Laser cladding is one of the advanced thermal techniques used to repair or modify the surface pro... more Laser cladding is one of the advanced thermal techniques used to repair or modify the surface properties of high-value components such as tools, military and aerospace parts. Unfortunately, tensile residual stresses are generated in the thermally treated area of this process. This work focuses on to investigate the key factors for the formation of tensile residual stress and how to minimize it in the clad when using dissimilar substrate and clad materials. To predict the tensile residual stress, a onedimensional analytical model has been adopted. Four cladding materials (Al 2 O 3 , TiC, TiO 2 , ZrO 2) on the H13 tool steel substrate and a range of preheating temperatures of the substrate, from 300 to 1200 K, have been investigated. Thermal strain and Young's modulus are found to be the key factors of formation of tensile residual stresses. Additionally, it is found that using a preheating temperature of the substrate immediately before laser cladding showed the reduction of residual stress.
Journal of the Mechanics and Physics of Solids, 2013
Creative Commons Legal Code AttributionNonCommercialNoDerivatives 4.0 International Official tran... more Creative Commons Legal Code AttributionNonCommercialNoDerivatives 4.0 International Official translations of this license are available in other languages. Creative Commons Corporation ("Creative Commons") is not a law firm and does not provide legal services or legal advice. Distribution of Creative Commons public licenses does not create a lawyer client or other relationship. Creative Commons makes its licenses and related information available on an "asis" basis. Creative Commons gives no warranties regarding its licenses, any material licensed under their terms and conditions, or any related information. Creative Commons disclaims all liability for damages resulting from their use to the fullest extent possible. Using Creative Commons Public Licenses Creative Commons public licenses provide a standard set of terms and conditions that creators and other rights holders may use to share original works of authorship and other material subject to copyright and certain other rights specified in the public license below. The following considerations are for informational purposes only, are not exhaustive, and do not form part of our licenses.
Abstract Finding a means of quantifying the structural deterioration suffered by concrete buildin... more Abstract Finding a means of quantifying the structural deterioration suffered by concrete buildings, bridges and dams is a high priority for studies of Alkali Aggregate Reaction (AAR). In this paper we are primarily interested in linking the change in the deformation properties to a damage index. First, a simple model is built, relating the initial elastic stiffness of a nominally undamaged cementitious system with its phase proportions, degree of hydration and aggregate modulus. Comparisons are made between the model predictions and earlier data. New results on the measured stiffness of damaged and undamaged concrete, cored from existing AAR affected structures, are then presented and an interpretation made using the model. The work attempts to provide a connection between the fabric of the material, the internal mechanisms and the macro-continuum response. The technique has potential for evaluating damage to concrete from other sources of deterioration.
In geotechnical analysis continuum idealisations of the bulk material still provide the most appr... more In geotechnical analysis continuum idealisations of the bulk material still provide the most appropriate approach for engineers designing large-scale structures. In this area, the most successful framework for describing the behaviour of soils is Critical State (CS) soil mechanics. However, the findings from discrete element method (DEM) analysis, such as the uniqueness of the CS, can provide invaluable information in the development such models. This paper details the key concepts behind a two-surface hyperplasticity model (?) whose development was informed by recent DEM findings on the uniqueness of the CS. Asymptotic states of the model will be confirmed and the DEM-continuum-experimental loop will be closed through comparison of the developed model with experimental data on coarse-grained particulate media. This will demonstrate, that providing the previous stress history is accounted for, the proposed model is suitable for a variety of particulate media.
Unsaturated Soils. Advances in Geo-Engineering, 2008
One of the sources of discrepancy between laboratory observations and the predicted behaviour of ... more One of the sources of discrepancy between laboratory observations and the predicted behaviour of unsaturated soil (by many existing constitutive models) is the sharp transition between the elastic and elastoplastic regimes exhibited by the latter but not the former. Such a transition is present in, for example, the Barcelona Basic Model. This paper suggests that by using the water retention curve and a multi-cell approach it is possible to overcome this limitation. The proposed enhancement may be incorporated relatively easily into many existing elasto-plastic models for unsaturated soils without addition of any new constitutive variables. The introduction of the algorithm presented here can improve the predictions in pre-yield states. In this paper the multi-cell approach has been implemented into the Barcelona Basic Model (BBM). The algorithm is described in detail using an illustrative stress path that involves hydrostatic compression, drying and wetting. The paper closes with a comparison of the modified model with the original BBM.
Biodegrading plumes in groundwater are often typified by relatively reactive zones around the fri... more Biodegrading plumes in groundwater are often typified by relatively reactive zones around the fringes and less reactive zones in the core. A high degree of refinement is required at the fringes if a model is to be of use in improving the conceptual understanding of plumes. Two strategies for dealing with the potentially high computational demands are (i) parallel processing, where the workload is shared between multiple processors, and (ii) locally adaptive remeshing, where a refined area of the grid tracks the moving plume fringes through the domain. The partial differential equation toolbox, UG (Unstructured Grids) offers advanced numerical tools including adaptive remeshing, sparse matrix storage schemes, and multigrid solvers. It embraces many of these features within a parallel processing environment. This paper reports on a recent development of UG to simulate field scale reactive biogeochemistry including Monod kinetics, NAPL dissolution, mineral precipitation/dissolution and ion exchange. The non-linear multicomponent reactive transport system is solved with the fully coupled method. Test cases have been used for verification of the new capability. The paper illustrates an application to a 3D field site. It is demonstrated that both adaptive remeshing and parallel processing can improve efficiency and in turn facilitate the incorporation of a more complex set of species and reactions such that understanding of plume processes is enhanced.
Laser Cladding is one of the developing manufacturing techniques used for diverse applications su... more Laser Cladding is one of the developing manufacturing techniques used for diverse applications such as coating, repairing and prototyping. Complex processing phenomena and the formation and growth of thin clad of few micrometers to millimeters range in most cases are yet to be fully understood. However, in recent past, several numerical models have been reported to get some understanding of physical, dynamic and metallurgical phenomena of this process. This article reviews the progress of numerical simulations spanning over three distinct stages of the process to model powder flow dynamics, melt pool and clad properties. For each stage, the governing equations, the effect of process variables and experimental validation techniques have been discussed. Specifically, we have outlined some of the underlying assumptions in the current numerical models which can act as pointers for further improvement of the existing numerical models. Authors recommend that numerical simulation results have to be complemented with experimental results to achieve better clad properties.
Modeling the complex decision problems faced in the coordination of disaster response as a schedu... more Modeling the complex decision problems faced in the coordination of disaster response as a scheduling problem to be solved using an optimization algorithm has the potential to deliver efficient and effective support to decision makers. However, much of the utility of such a model lies in its ability to accurately predict the outcome of any proposed solution. The stochastic nature of the disaster response environment can make such prediction difficult. In this paper we examine the effect of unknown disruptions to the road transport network on the utility of a disaster response scheduling model. The effects of several levels of disruption are measured empirically and the potential of using real-time information to revise model parameters, and thereby improve predictive performance, is evaluated.
There are few free open-source FE programs for 3D geometrically nonlinear shell elements that all... more There are few free open-source FE programs for 3D geometrically nonlinear shell elements that allow users to modify or extend the code. This paper presents such a MATLAB code which draws heavily on the work of Coombs and Coombs et al.
The yielding of soil exhibits both a Lode angle dependency and a dependency on the intermediate p... more The yielding of soil exhibits both a Lode angle dependency and a dependency on the intermediate principal stress. Ignoring these leads to a loss of realism in geotechnical analysis, yet neither of the widely used Mohr-Coulomb (M-C) or Drucker-Prager (D-P) models include both. This paper presents a simple pressure-dependent plasticity model based on a modified Reuleaux (mR) triangle which overcomes these limitations and yet (like the M-C and D-P formulations) allows for an analytical backward-Euler stress integration solution scheme. This latter feature is not found in more sophisticated (and computationally expensive) models. The mR deviatoric function is shown to provide a significantly improved fit to experimental data when compared with the M-C and D-P functions. Finite deformation finite-element analysis of the expansion of a cylindrical cavity is presented, verifying the use of the mR constitutive model for practical analyses.
When attempting to determine how to respond optimally to a large-scale emergency, the ability to ... more When attempting to determine how to respond optimally to a large-scale emergency, the ability to predict the consequences of certain courses of action in silico is of great utility. Agent-based simulations (ABSs) have become the de facto tool for this purpose; however, they may be used and implemented in a variety of ways. This article reviews existing implementations of ABSs for large-scale emergency response, and presents a taxonomy classifying them by usage. Opportunities for improving ABS for large-scale emergency response are identified.
Determining the stiffness (or compliance) of biological vascular vessels is of importance when in... more Determining the stiffness (or compliance) of biological vascular vessels is of importance when investigating pathological conditions, the design of stents, vascular grafts, distal anastomotic connectors in coronary artery bypass surgery, and understanding of ...
Over the last 30 years there has been considerable interest in the extension of the modifed Cam C... more Over the last 30 years there has been considerable interest in the extension of the modifed Cam Clay constitutive model to include anisotropic material behaviour. Many formulations have been proposed that allow the elliptical yield (or bounding) surface to rotate off the hydrostatic axis, thereby introducing anisotropy. Several of these extensions have included one or more inner yield surfaces that produce inelastic behaviour within the outer surface. However, a Lode angle dependency (LAD), essential for reproducing the behaviour of particulate media, has not been introduced in a satisfactory way in the existing models. Rotating a LAD yield surface has presented challenges for earlier workers, such as maintaining convexity of that surface and uniqueness of the Critical State (CS) cone for differing degrees of anisotropy [1]. This paper overcomes the above difficulties by presenting a two surface anisotropic hyperplasticity model that includes a deviatoric section with a dependency o...
Final report and appendices of the European project Proverbs on tools for the design of vertical ... more Final report and appendices of the European project Proverbs on tools for the design of vertical breakwaters (caisson type breakwaters) and similar hydraulic structures in the coastal zone. It includes the loads (waves) as well as the strength of the structure (geotechnial aspects, structural aspects)
Three dimensional analyses including geometric and material non--linearity require robust, effici... more Three dimensional analyses including geometric and material non--linearity require robust, efficient constitutive models able to simulate engineering materials. However, many existing constitutive models have not gained widespread use due to their computational burden and lack of guidance on choosing appropriate material constants. Here we offer a simple cone-type elasto-plastic formulation with a new deviatoric yielding criterion based on a modified Reuleaux triangle. The perfect plasticity model may be thought of as a hybrid between Drucker-Prager (D-P) and Mohr-Coulomb (M-C) that provides control over the internal friction angle independent of the shape of the deviatoric section. This surface allows an analytical backward Euler stress integration on the curved surface and exact integration in the regions where singularities appear. The attraction of the proposed algorithm is the improved fit to deviatoric yielding and the one--step integration scheme, plus a fully defined consist...
The element-free Galerkin method (EFGM) is superior to its counterpart the finite element method ... more The element-free Galerkin method (EFGM) is superior to its counterpart the finite element method (FEM) in terms of accuracy and convergence but is computationally expensive. Therefore it is more practical to use the EFGM only in a region, which is difficult to model using the FEM, while the FEM can be used in the remaining part of the problem domain. In the conventional EFGM, moving least squares (MLS) shape functions are used for the approximation of the field variables. These shape functions do not possess the Kronecker delta property and it is therefore not straightforward to couple the EFGM with the FEM. Local maximum entropy (max-ent) shape functions are an alternative way to couple the EFGM and FEM. These shape functions possess a weak Kronecker delta property at boundaries, which provides a natural way to couple the EFGM with the FEM as compared to the MLS basis functions, which need extra care to properly couple the two regions. This formulation removes the need for interfac...
Laser cladding is one of the advanced thermal techniques used to repair or modify the surface pro... more Laser cladding is one of the advanced thermal techniques used to repair or modify the surface properties of high-value components such as tools, military and aerospace parts. Unfortunately, tensile residual stresses are generated in the thermally treated area of this process. This work focuses on to investigate the key factors for the formation of tensile residual stress and how to minimize it in the clad when using dissimilar substrate and clad materials. To predict the tensile residual stress, a onedimensional analytical model has been adopted. Four cladding materials (Al 2 O 3 , TiC, TiO 2 , ZrO 2) on the H13 tool steel substrate and a range of preheating temperatures of the substrate, from 300 to 1200 K, have been investigated. Thermal strain and Young's modulus are found to be the key factors of formation of tensile residual stresses. Additionally, it is found that using a preheating temperature of the substrate immediately before laser cladding showed the reduction of residual stress.
Journal of the Mechanics and Physics of Solids, 2013
Creative Commons Legal Code AttributionNonCommercialNoDerivatives 4.0 International Official tran... more Creative Commons Legal Code AttributionNonCommercialNoDerivatives 4.0 International Official translations of this license are available in other languages. Creative Commons Corporation ("Creative Commons") is not a law firm and does not provide legal services or legal advice. Distribution of Creative Commons public licenses does not create a lawyer client or other relationship. Creative Commons makes its licenses and related information available on an "asis" basis. Creative Commons gives no warranties regarding its licenses, any material licensed under their terms and conditions, or any related information. Creative Commons disclaims all liability for damages resulting from their use to the fullest extent possible. Using Creative Commons Public Licenses Creative Commons public licenses provide a standard set of terms and conditions that creators and other rights holders may use to share original works of authorship and other material subject to copyright and certain other rights specified in the public license below. The following considerations are for informational purposes only, are not exhaustive, and do not form part of our licenses.
Abstract Finding a means of quantifying the structural deterioration suffered by concrete buildin... more Abstract Finding a means of quantifying the structural deterioration suffered by concrete buildings, bridges and dams is a high priority for studies of Alkali Aggregate Reaction (AAR). In this paper we are primarily interested in linking the change in the deformation properties to a damage index. First, a simple model is built, relating the initial elastic stiffness of a nominally undamaged cementitious system with its phase proportions, degree of hydration and aggregate modulus. Comparisons are made between the model predictions and earlier data. New results on the measured stiffness of damaged and undamaged concrete, cored from existing AAR affected structures, are then presented and an interpretation made using the model. The work attempts to provide a connection between the fabric of the material, the internal mechanisms and the macro-continuum response. The technique has potential for evaluating damage to concrete from other sources of deterioration.
In geotechnical analysis continuum idealisations of the bulk material still provide the most appr... more In geotechnical analysis continuum idealisations of the bulk material still provide the most appropriate approach for engineers designing large-scale structures. In this area, the most successful framework for describing the behaviour of soils is Critical State (CS) soil mechanics. However, the findings from discrete element method (DEM) analysis, such as the uniqueness of the CS, can provide invaluable information in the development such models. This paper details the key concepts behind a two-surface hyperplasticity model (?) whose development was informed by recent DEM findings on the uniqueness of the CS. Asymptotic states of the model will be confirmed and the DEM-continuum-experimental loop will be closed through comparison of the developed model with experimental data on coarse-grained particulate media. This will demonstrate, that providing the previous stress history is accounted for, the proposed model is suitable for a variety of particulate media.
Unsaturated Soils. Advances in Geo-Engineering, 2008
One of the sources of discrepancy between laboratory observations and the predicted behaviour of ... more One of the sources of discrepancy between laboratory observations and the predicted behaviour of unsaturated soil (by many existing constitutive models) is the sharp transition between the elastic and elastoplastic regimes exhibited by the latter but not the former. Such a transition is present in, for example, the Barcelona Basic Model. This paper suggests that by using the water retention curve and a multi-cell approach it is possible to overcome this limitation. The proposed enhancement may be incorporated relatively easily into many existing elasto-plastic models for unsaturated soils without addition of any new constitutive variables. The introduction of the algorithm presented here can improve the predictions in pre-yield states. In this paper the multi-cell approach has been implemented into the Barcelona Basic Model (BBM). The algorithm is described in detail using an illustrative stress path that involves hydrostatic compression, drying and wetting. The paper closes with a comparison of the modified model with the original BBM.
Biodegrading plumes in groundwater are often typified by relatively reactive zones around the fri... more Biodegrading plumes in groundwater are often typified by relatively reactive zones around the fringes and less reactive zones in the core. A high degree of refinement is required at the fringes if a model is to be of use in improving the conceptual understanding of plumes. Two strategies for dealing with the potentially high computational demands are (i) parallel processing, where the workload is shared between multiple processors, and (ii) locally adaptive remeshing, where a refined area of the grid tracks the moving plume fringes through the domain. The partial differential equation toolbox, UG (Unstructured Grids) offers advanced numerical tools including adaptive remeshing, sparse matrix storage schemes, and multigrid solvers. It embraces many of these features within a parallel processing environment. This paper reports on a recent development of UG to simulate field scale reactive biogeochemistry including Monod kinetics, NAPL dissolution, mineral precipitation/dissolution and ion exchange. The non-linear multicomponent reactive transport system is solved with the fully coupled method. Test cases have been used for verification of the new capability. The paper illustrates an application to a 3D field site. It is demonstrated that both adaptive remeshing and parallel processing can improve efficiency and in turn facilitate the incorporation of a more complex set of species and reactions such that understanding of plume processes is enhanced.
Laser Cladding is one of the developing manufacturing techniques used for diverse applications su... more Laser Cladding is one of the developing manufacturing techniques used for diverse applications such as coating, repairing and prototyping. Complex processing phenomena and the formation and growth of thin clad of few micrometers to millimeters range in most cases are yet to be fully understood. However, in recent past, several numerical models have been reported to get some understanding of physical, dynamic and metallurgical phenomena of this process. This article reviews the progress of numerical simulations spanning over three distinct stages of the process to model powder flow dynamics, melt pool and clad properties. For each stage, the governing equations, the effect of process variables and experimental validation techniques have been discussed. Specifically, we have outlined some of the underlying assumptions in the current numerical models which can act as pointers for further improvement of the existing numerical models. Authors recommend that numerical simulation results have to be complemented with experimental results to achieve better clad properties.
Modeling the complex decision problems faced in the coordination of disaster response as a schedu... more Modeling the complex decision problems faced in the coordination of disaster response as a scheduling problem to be solved using an optimization algorithm has the potential to deliver efficient and effective support to decision makers. However, much of the utility of such a model lies in its ability to accurately predict the outcome of any proposed solution. The stochastic nature of the disaster response environment can make such prediction difficult. In this paper we examine the effect of unknown disruptions to the road transport network on the utility of a disaster response scheduling model. The effects of several levels of disruption are measured empirically and the potential of using real-time information to revise model parameters, and thereby improve predictive performance, is evaluated.
There are few free open-source FE programs for 3D geometrically nonlinear shell elements that all... more There are few free open-source FE programs for 3D geometrically nonlinear shell elements that allow users to modify or extend the code. This paper presents such a MATLAB code which draws heavily on the work of Coombs and Coombs et al.
The yielding of soil exhibits both a Lode angle dependency and a dependency on the intermediate p... more The yielding of soil exhibits both a Lode angle dependency and a dependency on the intermediate principal stress. Ignoring these leads to a loss of realism in geotechnical analysis, yet neither of the widely used Mohr-Coulomb (M-C) or Drucker-Prager (D-P) models include both. This paper presents a simple pressure-dependent plasticity model based on a modified Reuleaux (mR) triangle which overcomes these limitations and yet (like the M-C and D-P formulations) allows for an analytical backward-Euler stress integration solution scheme. This latter feature is not found in more sophisticated (and computationally expensive) models. The mR deviatoric function is shown to provide a significantly improved fit to experimental data when compared with the M-C and D-P functions. Finite deformation finite-element analysis of the expansion of a cylindrical cavity is presented, verifying the use of the mR constitutive model for practical analyses.
When attempting to determine how to respond optimally to a large-scale emergency, the ability to ... more When attempting to determine how to respond optimally to a large-scale emergency, the ability to predict the consequences of certain courses of action in silico is of great utility. Agent-based simulations (ABSs) have become the de facto tool for this purpose; however, they may be used and implemented in a variety of ways. This article reviews existing implementations of ABSs for large-scale emergency response, and presents a taxonomy classifying them by usage. Opportunities for improving ABS for large-scale emergency response are identified.
Determining the stiffness (or compliance) of biological vascular vessels is of importance when in... more Determining the stiffness (or compliance) of biological vascular vessels is of importance when investigating pathological conditions, the design of stents, vascular grafts, distal anastomotic connectors in coronary artery bypass surgery, and understanding of ...
Over the last 30 years there has been considerable interest in the extension of the modifed Cam C... more Over the last 30 years there has been considerable interest in the extension of the modifed Cam Clay constitutive model to include anisotropic material behaviour. Many formulations have been proposed that allow the elliptical yield (or bounding) surface to rotate off the hydrostatic axis, thereby introducing anisotropy. Several of these extensions have included one or more inner yield surfaces that produce inelastic behaviour within the outer surface. However, a Lode angle dependency (LAD), essential for reproducing the behaviour of particulate media, has not been introduced in a satisfactory way in the existing models. Rotating a LAD yield surface has presented challenges for earlier workers, such as maintaining convexity of that surface and uniqueness of the Critical State (CS) cone for differing degrees of anisotropy [1]. This paper overcomes the above difficulties by presenting a two surface anisotropic hyperplasticity model that includes a deviatoric section with a dependency o...
Final report and appendices of the European project Proverbs on tools for the design of vertical ... more Final report and appendices of the European project Proverbs on tools for the design of vertical breakwaters (caisson type breakwaters) and similar hydraulic structures in the coastal zone. It includes the loads (waves) as well as the strength of the structure (geotechnial aspects, structural aspects)
Three dimensional analyses including geometric and material non--linearity require robust, effici... more Three dimensional analyses including geometric and material non--linearity require robust, efficient constitutive models able to simulate engineering materials. However, many existing constitutive models have not gained widespread use due to their computational burden and lack of guidance on choosing appropriate material constants. Here we offer a simple cone-type elasto-plastic formulation with a new deviatoric yielding criterion based on a modified Reuleaux triangle. The perfect plasticity model may be thought of as a hybrid between Drucker-Prager (D-P) and Mohr-Coulomb (M-C) that provides control over the internal friction angle independent of the shape of the deviatoric section. This surface allows an analytical backward Euler stress integration on the curved surface and exact integration in the regions where singularities appear. The attraction of the proposed algorithm is the improved fit to deviatoric yielding and the one--step integration scheme, plus a fully defined consist...
The element-free Galerkin method (EFGM) is superior to its counterpart the finite element method ... more The element-free Galerkin method (EFGM) is superior to its counterpart the finite element method (FEM) in terms of accuracy and convergence but is computationally expensive. Therefore it is more practical to use the EFGM only in a region, which is difficult to model using the FEM, while the FEM can be used in the remaining part of the problem domain. In the conventional EFGM, moving least squares (MLS) shape functions are used for the approximation of the field variables. These shape functions do not possess the Kronecker delta property and it is therefore not straightforward to couple the EFGM with the FEM. Local maximum entropy (max-ent) shape functions are an alternative way to couple the EFGM and FEM. These shape functions possess a weak Kronecker delta property at boundaries, which provides a natural way to couple the EFGM with the FEM as compared to the MLS basis functions, which need extra care to properly couple the two regions. This formulation removes the need for interfac...
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Papers by Roger Crouch