International Journal of Solids and Structures, May 1, 2008
The influence of indenter elasticity on Hertzian fracture initiation at frictional dissimilar ela... more The influence of indenter elasticity on Hertzian fracture initiation at frictional dissimilar elastic contact has been examined experimentally and numerically. In flat float glass specimens initiation of cone cracks has been observed and fracture loads measured with steel and tungsten carbide indenters at monotonically increasing loading and during a load cycle. The observed effect of indenter elasticity on fracture loads was found to be qualitatively different from the one predicted by the Hertz contact theory. This discrepancy may be explained by the presence of interfacial friction. The friction coefficient between the indenters and the specimen was measured and a contact cycle at finite Coulomb friction has been analyzed numerically. The influence of the indenter elasticity and the friction coefficient on the surface maximum tensile stress has been investigated and the results concerning the influence of these parameters on the fracture loads as given based on a critical stress fracture criterion. The obtained computational results were found to be in better agreement with experimental findings as compared to the predictions based on the frictionless contact theory. A remaining quantitative discrepancy was attributed to the well-known fact that a Hertzian macro-crack initiates from pre-existing defects on the specimen's surface. In order to account for the influence of the random distribution of these defects a Weibull statistics was introduced. The predicted critical loads corresponding to the 50% failure probability were found to be in close agreement with experimentally observed ones.
Friction has a profound influence on Hertzian fracture initiation when dissimilar materials are i... more Friction has a profound influence on Hertzian fracture initiation when dissimilar materials are involved. Experimental studies show that the presence of friction results in higher fracture loads and fracture radii as compared to the frictionless case. It has also been shown recently that the experimental observations concerning Hertzian fracture initiation at unloading may be explained by the effect friction has on a surface tensile stress distribution. Presently a contact cycle between two dissimilar elastic bodies at finite Coulomb friction has been investigated numerically for a wide range of material parameters and contact geometries. Emphasis has been given to the surface tensile stress distribution which is assumed to be a governing parameter for Hertzian fracture initiation. In particular it was found that during loading the contact region divides into invariant stick and inward slip regions and the presence of outward frictional shear tractions reduces the maximum surface tensile stress and shifts it away from the contact contour as compared to the frictionless case. At unloading, the distributions of stick-slip zones were found to be severely history-and geometry-dependent and shear tractions reversed their direction over part of the contact area. Consequently, tensile stresses were found to grow at unloading. Results concerning the influence of the friction coefficient, Dundur's parameter and the specimen's Poisson's ratio on the absolute maximum surface tensile stress obtained at a frictional contact cycle are reported along with the magnitudes of the relative increase of maximum tensile stresses at unloading. Based on a critical stress fracture criterion it is discussed how the predicted increases will influence the critical loads required for crack initiation.
The influence of indenter elasticity on Hertzian fracture initiation at frictional dissimilar ela... more The influence of indenter elasticity on Hertzian fracture initiation at frictional dissimilar elastic contact has been examined experimentally and numerically. In flat float glass specimens initiation of cone cracks has been observed and fracture loads measured with steel and tungsten carbide indenters at monotonically increasing loading and during a load cycle. The observed effect of indenter elasticity on fracture loads was found to be qualitatively different from the one predicted by the Hertz contact theory. This discrepancy may be explained by the presence of interfacial friction. The friction coefficient between the indenters and the specimen was measured and a contact cycle at finite Coulomb friction has been analyzed numerically. The influence of the indenter elasticity and the friction coefficient on the surface maximum tensile stress has been investigated and the results concerning the influence of these parameters on the fracture loads as given based on a critical stress fracture criterion. The obtained computational results were found to be in better agreement with experimental findings as compared to the predictions based on the frictionless contact theory. A remaining quantitative discrepancy was attributed to the well-known fact that a Hertzian macro-crack initiates from pre-existing defects on the specimen's surface. In order to account for the influence of the random distribution of these defects a Weibull statistics was introduced. The predicted critical loads corresponding to the 50% failure probability were found to be in close agreement with experimentally observed ones.
7th RILEM International Conference on Cracking in Pavements, 2012
In an effort to understand the effect of low temperature fatigue cracking, atomic force microscop... more In an effort to understand the effect of low temperature fatigue cracking, atomic force microscopy (AFM) was used to characterize the morphology of bitumen. In addition, thermal analysis and chemical characterization was done using differential scanning calorimetry (DSC) and thin-layer chromatography / flame ionization detection (TLC/FID), respectively. The AFM topographic and phase contrast image confirmed the existence of bee-shaped microstructure and different phases. The bitumen samples were subjected to both environmental and mechanical loading and after loading, micro-cracks appeared in the interfaces of the bitumen surface, confirming bitumen itself may also crack. It was also found that the presence of wax and wax crystallization plays a vital role in low temperature cracking performance of bitumen.
7th RILEM International Conference on Cracking in Pavements, 2012
Cracking is very common in areas having large variations in the daily temperatures and can cause ... more Cracking is very common in areas having large variations in the daily temperatures and can cause large discomfort to the users. To improve the binder properties against cracking and rutting, researchers have studied for many years the behaviour of different binder additives such as polymers. It is quite complex, however, to decide on the benefits of a more expensive solution without looking at the long term performance. Life cycle assessment (LCA) studies can help to develop this long term perspective, linking performance to minimizing the overall energy consumption, use of resources and emissions. To demonstrate this, LCA of an unmodified and polymer modified asphalt pavement using a newly developed open LCA framework has been performed. It is shown how polymer modification for improved performance affects the energy consumption and emissions during the life cycle of a road. Furthermore, it is concluded that better understanding of the binder would lead to better optimized pavement design, hence reducing the energy consumption and emissions. A limit in terms of energy and emissions for the production of the polymer was also found which could help the polymer producers to improve their manufacturing processes, making them efficient enough to be beneficial from a pavement life cycle point of view.
Multi-Scale Modeling and Characterization of Infrastructure Materials, 2013
In this paper, detailed study is carried out to develop a new workflow from image acquisition to ... more In this paper, detailed study is carried out to develop a new workflow from image acquisition to numerical simulation for the asphalt concrete microstructures. High resolution computed tomography scanned images are acquired and the image quality is improved using digital image processing techniques. Nonuniform illumination is corrected by applying an illumination profile to correct the background and flat-fields in the image. Distance map based watershed segmentation are used to segment the phases and separate the aggregates. Quantitative analysis of the micro-structure is used to determine the phase volumetric relationship and aggregates characteristics. The result of the quantitative analysis showed a very high level of reliability. Finite Element simulations were carried out with the developed micro-mechanical meshes to capture the strength and deformation mechanisms of the asphalt concrete micro-structure. From the micro-mechanical investigation the load transfer chains, higher strength characteristics and high stress localization at the mastic interface between adjacent aggregates was shown.
Film thickness describes the coating around aggregate particles on asphalt mixtures. The standard... more Film thickness describes the coating around aggregate particles on asphalt mixtures. The standard method of calculating film thickness has proven to present several limitations, such as assuming an average thickness independent of particle size, being completely independent to the porosity of the mixture and considering only one mineral type. In this paper, a binder distribution model is developed for aggregates according to size and role in the structure. The aggregates are separated into two different structures: primary structure, the load bearing one, and secondary structure, smaller material that provides stability to the skeleton. A coating thickness for these two structures is calculated from a geometrical consideration that includes the packing arrangement of particles and the effect of overlapping as the film grows. The results were compared with known rutting performance of field mixtures and moisture conditioned laboratory mixtures, showing a good correlation between film thickness and resistance to failure.
ABSTRACT With recent advancement in the use of X-Ray Computed Tomography to capture the internal ... more ABSTRACT With recent advancement in the use of X-Ray Computed Tomography to capture the internal structure of asphalt concrete (AC), results have shown several possibilities to account for the distribution of the different phases in the mix and quantify them in a reliable way. The morphology of asphalt mixtures which includes the aggregate size gradation and the distribution of the air-voids and bitumen phase are captured in a single morphological parameter called the Primary Structure (PS) coating thickness - (Tps). In this study, the effect of variations in the morphological micro-structural property on the meso-scale response of three (3) AC samples is examined using the 3D Finite Element Method (FEM). The AC internal geometry is acquired using X-Ray Computed Tomography (CT); the distribution of the aggregates, mastic and air-voids phase is considered and obtained using Digital Imaging Processing (DIP) techniques. Using a surface-based cohesive behavior and assuming a predominant adhesive failure at the interface between the mastic and aggregate, a maximum traction criterion is used to obtain the damage propensity of the different mixtures. The result of the analysis shows that the microstructural morphological parameter Tps adequately captures the meso-scale response of the mixtures; there exist an inverse relationship between mixture strength characterization and the morphological parameter Tps.
Indentation of elastic materials is investigated numerically using the finite element method. Lar... more Indentation of elastic materials is investigated numerically using the finite element method. Large deformation theory is relied upon for accuracy. The study focuses on nonlocal frictional effects on relevant indentation quantities in the microindentation regime. The indentation quantities investigated include both local and global ones. It is shown that nonlocal frictional effects are small when global quantities are at issue, as is the case when conventional (Coulomb) theory of friction is used, also when these features are introduced at the ridges of a Vickers indenter where stress gradients are substantial. These effects are, however, shown to be of importance for particular indenter geometries as far as local field variables are concerned.
International Journal of Pavement Engineering, 2014
Enhancing the quality of granular layers is fundamental to optimise the structural performance of... more Enhancing the quality of granular layers is fundamental to optimise the structural performance of the pavements. The objective of this study is to investigate whether previously developed packing theory-based aggregate parameters can evaluate the resilient modulus of unbound granular materials. In this study, 19 differently graded unbound granular materials from two countries (USA and Sweden) were evaluated. This study validated both porosity of primary structure (PS) and contact points per particle (coordination number) as key parameters for evaluating the resilient modulus of unbound granular materials. This study showed that decreasing the PS porosity -higher coordination number -calculated based on the proposed gradation model, yields higher resilient modulus. Good correlation was observed between the proposed packing parameters and resilient modulus of several types of aggregates. The packing theory-based framework successfully recognised granular materials that exhibited poor performance in terms of resilient modulus.
International Journal of Pavement Engineering, 2013
Permanent deformation of unbound granular materials plays an essential role in the long-term perf... more Permanent deformation of unbound granular materials plays an essential role in the long-term performance of a pavement structure. Stability of unbound granular materials is defined by the particle-to-particle contact of the system, the particle size distribution and the packing arrangement. This paper presents a gradation model based on packing theory to evaluate permanent deformation of unbound granular materials. The framework was evaluated by using 10 unbound granular materials from different countries. The disruption potential, which determines the ability of secondary structure (SS) to disrupt the primary structure (PS), is introduced. This study also identified the amount of PS and SS that may eventually be used as a design parameter for permanent deformation of unbound road layers. The evaluation of the model regarding permanent deformation behaviour of granular materials is found to compare favourably with experimental results.
In order to perform thickness design of flexible pavements based on the hot mix asphalt (HMA) fra... more In order to perform thickness design of flexible pavements based on the hot mix asphalt (HMA) fracture mechanics framework prior to construction, the tensile strength and creep power law parameters of the mixture has to be determined. As samples are normally not available in the design stage, these properties will have to be modelled rather than determined in a laboratory. This paper evaluates material models based on the relationship between dynamic (complex) modulus and creep compliance in order to predict tensile strength and creep power law parameters for a mixture. For this evaluation, fourteen field sections from the United States have been used for verification and the results indicate that the crack resistance of a mixture can indeed be estimated without a development of extensive empirical relations between mixture properties and crack-resistance.
Sweden has initiated the development of a new calibrated-mechanistic pavement design procedure to... more Sweden has initiated the development of a new calibrated-mechanistic pavement design procedure to replace the current mechanical-empirical pavement procedure entitled 'PMS Objekt'. The first phase was focused on the implementation and calibration of the viscoelastic fracture mechanics framework entitled 'HMA Fracture Mechanics', developed at the University of Florida. This paper outlines the implementation and calibration of a new pavement design module for Sweden that is based on the HMA fracture mechanics framework. Both the developed design module, as well as the reference model used for calibration (PMS Objekt), is presented in this paper. The results in thickness design after calibration of the design module indicate that the framework is clearly applicable for common Swedish conditions and design standards.
International Journal of Solids and Structures, May 1, 2008
The influence of indenter elasticity on Hertzian fracture initiation at frictional dissimilar ela... more The influence of indenter elasticity on Hertzian fracture initiation at frictional dissimilar elastic contact has been examined experimentally and numerically. In flat float glass specimens initiation of cone cracks has been observed and fracture loads measured with steel and tungsten carbide indenters at monotonically increasing loading and during a load cycle. The observed effect of indenter elasticity on fracture loads was found to be qualitatively different from the one predicted by the Hertz contact theory. This discrepancy may be explained by the presence of interfacial friction. The friction coefficient between the indenters and the specimen was measured and a contact cycle at finite Coulomb friction has been analyzed numerically. The influence of the indenter elasticity and the friction coefficient on the surface maximum tensile stress has been investigated and the results concerning the influence of these parameters on the fracture loads as given based on a critical stress fracture criterion. The obtained computational results were found to be in better agreement with experimental findings as compared to the predictions based on the frictionless contact theory. A remaining quantitative discrepancy was attributed to the well-known fact that a Hertzian macro-crack initiates from pre-existing defects on the specimen's surface. In order to account for the influence of the random distribution of these defects a Weibull statistics was introduced. The predicted critical loads corresponding to the 50% failure probability were found to be in close agreement with experimentally observed ones.
Friction has a profound influence on Hertzian fracture initiation when dissimilar materials are i... more Friction has a profound influence on Hertzian fracture initiation when dissimilar materials are involved. Experimental studies show that the presence of friction results in higher fracture loads and fracture radii as compared to the frictionless case. It has also been shown recently that the experimental observations concerning Hertzian fracture initiation at unloading may be explained by the effect friction has on a surface tensile stress distribution. Presently a contact cycle between two dissimilar elastic bodies at finite Coulomb friction has been investigated numerically for a wide range of material parameters and contact geometries. Emphasis has been given to the surface tensile stress distribution which is assumed to be a governing parameter for Hertzian fracture initiation. In particular it was found that during loading the contact region divides into invariant stick and inward slip regions and the presence of outward frictional shear tractions reduces the maximum surface tensile stress and shifts it away from the contact contour as compared to the frictionless case. At unloading, the distributions of stick-slip zones were found to be severely history-and geometry-dependent and shear tractions reversed their direction over part of the contact area. Consequently, tensile stresses were found to grow at unloading. Results concerning the influence of the friction coefficient, Dundur's parameter and the specimen's Poisson's ratio on the absolute maximum surface tensile stress obtained at a frictional contact cycle are reported along with the magnitudes of the relative increase of maximum tensile stresses at unloading. Based on a critical stress fracture criterion it is discussed how the predicted increases will influence the critical loads required for crack initiation.
The influence of indenter elasticity on Hertzian fracture initiation at frictional dissimilar ela... more The influence of indenter elasticity on Hertzian fracture initiation at frictional dissimilar elastic contact has been examined experimentally and numerically. In flat float glass specimens initiation of cone cracks has been observed and fracture loads measured with steel and tungsten carbide indenters at monotonically increasing loading and during a load cycle. The observed effect of indenter elasticity on fracture loads was found to be qualitatively different from the one predicted by the Hertz contact theory. This discrepancy may be explained by the presence of interfacial friction. The friction coefficient between the indenters and the specimen was measured and a contact cycle at finite Coulomb friction has been analyzed numerically. The influence of the indenter elasticity and the friction coefficient on the surface maximum tensile stress has been investigated and the results concerning the influence of these parameters on the fracture loads as given based on a critical stress fracture criterion. The obtained computational results were found to be in better agreement with experimental findings as compared to the predictions based on the frictionless contact theory. A remaining quantitative discrepancy was attributed to the well-known fact that a Hertzian macro-crack initiates from pre-existing defects on the specimen's surface. In order to account for the influence of the random distribution of these defects a Weibull statistics was introduced. The predicted critical loads corresponding to the 50% failure probability were found to be in close agreement with experimentally observed ones.
7th RILEM International Conference on Cracking in Pavements, 2012
In an effort to understand the effect of low temperature fatigue cracking, atomic force microscop... more In an effort to understand the effect of low temperature fatigue cracking, atomic force microscopy (AFM) was used to characterize the morphology of bitumen. In addition, thermal analysis and chemical characterization was done using differential scanning calorimetry (DSC) and thin-layer chromatography / flame ionization detection (TLC/FID), respectively. The AFM topographic and phase contrast image confirmed the existence of bee-shaped microstructure and different phases. The bitumen samples were subjected to both environmental and mechanical loading and after loading, micro-cracks appeared in the interfaces of the bitumen surface, confirming bitumen itself may also crack. It was also found that the presence of wax and wax crystallization plays a vital role in low temperature cracking performance of bitumen.
7th RILEM International Conference on Cracking in Pavements, 2012
Cracking is very common in areas having large variations in the daily temperatures and can cause ... more Cracking is very common in areas having large variations in the daily temperatures and can cause large discomfort to the users. To improve the binder properties against cracking and rutting, researchers have studied for many years the behaviour of different binder additives such as polymers. It is quite complex, however, to decide on the benefits of a more expensive solution without looking at the long term performance. Life cycle assessment (LCA) studies can help to develop this long term perspective, linking performance to minimizing the overall energy consumption, use of resources and emissions. To demonstrate this, LCA of an unmodified and polymer modified asphalt pavement using a newly developed open LCA framework has been performed. It is shown how polymer modification for improved performance affects the energy consumption and emissions during the life cycle of a road. Furthermore, it is concluded that better understanding of the binder would lead to better optimized pavement design, hence reducing the energy consumption and emissions. A limit in terms of energy and emissions for the production of the polymer was also found which could help the polymer producers to improve their manufacturing processes, making them efficient enough to be beneficial from a pavement life cycle point of view.
Multi-Scale Modeling and Characterization of Infrastructure Materials, 2013
In this paper, detailed study is carried out to develop a new workflow from image acquisition to ... more In this paper, detailed study is carried out to develop a new workflow from image acquisition to numerical simulation for the asphalt concrete microstructures. High resolution computed tomography scanned images are acquired and the image quality is improved using digital image processing techniques. Nonuniform illumination is corrected by applying an illumination profile to correct the background and flat-fields in the image. Distance map based watershed segmentation are used to segment the phases and separate the aggregates. Quantitative analysis of the micro-structure is used to determine the phase volumetric relationship and aggregates characteristics. The result of the quantitative analysis showed a very high level of reliability. Finite Element simulations were carried out with the developed micro-mechanical meshes to capture the strength and deformation mechanisms of the asphalt concrete micro-structure. From the micro-mechanical investigation the load transfer chains, higher strength characteristics and high stress localization at the mastic interface between adjacent aggregates was shown.
Film thickness describes the coating around aggregate particles on asphalt mixtures. The standard... more Film thickness describes the coating around aggregate particles on asphalt mixtures. The standard method of calculating film thickness has proven to present several limitations, such as assuming an average thickness independent of particle size, being completely independent to the porosity of the mixture and considering only one mineral type. In this paper, a binder distribution model is developed for aggregates according to size and role in the structure. The aggregates are separated into two different structures: primary structure, the load bearing one, and secondary structure, smaller material that provides stability to the skeleton. A coating thickness for these two structures is calculated from a geometrical consideration that includes the packing arrangement of particles and the effect of overlapping as the film grows. The results were compared with known rutting performance of field mixtures and moisture conditioned laboratory mixtures, showing a good correlation between film thickness and resistance to failure.
ABSTRACT With recent advancement in the use of X-Ray Computed Tomography to capture the internal ... more ABSTRACT With recent advancement in the use of X-Ray Computed Tomography to capture the internal structure of asphalt concrete (AC), results have shown several possibilities to account for the distribution of the different phases in the mix and quantify them in a reliable way. The morphology of asphalt mixtures which includes the aggregate size gradation and the distribution of the air-voids and bitumen phase are captured in a single morphological parameter called the Primary Structure (PS) coating thickness - (Tps). In this study, the effect of variations in the morphological micro-structural property on the meso-scale response of three (3) AC samples is examined using the 3D Finite Element Method (FEM). The AC internal geometry is acquired using X-Ray Computed Tomography (CT); the distribution of the aggregates, mastic and air-voids phase is considered and obtained using Digital Imaging Processing (DIP) techniques. Using a surface-based cohesive behavior and assuming a predominant adhesive failure at the interface between the mastic and aggregate, a maximum traction criterion is used to obtain the damage propensity of the different mixtures. The result of the analysis shows that the microstructural morphological parameter Tps adequately captures the meso-scale response of the mixtures; there exist an inverse relationship between mixture strength characterization and the morphological parameter Tps.
Indentation of elastic materials is investigated numerically using the finite element method. Lar... more Indentation of elastic materials is investigated numerically using the finite element method. Large deformation theory is relied upon for accuracy. The study focuses on nonlocal frictional effects on relevant indentation quantities in the microindentation regime. The indentation quantities investigated include both local and global ones. It is shown that nonlocal frictional effects are small when global quantities are at issue, as is the case when conventional (Coulomb) theory of friction is used, also when these features are introduced at the ridges of a Vickers indenter where stress gradients are substantial. These effects are, however, shown to be of importance for particular indenter geometries as far as local field variables are concerned.
International Journal of Pavement Engineering, 2014
Enhancing the quality of granular layers is fundamental to optimise the structural performance of... more Enhancing the quality of granular layers is fundamental to optimise the structural performance of the pavements. The objective of this study is to investigate whether previously developed packing theory-based aggregate parameters can evaluate the resilient modulus of unbound granular materials. In this study, 19 differently graded unbound granular materials from two countries (USA and Sweden) were evaluated. This study validated both porosity of primary structure (PS) and contact points per particle (coordination number) as key parameters for evaluating the resilient modulus of unbound granular materials. This study showed that decreasing the PS porosity -higher coordination number -calculated based on the proposed gradation model, yields higher resilient modulus. Good correlation was observed between the proposed packing parameters and resilient modulus of several types of aggregates. The packing theory-based framework successfully recognised granular materials that exhibited poor performance in terms of resilient modulus.
International Journal of Pavement Engineering, 2013
Permanent deformation of unbound granular materials plays an essential role in the long-term perf... more Permanent deformation of unbound granular materials plays an essential role in the long-term performance of a pavement structure. Stability of unbound granular materials is defined by the particle-to-particle contact of the system, the particle size distribution and the packing arrangement. This paper presents a gradation model based on packing theory to evaluate permanent deformation of unbound granular materials. The framework was evaluated by using 10 unbound granular materials from different countries. The disruption potential, which determines the ability of secondary structure (SS) to disrupt the primary structure (PS), is introduced. This study also identified the amount of PS and SS that may eventually be used as a design parameter for permanent deformation of unbound road layers. The evaluation of the model regarding permanent deformation behaviour of granular materials is found to compare favourably with experimental results.
In order to perform thickness design of flexible pavements based on the hot mix asphalt (HMA) fra... more In order to perform thickness design of flexible pavements based on the hot mix asphalt (HMA) fracture mechanics framework prior to construction, the tensile strength and creep power law parameters of the mixture has to be determined. As samples are normally not available in the design stage, these properties will have to be modelled rather than determined in a laboratory. This paper evaluates material models based on the relationship between dynamic (complex) modulus and creep compliance in order to predict tensile strength and creep power law parameters for a mixture. For this evaluation, fourteen field sections from the United States have been used for verification and the results indicate that the crack resistance of a mixture can indeed be estimated without a development of extensive empirical relations between mixture properties and crack-resistance.
Sweden has initiated the development of a new calibrated-mechanistic pavement design procedure to... more Sweden has initiated the development of a new calibrated-mechanistic pavement design procedure to replace the current mechanical-empirical pavement procedure entitled 'PMS Objekt'. The first phase was focused on the implementation and calibration of the viscoelastic fracture mechanics framework entitled 'HMA Fracture Mechanics', developed at the University of Florida. This paper outlines the implementation and calibration of a new pavement design module for Sweden that is based on the HMA fracture mechanics framework. Both the developed design module, as well as the reference model used for calibration (PMS Objekt), is presented in this paper. The results in thickness design after calibration of the design module indicate that the framework is clearly applicable for common Swedish conditions and design standards.
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Papers by Denis Jelagin