CONSTITUTIVE MODELING OF ALLOYS UNDER HIGH TEMPERATURE LOW-CYCLE AND THERMAL- MECHANICAL FATIGUE: A KEY ISSUE IN COMPONENT DESIGN [Keynote]

Dissipated plastic energy is a convenient and widely used criterion to assess the life of components experiencing high temperature low-cycle fatigue and thermal-mechanical fatigue. However, component design relies on efficient and accurate constitutive models. Elasto-viscoplastic models are enriched using dislocation density as an internal variable to account for recovery or overaging effects in precipitate strengthened alloys. Examples are shown for components made of cast iron, welded stainless steels and cast aluminum alloys.
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FATIGUE ANALYSIS WITHOUT CYCLE COUNTING: SUBCYCLE FATIGUE CRACK GROWTH AND EQUIVALENT INITIAL FLAW SIZE MODEL

Threshold and near-threshold fatigue crack growth (FCG) is critical for the total life prediction as majority of time is spent in this regime. The proposed study includes the fatigue crack growth near-threshold in the time-based subcycle model for fatigue life prediction under arbitrary loading conditions. A novel fatigue-life prediction methodology combining a subcycle fatigue crack growth analysis and equivalent initial flaw size (EIFS) concept is proposed. A previously developed time-based subcycle fatigue crack growth model is extended to near threshold regime and under multiaxial loadings. A new temporal kernel function to include intensity factor corresponding to near threshold region is proposed. The multiaxial load scenario is considered for mixed-mode FCG using a critical plane approach. Model predictions under arbitrary are compared with experimental data from open literatures and internal testing. Most of the predicted fatigue life results lie with error factor range of 2, which shows a good prediction for fatigue life.
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PROPOSAL OF FATIGUE DESIGN METHOD FOR STRUCTURAL DISCONTINUITES CONSIDERING STRES GRADIENT

This paper discusses a method that focuses not only on peak stresses but also on stress gradients to rationalize fatigue design using a low-alloy steels. First, fatigue strength reduction ratios are associated with stress gradients rather than stress concentration factors. Next, to verify the stress gradient method, fatigue tests were conducted on hole-notched specimens. Finally, the fatigue life was predicted, considering the stress gradient at the notch root. The predicted atigue lives agreed well with the experimental results. It was confirmed that the fatigue life can be predicted more accurately than the conventional peak stress method.
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EFFECT OF PRE-ACCUMULATED PLASTIC STRAIN ON STRESS CORROSION CRACKING AND FATIGUE LIFE OF STEELS; EXPERIMENT AND MODELING

Steel structures may experience localized plastic strains, arising from wide range of service anomalies. Regions of accumulated plastic strain are more prone to accelerated stress corrosion cracking and reduced fatigue life. In this work, we systematically analyzed the intergranular corrosion (IGC) under combined oscillatory mechanical loading and active electrochemical environment in a specially designed experimental apparatus. Loading cycles were design to mimic both the low amplitude high frequency vibration loads and the low frequency-high amplitude structural duty cycles. Electrochemical potentials were maintained for active dissolution in moderately alkaline carbonate-bicarbonate solutions and under pre-accumulated plastic strain of 0-4%. We observed grain boundary softening, directly arising from vacncies formed by silicon oxidation. Triangular wedges were formed and correlated with the level of the accumulated plastic strains and the load profile. A three-dimensional elasto-plastic continuum damage mechanics model is developed to account for both, the pre-accumulated plastic strain, and the induced elasto-plastic fatigue strains to accelerate the evolution of damage accumulation. Upto 90% of life reduction is observed with 4% of pre-accumulated plastic strain. These findings can be used to advance the understanding of the combined effect of damage and corrosion on the remaining fatigue life of energy materials.
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MONITORING FATIGUE DAMAGE IN HYPOEUTECTIC AL-SI CASTINGS WITH VARYING MICROSTRUCTURE CHARACTERISTICS [Keynote]

Due to their low density, good recyclability and producibility of complex net shapes, cast aluminium alloys are promising candidates for many demanding applications in mobility, power generation and machinery. The inherent microstructure inhomogeneity is the most striking challenge in placing cast Al alloys in cyclically loaded components. Therefore, obtaining a quantitative understanding of the correlation between casting process, microstructure parameters (dendrite arm spacing (DAS), size and shape of (i) the eutectic silicon, (ii) the gas porosity, and (iii) the shrinkage porosity) and fatigue properties (fatigue limit, fracture mechanical data) is the aim of the present study. The adjustment of these microstructure parameters by tailored casting systems and fatigue testing revealed that the fatigue limit increases and the threshold of the stress intensity range Delta K th decreases with decreasing DAS (microstructure refinement). Microscopic in-situ-tracking of fatigue damage yields a detailed understanding of the fatigue mechanisms that will be the basis of a numerical short crack modeling approach in the future.
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CRYSTALLOGRAPHIC ORIENTATION ANALYSIS OF FATIGUE CRACK SURFACE FROM AA7050 SAMPLES WITH MULTI-STAGE AGING TREATMENTS

The retrogression and re-aging (RRA) and interrupted aging heat treatments (T6I4 conditions) lead to a bimodal microstructure feature within the 7050-aluminum alloy. Microstructural effects have a strong influence on fatigue crack nucleation and short crack growth via the activation of competing mechanisms operating during fatigue crack nucleation. Electron Back Scatter Diffraction (EBSD) technique permits the study of the effects of fatigue-induced crystal defects on the crack path and the pre-existing defect structure. Furthermore, few studies have explored the potential of multi-stage aging treatments on the fatigue crack nucleation paths using the EBSD technique. The purpose of this study is to elucidate the role of the microstructure on small fatigue crack crystallography in samples of RRA, T6I4-65 and T7451 conditions made from AA 7050. Fatigue crack surface crystallography will be determined near the vicinity of crack initiation sites and within the early crack growth regime using focused ion beam (FIB) surface preparation combined with electron back-scattered diffraction (EBSD). All the small fatigue cracks were initiated at the bore-hole surfaces on the gage length of specimen geometry, i.e., double-edge notches with two 4.8 mm diameters. The bore-hole surfaces of samples were electropolished prior to fatigue loading to remove bore-hole surface machining damage.
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ORIENTATION-DEPENDENT FATIGUE ASSESSMENT OF TI6AL4V MANUFACTURED BY L-PBF

The fatigue behaviour of as-built parts produced by means of Laser-Powder Bed Fusion process (L-PBF) is primarily influenced by the presence of stress raisers on the surface, whose morphology strongly depends on the relative orientation between the surface and the build direction. This study aims to shed light into the factors representing the surface morphology that correlate with the fatigue performance of L-PBF Ti6Al4V specimens manufactured in different orientations. A fracture mechanics model based on measurable roughness parameters was employed for the prediction of the fatigue properties in both the finite life and endurance limit regions.
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FUNCTIONAL FATIGUE PROPERTIES OF TINIZRSN BIOCOMPATIBLE SHAPE MEMORY ALLOY

Functional fatigue degrades the superelastic properties of shape memory alloys under cyclic loading. In the presence of geometric stress concentrations, the local stress fields are amplified resulting in local accumulation of irrecoverable strains and consequently loss of functionality. For the biocompatible TiNbZrSn system, grain size and solution treatment temperature play a major role in affecting the level of pseudoelastic strains and their evolution upon cycling. These aspects are quantitatively investigated in this work. Dogbone tensile specimens and samples with drilled circular holes are considered in this work and full field strain measurements are employed to quantitatively evaluate the localization in response leading to loss of functionality.
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NUMERICAL ANALYSIS OF ROLLING CONTACT FATIGUE CRACK GROWTH ON CURVED RAILWAY TRACKS

In this study, numerical analyses were conducted to investigate the non-proportional mixed-mode RCF crack growth behaviour in the presence of severe longitudinal, lateral and spin creepages. The whole procedure combined multi-body dynamic simulation (MBDS) and the extended finite element method (XFEM) in an indirectly coupled way. Attempts were also made to modify the FaStrip theory to obtain traction distributions based on elastoplastic contact pressures which were then applied in an XFEM model to predict surface crack growth directions. Parametric studies were also conducted to further quantify the influence of different creepage combinations on both crack growth directions at rail surface and crack growth rate at crack tips. It is concluded that the increase of either of the three creepages can significantly influence the phase and magnitude of stress intensity factor histories, albeit to different extents.
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