Short Fatigue Cracks Research Articles

Application of fracture mechanics to weld fatigue

The application of fracture mechanics to the determination of the fatigue behavior of weldments is discussed with the focus on classic fatigue, i.e., the overall fatigue life and the fatigue strength in terms of an S-N curve and the endurance limit. The following issues are addressed: specific features of short fatigue crack propagation, an adequate initial crack size, multiple crack propagation and its statistical treatment as well as welding residual stresses. As an example, an approach of the authors is applied to the determination of FAT classes for a butt weld with varying weld toe geometry.

Effect of microstructural features on short fatigue crack growth behaviour in SA508 Grade 3 Class I low alloy steel

Aim of the paper is to understand the effect of microstructural features of SA508 Grade 3 Class I low alloy steel (LAS) on short crack propagation rate under cyclic loading. The complex upper bainitic microstructure of this LAS consists of low angle bainitic ferrite lath boundaries and high angle prior austenite grain boundaries (PAGBs). Compared to bainitic ferrite lath boundaries, the PAGBs provided major hindrance to short fatigue crack propagation in the subject LAS. The high angle PAGBs strongly resist the dislocation motion ahead of the crack tip as the crack tip approaches the PAGBs compared to that of low angle bainitic ferrite lath boundaries. This restriction of dislocation motion ahead of the crack tip based on hindrance provided by PAGBs resulted in retardation in short fatigue crack propagation rate along the crack path. The short fatigue crack propagated at stress intensity factor (SIF) range ‘ΔK’ values lower than threshold SIF range ‘ΔKth’ for the long cracks. The growth rate of short fatigue cracks cannot be predicted by Paris law which is applicable for long crack growth. This is due to the fact that crack growth rate undergoes acceleration and retardation in short crack regime because of microstructural effects.

Nucleation and propagation modeling of short fatigue crack in rolled bi-modal Ti–6Al–4V alloy

A numerical procedure centered around the crystal plasticity finite element method was developed for the simulation of fatigue crack initiation and propagation in metallic materials. Damage indicators were derived from theoretical dislocation-based models for the prediction of the crack initiation, crack path and crack growth rate, taking into account slip transfer across grain boundaries. They were incorporated into a numerical framework in which an explicitly growing crack was considered via remeshing technique to account for the redistribution of the micromechanical fields ahead of the crack tip. In this regard, a non-local averaging scheme was applied to account for both the spatial heterogeneity of the stress–strain fields and the grains morphology. The proposed framework was tested on a rolled bi-modal Ti-6Al-4V alloy subjected to a fully reversed uniaxial loading condition. Statistically representative synthetic microstructures were generated by a multi-scale anisotropic tessellation procedure considering the presence of multiple lamellar variants within a prior β-phase grain sharing the same orientation relationship. The predictions of crack initiation, crack path and growth rate were statistically compared with experimental data from prior study to emphasize the predictive capabilities and limitations of the proposed approach.

Fatigue short crack propagation behavior of selective laser melted Inconel 718 alloy by in-situ SEM study: Influence of orientation and temperature

The influence of orientation and temperature on fatigue short crack propagation behavior of Inconel 718 alloy was studied at both 25 °C and 650 °C by in-situ fatigue testing under scanning electron microscope. The fatigue crack growth rates (FCGR) showed evident dependence on orientation at both temperatures, following: XZ < X < Z. The FCGR difference of XZ and Z orientation became less pronounced at 650 °C. Fatigue cracks exhibited commonly transgranular fracture at both temperatures, with enhanced FCGR at 650 °C. Microstructurally short crack propagation showed evident fluctuations, which were closely related to the blocking effects of grain boundaries.

Generalization of the equivalent area method for the case of short fatigue cracks in a three-dimensional body

In industry, structural elements with small cracks are widely used; however, little research has been done on relatively simple methods for lifetime evaluation of such structural elements. This study proposes a generalization of the equivalent area method for a case of small fatigue cracks using an alternative representation of the relation between the crack propagation rate and the crack tip opening. A tension problem for a three-dimensional body with an elliptical crack has been solved analytically by applying the generalized equivalent area method and numerically by the Runge-Kutta method. The comparison of the obtained solutions has shown a good correlation of the results that confirms the potential application of the generalized equivalent area method to short crack problems.

Crystal plasticity finite‐element modelling of cyclic deformation and crack initiation in a nickel‐based single‐crystal superalloy under low‐cycle fatigue

AbstractNickel‐based single‐crystal superalloys are predominantly used for turbine blades in aircraft engines and land‐based gas turbines. Understanding and predicting the fatigue failure of Ni‐based single‐crystal superalloys are critical to ensure the safety of these components during operation. In this paper, low‐cycle fatigue experiments were carried out to investigate cyclic deformation of a nickel‐based single‐crystal superalloy MD2, recently developed by GE Power, with different crystallographic orientations. Specialty in situ scanning electron microscope (SEM) tests were also conducted to study the slip‐controlled initiation of short cracks under low‐cycle fatigue. In particular, the stress–strain response for both [001] and [111] orientations was used to calibrate a crystal plasticity model, which allowed us to simulate the activation of crystallographic slip systems and predict the initiation of short fatigue crack. Using the accumulated shear strain as a criterion, the simulations confirmed that the slip system with the maximum accumulated shear strain appeared to control the crack initiation. The location and direction of slip traces and short cracks, captured by the crystal plasticity finite‐element simulations, agreed with the in situ SEM observations. The modelling tool will be valuable for assessing the structural integrity of critical gas turbine blades.

Methodical approach for studying kinetics of short and long fatigue cracks growth for irradiated reactor materials. Part 2. Сonstruction of fatique propagation rate diagram on the basis of test of precracked charpy specimens

The finite element technique has been used for estimating the stress intensity factors (SIF) for short cracks emanating from a notch of single-edge bend specimen (SE(B)) and also weight function for this type of the specimen have been determined. On the basis of the received results the uniform equation for calculations SIF for short and long cracks in a specimen such as SEB (at h/W = 0.3) has been offered. The technique of construction of full kinetic diagrams of growth of the short and long fatigue cracks, initiated from the notch, is developed and approved. Full kinetic diagrams of growth of fatigue cracks (short and long) in steel Х18Н10Т and in weld metal in initial and irradiated (up to 40 dpa) conditions were built.

Physically short fatigue crack growth from notch described by plasticity-corrected stress intensity factor

When a physically short fatigue crack propagates from a notch root, the crack growth rate shows a “V-shaped” behavior; i.e., the rate firstly decreases and then increases as the crack propagates. In this study, a plasticity-corrected stress intensity factor (PC-SIF) is applied to describe the effects of both notch's plastic zone and crack-tip's plastic zone in ductile materials under cyclic loading. Experiments in previous study for disk-shaped compact tension (CT) specimens made of 1070 steel with different notch sizes and load ratios, as well as single-edged specimens made of low-carbon steel with U-shaped notches, are examined by numerical analysis in present paper. The V-shaped curves of fatigue crack growth data can be summarized by straight lines if the PC-SIF range is applied which considers the influence of plastic zone near the notch roots. The plastic zone near the notch root plays an important role in the V-shaped behavior for the physically fatigue short crack, and the evolution of the plastic zone is discussed in detail. The ratio of PC-SIF range to the stress intensity factor (SIF) range was fitted by the power functions and the fitting expressions before and after the turning point of the V-shaped curve exhibit a significant difference.

Microstructure-interacting short crack growth in blocky alpha Zircaloy-4

Microstructurally short fatigue crack growth in blocky alpha Zircaloy-4 is experimentally investigated in cyclic three-point bend testing. The short crack propagation is sensitive to the local microstructure with respect to grain crystallographic orientation and grain boundaries. Polycrystals with predominant c-axis texture aligned out-of-plane and normal to loading give alternating crack paths along prismatic planes. Samples with c-axis texture aligned in plane and normal to loading typically show straight paths along prismatic planes, sometimes tortuous paths, but always crystallographic. Prismatic <a>-direction crack growth rate is low compared to that for prismatic <c>-direction growth for given loading. Hence the crystallographic plane within which cracks grow is important for determining overall growth rate. For tortuous cracks, with the predominant c-axis texture in plane and normal to loading, crack growth occurs along basal, prismatic and pyramidal planes, deflecting from one slip plane to another during transgranular propagation.

Cracking simulation‐based fatigue life assessment

AbstractA fracture mechanics numerical model is developed to simulate the collective behavior of growing short fatigue cracks originating from the surface of unnotched round specimens made of a two‐phase alloy. The specimen surface roughness is considered resembling microcracks of different sizes and locations along the minimum specimen circumference. Material grains of different phases, sizes, and strengths are randomly distributed over that circumference. Variations in mechanical and microstructural features of grains are randomly distributed. Possible activities of surface cracks are predicted against loading cycles till either fracture occurs or all existing cracks become nonpropagating. The material's S‐N curve and fatigue limit can, thus, be assessed. Published experimental data on ferritic‐pearlitic steel specimens in push‐pull constant amplitude loading (CAL) were utilized. Different specimens were randomly configured and virtually tested. Comparison of experimental results and corresponding predictions validates the model, which, further, recognizes the effect of surface roughness, specimen size, and mean stress on lives.

Study on Short Fatigue Crack Behaviour of LZ50 Steel Under Non-Proportional Loading.

The low cycle fatigue tests using the replica technique for LZ50 steel under non-proportional cyclic loading were carried out, and eight groups of effective test data were obtained. The evolution behaviour of short cracks was studied based on the effective short cracks criterion. The results show that short cracks generally originate in the grain or along the grain boundary. At the microstructural short crack stage, the crack propagation is influenced strongly by the microstructure of the material, and the growth rate of the short crack slows down several times according to the number of obstacles encountered. At the physical short crack stage, the crack propagation breaks through the banded structure of pearlite. Thus, the dominant effective short fatigue crack is formed, and the crack growth rate increases rapidly. Based on the modified parameters of the uniaxial short crack model, an approach is presented to calculate the growth rate of short cracks under multi-axial non-proportional loadings, and the new model can consider the non-proportional factor F. The fitting results of the multi-axial microstructural obstacles model are compared with test data. The comparison results show that this model can reflect the trend of short fatigue crack propagation rate under non-proportional loadings.

An iterative technique to assess the fatigue strength of notched components

The present work provides an efficient formulation to assess the growth of short fatigue cracks in metallic components. The proposed technique consists on the iterative combination of a micromechanical short-crack growth model and the Finite Elements Method. The interaction of the crack with the microstructure of the material is evaluated through the dislocations distribution technique. The finite elements analysis of the problem is needed to obtain the stress gradient ahead of the notch. The division of the main problem into simpler scenarios makes the resolution of the method easier since cases with known solutions are required exclusively. The iterative method formulation is properly described and application examples are given in order to show its usefulness.

Short fatigue crack behaviour of LZ50 railway axle steel under multi-axial loading in low-cycle fatigue

An experimental study was carried out on the low-cycle fatigue of LZ50 railway axle steel under multi-axial loading. The initiation and propagation behaviour of a short fatigue crack was observed and analysed as per the principles of effective short fatigue crack. Our results showed that in the microstructurally short crack (MSC) stage, the crack propagation was strongly influenced by the microstructural features of the material and did not satisfy the linear elastic fracture mechanics. At the physically short crack (PSC) stage, the crack was long enough to not be directly influenced by the microstructures; nevertheless, linear elastic fracture mechanics still could not explain the law of crack propagation properly owing to the large-scale yield range of the crack tip. Based on these observations, a crack growth rate model under multi-axial loading was presented to explain the short fatigue crack behaviour. The non-linear elastic-plastic fracture mechanics parameter, J-integral (cyclic), was adopted as the driving parameter in this model and the crack closure effect was also taken into account. In the MSC stage, the crack growth rate fluctuated significantly when the cyclic J-integral was used owing to the influence of the microstructure. However, at the PSC stage, the crack growth rate fluctuations were small. A linear correlation for the PSC was obtained with R = 0.934 and this model could be applied to describe the short fatigue crack behaviour in the PSC stage.

Strain visualization of growing short fatigue cracks in the heat-affected zone of a Ni–Cr–Mo–V steel welded joint: Intergranular cracking and crack closure

Physically short fatigue crack growth behavior in the heat-affected zone of a Ni–Cr–Mo–V steel welded joint was investigated in-situ in SEM to reveal effects of crack closure and tortuous crack growth path on crack-tip straining behavior with the help of high resolution digital image correlation technique. The crack growth path and associated fatigue damage at microstructural scale was characterized by electron backscatter diffraction and electron channeling contrast imaging techniques. Results showed that the short crack growth was influenced by both local microstructure and global strength gradient. The short crack could deflect intergranularly when meeting with one large grain due to misfit deformation and strain localization. The crack closure, which was accompanied during loading and unloading in terms of crack surface contact, shielded the real crack-tip and manifested itself well in both compressive strain at crack wake and strain development at crack-tip. It is indicated that strain localization at lathy boundaries and formation of sub-grains was responsible for the fatigue of short cracks.

Fatigue crack propagation analysis in structures with random parameters based on polynomial chaos expansion method

Fatigue fracture is the most common failure for aircraft structures, therefore fatigue life prediction plays an important role in structure design. However, uncertainties widely existing in engineering structures make it difficult to analyze crack propagation and predict fatigue life. This paper presents a novel efficient and accurate approach to crack propagation analysis in structures with random uncertainties based on polynomial chaos expansion (PCE) method. For short fatigue crack in aircraft skin or armor plate of tank, the structure can be regarded as an infinite plate. In this situation, the stress intensity factor range has a simple analytical expression and a basic crack propagation equation is derived based on it. Considering uncertain parameters, crack length is regarded as a random variable and expanded into orthogonal polynomial series after introducing Hermite polynomials. Because crack length is a time-dependent variable, the coefficients in polynomial series are regarded to be time-dependent. In order to determine values of these coefficients, orthogonal polynomial series of crack length is substituted into Paris law and a series equations are obtained considering the orthogonality of Hermite polynomials. Time history of the coefficients is acquired by solving these equations using numerical algorithm. Furthermore, expectation and standard deviation of crack length can be easily obtained. The proposed method is verified based on three numerical examples and Monte-Carlo Simulation (MCS) is applied for the sake of validation. The results demonstrated the accuracy and efficiency of the proposed method for crack propagation analysis of structures with random parameters.

In situ synchrotron ultrasonic fatigue testing device for 3D characterisation of internal crack initiation and growth

AbstractThis work presents a new ultrasonic fatigue testing device for studying the initiation and propagation mechanisms of internal microstructurally short fatigue cracks using in situ synchrotron tomography. Its principle is described as well as the method used for automatically detecting crack initiation and its subsequent growth. To promote internal crack initiation, specimens containing internal casting defects were tested between the high cycle and very high cycle fatigue regimes (107‐109cycles). Preliminary results show the ability of this new device to initiate an internal microstructurally short crack in a reasonable testing time and monitor its growth.

Fractographic Study on Naturally Initiated Short Fatigue Cracks in a Near-Lamellar TiAl Alloy at Room Temperature

Short crack phenomena are considered important for lamellar structures in γ-TiAl alloys and have been thoroughly investigated in the past. However, the short cracks in the previous studies were nearly all introduced artificially. No particular investigations have looked into the initiation of fatigue short cracks. Therefore, naturally initiated short fatigue cracks at room temperature under two different stress ratios (0.1 and 0.5) were investigated in a near-lamellar γ-TiAl alloy (Ti-45Al-2Mn-2Nb) in this study. The observations show that the fatigue crack initiation behaved differently at low and high stress ratios. At low stress ratio, the specimens failed at lower ultimate stress levels (σmax = 450 and 475 MPa), and the crack initiated from the cluster of interlamellar fracture near mode-I orientation or stress concentration areas. At the higher stress ratio, the specimens failed at higher but consistent stress levels (σmax = 560 and 570 MPa), and in the specimen crack initiation areas, the interlamellar fractures were still the primary fracture mode, whereas some were found at tilted angles due to shear deformation. The results suggest that short fatigue cracks can naturally initiate in lamellar γ-TiAl alloys, thus attention should be paid to their microstructure design, surface finishing and cleanliness.

On the numerical modeling of nucleation and growth of microstructurally short cracks in polycrystals under cyclic loading

Abstract

Methodical approach for studying kinetics of short and long fatigue cracks growth for irradiated reactor materials. Part 1. Statement of problem. The effect of the initial notch acuity on the fatigue crack rate on small-sized specimens

The paper considers methodical issues in the experimental research of fatigue crack growth kinetics when testing irradiated small-sized specimens. The effect of the initial notch acuity is studied on the long crack growth rate. The stress concentration zone sizes are estimated for notches of various types. A brif literature review of the main problems in the study of the growth kinetics of short fatigue cracks has been performed. The tasks of further research are formulated.

Short review on multiscale short fatigue crack growth model

Short review on multiscale short fatigue crack growth model