Fatigue Crack Closure Behavior Research Articles

Direct observation of three-dimensional short fatigue crack closure behavior in Ti-6Al-4V alloy using ultra-high-resolution X-ray microtomography

Local 3D short fatigue crack closure behavior was investigated in Ti-6Al-4V alloy using ultra-high-resolution X-ray microtomography (XMT). The results show that the inhomogeneous distribution of plasticity-induced and roughness-induced crack closure is caused by the variation of crack path morphologies. These crack path morphologies exhibited heterogeneous plastic deformation at the crack-tips due to the anisotropic nature of α grains and varying extents of crack tilting and twisting caused by the interaction of the crack front with α/α boundary or α/α + β interface. It was revealed via surrogate-based statistical analysis that the Schmid factor has the strongest effect on crack growth rate.

Elasto-Plastic Fatigue Crack Growth Behavior of Extruded Mg Alloy with Deformation Anisotropy Due to Stress Ratio Fluctuation.

Fatigue crack growth (FCG) experiments were performed using a low-temperature extruded magnesium alloy AZ31 with texture. Under a constant maximum stress intensity factor (Kmax), the stress ratio R was changed from 0.1 to −1 during the fatigue crack growth process, and the FCG behavior before and after the R change was investigated. As a result, tensile twins were generated owing to the fatigue load on the compression side of R = −1, and the FCG velocity was accelerated. In addition, when the maximum compressive stress at R = −1 (|(σmin)R = −1|) exceeded the compressive yield strength of the material (σcy), the FCG velocity after R fluctuation greatly accelerated. On the other hand, under the condition |(σmin)R = −1| < σcy, the degree of acceleration of the FCG velocity due to R fluctuation was small. In either case, the degree of acceleration in the FCG increased as the Kmax value increased. The above FCG acceleration mechanism due to the R fluctuation was considered based on the observation of the deformation and twinning states of the fatigue crack tip, the fatigue crack closure behavior, and the cyclic stress–strain curve of the fatigue process. The FCG acceleration mechanism was as follows: First, the driving force of the FCG increased owing to the increase in crack opening displacement due to the generation of tensile twins. Second, the coalescence of the main crack and a plurality of microcracks were generated at the twin interface. The elasto-plastic FCG behavior after the stress ratio fluctuations is defined by the effective J-integral range ΔJeff.

Effect of twins and plastic-deformation anisotropy of extruded magnesium alloy on fatigue crack growth and crack closure behavior

To study the effect of twins and plastic deformation anisotropy on the fatigue crack growth (FCG) behavior and crack closure behavior of extruded AZ31B magnesium alloys, two types of single-edge notched tensile specimens were prepared, with loading directions parallel (specimen E) and perpendicular (specimen T) to the extrusion direction (ED). FCG experiments using these specimens were performed at stress ratios R = 0.1 and −1, and cyclic speed of 10 Hz.In extruded magnesium alloy materials, the c-axis of the crystal is radially oriented on a plane perpendicular to the ED, indicating the generation of a strong texture.In the FCG experiment at R = −1 for specimen E, elongation in the c-axis direction occurred under compressive load, which generated {101‾2} <101‾1> tensile twins. The tensile twins increased the material deformation and fatigue crack opening distance. As a result, the effective stress intensity factor range ΔKeff increased, leading to the acceleration of the FCG velocity. At R = 0.1, tensile twins were not generated because the load waveform had no compressive load component; there was no change in ΔKeff and no acceleration of FCG.For specimen T at R = 0.1 and −1, tensile twin generation was expected because elongation in the c-axis direction occurred under tensile load. However, tensile twins did not occur, which is attributed to the effect of plastic deformation anisotropy. Therefore, for specimen T, the increase in ΔKeff and FCG acceleration did not occur, and no difference due to the R value was observed.

3D short fatigue crack closure behavior in Ti-6Al-4V alloy investigated using in-situ high resolution synchrotron X-ray tomography

In-situ synchrotron X-ray microtomography has been employed to investigate the three-dimensional (3D) closure behavior of short fatigue crack in Ti-6Al-4V alloy. At a spatial resolution level of 1 µm, tomography datasets were taken at two short crack fronts to observe the time evolution behavior of closure (sometimes called 4D imaging). Owing to the high spatial resolution level, the complex 3D crack morphology consisting of deflections, facet paths and crack branching/overlap caused by the 3D variation in the microstructural features were readily observed together with the occurrence of crack closure. This study separated the contributions of near-tip and behind-tip crack closure and observed their behavior with crack growth. It was deduced that the evolution of behind-tip closure with increase in crack length predominantly affects the overall closure behavior in the short crack regime. Crack front closure behavior variation was found to be attributable to the existence of different crack path morphologies which showed varied degree of fracture surface roughness due to Mode II and Mode III displacements. The relationship between closure, crack tip opening displacement range (ΔCTOD) and crack growth rate are also discussed.

Small Fatigue Crack Initiation and Growth Behavior in Notch Root for Low Carbon Steel

To clarify the small fatigue crack initiation, growth and crack closure behavior at the stress concentration area of welding, microscopic successive observation with automatic microscope system was conducted for notch root of heat treated low carbon steel, which simulate coarse grain (CGHAZ) and fine grain (FGHAZ) heat affected zone (HAZ) microstructure. As a result, fatigue life of notched CGHAZ specimen was shorter than that of FGHAZ. In addition, to evaluate the small fatigue crack growth behavior at notch root, this study proposed to use crack opening displacement (COD) as a related value of the crack depth. The COD, which is related to small fatigue crack size, of CGHAZ grow faster than that of FGHAZ microstructure. Furthermore, small fatigue crack opening stress were directly measured with digital image correlation (DIC) analysis. Small fatigue crack closure at notch root were successfully measured with microscope system with DIC analysis. As a result, CGHAZ microstructure shows lower crack opening stress than FGHAZ. This is one of the reason why CGHAZ shows shorter fatigue life and faster COD growth rate than FGHAZ.

Experimental and Numerical Simulation Study of Plasticity-Induced and Roughness-Induced Fatigue Crack Closure

In this study, fatigue crack closure behavior was investigated in the aluminum alloy 6061-T6 and the carbon steel JIS. S25C. It was found that crack closure in the aluminum alloy 6061-T6 showed the characteristics of plasticity-induced fatigue crack closure (PIFCC), whereas the carbon steel JIS. S25C showed the characteristics of roughness-induced fatigue crack closure (RIFCC). The experiments included the determination of the crack-opening levels Kop as a function of stress intensity factor range ΔK and the effect of surface removal on the crack-opening level. In order to simulate the behaviors of the plasticity-and the roughness-induced fatigue crack closure, the finite element method was adopted. The results of FEM were in good agreement with the experimental results. It was cconcluded that at a given yield strength level , a low Youngs modulus and a low work-hardening coefficient will favor PIFCC, whereas a high Youngs modulus and a high work-hardening coefficient will favor RIFCC.

Assessment of the interaction between two collinear cracks in plates of arbitrary thickness using a plasticity‐induced crack closure model

ABSTRACTFracture and fatigue assessment of structures weakened by multiple site damage, such as two or more interacting cracks, is currently a very challenging problem. The main objective of this paper is to develop a mathematical model and an approach to investigate fatigue crack closure behaviour of two through‐the‐thickness collinear cracks of equal length in a plate of arbitrary thickness under remote tensile cyclic loading. The developed mathematical model of the problem under consideration is based on the Dugdale strip yield model and plasticity‐induced crack closure concept. The approach utilises the fundamental solution for an edge dislocation in a plate of finite thickness and the distributed dislocation technique to obtain an effective and accurate solution to the system of governing equations. The obtained results show a very good agreement with the previously published analytical solutions for limiting cases. In particular, the new results confirm that the crack closure behaviour and the opening stress variation in the case of two collinear cracks are significantly dependent on the separation gap between two cracks as well as the plate thickness.

Fatigue crack growth and closure behavior under random loadings in 7475-T7351 aluminum alloy

Fatigue crack growth and closure behavior under random loadings in 7475-T7351 aluminum alloy is investigated by performing various random loading tests. The effects of random load spectrum, history length, and stress ratio on the crack growth and closure behavior are discussed. The crack opening load was automatically determined by using the normalized-extended ASTM method. The crack opening load is nearly constant during a random loading block and is mainly governed by the largest load cycle in a random load history, irrespective of random load spectrum and history length. The fatigue crack growth under random loading can be well described by the crack closure concept.

603 厚さの異なる炭素鋼並びにアルミニウム合金の疲労き裂閉口挙動とそのメカニズムに関する検討(GS21-1-1 材料力学/材料加工, 疲労特性)

603 厚さの異なる炭素鋼並びにアルミニウム合金の疲労き裂閉口挙動とそのメカニズムに関する検討(GS21-1-1 材料力学/材料加工, 疲労特性)

Fatigue crack growth and closure behaviour under multiaxial loading

The three dimensional finite- element analysis of fatigue crack growth and closure was used to investigate crack- opening and closure stresses of elastic-perfectly plastic Aluminium 2024 alloy under constant amplitude crack extensions.. The 3D analysis was applied to the middle-crack specimen with a specified thickness value under multiaxial and uniaxial loadings. Then, crackopening and closure stresses and the v-displacements of nodes on the crack surface plane for both multiaxial and uniaxial cases were investigated.The finite element analysis was developed using isoparametric eight nodded hexahedron elements. The plasticity part of the program uses initial stress approach. In the analysis, the crack was advanced one element size as the applied load reached the maximum Value of each load cycle.

Fatigue properties and crack propagation behavior of stainless cast steel for turbine runner of hydraulic power generation

In order to clarify the fatigue properties of stainless cast steel JIS SCS6 used as the turbine runner of hydraulic power generation for about 27 years, fatigue tests were carried out by using tension-compression specimens, plane bending specimen and CT specimens. The results are summarized as follows: The fatigue strength of tension-compression was lower than that of plane bending. In order to examine the reason for that, fracture surfaces were observed by SEM. It is because that in the case of tension-compression fatigue tests, the increase of volume of dangerous layer which was applied high stress led the increase of the size of the casting defect included there for the loading mode and the shape of specimens. It suggests that casting defects strongly affect the fatigue properties of aging stainless cast steel. Furthermore, according to the estimation of fatigue life, it is clear that fatigue life of aged stainless cast steel can evaluate by using the casting defect size at crack initiation site and the stress intensity factor calculated by defect size. In addition, to clear the fatigue crack propagation behavior, the relationship between the fatigue crack growth rate and effective stress intensity range was examined by using the unloading elastic compliance method which can observe the detailed fatigue crack closure behavior.

Effect of Absorbed and Environmental Hydrogen on Short Fatigue Crack Propagation near Threshold in Low Alloy Steel

The effects of hydrogen on fatigue crack propagation behavior of short fatigue crack and crack closure behavior were studied using low alloy steel SCM440H. Fatigue crack propagation test using specimen which has a 50µm deep pre-crack was conducted at a frequency of 28Hz in air, vacuum and hydrogen gas. Hydrogen pre-charge was done by cathodic polarization method. The concentration of absorbed hydrogen was 0.83 ppm in hydrogen charged specimen and 0.06 ppm in uncharged specimen. The fatigue crack propagation rate of uncharged material was dependent on environments. The acceleration of fatigue crack propagation rate by absorbed hydrogen was about 2 to 5 times irrespective of environment. As a consequence, the fatigue crack propagation rate of hydrogen pre-charged material tested in air was the highest. The crack opening stress was higher in hydrogen gas and in vacuum compared with that in air. Fatigue crack propagation rate was summarized in two independent bands when plotted against effective stress intensity factor range. This means that the effects of environment (vacuum, air and hydrogen) and stress ratio (R = -1, 0, 0.6) could be explained by crack closure phenomenon. However, the acceleration of crack propagation by absorbed hydrogen could not be explained only by crack closure phenomenon.

低合金鋼の下限界近傍の微小疲労き裂進展に及ぼす水素侵入と水素雰囲気の影響

The effects of hydrogen on fatigue crack propagation behavior of short fatigue crack and crack closure behavior were studied using low alloy steel SCM440H. Fatigue crack propagation test using specimen which has a 50µm deep pre-crack was conducted at a frequency of 28Hz in air, vacuum and hydrogen gas. Hydrogen pre-charge was done by cathodic polarization method. The concentration of absorbed hydrogen was 0.83 ppm in hydrogen charged specimen and 0.06 ppm in uncharged specimen. The fatigue crack propagation rate of uncharged material was dependent on environments. The acceleration of fatigue crack propagation rate by absorbed hydrogen was about 2 to 5 times irrespective of environment. As a consequence, the fatigue crack propagation rate of hydrogen pre-charged material tested in air was the highest. The crack opening stress was higher in hydrogen gas and in vacuum compared with that in air. Fatigue crack propagation rate was summarized in two independent bands when plotted against effective stress intensity factor range. This means that the effects of environment (vacuum, air and hydrogen) and stress ratio (R = -1, 0, 0.6) could be explained by crack closure phenomenon. However, the acceleration of crack propagation by absorbed hydrogen could not be explained only by crack closure phenomenon.

Tri-dimensional numerical modelling of plasticity induced fatigue crack closure

Fatigue crack closure has been studied numerically with 2D finite element models as alternative to experimental methods. In a few cases fatigue crack closure behaviour has been analyzed with tri-dimensional models. In this work, a compact C(T) aluminium specimen has been modelled tri-dimensionally. The minimum element size is smaller than common accepted recommendation. This parameter limits the mesh size along the thickness. The number of divisions of the thickness is a special issue studied in this work. Different thicknesses, range of loads and stress relations have been calculated. The results show the crack behaviour through the thickness. The plastic zone is visualized and quantified. An abrupt transition is observed in a thin external slice of the specimen; only captured with a fine meshing of the thickness. Crack closure/opening along the thickness have been computed. Results show that closure is only relevant in this small external area and its size is not affected by the thickness. Results match experimental observation and have been correlated with experimental tests.

Understanding fatigue crack propagation in AISI 316 (N) weld using Elber’s crack closure concept: Experimental results from GCMOD and acoustic emission techniques

An experimental study of fatigue crack propagation and crack closure behaviour, in compact tension specimens of AISI 316 (N) weld has been conducted. The crack closure load was determined from the changes in the slope of the load–displacement curves using global crack mouth opening displacement (GCMOD) type gauge. The results were compared with those measured by acoustic emission technique which showed good agreement with each other. The experimental data bear clear evidence of fatigue crack closure. The crack opening force was found to increase moderately with crack length and increasing R-ratio, under a constant P max of 5 kN. Above a critical R-ratio of 0.45 (approximately), the crack closure load is smaller than the minimum applied load. A good correlation was obtained for Δ K eff/Δ K = 0.6684 – 2.4135 R + 7.0077 R 2 in the range 0 ⩽ R ⩽ 0.5. The magnitude of crack closure is used to interpret observed crack growth behaviour at different R-ratios.

Elucidation of fatigue crack closure behaviour in surface crack by 3-D finite element analysis

A 3-D elastic–plastic finite element analysis of fatigue crack closure for semi-circular crack is performed. The stabilization behaviour of crack opening level, the effect of mesh size, the effect of initial crack length, the opening process of crack surface and the shortcoming and sensitivity of experimental measurement methods are investigated. The crack opening load tends to increase with the decrease of mesh size. Irrespective of an initial crack length, all the first nodes behind the crack tip along the front of surface crack are the last to open. The plane strain-opening load at the deepest point measured by extensometer at the midpoint of crack mouth may be estimated to be higher than the actual. The crack opening load experimentally measured by strain gauge boned in the vicinity of the surface intersection point can successfully represent the plane stress crack opening of the free surface.

B22 アルミニウム合金の疲労き裂開閉口挙動に及ぼす圧縮最小応力の影響(B2 材料力学6)

B22 アルミニウム合金の疲労き裂開閉口挙動に及ぼす圧縮最小応力の影響(B2 材料力学6)

Fatigue crack growth resistance and crack closure behavior in two aluminum alloys for aeronautical applications

Aluminum-lithium alloys are candidate materials for many aerospace applications because of their high specific strength and elastic modulus. These alloys have several unique characteristics such as excellent fatigue crack growth resistance when compared with that of the conventional 2000 and 7000 series alloys. In this study, fatigue crack propagation behavior has been examined in a commercial thin plate of Al-Li-Cu-Mg alloy (8090), with specific emphasis at the fatigue threshold. The results are compared with those of the traditional Al-Cu-Mg alloy (2024). Fatigue crack closure is used to explain the different behavior of the compared alloys.

Numerical analysis for prediction of fatigue crack opening level

Finite element analysis (FEA) is the most popular numerical method to simulate plasticity-induced fatigue crack closure and can predict fatigue crack closure behavior. Finite element analysis under plane stress state using 4-node isoparametric elements is performed to investigate the detailed closure behavior of fatigue cracks and the numerical results are compared with experimental results. The mesh of constant size elements on the crack surface can not correctly predict the opening level for fatigue crack as shown in the previous works. The crack opening behavior for the size mesh with a linear change shows almost flat stress level after a crack tip has passed by the monotonic plastic zone. The prediction of crack opening level presents a good agreement with published experimental data regardless of stress ratios, which are using the mesh of the elements that are in proportion to the reversed plastic zone size considering the opening stress intensity factors. Numerical interpolation results of finite element analysis can precisely predict the crack opening level. This method shows a good agreement with the experimental data regardless of the stress ratios and kinds of materials.

Three-dimensional finite element analysis of fatigue crack closure behavior in surface flaws

Finite element technique has been widely used in 2-D fatigue crack closure problems for almost three decades. However, the reported 3-D surface crack closure results obtained from this technique are scarce. One possible cause could be the meshing difficulty. In the present study, meshing issues are discussed. Subsequently, both semi-circular and semi-elliptical crack closure behaviors at various loading conditions are calculated. The calculated results show that the closure and opening phenomena of surface cracks are caused by two types of plastic wake, that is, the near-surface type and the interior region type. In the stress ratio R=0 cases, the final crack-front opening always takes place at the free surface. However, the calculated R=−1 results show a different crack opening sequence. In the semi-elliptical crack results, along the crack front, different opening levels are found at various locations of the interior region. Finally, surface crack shape development incorporating the crack closure effect is also briefly discussed.