Themes: JoDean Morrow & Paul Paris Memorial Symposium on Fatigue & Fracture

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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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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CRACK TIP ENHANCED CRYSTAL PLASTICITY PHASE FIELD MODEL FOR CRACK PROPAGATION IN TI64 ALLOYS

This work introduces a computational fracture model for Ti64 alloy based on coupled Crystal Plasticity Phase Field model for fracture but also considers the atomistic mechanisms of plasticity at the crack tip. Atomistic simulations are conducted to identify the crack-tip mechanisms of plasticity and the continuum scale phase field model is augmented to account for this. Using the data generated using atomic scale Molecular Dynamic simulations, a functional form describing the evolution of dislocation density nucleating from the crack tip is obtained using Bayesian Inference and Genetic Programming based Symbolic Regression. The effect of nucleated dislocations in crack path and rate of crack propagation is evaluated. The additional plastic strain at the crack tip is also validated with results from Molecular Dynamics.
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A GENERALIZED TWO-PARAMETER DRIVING FORCE MODEL FOR SHORT AND LONG FATIGUE CRACK PROPAGATION

Numerous different crack growth modeling approaches have been developed to consider the short crack and long crack behaviors by accounting for the stress intensity range-based crack driving forces or the crack closure concept. However, those methods lacked a proper systematic approach to accurately account for the behavior of short cracks. Based on the recent systematic study performed in the authors’ group, a new generalized two-parameter driving force model is proposed to account for crack growth driving forces and corresponding crack growth thresholds to predict both short crack and long crack propagation behaviors. The model predicted crack growth rates are compared with crack growth data set of Ti-6Al-4V titanium and 2024-T3 aluminum alloys. Predicted results show good agreement with experimental crack growth data for these materials.
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INFRARED TEMPERATURE MEASUREMENT AND X-RAY TOMOGRAPHY FOR INTERNAL FATIGUE CRACK MONITORING DURING ULTRASONIC FATIGUE TESTS [Keynote]

The observation of fatigue cracks in the gigacycle fatigue regime is very difficult because they are very often initiating and propagating in the core of the specimens. This paper presents a methodology for detecting and monitoring internal fatigue cracks during ultrasonic fatigue tests. Using both the heat source located in the reverse cyclic plastic zone at the crack tip and the 3D geometry of the crack (from X-Ray tomography), finite element analysis is done to solve the heat transfert problem. This allow us to related the internal crack growth rate and the temperature field evolution versus time at the surface of the specimen. This proposed method has been successfully applied on specimens in cast aluminum alloy.
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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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