Styrene-Butadiene Rubber (SBR) is a form of rubber compound that is widely used in the tire industry. This is due to some of their unique characteristics such as high strength, high elasticity and resilience, high abrasion resistance, ability to absorb and dissipate shocks and vibrations, low plastic deformation, high deformation at low levels of stresses, and high product life. One of the most important and often overlooked causes of SBR degradation and eventual tire failure is 'rubber aging.' It can be defined as an alteration in the mechanical, chemical, physical, or morphological properties of elastomers under the influence of various environmental factors during processing, storing and use. Some of these environmental factors are humidity, ozone, oxygen, temperature, radiation (UV rays), etc. This study focuses on the effects of two of these factors acting in tandem, oxygen and temperature. In the past, studies have been conducted to observe the effects of rubber aging on the mechanical and wear properties of rubber. Studies have also been conducted to study the reactions taking place in rubber during aging and changes in its chemical structure. These studies use different modelling techniques and experiments to quantify the effects of aging. In this study, a material aging model that can predict the hyperplastic response of styrene-butadiene rubber (SBR) was mathematically developed using an integrated testing and continuum damage model framework. Coupling between the mechanical changes of SBR to the change in the chemical properties, specifically crosslink density (CLD) was also investigated. SBR dogbone shaped samples were accelerated aged in an aging oven at various temperatures and aging periods. Subsequently, hyperelastic tests were conducted to obtain the high strain response taking the 'Mullin's effect' into consideration. These responses were calibrated to different hyperelastic material models and the Arruda-Boyce model was chosen, due to its stable behavior and optimal fit. An aging evolution function was developed based on the variation in the model coefficients. This damage model is able to predict the hyperelastic response of SBR as it ages. A user material subroutine (UMAT) was also implemented in Abaqus based on the obtained aging evolution function to predict the stress response of SBR for varied applications. Additionally, to couple the chemical variations with the hyperelastic response, the rubber structure and composition was probed using Fourier-transform infrared spectroscopy (FTIR). The degradation of additives and SBR polymer chains were analyzed microscopically to explain the impact on the macroscopic properties. This study helps to correlate the change in crosslink density to ameliorate mechanical properties, such as strain at break, modulus, and stiffness. The effects of aging on the viscoelastic properties of SBR were also studied. Dynamic Mechanical Analysis (DMA) was used to characterize the viscoelastic response. Master curves of storage and loss modulus were generated using the time-temperature superposition principle (TTSP). The friction coefficient was estimated from the storage and loss modulus using a simplified form of the Persson equation [1]. CLD was also estimated from DMA data. Wear experiments were conducted on the Dynamic Friction Tester (DFT) for various aging conditions. The estimated friction coefficient was compared to the one from the experiments. Archard's law was used to correlate the frictional energy to the volume loss during wear experiments. Correlation between the wear and the viscoelastic properties of SBR is also studied. Finally, the lifetime of SBR for various aging temperatures is predicted using various models. [1] M. Ciavarella, "A Simplified Version of Persson's Multiscale Theory for Rubber Friction Due to Viscoelastic Losses," J. Tribol., vol. 140, no. 1, 2018, doi: 10.1115/1.4036917. / Master of Science / Elastomers or rubbers are they are generally referred to are an indispensable part of human life. They are made up of long-chain polymer units linked to one other through crosslinks. This peculiar morphology of rubbers is what gives them their unique characteristics. There are as many as 40,000 known products that use some form of rubber as the primary raw material. Apart, from this, they are also widely used in aviation and aerospace, automobiles, dampers and absorbers, civil engineering, electronics, medical, toys, clothing, sports, footwear, and so on. This is due to some of their unique characteristics such as high strength, high elasticity and resilience, high abrasion resistance, ability to absorb and dissipate shocks and vibrations, low plastic deformation, high deformation at low levels of stresses, and high product life. Over the last couple of years, it has also played a pivotal role in personal protective equipment (PPE) and masks worn by billions of people and frontline workers all over the globe. The fact that rubber is included in the EU's list of critical raw materials highlights its global importance. However, over the past several years, the rubber supply has dwindled. COVID-19 also caused disruptions in the supply chain of rubber. As the effects of COVID-19 are fading, there has been a spike in the demand for rubber; the primary reason being automotive tires! Even though substantial research is being conducted to try and replace rubber as a raw material with synthetic alternatives such as polyurethane, the excellent blend of damping, friction and wear characteristics, heat dissipation provided by natural rubber cannot be replicated by any of these laboratory compounds. Hence, at this time, there is an increased need to conserve and improve the longevity of rubber compounds. Styrene-Butadiene Rubber (SBR) is a form of a rubber compound that is widely used in the tire industry. One of the most important and often overlooked causes of SBR degradation and eventual tire failure is 'rubber aging.' It can be defined as an alteration in the mechanical, chemical, physical, or morphological properties of elastomers under the influence of various environmental factors during processing, storing and use. Some of these environmental factors are humidity, ozone, oxygen, temperature, radiation (UV rays), etc. This study focuses on the effects of two of these factors acting in tandem, oxygen and temperature. In the past, studies have been conducted to observe the effects of rubber aging on the mechanical and wear properties of rubber. Studies have also been conducted to study the reactions taking place in rubber during aging and changes in its chemical structure. These studies use different modelling techniques and experiments to quantify the effects of aging. The present study aims to model changes in the hyperelastic (large stretching) behavior of SBR using a Continuum Damage Mechanics (CDM) approach. This mathematical model is translated into ABAQUS, a finite element analysis software to study the mechanical response of components with various geometries and loading conditions. Secondly, the effects of aging on the viscoelastic behavior of SBR is studied. This helps us to estimate the cross-link density (CLD) as well as the friction coefficient of SBR as it is aged. The impact of aging on the wear and friction properties of SBR is studied experimentally. Finally, using various mechanical and chemical models the lifetime of SBR is estimated for various aging temperatures. Thus, the end goal of the study is to drive the development of new rubber compounds that will help improve the service life of rubbers and also have a positive impact on the environment.

Ultrasonic measurements of the preload in bolted joints is a very accurate method since it does not depend on the friction and other factors which cause difficulties for common methods. The ultrasonic method works by emitting an ultrasonic pulse into the bolt which is reflected at the end and returned to the transducer, the change in the time of flight (TOF) can be related to the elongation of the bolt and therefore the preload. One must account for the acoustoelastic effect which is the change in sound speed due to an initial stress state. The goal of this thesis project was to implement a Murnaghan hyperelastic material model in order to account for the acoustoelastic effect when conducting a numerical simulation using the finite element method (FEM). An experiment was also performed to validate the numerical simulation. The DeltaTOF as a function of a tensile force was obtained for an M8 and M10 test piece from the experiment. The material model was implemented by creating a user subroutine written in Fortran for the explicit solver Radioss. Hypermesh was used to set-up the numerical simulation. The material model has shown an expected behavior with an increased sound speed with compressive stresses and a decreased speed with tensile stresses. The numerical simulation showed a good correspondence to the experimental results. / Ultraljudsmätning av klämklraften i skruvförband är en väldigt noggrann metod eftersom att metoden inte påverkas av friktion eller andra faktorer som innebär svårigheter för vanliga metoder. Ultraljudsmetoden fungerar genom att skicka in en ultraljudsvåg i skruven som reflekteras i botten och återvänder tillbaka till sensorn. Skillnaden i tiden för ekot att återvända kan relateras till förlängningen av skruven och därmed klämkraften. Det är viktigt att ta hänsyn till den akustoelastiska effekten, som är fenomenet där ljudhastigheten av en våg i en solid förändras med spänningstillståndet. Målet med det här arbetet är att implementera en hyperelastisk Murnaghan modell som tar hänsyn till den akustoelastiska effekten med FEM simuleringar. Ett experiment har också genomförts för att validera beräkningsmodellen. Tidsfördröjningen som en funktion av förspänningskraften togs fram för ett M8 och M10 provobjekt. Murnaghans hyperelastiska materialmodell implementerades genom att skapa ett användar material skriven i programmeringsspråket Fortran för den explicita lösaren Radioss. Hypermesh användes för att ställa upp FEM simuleringen. Materialmodellen har visat ett väntat beteende med en ökad ljudhastighet med tryckspänningar och minskad ljudhastighet med dragspänningar. Beräkningsmodellen visade en god överenstämmelse med resultatet från experimentet.

This thesis is dedicated to the multiaxial constitutive modeling of basal textured wrought magnesium (Mg) alloys. Twin-roll-cast Mg alloys typically exhibit a significant basal texture as a result of the manufacturing process. Depending on the direction of load, plastic deformation can occur by activating dislocation slip or twinning. The pronounced basal texture and load direction-dependent plastic deformation mechanisms lead to anisotropic and asymmetric plastic material behavior. Furthermore, in recent research, a pronounced strain localization has been observed in the form of macroscopic bands of twinned grains (BTGs), where the compressive strain is approximately 600 % higher than in adjacent areas. The fatigue behavior is significantly affected by BTGs. However, their local elasto-plastic material behavior remains largely unexplored both experimentally and numerically. The objective of this study is to conduct an experimental analysis of the global and local elasto-plastic material behavior for uniaxial and biaxial stress states, with particular emphasis on anisotropic and asymmetric properties. Additionally, the study aims to establish a constitutive model for finite element method simulation, which encompasses both the anisotropic and asymmetric material behavior as well as the pronounced strain localization. Therefore, strain-controlled uniaxial tensile and compression tests were performed in both the rolling (RD) and transverse directions (TD). In-situ strain field measurements were performed using digital image correlation (DIC) to locally evaluate the three-dimensional evolution of plastic deformations in the BTGs. The results of the uniaxial compression tests demonstrate that macroscopic plastification takes place exclusively in the BTGs, whereas the adjacent areas are mainly elastically deformed. Additionally, the BTGs exhibit a strong anisotropy in lateral plastic strains. The plastic Poisson’s ratio is 0 in the sheet mid-plane and 1 perpendicular to the sheet mid-plane. It can be stated that this is a fundamental plastic property for twinning. The deviation between the measured plastic Poisson’s ratios and the theoretical values is dependent on the intensity of the basal texture. In contrast, the gauge area of the uniaxial tensile sample exhibits a homogeneous strain field with nearly isotropic material behavior. Both the tensile and compression tests demonstrate plastic volume constancy. A testing device is presented that allows not only biaxial tensile tests but also, for the first time, biaxial compression tests on thin-walled sheets. A cruciform sample was developed that ensures an almost homogeneous biaxial stress state in the gauge area. Furthermore, a novel anti-buckling device was developed to prevent the biaxial sample from buckling under compressive loading. In analogy to the uniaxial and shear tests, in-situ strain field measurements were performed in the gauge area to study the evolution of plastic deformations. The initial yield loci for calibrating the yield surface were determined using a numerical-experimental method. The findings from the equi-biaxial compression test prove that BTGs form similar to the uniaxial compression tests. Initially, the BTGs form perpendicular to the TD and exhibit a plastic Poisson’s ratio in the sheet plane of approx. 0. The initial biaxial yield stresses correspond approximately to the uniaxial compressive yield stresses. It was proven that the elasto-plastic material behavior can be modeled with a convex yield surface. At higher loads, BTGs are also formed perpendicular to the RD. A three-dimensional, phenomenological, constitutive model was developed that accounts for the anisotropic and asymmetric plastic material behavior and the potential strain localization. The constitutive model was calibrated through the uniaxial and biaxial tests and subsequently validated with ten different FEM simulations. Tests of the constitutive model using one finite element show that the stress-strain relationship and plastic deformations can be validly calculated, for example, a total of four uniaxial and three biaxial stress states. The FEM simulation of a uniaxial compression sample shows that the discontinuous strain localization can be simulated on the macroscopic scale for the first time. This results in macroscopic bands with high plastic strain (BPDs) whose shape, volume, and propagation match those of the BTGs. Moreover, local state variables, such as stresses, plastic strains, and flow vectors in the BPD, correspond remarkably well to the experimentally measured state variables in the BTG. The validation of a notched specimen from the literature demonstrated that the cross-shaped strain localization in the form of BTGs can be reasonably well calculated. The accurate determination of the field quantities in the BTGs through FEM simulation, which is crucial for fatigue modeling, underscores the capability of the constitutive model.