The demand for higher velocities and heavier axle loads for freight trains leads to higher forces on the railway wheels which in turn lead to an increase in stresses on and below the surface of the wheel-rail contact. By time, this induces wear on the wheels which consequently lead to higher maintenance costs and in some cases accidents. The ability to predict the evolution of wheel profiles due to uniform wear has been demonstrated with a rather accurate precision in most operational conditions. These wear models are based on wear coefficients and since they are not usually valid for real operational conditions, the models are generally calibrated against real-life scenarios in order to adjust the coefficients from test conditions to real-life lubrication conditions. This engineering approach can be useful in prediction of wear in systems where the materials and contact conditions do not vary. However, when addressing material development focused on reducing specific damage modes, the approach is of limited use because the obtained wear coefficients are not directly related to material properties. Therefore, attempts towards developing physical fracture propagation models that relates to the contact conditions and material properties have been made. The purpose has been to retrieve vital information about where a fracture initiates and how it propagates. In the long run, it is of great interest to be able to attain information about how a material particle is removed from the contact surface. Studies for this type of model was done in the 70’s and 80’s mainly with pin-disk experiments but has not been utilized in the specific field of wheel-rail contact. The thesis is part of the FR8RAIL project arranged by the European rail initiative Shift2Rail. Literature studies have been the basis for the thesis in order to gain vital insights into fracture mechanics and other related fields. The physical fracture propagation models have been constructed in the FE software Abaqus with the implementation of the XFEM. For the 2D model, the fracture initiates at the top of the implanted inclusion when the friction coefficient is  and propagates upwards a few elements. For , the fracture initiates at the right surface boundary where the pressure distribution and traction is applied. The fracture propagation angle increases relative to the surface as the friction coefficient value is increased. The fracture for the 3D model extends broader compared to the 2D model at the top of the inclusion in the case of . The fracture initiates at the same surface location as for the 2D model for . The fracture propagation is however non-existent due to convergence problems. The FE-models constructed are initial steps towards analysing the fracture propagation and closely related phenomena for a railway freight wheel in detail. At the end of the thesis, the simplified models give mainly information about the fracture initiation, propagation and its patterns. From this first phase, further adjustments and improvements can take place in order to eliminate the margins of error. In the long run, fully integrated models with further implementations such as detailed microstructure for the contact conditions, plastic behaviour for the material, and complete three-dimensional models can finally be employed. / Efterfrågan på högre hastigheter och tyngre axelbelastningar för godståg leder till högre krafter på järnvägshjulen som i sin tur leder till ökade spänningar på och under ytan vid hjul-räl-kontakten. Med tiden induceras slitage på hjulen som följaktligen leder till höga underhållskostnader och i vissa fall olyckor. Förmågan att förutse utvecklingen av hjulprofiler på grund av enhetligt slitage har visats kunna ske med en noggrann precision under de flesta driftsförhållanden. Dessa slitagemodeller bygger på slitagekoefficienter, och eftersom de vanligtvis inte är giltiga under realistiska driftsförhållanden är modellerna i allmänhet kalibrerade mot verkliga händelseförlopp för att justera koefficienterna från testförhållandena till realistiska smörjförhållanden. Detta tekniska tillvägagångssätt kan vara användbart vid prognos av slitage i system där material och kontaktförhållanden inte varierar. När man addresserar materialutveckling inriktad på att reducera specifika skadelägen är emellertid tillvägagångssättet av begränsad användning eftersom de erhållna slitagekoefficienterna inte är direkt relaterade till materialegenskaper. Därför har försök gjorts till att utveckla fysikaliska sprickbildningsmodeller som relateras till kontaktförhållanden och materialegenskaper. Syftet har varit att erhålla viktig information om var en spricka initieras och hur den fortskrider. I det långa loppet är det även av stor vikt att kunna erhålla information om hur en materialpartikel avlägsnas från kontaktytan. Studier för denna typ av modeller har gjorts på 70- och 80-talet i huvudsak med stift- och skivexperiment men har inte använts inom det specifika området för hjul-räl-kontakt. Avhandligen ingår i FR8RAIL-projektet som arrangeras av det europeiska järnvägsinitiativet Shift2Rail. Literaturstudier har varit grunden för avhandlingen för att få väsentlig insikt i frakturmekanik och andra relaterade områden. De fysiska sprickbildningsmodellerna har konstrueras i FE-mjukvaran Abaqus med XFEM som implementering. För 2D-modellen initieras sprickan överst vid den implanterade imperfektionen när friktionskoefficienten är  och propagerar uppåt några få element. För  initieras sprickan på högra ytgränsen där tryckfördelning och friktionskraft appliceras. Utbredningsvinkeln för sprickan ökar relativt till ytan då friktionskoefficienten ökar. Sprickan för 3D-modellen breder ut sig mer jämfört med 2D-modellen överst vid imperfektionen då . Sprickan initieras på samma ytplats som för 2D-modellen vid . Sprickbildningen är dock obefintlig på grund av konvergensproblem. De konstruerade FE-modellerna är initiala steg mot att analysera sprickutbredningen och närbesläktade fenomen för ett godstågs järnvägshjul i detalj. I slutet av avhandlingen ger de förenklade modellerna huvudsakligen information om sprickinitiering, utbredning och dess mönster. Ytterligare justeringar och förbättringar kan ske efter denna första fas i syfte att eliminera felmarginalerna. På lång sikt kan slutligen helt integrerande modeller med ytterligare implementeringar såsom detaljerad mikrostruktur för kontaktförhållandena, oelastiskt materialbeteende och kompletta tredimensionella modeller användas. / FR8RAIL

<div>The mechanical behaviors of different kinds of biological tissues, including muscle tissues, cortical bones, cancellous bones and skulls, were studied under various loading conditions to investigate their strain-rate sensitivities and loading-direction dependencies. Specifically, the compressive mechanical behaviors of porcine muscle were studied at quasi-static (<1/s) and intermediate (1/s─10^2/s) strain rates. Both the compressive and tensile mechanical behaviors of human muscle were investigated at quasi-static and intermediate strain rates. The effect of strain-rate and loading-direction on the compressive mechanical behaviors of human frontal skulls, with its entire sandwich structure intact, were also studied at quasi-static, intermediate and high (10^2/s─10^3/s) strain rates. The fracture behaviors of porcine cortical bone and cancellous bone were investigated at both quasi-static (0.01mm/s) and dynamic (~6.1 m/s) loading rates, with the entire failure process visualized, in real-time, using the phase contrast imaging technique. Research effort was also focused on studying the dynamic fracture behaviors, in terms of fracture initiation toughness and crack-growth resistance curve (R-curve), of porcine cortical bone in three loading directions: in-plane transverse, out-of-plane transverse and in-plane longitudinal. A hydraulic material testing system (MTS) was used to load all the biological tissues at quasi-static and intermediate loading rates. Experiments at high loading rates were performed on regular or modified Kolsky bars. Tomography of bone specimens was also performed to help understand their microstructures and obtain the basic material properties before mechanical characterizations. Experimental results found that both porcine muscle and human muscle exhibited non-linear and strain-rate dependent mechanical behaviors in the range from quasi-static (10^(-2)/s─1/s) to intermediate (1/s─10^2/s) loading rates. The porcine muscle showed no significant difference in the stress-strain curve between the along-fiber and transverse-to-fiber orientation, while it was found the human muscle was stiffer and stronger along fiber direction in tension than transverse-to fiber direction in compression. The human frontal skulls exhibited a highly loading-direction dependent mechanical behavior: higher ultimate strength, with an increasing ratio of 2, and higher elastic modulus, with an increasing ratio of 3, were found in tangential loading direction when compared with those in the radial direction. A transition from quasi-ductile to brittle compressive mechanical behaviors of human frontal skulls was also observed as loading rate increased from quasi-static to dynamic, as the elastic modulus was increased by factors of 4 and 2.5 in the radial and tangential loading directions, respectively. Experimental results also suggested that the strength in the radial direction was mainly depended on the diploë porosity while the diploë layer ratio played the predominant role in the tangential direction. For the fracture behaviors of bones, straight-through crack paths were observed in both the in-plane longitudinal cortical bone specimens and cancellous bone specimens, while the cracks were highly tortuous in the in-plane transverse cortical bone specimens. Although the extent of toughening mechanisms at dynamic loading rate was comparatively diminished, crack deflections and twists at osteon cement lines were still observed in the transversely oriented cortical bone specimens at not only quasi-static loading rate but also dynamic loading rate. The locations of fracture initiations were found statistical independent on the bone type, while the propagation direction of incipient crack was significantly dependent on the loading direction in cortical bone and largely varied among different types of bones (cortical bone and cancellous bone). In addition, the crack propagation velocities were dependent on crack extension over the entire crack path for all the three loading directions while the initial velocity for in-plane direction was lower than the other two directions. Both the cortical bone and cancellous bone exhibited higher fracture initiation toughness and steeper R-curves at the quasi-static loading rate than the dynamic loading rate. For cortical bone at a dynamic loading rate (5.4 m/s), the R-curves were steepest, and the crack surfaces were most tortuous in the in-plane transverse direction while highly smooth crack paths and slowly growing R-curves were found in the in-plane longitudinal direction, suggesting an overall transition from brittle to ductile-like fracture behaviors as the osteon orientation varies from in-plane longitudinal to out-of-plane transverse, and to in-plane transverse eventually.</div>