Connecting 3D printing parameters and mechanical properties of FDM polymers: experiments and modelling

Los polímeros tradicionales presentan un comportamiento mecánico no lineal y dependiente de la temperatura y velocidad de deformación. A este complejo comportamiento, hay que añadirle una dependencia extra con los parámetros de impresión cuando se trata con piezas fabricadas por impresión 3D. Todas estas dependencias hacen que la caracterización mecánica y el modelado de polímeros impresos por 3D sean complejos. Entre todas las tecnologías de impresión 3D, el modelado por deposición fundida (FDM) es la más común para trabajar con polímeros termoplásticos. Toda pieza fabricada por FDM presenta cierta porosidad y una respuesta mecánica anisótropa debidas al propio proceso de fabricación. Sin embargo, lejos de ver esto como una desventaja, FDM puede permitir fabricar piezas con propiedades mecánicas a medida mediante el control de los parámetros del proceso de impresión. Esta tesis doctoral profundiza en el estudio de la relación entre los parámetros de impresión y las propiedades mecánicas de piezas poliméricas fabricadas por FDM. Para ello, es necesario estudiar y comprender la mecánica y termodinámica del proceso de impresión para avanzar en el conocimiento de la relación última entre parámetros de impresión y propiedades mecánicas. El objetivo final de esta tesis ha sido desarrollar herramientas de análisis y un modelo constitutivo para predecir la respuesta mecánica de termoplásticos fabricados por impresión 3D. Para alcanzar este objetivo, esta tesis se ha dividido en tres bloques principales: i. El primer bloque proporciona una caracterización experimental de la mesostructura y del comportamiento mecánico de probetas fabricadas por FDM. Se ha estudiado la influencia del espesor de capa, la orientación de impresión y el número de capas. Además, a partir de observaciones experimentales, se han desarrollado expresiones analíticas para la predicción de la densidad de vacíos y las propiedades mecánicas. ii. En el segundo bloque se han desarrollado modelos térmicos y de sinterización que permiten analizar la mecánica y termodinámica del proceso de impresión. Estos modelos han permitido estudiar el proceso de unión entre filamentos que tiene lugar durante la fabricación y la influencia de los parámetros de impresión sobre la mesostructura de piezas fabricadas por FDM. La combinación de estos modelos con las expresiones analíticas propuestas en el bloque previo, han permitido crear una metodología para predecir las propiedades mecánicas en función de los parámetros de impresión. Esta metodología proporciona los fundamentos sobre las características de los termoplásticos fabricados por FDM que motiva futuros modelos para la optimización de las propiedades mecánicas en función de los requisitos de servicio. iii. Finalmente, basado en observaciones experimentales, el tercer bloque presenta un modelo anisótropo visco-hiperelástico del continuo para polímeros fabricados por FDM. Este modelo incluye la dependencia de la respuesta mecánica con los parámetros de impresión analizados. La metodología propuesta en esta tesis se ha aplicado a un caso específico, incluyendo experimentación y simulaciones computacionales sobre el Acrilonitrilo Butadieno Estireno (ABS). Este termoplástico ha sido elegido, sin pérdida de generalidad, por ser uno de los materiales más empleados en FDM.Traditional polymers present a complex mechanical behaviour by means of nonlinearity, temperature and rate dependencies. To these complexities, we need to add extra dependencies on printing process parameters when dealing with components manufactured by 3D printing. All these dependencies together make the characterisation and modelling of 3D printed polymers very challenging. Among all the 3D printing techniques, fused deposition modelling (FDM) is the most common for thermoplastic components. FDM components are porous materials with an anisotropic behaviour derived from the manufacturing process. However, despite it can seem a disadvantage, customised mechanical properties can be obtained by controlling these variables through the printing process. This doctoral dissertation deepens in the understanding of the relationship between the printing parameters and mechanical properties of FDM polymers. In this regard, it is also necessary to deal with the study of the mechanics and thermodynamics of the printing process. The final aim of this thesis is to provide analytical tools and a continuum mechanics constitutive model to predict the mechanical response of 3D printed thermoplastics. To this end, this thesis is composed of three principal blocks: i. The first block provides an experimental characterisation of the mesostructure and mechanical performance of FDM components. The influence of layer height, raster orientation and number of layers is studied. On the other hand, from the experimental observations, analytical expressions for the prediction of the void density and mechanical properties are developed. ii. In the second block, the mechanics and thermodynamics of the FDM process are studied through the development of thermal and sintering models. These permit to analyse the bonding process that takes place during the manufacturing process and study the influence of the printing parameters on the mesostructure of the FDM components. The combination of these models with the analytical expressions developed in the previous block allows for the proposal of a methodology to predict the final mechanical properties as a function of the printing parameters. This methodology provides the fundamentals of FDM thermoplastic characteristics to motivate further models and tools for optimisation of the mechanical properties as a function of the final requirements in service. iii. Finally, based on experimental observations, an anisotropic viscous-hyperelastic constitutive model for FDM polymers is developed in the third block. This model includes the dependence of the printing parameters on the mechanical behaviour. The methodology proposed in this thesis has been applied to a specific case, including experiments and computational simulations on Acrylonitrile Butadiene Styrene (ABS). This FDM thermoplastic has been chosen, without loss of generality, because it is one of the most common solutions for FDM components.Programa de Doctorado en Ingeniería Mecánica y de Organización Industrial por la Universidad Carlos III de MadridPresidente: Ramón Eulalio Zaera Polo.- Secretario: David Ángel Cendón Franco.- Vocal: Alexis Rusine

A new constitutive model for polymeric matrices: Application to biomedical materials

Semi crystalline polymeric composites are increasingly used as bearing material in the biomedical sector, mainly because of their specific mechanical properties and the new advances in 3D printing technologies that allows for customised devices. Among these applications, total or partial prostheses for surgical purposes must consider the influence of temperature and loading rate. This paper proposes a new constitutive model for semi-crystalline polymers, commonly used as matrix material in a wide variety of biomedical composites, that enables reliable predictions under a wide range of loading conditions. Most of the recent models present limitations to predict the non-linear behaviour of the polymer when it is exposed to large deformations at high strain rates. The proposed model takes into account characteristic behaviours of injected and 3D printed thermoplastic polymers such as material hardening due to strain rate sensitivity, thermal softening, thermal expansion and combines viscoelastic and viscoplastic responses. These viscous-behaviours are relevant for biomedical applications where temperature evolution is expected during the deformation process due to heat generation induced by inelastic dissipation, being essential the thermo-mechanical coupling consideration. The constitutive model is formulated for finite deformations within a thermodynamically consistent framework. Additionally, the model is implemented in a finite element code and its parameters are identified for two biomedical polymers: ultra-high-molecular-weight polyethylene (UHMWPE) and high density polyethylene (HDPE). Finally, the influence of viscous behaviours on dynamic deformation of semi-crystalline polymeric matrices is analysed

Multi-impact mechanical behaviour of short fibre reinforced composites

High velocity transverse impact on reinforced composites is a matter of interest in the automotive, aeronautical and biomedical sectors. Most existing studies have addressed this problem by single isolated impacts; however, this work deals with the distinction between single, sequential and simultaneous impacts on composite structures. This paper proposes an experimental methodology to study the mechanical behaviour of materials under single and multi-impact loadings. The overall objective is to investigate the mechanical response of short carbon fibre reinforced PEEK when is subjected to single and multiple high velocity impacts. Experimental tests are conducted covering impact velocities from 90 m/s to 470 m/s. Energy absorption, damage extension and failure mechanisms are compared to assess additive and cumulative effects in high velocity impact scenarios. Experimental results show that the specific deformation and fracture mechanisms observed during multi-hitting events change with impact velocity. Compared to the behaviour of unreinforced thermoplastics, short fibre reinforced composites present significant limitations at velocities close to the ballistic limit, but multi-hit capability is observed at high impact velocity when the damage is mainly local. As key conclusion, the ballistic limit obtained in single impact test cannot be extrapolated to sequential and simultaneous tests. Multi-impact tests, especially close to the ballistic limit, are necessary to guarantee the structural integrity of composite structures in realistic impact scenarios.The researchers are indebted to Ministerio de Economía y Competitividad de España (Project DPI2014-57989-P) and Vicerrectorado de Política Científica UC3M (Project 2013-00219-002) for financial support

A continuum constitutive model for FDM 3D printed thermoplastics

Fused deposition modelling (FDM) is the most common additive manufacturing technology used for thermoplastic components. This layers-based manufacturing process results into direct links between printing parameters and the polymer mesostructure by means of porosity and structural anisotropy. These dependencies along with other features of thermoplastic polymers (i.e., nonlinearities, viscous and thermal responses) makes its constitutive modelling very challenging. This work distances from studies that model the 3D printing process. Instead, we aim at complementing such approaches with a continuum model to describe the macroscopic behaviour of FDM thermoplastics while preserving links with printing parameters. Prior to the modelling conceptualisation, experimental characterisation tests are conducted on ABS specimens to evaluate the influence of printing parameters on the macroscopic mechanical response. The physical fundamentals behind the deformation and failure mechanisms are identified and motivate the new constitutive model. This model is formulated for finite deformations within a thermodynamically consistent framework. The model accounts for: nonlinear response; anisotropic hyperelasticity related to a transversely isotropic distribution of porous; strain rate dependency; macroscopic stiffness dependent on 3D printing processing. Finally, the model is numerically implemented and calibrated for ABS with original experiments, demonstrating its suitability.The authors acknowledge support from Ministerio de Ciencia, Innovación y Universidades, Spain, Agencia Estatal de Investigación y Fondo Europeo de Desarrollo Regional, Spain, como entidades financiadoras (RTI2018-094318-B-I00). D.G.-G., S.G.-H. and A.A. acknowledge support from Programa de Apoyo a la Realización de Proyectos Interdisciplinares de I+D para Jóvenes Investigadores de la Universidad Carlos III de Madrid (BIOMASKIN-CM-UC3M). D.G.-G. acknowledges support from the Talent Attraction grant (CM 2018 - 2018-T2/IND- 9992) from the Comunidad de Madrid, Spain

Design of FDM 3D printed polymers: An experimental-modelling methodology for the prediction of mechanical properties

Additive manufacturing technologies provide new opportunities for the manufacturing of components with customisable geometries and mechanical properties. In particular, fused deposition modelling (FDM) allows for customisable mechanical properties by controlling the void density and filament orientation. In this work, a methodology is provided for the prediction of the mechanical properties and mesostructure of FDM polymers. To this end, we propose a computational framework for the simulation of the printing process taking as input data specific manufacturing parameters and filament properties. A new two-stage thermal and sintering model is developed to predict the bond formation process between filaments. The model predictions are validated against original experimental data for acrylonitrile butadiene styrene (ABS) components manufactured by FDM. A parametric study is finally presented to interpret the effects of different manufacturing parameters on the mechanical performance of ABS specimens. Overall, the proposed framework offers new avenues for the design of 3D printed polymeric components with custom properties, directly in terms of manufacturing settings.D. Garcia-Gonzalez acknowledges support from the Talent Attraction grant (CM 2018 - 2018-T2/IND-9992) from the Comunidad de Madrid. S. Garzon-Hernandez, D. Garcia-Gonzalez and A. Arias acknowledge support from Ministerio de Ciencia, Innovación y Universidades, Agencia Estatal de Investigación y Fondo Europeo de Desarrollo Regional,comoentidades financiadoras (RTI2018-094318- B-I00)

A continuum mechanics constitutive framework for transverse isotropic soft tissues

In this work, a continuum constitutive framework for the mechanical modelling of soft tissues that incorporates strain rate and temperature dependencies as well as the transverse isotropy arising from fibres embedded into a soft matrix is developed. The constitutive formulation is based on a Helmholtz free energy function decoupled into the contribution of a viscous-hyperelastic matrix and the contribution of fibres introducing dispersion dependent transverse isotropy. The proposed framework considers finite deformation kinematics, is thermodynamically consistent and allows for the particularisation of the energy potentials and flow equations of each constitutive branch. In this regard, the approach developed herein provides the basis on which specific constitutive models can be potentially formulated for a wide variety of soft tissues. To illustrate this versatility, the constitutive framework is particularised here for animal and human white matter and skin, for which constitutive models are provided. In both cases, different energy functions are considered: Neo-Hookean, Gent and Ogden. Finally, the ability of the approach at capturing the experimental behaviour of the two soft tissues is confirmed.The researchers are indebted to the Ministerio de Economía y Competitividad de España (Projects DPI2014-57989-P and DPI2017-85970-R) for the financial support which permitted to conduct part of this work. D.G.-G. and A.J. acknowledge funding from the European Union’s Seventh Framework Programme ( FP7 2007-2013 ) ERC Grant Agreement No. 306587

Innovative acrylic thermoplastic composites versus conventional composites: Improving the impact performances

This study focuses on the benefits to the mechanical performance against impact loading offered by glass fiber reinforced (GFR) acrylic thermoplastic polymers, based on new room temperature cure methyl-methacrylate (MMA) matrix. Glass fiber reinforcement is a common solution for a wide variety of engineering applications based on thermoset matrices. However, its use presents some disadvantages such as adequate control of manufacturing temperature, problematic recycling and low damage tolerance. In contrast, acrylic polymers presents a high potential as an alternative matrix for thermoset composites due to their superior mechanical properties, manufacturing at low temperatures and recycled possibilities. In order to compare the mechanical behavior under impact loading of acrylic thermoplastic composites versus conventional composites, Charpy impact test and low velocity impact tests are carried out. The GFR acrylic laminate composites considered are compared to conventional composites manufactured with epoxy and polyester resins in terms of impact resistance and damage evolution. This study covers an impact energy rate from 10 to 60 J and analyses the maximum load, deflection, absorbed energy and associated damage, showing a better performance of the new GPR. acrylic thermoplastic polymers with respect to conventional GFR composites

Conductive 3D printed PLA composites: On the interplay of mechanical, electrical and thermal behaviours

Additive manufacturing (AM) techniques represent a real challenge to manufacture novel composites with cou- pled multifunctional properties. This work focuses on the mechanical, electrical and thermal behaviours of 3D printed polymeric composites of polylactic acid (PLA) filled with carbon black (CB) conductive particles. The incorporation of conductive particles within the polymer matrix allows for programmable conduction paths via the printing process, whose electric properties are intimately coupled to thermo‐mechanical processes. In this study, samples were prepared using a fused deposition modelling (FDM) printer, controlling the filament ori- entation to manufacture three different types: longitudinal (0°); transverse (90°); oblique (±45°) printing ori- entations. Different types of multifunctional characterisation have been made: (i) electro‐thermal tests, evaluating the influence of electrical conductivity on the sample temperature due to Joule’s heating; (ii) thermo‐electrical tests, analysing the influence of temperature on the DC resistance of the samples; (iii) mechano‐electrical tests, analysing the effect of mechanical deformation on the specimens’ electric resistance. The results show a strong dependence of printing direction on the material properties of 3D printed conductive‐ PLA and identify strong thermo‐electro‐mechanical interplays. The results of this work will contribute to the AM progress in functional electro‐mechanical components with potential applications in biosensing devices, composite sensors, 3D electrodes and soft robotic industry.The authors acknowledge financial support from Ministerio de Ciencia, Innovación y Universidades, Spain, Agencia Estatal de Inves- tigación y Fondo Europeo de Desarrollo Regional, Spain, under Grant number RTI2018‐094318‐BI00. D.G.‐G., S.G.‐H. and A.A. acknowledge support from Programa de Apoyo a la Realización de Proyectos Inter- disciplinares de I + D para Jóvenes Investigadores de la Universidad Carlos III de Madrid (BIOMASKIN‐CM‐UC3M). D.G.‐G. acknowledges support from the Talent Attraction grant (CM 2018 ‐ 2018‐T2/ IND9992) from the Comunidad de Madrid, Spain. Some tests were made in the High Voltage Research and Test Laboratory (LINEALT) at UC3M

Use of microhabit by anurans in an intervened dry forest fragment of the Magdalena medio area in Guarinocito, Caldas

Se compararon los ensamblajes de anuros de tres hábitats (bosque, lago y cantera) con diferente grado de perturbación antrópica presentes en un fragmento de bosque seco en la hacienda La Española, corregimiento de Guarinocito, Caldas. Se realizaron muestreos de recorridos extensivos usando el método de registro por relevamiento por encuentros visuales (REV), a partir de los cuales se registró la abundancia y uso del microhábitat para cada una de las especies dentro de los diferentes hábitats, y se realizó el registro de variables ambientales descriptoras del sitio de muestreo, tales como temperatura, humedad relativa y perímetro de espejos de agua. Se encontraron 13 especies de anuros, entre las cuales Engystomops pustulosus registró la mayor abundancia relativa, con una tasa de captura de 5,16 individuos por hora. Se registraron diferencias significativas en la diversidad entre los hábitats de estudio, siendo menor en el bosque, en donde hubo menor riqueza, pero mayor dominancia. La tasa de recambio de especies entre hábitats fue del 71%, exhibiendo Lago y Cantera la mayor similitud en composición de la anurofauna. El uso de sustrato por parte de cada especie en los diferentes hábitats registró diferencias significativas (X², p<0,05) y solo dos especies (Leptodactylus fragilis y L. colombiensis) presentaron alta variabilidad en los sustratos usados.The anuran assemblages in three habitats (forest, lake and quarry) with different degrees of human disturbance present in a dry forest fragment in the Farm La Española, Guarinocito, Caldas, were compared. Extensive routes samplings were conducted using visual encounter surveys (VES), from which abundance and microhabitat use were recorded for each one of the species within the different habitats, and the record of environmental variables describing the sampling area such as temperature, relative humidity, and water ponds perimeter, was carried out. A total of 13 anurans species were found, among which Engystomops pustulosus recorded the most relative abundance with a catch rate of 5.16 individuals per hour. Significant differences were found in diversity between the studied habitats being lower in the forest where there were lower richness but better dominance. The rate of replacement of species between habitats was 71%, showing pond and quarry the greater similarity in anurofauna composition. The use of substrate by each species was different between habitats (X², p<0.05) and only two species (Leptodactylus fragilis and L. colombiensis) showed high variability in the modified habitats