Small Fatigue Crack Propagation Research Articles

Effect of microstructure on small fatigue crack initiation and early propagation behavior in super austenitic stainless steel 654SMO

The objective of this study was to investigate the cyclic deformation mechanisms at the microstructural level and their correlation with the initiation and early propagation of small fatigue cracks in super austenitic stainless steel 654SMO. The findings revealed that localized microstructural variations in grain size and orientation led to strain concentration, which subsequently triggered crack initiation. The behavior of surrounding microstructural evolution influences the crack propagation mode, further affecting the crack propagation rate. The retarding effect of small grain cluster regions on grain growth is weaker compared to high-angle grain boundaries and pinning by small grains, and multiple slip will trigger cross slip, causing uneven local strain within the grains, thus altering the crack propagation path.

A new data-driven probabilistic fatigue life prediction framework informed by experiments and multiscale simulation

Traditional probabilistic fatigue life test requires long time, a large number of samples and high cost due to dispersion, randomness and complexity. A new data-driven probabilistic fatigue life prediction framework informed by experiments and multiscale simulation, was proposed and applied to a Ni-based superalloy FGH96. It exhibits the advantages of less tests and fully consideration of the effects of microstructure dispersion on fatigue crack initiation, small fatigue crack (SFC) and long fatigue crack (LFC) propagation. The fatigue crack initiation life was accurately predicted by a stored energy model of persistent slip band (PSB) based on microscale crystal plasticity (CP) theory. The usage of artificial neural network (ANN) leads to the capability of rapidly obtaining CP parameters through the macroscopic Ramberg-Osgood (RO) relationship. Bayesian neural networks (BNN) was trained to predict probabilistic fatigue crack initiation life, SFC and LFC growth rates based on results obtained from CP modellings and carefully designed experiments. With BNN as a probabilistic distribution generator, a Monte Carlo (MCs) model is developed to capture the complete fatigue failure processes, including the crack initiation, SFC and LFC propagation. The total fatigue life and the proportions of life at different stages of superalloy FGH96 were predicted by MCs, which agree well with the experimental data. This work provides a new pathway for structural safety assessment and damage tolerance design.

Evaluation of the influences of the stress ratio, temperatures, and local microstructure on small fatigue crack propagation behavior of the FGH96 superalloy

Here, the influences of the stress ratios, temperatures, and local microstructure on the propagation behavior of small fatigue crack for FGH96 superalloy are quantitatively investigated. Following an increase in the stress ratio and temperature, the small fatigue crack growth rates increased. The influences of the local microstructure on the restrained ability for small fatigue crack propagation are analyzed, and the critical stress range of small fatigue crack propagation for the FGH96 superalloy at different stress ratios is proposed based on an amendatory-three-dimensional Kitagawa–Takahashi diagram. These results can aid in the fatigue damage tolerance design of FGH96 superalloy components.

Experimental and modeling study on small fatigue crack initiation and propagation behavior of Inconel 617

Small fatigue crack initiation and propagation behavior of Inconel 617 was studied by in-situ SEM observation, electron back-scattered diffraction (EBSD), digital image correlation (DIC), crystal plasticity finite element method (CPFEM) and extended finite element method (XFEM) in this paper. The experimental results indicated that different surface corrosion methods used to display surface microstructure will affect the small fatigue cracking mode of Inconel 617. The local micro-scale strain field of Inconel 617 near the notch and crack tip was quantitatively measured by a new grain scale DIC method. The CPFEM-XFEM simulation model of small fatigue crack initiation and propagation characteristics of Inconel 617 was established. A new framework for small fatigue crack path and lifetime prediction at the polycrystalline scale was constructed, which includes the influences of slip system and grain boundary on small fatigue crack initiation and growth rate.

Small fatigue crack propagation rate and behaviours in a powder metallurgy superalloy: Role of stress ratio and local microstructure

The role of stress ratio and local microstructure on small crack propagation (0.065–1.0 mm) behaviours in superalloy FGH4096 were investigated using the Photomicroscopy method. ΔK correlated the growth rates of small cracks with different stress ratios well, which is attributed to the low crack closure level. Statistical and crystallographic analysis revealed that grain boundaries resulted in crack temporary arrests, which is independent of loading conditions and crack length. Temporarily arrested cracks propagated further by connecting to the secondary crack that initiated on the slip plane that parallels to the twin boundary or in the grain with a local highest Taylor factor.

Identification of fatigue damage mechanism in PA38-T6 aluminum alloy under multiaxial loadings – initial research

This paper presents results of the initial research which originated from previous work, which was aimed to study the effect of asynchronous loadings in elastic-plastic strain range. Among the others, a study of fatigue cracks on the fatigued specimens’ surfaces, was performed.The aim of the present work is to identify the mechanisms of initiation and propagation of small fatigue cracks in selected materials under multiaxial loadings, including asynchronous cases. To study the evolution of small cracks and main crack formation, the cellulose acetate thin foil replication technique was utilized. Fatigue tests were performed on thin-walled tubular specimens, which were mirror-polished after CNC machining, to avoid the influence of unwanted factors.This paper presents the first part of fatigue tests, performed on specimens manufactured from PA38-T6 (AW 6060-T6) aluminum alloy. Fully reversed axial, torsional and 90º out-of-phase loadings were applied in elastic-plastic strain regime, with strain control. For these loading cases a shear damage mechanism was identified, based on the observation of small cracks. The small cracks initiated ate grew on maximum shear strain planes. The main crack formed at the very end of fatigue life by coalescence of small cracks of high density, regardless the applied loading case and level. An interesting difference in cracking behavior was observed in case of out-of-phase loading, on the low loading level. Small cracks propagated, and the main crack formed by linking of a few propagating cracks.The research is in progress and will be extended to the lower levels of strain as well as other cases of loadings, including asynchronous loadings. It is planned to conduct the research using other materials, for example non-alloy steel and austenitic stainless steel.

Planar slip-driven fatigue crack initiation and propagation in an equiatomic CrMnFeCoNi high-entropy alloy

High-cycle fatigue crack initiation and propagation in an equiatomic CrMnFeCoNi high-entropy alloy were investigated using smooth specimens. The microstructural deformation characteristics, i.e., planar dislocation slip, significantly affected the fatigue crack initiation and small fatigue crack propagation. The deformation localization associated with dislocation planarity led to multiple crack initiation on the slip planes. The crack propagation mechanism comprised crack formation on slip planes around the main crack tip and subsequent coalescence. The fatigue crack propagation mechanism shifted to Mode I type as the crack length increased.

Influence of grain size on the small fatigue crack initiation and propagation behaviors of a nickel-based superalloy at 650 °C

GH4169 at 650 °C in atmosphere was investigated by using single edge notch tensile specimens. The number of main cracks and crack initiation mechanisms at the notch surface strongly depended on the grain size. The crack initiation life accounted for more percentages of the total fatigue life for the alloy with smaller grain size. The fatigue life generally increased with increasing crack initiation life. The small crack transited to long crack when its length reached ˜10 times the grain size.

Full-field Strain Measurements for Microstructurally Small Fatigue Crack Propagation Using Digital Image Correlation Method.

A novel measurement approach is used to reveal the cumulative deformation field at a sub-grain level and to study the influence of microstructure on the growth of microstructurally small fatigue cracks. The proposed strain field analysis methodology is based on the use of a unique pattering technique with a characteristic speckle size of approximately 10 µm. The developed methodology is applied to study the small fatigue crack behavior in body centered cubic (bcc) Fe-Cr ferritic stainless steel with a relatively large grain size allowing a high spatial measurement accuracy at the sub-grain level. This methodology allows the measurement of small fatigue crack growth retardation events and associated intermittent shear strain localization zones ahead of the crack tip. In addition, this can be correlated with the grain orientation and size. Thus, the developed methodology can provide a deeper fundamental understanding of the small fatigue crack growth behavior, required for the development of robust theoretical models for the small fatigue crackpropagation in polycrystalline materials.

Full-field Strain Measurements for Microstructurally Small Fatigue Crack Propagation Using Digital Image Correlation Method

Full-field Strain Measurements for Microstructurally Small Fatigue Crack Propagation Using Digital Image Correlation Method

Role of Microstructure on Small Fatigue Crack Initiation and Propagation behavior of Rolled and Forged Ti-6Al-4V Alloy

Role of Microstructure on Small Fatigue Crack Initiation and Propagation behavior of Rolled and Forged Ti-6Al-4V Alloy

Small crack initiation and early propagation in an as-extruded Mg-10Gd-3Y-0.5Zr alloy in high cycle fatigue regime

High cycle fatigue testing was performed on an as-extruded Mg-10Gd-3Y-0.5Zr alloy to investigate its fatigue crack initiation and early propagation behaviors. Experimental results showed that only basal slip was activated at a low cyclic stress amplitude. Fatigue crack initiated from the slip bands along the basal plane, leading to the formation of cleavage-like facets on the fracture surface. The formation of facets around the crack initiation sites consumed a vast majority of the cyclic loadings in high cycle fatigue regime, and small fatigue crack propagation was significantly retarded by local microstructure heterogeneity of neighboring grains. This can be ascribed to the incompatible deformation at low cyclic stresses, resulting in the localization of slip bands within isolated grains.

3D characterization of the propagation of small fatigue cracks in steels with different forging conditions

The influence of forging conditions on the propagation of physically small fatigue cracks has been studied for two high strength steels. Two surface conditions were produced after the forging process. The subsurface microstructure of the materials has been characterized by EBSD. Small samples extracted from the original specimens were used to perform in situ fatigue tests monitored by high resolution synchrotron X-ray tomography. Fatigue cracks were initiated from an artificial defect (100 μm × 50 μm deep) introduced in the forging skin by laser machining. 3D images of the initiation and growth of those physically small fatigue cracks have been obtained. It was found that the presence of a shot-blasted skin containing a hardness and microstructure gradient influences the 3D crack shape during propagation in comparison with the materials without material properties gradient. The 3D crack shapes are rationalized in terms of crack closure effects induced by the forging processes, close to the surface.

A recursive Bayesian approach to small fatigue crack propagation and detection modeling

Engineers have witnessed much advancement in the study of fatigue crack detection and propagation modeling. More recently, the use of certain damage precursors such as acoustic emission signals to assess the integrity of structures has been proposed for application to prognosis and health management of structures. However, due to uncertainties associated with small crack detection of damage precursors and crack size measurement errors of the detection technology used, applications of prognosis and health management assessments have been limited. In this article, a methodology is developed for the purpose of assessment of crack detection and propagation parameters and the minimization of uncertainties including detection and sizing errors associated with a series of known crack detection and propagation models that use acoustic emission as the precursor to fatigue cracking. The methodology is facilitated by the Bayesian inference of a joint-likelihood model which includes sizing and detection models. Examples where several dog-bone Al 7075T6 specimens are tested to produce fatigue crack initiation and propagation data and estimates based on remaining useful life support the effectiveness and usefulness of the proposed methodology.

Fractographic analysis of fatigue crack initiation and propagation in CP titanium with a bimodal harmonic structure

Commercially pure (CP) titanium with a bimodal harmonic structure, which is defined as coarse grained regions surrounded by a network of fine grains, was fabricated by consolidating mechanically milled powders to improve both strength and ductility. In order to determine the fatigue properties of bimodal harmonic structured CP titanium, four-point bending fatigue tests were conducted at a stress ratio of 0.1 under ambient conditions. The fracture surfaces were analyzed by scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD) to identify the mechanisms of small fatigue crack initiation and propagation on the basis of fractography and crystallography. The fatigue crack initiation resistance was increased compared to sintered compacts fabricated from as-received initial powders, which was attributed to grain refinement during mechanical milling. Transgranular fracture occurred from coarse grains with a basal plane perpendicular to the loading direction. The fatigue crack path in harmonic structured CP titanium was not influenced by the microstructure.

Small Fatigue Crack Initiation and Propagation in Thermo-mechanical Processed Commercially Pure Titanium with Harmonic Structure

Small Fatigue Crack Initiation and Propagation in Thermo-mechanical Processed Commercially Pure Titanium with Harmonic Structure

Microstructurally small fatigue crack propagation and retardation at grain boundary in a polycrystalline Ni-base superalloy

Microstructurally small fatigue crack propagation and retardation at grain boundary in a polycrystalline Ni-base superalloy

Statistical fatigue properties and small fatigue crack propagation in bimodal harmonic structured Ti-6Al-4V alloy under four-point bending

Small fatigue crack propagation in bimodal harmonic structured titanium alloy (Ti-6Al-4V) with high strength and ductility was examined under four-point bending at a stress ratio of 0.1 in the ambient laboratory atmosphere. The crack profiles were observed using optical microscopy and scanning electron microscopy, and analyzed using an electron backscattered diffraction to examine the mechanism of small fatigue crack propagation. Fatigue crack paths were not influenced by the bimodal harmonic structure, and the crack growth rates, da/dN, in the harmonic structured Ti-6Al-4V were almost the same as those in a material with coarse acicular microstructure for comparable values of stress intensity range, ΔK. In contrast, the harmonic structured Ti-6Al-4V had a higher resistance of fatigue crack initiation due to the grain refinement induced by mechanical milling, which resulted in an increase of the fatigue life and fatigue limit. Furthermore, the statistical fatigue properties of Ti-6Al-4V alloy were analyzed using the stress dependence of Weibull parameters to quantitatively examine the effects of the bimodal harmonic structure on its fatigue life.

Strain accumulation during microstructurally small fatigue crack propagation in bcc Fe-Cr ferritic stainless steel

Strain accumulation was studied by digital image correlation technique (DIC) during microstructurally small fatigue crack propagation in polycrystalline 18%Cr ferritic stainless steel. Load-controlled fatigue testing was performed with R-ratio of 0.1 and frequency 10 Hz. The maximum applied stress was well below the yield stress of the studied material. The effect of the observed strain field on crack growth rate variation is discussed. Fracture surfaces were studied by scanning electron microscopy (SEM) evidencing the connection between the mechanism of the fatigue crack growth, accumulated strain and crack growth rate. Detailed study of fracture surface morphology was carried out by atomic force microscopy (AFM). Results indicate two processes of material damage accumulation and failure during cyclic loading: 1) local shear strain zones form successively ahead of the crack tip, and 2) fatigue crack growth occurs by both single- and multiple-slip mechanisms. The place and intensity of shear strain localization zones vary during the crack growth that is related closely to the local variation of crack growth rate.

Experimental investigation of short crack growth at notches in 7475-T761

Fatigue crack propagation experiments on AA7475-T761 have been undertaken using a DC potential drop measurement for determining the crack length. The signal of the potential drop measurement was smoothed using a LOESS filter algorithm. This method enhances the detection limit for small fatigue cracks and allows to define a lower limit for the beginning of the crack propagation. The investigation of short fatigue cracks is difficult, because of the geometry of the short cracks emanating from both corners of the notch in form of quarter elliptical cracks. The crack propagation in the notch root is accelerated by a higher stress concentration compared to the crack propagation on the specimen flanks. The crack propagation in the notch root is impeded by the slower crack propagation on the specimen surface. Therefore, a 2-dimensional model for the description of the propagation of small fatigue cracks at notches has to be developed.