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Papers by roberto brighenti

Fatigue & Fracture of Engineering Materials & Structures, Apr 1, 2000

The high-cycle fatigue behaviour of metals under multiaxial loading is examined. By employing the... more The high-cycle fatigue behaviour of metals under multiaxial loading is examined. By employing the weight function method, the authors propose to correlate the fatigue fracture plane orientation with the averaged principal stress directions. The results derived by applying such an approach are compared with the experimental data collected from the relevant literature, concerning different types of metals under in-phase or out-of-phase sinusoidal biaxial normal and shear stress states. Theoretical results determined by McDiarmid are also reported.

Principles and Practice of Constraint Programming, Mar 6, 2013

The fatigue assessment of metallic structural components under uniaxial cyclic loading is traditi... more The fatigue assessment of metallic structural components under uniaxial cyclic loading is traditionally tackled through experimental methods such as the S-N curve approach. For more complex variable stress states, such as multiaxial stress histories, the fatigue safety can be analysed by employing a physics-based damage mechanics approach. On the other hand, fatigue failure can be recognized as the result of a stable crack propagation up to a critical condition and, in this context, the availability of suitable laws to properly describe and quantify the crack propagation is a crucial aspect. In the present paper, a fatigue crack propagation law for both short (Low-Cycle-Fatigue) and long crack regime (High-Cycle-Fatigue) is discussed based on damage mechanics concepts. Fatigue crack growth law and damage mechanics approach are compared in order to determine both the damage value according to a given fatigue crack growth (FCG) law and the crack length associated to a given mechanical damage of the fatigued material. Such two methods are shown to be different formulations of the same physics-based approach to fatigue phenomena.

International Journal of Pressure Vessels and Piping, Apr 1, 2013

The fatigue assessment of structural components under uniaxial or multiaxial stress histories can... more The fatigue assessment of structural components under uniaxial or multiaxial stress histories can be performed by employing damage mechanics concepts and a physics-based approach. A model for fatigue damage evaluation for an arbitrary loading (uniaxial or multiaxial, cyclic or random) has recently been proposed by the authors using an endurance function which quantifies the damage accumulation in the material up to the final failure. On the other hand, the approach based on the Paris law interprets fatigue failure as the result of the crack propagation inside the material up to the final collapse. In the context of damage mechanics, the structure collapse is assumed to occur when a scalar damage parameter (evaluated by using a proper damage accumulation law depending on the material parameters) is equal to the unity, whereas the final failure according to the Paris law is assumed to occur when the growing crack reaches the 'critical size' which depends on the mechanical properties of the material. In the present paper, these two fatigue assessment methods (damage model and Paris approach) are compared in order to determine both a damage value according to the Paris law and the crack length corresponding to a given damage. Such two methods are shown to be different formulations of the same physics-based approach to fatigue.

Auxetics are materials characterized by a negative Poisson’s ratio (NPR), an uncommon mechanical ... more Auxetics are materials characterized by a negative Poisson’s ratio (NPR), an uncommon mechanical behavior corresponding to a transversal deformation tendency opposite to the traditional materials. Here we present the first example of a 3D synthetic molecular auxetic polymer, obtained by embedding a conformationally expandable cavitand as crosslinker into a rigid polymer of intrinsic microporosity (PIM). The rigidity and microporosity of the polymeric matrix are pivotal to maximize the expansion effect of the cavitand that, under mechanical stress, can assume two different conformations: a compact vase one and an extended kite form. The auxetic behavior and the corresponding NPR of the proposed material is predicted by a specific micromechanical model that considers the cavitand volume expansion ratio, the fraction of the cavitand crosslinker in the polymer, and the mechanical characteristics of the polymer backbone. The reversible auxetic behavior of the material is experimentally v...

Frattura ed Integrità Strutturale, 2015

Structural components made of fibre-reinforced materials are frequently used in engineering appli... more Structural components made of fibre-reinforced materials are frequently used in engineering applications. Fibre-reinforced composites are multiphase materials, and complex mechanical phenomena take place at limit conditions but also during normal service situations, especially under fatigue loading, causing a progressive deterioration and damage. Under repeated loading, the degradation mainly occurs in the matrix material and at the fibre-matrix interface, and such a degradation has to be quantified for design structural assessment purposes. To this end, damage mechanics and fracture mechanics theories can be suitably applied to examine such a problem. Damage concepts can be applied to the matrix mechanical characteristics and, by adopting a 3-D mixed mode fracture description of the fibre-matrix detachment, fatigue fracture mechanics concepts can be used to determine the progressive fibre debonding responsible for the loss of load bearing capacity of the reinforcing phase. In the p...

Frontiers in Materials, 2020

Responsive materials, as well as active structural systems, are today widely used to develop unpr... more Responsive materials, as well as active structural systems, are today widely used to develop unprecedented smart devices, sensors, or actuators; their functionalities come from the ability to respond to environmental stimuli with a detectable reaction. Depending on the responsive material under study, the triggering stimuli can have a different nature, ranging from physical (temperature, light, electric or magnetic field, mechanical stress, etc.), chemical (pH, ligands, etc.), or biological (enzymes, etc.) type. Such a responsiveness can be obtained by properly designing the meso-or macroscopic arrangement of the constitutive elements, as occurs in metamaterials, or can be obtained by using responsive materials per se, whose responsiveness comes from the chemistry underlying their microstructure. In fact, when the responsiveness at the molecular level is properly organized, the nanoscale response can be collectively detected at the macroscale, leading to a responsive material. In the present article, we review the enormous world of responsive polymers, by outlining the main features, characteristics, and responsive mechanisms of smart polymers and by providing a mechanical modeling perspective, both at the molecular as well as at the continuum scale level. We aim at providing a comprehensive overview of the main features and modeling aspects of the most diffused smart polymers. The quantitative mechanical description of active materials plays a key role in their development and use, enabling the design of advanced devices as well as to engineer the materials' microstructure according to the desired functionality.

Frattura ed Integrità Strutturale, 2019

Highly deformable materials (elastomers, gels, biological tissues, etc.) are ubiquitous in nature... more Highly deformable materials (elastomers, gels, biological tissues, etc.) are ubiquitous in nature as well as in technology. The understanding of their flaw sensitivity is crucial to ensure a desired safety level. Fracture failure in soft materials usually occurs after the development of an uncommon crack path because of the non-classical near-tip stress field and the viscous effects. In a neo-Hookean material, the true opening stress singularity along the crack path (evaluated normal to the crack line) is of the order 2 r  , while it is of the order 1/2 s  ahead of the crack tip, promoting the appearance of a crack tip splitting leading to a tortuous crack. In the present paper, experimental tests concerning the fracture behavior of highly deformable thin sheets under tension are discussed, and the observed crack paths are interpreted according to the crack tip stress field arising for large deformations. The study reveals that higher strain rates facilitate the development of a simple Mode I crack path, while lower strain rates induce a mixed Mode in the first crack propagation stage, leading to the formation of new crack tips. The above described behavior seems to not be affected by the initial crack size.

Frattura ed Integrità Strutturale, 2015

The present paper aims to assess the effect of freeze/thaw cycles on fracture behaviour of a natu... more The present paper aims to assess the effect of freeze/thaw cycles on fracture behaviour of a natural stone: the red Verona marble. A wide variety of specimen types and methods to determine Mode I fracture toughness of natural stones are available in the literature and, in this context, the model originally proposed for plain concrete, i.e. the Two-Parameter Model (TPM), is adopted. Such a method is able to take into account the slow nonlinear crack growth occurring before the peak load, typical of quasi-brittle materials, with the advantage of easy specimen preparation and simple test configuration. In the present paper, the atmospheric ageing is simulated by means of thermal pre-treatments consisting of freeze/thaw cycles. Experimental tests are carried out using three-point bending Single-Edge Notched (SEN) specimens, according to the TPM procedure. The effects of thermal treatment on both mechanical and fracture parameters are examined in terms of elastic modulus and fracture tou...

Journal of the Mechanics and Physics of Solids, 2018

Active materials are those capable of giving some physical reaction under external stimuli coming... more Active materials are those capable of giving some physical reaction under external stimuli coming from the environment such as temperature, pH, light, mechanical stress, etc. Reactive polymeric materials can be obtained through the introduction of switchable molecules in their network, i.e. molecules having two distinct stable conformations: if properly linked to the hosting polymer chains, the switching from one state to the other can promote a mechanical reaction of the material, detectable at the macroscale, and thus enables us to tune the response according to a desired functionality. In the present paper, the main aspects of the mechanical behavior of polymeric materials with embedded switchable molecules-properly linked to the polymer's chains-are presented and discussed. Starting from the micro mechanisms occurring in such active material, a continuum model is developed, providing a straightforward implementation in computational approaches. Finally, some experimental outcomes related to a switchable molecules (known as quinoxaline cavitands) added to an elastomeric PDMS under chemical stimuli, are presented and quantitatively discussed through the use of the developed mechanical framework.

Synlett, 2018

While pH-driven interconversion of tetraquinoxaline cavitands (QxCav) from vase to kite conformat... more While pH-driven interconversion of tetraquinoxaline cavitands (QxCav) from vase to kite conformation has been extensively studied both in solution and at interfaces, cavitands behavior in solid matrices is still unexplored. Therefore, the synthesis of a new class of quinoxaline cavitand based copolymers is here reported; a soluble linear poly(butyl methacrylate) (PBMA) and an insoluble cross-linked polydimethylsiloxane (PDMS), ensuring a convenient incorporation of the switchable unit, were chosen as polymer matrices. Conformational studies, performed both in solution and at the solid state, confirmed the retention of vase → kite switching behavior when moving from monomeric units to polymeric structures.

Macromolecules, 2018

The macroscopic mechanical response of polymers can be traced down to the microscale physics of t... more The macroscopic mechanical response of polymers can be traced down to the microscale physics of the network by using a statistical approach for the description of the configuration state of the polymer chains. In this paper we present a micromechanical model to capture the macroscopic behavior of polymers by tracking the evolution of a distribution function describing chain configurations, more specifically the statistics of the end-to-end distance on the network chains. Damage, manifested in the softening and hysteresis under cyclic loading, is accounted for through the scission of chains, whose occurrence is evaluated on the basis of the probability of failure, also settled in the configuration space. The proposed micromechanical model can easily accommodate also the mechanics of dynamic network with reversible cross-links, thereby providing a general and physics-based approach to the study of polymers and polymer-like materials.

Structural Integrity and Durability of Advanced Composites, 2015

This chapter presents some of the most recent and relevant computational techniques for modelling... more This chapter presents some of the most recent and relevant computational techniques for modelling and simulation of damage and/or failure on composite materials. In the last few years, the number of computer methods dedicated to virtual composite damage simulation has exploded as a consequence of progress on a number of numerical methods such as the Partition of Unity Methods (Extended Finite Element Method, Phantom Node Method,...), meshfree methods (Particle Methods, Element Free Galerkin Method,..) or semi-numerical approaches linking novel strategies for computation of damage based on phenomenological theories and effective replication of cracks embedded within the Finite Element Method. Although, this chapter deals mainly with computer methods applied to fibre reinforced composites based on a polymeric matrix, many of them are applicable to a broader range of composite materials as well as other anisotropic materials. j a , k  b FE additional degrees of freedom for the displacement approximation  B support functions  Ultimate fibre-matrix shear stress

In the present paper, both a continuum FE approach and a lattice-based micromechanical approach a... more In the present paper, both a continuum FE approach and a lattice-based micromechanical approach are employed to analyse fibre-reinforced brittle-matrix materials by adopting a cohesive-like fracture behaviour, properly modified taking into account the fibre bridging effect. The basic assumptions and theoretical background of such two computational approaches are outlined, and some benchmark analyses related to random and unidirectional fibre-reinforced brittle-matrix structural components under monotonic tensile loading are discussed.

The fatigue assessment of metallic structural components under uniaxial cyclic loading is traditi... more The fatigue assessment of metallic structural components under uniaxial cyclic loading is traditionally tackled through experimental methods such as the S-N curve approach. For more complex variable stress states, such as multiaxial stress histories, the fatigue safety can be analysed by employing a physics-based damage mechanics approach. On the other hand, fatigue failure can be recognized as the result of a stable crack propagation up to a critical condition and, in this context, the availability of suitable laws to properly describe and quantify the crack propagation is a crucial aspect. In the present paper, a fatigue crack propagation law for both short (Low-Cycle-Fatigue) and long crack regime (High-Cycle-Fatigue) is discussed based on damage mechanics concepts. Fatigue crack growth law and damage mechanics approach are compared in order to determine both the damage value according to a given fatigue crack growth (FCG) law and the crack length associated to a given mechanical damage of the fatigued material. Such two methods are shown to be different formulations of the same physics-based approach to fatigue phenomena.

Tensile Cracking Behaviour of Strain-Hardening Cement-Based Composites using a Micromechanical Lattice Model

It has long been recognized that the fatigue growth behaviour of cracks having a length comparabl... more It has long been recognized that the fatigue growth behaviour of cracks having a length comparable with the material microstructure size (the so-called short or small cracks) is remarkably different from that of long cracks. In particular, the threshold condition of fatigue crack growth is seen to be correlated to the crack length and the material microstructure. The well-known "Kitagawa diagram" describes the variation of the threshold stress intensity range against the crack length, showing the existence of a transition value of length beyond which the threshold of fatigue crack growth is governed by linear elastic fracture mechanics. In the present paper, the crack surface is firstly treated as a self-similar invasive fractal set (which is characterized by a uniform fractal dimension) and, owing to the fractional physical dimension of the fracture surface, the stress intensity factor is shown to be a function of the crack length. Consequently, the threshold stress intensity range is deduced to be a function of the crack length. Then the fractal dimensional increment is assumed to vary from 0 to 1 since, in the physical reality, the fractal dimension of the crack surface may change with the crack length. This allows us to put forward a new interpretation of the Kitagawa diagram within the framework of the fractal geometry.

Crack path dependence on inhomogeneities of material microstructure

Frattura ed Integrita Strutturale, 2012

Journal of the Mechanics and Physics of Solids, 2021

The photopolymerization process used for the production of additively manufactured polymers emplo... more The photopolymerization process used for the production of additively manufactured polymers employed in advanced applications, enables obtaining objects spanning a large dimensional scale thanks to the molecular size achievable by the solidification process. In fact, the photopolymerization is based on the physical-chemical network cross-linking mechanism taking place at the nanoscale. Since the starting raw material is a liquid resin that progressively becomes solid upon the irradiation by a suitable light source, the mechanical propertiesand so the corresponding mechanical response of the final printed structural materialheavily depend on the degree and distribution of the polymerization induced in the material itself. In the present study, starting from the governing equations of the light-induced polymerization process, we determine the chain density formed within the solid domain. Then, the mechanical response of photopolymerized elements obtained with different photopolymerization parameters is investigated. Moreover, the microstructure optimization of polymeric elements in relation to the achievement of the desired mechanical response with the least energy spent in the polymer's formation, is studied. Finally, some interesting considerations related to the modeling of the photopolymerization process are outlined.

Frattura ed Integrità Strutturale, 2015

A new experimental method for measuring strain fields in highly deformable materials has been dev... more A new experimental method for measuring strain fields in highly deformable materials has been developed. This technique is based on an in-house developed Digital Image Correlation (DIC) system capable of accurately capturing localized or non-uniform strain distributions. Thanks to the implemented algorithm based on a Semi-Global Matching (SGM) approach, it is possible to constraint the regularity of the displacement field in order to significantly improve the reliability of the evaluated strains, especially in highly deformable materials. Being originally introduced for Digital Surface Modelling from stereo pairs, SGM is conceived for performing a one-dimensional search of displacements between images, but here a novel implementation for 2D displacement solution space is introduced. SGM approach is compared with the previously in-house developed implementation based on a local Least Squares Matching (LSM) approach. A comparison with the open source code Ncorr and with some FEM results is also presented. The investigation using the present DIC method focuses on 2D full-field strain maps of plain and notched specimens under tensile loading made of two different highly deformable materials: hot mix asphalt and thermoplastic composites for 3D-printing applications. In the latter specimens, an elliptical hole is introduced to assess the potentiality of the method in experimentally capturing high strain gradients in mixed-mode fracture situations.

Geometrically non-linear bending of plates: Implications in curved building façades

Curved glazed facades of buildings are often realized by cold bending of flat plates. This paper ... more Curved glazed facades of buildings are often realized by cold bending of flat plates. This paper is devoted to experimental tests under static loading on the geometrically non-linear anticlastic bending of aluminium plates, whose constituent material is used as a phantom material to mimic the elastic mechanical behaviour of glass. The 3D displacement field of the plates is reconstructed through the use of the DIC (Digital Image Correlation) technique and several numerical FE analyses are performed to simulate the experimental results. A critical value, corresponding to a configuration of the plate where a change in its curvature sign occurs, of the applied displacement at the plate corner is determined. The results are discussed in terms of critical applied displacement against the plate thickness for different plate sizes. It is shown that the critical displacements are almost independent on the plate size, but linearly depend on the plate thickness.

Fatigue & Fracture of Engineering Materials & Structures, Apr 1, 2000

The high-cycle fatigue behaviour of metals under multiaxial loading is examined. By employing the... more The high-cycle fatigue behaviour of metals under multiaxial loading is examined. By employing the weight function method, the authors propose to correlate the fatigue fracture plane orientation with the averaged principal stress directions. The results derived by applying such an approach are compared with the experimental data collected from the relevant literature, concerning different types of metals under in-phase or out-of-phase sinusoidal biaxial normal and shear stress states. Theoretical results determined by McDiarmid are also reported.

Principles and Practice of Constraint Programming, Mar 6, 2013

The fatigue assessment of metallic structural components under uniaxial cyclic loading is traditi... more The fatigue assessment of metallic structural components under uniaxial cyclic loading is traditionally tackled through experimental methods such as the S-N curve approach. For more complex variable stress states, such as multiaxial stress histories, the fatigue safety can be analysed by employing a physics-based damage mechanics approach. On the other hand, fatigue failure can be recognized as the result of a stable crack propagation up to a critical condition and, in this context, the availability of suitable laws to properly describe and quantify the crack propagation is a crucial aspect. In the present paper, a fatigue crack propagation law for both short (Low-Cycle-Fatigue) and long crack regime (High-Cycle-Fatigue) is discussed based on damage mechanics concepts. Fatigue crack growth law and damage mechanics approach are compared in order to determine both the damage value according to a given fatigue crack growth (FCG) law and the crack length associated to a given mechanical damage of the fatigued material. Such two methods are shown to be different formulations of the same physics-based approach to fatigue phenomena.

International Journal of Pressure Vessels and Piping, Apr 1, 2013

The fatigue assessment of structural components under uniaxial or multiaxial stress histories can... more The fatigue assessment of structural components under uniaxial or multiaxial stress histories can be performed by employing damage mechanics concepts and a physics-based approach. A model for fatigue damage evaluation for an arbitrary loading (uniaxial or multiaxial, cyclic or random) has recently been proposed by the authors using an endurance function which quantifies the damage accumulation in the material up to the final failure. On the other hand, the approach based on the Paris law interprets fatigue failure as the result of the crack propagation inside the material up to the final collapse. In the context of damage mechanics, the structure collapse is assumed to occur when a scalar damage parameter (evaluated by using a proper damage accumulation law depending on the material parameters) is equal to the unity, whereas the final failure according to the Paris law is assumed to occur when the growing crack reaches the 'critical size' which depends on the mechanical properties of the material. In the present paper, these two fatigue assessment methods (damage model and Paris approach) are compared in order to determine both a damage value according to the Paris law and the crack length corresponding to a given damage. Such two methods are shown to be different formulations of the same physics-based approach to fatigue.

Auxetics are materials characterized by a negative Poisson’s ratio (NPR), an uncommon mechanical ... more Auxetics are materials characterized by a negative Poisson’s ratio (NPR), an uncommon mechanical behavior corresponding to a transversal deformation tendency opposite to the traditional materials. Here we present the first example of a 3D synthetic molecular auxetic polymer, obtained by embedding a conformationally expandable cavitand as crosslinker into a rigid polymer of intrinsic microporosity (PIM). The rigidity and microporosity of the polymeric matrix are pivotal to maximize the expansion effect of the cavitand that, under mechanical stress, can assume two different conformations: a compact vase one and an extended kite form. The auxetic behavior and the corresponding NPR of the proposed material is predicted by a specific micromechanical model that considers the cavitand volume expansion ratio, the fraction of the cavitand crosslinker in the polymer, and the mechanical characteristics of the polymer backbone. The reversible auxetic behavior of the material is experimentally v...

Frattura ed Integrità Strutturale, 2015

Structural components made of fibre-reinforced materials are frequently used in engineering appli... more Structural components made of fibre-reinforced materials are frequently used in engineering applications. Fibre-reinforced composites are multiphase materials, and complex mechanical phenomena take place at limit conditions but also during normal service situations, especially under fatigue loading, causing a progressive deterioration and damage. Under repeated loading, the degradation mainly occurs in the matrix material and at the fibre-matrix interface, and such a degradation has to be quantified for design structural assessment purposes. To this end, damage mechanics and fracture mechanics theories can be suitably applied to examine such a problem. Damage concepts can be applied to the matrix mechanical characteristics and, by adopting a 3-D mixed mode fracture description of the fibre-matrix detachment, fatigue fracture mechanics concepts can be used to determine the progressive fibre debonding responsible for the loss of load bearing capacity of the reinforcing phase. In the p...

Frontiers in Materials, 2020

Responsive materials, as well as active structural systems, are today widely used to develop unpr... more Responsive materials, as well as active structural systems, are today widely used to develop unprecedented smart devices, sensors, or actuators; their functionalities come from the ability to respond to environmental stimuli with a detectable reaction. Depending on the responsive material under study, the triggering stimuli can have a different nature, ranging from physical (temperature, light, electric or magnetic field, mechanical stress, etc.), chemical (pH, ligands, etc.), or biological (enzymes, etc.) type. Such a responsiveness can be obtained by properly designing the meso-or macroscopic arrangement of the constitutive elements, as occurs in metamaterials, or can be obtained by using responsive materials per se, whose responsiveness comes from the chemistry underlying their microstructure. In fact, when the responsiveness at the molecular level is properly organized, the nanoscale response can be collectively detected at the macroscale, leading to a responsive material. In the present article, we review the enormous world of responsive polymers, by outlining the main features, characteristics, and responsive mechanisms of smart polymers and by providing a mechanical modeling perspective, both at the molecular as well as at the continuum scale level. We aim at providing a comprehensive overview of the main features and modeling aspects of the most diffused smart polymers. The quantitative mechanical description of active materials plays a key role in their development and use, enabling the design of advanced devices as well as to engineer the materials' microstructure according to the desired functionality.

Frattura ed Integrità Strutturale, 2019

Highly deformable materials (elastomers, gels, biological tissues, etc.) are ubiquitous in nature... more Highly deformable materials (elastomers, gels, biological tissues, etc.) are ubiquitous in nature as well as in technology. The understanding of their flaw sensitivity is crucial to ensure a desired safety level. Fracture failure in soft materials usually occurs after the development of an uncommon crack path because of the non-classical near-tip stress field and the viscous effects. In a neo-Hookean material, the true opening stress singularity along the crack path (evaluated normal to the crack line) is of the order 2 r  , while it is of the order 1/2 s  ahead of the crack tip, promoting the appearance of a crack tip splitting leading to a tortuous crack. In the present paper, experimental tests concerning the fracture behavior of highly deformable thin sheets under tension are discussed, and the observed crack paths are interpreted according to the crack tip stress field arising for large deformations. The study reveals that higher strain rates facilitate the development of a simple Mode I crack path, while lower strain rates induce a mixed Mode in the first crack propagation stage, leading to the formation of new crack tips. The above described behavior seems to not be affected by the initial crack size.

Frattura ed Integrità Strutturale, 2015

The present paper aims to assess the effect of freeze/thaw cycles on fracture behaviour of a natu... more The present paper aims to assess the effect of freeze/thaw cycles on fracture behaviour of a natural stone: the red Verona marble. A wide variety of specimen types and methods to determine Mode I fracture toughness of natural stones are available in the literature and, in this context, the model originally proposed for plain concrete, i.e. the Two-Parameter Model (TPM), is adopted. Such a method is able to take into account the slow nonlinear crack growth occurring before the peak load, typical of quasi-brittle materials, with the advantage of easy specimen preparation and simple test configuration. In the present paper, the atmospheric ageing is simulated by means of thermal pre-treatments consisting of freeze/thaw cycles. Experimental tests are carried out using three-point bending Single-Edge Notched (SEN) specimens, according to the TPM procedure. The effects of thermal treatment on both mechanical and fracture parameters are examined in terms of elastic modulus and fracture tou...

Journal of the Mechanics and Physics of Solids, 2018

Active materials are those capable of giving some physical reaction under external stimuli coming... more Active materials are those capable of giving some physical reaction under external stimuli coming from the environment such as temperature, pH, light, mechanical stress, etc. Reactive polymeric materials can be obtained through the introduction of switchable molecules in their network, i.e. molecules having two distinct stable conformations: if properly linked to the hosting polymer chains, the switching from one state to the other can promote a mechanical reaction of the material, detectable at the macroscale, and thus enables us to tune the response according to a desired functionality. In the present paper, the main aspects of the mechanical behavior of polymeric materials with embedded switchable molecules-properly linked to the polymer's chains-are presented and discussed. Starting from the micro mechanisms occurring in such active material, a continuum model is developed, providing a straightforward implementation in computational approaches. Finally, some experimental outcomes related to a switchable molecules (known as quinoxaline cavitands) added to an elastomeric PDMS under chemical stimuli, are presented and quantitatively discussed through the use of the developed mechanical framework.

Synlett, 2018

While pH-driven interconversion of tetraquinoxaline cavitands (QxCav) from vase to kite conformat... more While pH-driven interconversion of tetraquinoxaline cavitands (QxCav) from vase to kite conformation has been extensively studied both in solution and at interfaces, cavitands behavior in solid matrices is still unexplored. Therefore, the synthesis of a new class of quinoxaline cavitand based copolymers is here reported; a soluble linear poly(butyl methacrylate) (PBMA) and an insoluble cross-linked polydimethylsiloxane (PDMS), ensuring a convenient incorporation of the switchable unit, were chosen as polymer matrices. Conformational studies, performed both in solution and at the solid state, confirmed the retention of vase → kite switching behavior when moving from monomeric units to polymeric structures.

Macromolecules, 2018

The macroscopic mechanical response of polymers can be traced down to the microscale physics of t... more The macroscopic mechanical response of polymers can be traced down to the microscale physics of the network by using a statistical approach for the description of the configuration state of the polymer chains. In this paper we present a micromechanical model to capture the macroscopic behavior of polymers by tracking the evolution of a distribution function describing chain configurations, more specifically the statistics of the end-to-end distance on the network chains. Damage, manifested in the softening and hysteresis under cyclic loading, is accounted for through the scission of chains, whose occurrence is evaluated on the basis of the probability of failure, also settled in the configuration space. The proposed micromechanical model can easily accommodate also the mechanics of dynamic network with reversible cross-links, thereby providing a general and physics-based approach to the study of polymers and polymer-like materials.

Structural Integrity and Durability of Advanced Composites, 2015

This chapter presents some of the most recent and relevant computational techniques for modelling... more This chapter presents some of the most recent and relevant computational techniques for modelling and simulation of damage and/or failure on composite materials. In the last few years, the number of computer methods dedicated to virtual composite damage simulation has exploded as a consequence of progress on a number of numerical methods such as the Partition of Unity Methods (Extended Finite Element Method, Phantom Node Method,...), meshfree methods (Particle Methods, Element Free Galerkin Method,..) or semi-numerical approaches linking novel strategies for computation of damage based on phenomenological theories and effective replication of cracks embedded within the Finite Element Method. Although, this chapter deals mainly with computer methods applied to fibre reinforced composites based on a polymeric matrix, many of them are applicable to a broader range of composite materials as well as other anisotropic materials. j a , k  b FE additional degrees of freedom for the displacement approximation  B support functions  Ultimate fibre-matrix shear stress

In the present paper, both a continuum FE approach and a lattice-based micromechanical approach a... more In the present paper, both a continuum FE approach and a lattice-based micromechanical approach are employed to analyse fibre-reinforced brittle-matrix materials by adopting a cohesive-like fracture behaviour, properly modified taking into account the fibre bridging effect. The basic assumptions and theoretical background of such two computational approaches are outlined, and some benchmark analyses related to random and unidirectional fibre-reinforced brittle-matrix structural components under monotonic tensile loading are discussed.

The fatigue assessment of metallic structural components under uniaxial cyclic loading is traditi... more The fatigue assessment of metallic structural components under uniaxial cyclic loading is traditionally tackled through experimental methods such as the S-N curve approach. For more complex variable stress states, such as multiaxial stress histories, the fatigue safety can be analysed by employing a physics-based damage mechanics approach. On the other hand, fatigue failure can be recognized as the result of a stable crack propagation up to a critical condition and, in this context, the availability of suitable laws to properly describe and quantify the crack propagation is a crucial aspect. In the present paper, a fatigue crack propagation law for both short (Low-Cycle-Fatigue) and long crack regime (High-Cycle-Fatigue) is discussed based on damage mechanics concepts. Fatigue crack growth law and damage mechanics approach are compared in order to determine both the damage value according to a given fatigue crack growth (FCG) law and the crack length associated to a given mechanical damage of the fatigued material. Such two methods are shown to be different formulations of the same physics-based approach to fatigue phenomena.

Tensile Cracking Behaviour of Strain-Hardening Cement-Based Composites using a Micromechanical Lattice Model

It has long been recognized that the fatigue growth behaviour of cracks having a length comparabl... more It has long been recognized that the fatigue growth behaviour of cracks having a length comparable with the material microstructure size (the so-called short or small cracks) is remarkably different from that of long cracks. In particular, the threshold condition of fatigue crack growth is seen to be correlated to the crack length and the material microstructure. The well-known "Kitagawa diagram" describes the variation of the threshold stress intensity range against the crack length, showing the existence of a transition value of length beyond which the threshold of fatigue crack growth is governed by linear elastic fracture mechanics. In the present paper, the crack surface is firstly treated as a self-similar invasive fractal set (which is characterized by a uniform fractal dimension) and, owing to the fractional physical dimension of the fracture surface, the stress intensity factor is shown to be a function of the crack length. Consequently, the threshold stress intensity range is deduced to be a function of the crack length. Then the fractal dimensional increment is assumed to vary from 0 to 1 since, in the physical reality, the fractal dimension of the crack surface may change with the crack length. This allows us to put forward a new interpretation of the Kitagawa diagram within the framework of the fractal geometry.

Crack path dependence on inhomogeneities of material microstructure

Frattura ed Integrita Strutturale, 2012

Journal of the Mechanics and Physics of Solids, 2021

The photopolymerization process used for the production of additively manufactured polymers emplo... more The photopolymerization process used for the production of additively manufactured polymers employed in advanced applications, enables obtaining objects spanning a large dimensional scale thanks to the molecular size achievable by the solidification process. In fact, the photopolymerization is based on the physical-chemical network cross-linking mechanism taking place at the nanoscale. Since the starting raw material is a liquid resin that progressively becomes solid upon the irradiation by a suitable light source, the mechanical propertiesand so the corresponding mechanical response of the final printed structural materialheavily depend on the degree and distribution of the polymerization induced in the material itself. In the present study, starting from the governing equations of the light-induced polymerization process, we determine the chain density formed within the solid domain. Then, the mechanical response of photopolymerized elements obtained with different photopolymerization parameters is investigated. Moreover, the microstructure optimization of polymeric elements in relation to the achievement of the desired mechanical response with the least energy spent in the polymer's formation, is studied. Finally, some interesting considerations related to the modeling of the photopolymerization process are outlined.

Frattura ed Integrità Strutturale, 2015

A new experimental method for measuring strain fields in highly deformable materials has been dev... more A new experimental method for measuring strain fields in highly deformable materials has been developed. This technique is based on an in-house developed Digital Image Correlation (DIC) system capable of accurately capturing localized or non-uniform strain distributions. Thanks to the implemented algorithm based on a Semi-Global Matching (SGM) approach, it is possible to constraint the regularity of the displacement field in order to significantly improve the reliability of the evaluated strains, especially in highly deformable materials. Being originally introduced for Digital Surface Modelling from stereo pairs, SGM is conceived for performing a one-dimensional search of displacements between images, but here a novel implementation for 2D displacement solution space is introduced. SGM approach is compared with the previously in-house developed implementation based on a local Least Squares Matching (LSM) approach. A comparison with the open source code Ncorr and with some FEM results is also presented. The investigation using the present DIC method focuses on 2D full-field strain maps of plain and notched specimens under tensile loading made of two different highly deformable materials: hot mix asphalt and thermoplastic composites for 3D-printing applications. In the latter specimens, an elliptical hole is introduced to assess the potentiality of the method in experimentally capturing high strain gradients in mixed-mode fracture situations.

Geometrically non-linear bending of plates: Implications in curved building façades

Curved glazed facades of buildings are often realized by cold bending of flat plates. This paper ... more Curved glazed facades of buildings are often realized by cold bending of flat plates. This paper is devoted to experimental tests under static loading on the geometrically non-linear anticlastic bending of aluminium plates, whose constituent material is used as a phantom material to mimic the elastic mechanical behaviour of glass. The 3D displacement field of the plates is reconstructed through the use of the DIC (Digital Image Correlation) technique and several numerical FE analyses are performed to simulate the experimental results. A critical value, corresponding to a configuration of the plate where a change in its curvature sign occurs, of the applied displacement at the plate corner is determined. The results are discussed in terms of critical applied displacement against the plate thickness for different plate sizes. It is shown that the critical displacements are almost independent on the plate size, but linearly depend on the plate thickness.