Load induced top-down fatigue cracking has been recognized recently as a major distress phenomenon in asphalt pavements. This paper presents a mechanics-based design framework in load and resistance factor design (LRFD) format for the top-down fatigue cracking performance evaluation of flexible pavements. This was achieved by enhancing further the hot mix asphalt fracture mechanics (HMA-FM) model through the incorporation of mixture morphology influence on key fracture properties, and incorporating partial safety factors to account for variabilities and uncertainties. The analysis framework was calibrated and validated using pavement sections that have high quality laboratory data and well documented field performance histories. Moreover, as traffic volume was identified in having a dominant influence on predicted performance, a further investigation was performed to establish and evaluate truck traffic characterization parameters effect on predicted results. A two-component reliability analysis methodology, which uses central composite design (CCD) based response surface approach for surrogate model generation and the first order reliability method (FORM) for reliability estimation was used for the development of the LRFD mechanics-based design framework. The effectiveness of the design framework was investigated through design examples, and the results have shown that the formulated partial safety factors have accounted effectively the variabilities involved in the design process. Further investigation was performed to establish the influence design inputs variabilities have on target reliabilities through case studies that combine input variabilities in a systematic way. It was observed from the results that the coefficient of variation (COV) level of the variability irrespective of the distribution type used have a significant influence on estimated target reliability. / Lastinducerade utmattningssprickor från ytan har nyligen identifierats som en betydande brottmekanism i asfaltbeläggningar. Denna avhandling presenterar ett mekanik-baserat ramverk i last- och motståndsfaktorer konstruktionsformat (LRFD) för utvärdering av utmattningssprickor från ytan för asfaltsbeläggningar. Detta uppnåddes genom att ytterligare förbättra brottmekanikmodellen för varmblandad asfalt (HMA-FM) genom integreringen av blandingens morfologieffekt på brottegenskaper samt säkerhetsfaktorer för att beakta variabilitet och osäkerheter. Ramverket kalibrerades och validerades med hjälp av vägar med säkerställda egenskaper och väldokumenterad prestanda. Eftersom trafikvolymen identifierades att ha ett dominerande inflytande på förväntad prestanda, utfördes vidare en utredning för att fastställa effekten av lastbilstrafikens karakteriseringsparametrar på förutsedd prestanda. Tillförlitlighetsanalysen uppnåddes genom att implementera en tvåkomponents tillförlitlighetsanalys där en metod som kallas ”central composite design” (CCD), vilken skapar en surrogatmodell, och första ordningens tillförlitlighetsmetod (FORM) för beräkning av tillförlitligheten. Effektiviteten i det mekanik-baserade konstruktionsramverket undersöktes genom konstruktionsexempel och resultaten har tydligt visat att de formulerade partialkoefficienter effektivt har beaktat variabiliteten inom konstruktionsprocessen. Ytterligare undersökningar utfördes för att fastställa påverkan som indatas variabilitet har på den förutbestämda tillförlitligheten genom fallstudier där indatans variabilitet ändras systematiskt. Det observerades från resultaten att variabilitetens variationskoefficient (COV) har en signifikant inverkan på tillförlitligheten, oberoende av fördelningstyp. / <p>QC 20161125</p>

Ti-6Al-4V manufactured by electron beam melting Keywords: Additive manufacturing, high-temperature tensile properties, low cycle fatigue, neutron diffraction, fatigue crack growth ISBN: 978-91-89325-27-2 (Printed) 978-91-89325-26-5 (Electronic) Ti-6Al-4V is the most widely used α+β titanium alloy in aerospace engine applications due to its high specific strength. Typically, the alloy is manufactured as castings or forgings and then machined to final geometry. These conventional manufacturing processes do however generate a lot of waste material, whereas additive manufacturing (AM) can potentially produce a near-net-shape geometry directly from the feedstock. In the past decade, electron beam melting (EBM), one of the powder bed fusion techniques, has been widely researched to build Ti[1]6Al-4V components. Still, the as-built material can contain defects such as gas pores that require post-processing, such as hot isostatic pressing (HIP) to produce nearly fully dense components. HIP treatment of conventionally cast Ti-6Al-4V is normally performed at 920 ˚C, 100 MPa for 2 hours. This same HIP treatment has then been adapted also for EBM-manufactured Ti-6Al-4V, which however results in coarsening of α laths and reduction of yield strength. Therefore, finding a more appropriate HIP treatment for this new type of Ti-6Al-4V material, i.e. EBM manufactured, would be of great benefit for the industry. Lowering the HIP treatment temperature to 800 ˚C and increasing the pressure to 200 MPa has recently been proven to close the porosity to a high degree while sustaining the high yield strength. In this thesis, the high-temperature tensile properties of EBM-manufactured Ti[1]6Al-4V subjected to a low-temperature (800 ˚C) HIP treatment were evaluated and compared with standard HIP-treated (920 ˚C) materials. Metallurgical characterization of the as-built, HIP-treated materials have been carried out to understand the effect of temperature on the microstructures. The standard HIP[1]treated material measured about 1.4x - 1.7x wider α laths than those in the low[1]temperature HIP treated and as-built samples, respectively. The standard HIP[1]treated material showed about 10 - 14% lower yield strength than other HIP treated materials. At 350 ˚C the yield strength decreases to about 65% compared to the room temperature strength for all tested materials. An increase in ductility vi programvaran NASGRO där livsförutsägelserna visade god överensstämmelse med experimentella livscykler i de flesta fall. vii Abstract Title: Microstructure and mechanical properties of low-temperature hot isostatic pressed Ti-6Al-4V manufactured by electron beam melting Keywords: Additive manufacturing, high-temperature tensile properties, low cycle fatigue, neutron diffraction, fatigue crack growth ISBN: 978-91-89325-27-2 (Printed) 978-91-89325-26-5 (Electronic) Ti-6Al-4V is the most widely used α+β titanium alloy in aerospace engine applications due to its high specific strength. Typically, the alloy is manufactured as castings or forgings and then machined to final geometry. These conventional manufacturing processes do however generate a lot of waste material, whereas additive manufacturing (AM) can potentially produce a near-net-shape geometry directly from the feedstock. In the past decade, electron beam melting (EBM), one of the powder bed fusion techniques, has been widely researched to build Ti[1]6Al-4V components. Still, the as-built material can contain defects such as gas pores that require post-processing, such as hot isostatic pressing (HIP) to produce nearly fully dense components. HIP treatment of conventionally cast Ti-6Al-4V is normally performed at 920 ˚C, 100 MPa for 2 hours. This same HIP treatment has then been adapted also for EBM-manufactured Ti-6Al-4V, which however results in coarsening of α laths and reduction of yield strength. Therefore, finding a more appropriate HIP treatment for this new type of Ti-6Al-4V material, i.e. EBM manufactured, would be of great benefit for the industry. Lowering the HIP treatment temperature to 800 ˚C and increasing the pressure to 200 MPa has recently been proven to close the porosity to a high degree while sustaining the high yield strength. In this thesis, the high-temperature tensile properties of EBM-manufactured Ti[1]6Al-4V subjected to a low-temperature (800 ˚C) HIP treatment were evaluated and compared with standard HIP-treated (920 ˚C) materials. Metallurgical characterization of the as-built, HIP-treated materials have been carried out to understand the effect of temperature on the microstructures. The standard HIP[1]treated material measured about 1.4x - 1.7x wider α laths than those in the low[1]temperature HIP treated and as-built samples, respectively. The standard HIP[1]treated material showed about 10 - 14% lower yield strength than other HIP treated materials. At 350 ˚C the yield strength decreases to about 65% compared to the room temperature strength for all tested materials. An increase in ductility viii was observed at 150 ˚C compared to that at room temperature, but the ductility decreased between 150 - 350 ˚C because of activation of different slip systems. The low cycle fatigue (LCF) behavior of such a modified HIP (low-temperature HIP) material is assessed at two different strain levels and compared with the corresponding LCF properties for the standard HIP material. Even though the modified HIP material had lowest minimum life cycles to failure, the overall fatigue performance is comparable with that of the standard HIP material. Also, fatigue life predictions were made from the measured defect size at the crack initiation site using NASGRO. The calculated life predictions showed good agreement with the experimental values in most cases. In-situ neutron diffraction measurements on tensile test specimens were conducted, at both room temperature and at 350˚ C, for the standard and modified HIP-treated materials. The objective was to gain essential insights on how the crystal lattice strains relate to the macroscopic strengths in these specific microstructures. This investigation helped to understand the load partitioning between different slip planes and constituent phases in the microstructure at different temperatures. / Ti-6Al-4V är den mest använda α+β titanlegeringen i flygmotortillämpningar på grund av sin höga specifika hållfasthet. Vanligtvis tillverkas legeringen som gjutgods eller smide och bearbetas sedan till slutlig geometri. Dessa konventionella tillverkningsprocesser genererar dock en hel del avfallsmaterial, medan additiv tillverkning (AM) potentiellt kan producera en nästan slutgiltlig geometri direkt från råvaran. Under det senaste decenniet har elektronstrålesmältning (EBM), en av pulverbäddsfusionsteknikerna, undersökts mycket för att bygga Ti-6Al-4V-komponenter. Ändå kan det byggda materialet innehålla defekter såsom gasporer som kräver efterbearbetning, såsom varm isostatisk pressning (HIP) för att producera nästan helt täta komponenter. HIP[1]behandling av konventionellt gjutet Ti-6Al-4V utförs normalt vid 920 ˚C, 100 MPa under 2 timmar. Samma HIP-behandling har sedan anpassats även för EBM[1]tillverkat Ti-6Al-4V, vilket dock resulterar i förgrovning av α-lameller och minskning av sträckgränsen. Att hitta en mer lämplig HIP-behandling för denna nya typ av Ti-6Al-4V-material, dvs EBM-tillverkat, skulle därför vara till stor fördel för industrin. Att sänka HIP-behandlingstemperaturen till 800 ˚C och öka trycket till 200 MPa har nyligen visat sig stänga porositeten i hög grad samtidigt som den höga sträckgränsen bibehålls. Ti-6Al-4V används huvudsakligen i applikationer för flygmotorer upp till en maximal driftstemperatur på 300 ˚C. Därför studerades högtemperaturdragegenskaperna hos de olika HIP-behandlade EBM[1]byggmaterialen i detta forskningsarbete. Denna studie visade att duktiliteten påverkas av aktiveringen av olika glidsystem baserat på temperatur. Ytterligare neutrondiffraktionsexperiment utfördes tillsammans med in-situ dragprovning för att bestämma det aktiva glidsystemet vid en specifik temperatur. Utmattningsbeteendet hos det lågtemperaturbehandlade HIP-materialet utvärderas också genom lågcykelutmattningstestning och utmattningsspricktillväxttest. Utmattningsprestandan för det modifierade HIP[1]materialet utvärderades mot standard HIP- material och visade sig ha jämförbara utmattningsegenskaper. Förutsägelser om utmattningsliv utfördes med hjälp av vi programvaran NASGRO där livsförutsägelserna visade god överensstämmelse med experimentella livscykler i de flesta fall. / <p>Submitted papers or manuscripts have been excluded from the fulltext file.</p>