Symposium 2: JoDean Morrow & Paul Paris Memorial Symposium on Fatigue & Fracture

Multiaxial fatigue testing generates curved cracks that are extremely difficult to characterize using standard
compliance based and potential drop-based methods. Therefore, an automated online system was
developed to monitor the crack tips positions in plate cruciform specimens. The system periodically
evaluates the deformation fields at small areas around the crack tips by performing digital image correlation
(DIC) on images obtained by a moving camera triggered at desired phases of the loading cycle. The
displacement field obtained by DIC is fitted by a simple model that specifies the crack propagation direction
and enables it to iteratively find new crack tip positions. Moreover, the model is capable of computing the
crack opening displacements near the crack tip and thus characterizes the local loading. This approach
enables fully automated multiaxial testing with controlled crack length and crack tip loading. The method
was successfully tested on an AA5754 aluminium alloy sheet..Multiaxial fatigue testing generates curved cracks that are extremely difficult to characterize using standard
compliance based and potential drop-based methods. Therefore, an automated online system was
developed to monitor the crack tips positions in plate cruciform specimens. The system periodically
evaluates the deformation fields at small areas around the crack tips by performing digital
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The field of fracture mechanics started with Griffith’s energy concept for brittle fracture in 1920. In 1963, Paris et al. used a fracture mechanics’ parameter to introduce an equation for the fatigue crack growth rate in ductile materials and this equation is now commonly known as the ‘Paris law’. However, the Paris law and the semi-empirical models that followed ever since do not fully account for the main intrinsic and extrinsic properties involved with fatigue crack growth in metallic alloys. In contrast, here we introduce a dimensionally correct fatigue crack growth rate equation that is based on the original crack driving force as introduced by Griffith and the presence of plasticity in a metal to withstand crack propagation.
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The present work is dedicated to a comparative analysis of strain accumulation and damage initiation in a forged Ti-6Al-4V alloy subjected to either fatigue or dwell-fatigue condition. To this end, high-resolution digital image correlation analyses were carried out on fatigue specimens interrupted at different number of cycles to clarify the grain-scale strain activity and correlate it with local micro-mechanical features and crack initiation sites.
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The major challenge in the mechanics of elastic-plastic fatigue crack growth (FCG) is to find a physically based driving force to correlate the crack growth rates under stress-controlled and strain-controlled conditions. Specifically, a parameter capable of providing a single-valued correlation of crack growth rate, regardless of applied fatigue stress/strain values, is needed. Approaches of the past used either cyclic strain (strain intensity factor) or nonlinear fracture mechanics based (cyclic J-integral, ∆J) parameter, to correlate fatigue crack growth. The latter, however, requires experimental load-deflection curve after every crack length increment and geometry correction factors, which are complex. In the present work, it is shown that a new and physically based approach, based on the cumulative change in the cyclic strain energy of the net-section, is used to successfully correlate fatigue crack growth in a variety of loading elastic-plastic loading situations. The change in the cyclic strain energy is determined analytically from tensile elastic-plastic behavior of material and from the relative sizes of cracked and uncracked sections in the crack plane. Remarkably, excellent correlations of fatigue crack growth data in a variety of specimen geometries and stress/strain levels have been found for both stress- and strain-controlled fatigue conditions.
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Martensite is applied as the main structure of high-strength steel to satisfy the demand for lightweight machines. As the fatigue crack extension life takes almost the whole fatigue life, the complex fatigue extension behavior needs to be further studied. This paper is planned to clarify the effect of the hierarchical microstructures and their interfaces on the fatigue extension and the fatigue crack extension mode for the safe and long-lasting use of this material. To achieve the proposal, the rotating bending fatigue tests of 18% Ni martenstic steel was carried out. Fatigue crack behavior and micostrcture near crack path was observed on the specimen surface. The crack extension was found to be discontinuous and was processed by the sub-crack initiation and coalescence with main crack. By the observation of the microstructure around crack path, the observed sub-crack was found to be the inter-granular crack. The proposed reason for the extension process was thought to be the strain localization by the slip along {110} plane and high angle microstructure intrefcae resistance to dislocation motion. Besides, the crack path included inter-granular and trans-granular crack. And the crack extension mode in this material was thought to be damage accumulation mode.
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To achieve high-strength steels, their microstructures are complicated. However, with effort, these high-strength steels do not exhibit the fatigue limits expected from their hardness or tensile strength. The low fatigue limit due to inclusions in the steels can be predicted as a fatigue limit problem for metallic materials with small defects. However, the threshold stress intensity factor range of high-strength steel of a long crack is still not as high as expected from the hardness. Currently, there is no clear explanation for this reason. Therefore, the material cannot be used with confidence. The authors propose that this is due to a different crack extension mechanism. In other words, the authors point out the existence of a different mechanism of fatigue crack extension from the generally accepted mechanism of fatigue crack extension due to plastic deformation by alternating slip. Based on the mechanism, the mode of fatigue crack extension is called damage accumulation mode fatigue crack propagation. This name differs from the conventional name focusing on the loading mode, i.e., Modes I, II, and III, and is focused on the extension mechanism. This study discusses a method to predict the fatigue crack propagation behavior.
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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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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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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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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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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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