FATIGUE OF HUMAN RED BLOOD CELLS IN HEALTH AND DISEASE

Human red blood cells (RBCs) are responsible for delivering oxygen to the organs and tissues from the lungs. During its lifespan, an RBC needs to squeeze through the smallest openings (i.e., smallest capillaries and splenic interendothelial slits) in the human body many times, and go through repeated hypoxia-normoxia cycles. Using our established microfluidic platform, we have shown that both mechanical fatigue and hypoxia-normoxia fatigue (through hypoxia-normoxia cycles) may cause significant mechanical degradation of RBCs. The results are compared between healthy RBCs and sickle cell disease (SCD) RBCs, and provide underlying mechanisms for a much shorter lifespan of SCD RBCs.
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THERMO-MECHANICAL FATIGUE CRACK GROWTH INVESTIGATION FOR CAST AUSTENITIC STAINLESS STEEL

This paper describes a complete experimental program and its numerical counterpart to investigate and predict failure analysis (crack initiation and propagation) of a cast 1.4837 heat-resistant austenitic stainless steel commonly used for automotive turbochargers. Fatigue crack growth analysis is the focus of this paper considering both isothermal and anisothermal loading for both experimental and finite element analysis. On this basis fatigue crack growth rate model is derived accounting for complex interaction of large levels of plasticity and subsequent crack closure.
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CRYSTAL PLASTICITY MODELING OF FATIGUE CRACK GROWTH IN STAINLESS STEEL

Predicting the crack behavior under monotonic and cyclic loading is essential for an accurate assessment of the reliability of engineering structures. This work is concerned with the deformation fields in crack tip grains and their effects on fatigue crack growth rates under cyclic loading. We develop a cyclic crystal plasticity finite element (CPFE) model to characterize the mechanical behavior of 316L stainless steel. The deformation fields in crystal grains near crack tips under monotonic and cyclic loading are studied for two crack tip grain orientations using CPFE simulations. The CPFE results under monotonic loading are consistent with previous theoretical and experimental results. The CPFE results under cyclic loading match those from cyclic J2 plasticity finite element (JPFE) simulations. Based on the accumulated plastic work, cyclic CPFE simulations predict the fatigue crack growth rate as a function of stress intensity factor. The predicted Paris law exponent is consistent with the experimental value. This work demonstrates a new CPFE approach to predict both the deformation field and fatigue crack growth rate in metal alloys. This approach may be further generalized to investigate the time dependent crack growth that can be strongly influenced by the crystallographic effects of crack tip grains.
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DEVELOPMENT OF THE NOVEL MIXED MODE ULTRASONIC FATIGUE TEST SYSTEM BASED ON FREQUENCY RESPONSE FUNCTION AND DYNAMIC MODAL ANALYSIS

In case it is important to characterizze the ultra-high fatigue behaviors of a metal, ultrasonic fatigue tests can be considered due to high test frequences. Moreover, it is quite important to understand the ultra-high fatigue life of metals under multi-axial stress status practically. This research demonstrates how to develop a novel mixed mode ultrasonic fatigue test system based on Frequency Response Function and Dynamics Modal Analysis, and the compatibility of the fatigue system is validated by experiments.
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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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