Simulating Fractures with Bonded Discrete Element Method

Along with motion and deformation, fracture is a fundamental behaviour for solid materials, playing a critical role in physically-based animation. Many simulation methods including both continuum and discrete approaches have been used by the graphics community to animate fractures for various materials. However, compared with motion and deformation, fracture remains a challenging task for simulation, because the material's geometry, topology and mechanical states all undergo continuous (and sometimes chaotic) changes as fragmentation develops. Recognizing the discontinuous nature of fragmentation, we propose a discrete approach, namely the Bonded Discrete Element Method (BDEM), for fracture simulation. The research of BDEM in engineering has been growing rapidly in recent years, while its potential in graphics has not been explored. We also introduce several novel changes to BDEM to make it more suitable for animation design. Compared with other fracture simulation methods, the BDEM has some attractive benefits, e.g. efficient handling of multiple fractures, simple formulation and implementation, and good scaling consistency. But it also has some critical weaknesses, e.g. high computational cost, which demand further research. A number of examples are presented to demonstrate the pros and cons, which are then highlighted in the conclusion and discussion

Development of BEM for ceramic composites

It is evident that for proper micromechanical analysis of ceramic composites, one needs to use a numerical method that is capable of idealizing the individual fibers or individual bundles of fibers embedded within a three-dimensional ceramic matrix. The analysis must be able to account for high stress or temperature gradients from diffusion of stress or temperature from the fiber to the ceramic matrix and allow for interaction between the fibers through the ceramic matrix. The analysis must be sophisticated enough to deal with the failure of fibers described by a series of increasingly sophisticated constitutive models. Finally, the analysis must deal with micromechanical modeling of the composite under nonlinear thermal and dynamic loading. This report details progress made towards the development of a boundary element code designed for the micromechanical studies of an advanced ceramic composite. Additional effort has been made in generalizing the implementation to allow the program to be applicable to real problems in the aerospace industry

Crack growth in adhesively bonded joints under quasi-static and fatigue loading

Adhesively bonded joints are attracting increasing interest in the aerospace industry. However, incomplete knowledge of fatigue crack growth in adhesive bonds is a major concern to their application. This thesis investigates several aspects of crack growth in adhesively bonded joints. The influence of adhesive thickness on fatigue crack growth under mode I loading was addressed by a combination of experimental tests and numerical simulations. Increased crack growth was found in thicker specimens. This was explained as a result of increased energy available for crack growth in thicker adhesives, while the crack growth resistance was found not to be affected by the thickness. Formation of micro-cracks promoted by increased plasticity is thought to be the source of increased crack growth. Cohesive zone models were applied to the study of mode I and mode II quasi-static crack growth. A strong dependence on the input parameters was observed. In particular, the effect of viscous regularization on the solution was investigated. A proof of consistency of the viscous solution was proposed. It was shown that a low value of viscosity is needed to obtain consistent results. Finally, disbond arrest in bonded GLARE was studied by means of fatigue tests on bolted cracked lap shear specimens. The experiments evidenced a moderate decrease of the crack growth rate near the bolt. This was further investigated by numerical computations, which showed a significant change of the strain energy release rate around the bolt from mixed mode I/II to almost pure mode II. Outside this region, good predictions of the fatigue crack growth rate could be obtained by a combination of existing models from the literature. Extensive adherent cracking was observed, which led to the conclusion that crack arrest in GLARE comes from a balance of adhesive crack growth retardation and adherent cracking

Advances in the development of the discrete element method for excavation processes

This work presents new developments of the discrete element method improving e ciency and accuracy of modelling of rock-like materials, especially in excavation processes.Postprint (published version

Near Infrared Laser Sensor System for In-Line Detection of Conversion in UV-Cured Polymer Coatings

The work describes a method for the determination of the conversion by radical photopolymerization of acrylic coatings that is suitable for an in situ monitoring during the coating process.The applied method is based on the 1620 nm overtone absorption of the acrylate end group. The capability of the sensor to discriminate between polymerized and unpolymerized coatings on metal substrates down to a coating thickness of less than 16µm is demonstrated and and proved by reference measurements

A novel numerical framework for simulation of multiscale spatio-temporally non-linear systems in additive manufacturing processes.

New computationally efficient numerical techniques have been formulated for multi-scale analysis in order to bridge mesoscopic and macroscopic scales of thermal and mechanical responses of a material. These numerical techniques will reduce computational efforts required to simulate metal based Additive Manufacturing (AM) processes. Considering the availability of physics based constitutive models for response at mesoscopic scales, these techniques will help in the evaluation of the thermal response and mechanical properties during layer-by-layer processing in AM. Two classes of numerical techniques have been explored. The first class of numerical techniques has been developed for evaluating the periodic spatiotemporal thermal response involving multiple time and spatial scales at the continuum level. The second class of numerical techniques is targeted at modeling multi-scale multi-energy dissipative phenomena during the solid state Ultrasonic Consolidation process. This includes bridging the mesoscopic response of a crystal plasticity finite element framework at inter- and intragranular scales and a point at the macroscopic scale. This response has been used to develop an energy dissipative constitutive model for a multi-surface interface at the macroscopic scale. An adaptive dynamic meshing strategy as a part of first class of numerical techniques has been developed which reduces computational cost by efficient node element renumbering and assembly of stiffness matrices. This strategy has been able to reduce the computational cost for solving thermal simulation of Selective Laser Melting process by ~100 times. This method is not limited to SLM processes and can be extended to any other fusion based additive manufacturing process and more generally to any moving energy source finite element problem. Novel FEM based beam theories have been formulated which are more general in nature compared to traditional beam theories for solid deformation. These theories have been the first to simulate thermal problems similar to a solid beam analysis approach. These are more general in nature and are capable of simulating general cross-section beams with an ability to match results for complete three dimensional analysis. In addition to this, a traditional Cholesky decomposition algorithm has been modified to reduce the computational cost of solving simultaneous equations involved in FEM simulations. Solid state processes have been simulated with crystal plasticity based nonlinear finite element algorithms. This algorithm has been further sped up by introduction of an interfacial contact constitutive model formulation. This framework has been supported by a novel methodology to solve contact problems without additional computational overhead to incorporate constraint equations averting the usage of penalty springs

Advances in the development of the discrete element method for excavation processes

Modelling of granular materials, soils and rocks has been a challenging topic of investigation for decades. Classical continuum mechanics has been used to idealize soils and rocks, and numerical solution techniques such as finite element method (FEM) has been used to model these materials. Considering the idealization of the material, continuum mechanics allows the analysis of phenomena with discontinuous nature such as fracture in rock or soil via damage models. However, in more complex processes like rock milling or crushing, this kind of models are usually not suitable. Discrete models are more appropriate for problems with multiple discontinuities and particulate materials. The discrete element method (DEM) has been gaining popularity in analysis of granular materials, soils and rocks. Many aspects of this method still require more profound investigation. This thesis presents new developments of the discrete element method improving effi ciency and accuracy of modelling of rock-like materials, especially in excavation processes. All the numerical algorithms has been implemented in an in-house software, which was then used to run numerical examples. The basic formulation of DEM with linear elastic-perfectly brittle contact model is presented. The main di erence with other models found in the literature is the consideration of global sti ness and strength parameters that are constants in the whole model. The result of a simulations is strongly related with the con guration of the particle assembly used. Particle assemblies should be su ciently compact and ensure the isotropy to reproduce the physical properties of the modelled material. This thesis presents a novel technique for the generation of highly dense particle assemblies in arbitrary geometries, satisfying all the requirements for accurate discrete element simulations. One of the key issues in the use of the DEM is the estimation of the contact model parameters. A methodology is proposed for the estimation of the contact model parameters yielding required macroscopic properties of the material. The relationships between the contact model parameters and the mechanical properties of brittle materials, as well as the influence of the particles assembly con guration on the macroscopic properties, are analysed. A major di culty in the application of the DEM to real engineering problems is the high computational cost in simulation involving a large number of particles. The most common way to solve this is the use of parallel computing techniques, where multiple processors are used. As an alternative, a coupling scheme between DEM and the finite element method (FEM) is proposed in the thesis. Within the hybrid DEM/FEM model, DEM is only used in the region of the domain where it provides an advantage over a continuum-based approach, as the FEM. The coupling is dynamically adapted, starting with the whole domain discretized with FEM. During the simulation, in the regions where a high stress level are found, a change of modelling method from continuum FEM to the discrete DEM is employed. Finally, all the developments are applied to the simulation of a real excavation process. An analysis of the rock cutting process with TBM disc cutters is performed, where DEM and the DEM/FEM coupling technique presents an important advantage over other simulation techniques.La modelación de materiales granulares, terrenos y rocas ha sido un desafío para la investigación por décadas. La mecánica del continuo clásica ha sido utilizada para idealizar terrenos y rocas, y técnicas numéricas de solución, como el método de los elementos finitos (FEM), han sido usadas para modelar estos materiales. Considerando la idealización del material, la mecánica del continuo permite el análisis de fenómenos de naturaleza discontinua como la fractura en rocas y terreno mediante modelos de daño. Sin embargo, en procesos mas complejos como la molienda o trituración de roca, este tipo de modelos no suelen ser adecuados. Los modelos discretos son mas apropiados para problemas con múltiples discontinuidades y material particulado. El método de los elementos discretos (DEM) ha ido ganando popularidad en el análisis de materiales granulares, terrenos y rocas. Sin embargo, muchos aspectos de este método todavía requieren una investigación mas profunda. Esta tesis presenta nuevos desarrollos del método de los elementos discretos para mejorar su eficiencia y precisión en el modelado de materiales como roca, especialmente para procesos de excavación. Todos los algoritmos numéricos se han implementado en el programa propio, que ha sido utilizado para probar distintos ejemplos. La formulación básica del DEM, con un modelo lineal de contacto elástico perfectamente frágil ha sido utilizado en el presente trabajo. La principal diferencia con otros modelos de la literatura es la consideración de que los parámetros de rigidez y fuerzas máximas son valores globales y constantes en todo el modelo. El resultado de la simulación está fuertemente relacionado con la configuración del ensamblaje de partículas utilizado. El ensamblaje de partículas debe ser suficientemente compacto y asegurar la isotropía de las propiedades físicas del material modelado. La tesis presenta una nueva técnica para la generación de ensamblajes de partículas de alta densidad para geometrías arbitrarias, satisfaciendo todos los requisitos para una simulación con elementos discretos correcta. Uno de los temas clave en el uso del DEM es la estimación de los parámetros del modelo de contacto. Se propone una metodología para la estimación de los parámetros del modelo de contacto siguiendo las propiedades macroscópicas requeridas en el material Las relaciones entre los parámetros del modelo y las propiedades mecánicas de materiales frágiles, así como su la influencia de la configuración del ensamblaje de partículas son analizadas. Una gran dificultad en la aplicación del DEM en problemas reales de ingeniería es el alto costo computacional de simulaciones que consideran un gran número de partículas. La solución mas común para resolver esto es el uso de técnicas de computación paralela, donde se utiliza un gran número de procesadores. Como vía alternativa, un esquema acoplado entre el DEM y el FEM expuesto en la tesis. Con el modelo híbrido DEM/FEM, el DEM es usado solo en las partes del dominio donde presenta ventajas sobre el enfoque continuo del FEM. El acoplamiento puede ser adaptado dinámicamente, comenzando con todo el dominio discretizado con FEM, y durante la simulación, en las regiones donde se encuentran altos niveles de tensión, se emplea un cambio del método de simulación de continuo (FEM) a discreto (DEM). Finalmente, todos los desarrollos son aplicados a la simulación de un proceso excavación real. Se realiza un estudio del proceso de corte de roca con discos costadores, utilizados en tuneladoras, donde el DEM y la técnica de acoplamiento presentan una importante ventaja sobre otras técnicas de simulación

Near Infrared Laser Sensor System for In-Line Detection of Conversion in UV-Cured Polymer Coatings

The work describes a method for the determination of the conversion by radical photopolymerization of acrylic coatings that is suitable for an in situ monitoring during the coating process. The applied method is based on the 1620 nm overtone absorption of the acrylate end group. The capability of the sensor to discriminate between polymerized and unpolymerized coatings on metal substrates down to a coating thickness of less than 16µm is demonstrated and and proved by reference measurements

INFLUENCE AND ON-SITE ASSESSMENT OF LONG-TERM PRESTRESSING LOSSES ON SHEAR STRENGTH OF BRIDGE GIRDERS

La valutazione della capacità portante residua di un impalcato da ponte in calcestruzzo armato precompresso (CAP), dopo oltre 50 anni di esercizio può risultare difficile, soprattutto se esso mostra difetti di degrado a causa della scarsa/assenza di manutenzione o addirittura esibisce un quadro fessurativo anomalo. A seguito di una campagna di ispezioni svolta su oltre 400 ponti della Provincia di Brescia, è emerso come il 6% dei ponti in CAP, che corrispondono a circa il 44% del patrimonio infrastrutturale del gestore, mostra fessure a taglio nelle anime, anomale per le condizioni di carico a cui il ponte solitamente è sottoposto. La natura di queste fessure può essere di varia origine, tra cui una sovrastima in fase progettuale del contributo a resistenza a taglio fornito dalla precompressione dell’elemento. Indagando in campo e in letteratura, si è scoperto che ditte di prefabbricazione negli anni 70-80, realizzavano elementi precompressi con scarsa armatura a taglio, se non addirittura assenza in alcuni casi. Notizia ritrovata anche nella normativa tedesca antecedente gli anni 70 per la realizzazione di elementi precompressi. Questo perché, essendo l’elemento precompresso, parte della resistenza a taglio veniva destinata al contributo della precompressione e quindi si poteva «risparmiare» in termini di armatura trasversale. Il presente lavoro di ricerca intende approfondire due aspetti: la valutazione della bontà di alcune tecniche diagnostiche proposte in letteratura per la valutazione della precompressione in situ e l’influenza delle perdite da precompressione sul quadro fessurativo e sulla resistenza a taglio di travi da ponte in scala reale. Per questi scopi è stata condotta una campagna sperimentale indirizzata alla realizzazione di 4 travi in CAP della lunghezza di 10 m, sezione a “I” alta 80 cm e con staffatura minima. Due travi sono state realizzate con la tecnologia di trefoli pre-tesi aderenti, che differiscono tra loro per un 30% di livello di precompressione assegnata. Le altre due travi, identiche, sono state progettate con un sistema di post-tensione dei cavi (non aderenti) che permette la variazione del livello di precompressione in modo controllato, coprendo così più scenari di perdite a lungo termine. Su questi elementi sono stati applicati 3 metodi semi-distruttivi per la valutazione della precompressione in situ basati sul rilascio tensionale, ovvero: carota strumentata, tagli paralleli all’intradosso e provino tronco piramidale. In aggiunta viene proposto e validato un nuovo metodo, simile ai tagli paralleli all’intradosso, ma eseguito sull’anima. Su questo, dimostratosi il più affidabile tra i metodi testati, sono stati realizzati modelli ad elementi finiti in 2D e 3D in campo elastico. Successivamente, è stata eseguita una prova di carico in 3 punti su ciascuna trave, valutando l’evoluzione del quadro fessurativo con la tecnica del Digital Image Correlation (DIC) e confrontando i risultati con quelli ottenuti dalla simulazione numerica eseguita con il software VecTor 2, basato sulla Modified Compression Field Theroy (MCFT). Durante le varie fasi è stato valutato se un’indagine non distruttiva come l’identificazione dinamica possa cogliere danneggiamenti dovuti a lesioni a taglio e una riduzione della perdita di precompressione. Infine, essendo le formule di verifica della resistenza a taglio proposte dai modelli delle varie norme (e.g. Eurocodice 2, fib Model Code, CSA, ACI) molto conservative per gli elementi precompressi con staffe, viene proposta una formulazione analitica basata sulla bozza del fib Model Code 2020 LoA IIb. La formulazione è stata validata grazie all’applicazione ad alcune prove sperimentali presenti in letteratura, ottenendo buoni risultati.The assessment of the residual load-bearing capacity of a precast prestressed reinforced concrete bridge deck (PRC) after more than 50 years of service can be challenging, especially if it exhibits degradation defects due to poor or missing maintenance or even if it is affected by any kind of cracking due to stress. Following an inspection program carried out on over 400 bridges in the Province of Brescia, it emerged that 6% of the PRC bridges, corresponding to approximately 44% of the infrastructure assets of the bridge manager, exhibit shear cracks in the webs that are unexpected for the load conditions to which the bridge is usually subjected. The nature of these cracks can have various causes, including an overestimation of the contribution to shear resistance provided by the prestressing of the element during the design phase. From a survey in the field and the literature, it was discovered that precast companies in the 1970s-1980s produced prestressed elements with low shear reinforcement, or even without reinforcement in some cases. This was also found in German regulations before the 1970s for the production of prestressed elements. This is because, being a prestressed element, part of the shear resistance was given to the contribution of prestressing, providing a significant saving in terms of transverse reinforcement. This research aims to investigate two aspects: the evaluation of the reliability of some diagnostic techniques proposed in the literature for the assessment of in-situ prestressing and the influence of prestress losses on the crack pattern and shear strength of full-scale bridge beams. For these purposes, an experimental program was conducted on 4 PRC beams with a length of 10 m, an 80 cm high I-section, and minimum web reinforcement. Two beams were constructed pre-stresses strands, which differ from each other by 30% of the assigned level of prestress. The other two identical beams were designed with a system of post-tensioning stands that allows the variation of the level of prestress in a controlled way, covering several long-term loss scenarios during the experiments. On these elements, three semi-destructive methods for in-situ prestressing evaluation based on tension release were applied: core trepanning, saw-cut at intrados, and blunt pyramidal specimen. In addition, a new method, similar to the parallel saw cuts at intrados, but performed on the web, is proposed and assessed. On this method, which proved to be the most reliable among the tested methods, 2D and 3D finite element models were also performed and discussed. Subsequently, a 3-point loading test was performed on each beam, evaluating the evolution of the crack pattern with the Digital Image Correlation (DIC) technique and comparing the results with those obtained from the numerical simulation performed with the VecTor 2 software based on Modified Compression Field theory (MCFT). During the various loading phases, it was evaluated whether a non-destructive investigation such as dynamic identification could detect damage due to shear cracking and a reduction in prestress loss. Finally, since the verification formulas for shear strength proposed by the models of various codes (e.g. Eurocode 2, fib Model Code, CSA, ACI) are very conservative for prestressed elements with stirrups, an analytical formulation based on the draft of the fib Model Code 2020 LoA IIb is proposed. The formulation was validated against to some experimental tests in the literature, obtaining good results

Use of X-ray K-edge Tomography and Interferometry Imaging Techniques for the Studies of Brominated Flame Retardants

The work presented in this dissertation is based on the studies of flame retardancy performance of various formulations consisting of brominated flame retardants (BFRs: Saytex 8010 and Green Armor) and their synergist, antimony trioxide (Sb2O3) in high impact polystyrene (HIPS). Chemical flame retardants are incorporated in polymers to improve their flame inhibition for optimal applications in electrical and electronic devices, furniture, printers and more. These flame retardant polymer blends are studied using the Underwriters Laboratory vertical burn test (UL 94) and X-ray imaging techniques such as X-ray K-edge absorption tomography and X-ray grating interferometry. The UL 94 burn test is initially performed to assess the flammability behavior of flame retardant samples before X-ray imaging methods of burnt and pristine polymer blends. Because the UL 94 test bars are formulated with varying concentrations of a brominated flame retardant (Saytex 8010® or Green Armor®) and a synergist, Sb2O3 into a high impact polystyrene (HIPS), samples pass or fail the UL 94 plastics flammability test based on the burn time and other factors. Then, the X-ray imaging techniques are used to reveal internal features for the flame retardant performance during the burn. The Underwriters Laboratory 94 test bars are imaged with X-ray K-edge absorption tomography between 12 to 32 keV to assess the bromine and antimony concentration gradient across char layers of partially burnt samples. X-ray grating interferometry on partially burnt samples shows gas bubbles and dark-field scattering ascribed to residual blend inhomogeneity. In addition, X-ray single-shot grating interferometry is used to record X-ray movies of test samples during heating intended to mimic the UL 94 plastics flammability test. Key features such as char layer, gas bubble formation, micro-cracks, and dissolution of the flame retardant in the char layer regions are used in understanding the efficiency of the flame retardant and synergist. The samples that pass the UL 94 test have a thick, highly visible char layer, low bromine and antimony concentration in the char layer as well as an interior rich in gas bubbles. Growth of gas bubbles from flame retardant thermal decomposition is noted in the X-ray phase contrast movies