S. Kumar
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22 records found
1
This work investigates the healing performance of an adipic acid vitrimer integrated into two distinct 2D cellular architectures. Specimens were manufactured via waterjet cutting and subjected to cyclic loading to observe crack initiation and propagation throughout the lattice. Subsequent healing cycles were performed using a mold and heat press system to apply the pressure required for crack-face closure and bond-exchange activation. The more compliant architecture accumulated greater damage and was consequently more challenging to heal, whereas the stiffer design recovered its stiffness more readily. Overall, the results demonstrate the feasibility of incorporating vitrimers and other self-healing polymers into foams and metamaterials while highlighting the process-design considerations and limitations that must be addressed to achieve reliable mechanical recovery. ...
This work investigates the healing performance of an adipic acid vitrimer integrated into two distinct 2D cellular architectures. Specimens were manufactured via waterjet cutting and subjected to cyclic loading to observe crack initiation and propagation throughout the lattice. Subsequent healing cycles were performed using a mold and heat press system to apply the pressure required for crack-face closure and bond-exchange activation. The more compliant architecture accumulated greater damage and was consequently more challenging to heal, whereas the stiffer design recovered its stiffness more readily. Overall, the results demonstrate the feasibility of incorporating vitrimers and other self-healing polymers into foams and metamaterials while highlighting the process-design considerations and limitations that must be addressed to achieve reliable mechanical recovery.
Hybrid Beam-3D Continuum Nonlinear Modeling of Architected Truss Metamaterials
Insights into Joint Geometry
Previous research has shown that their mechanical performance depends not only on the beams themselves, but also on parameters such as material choice, manufacturing method, lattice architecture, and the shape of the joints where beams meet. This thesis extends existing joint-resolved modeling approaches beyond the linear elastic range into the nonlinear regime, where large deformations and buckling become important. To do so, it develops a hybrid finite element framework in which joints are represented by detailed 3D solid elements and struts by efficient beam elements, combining improved physical realism with manageable computational cost. The thesis presents a nonlinear solver, validation metrics, an accuracy and computational efficiency study, and outlines future developments toward monolithic and data-driven modeling approaches. ...
Previous research has shown that their mechanical performance depends not only on the beams themselves, but also on parameters such as material choice, manufacturing method, lattice architecture, and the shape of the joints where beams meet. This thesis extends existing joint-resolved modeling approaches beyond the linear elastic range into the nonlinear regime, where large deformations and buckling become important. To do so, it develops a hybrid finite element framework in which joints are represented by detailed 3D solid elements and struts by efficient beam elements, combining improved physical realism with manageable computational cost. The thesis presents a nonlinear solver, validation metrics, an accuracy and computational efficiency study, and outlines future developments toward monolithic and data-driven modeling approaches.
Mechanochemical Stress Sensing
A correlation of strain field and Spiropyran activation in elastomers
Unveiling the Free-Energy Landscape
Descriptor-Based Inference in Cahn–Hilliard Phase- Field Simulations
In this study, Gas Chromatography-Mass Spectrometry (GC/MS) was employed to analyze the chemical composition of the adhesives. To classify different production methods, machine learning techniques—including Principal Component Analysis (PCA), Linear Discriminant Analysis (LDA), and K-Nearest Neighbors (KNN)—were applied. The results indicate that LDA successfully differentiates between production techniques, suggesting its potential for identifying variations in tar preparation. However, since this study is based on experimentally produced samples, its application to archaeological specimens requires further validation. ...
In this study, Gas Chromatography-Mass Spectrometry (GC/MS) was employed to analyze the chemical composition of the adhesives. To classify different production methods, machine learning techniques—including Principal Component Analysis (PCA), Linear Discriminant Analysis (LDA), and K-Nearest Neighbors (KNN)—were applied. The results indicate that LDA successfully differentiates between production techniques, suggesting its potential for identifying variations in tar preparation. However, since this study is based on experimentally produced samples, its application to archaeological specimens requires further validation.
A surrogate for such simulation methods that can benefit from a data-driven approach could be deep learning through artificial neural networks. The aim of this study is to leverage a suitable deep learning neural network model and assess its ability to capture the complexity of texture. As such, Normalizing Flows (NF), a generative deep learning model, is employed to learn textures by capturing the multi-modal distribution of discrete sets of orientation matrices that represent those textures. The performance of the model is assessed for two types of data, matrices that belong to a fixed texture (unconditional modelling) and matrices paired with simple conditions (conditional modelling) that produce textures based on the conditions. The training data for the neural network is synthetically generated in a microstructure modelling software. The model is trained on this synthetic data and subsequently used to generate samples on the distribution it has learned. These samples and ground truth data are plotted in pole figures to compare the distribution of the generated data and the ground truth data, respectively, to visualize the model’s generative capability.
It is found that the model closely approximates the distribution of a given texture, capturing the structure of the distribution, albeit having a reduced density in the modes of the distribution. The conditional model is also capable of generating the approximate texture relevant to the condition given to it during evaluation. These findings indicate that the model is capable of learning texture, and further improvements in the model’s architecture could make the model highly robust. ...
A surrogate for such simulation methods that can benefit from a data-driven approach could be deep learning through artificial neural networks. The aim of this study is to leverage a suitable deep learning neural network model and assess its ability to capture the complexity of texture. As such, Normalizing Flows (NF), a generative deep learning model, is employed to learn textures by capturing the multi-modal distribution of discrete sets of orientation matrices that represent those textures. The performance of the model is assessed for two types of data, matrices that belong to a fixed texture (unconditional modelling) and matrices paired with simple conditions (conditional modelling) that produce textures based on the conditions. The training data for the neural network is synthetically generated in a microstructure modelling software. The model is trained on this synthetic data and subsequently used to generate samples on the distribution it has learned. These samples and ground truth data are plotted in pole figures to compare the distribution of the generated data and the ground truth data, respectively, to visualize the model’s generative capability.
It is found that the model closely approximates the distribution of a given texture, capturing the structure of the distribution, albeit having a reduced density in the modes of the distribution. The conditional model is also capable of generating the approximate texture relevant to the condition given to it during evaluation. These findings indicate that the model is capable of learning texture, and further improvements in the model’s architecture could make the model highly robust.
This thesis develops physics-guided machine learning approaches to accelerate the design, modeling, and interpretation of materials across scales and material classes. It introduces neural network architectures for metamaterial design that learn directly from data while remaining physically consistent. These models retrieve structural designs that achieve specified target properties instantly, enabling rapid exploration of the structure–property landscape and supporting scenarios with multiple design goals more efficiently than traditional optimization methods. For material modeling, this work addresses the limitation that stress fields are not directly accessible in experiments. The proposed frameworks instead learn constitutive laws from measurable quantities such as displacements and forces, while preserving essential physical principles such as thermodynamic consistency. This work further demonstrates how data-driven approaches reveal previously unknown structure–property relationships, such as the role of dangling atomic masses in the thermal transport of crystalline nanoporous materials. Finally, it introduces a chemistry-constrained generative framework that proposes synthesizable, diverse, and novel molecular structures using limited data, while providing interpretable representations of the molecular generation.
Together, these contributions establish that physics-guided machine learning can complement and extend traditional materials science by delivering reliable, interpretable, and generalizable solutions to longstanding challenges in design and modeling. ...
This thesis develops physics-guided machine learning approaches to accelerate the design, modeling, and interpretation of materials across scales and material classes. It introduces neural network architectures for metamaterial design that learn directly from data while remaining physically consistent. These models retrieve structural designs that achieve specified target properties instantly, enabling rapid exploration of the structure–property landscape and supporting scenarios with multiple design goals more efficiently than traditional optimization methods. For material modeling, this work addresses the limitation that stress fields are not directly accessible in experiments. The proposed frameworks instead learn constitutive laws from measurable quantities such as displacements and forces, while preserving essential physical principles such as thermodynamic consistency. This work further demonstrates how data-driven approaches reveal previously unknown structure–property relationships, such as the role of dangling atomic masses in the thermal transport of crystalline nanoporous materials. Finally, it introduces a chemistry-constrained generative framework that proposes synthesizable, diverse, and novel molecular structures using limited data, while providing interpretable representations of the molecular generation.
Together, these contributions establish that physics-guided machine learning can complement and extend traditional materials science by delivering reliable, interpretable, and generalizable solutions to longstanding challenges in design and modeling.
tested using validation datasets of a full cycle prediction of the torpedo ladle car process. The RMSE and R2 of the validation prediction of each model were obtained and compared. Results show that RF-SVDmodel, NN-SVDmodel and RNN-model exhibit some challenges such as potential overfitting and inconsistency in performance. The NN-model is the most promising option with robust performance, high generalization abilities, and competitive accuracy to the AnsysTwinbuilder. ...
tested using validation datasets of a full cycle prediction of the torpedo ladle car process. The RMSE and R2 of the validation prediction of each model were obtained and compared. Results show that RF-SVDmodel, NN-SVDmodel and RNN-model exhibit some challenges such as potential overfitting and inconsistency in performance. The NN-model is the most promising option with robust performance, high generalization abilities, and competitive accuracy to the AnsysTwinbuilder.
Using a multiphysics finite element framework based on Euler-Bernoulli beam theory, we develop a robust model to capture 3D deformation and apply the Wave Finite Element Method with Bloch boundary conditions to analyze dispersion relations. Mode identification is performed to investigate the effects of piezoelectricity on both in-plane and out-of-plane wave modes. Two distinct lattice configurations—rectangular with varying stretch ratios and hexagonal with different internal angles—are systematically analyzed to elucidate their wave propagation behaviour.
The findings reveal that piezoelectricity significantly impacts in-plane modes by reducing group velocity and modifying bandgap structures. Conversely, out-of-plane modes remain unaffected. Notably, piezoelectricity strongly suppresses energy flow in the direction of poling, enabling directional wave control and facilitating wave beaming phenomena. These results highlight the potential of piezoelectric metamaterials in achieving tunable wave propagation and directional energy flow.
To the best of our knowledge, this study marks the first comprehensive investigation into wave dispersion in piezoelectric truss metamaterials. The developed framework, rigorously validated against established results for purely mechanical metamaterials, demonstrates robustness and reliability. By advancing the theoretical modeling of piezoelectric truss lattices, this work bridges the gap between foundational research and transformative applications, including energy harvesting, wave control, and sensing technologies. These findings establish a solid platform for future exploration of 3D piezoelectric truss lattices and the strategic use of dynamic stimuli to engineer tunable bandgap materials.
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Using a multiphysics finite element framework based on Euler-Bernoulli beam theory, we develop a robust model to capture 3D deformation and apply the Wave Finite Element Method with Bloch boundary conditions to analyze dispersion relations. Mode identification is performed to investigate the effects of piezoelectricity on both in-plane and out-of-plane wave modes. Two distinct lattice configurations—rectangular with varying stretch ratios and hexagonal with different internal angles—are systematically analyzed to elucidate their wave propagation behaviour.
The findings reveal that piezoelectricity significantly impacts in-plane modes by reducing group velocity and modifying bandgap structures. Conversely, out-of-plane modes remain unaffected. Notably, piezoelectricity strongly suppresses energy flow in the direction of poling, enabling directional wave control and facilitating wave beaming phenomena. These results highlight the potential of piezoelectric metamaterials in achieving tunable wave propagation and directional energy flow.
To the best of our knowledge, this study marks the first comprehensive investigation into wave dispersion in piezoelectric truss metamaterials. The developed framework, rigorously validated against established results for purely mechanical metamaterials, demonstrates robustness and reliability. By advancing the theoretical modeling of piezoelectric truss lattices, this work bridges the gap between foundational research and transformative applications, including energy harvesting, wave control, and sensing technologies. These findings establish a solid platform for future exploration of 3D piezoelectric truss lattices and the strategic use of dynamic stimuli to engineer tunable bandgap materials.
The development of spinodoid topologies has allowed for extensive design exploration and property-structure investigations, important in the context of many potential future applications, such as bone biomaterials design. Until now, the majority of studies performed on spinodoid metamaterials were limited to the elastic regime. However, many materials, especially biological tissues such as bone, rarely exhibit purely elastic behaviour. Thus, exploring other material regimes is of great interest. This work attempts a finite element analysis of three different types of spinodoid structures: lamellar, cubic, and columnar. These structures' properties are based on a generalized Maxwell model for cortical bone. The study explores how tuning the design parameters influences their mechanical behaviour in a viscoelastic regime. ...
The development of spinodoid topologies has allowed for extensive design exploration and property-structure investigations, important in the context of many potential future applications, such as bone biomaterials design. Until now, the majority of studies performed on spinodoid metamaterials were limited to the elastic regime. However, many materials, especially biological tissues such as bone, rarely exhibit purely elastic behaviour. Thus, exploring other material regimes is of great interest. This work attempts a finite element analysis of three different types of spinodoid structures: lamellar, cubic, and columnar. These structures' properties are based on a generalized Maxwell model for cortical bone. The study explores how tuning the design parameters influences their mechanical behaviour in a viscoelastic regime.
Additive manufacturing (AM) is considered an environmentally friendly manufacturing process that builds solid 3D structures layer-by-layer from a computer-aided design (CAD), resulting in reduced waste compared to conventional subtractive manufacturing. However, a significant challenge in AM is the extensive use of plastics, which lack a standardized recycling process. Insufficient adhesion between printed layers contributes to waste generated with AM. Therefore, it is important to prioritize sustainability as a design parameter for future AM materials.
Dynamic Covalent Networks (DCNs) offer a potential solution by combining the desirable characteristics of thermosets with the recyclability of thermoplastics. DCNs undergo bond rearrangement reactions influenced by external stimuli such as heat or light, leading to changes in their topology. Bond exchange reactions can occur through two mechanisms: dissociative, involving separate steps of bond breaking and forming, or associative, involving simultaneous bond breaking and forming.
Recently, a phosphate triester-based DCN was developed, obtaining a neighboring β-hydroxyl group which could perform transesterification exchange reactions within the network via the formation of a cyclic phosphate triester intermediate in a dissociative manner. Based on this network rearrangement, we synthesized a network by reacting phosphoric acid with a diglycidyl ether to perform ring-opening reactions, creating a pendent β-hydroxyl functional group that assists in neighboring group transesterification.
The reversibility of the bond exchange reaction within the network was investigated using a heating and cooling cycle with variable temperature (VT) ³¹P solid-state NMR (SSNMR). Furthermore, the network was reprocessable using compression molding at 130 °C. Fast relaxation times of 5 seconds at 200 °C were observed. Additionally, frequency sweep and dynamic mechanical temperature analysis (DMTA) experiments showed profiles that were expected for a dissociative bond exchange mechanism.
However, an increase in the storage modulus at 150 °C was observed, indicating a curing process of the network. Subsequently, similar experiments were performed on the cured network, in which a reduction of the dynamic properties of the network was noted, with a stress relaxation time of 114 seconds at 200 °C. DMTA and frequency sweep experiments confirmed its increased storage modulus as well.
Nevertheless, the uncured network was reprocessable via extrusion at 200 °C. However, it required at least 50 minutes for the network to obtain a viscous flow behavior suitable for optimal extrusion.
Currently, the epoxy adhesive film is used for these types of joints. Adhesive co-bonding raises challenges due to the limited access to the mast. Challenges concerning surface preparation, low mechanical strength, reliability and labour intensity. Other techniques will be reviewed for the internally bonding limited access hollow tubes. The aim is that no new tooling is required, and the currently used materials and process parameters can be used. The thermoset resin used is an epoxy carbon fibre prepreg.
According to the literature study, potential joining techniques for this application are joining with partially cured thermosets and joining with fusion welding of thermoset composites. Differential scanning calorimetry measurements are performed to analyse the thermoset resin. Material characterisation is required to apply the joining methods with the currently used material and process parameters of Hall Spars. Process parameters for the curing model, and glass transition temperature model are derived. Using this material model allows creating the insight that partial cured joining with the thermoset resin of Hall Spars is not feasible. The structural integrity of the joined parts can not be guaranteed in the curing cycle.
Polyetherimide is used as thermoplastic material to apply the fusion welding technique on the thermoset. This thermoplastic film is co-cured with the thermoset and used as a coupling layer to join the two thermoset parts. Performing an interphase analysis with the scanning electron microscope indicates interphase formation at the PEI/epoxy interface. The higher the isothermal curing temperature, the thicker this interphase. Gelation causes the interphase formation to slow down. The interphase formation at the interface is not been observed as expected based on the literature review. This required a test if the interphase formation would have occurred at a smaller scale than the scanning electron microscopy could observe. This can be achieved by performing single-lap shear experiments. These joints are processed according to Hall Spars process parameters. Joints created with the fusion coupling technique with the PEI film show promising results. However, due to high void content, a lower ultimate lap shear strength is observed. Therefore it is advised to investigate a curing cycle with a second dwell phase. This changes the viscosity profile of the resin. Therefore allows the trapped air at the PEI/epoxy interface to leave the joint. This could potentially lead to more reliable joints. ...
Currently, the epoxy adhesive film is used for these types of joints. Adhesive co-bonding raises challenges due to the limited access to the mast. Challenges concerning surface preparation, low mechanical strength, reliability and labour intensity. Other techniques will be reviewed for the internally bonding limited access hollow tubes. The aim is that no new tooling is required, and the currently used materials and process parameters can be used. The thermoset resin used is an epoxy carbon fibre prepreg.
According to the literature study, potential joining techniques for this application are joining with partially cured thermosets and joining with fusion welding of thermoset composites. Differential scanning calorimetry measurements are performed to analyse the thermoset resin. Material characterisation is required to apply the joining methods with the currently used material and process parameters of Hall Spars. Process parameters for the curing model, and glass transition temperature model are derived. Using this material model allows creating the insight that partial cured joining with the thermoset resin of Hall Spars is not feasible. The structural integrity of the joined parts can not be guaranteed in the curing cycle.
Polyetherimide is used as thermoplastic material to apply the fusion welding technique on the thermoset. This thermoplastic film is co-cured with the thermoset and used as a coupling layer to join the two thermoset parts. Performing an interphase analysis with the scanning electron microscope indicates interphase formation at the PEI/epoxy interface. The higher the isothermal curing temperature, the thicker this interphase. Gelation causes the interphase formation to slow down. The interphase formation at the interface is not been observed as expected based on the literature review. This required a test if the interphase formation would have occurred at a smaller scale than the scanning electron microscopy could observe. This can be achieved by performing single-lap shear experiments. These joints are processed according to Hall Spars process parameters. Joints created with the fusion coupling technique with the PEI film show promising results. However, due to high void content, a lower ultimate lap shear strength is observed. Therefore it is advised to investigate a curing cycle with a second dwell phase. This changes the viscosity profile of the resin. Therefore allows the trapped air at the PEI/epoxy interface to leave the joint. This could potentially lead to more reliable joints.
In the context of this thesis, a computational model is developed, based on the framework of continuum damage mechanics and general plasticity theory. A potential function inspired by the Tsai-Wu criterion combined with a damage model is used to predict the strain and damage evolution. An exponential damage evolution law is introduced while the coupling of different damage modes is also considered. Moreover, an experimental investigation on the macroscopic mechanical behavior and damage mechanisms of C/C-SiC under in-plane onand off-axis loading conditions is performed. Specimens with 0\, 30\ and 45\ on- and off-axis angles were manufactured and tested under monotonic and cyclic tensile and compression loads. Furthermore, the microstructure of the pristine material and the fracture surfaces of the tested specimens are studied through scanning electron microscopy (SEM). A Bayesian optimization algorithm is finally used to optimize simultaneously the different material parameters based on the experimental test data.
The predicted stress-strain curves are in good agreement with the experimental curves, especially in the case of monotonic tensile loading. Both damage initiation and evolution are predicted accurately by the chosen laws and coupling functions. Moreover, the combination of the Tsai-Wu criterion with a damage evolution law is proven to predict the ultimate strength well. Fiber pull-out is observed in tension, while interlaminar and translaminar cracks in compression.
This study thus provides an accurate constitutive model, a complete mechanical characterization of the in-plane behavior and a better understanding of the fracture mechanisms of C/C-SiC. ...
In the context of this thesis, a computational model is developed, based on the framework of continuum damage mechanics and general plasticity theory. A potential function inspired by the Tsai-Wu criterion combined with a damage model is used to predict the strain and damage evolution. An exponential damage evolution law is introduced while the coupling of different damage modes is also considered. Moreover, an experimental investigation on the macroscopic mechanical behavior and damage mechanisms of C/C-SiC under in-plane onand off-axis loading conditions is performed. Specimens with 0\, 30\ and 45\ on- and off-axis angles were manufactured and tested under monotonic and cyclic tensile and compression loads. Furthermore, the microstructure of the pristine material and the fracture surfaces of the tested specimens are studied through scanning electron microscopy (SEM). A Bayesian optimization algorithm is finally used to optimize simultaneously the different material parameters based on the experimental test data.
The predicted stress-strain curves are in good agreement with the experimental curves, especially in the case of monotonic tensile loading. Both damage initiation and evolution are predicted accurately by the chosen laws and coupling functions. Moreover, the combination of the Tsai-Wu criterion with a damage evolution law is proven to predict the ultimate strength well. Fiber pull-out is observed in tension, while interlaminar and translaminar cracks in compression.
This study thus provides an accurate constitutive model, a complete mechanical characterization of the in-plane behavior and a better understanding of the fracture mechanisms of C/C-SiC.