Aços microligados pertencem à classe dos aços ARBL contendo baixa ou média quantidade de carbono e pequena adição de elementos de liga tais como Mn, Nb, Mo, V e Ti. A variedade microestrutural desses aços pode ser obtida dependendo da temperatura de conformação, taxa de resfriamento e composição química. Os tratamentos intercríticos e isotérmicos produzem microestruturas multifásicas com diferentes quantidades de ferrita, martensita, bainita e austenita retida. A presença de diferentes fases nestes materiais, com morfologias distintas, pode afetar de modo significativo seu comportamento mecânico, afetando, por exemplo, o fechamento da trinca e resultando em mudanças na taxa de crescimento da mesma. O objetivo deste trabalho é avaliar as propriedades de tração e a resistência ao crescimento da trinca por fadiga de um aço microligado RD 480 com 0.08%C-1, 5%Mn (p), correlacionando-as com suas características microestruturais. Esse aço, desenvolvido recentemente pela CSN (Companhia Siderúrgica Nacional), é considerado promissor como alternativa para substituir o aço de baixo carbono utilizado em componentes de rodas na indústria automotiva. Distintas condições microestruturais foram obtidas por meio de tratamentos térmicos seguidos de resfriamento em água. As condições de tratamento intercrítico e têmpera simples foram escolhidas para se avaliar a resistência à propagação da trinca por fadiga. Os resultados dos ensaios foram sintetizados em termos da taxa de crescimento da trinca (da/dN) versus a variação do Fator Intensidade de Tensão (_K) no ciclo de carregamento. Para descrever o comportamento das trincas foram utilizados dois modelos: a equação convencional de Paris e um novo modelo exponencial que mostra o comportamento não linear das curvas de fadiga. Os resultados mostraram que uma microestrutura combinando ferrita de aspecto acicular e fases duras (martensita/bainita) resultou em menores taxas de crescimento da trinca. No entanto, a melhor combinação entre as propriedades de tração (limite de escoamento, resistência e ductilidade) e fadiga foi obtida com uma microestrura bifásica contendo martensita dispersa em uma matriz ferrítica. Observou-se uma transição nas curvas de crescimento da trinca para todas as condições tratadas termicamente e, por conta disto, as curvas das condições microestruturais bifásicas e multifásicas foram melhores modeladas quando divididas em duas regiões. As superfícies de fratura dessas amostras, bem como o caminho percorrido pela trinca, foram analisados via MEV e MO. / Microalloyed steels are a class of HSLA steels with low or medium carbon content and small additions of alloy elements such as Mn, Nb, Mo, V and Ti. A variety of microstructures in microalloyed steels can be obtained depending on the deformation temperature, cooling rate and chemical composition. Heat treatments and isothermal transformation on these materials, with various temperatures and holding times, produce multiphase microstructures with different amounts of ferrite, martensite, bainite and retained austenite. These different phases, with distinct morphologies, are determinant of the mechanical behavior of the steel and can, for instance, affect crack closure or promote crack shielding, thus resulting in changes on its propagation rate under cyclic loading. The aim of this study is to evaluate the tensile properties and resistance to fatigue crack growth in a microalloyed steel RD 480 with 0.08%C-1, 5% Mn (wt), correlating with their microstructural characteristics. This steel, recently developed by CSN (Companhia Siderurgica Nacional), is being considered as a promising alternative to replace low carbon steel in wheel components for the automotive industry. Distinct microstructural conditions were obtained by means of heat treatments followed by water quench. The intercritical treatment and quenching conditions were chosen to evaluate the strength to crack propagation. The crack propagation test results were summarized in terms of FCG rate (da/dN) versus stress intensity factor range (?K) curves. In order to describe the FCG behavior, two models were tested: the conventional Paris equation and a new exponential equation developed for materials showing non-linear FCG behavior. The results showed that a microstructure combining aspect acicular ferrite and hard phases (martensite / bainite) resulted in lower rates of crack growth. However, the best combination between the tensile properties (yield stress, tensile strength and ductility) and fatigue was obtained with a dual phase steel microstructure containing martensite dispersed in a ferrite matrix. It was observed a transition in the crack growth curves for all heat treated conditions, so the curves of the dual and multiphase microstructural conditions were better modeled by dividing them in two regions. The fracture planes of the fatigued specimens, as well as the crack path, were examined using a scanning electron microscope (SEM) and optical micrography (OM).

O presente trabalho visa avaliar a microestrutura em chapas de um aço de alta resistência e baixa liga após soldagem por centelhamento e quantificar a resistência ao crescimento de trincas por fadiga ao longo do cordão de solda e da zona termicamente afetada, comparando-a ao comportamento do material base. O aço em estudo, recentemente desenvolvido pela Companhia Siderúrgica Nacional sob a designação RD480, foi fornecido na forma de chapas com 5,0 mm de espessura e soldado pelo processo de centelhamento na divisão de rodas e chassis da IOCHPE-MAXION. A avaliação microestrutural do aço após o processo de soldagem por centelhamento foi realizada nas regiões do metal de base, zona termicamente afetada e metal de solda, utilizando microscopia ótica com diferentes ataques químicos e microscopia eletrônica de varredura. A caracterização mecânica foi realiza com ensaio de tração, teste de dureza Vickers e ensaio de propagação de trincas por fadiga. Para este último foram adotados corpos-de-prova do tipo compacto C(T) e carregamento senoidal de amplitude constante com frequencia 10 Hz e razão de tensão R= 0,1 e as curvas obtidas deste ensaio foram avaliadas em relação a dois modelos matemáticos, o de Paris-Erdogan e o exponencial. Após o ensaio de propagação de trincas por fadiga, as superfícies dos corpos-de-prova foram avaliadas por microscopia eletrônica de varredura. Os resultados encontrados permitiram correlacionar a resistência mecânica e a taxa de crescimento da trinca com as características microestruturais resultantes do processo de soldagem. / This study aims to evaluate the microstructure of a high strength, low alloy (HSLA) steel after flash welding and to quantify the resistance to fatigue crack growth along the weld and heat affected zone, comparing it to the behavior of the base material. The steel under study, recently developed by the Companhia Siderúrgica Nacional under the designation RD480, was provided in the form of plates with 5.0 mm in thickness and welded by IOCHPE-MAXION, division of wheels and chassis. The microstructural evaluation of the steel after the flash welding process was performed at the base metal, heat affected zone and weld metal, using optical microscopy with different chemical attacks and scanning electron microscopy (SEM). The mechanical characterization was performed by means of tensile test, Vickers hardness measurement and fatigue crack growth (FCG) test. Compact Tension C(T) specimens were chosen for the fatigue tests, and the loading was sinusoidal with constant amplitude, frequency of 10 Hz and stress ratio R = 0.1. The crack propagation test results were summarized in terms of FCG rate (da/dN) versus stress intensity factor range (?K) curves. In order to describe the FCG behavior, two models were tested: the conventional Paris equation and a new exponential equation developed for materials showing non-linear FCG behavior. The fracture surfaces of the fatigued specimens were examined via SEM in the secondary electrons regime. The results allowed correlating the mechanical strength and crack growth rate with the microestrutural characteristics resulting from the welding process.

AA6061 Al alloys modified with addition of Mn, Cr and Cu were homogenized at temperatures between 350 ºC and 550 ºC after casting. STEM experiments revealed that the formation of α-Al(MnFeCr)Si dispersoids during homogenization were strongly affected by various factors such as heating rate, concentration of Mn, low temperature pre-nucleation treatment and homogenization temperature. Through analysis of the STEM results using an image software Image-Pro, the size distributions and number densities of the dispersoids formed during different annealing treatments were quantitatively measured. It was revealed that increasing the heating rate or homogenization temperature led to a reduction of the number density and an increase in size of the dispersoids. The number density of dispersoids could be markedly increased through a low temperature pre-nucleation treatment. A higher Mn level resulted in the larger number density, equivalent size and length/width ratio of the dispersoids in the alloy. Upsetting tests on two of these Mn and Cr-containing AA6061 (Al-Mg-Si-Cu) Al alloys with distinctive Mn contents were carried out at a speed of 15 mm s-1 under upsetting temperature of 450 ºC after casting and subsequent homogenization heat treatment using a 300-Tone hydraulic press. STEM experiments revealed that the finely distributed α-Al(MnFeCr)Si dispersoids formed during homogenization showed a strong pinning effect on dislocations and grain boundaries, which could effectively inhibit recovery and recrystallization during hot deformation in the two alloys. The fractions of recrystallization after hot deformation and following solution heat treatment were measured in the two alloys with EBSD. It was found that the recrystallization fractions of the two alloys were less than 30%. This implied that the finely distributed α-dispersoids were rather stable against coarsening and they stabilized the microstructure by inhibiting recovery and recrystallization by pinning dislocations during deformation and annealing at elevated temperatures. By increasing the content of Mn, the effect of retardation on recrystallization were further enhanced due to the formation of higher number density of the dispersoids. STEM and 3-D atom probe tomography experiments revealed that α-Al(MnFeCr)Si dispersoids were formed upon dissolution of lathe-shaped Q-AlMgSiCu phase during homogenization of the modified AA6061 Al alloy. It was, for the first time, observed that Mn segregated at the Q-phase/matrix interfaces in Mn-rich regions in the early stage of homogenization, triggering the transformation of Q-phase into strings of Mn-rich dispersoids afterwards. Meanwhile, in Mn-depleted regions the Q-phase remained unchanged without segregation of Mn at the Q-phase/matrix interfaces. Upon completion of α-phase transformation, the atomic ratio of Mn and Si was found to be 1:1 in the α-phase. The strengthening mechanisms in the alloy were also quantitatively interpreted, based on the measurements of chemical compositions, dispersoids density and size, alloy hardness and resistivity as a function of the annealing temperature. This study clarified the previous confusion about the formation mechanism of α-dispersoids in 6xxx series Al alloys. Four-point bend fatigue tests on two modified AA6061 Al alloys with different Si contents (0.80 and 1.24 wt%, respectively) were carried out at room temperature, f = 20 Hz, R = 0.1, and in ambient air. The stress-number of cycles to failure (S-N) curves of the two alloys were characterized. The alloys were solution heat treated, quenched in water, and peak aged. Optical microscopy and scanning electron microscopy were employed to capture a detailed view of the fatigue crack initiation behaviors of the alloys. Fatigue limits of the two alloys with the Si contents of 0.80 and 1.24 wt% were measured to be approximately 224 and 283.5 MPa, respectively. The number of cracks found on surface was very small (1~3) and barely increased with the applied stress, when the applied stress was below the yield strength. However, it was increased sharply with increase of the applied stress to approximately the ultimate tensile strength. Fatigue crack initiation was predominantly associated with the micro-pores in the alloys. SEM examination of the fracture surfaces of the fatigued samples showed that the crack initiation pores were always aspheric in shape with the larger dimension in depth from the sample surface. These tunnel-shaped pores might be formed along grain boundaries during solidification or due to overheating of the Si-containing particles during homogenization. A quantitative model, which took into account the 3-D effects of pores on the local stress/strain fields in surface, was applied to quantification of the fatigue crack population in a modified AA6061 Al alloy under cyclic loading. The pores used in the model were spherical in shape, for simplicity, with the same size of 7 μm in diameter. The total volume fraction of the pores in the model were same as the area fraction of the pores measured experimentally in the alloy. The stress and strain fields around each pore near the randomly selected surface in a reconstructed digital pore structure of the alloy were quantified as a function of pore position in depth from the surface using a 3-D finite element model under different stress levels. A micro-scale Manson-Coffin equation was used to estimate the fatigue crack incubation life at each of the pores in the surface and subsurface. The population of fatigue cracks initiated at an applied cyclic loading could be subsequently quantified. The simulated results were consistent with those experimentally measured, when the applied maximum cyclic stress was below the yield strength, but the model could not capture the sudden increase in crack population at UTS, as observed in the alloy. This discrepancy in crack population was likely to be due to the use of the spherical pores in the model, as these simplified pores could not show the effects of pore shape and their orientations on crack initiation at the pores near surface. Although it is presently very time-consuming to calculate the crack population as a function of pore size and shape in the alloy with the current model, it would still be desirable to incorporate the effects of shape and orientation of the tunnel-shaped pores into the model, in the future, in order to simulate the fatigue crack initiation more accurately in the alloy.

A study was done to determine the temperature and load interaction effects on the fatigue crack growth rate of polycrystalline superalloy IN100. Temperature interaction testing was performed by cycling between 316°C and 649°C in blocks of 1, 10 and 100 cycles. Load interaction testing in the form of single overloads was performed at 316°C and 649°C. After compiling a database of constant temperature, constant amplitude FCGR data for IN100, fatigue crack growth predictions assuming no load or temperature interactions were made. Experimental fatigue crack propagation data was then compared and contrasted with these predictions. Through the aid of scanning electron microscopy the fracture mechanisms observed during interaction testing were compared with the mechanisms present during constant temperature, constant amplitude testing. One block alternating temperature interaction testing grew significantly faster than the non-interaction prediction, while ten block alternating temperature interaction testing also grew faster but not to the same extent. One hundred block alternating testing grew slower than non-interaction predictions. It was found that as the number of alternating temperature cycles increased, changes in the gamma prime morphology (and hence deformation mode) caused changes in the environmental interactions thus demonstrating the sensitivity of the environmental interaction on the details of the deformation mode. SEM fractography was used to show that at low alternating cycles, 316°C crack growth was accelerated due to crack tip embrittlement caused by 649°C cycling. At higher alternating cycles the 316°C cycling quickly grew through the embrittled crack tip but then grew slower than expected due to the possible formation of Kear-Wilsdorf locks at 649°C. Overload interaction testing led to full crack retardation at 2.0x overloads for both 316°C and 649°C testing. 1.6x overloading at both temperatures led to retarded crack growth whereas 1.3x overloads at 649°C created accelerated crack growth and at 316°C the crack growth was retarded.

An extensive review of probabilistic techniques in fatigue analysis indicates that there is a need for new microstructure-sensitive methods in describing the effects of notches on the fatigue life reduction in cyclically loaded components. Of special interest are notched components made from polycrystalline nickel-base superalloys, which are used for high temperature applications in aircraft gas turbine engine disks. Microstructure-sensitive computational crystal plasticity is combined with novel probabilistic techniques to determine the probability of failure of notched components based on the distribution of slip within the notch root region and small crack initiation processes. The key microstructure features of two Ni-base superalloys, a fine and coarse grain IN100, are reviewed and the method in which these alloys are computationally modeled is presented. Next, the geometric model of the notched specimens and method of finite element polycrystalline reconstruction is demonstrated. Shear-based fatigue indicator parameters are used to characterize the shear-based, mode I formation and propagation of fatigue cracks. Finally, two different probabilistic approaches are described in this work including a grain-scale approach, which describes the probability of forming a crack on the order of grain size, and a transition crack length approach, which describes the probability of forming and propagating a crack to the transition crack length. These approaches are used to construct cumulative distribution functions for the probability of failure as a function of various notch root sizes and strain load amplitudes.

This doctoral thesis is concerned with fatigue life of welded structures. Several topics related to fatigue of welded structures are treated such as; weld defects and their influence on fatigue performance of welded structures, fatigue life prediction using LEFM (Linear Elastic Fracture Mechanics), fatigue testing, welding simulation, residual stress prediction and measurement and their influence on fatigue life. The work that is reported in this doctoral thesis is part results of the Nordic R&D project QFAB (Quality and Cost of Fabricated Advanced Welded Structures) and the Swedish R&D project LOST (Light Optimized Welded Structures). One of the main objectives is to compare different welding processes for the fatigue performance, weld quality and gain understanding of the weld defects, their appearance in different welding processes and their effect on fatigue life. Another main objective is to study welding residual stresses and their effect on fatigue. The design rules are in some cases conservative and especially on the weld root sides the knowledge about the residual stress field may improve the life prediction. The aim is to develop simplified procedures for analysis of residual stresses, their relaxation and influence on fatigue life. Fatigue testing of Hybrid Nd: YAG laser/MAG and MAG welded (tandem arc solid wire, flux cored wire, tandem flux cored wire) non-load carrying cruciform joints was carried out. Four batches were produced, tested and the results were compared. The local weld geometry of the cruciform welded joints was measured and analyzed. Residual stress measurement was carried out close to the toe region using X-ray diffraction. Weld defects, in most cases cold laps, in the cracked specimens were measured. Further fatigue testing, weld defect assessment and residual stress and local weld geometry measurements were carried out on joints welded with flux cored and metal cored arc wires. Two-and three dimensional LEFM crack growth analysis were carried out in order to predict the influence of weld defects, local weld geometry and residual stresses. Residual stresses in multi-pass welded tube-to-plates were studied for two different tubular joint configurations; a three-pass single-U weld groove for maximum weld penetration and a two-pass fillet (no groove) welded tube-to-plates for minimum weld penetration. Torsion fatigue tests were performed in order to study crack propagation from the weld root. Mode III propagation from the lower and upper weld toe on the same tubular joints was also studied. Some tubes were stress relieved (PWHT) and some were fatigue tested with internal static pressure. A three dimensional finite element welding simulation of the multi-pass welded tubular joint was carried out. The calculated temperatures in the transient thermal analysis were compared with measured temperatures. The FE predicted residual stresses in the as-welded conditions were verified with hole drilling strain gage measurements. The residual stresses were used as internal stresses in the finite element model for the torsion fatigue simulation in order to study the cycle by cycle relaxation of the residual stresses in constant amplitude torsion loading. A two dimensional finite element welding simulation procedure was developed in order to predict welding residual stress. The predicted residual stresses were used together with a developed 2D LEFM subroutine to predict the fatigue life, crack path and the effect of residual stresses on weld root defects. The developed simulation subroutines were validated with results found in the literature. Residual stresses measurement, two-and three dimensional welding simulations were carried out in fillet welded joints in order to study the three dimensional effects of the welding process, boundary conditions and modelling technique on the formation of residual stresses. / QC 20100706

The studied material is an A356 Al alloy, used to produce engine cylinder heads for the automotive industry by die casting process. The material displays a quite coarse dendritic microstructure in a eutectic matrix, with a mean grains size of 25 microns, intemetallic precipitates and porosities. The tensile properties are strongly affected by testing temperature, with a quite sensitive drop of the Young's modulus, the Yield stress as the temperature was raised. The isothermal fatigue life dropped of markedly (approximately 10 times) when the testing temperature is raised from 120 to 280 °C, under strain control. From the themomechanical in-phase cyclic tests, with temperature varying from (120 to 280 oC), it was possible to observe that life is quite similar to the isothermal fatigue test at 280 oC. In this case, the more sensitive damage caused the in-phase mechanical and thermal cycle take place at the highest temperature. Relaxation tests indicated two distinct behaviors, with the temperature of 240°C being a threshold. At lower temperatures, the material hardens cyclically whereas it softens cyclically at higher temperatures. From the fatigue crack growth results, it was observed that temperature and wave shape has a strong influence on the crack growth rate as well as on the stress intensity threshold. Considering sinusoidal wave shape (10 Hz), as the temperature increased the DKth decreased and the crack propagation rate increased. However, the rate as da/dN change with temperature is quite similar, as an indicative that the micromechanism of crack growth has not changed due to the high frequency used, and it was due only to loss of mechanical strength. An elastic-visco-plastic non-isothermal constitutive law was identified for the material. For the cast material studied in this work, the mechanical behavior parameters are statistically distributed. However, it was shown that the model was able to reproduce, with a reasonable approximation, the stress - strain relationship at different temperatures, for the isothermal and anisothermal cases.

The mechanical behavior of a high pressure die cast AM60B Mg alloy is studied. Constant load amplitude fatigue tests were conducted at room, elevated and cold temperatures, with a stress ratio of R=0.1, and frequency of 30 Hz. The objective was to identify the possible effects of temperature on fatigue life cycle. In addition, fatigue crack propagation tests were conducted to ascertain the fatigue response of the alloy and determine its fatigue crack growth rate as a function of the applied stress ratio, experimentally, analytically and computationally, using Walker’s model. The results demonstrated that temperature had a significant influence on the fatigue life, and that the life increased at cold temperature but decreased at elevated temperature as compared to that evaluated at room temperature. In this study, the limit for applicability of LEFM was established for AM60B magnesium alloy. In addition, fatigue crack propagation test results were used to evaluate the coefficients of the Paris model.

The main objective of the project was to establish a fracture-mechanics energy-based approach for the design of structural adhesive joints under cyclic loading. This required understanding how an adhesive system behaved near its fatigue threshold, and how the key factors affected this behavior in a fresh undegraded joint. The investigated factors were mode ratio (phase angle), substrate material, surface treatment and surface roughness (both affecting the bond strength), bondline thickness and load ratio. It was first required to understand how the adhesive system behaved under quasi-static loading by examining a fracture mechanics-based design approach for adhesive systems with different substrate materials and geometries. Experiments were initially performed to characterize the strength of aluminum and steel adhesive systems based on the fracture envelope, critical strain energy release rate as a function of the mode ratio. Ultimate failure loads of aluminum and steel adhesive joints, having different overlap end conditions and different geometries were then experimentally measured. These values were compared with the failure loads extracted from the fracture envelope. Considering the toughening behavior of the adhesive in the fracture mechanics analyses, a very good agreement (average of 6%) was achieved between the predictions and experiments for all types of overlap end conditions and geometries. Different fatigue threshold testing approaches, which are commonly used in the literature or suggested by the ASTM standard, were evaluated for the cracked and intact fillet joints. Based on the experimental and analytical studies, the most appropriate technique for fatigue testing and characterization of adhesive systems was suggested. Comparing the mixed-mode near-threshold behavior of different adhesive systems with the fracture behavior and fatigue mode-I and mixed-mode high crack growth rates showed the high sensitivity of the mixed-mode near-threshold fatigue to the subtle changes in the interfacial bond strength. In order to make a baseline for the design of adhesive joints under cyclic loading, similar to the previous fracture tests and following the energy-based approach, fatigue behavior was characterized as a function of the loading mode ratio for aluminum and steel adhesive joints. The effect of substrate material, surface treatment, bondline thickness, surface roughness and fatigue testing load ratio on the near-threshold fatigue behavior of adhesives joints was evaluated experimentally. The experimental observations were then explained using finite element modeling. To generalize the conclusions, the majority of experiments and studies covered a broad range of crack growth rates, as low as fatigue threshold and as high as 10-2 mm/cycle. Having understood the significant testing and design parameters, an adhesive system can be designed based on a safe cyclic load that produces an insignificant (for automotive industry) or reasonably low but known crack growth rate (for aerospace industry).

The effect of retrogression and reaging heat treatment on corrosion fatigue crack growth behavior on AA7050 T73651 aluminum alloy is investigated. CT (Compact Tension) specimens are prepared in LS direction for fatigue crack growth tests . Samples are solution heat treated at 477 &deg / C and aged at 120 &deg / C for 24 h (T6 condition). After that, samples are retrogressed at 200 &deg / C for times of 1, 5, 30, 55 and 80 minutes in a circulating oil bath. Then, samples are re-aged at 120 &deg / C for 24 h (T6 condition). Hardness measurements are taken at different retrogression times and at the end of the heat treatment. Fatigue crack growth tests are performed at as received condition and at different retrogression times with sinusoidal loading of R=0.1 and f=1 in both laboratory air and corrosive environment of 3.5% NaCl solution. The highest fatigue crack growth resistance is observed for 30 min. and 5 min. retrogression for laboratory air and corrosive environment respectively. It is concluded that RRA can successfully be used to improve fatigue performance of this alloy.