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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Themes: JoDean Morrow & Paul Paris Memorial Symposium on Fatigue & Fracture
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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BIAXIAL LOADING IMPACT ON FATIGUE CRACK PROPAGATION IN METALLIC MATERIALS
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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BACK TO BASICS FOR THE FATIGUE CRACK GROWTH RATE IN METALLIC ALLOYS
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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FATIGUE AND DWELL-FATIGUE BEHAVIOR OF A FORGED TI-6AL-4V ALLOY INVESTIGATED BY HIGH-RESOLUTION DIGITAL IMAGE CORRELATION
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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A DIRECT APPROACH TO FATIGUE CRACK GROWTH UNDER LARGE SCALE PLASTICITY (PRESENTATION IN HONOR OF JODEAN MORROW, UNIVERSITY OF ILLINOIS)
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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EFFECT OF DYNAMIC EMBRITTLEMENT ON FATIGUE CRACK PROPAGATION MECHANISM AND CRACK GROWTH RATE IN IN718 [Keynote]
IN718 is a commonly used nickel-base alloy for high temperature applications, e.g., in gas and steam turbines. At elevated temperatures, this and other superalloys are prone to the failure mechanism “dynamic embrittlement”. In order to reveal the mechanism of dynamic embrittlement, high-temperature fatigue crack propagation tests were carried out at 650°C applying various dwell times and testing frequencies. Most of the tests were performed in laboratory air, but some experiments were run in vacuum as well, in order to eliminate environmental effects and, hence, to define the reference fatigue crack propagation behavior. Based on the results obtained, a model was developed for the range of test parameters, where intergranular and transgranular areas exist side by side in the fracture surface. This model provides a quantitative mechanismen-related description of the effect of dynamic embrittlement on fatigue crack propagation rate.
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FATIGUE CRACK EXTENSION MODE OF 18%NI MARTENSITIC STEEL
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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