Fatigue Crack Initiation In Metals Research Articles

Hydrogen-assisted intergranular fatigue crack initiation in metals: Role of grain boundaries and triple junctions

In this work, small-scale, low-cycle fatigue experiments on hydrogen charged nickel specimens are performed that highlight particular grain boundaries (GBs) and triple junctions as potential intergranular crack initiation sites. To understand the micromechanics and underlying physics, a dislocation density-based crystal plasticity model coupled with slip-rate based hydrogen transport model is developed. A fatigue indicator parameter (FIP) is also developed that models the crack initiation process by considering the contributions of accumulated plastic slip, GB normal stress, and local hydrogen concentration. Depending on the diffusivity, hydrogen binding energy, and misorientation, GBs are categorized as ‘special’ or ‘random’, and their role on hydrogen distribution is analysed using a model microstructure. Special GBs are ones with low diffusivity and high hydrogen binding energy whereas the random GBs have high diffusivity but low hydrogen binding energy. Complying with the experimental observations, the evolution of FIP with load cycles suggests certain triple junction configurations in the microstructure involving in the crack initiation process. For the case of uniform initial hydrogen concentration, special GBs are found to retain more hydrogen with load cycles primarily due to their low hydrogen diffusivity whereas the random GBs diffuse hydrogen out quickly to the bulk. The high hydrogen concentration and favourable stress state in the form of high hydrostatic stress spots generally found at triple junction of special/random GBs fulfil the necessary condition leading to an intergranular crack initiation.

A Probabilistic Fatigue Framework to Enable Location-Specific Lifing for Critical Thermo-mechanical Engineering Applications

The present paper describes a probabilistic framework to predict the fatigue life and failure mode under various thermo-mechanical loading conditions. Specifically, inclusion- and matrix-driven competing failure modes are examined within nickel-based superalloys. The critical accumulated plastic strain energy density (APSED) is employed as a unified metric to predict fatigue crack initiation in metals, which is favorable due to the usage of a single unknown parameter and its capability to predict failure across loading conditions and failure modes. In this research, we characterize the temperature-dependent variation of the critical APSED using a Bayesian inference framework and predict the competing failure modes in a coarse grain variant of RR1000 with varying strain range and temperature. The critical APSED appears to decrease along a vertically reflected sigmoidal curve with increasing temperature. Further, (a) the prediction of a failure mode, (b) failure mode associated with the minimum life, and (c) the change in the location associated with the matrix-driven failure mode with increasing temperature and decreasing strain range are consistent with the experimentally observed trends in RR1000, as well as other Nickel-based superalloys, documented in the literature. Finally, for each simulated loading condition, the uncertainty in the fatigue life is quantified as a prediction interval computed based on a 95% confidence level of the critical APSED and the computed APSED from simulations. The overall framework provides a promising step towards microstructural-based fatigue life determination of components and enables a location-specific lifing approach.

Coupling Geometric and Microstructure Gradients in Fatigue Crack Formation

Scribe marks on aircraft structures are micron size surface scratches that usually result from removing sealant prior repainting. Some efforts revealed that these scratches can nucleate multiple shallow cracks through thickness and consequently reduce the fatigue life of aircraft components. Since the size of scribe marks and microstructure (e.g., grains) are of comparable dimensions, they both play a role in fatigue crack initiation in metals. This presentation introduces an approach that enables the quantification of microstructural effects in fatigue crack growth from notches with a low computational cost.

Towards the prediction of intergranular fatigue crack initiation in metals due to hydrogen

Hydrogen deteriorates the fatigue behaviour of metals by attacking their microstructure in different ways. For better clarity about hydrogen-based degradation of metals, it is vital to identify the microstructural configurations that promote fatigue crack initiation (FCI) in the presence of hydrogen. Intergranular regions in polycrystalline metals are more prone to hydrogen attack due to the prevailing atomic structure, elastic anisotropy, and plastic inhomogeneities causing increased hydrogen accumulation in this region. In this work, FCI is studied in a model material nickel in the uncharged and hydrogen charged state during in-situ strain controlled low cycle fatigue (LCF) testing under a scanning electron microscope (SEM). Crack initiation sites are characterized by investigating the elastic modulus in the loading direction as well as the maximum Schmid factor of the crack neighbouring grains extracted through the electron backscattered diffraction (EBSD) data. The crack frequency for the uncharged and hydrogen charged specimens is then analyzed using the difference in the elastic modulus (ΔE), the difference in the maximum Schmid factor (Δm), and ΔE/Δm ratio between the crack neighbouring grains. The comparison shows that for the hydrogen charged specimens, intergranular FCI sites show high values of ΔE/Δm compared to the uncharged specimens. These findings provide a predictive model for hydrogen linked FCI in metals. In addition, the synergistic role of the Hydrogen Enhanced Local Plasticity (HELP) mediated Hydrogen Enhanced Decohesion (HEDE) mechanism responsible for FCI is also demonstrated.

Analysis of fatigue crack initiation in cyclic microplasticity regime

The present work provides description of fatigue crack initiation in metals subjected to cyclic loading within the nominal elastic or initial elastic-plastic regimes next passing to elastic response during cyclic deformation and shake down process. It is assumed that damage growth proceeds due to action of local stress, specified as the sum of mean stress and its fluctuations induced by material inhomogeneities such as grain boundaries, inclusions, cavities, boundary asperities, also due to design notches or holes introduced into the element. The damage growth model is proposed, based on the critical plane concept. The macrocrack initiation then corresponds to a critical value of accumulated damage. The modelling of damage growth is supported by Electronic Speckle Pattern Interferometry (ESPI) apparatus using the coherent laser light. The damage growth effect is analysed by microindentation tests. The fatigue tests are performed for high strength steel specimens with central hole.

Another Mechanism for Fatigue Strength Improvement of Metallic Parts by Shot Peening

Fatigue crack source in shot-peened specimens may be located either at the surface or in the interior, beneath the hardened layer. In this paper, the mechanism for fatigue strength improvement of shot-peened specimens with internal fatigue crack source was studied. Un-peened and shot-peened specimens made of quenched and low-temperature tempered 40CrNi2Si2Mo2V steel were used. The fatigue crack source in shot-peened specimen is found in the interior beneath the hardened layer. X-ray diffraction analyses of both kinds of specimen fatigue tested at stress equal to their apparent fatigue limit show that obvious changes have taken place in the surface layer for un-peened specimens, while for shot-peened specimens, such changes are observed in the sub-surface layer beneath the hardened layer. The calculated actual critical stress at the fatigue source position (the “internal fatigue limit”) for shot-peened specimen is about 138% of the (surface) fatigue limit of un-peened specimen. According to an analysis about the micromeso-processes of fatigue crack initiation in metals, a concept of “internal and surface fatigue limits of metal” has been proposed. It is believed that the fatigue crack source transfers into the interior, Also, the internal fatigue limit of metal is higher than its surface fatigue limit, and is another mechanism for the improvement of apparent fatigue limit of shot-peened specimen.

Atomic force microscopy and modeling of fatigue crack initiation in metals

We have obtained the first atomic force microscope (AFM) images of cell like boundaries and slip band emergence of fatigued metals surfaces (titanium and high strength-low alloy steel). The AFM has been used to obtain quantitative information about slip spacings and slip height displacements. Using these measurements we have determined the fraction of plastic strain that contributes to surface displacement upset. Values of 1.6% for Ti and 0.9% for HSLA resulted for completely reversed plastic strain cycling. We introduce a model for fatigue crack initiation based on slip band spacings, slip height displacement, and cumulative plastic strain. To first order these compare well with experimental initiation data obtained by other investigators. The relative ease of sample preparation for AFM, along with its outstanding height resolution, offers a higher degree of simplicity and precision in fatigue crack initiation studies compared to more conventional techniques.

Mechanisms of fatigue crack initiation in metals: role of aqueous environments

Fatigue crack initiation in engineering materials has been the subject of considerable research. Most of these investigations focused on gaseous environment effects, and extensive review articles have appeared in recent times discussing the role of gaseous environments on crack initiation. Because of experimental difficulties, the effect of aqueous environments on mechanisms of fatigue crack initiation has received little attention, despite their unquestionable importance from an engineering standpoint. In this review, several of the fatigue crack initiation models are examined in detail and their anomalies discussed. The physics and micromechanisms of crack initiation during cyclic deformation in aqueous environments which are highly corrosive in nature are examined. The characteristics of the crack initiation process in aqueous environments are critically reviewed in the light of the specific role of several concurrent factors involving the nature of the aqueous medium, corrosion interactions, alloy chemistry, processing treatments, intrinsic microstructural effects and test variables.[/p]

Fatigue Fracture Initiation in Notched Graphite-Epoxy Specimens

An experimental program on fatigue of doubly notched [0/-60/+60] s graphite-epoxy specimens was performed which had two major objectives: 1. To adapt a photomicrographic technique, previously developed for detecting fatigue crack initiation in metals, for use with composite mate rials ; 2. To demonstrate the use of the technique in an exploratory fatigue program. Examination of enlarged images of photomicrographs, auto matically exposed at periodic interval during the tests, revealed that fatigue cracks developed early in the test. It was also evident that the test material had many manufacturing irregularities such as non-uniform ply-thicknesses and fiber distribution, incomplete ply bonding, and random voids. Two fatigue specimens sustained 10 Kc of high level cyclic loading and had a residual strength of approximately the static value; a third specimen frac tured on cycle 1 at 75% of the static strength. The inconsistent fatigue behavior may be due to the observed random distribution of manufactur ing irregularities.