- Loughborough, Leicestershire, LE11 3TU, UK
Polyurea elastomer is known to exhibit advantageous impact-mitigation characteristics and thus can improve the dynamic performance of various components and structures. This study identifies the mechanisms of dynamic response of thin... more
Polyurea elastomer is known to exhibit advantageous impact-mitigation characteristics and thus can improve the dynamic performance of various components and structures. This study identifies the mechanisms of dynamic response of thin metallic plates, covered by a frontal polyurea layer, using a physically verified, custom material model for two-part polyurea implemented within a finite-element-method framework. A linear increase in the ballistic performance of a target with polymer coating is consistent with experimental work captured for the first time in a numerical study. A reported ballistic-limit improvement of 7.4 m s-1 per millimetre increase of polyurea thickness for frontal-layer thicknesses higher than 4 mm on the thin monolithic plate was established. In contrast, the application of polyurea coating thinner than 4 mm resulted in a diminished ballistic performance of the target. These outcomes are attributed to significant alterations in the energy-absorbing capacity of thin plates with the introduction of the polyurea layer that strongly depend on the impact velocity, polymer thickness, and interfacial interactions.
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Short cracks appearing under fatigue conditions are of major concern for safety-critical components. In this paper, a computational approach based on crystal plasticity and extended finite element method is developed to predict the... more
Short cracks appearing under fatigue conditions are of major concern for safety-critical components. In this paper, a computational approach based on crystal plasticity and extended finite element method is developed to predict the slip-controlled short crack growth in a single crystal nickel-based superalloy. The onset of fracture is controlled by cumulative shear strain of individual slip system and the direction of crack growth follows crystallographic slip plane. Simulations are carried out for [111] orientation at 24 °C and 650 °C, and the results confirm the capability of this approach in predicting the tortuous crack path and irregular propagation rate.
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Short cracks propagating under fatigue conditions are major concerns for the structural integrity of safety-critical applications. These defects tend to grow at high and irregular rates compared to long cracks under similar load, making... more
Short cracks propagating under fatigue conditions are major concerns for the structural integrity of safety-critical applications. These defects tend to grow at high and irregular rates compared to long cracks under similar load, making the prediction of their evolution a challenging task. In this study, a computational approach comprising a crystal plasticity constitutive model and the Extended Finite Element Method (XFEM) is developed to investigate the slip-controlled short crack growth in a single crystal Ni-based superalloy. The onset of fracture is controlled by the cumulative shear strain of individual slip systems and crack develops along crystallographic directions. The model is calibrated from low-cycle fatigue experiments and used to evaluate short crack growth paths and rates in [111] and [001] orientations at 24 °C and 650 °C. Furthermore, the slip behaviour around cracks is investigated. The obtained results show that this modelling approach can capture the tortuous short crack paths and predict the fluctuating propagation rates. Abstract Short cracks propagating under fatigue conditions are major concerns for the structural integrity of safety-critical applications. These defects tend to grow at high and irregular rates compared to long cracks under similar load, making the prediction of their evolution a challenging task. In this study, a computational approach comprising a crystal plasticity constitutive model and the Extended Finite Element Method (XFEM) is developed to investigate the slip-controlled short crack growth in a single crystal Ni-based superalloy. The onset of fracture is controlled by the cumulative shear strain of individual slip systems and crack develops along crystallographic directions. The model is calibrated from low-cycle fatigue experiments and used to evaluate short crack growth paths and rates in [111] and [001] orientations at 24 °C and 650 °C. Furthermore, the slip behaviour around cracks is investigated. The obtained results show that this modelling approach can capture the tortuous short crack paths and predict the fluctuating propagation rates.
Research Interests:
Polyurea elastomer exhibits desirable characteristics for impact mitigation, with varying stoichiometric-dependent properties that can be tailored for specific applications and applied to reinforce existing and new structural components.... more
Polyurea elastomer exhibits desirable characteristics for impact mitigation, with varying stoichiometric-dependent properties that can be tailored for specific applications and applied to reinforce existing and new structural components. This numerical study aims to investigate the ballistic performance of polyurea-aluminium laminate targets, employing a user-defined material model for polyurea elastomer developed in a finite-element (FE) framework. The model consists of a rigid spherical projectile impacting the considered target plate. A linear increase in the ballistic performance with a growing thickness of polymer coating was observed and is consistent with previously conducted experimental work. The ballistic limit is increased by some 5% per millimetre of polymer coating thickness, when compared to the monolithic metallic plate. The presence of the polymer layer significantly affects the dynamic response mechanisms of the component during bending due to impact. The result is a more localised deformation compared to global bending of the target. Abstract Polyurea elastomer exhibits desirable characteristics for impact mitigation, with varying stoichiometric-dependent properties that can be tailored for specific applications and applied to reinforce existing and new structural components. This numerical study aims to investigate the ballistic performance of polyurea-aluminium laminate targets, employing a user-defined material model for polyurea elastomer developed in a finite-element (FE) framework. The model consists of a rigid spherical projectile impacting the considered target plate. A linear increase in the ballistic performance with a growing thickness of polymer coating was observed and is consistent with previously conducted experimental work. The ballistic limit is increased by some 5% per millimetre of polymer coating thickness, when compared to the monolithic metallic plate. The presence of the polymer layer significantly affects the dynamic response mechanisms of the component during bending due to impact. The result is a more localised deformation compared to global bending of the target.
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One of the promising methods to increase the resistance of polymer-matrix composite materials to impact damage is the use of protective coatings. In this work, the effect of polyurea coating on impact-performance parameters of a woven... more
One of the promising methods to increase the resistance of polymer-matrix composite materials to impact damage is the use of protective coatings. In this work, the effect of polyurea coating on impact-performance parameters of a woven glass-fibre-reinforced laminate is studied. The study was performed on a specially developed ballistic experimental test rig employing a pneumatic gun. Eleven polymer composite targets with dimensions 200 mm x 300 mm x 8 mm were impacted orthogonally with a steel projectile with 23.8 mm diameter and weight 54.7 g in the range of the impact speed up to 150 m/s. A comparative assessment of the ballistic limit for targets with a 1.2 mm protective coating on the front and rear faces of the target, as well as for samples without any protective coating, was performed. The impact process was captured using two high-speed cameras for filming the front and top views at 25,000 frames per second. Experimental data on the ballistic limit for uncoated and polyurea coated fiberglass plates on the front and back surfaces were obtained. It was shown that 1.2 mm thick coating on the face surface increases the ballistic limit by 20%. The nature of the damage of the GRP base plate and coating has been analyzed. The obtained data can be used for validation of numerical models of ballistic impacts of polyurea-coated laminates.
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In this work, graphite debonding under thermal loading in compacted graphite iron (CGI) is investigated, employing a microstructure-based modelling approach. CGI has a complex microstructure, comprising graphite particles of different... more
In this work, graphite debonding under thermal loading in compacted graphite iron (CGI) is investigated, employing a microstructure-based modelling approach. CGI has a complex microstructure, comprising graphite particles of different shapes, sizes and orientations embedded in an iron matrix. As a result of mismatch in coefficients of thermal expansion of constituents, thermal load can result in damage due to interfacial debonding. To evaluate this phenomenon, representative volume elements of CGI microstructures are studied using finite-element simulations. Specific inputs in the model are provided through statistical analysis of SEM micrographs. Further, the influence of boundary conditions and incorporation of an interfacial layer is discussed. The obtained results demonstrate that the onset of matrix plasticisation and graphite decohesion are sensitive to the adopted modelling assumptions.
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Damage initiation and progression in precipitate hardened alloys is typically linked to the failure of second phase particles that result from the precipitation process. These particles have been shown to be stress concentrators and crack... more
Damage initiation and progression in precipitate hardened alloys is typically linked to the failure of second phase particles that result from the precipitation process. These particles have been shown to be stress concentrators and crack starters as a result of both particle debonding and fracture. In this investigation, a precipitate hardened aluminum alloy (Al 2024-T3) is loaded monotonically to investigate the role the particles have in the progressive failure process. The damage process was monitored continuously by combining the Acoustic Emission (AE) method either with in situ Scanning Electron Microscopy (SEM) or X-ray Microcomputed Tomography (X-ray µ-CT) to obtain both surface and volume microstructural information. Particles were observed to fracture only in the elastic regime of the material response while void growth at locations predominantly near particles were found to be associated with progressive failure in the plastic region of the macroscopic response. Experimental findings were validated by fracture simulations at the scale of particle-matrix interface.
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Aim: This paper aims to evaluate the effectiveness of MitraClip implantation as a solution to severe mitral regurgitation (MR) in the case of posterior leaflet prolapse due to hypertrophic obstructive cardiomyopathy and chordae rupture.... more
Aim: This paper aims to evaluate the effectiveness of MitraClip implantation as a solution to severe mitral regurgitation (MR) in the case of posterior leaflet prolapse due to hypertrophic obstructive cardiomyopathy and chordae rupture. Methods: NX CAD software was used to create a surface geometric model for the mitral valve (MV). A hyperelastic material model, calibrated against experimental results, was used to describe stress-strain responses of the MV leaflets, and a spring element approach was used to describe chordae response. Abaqus CAE was employed to create a finite element model for diseased MV suffering from MR. The effectiveness of MitraClip implantation on valve function was investigated by simulating the deformation of diseased valve, with and without MitraClip repair, during peak systole and diastole. Leaflet deformation and stress distributions were used to assess the effectiveness of the procedure.
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In this work interaction problems between a finite-length crack with plane and antiplane crystal defects in the context of couple-stress elasticity are presented. Two alternative yet equivalent approaches for the formulation of crack... more
In this work interaction problems between a finite-length crack with plane and antiplane crystal defects in the context of couple-stress elasticity are presented. Two alternative yet equivalent approaches for the formulation of crack problems are discussed based on the distributed dislocation technique. To this aim, the stress fields of climb and screw dislocation dipoles are derived within couple-stress theory and new 'constrained' rotational defects are introduced to satisfy the boundary conditions of the opening mode problem. Eventually, all interaction problems are described by single or systems of singular integral equations that are solved numerically using appropriate collocation techniques. The obtained results aim to highlight the deviation from classical elasticity solutions and underline the differences in interactions of cracks with single dislocations and dislocation dipoles. In general, it is concluded that the cracked body behaves in a more rigid way when couple-stresses are considered. Also, the stress level is significantly higher than the classical elasticity prediction. Moreover, the configurational forces acting on the defects are evaluated and their dependence on the characteristic material length of couple-stress theory and the distance between the defect and the crack-tip is discussed. This investigation reveals either a strengthening or a weakening effect in the opening mode problem while in the antiplane mode a strengthening effect is always obtained.
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Interaction problems of a finite-length crack with plane and antiplane dislocation dipoles in the context of couple-stress elasticity are presented in this study. The analysis is based on the distributed dislocation technique where... more
Interaction problems of a finite-length crack with plane and antiplane dislocation dipoles in the context of couple-stress elasticity are presented in this study. The analysis is based on the distributed dislocation technique where infinitesimal dislocation dipoles are used as strain nuclei. The stress fields of these area defects are provided for the first time in the framework of couple-stress elasticity theory. In addition, a new rotational defect is introduced to satisfy the boundary conditions of the opening mode problem. This formulation leads to displacement-based hyper-singular integral equations that govern the crack problems, which are solved numerically. It is further shown that this method has several advantages over the slope formulation. Based on the obtained results, it is deduced that in all cases the cracked body behaves in a more rigid way when couple-stresses are considered. The effect of couple-stresses is highlighted in a small zone ahead of the crack-tip and around the dislocation dipole, where the stress level is significantly higher than the classical elasticity prediction. Further, the dependence of the energy release rate and the configurational force exerted on the defect on the characteristic material length and the distance between the defect and the crack-tip is discussed. In the plane problems, couple-stress theory predicts either strengthening or weakening effects while in the antiplane mode a strengthening effect is predicted.
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Acoustic emission (AE) is a common nondestructive evaluation tool that has been used to monitor fracture in materials and structures. The direct connection between AE events and their source, however, is difficult because of material,... more
Acoustic emission (AE) is a common nondestructive evaluation tool that has been used to monitor fracture in materials and structures. The direct connection between AE events and their source, however, is difficult because of material, geometry and sensor contributions to the recorded signals. Moreover, the recorded AE activity is affected by several noise sources which further complicate the identification process. This article uses a combination of in situ experiments inside the scanning electron microscope to observe fracture in an aluminum alloy at the time and scale it occurs and a novel AE signal processing framework to identify characteristics that correlate with fracture events. Specifically, a signal processing method is designed to cluster AE activity based on the selection of a subset of features objectively identified by examining their correlation and variance. The identified clusters are then compared to both mechanical and in situ observed microstructural damage. Results from a set of nanoindentation tests as well as a carefully designed computational model are also presented to validate the conclusions drawn from signal processing.
A computational damage model which is driven by material, mechanical behavior and nondestructive evaluation data is presented in this study. To collect material and mechanical behavior damage data, an aerospace grade precipitate-hardened... more
A computational damage model which is driven by material, mechanical behavior and nondestructive evaluation data is presented in this study. To collect material and mechanical behavior damage data, an aerospace grade precipitate-hardened aluminum alloy was mechanically loaded under monotonic conditions inside a Scanning Electron Microscope, while acoustic and optical methods were used to track the damage accumulation process. In addition, to obtain experimental information about damage accumulation at the laboratory scale, a set of cyclic loading experiments was completed using 3-point bending specimens made out of the same aluminum alloy and by employing the same nondestructive methods. The ensemble of recorded data for both cases was then used in a post-processing scheme based on outlier analysis to form damage progression curves which were subsequently used as custom damage laws in finite element simulations. Specifically , a plasticity model coupled with stiffness degradation triggered by the experimentally defined damage curves was used in custom subroutines. The results highlight the effect of the data-driven damage model on the simulated mechanical response of the geometries considered and provide an information workflow that is capable of coupling experiments with simulations that can be used for remaining useful life estimations.
Strain localization bands (SLBs) that appear at early stages of deformation of magnesium alloys have been recently associated with heterogeneous activation of deformation twinning. Experimental evidence has demonstrated that such... more
Strain localization bands (SLBs) that appear at early stages of deformation of magnesium alloys have been recently associated with heterogeneous activation of deformation twinning. Experimental evidence has demonstrated that such "Lüders-type" band formations dominate the overall mechanical behavior of these alloys resulting in sigmoidal type stress-strain curves with a distinct plateau followed by pronounced anisotropic hardening. To evaluate the role of SLB formation on the local and global mechanical behavior of magnesium alloys, an integrated experimental/compu-tational approach is presented. The computational part is developed based on custom subroutines implemented in a Finite Element Method that combine a plasticity model with a stiffness degradation approach. Specific inputs from the characterization and testing measurements to the computational approach are discussed while the numerical results are validated against such available experimental information, confirming the existence of load drops and the intensification of strain accumulation at the time of SLB initiation.
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In the present work the interaction of a finite-length crack with a discrete climb dislocation is studied within the framework of the generalized continuum theory of couple-stress elasticity. The climb dislocation is placed on the crack... more
In the present work the interaction of a finite-length crack with a discrete climb dislocation is studied within the framework of the generalized continuum theory of couple-stress elasticity. The climb dislocation is placed on the crack plane resulting in an opening crack mode. For the solution of the crack problem the distributed dislocation technique is employed. Due to the nature of the boundary conditions that arise in couple-stress elasticity, the crack is modeled by a continuous distribution of translational and rotational defects. The distribution of these defects produces both stresses and couple stresses in the body. It is shown that the interaction problem is governed by a system of coupled singular integral equations with both Cauchy and logarithmic kernels which is solved numerically using an appropriate collocation technique. The results for the near-tip fields differ in several respects from the predictions of classical fracture mechanics. It is shown that a cracked couple-stress solid behaves in a more rigid way compared to one governed by classical elasticity. Moreover, the evaluation of the energy release rate in the crack-tips and the associated driving force exerted on the dislocation reveals an interesting 'alternating' behavior between strengthening and weakening of the crack, depending on the distance of the crack-tip to the dislocation core as well as on ratio of the material length, introduced by the couple-stress theory, to the length of the crack.
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In the second part of this study, the interaction of a finite-length crack with a glide and a screw dislocation is examined within the framework of couple-stress elasticity. The loading from the two defects on the crack results to plane... more
In the second part of this study, the interaction of a finite-length crack with a glide and a screw dislocation is examined within the framework of couple-stress elasticity. The loading from the two defects on the crack results to plane and antiplane shear modes of fracture, respectively. Both problems are attacked using the distributed dislocation technique and the cracks are modeled using distributions of discrete glide or screw dislocations. The antiplane strain case is governed by a single hyper-singular integral equation with a cubic singularity, whereas the plane strain case by a singular integral equation. In both cases, the integral equations are numerically solved using appropriate collocation techniques. The results obtained herein show that a crack under antiplane conditions closes in a smoother way as compared to the classical elasticity result. Further, the evaluation of the energy release rate in the crack tips reveals an 'alternating' behavior between strengthening and weakening effects in the plane strain case, depending on the defect's distance from the crack tip and the magnitude of the characteristic material length. On the other hand, the energy release rate in the antiplane mode shows a strengthening effect when couple-stresses are considered.
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The evolution of crack tip displacement and strain fields during uniaxial, room temperature , low cycle fatigue experiments of Nickel superalloy compact tension specimens were measured by a digital image correlation approach and were... more
The evolution of crack tip displacement and strain fields during uniaxial, room temperature , low cycle fatigue experiments of Nickel superalloy compact tension specimens were measured by a digital image correlation approach and were further used to validate a cyclic plasticity model and corresponding deformation calculations made by a finite elements methodology. The experimental results provided data trends for the opening displacements and near crack tip strains as function of cycles. The calculated full field deformation data were then used to validate the corresponding results obtained by a confined crack tip plasticity model which was calibrated using stress-strain measurements on round-bar specimens. This type of direct comparison demonstrated that the computational model was capable to adequately capture the crack opening displacements at various stages of the specimen's fatigue life, providing in this way a tool for quantitative cyclic plasticity model validation. In addition, this integrated experimental-computational approach provides a framework to accelerate our understanding related to interactions of fatigue test data and models, as well as ways to inform one another.
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This article presents a modeling approach to estimate the energy release due to ductile crack initiation in conjunction to the energy dissipation associated with the formation and propagation of transient stress waves typically referred... more
This article presents a modeling approach to estimate the energy release due to ductile crack initiation in conjunction to the energy dissipation associated with the formation and propagation of transient stress waves typically referred to as Acoustic Emission. To achieve this goal, a ductile fracture problem is investigated computationally using the Finite Elements Method based on a compact tension geometry under Mode I loading conditions. To quantify the energy dissipation associated with Acoustic Emission, a crack increment is produced given a predetermined notch size in a 3D cohesive-based extended finite element model. The computational modeling methodology consists of defining a damage initiation state from static simulations and linking such state to a dynamic formulation used to evaluate wave propagation and related energy redistribution effects. The model relies on a custom traction separation law constructed using full field deformation measurements obtained experimentally using the Digital Image Correlation method. The amount of energy release due to the investigated first crack increment is evaluated through three different approaches both for verification purposes and to produce an estimate of the portion of the energy that radiates away from the crack source in the form of transient waves. The results presented herein propose an upper bound for the energy dissipation associated to Acoustic Emission, which could assist the interpretation and implementation of relevant nondestructive evaluation methods and the further enrichment of the understanding of effects associated with fracture.
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We present a finite element description of Volterra dislocations using a thermal analogue and the integral representation of dislocations through stresses in the context of linear elasticity. Several analytical results are fully recovered... more
We present a finite element description of Volterra dislocations using a thermal analogue and the integral representation of dislocations through stresses in the context of linear elasticity. Several analytical results are fully recovered for two dimensional edge dislocations. The full fields are reproduced for edge dislocations in isotropic and anisotropic bodies and for different configurations. Problems with dislocations in infinite medium, near free surfaces or bimaterial interfaces are studied. The efficiency of the proposed method is examined in more complex problems such as interactions of dislocations with inclusions and cracks and multiple dislocation problems. The configurational (Peach-Koehler) force of the dislocations is calculated numerically based on energy considerations (Parks method). Some important integral conservation laws of elastostatics are considered and the connection between the material forces and the conserved integrals (J and M) is presented. The variable core model of Lubarda and Markenscoff is introduced to model the dislocation core area that is indeterminate by the classical theory.
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Indentation tests have long been a standard method for material characterization due to the fact that they provide an easy, inexpensive, non-destructive and objective method of evaluating basic properties from small volumes of materials.... more
Indentation tests have long been a standard method for material characterization due to the fact that they provide an easy, inexpensive, non-destructive and objective method of evaluating basic properties from small volumes of materials. As the contact scales in such experiments reduce progressively (micro to nano-scales) the internal material lengths become important and their effect upon the macroscopic response cannot be ignored. In the present study, we derive general solutions for three basic two-dimensional (2D) plane-strain contact problems within the framework of the generalized continuum theory of couple-stress elasticity. This theory introduces characteristic material lengths in order to describe the pertinent scale effects that emerge from the underlying microstructure and has proved to be very effective for modeling microstructured materials. By using this theory, we initially study the problem of the indentation of a deformable elastic half-plane by a flat punch, then by a cylindrical indentor, and finally by a shallow wedge indentor. Our approach is based on singular integral equations which have resulted from a treatment of the mixed boundary value problems via integral transforms and generalized functions. The results show significant departure from the predictions of classical elasticity revealing that it is inadequate to analyze indentation problems in microstructured materials employing only classical contact mechanics.
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Research Interests:
The present work gives a systematic and rigorous implementation of (edge type) circular Volterra dislocation loops in ordinary axisymmetric finite elements using the thermal analogue and the integral representation of dislocations through... more
The present work gives a systematic and rigorous implementation of (edge type) circular Volterra dislocation loops in ordinary axisymmetric finite elements using the thermal analogue and the integral representation of dislocations through stresses. The accuracy of the proposed method is studied in problems where analytical solutions exist. The full fields are given for loop dislocations in isotropic and anisotropic crystals and the Peach-Koehler forces are calculated for loops approaching free surfaces and bimaterial interfaces. The results are expected to be very important in the analysis of plastic yield strength, giving quantitative results regarding the influence of grain boundaries, interstitial particles, microvoids, thin film constraints and nano-indentation phenomena. The interaction of few dislocations with various inhomogeneities gives rise to size effects in the yield strength which are of great importance in nano-mechanics.
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The present work gives a systematic and rigorous implementation of Volterra dislocations in ordinary two-dimensional finite elements using the thermal analogue and the integral representation of dislocations through the stresses. The full... more
The present work gives a systematic and rigorous implementation of Volterra dislocations in ordinary two-dimensional finite elements using the thermal analogue and the integral representation of dislocations through the stresses. The full fields are given for edge dislocations in anisotropic crystals and the Peach-Koehler forces are found for some important examples.
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The use of fibre-reinforced plastics (FRPs) in sandwich structures increased for various industrial applications thanks to their strength-to-weight ratio which provides designers with advanced options for modern structures. FRP Sandwich... more
The use of fibre-reinforced plastics (FRPs) in sandwich structures increased for various industrial applications thanks to their strength-to-weight ratio which provides designers with advanced options for modern structures. FRP Sandwich Structures (FRPSS) are often used in aerospace, biomedical, defence, and marine products, where their high structural performance is required to sustain complex in-service loads and withstand varying environmental conditions. Progressive degradation of FRPSS under such circumstances has been a subject of interest for researchers owing to safety requirements for products with FRP. This paper reviews the state-of-the-art of the mechanical behaviour of FRPSS subjected to various loading regimes. It highlights the variation in structural performance, viscoelastic properties, damage resistance, and sequence of environmental degradation of FRPSS. Numerical methods and damage algorithms used to predict failures are also presented to provide sufficient knowl...
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A finite element description of variable core edge dislocations in the context of linear elasticity is presented in this work. The approach followed is based on a thermal analogue and the integral representation of dislocations through... more
A finite element description of variable core edge dislocations in the context of linear elasticity is presented in this work. The approach followed is based on a thermal analogue and the integral representation of dislocations through stresses. The objective of a variable core defect concept is to eliminate the stress singularity experienced at the dislocation core. This is accomplished assuming that the displacement discontinuity is achieved gradually over some distance. To implement this concept in a finite element scheme, we first model purely rotational crystal defects considering an appropriate pseudo-temperature distribution, which produces a dislocation array of increasing width. Accordingly, we simulate a discrete edge dislocation of linearly increasing width. This description of dislocation core is closer to experimental observations and has a physically anticipated behaviour reproducing the Volterra dislocation away from the core. Further, interactions of variable core di...
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Closed access. This is an erratum to: "Finite Element Analysis of Discrete Circular Dislocations" [CMES, vol. 60, no. 2, pp. 181-198, 2010]. The paper is at: https://dspace.lboro.ac.uk/2134/25963
Strain mapping at various length scales and its relationship to both microstructural features and mechanical behavior has been greatly advanced over the past years through the use of optical, non-contact full-field measurement techniques,... more
Strain mapping at various length scales and its relationship to both microstructural features and mechanical behavior has been greatly advanced over the past years through the use of optical, non-contact full-field measurement techniques, capable of measuring 2D and 3D surface deformations. An integrated experimental and numerical approach for the characterization of small scale plasticity under monotonic and cyclic loading is presented in this work.
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A computational damage model which is driven by material, mechanical behavior and nondestructive evaluation data is presented in this study. To collect material and mechanical behavior damage data, an aerospace grade precipitate-hardened... more
A computational damage model which is driven by material, mechanical behavior and nondestructive evaluation data is presented in this study. To collect material and mechanical behavior damage data, an aerospace grade precipitate-hardened aluminum alloy was mechanically loaded under monotonic conditions inside a Scanning Electron Microscope, while acoustic and optical methods were used to track the damage accumulation process. In addition, to obtain experimental information about damage accumulation at the laboratory scale, a set of cyclic loading experiments was completed using 3-point bending specimens made out of the same aluminum alloy and by employing the same nondestructive methods. The ensemble of recorded data for both cases was then used in a post-processing scheme based on outlier analysis to form damage progression curves which were subsequently used as custom damage laws in finite element simulations. Specifically, a plasticity model coupled with stiffness degradation tri...
Short cracks propagating under fatigue conditions are major concerns for the structural integrity of safety-critical applications. These defects tend to grow at high and irregular rates compared to long cracks under similar load, making... more
Short cracks propagating under fatigue conditions are major concerns for the structural integrity of safety-critical applications. These defects tend to grow at high and irregular rates compared to long cracks under similar load, making the prediction of their evolution a challenging task. In this study, a computational approach comprising a crystal plasticity constitutive model and the Extended Finite Element Method (XFEM) is developed to investigate the slip-controlled short crack growth in a single crystal Ni-based superalloy. The onset of fracture is controlled by the cumulative shear strain of individual slip systems and crack develops along crystallographic directions. The model is calibrated from low-cycle fatigue experiments and used to evaluate short crack growth paths and rates in [111] and [001] orientations at 24 °C and 650 °C. Furthermore, the slip behaviour around cracks is investigated. The obtained results show that this modelling approach can capture the tortuous short crack paths and predict the fluctuating propagation rates. Abstract Short cracks propagating under fatigue conditions are major concerns for the structural integrity of safety-critical applications. These defects tend to grow at high and irregular rates compared to long cracks under similar load, making the prediction of their evolution a challenging task. In this study, a computational approach comprising a crystal plasticity constitutive model and the Extended Finite Element Method (XFEM) is developed to investigate the slip-controlled short crack growth in a single crystal Ni-based superalloy. The onset of fracture is controlled by the cumulative shear strain of individual slip systems and crack develops along crystallographic directions. The model is calibrated from low-cycle fatigue experiments and used to evaluate short crack growth paths and rates in [111] and [001] orientations at 24 °C and 650 °C. Furthermore, the slip behaviour around cracks is investigated. The obtained results show that this modelling approach can capture the tortuous short crack paths and predict the fluctuating propagation rates.
Research Interests:
Research Interests:
Polyurea elastomer exhibits desirable characteristics for impact mitigation, with varying stoichiometric-dependent properties that can be tailored for specific applications and applied to reinforce existing and new structural components.... more
Polyurea elastomer exhibits desirable characteristics for impact mitigation, with varying stoichiometric-dependent properties that can be tailored for specific applications and applied to reinforce existing and new structural components. This numerical study aims to investigate the ballistic performance of polyurea-aluminium laminate targets, employing a user-defined material model for polyurea elastomer developed in a finite-element (FE) framework. The model consists of a rigid spherical projectile impacting the considered target plate. A linear increase in the ballistic performance with a growing thickness of polymer coating was observed and is consistent with previously conducted experimental work. The ballistic limit is increased by some 5% per millimetre of polymer coating thickness, when compared to the monolithic metallic plate. The presence of the polymer layer significantly affects the dynamic response mechanisms of the component during bending due to impact. The result is a more localised deformation compared to global bending of the target. Abstract Polyurea elastomer exhibits desirable characteristics for impact mitigation, with varying stoichiometric-dependent properties that can be tailored for specific applications and applied to reinforce existing and new structural components. This numerical study aims to investigate the ballistic performance of polyurea-aluminium laminate targets, employing a user-defined material model for polyurea elastomer developed in a finite-element (FE) framework. The model consists of a rigid spherical projectile impacting the considered target plate. A linear increase in the ballistic performance with a growing thickness of polymer coating was observed and is consistent with previously conducted experimental work. The ballistic limit is increased by some 5% per millimetre of polymer coating thickness, when compared to the monolithic metallic plate. The presence of the polymer layer significantly affects the dynamic response mechanisms of the component during bending due to impact. The result is a more localised deformation compared to global bending of the target.