M. Quaresimin - Academia.edu (original) (raw)
Papers by M. Quaresimin
Composites Science and Technology, 2017
An analytical and experimental study is carried out on the electrical response of a laminate with... more An analytical and experimental study is carried out on the electrical response of a laminate with a delamination. The present study represents a basis for the detection of delaminations in conductive laminates through electrical measurements. As an example of application, the case of a Double Cantilever Beam (DCB) specimen is considered, with the aim to calculate the electric potential of a point on the surface of the laminate, and its variation as a function of the delamination length, highlighting the most influencing parameters. The accuracy of the theoretical predictions is verified against a number of finite element analyses, showing an extremely satisfactory agreement. In addition to this, theoretical predictions are also validated by comparison with a bulk of experimental data coming from an ad-hoc campaign, as well as using data taken from the literature. In all cases a good agreement is found.
Composites Part A: Applied Science and Manufacturing, 2016
Abstract In this work an analytical solution is developed to accurately predict the stiffness red... more Abstract In this work an analytical solution is developed to accurately predict the stiffness reduction in conductive cross-ply laminates, caused by matrix cracking in the transverse layers, as a function of the electrical resistance change of the laminate itself. To this end a closed form solution is initially developed with the aim to link the density of transverse cracks to the electric resistance of the cross-ply laminate. Such an expression is later used within a further model which allows the stiffness degradation associated to a given crack density to be estimated. The accuracy of the proposed model is verified by comparison with a bulk of FE analyses.
Composites Part A: Applied Science and Manufacturing, 2014
In the present work, a model for fibre-matrix debonding initiation under the biaxial loading cond... more In the present work, a model for fibre-matrix debonding initiation under the biaxial loading condition (plane tension combined with out of plane shear) is proposed. The model is developed within the frame of Finite Fracture Mechanics and it is capable of predicting the critical stress value for the onset of debonding and the length of the initial defect. A parametric analysis is carried out to understand the influence of the main geometric and interface parameters on the critical debonding stress, which is found to be strongly dependent on the fibre radius. Satisfactory agreement with experimental data from the literature suggests that this approach, suitably adapted, might also be applied in the case of a unidirectional composite lamina, representing a very useful tool for modelling composite material behaviour under multiaxial loading conditions.
Composites Part A: Applied Science and Manufacturing, 2016
Abstract In the present work, the influence of manufacturing induced voids on damage mechanisms a... more Abstract In the present work, the influence of manufacturing induced voids on damage mechanisms at the microscopic scale was analysed on [45/−45/0] s laminates subjected to tension fatigue loading. Microscopic observations of the top surface of the 45° ply revealed that the first event of damage at the microscopic scale was the initiation of multiple micro-cracks in the matrix between the fibres, located preferentially in correspondence of the voids in that layer. The subsequent coalescence of these micro-cracks gave rise to the formation of a crack propagating in the 45° fibres direction. This is qualitatively the same scenario observed in void-free specimens in a recent work by the authors, thus confirming that the same crack initiation criterion can be applied in the absence and presence of voids. In addition, the micro-scale damage is shown to evolve faster and therefore off-axis cracks to initiate earlier and in a larger quantity in the presence of voids.
International Journal of Fatigue, 2017
Abstract In the present work, glass/epoxy panels with stacking sequence [0/902]S and [0/452/0/−45... more Abstract In the present work, glass/epoxy panels with stacking sequence [0/902]S and [0/452/0/−452]S were produced by vacuum resin infusion. Using different process parameters, which may lead to cost saving, also the presence of porosity was obtained in the laminates. Specimens were tested under uniaxial tensile fatigue loadings, monitoring the initiation and propagation of cracks in the off-axis plies. The performance of the material and the effect of porosity were evaluated in terms of life to crack initiation, crack growth rate, crack density evolution and stiffness degradation. A detrimental effect of the porosity was found even for a very low void content, highlighting the importance of properly accounting for their presence in the design of composite parts.
Composites Science and Technology, 2016
A successful exploitation of nanocomposite materials founds on the development of models capable ... more A successful exploitation of nanocomposite materials founds on the development of models capable of predicting the macroscopic mechanical behaviour as a function of the nanostructure. To this end the most critical issue to overcome is the identification of the inherent mechanisms at the very nanoscale which might depend on the type, the morphology and the functionalisation of the nanofilleras well as on the loading conditions. Within this scenario, this work aims to review the main damage mechanisms reported in the literature for nano-reinforced thermosetting polymers, to include new insights and to discuss predictive models incorporating these mechanisms.
Toughening Mechanisms in Composite Materials, 2015
The success of nanotechnology in structural applications is founded on the development of models,... more The success of nanotechnology in structural applications is founded on the development of models, analytical or numerical, to predict the macroscopic fracturing behavior as a function of the molecular structure of the polymer and the nanofiller. This is particularly challenging because the final process of failure is inherently a multiscale process involving several length and temporal scales and the formulation of physically based multiscale models with real predictive capability can but stand on the identification of the inherent mechanisms of damage at the nanoscale. The present work aims to review the main damage mechanisms reported in the literature for nanoparticle-reinforced polymers, to include new insights, and to discuss predictive models incorporating these mechanisms.
Composites Part A: Applied Science and Manufacturing, 2022
In this paper, the state of the fiber-matrix interface and the matrix behavior of a short glass f... more In this paper, the state of the fiber-matrix interface and the matrix behavior of a short glass fiber reinforced polyamide (PA66-GF35) under fatigue loading were investigated. Significant differences were observed between plain and notched specimens. Comparing the morphology of the fracture surface at crack initiation site, the notched specimens show a lower inelastic matrix deformation and a higher degree of fiber-matrix adhesion. The difference in terms of matrix behavior is attributed to the failure mode of the specimens. In the plain specimens, the damage nucleation, observed in form of a temperature spot, is followed by the unstable propagation of the crack; instead, in the notched specimens, a stable crack propagation phase was observed.
Composite Structures, 2021
Nomex honeycombs are widely used in lightweight structures due to their unique porous structure. ... more Nomex honeycombs are widely used in lightweight structures due to their unique porous structure. By adding an absorbent agent to Nomex honeycomb wall and filling carbon fiber reinforced polymer (CFRP) tube, a novel tube-reinforced absorbent honeycomb sandwich structure was proposed. Its compressive properties and energy absorption characteristics were studied experimentally and numerically. Compared with empty absorbent honeycomb, the normalized elastic modulus E, peak stress and energy absorption of tube-reinforced absorbent honeycomb can be increased by 55.74%, 621.69% and 327.86% respectively. Moreover, the energy absorption of the tube-reinforced absorbent honeycomb was increased by 30.34%, compared with the sum of empty absorbent honeycomb and CFRP tube tested separately. The interaction effects and enhancement mechanism between absorbent honeycomb and CFRP tube were also discussed. The excellent electromagnetic absorption and mechanical properties make this novel honeycomb sandwich structure much more competitive not only in electromagnetic wave stealth application but also in load-carry structures.
Proceedings of the 2006 International Conference on Quantitative InfraRed Thermography, 2006
Experimental estimation of the fatigue limit of metallic materials on the basis of temperature me... more Experimental estimation of the fatigue limit of metallic materials on the basis of temperature measurements is well documented in the literature [1]. Typically, infrared cameras are adopted in order to monitor surface temperature of specimens and components subjected to fatigue tests, thus leading to the so called "thermographic method". In summary, the thermographic methodology is sketched in fig. 1. When a force-controlled, constant amplitude fatigue test is running, temperature measured at the specimen surface is seen to increase at the beginning of the test and then, after a certain number of cycle N, stabilises so that it is possible to identify a stable temperature increase ∆TB stat B. The higher the stress amplitude σB a B the higher the stable temperature increase, as depicted in fig. 1a. Since fatigue failure is caused by microplastic strains which initiate a small fatigue crack, then temperature increments have been qualitatively interpreted as the manifestation of energy dissipation due to plastic hysteresis energy. A quantitative evaluation is reported in [2]. By plotting the stable temperature increments as a function of the applied stress amplitude, one can extrapolate the ∆TB stat B versus σB a B curve, as shown in fig. 1b. By so doing, one can estimate the fatigue limit σB 0,th
Modeling Damage, Fatigue and Failure of Composite Materials, 2016
Abstract In this chapter, the fatigue behavior of composite laminates under multiaxial loading is... more Abstract In this chapter, the fatigue behavior of composite laminates under multiaxial loading is analyzed and discussed. In the scientific literature it is widely shown that the multiaxial stress state as well as the load ratio (minimum to maximum load) are strong factors in determining the cycles spent for the final failure of laminates. Fewer studies report that also damage evolution, consisting of the initiation and propagation of multiple off-axis cracks, depends on the degree of multiaxiality. Understanding and predicting these phenomena, which are governed by matrix and interface properties, is of fundamental importance, mainly for a stiffness-based fatigue design. With the aim to provide a deeper insight into this area an extensive experimental study was carried out by the present authors using glass–epoxy tubular samples under combined tension and torsion. It was found that the life to crack initiation, the crack propagation rate, and the damage mechanisms at the microscopic scale are strongly dependent on the value of the shear stress relative to the transverse stress. In addition, the influence of the load ratio was investigated for three different multiaxial conditions. Finally, it will be shown that the characterization of multiaxial fatigue behavior, in terms of crack initiation and propagation, can be suitably conducted by means of properly designed flat laminates under uniaxial loading, exploiting the local multiaxial stress state arising from the material anisotropy.
Structural Integrity and Durability of Advanced Composites, 2015
This chapter presents first an extensive experimental investigation on composite bonded joints un... more This chapter presents first an extensive experimental investigation on composite bonded joints under mixed mode (I + II) static and cyclic loading conditions oriented to understand the influence of the mode mixity on the propagation of a bondline crack. double cantilever beam (DCB), end notch flexure (ENF) and mixed mode bending (MMB) tests were carried out on precracked samples, and fracture toughness and crack propagation resistance were seen to increase, for both static and fatigue loading, respectively, as the mode II contribution increases. Careful observations of crack propagation and damage evolution revealed a strong dependence of the propagation mode on the mode mixity. Then, a damage-based model was developed on the basis of the actual damage mechanisms occurring in a properly defined process zone. The proposed model has been validated on the experimental results obtained from DCB, ENF and MMB tests as well as from single lap bonded joints.
Multi-Scale Modelling of Composite Material Systems, 2005
Publisher Summary The simplest, traditional approach to the fatigue design based on nominal stres... more Publisher Summary The simplest, traditional approach to the fatigue design based on nominal stresses and fatigue curves turned out to be unsuitable to represent together data referred to joints of different geometry and to provide a model of general validity. Moreover, the use of fatigue curves, usually made on the basis of the number of cycles to failure, makes it difficult to describe and account for damage evolution. The available experimental results indicate the actual mechanics of fatigue damage evolution in the nucleation of one or more cracks at the critical locations in the joint and their subsequent propagation to the point of joint failure. The fatigue cracks grow at the laminate/adhesive interface or, less frequently, inside the adhesive layer. The majority of the methodologies available for the fatigue life prediction of bonded joints both in metallic or composite materials are based on a fracture mechanics approach, considering the fatigue life of the joint entirely spent in the crack propagation phase. On the assumption that this choice is on the conservative side, in some models the assessment of the nucleation phase is explicitly neglected, in others it is simply not considered.
The high fracture toughness improvements exhibited by nanofilled polymers is commonly thought of ... more The high fracture toughness improvements exhibited by nanofilled polymers is commonly thought of as due to the large amount of energy dissipated at the nanoscale. In the present work, a multiscale modelling strategy to assess the nanocomposite toughening is presented. The model accounts for the emergence of an interphase with mechanical properties different from those of the matrix.
In the present work the effects of nanoclay addition on fracture behaviour of an epoxy resin unde... more In the present work the effects of nanoclay addition on fracture behaviour of an epoxy resin under mixed mode (I plus II) loadings are studied by analysing the results from Single Edge Notch Bending (SENB) specimens. The results allow to conclude that, for weight contents lower than 5 wt.%, nanomodification significantly enhances the fracture toughness of the epoxy resin upon the entire range of mixed mode loadings, the improvements being dependent on the mode mixity ratio. Experimental results are later compared to the theoretical predictions based on different, well acknowledged, mixed mode fracture criteria. The results from specimens made of pure epoxy are well predicted, almost independently of the approach used for the synthesis. Conversely, as far as the results from specimens made of nanomodified polymer are concerned, the agreement with theoretical predictions is much poorer. This can be thought of as linked to the emergence, of different damaging mechanisms due to nanomodification depending on the mode mixity.
In the recent literature great attention has been paid to the prediction of mechanical properties... more In the recent literature great attention has been paid to the prediction of mechanical properties of polymer nanocomposites, however a comprehensive evaluation of the available mechanical models has not been presented yet. In this work we propose and discuss a classification of the available modelling strategies into three groups, depending on the scale used to address the problem: micro- mechanical, nano-structural and molecular models. The inadequacy of micromechanical models on predicting mechanical properties of nanocomposite is quite clear, since they make use of relationships developed for the microscopic scale. On the other hand, nanostructural models, while treating polymer matrix and nanofillers as continuum media, attempt to account for the nanostructure of fillers and its global impact on macroscopic properties. However, the molecular interactions between nanofillers and the polymer chains of the matrix may also play an important role in the global observed mechanical behaviour. These interactions are accounted for only by the molecular models. Most of the above mentioned models are mainly focused on the elastic properties of nanocomposites, whilst analytical formulations for toughening mechanisms accounting for the typical features and mechanisms of the nanoscale are still missing. Some of the existing toughening models will then be modified, according to the experimental evidence, in order to make them size-dependent. Accurate results will be presented and discussed.
In this paper a multiscale model is provided to assess the toughening improvements in nanoparticl... more In this paper a multiscale model is provided to assess the toughening improvements in nanoparticle filled polymers caused by the formation of localised plastic shear bands. The model quantifies the energy absorbed at the nanoscale and accounts for the emergence of an interphase zone surrounding the nanoparticles. It is proved that the interphase elastic properties, which are different from those of the matrix due to chemical interactions, highly affect the stress field rising around particles causing lower or higher energy dissipation at the nanoscale.
Composites Part B: Engineering, 2014
Composites Science and Technology, 2017
An analytical and experimental study is carried out on the electrical response of a laminate with... more An analytical and experimental study is carried out on the electrical response of a laminate with a delamination. The present study represents a basis for the detection of delaminations in conductive laminates through electrical measurements. As an example of application, the case of a Double Cantilever Beam (DCB) specimen is considered, with the aim to calculate the electric potential of a point on the surface of the laminate, and its variation as a function of the delamination length, highlighting the most influencing parameters. The accuracy of the theoretical predictions is verified against a number of finite element analyses, showing an extremely satisfactory agreement. In addition to this, theoretical predictions are also validated by comparison with a bulk of experimental data coming from an ad-hoc campaign, as well as using data taken from the literature. In all cases a good agreement is found.
Composites Part A: Applied Science and Manufacturing, 2016
Abstract In this work an analytical solution is developed to accurately predict the stiffness red... more Abstract In this work an analytical solution is developed to accurately predict the stiffness reduction in conductive cross-ply laminates, caused by matrix cracking in the transverse layers, as a function of the electrical resistance change of the laminate itself. To this end a closed form solution is initially developed with the aim to link the density of transverse cracks to the electric resistance of the cross-ply laminate. Such an expression is later used within a further model which allows the stiffness degradation associated to a given crack density to be estimated. The accuracy of the proposed model is verified by comparison with a bulk of FE analyses.
Composites Part A: Applied Science and Manufacturing, 2014
In the present work, a model for fibre-matrix debonding initiation under the biaxial loading cond... more In the present work, a model for fibre-matrix debonding initiation under the biaxial loading condition (plane tension combined with out of plane shear) is proposed. The model is developed within the frame of Finite Fracture Mechanics and it is capable of predicting the critical stress value for the onset of debonding and the length of the initial defect. A parametric analysis is carried out to understand the influence of the main geometric and interface parameters on the critical debonding stress, which is found to be strongly dependent on the fibre radius. Satisfactory agreement with experimental data from the literature suggests that this approach, suitably adapted, might also be applied in the case of a unidirectional composite lamina, representing a very useful tool for modelling composite material behaviour under multiaxial loading conditions.
Composites Part A: Applied Science and Manufacturing, 2016
Abstract In the present work, the influence of manufacturing induced voids on damage mechanisms a... more Abstract In the present work, the influence of manufacturing induced voids on damage mechanisms at the microscopic scale was analysed on [45/−45/0] s laminates subjected to tension fatigue loading. Microscopic observations of the top surface of the 45° ply revealed that the first event of damage at the microscopic scale was the initiation of multiple micro-cracks in the matrix between the fibres, located preferentially in correspondence of the voids in that layer. The subsequent coalescence of these micro-cracks gave rise to the formation of a crack propagating in the 45° fibres direction. This is qualitatively the same scenario observed in void-free specimens in a recent work by the authors, thus confirming that the same crack initiation criterion can be applied in the absence and presence of voids. In addition, the micro-scale damage is shown to evolve faster and therefore off-axis cracks to initiate earlier and in a larger quantity in the presence of voids.
International Journal of Fatigue, 2017
Abstract In the present work, glass/epoxy panels with stacking sequence [0/902]S and [0/452/0/−45... more Abstract In the present work, glass/epoxy panels with stacking sequence [0/902]S and [0/452/0/−452]S were produced by vacuum resin infusion. Using different process parameters, which may lead to cost saving, also the presence of porosity was obtained in the laminates. Specimens were tested under uniaxial tensile fatigue loadings, monitoring the initiation and propagation of cracks in the off-axis plies. The performance of the material and the effect of porosity were evaluated in terms of life to crack initiation, crack growth rate, crack density evolution and stiffness degradation. A detrimental effect of the porosity was found even for a very low void content, highlighting the importance of properly accounting for their presence in the design of composite parts.
Composites Science and Technology, 2016
A successful exploitation of nanocomposite materials founds on the development of models capable ... more A successful exploitation of nanocomposite materials founds on the development of models capable of predicting the macroscopic mechanical behaviour as a function of the nanostructure. To this end the most critical issue to overcome is the identification of the inherent mechanisms at the very nanoscale which might depend on the type, the morphology and the functionalisation of the nanofilleras well as on the loading conditions. Within this scenario, this work aims to review the main damage mechanisms reported in the literature for nano-reinforced thermosetting polymers, to include new insights and to discuss predictive models incorporating these mechanisms.
Toughening Mechanisms in Composite Materials, 2015
The success of nanotechnology in structural applications is founded on the development of models,... more The success of nanotechnology in structural applications is founded on the development of models, analytical or numerical, to predict the macroscopic fracturing behavior as a function of the molecular structure of the polymer and the nanofiller. This is particularly challenging because the final process of failure is inherently a multiscale process involving several length and temporal scales and the formulation of physically based multiscale models with real predictive capability can but stand on the identification of the inherent mechanisms of damage at the nanoscale. The present work aims to review the main damage mechanisms reported in the literature for nanoparticle-reinforced polymers, to include new insights, and to discuss predictive models incorporating these mechanisms.
Composites Part A: Applied Science and Manufacturing, 2022
In this paper, the state of the fiber-matrix interface and the matrix behavior of a short glass f... more In this paper, the state of the fiber-matrix interface and the matrix behavior of a short glass fiber reinforced polyamide (PA66-GF35) under fatigue loading were investigated. Significant differences were observed between plain and notched specimens. Comparing the morphology of the fracture surface at crack initiation site, the notched specimens show a lower inelastic matrix deformation and a higher degree of fiber-matrix adhesion. The difference in terms of matrix behavior is attributed to the failure mode of the specimens. In the plain specimens, the damage nucleation, observed in form of a temperature spot, is followed by the unstable propagation of the crack; instead, in the notched specimens, a stable crack propagation phase was observed.
Composite Structures, 2021
Nomex honeycombs are widely used in lightweight structures due to their unique porous structure. ... more Nomex honeycombs are widely used in lightweight structures due to their unique porous structure. By adding an absorbent agent to Nomex honeycomb wall and filling carbon fiber reinforced polymer (CFRP) tube, a novel tube-reinforced absorbent honeycomb sandwich structure was proposed. Its compressive properties and energy absorption characteristics were studied experimentally and numerically. Compared with empty absorbent honeycomb, the normalized elastic modulus E, peak stress and energy absorption of tube-reinforced absorbent honeycomb can be increased by 55.74%, 621.69% and 327.86% respectively. Moreover, the energy absorption of the tube-reinforced absorbent honeycomb was increased by 30.34%, compared with the sum of empty absorbent honeycomb and CFRP tube tested separately. The interaction effects and enhancement mechanism between absorbent honeycomb and CFRP tube were also discussed. The excellent electromagnetic absorption and mechanical properties make this novel honeycomb sandwich structure much more competitive not only in electromagnetic wave stealth application but also in load-carry structures.
Proceedings of the 2006 International Conference on Quantitative InfraRed Thermography, 2006
Experimental estimation of the fatigue limit of metallic materials on the basis of temperature me... more Experimental estimation of the fatigue limit of metallic materials on the basis of temperature measurements is well documented in the literature [1]. Typically, infrared cameras are adopted in order to monitor surface temperature of specimens and components subjected to fatigue tests, thus leading to the so called "thermographic method". In summary, the thermographic methodology is sketched in fig. 1. When a force-controlled, constant amplitude fatigue test is running, temperature measured at the specimen surface is seen to increase at the beginning of the test and then, after a certain number of cycle N, stabilises so that it is possible to identify a stable temperature increase ∆TB stat B. The higher the stress amplitude σB a B the higher the stable temperature increase, as depicted in fig. 1a. Since fatigue failure is caused by microplastic strains which initiate a small fatigue crack, then temperature increments have been qualitatively interpreted as the manifestation of energy dissipation due to plastic hysteresis energy. A quantitative evaluation is reported in [2]. By plotting the stable temperature increments as a function of the applied stress amplitude, one can extrapolate the ∆TB stat B versus σB a B curve, as shown in fig. 1b. By so doing, one can estimate the fatigue limit σB 0,th
Modeling Damage, Fatigue and Failure of Composite Materials, 2016
Abstract In this chapter, the fatigue behavior of composite laminates under multiaxial loading is... more Abstract In this chapter, the fatigue behavior of composite laminates under multiaxial loading is analyzed and discussed. In the scientific literature it is widely shown that the multiaxial stress state as well as the load ratio (minimum to maximum load) are strong factors in determining the cycles spent for the final failure of laminates. Fewer studies report that also damage evolution, consisting of the initiation and propagation of multiple off-axis cracks, depends on the degree of multiaxiality. Understanding and predicting these phenomena, which are governed by matrix and interface properties, is of fundamental importance, mainly for a stiffness-based fatigue design. With the aim to provide a deeper insight into this area an extensive experimental study was carried out by the present authors using glass–epoxy tubular samples under combined tension and torsion. It was found that the life to crack initiation, the crack propagation rate, and the damage mechanisms at the microscopic scale are strongly dependent on the value of the shear stress relative to the transverse stress. In addition, the influence of the load ratio was investigated for three different multiaxial conditions. Finally, it will be shown that the characterization of multiaxial fatigue behavior, in terms of crack initiation and propagation, can be suitably conducted by means of properly designed flat laminates under uniaxial loading, exploiting the local multiaxial stress state arising from the material anisotropy.
Structural Integrity and Durability of Advanced Composites, 2015
This chapter presents first an extensive experimental investigation on composite bonded joints un... more This chapter presents first an extensive experimental investigation on composite bonded joints under mixed mode (I + II) static and cyclic loading conditions oriented to understand the influence of the mode mixity on the propagation of a bondline crack. double cantilever beam (DCB), end notch flexure (ENF) and mixed mode bending (MMB) tests were carried out on precracked samples, and fracture toughness and crack propagation resistance were seen to increase, for both static and fatigue loading, respectively, as the mode II contribution increases. Careful observations of crack propagation and damage evolution revealed a strong dependence of the propagation mode on the mode mixity. Then, a damage-based model was developed on the basis of the actual damage mechanisms occurring in a properly defined process zone. The proposed model has been validated on the experimental results obtained from DCB, ENF and MMB tests as well as from single lap bonded joints.
Multi-Scale Modelling of Composite Material Systems, 2005
Publisher Summary The simplest, traditional approach to the fatigue design based on nominal stres... more Publisher Summary The simplest, traditional approach to the fatigue design based on nominal stresses and fatigue curves turned out to be unsuitable to represent together data referred to joints of different geometry and to provide a model of general validity. Moreover, the use of fatigue curves, usually made on the basis of the number of cycles to failure, makes it difficult to describe and account for damage evolution. The available experimental results indicate the actual mechanics of fatigue damage evolution in the nucleation of one or more cracks at the critical locations in the joint and their subsequent propagation to the point of joint failure. The fatigue cracks grow at the laminate/adhesive interface or, less frequently, inside the adhesive layer. The majority of the methodologies available for the fatigue life prediction of bonded joints both in metallic or composite materials are based on a fracture mechanics approach, considering the fatigue life of the joint entirely spent in the crack propagation phase. On the assumption that this choice is on the conservative side, in some models the assessment of the nucleation phase is explicitly neglected, in others it is simply not considered.
The high fracture toughness improvements exhibited by nanofilled polymers is commonly thought of ... more The high fracture toughness improvements exhibited by nanofilled polymers is commonly thought of as due to the large amount of energy dissipated at the nanoscale. In the present work, a multiscale modelling strategy to assess the nanocomposite toughening is presented. The model accounts for the emergence of an interphase with mechanical properties different from those of the matrix.
In the present work the effects of nanoclay addition on fracture behaviour of an epoxy resin unde... more In the present work the effects of nanoclay addition on fracture behaviour of an epoxy resin under mixed mode (I plus II) loadings are studied by analysing the results from Single Edge Notch Bending (SENB) specimens. The results allow to conclude that, for weight contents lower than 5 wt.%, nanomodification significantly enhances the fracture toughness of the epoxy resin upon the entire range of mixed mode loadings, the improvements being dependent on the mode mixity ratio. Experimental results are later compared to the theoretical predictions based on different, well acknowledged, mixed mode fracture criteria. The results from specimens made of pure epoxy are well predicted, almost independently of the approach used for the synthesis. Conversely, as far as the results from specimens made of nanomodified polymer are concerned, the agreement with theoretical predictions is much poorer. This can be thought of as linked to the emergence, of different damaging mechanisms due to nanomodification depending on the mode mixity.
In the recent literature great attention has been paid to the prediction of mechanical properties... more In the recent literature great attention has been paid to the prediction of mechanical properties of polymer nanocomposites, however a comprehensive evaluation of the available mechanical models has not been presented yet. In this work we propose and discuss a classification of the available modelling strategies into three groups, depending on the scale used to address the problem: micro- mechanical, nano-structural and molecular models. The inadequacy of micromechanical models on predicting mechanical properties of nanocomposite is quite clear, since they make use of relationships developed for the microscopic scale. On the other hand, nanostructural models, while treating polymer matrix and nanofillers as continuum media, attempt to account for the nanostructure of fillers and its global impact on macroscopic properties. However, the molecular interactions between nanofillers and the polymer chains of the matrix may also play an important role in the global observed mechanical behaviour. These interactions are accounted for only by the molecular models. Most of the above mentioned models are mainly focused on the elastic properties of nanocomposites, whilst analytical formulations for toughening mechanisms accounting for the typical features and mechanisms of the nanoscale are still missing. Some of the existing toughening models will then be modified, according to the experimental evidence, in order to make them size-dependent. Accurate results will be presented and discussed.
In this paper a multiscale model is provided to assess the toughening improvements in nanoparticl... more In this paper a multiscale model is provided to assess the toughening improvements in nanoparticle filled polymers caused by the formation of localised plastic shear bands. The model quantifies the energy absorbed at the nanoscale and accounts for the emergence of an interphase zone surrounding the nanoparticles. It is proved that the interphase elastic properties, which are different from those of the matrix due to chemical interactions, highly affect the stress field rising around particles causing lower or higher energy dissipation at the nanoscale.
Composites Part B: Engineering, 2014