended to the analysis of linear viscoelastic materials, by introducing time-dependent quantities; it is also used for nonlinear viscoelastic polymers, even if its accuracy in this case is still an open question. In this work the slow crack growth and the environmental stress cracking resistance of two high-density polyethylene grades were investigated, differing for their molecular weight distribution and fracture resistance. The description of the fracture behavior of the two materials provided by the stress intensity factor or the energy release rate turned out to be equivalent, despite the nonlinear mechanical behavior exhibited by the two polymers. Moreover, a time-dependent effective modulus, related to the two fracture parameters, was evaluated: its value was in good agreement with the modulus experimentally determined from tensile tests. A constant relevant effective strain was found despite the different testing conditions (i.e. applied mechanical loading, temperature and presence of an active environment), its value being equal for the two considered polyethylenes.

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.

High-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.

In polyjet printing photopolymer droplets are deposited on a build tray, leveled off by a roller and cured by UV light. This technique is attractive to fabricate heterogeneous architectures combining compliant and stiff constituents. Considering the layer-by-layer nature, interfaces between different photopolymers can be formed either before or after UV curing. We analyzed the properties of interfaces in 3D printed composites combining experiments with computer simulations. To investigate photopolymer blending, we characterized the mechanical properties of the so-called digital materials, obtained by mixing compliant and stiff voxels according to different volume fractions. We then used nanoindentation to measure the spatial variation in mechanical properties across bimaterial interfaces at the micrometer level. Finally, to characterize the impact of finite-size interfaces, we fabricated and tested composites having compliant and stiff layers alternating along different directions. We found that interfaces formed by deposition after curing were sharp whereas those formed before curing showed blending of the two materials over a length scale bigger than individual droplet size. We found structural and functional differences of the layered composites depending on the printing orientation and corresponding interface characteristics, which influenced deformation mechanisms. With the wide dissemination of 3D printing techniques, our results should be considered in the development of architectured materials with tailored interfaces between building blocks.

In this preliminary study, a set of self-lubricating copper-tungsten disulfide (Cu-WS2) composites are prepared via a powder metallurgy route to analyze the effects of solid lubricant concentration on their tribological, electrical and wettability properties. An extensive characterization is performed to preliminarily assess the potential application of these metal matrix composites (MMCs) in sliding electrical contacts working under harsh conditions. The experimental results reveal the beneficial effect of WS2 on the wear behavior of the prepared composites, as demonstrated by friction coefficient, specific wear rate and wear coefficients results. A second phase content in the 10–15 wt % range appears to guarantee the better combination of the desired features.

In many applications, polymeric foams (such as expanded polystyrene or expanded polypropylene) are used for protection from impacts. Standard design requires the foam to maximize the energy absorption, thus achieving large deformations (typically up to 25% and above in compression) while maintaining the stress level below a threshold value. In this work, steam chest-moulded EPS and EPP were characterized in relation to their density, microstructure and applied strain rate. Typical mechanical parameters (elastic moduli and plateau stress in compression) were compared with existing models and data in the literature. The strain-rate dependence was accurately described using Nagy’s phenomenological model. The mechanical behaviour of the foams was then correlated with their microstructure, as investigated using scanning electron microscopy and X-ray micro-tomography. Structural parameters were obtained using both (2D and 3D) techniques and relevant results were compared. © The Author(s) 2018.

This is an extensive study of the ageing of athletics tracks, approached with a variety of experimental techniques. The effects of environmental variables, such as UV radiation, relative humidity, water immersion and temperature, were investigated using several artificial ageing protocols, applied to six prefabricated tracks of different color and chemical formulation. A few effective techniques capable of detecting and monitoring the changes occurring in track materials because of ageing were identified among a broad range of available experimental tests. In particular, semi-quantitative colorimetric analysis and dispersive electron spectroscopy were successfully employed to investigate surface degradation phenomena, while uniaxial compression and thermo-gravimetric analysis allowed characterization of the underlying bulk material. The combination of accelerated ageing protocols and monitoring techniques proved to be a powerful tool to study ageing in athletics tracks, with the aim of developing new products with improved durability for installations in critical areas.

This paper reports the results of a laboratory investigation on the use of recycled fillers in bituminous mixtures for road pavements. The fillers were obtained by crushing and sieving (0.00–0.063 mm) Stabilized Bottom Ashes from municipal waste incinerators and Electric Arc Furnace Steel Slags. Moreover, a currently used calcareous filler was included in the research for comparative purposes. Two filler dosages were considered in the experiments. Laboratory tests aimed at investigating compaction properties, volumetric characteristics, and mechanical performance of the bituminous mixtures. The results suggest that both types of recycled filler are suitable to be used in bituminous mixtures, also demonstrating that in certain conditions the investigated fillers increase the performance of the corresponding mixtures in comparison to standard (calcareous) filler.

The experimental investigation herein described is aimed at understanding the effects of cellulose-based fibers on the mechanical properties of bituminous mastics for paving applications. Three bitumen (two of which modified with SBS polymers), a calcareous filler and four different types of fiber with varying content were used to prepare the investigated mastics. The filler to bitumen ratio was maintained constant. The laboratory investigations were focused on empirical tests (Needle Penetration and Ring and Ball – R&B – Temperature) and dynamic-mechanical tests, the latter performed in a wide range of temperatures. Results suggest that fibers improve the behavior of mastics for hot mix asphalts, particularly with respect to the prevention of rutting phenomena at high service temperatures.

In previous works, a finite element model of the shock absorbing characteristics of athletics tracks was developed, able to give sufficiently reliable predictions from laboratory tests performed on suitable material samples. The model proved to be effective in discriminating the effects of geometry (i.e. thickness) and material properties (essentially the elastic characteristics) on force reduction, thus allowing a first optimization of the tracks in view of their approval by the International Association of Athletics Federations (IAAF). This simplified 2D model neglected the real track structure, considering it as a single layer of material having homogenized properties. In the present study, a new 3D model was developed to accurately describe the structure of multi-layered tracks, with properties and geometrical construction (e.g. solid or honeycomb) differing from one layer to another. Several tracks having different combinations of top/bottom layers varying in both material formulation (i.e. chemical composition) and geometry were thus considered. Mechanical properties of the individual elements constituting the track were measured with small-scale laboratory tests, taking into account their strain-rate dependence. The 3D model allowed a complete representation of the loads acting on the track and it gave results which are in very good agreement with the experiments. This proves it to be a valuable tool for the purpose of optimizing the track in terms of material formulation as well as layer geometrical construction and arrangement: as an example, the effect of changing the cell size of the honeycomb pattern was investigated.

Bike and ski helmets are mainly made up of two layers: the external shell and the foam liner. The foam liner, typically made of expanded polystyrene (EPS) or polypropylene (EPP), is asked to provide energy absorption in case of impacts. Standard helmet design requires the foam to maximize this energy absorption, thus achieving large deformations (up to 25% in compression) while maintaining the stress level below a threshold value. To optimize the helmet construction in terms of foam composition, structure and density, reliable numerical models are required, which in turn need to be fed with accurate experimental data. A characterisation of several foams was performed, including EPS and EPP having varying densities, under tensile and compressive stress states at varying strain rates. Typical mechanical parameters (elastic moduli and plateau stress in compression, Poisson's ratio) were compared with literature data and applicability of existing models to experimental results was discussed. A marked strain rate dependence – very important for impact applications – was accurately described using the Nagy phenomenological model. The foam microstructure was investigated using scanning electron microscopy (SEM) to assess structural changes before and after compression. The aforementioned mechanical features were then adopted in a rate-dependent constitutive law for crushable foams, to model the shock attenuation properties of helmets and validate the approach against available data. Finally, a microelectromechanical system based in-helmet wireless micro monitoring system was developed and inserted in a helmet prototype. The system is capable of acquiring impact load spectra, providing valuable information to investigate generic impacts with varying angles and energy. In particular, it can monitor the effect of repeated micro-impacts on the residual energy absorption characteristics of the foam.

Peridynamics (Silling (2000)) is a non-local continuum theory that is particularly suited to handle discontinuities in the displacement field, such as those arising during fracture. Peridynamics prescribes that each material point interacts with all its neighbors contained in a sphere of given radius; this assumption introduces a characteristic length scale in the continuum description. In a nutshell, the interactions between material points depend on their relative distance; in the peridynamics framework this distance is called the “bond length”. The equations of motion, holding at each material point, link the material point acceleration to the integral over the point neighborhood of a force density field, whose strength depend on bond-stretches, i.e. the ratio of the actual bond-length over the initial one. In these equations the displacement gradient does not appear, thus naturally allowing for discontinuities in the displacement field to occur. As to failure, the simplest possible damage description is provided by an interaction law prescribing the force to vanish when a critical bond-stretch threshold is crossed; this parameter can be related to the Mode I critical strain energy release rate. A single parameter is needed to describe failure, in principle under every possible loading condition. In this work the predictive abilities of peridynamics were checked against experimental results in the case of mixed-mode failure of brittle polymers. Pre-cracked poly(methylmethacrylate) (PMMA) samples were tested using different specimens, in order to obtain Mode I, Mixed-Mode and Mode II loading conditions. The material was assumed to behave according to a peridynamics brittle elastic material model; the parameters needed to calibrate the elastic behavior were determined from Mode I tests, as was the critical stretch.The peridynamics simulations of mixed-mode tests were able to catch the correct fracture initiation load and to provide a fair description of the crack path under different conditions. The peridynamics model was also able to qualitatively capture the typical “nail” shape assumed by the crack front during propagation.

Continuous outdoor exposure of athletics tracks can lead to an important degradation of their mechanical and aesthetical properties. In this work, flat laboratory samples prepared from rubber blends of different colours were subjected to natural and artificial ageing, to investigate their effect on the surface properties. Compositional variations demonstrated a generalized oxidization of the outer (top) material layer, together with surfacing of inorganic additives; a small increase of the degradation temperature of the natural rubber component was reported, similar to the one previously observed on bulk track samples. The smooth surface of the present samples allowed their testing using a microscratching technique, able to mechanically probe the material within a few hundred microns below the top surface. The formation of a significantly harder outer crust layer was reported, potentially impacting the track performance since it is exactly the locus of interaction between the athlete and the sport surface. In particular, the increase in scratch hardness is accompanied by a significant reduction in the apparent friction coefficient. These surface modifications, previously unreported in the literature, are independent phenomena with respect to generalized bulk ageing. Microscratch data supported by microscopy evidenced a significantly varying sensitivity to ageing for the different colours (red, blue, green, neutral). Moreover, this sensitivity appeared strongly dependent on the applied ageing protocol (natural vs. artificial). In view of these results, care must be taken when accelerated artificial weathering is used to simulate long-term natural ageing of these materials.

International audience; Round-robin (RR) tests carried out under the direction of the Technical Committee 4, “Polymers, Polymer Composites and Adhesives,” of the European Structural Integrity Society (ESIS TC4) showed that the multispecimen methodology employed for the construction of the crack growth resistance curve (J vs crack extension, Δa) of polymers often does not provide reliable data because of the uncertainties associated with the measurement of Δa. With this in mind, the ESIS TC4 attention has been more recently focused on the analysis of a testing scheme based on the load separation criterion, which does not require the measurement of Δa. With the aim to employ this new approach into a standardized procedure, the degree of reproducibility of the results obtainable with the application of this testing scheme to ductile polymers has been assessed by means of multilaboratory RR testing exercises that started in 2011. An ESIS TC4 reference draft protocol was prepared and 10 laboratories participated in the RR activities. The present work describes the load separation criterion-based testing procedure recently examined by ESIS TC4 and gives a summary of the results obtained in the RR activities, which appear encouraging.

In this work, a three-dimensional finite-element model of athletic tracks is presented. The model is based on data from quasi-static compression tests performed on small laboratory samples, to tune the constitutive parameters. The model was validated on three different athletic tracks, considering their top and bottom layers. Model predictions compared well with the results of shock absorption tests performed using a standard artificial athlete system, with relative errors of a few percent in terms of shock absorption. The model was then used to investigate the effect of the geometric structure of different tracks on their shock absorption capabilities. In particular, a reduction in size of the bottom layer cell pattern increased cushioning; the same property was shown to depend on the pattern voids depth in a non-monotonic way. A maximum in shock absorption was found for a void depth value about 40% higher than the one currently used in the analysed track patterns.

In this work, finite element simulations of typical sports surfaces were performed to evaluate parameters, such as the loading rate and the energy absorbed by the surface, in relation to its characteristics (surface structure and material properties). Hence, possible relations between these quantities and the standard parameters used to characterize the shock absorbing characteristics of the athletics track (in particular, its force reduction) were investigated. The samples selected for this study were two common athletics tracks and a sheet of natural rubber. They were first characterized by quasi-static compression tests; their mechanical properties were extrapolated to the strain rate of interest and their dependence on the level of deformation was modelled with hyperelastic constitutive equations. Numerical simulations were carried out for varying sample thicknesses to understand the influence of track geometry on force reduction, loading rate and stored energy. A very good correlation was found between force reduction and the other relevant parameters, with the exception of the loading rate at the beginning of the impact.

In this work the environmental stress cracking resistance of two high-density polyethylene grades, employed for the manufacturing of bleach bottles, was investigated by using a fracture mechanics approach. Two aqueous solutions similar to common bleach products (with and without sodium hypochlorite) were considered as the active environment, whose effect on both the initiation and propagation phases of fracture was evaluated for the two materials. Tests at different temperatures were performed and a time-temperature superposition scheme was applied both in air and in the active environment. Temperature was shown to influence fracture behaviour only through the material inherent viscoelasticity.

In this work, the possibility of predicting the force reduction (FR) characterizing the shock absorption capability of track surfaces by finite element modeling was investigated. The mechanical responses of a typical sport surface and of a reference material were characterized by quasi-static uniaxial compression experiments and fitted by Neo-Hookean and Mooney–Rivlin’s hyperelastic models to select the more appropriate one. Furthermore, in order to examine the materials behavior at strain rates typical of athletics applications, the rate dependence of the constitutive parameters was investigated. A finite element model, taking into consideration the post-impact nonlinear dynamics of the track surface and of the system (track surface + artificial athlete), was developed and validated through comparison with the results of FR tests. The simulations showed a very good agreement with the experiments and allowed to interpret the experimentally observed combined effect of track thickness and material intrinsic properties on the overall surface behavior.

The Force Reduction (FR) impact test, performed by means of an apparatus called artificial athlete, has been chosen by IAAF (International Association of Athletics Federations) as a standard to evaluate the performance of athletics tracks. The test procedure consists in dropping a mass on the sample, recording the evolution of the impact force and taking its maximum value normalized with respect to a reference one. In this work a Finite Element (FE) model of the FR test was developed to investigate the effects of sample thickness and material properties. Two athletics tracks and, for comparison, a sample of natural rubber were considered. Their mechanical behaviour was characterized, extrapolated to the strain rate of interest and modelled using hyperelastic constitutive equations. With data so derived a number of numerical (FE) simulations of the FR test on the three materials with varying thickness were performed. The FR values predicted by the simulations resulted to be in very good agreement with experimental FR data, particularly for thicknesses of practical interest. Finally, the suitability of an alternative model based on a linear elastic constitutive relationship was considered and results were discussed.

Industry requires methods able to quickly characterize the environmental stress cracking (ESC) resistance of polymeric materials for R and D and quality control purposes. None of the available standards is completely satisfactory and their reliability is often not sufficient. In this paper the fracture mechanics (FM) approach has been successfully applied to characterize the ESC behaviour of two different high-impact polystyrene grades. Data in air were compared with those in an active environment (sunflower oil), adopting different test configurations (double cantilever beam, four point bending and buckled plate) under different loading histories (respectively constant displacement rate, creep and relaxation). A strong effect of the active environment was observed but only above a characteristic interaction time. FM parameters proved to be independent from the applied loading history/configuration, thus allowing ranking of different materials and components’ lifetime predictions.

Processing conditions have a strong effect on the final mechanical properties of products made of polymeric materials. Relevant phenomena most commonly include thermal stresses, physical ageing, frozen-in strains and molecular orientation. In this work, two different high-impact polystyrenes, processed by thermoforming, were considered: a “standard” one and a grade specifically resistant to Environmental Stress Cracking (ESC). The main effect induced by thermoforming was molecular orientation. The local degree of orientation was measured on a thermoformed product and its effect on the material ESC behavior in sunflower oil was studied. A Fracture Mechanics approach was applied to evaluate the fracture resistance of the two materials. Results show that a higher degree of orientation increases the fracture resistance in air but has no effect on the (expectedly lower) resistance in the active oil environment.

In this work, the viscoelastic behavior of open cell polyurethane foams used in noise control applications is investigated through dynamic mechanical analysis in compression. Several levels of static strains superimposed on a small dynamic one were considered in order to assess the effect of material non-linearity on the mechanical response. Further, a wide range of frequencies and temperatures was explored. For each static strain a different master curve for conservative component of complex modulus, E’, could be determined. Interestingly, the loss factor was the same at all static strains, indicating that the relative contribution of energy dissipation and conservation is unaffected by the static strain. Moreover, shift factors (and thus the bulk material relaxation times) turned out to be independent on static strain level. These results suggest that the non-linearity of the foam is linked to the change in foam structure with strain rather than to a non-linear behavior of the viscoelastic constituent m...

In this work two grades of Isotactic polybutene-1 with a different degree of isotacticity have been investigated; fracture tests have been performed at various temperatures and testing speeds on DCB and SENB samples. Optical methods have been used to record crack advancement. Results of the tests have been interpreted using the fracture mechanics framework; a time–temperature superposition scheme has been adopted to describe crack propagation behaviour over several decades of time-scale. An analytical model has been applied to predict the lifetime of pressurised pipes from experimental fracture data. There is good agreement between model predictions and experimental data obtained from full-scale tests on real pipes.

Il Polibutene-1 isotattico (i-PB1) è un materiale polimerico usato per la produzione di tubi per il trasporto di fluidi in pressione. In questo lavoro si sono studiati due tipi di i-PB1 prodotti da Basellche differiscono per grado di isotatticità.Si sono condotte prove di frattura a diverse temperature e velocità di spostamento imposte. Si è utilizzata una configurazione di flessione su provini con singolo intaglio (SENB) unitamente a quella di doppia trave a sbalzo (DCB), quest’ultima limitatamente allo studio della fase di propagazione. Al fine di individuare con precisione l’innesco della frattura e la velocità di propagazione della stessa si è fatto ricorso a metodi ottici. Dal punto di vista fenomenologico durante la propagazione si assiste alla formazione di zone in cui il materiale risulta fortemente stirato. La frattura in esse avanza con una lacerazione continua che si alterna a salti repentini in occasione del brusco cedimento di queste zone, associato a conseguenti cadute del carico. Questaparziale instabilità è stata osservata sui due materiali per entrambe le configurazioni di prova.I risultati ottenuti sono stati interpretati seguendo l’approccio della meccanica della frattura e applicando uno schema di riduzione di tipo tempo-temperatura che ha permesso di descrivere il comportamento viscoelastico del materiale su un intervallo temporale di diverse decadi.I risultati hanno permesso di applicare un modello analitico per la previsione della vita utile di tubi in pressione.Il modello si è mostrato in buon accordo con i dati sperimentali disponibili da prove condotte su tubi dello stesso materiale.

A finite element model has been used to study the sintering process of polytetrafluoroethylene (PTFE) cylinders in order to predict residual thermal stresses; both solid (rods) and hollow (billets) blocks were studied. The simulation of the process was performed considering three separate stages: thermal, deformation, and stress analysis. For each stage, relevant material properties were determined experimentally. In particular, the deformation behavior of PTFE was thoroughly investigated by means of thermo-mechanical analysis (TMA). It is shown that experimental results can be explained considering deformation recovery and orientation effects. Predictions of the model are compared with experimental measurements performed on real PTFE-sintered cylinders. Temperature and deformation distributions determined with the model agree well with experimental data. Fair agreement between predicted and experimentally measured residual stresses is obtained, and the influence of cylinder size and applied cooling rate on residual stresses is correctly predicted. Polym. Eng. Sci. 44:1368–1378, 2004. © 2004 Society of Plastics Engineers.

Variation in the scratch behaviour of polymers with incorporation of different types and amounts of fillers was investigated both in terms of scratch visibility and scratch hardness. Scratch tests were carried out by a microscratch tester. The effect of fillers on the scratch deformation mode and hence on scratch visibility was evaluated by examining recorded penetration depth and acoustic emission traces together with optical microscope images of the groove left on the surface. Scratch hardness was determined by using a recently proposed scratch model. Adequacy of the model to interpret the local phenomenon induced by addition of fillers was questioned. It was demonstrated that incorporation of hard fillers can enhance the scratch hardness of polymers as well as their basic mechanical properties but leads to an increase in scratch visibility due to a change in scratch deformation mode from ductile ploughing to a brittle failure with crack formation. On the contrary, addition of soft fillers causes a reduction in scratch hardness but it also reduces scratch visibility since it improves the recovery characteristics of the polymer.

In this study a robust strategy for 3D-Volume Digital Image Correlation DIC is presented, apt to provide accurate kinematic measurements within a loaded sample on the basis of three-dimensional digital images by X-ray computed micro-tomography. In the framework of a Galerkin, finite element discretization of the displacement field, the inverse problem of estimating 3D motion inside the bulk material is solved recursively on a hierarchical family of grids, linked by suitable restriction and prolongation operators. Such structured grids are defined over an image pyramid, which is generated starting from the raw tomographic reconstructions by a reiterated application of average filters and sub-sampling operators. To achieve robust estimates of the underlying displacement fields, multi-grid cycles are performed ascending and descending along the pyramid in a selected sequence. At each scale, the least-square matching function for DIC is enriched by means of a penalty term in the spirit of Tychonoff regularization, including as a priori information the estimate achieved at the previous grid and transferred to the current scale. For each grid only one Newton iteration can be considered, implying important time savings. Results are presented concerning a laboratory X-ray micro-tomography experiment on a polymeric foam sample, subjected to loading by an apparatus ad-hoc realized.

The present paper is aimed at clarifying the dependence of the force reduction ability of sport surfaces used in athletic tracks on the material's viscoelastic properties and on the geometry of the sample. The study is based on laboratory tests carried out with an “artificial athlete” apparatus and dynamic mechanical analysis. Seven different sport surfaces were tested; other polymeric materials were also examined in order to widen the property ranges covered. The results show a prominent effect of sample thickness on the measured value of force reduction; a method to relate it to the intrinsic properties of the material is proposed.

The scratch behaviour displayed by a series of polymers in constant penetration depth scratch tests conducted by a microscratch tester was examined in relation to their intrinsic rate-dependent mechanical properties. Scratch hardness, which is considered as a measure of the scratch performance, was assessed using a recently proposed scratch model. The adequacy of the model to explain deformation recovery and pile-up characteristics of the material in the scratch test was questioned by comparing the predictions of the model with residual profile measurements. The study demonstrated that the scratch phenomenon is dominated by the yield stress of the material. Besides, scratch hardness was found to be independent of the loading conditions. Indentation hardness and scratch hardness are comparable if the testing conditions are similar.

In this work we provide an experimental-numerical investigation of impact-induced failure of layered composites. An experimental campaign, consisting of multiple drop tests, was performed to determine the delamination threshold (in the low-velocity regime) and to assess how, beyond this limit, failure mode is affected by the impact energy. A three-dimensional finite element model of the laminated composite was built; in the numerical model complex delamination events are simulated by means of a rate-dependent damage interface law. Numerical simulations of the whole impact event are directly compared to experimental results and then critically discussed.

Fracture properties of isotactic polybutene-1 have been investigated. Fracture tests have been conducted and relevant properties at initiation have been determined according to linear elastic fracture mechanics. Two distinct fracture mechanisms have been identified, one of them causing partial instability during crack propagation. Numerical modelling has been performed using a cohesive zone approach. In particular, the identification of suitable cohesive laws has been tried using parametric identification and two different experimental methods. Results suggest that two different cohesive laws may be needed in order to describe crack initiation and crack propagation.

High-density polyethylene (HDPE) can be blow-molded and used for the production of bottles for aggressive products. These products can interact both chemically and physically with the polymer constituting these containers, leading to a decrease in the performance of the material and undermining the structural integrity of the component. A fracture mechanics approach was adopted to evaluate the Environmental Stress Cracking Resistance (ESCR) of two HDPE commercial grades used for bleach containers; two different solutions, with and without sodium hypochlorite (the main ingredient of commercial bleach solutions), were considered as aggressive environments. Size effects were studied using different test configurations and loading histories in air. The correlation between the stress intensity factor and the initiation time was found. A clear effect of the aggressive solutions on the fracture resistance of the two HDPEs was observed, irrespective of the presence of sodium hypochlorite; the effect therefore has to be ascribed to other bleach components.