Surface Crack Growth Rate Research Articles

A study of the morphological effect of an α-Al2O3 layer on the creep life for nickel-based superalloys using microstructure-based geometrical models

A study on the role of the irregular geometry of the α-Al2O3 film during the creep rupture test was conducted on samples of DD6 Ni-based single-crystal superalloy at 980 °C/250 MPa. This study involved experimental and finite element analyses of creep-oxidation specimens. The experimental results showed that the creep rupture life of the 0.67 mm-thin samples was shorter than that of the 0.81 and 1.12 mm specimens. Microscopic examination of the oxidation-affected zones showed that the morphology of the inner α-Al2O3 layer of a 0.67 mm-thin sample was discontinuous and incomplete, and the sample (1.12 mm thick) included a continuous and complete α-Al2O3 film. Hence, the influence of different morphologies of the α-Al2O3 layer on the stress distribution was studied numerically using the ABAQUS software. The finite element modeling results showed that the maximum stress was concentrated in the blocks of a discontinuous α-Al2O3 layer, which increased the diffusion of ions and metal cations. As a result, excessive growth of the oxidation-affected zone of the thin samples and the formation of AlN particles in high density occurred. It also contributed to an increase in the surface crack growth rate in the thin wall sample, which reaches the critical value of crack length in a shorter time than thick ones. Microstructure-based modeling has provided further insight into the role of the α-Al2O3 layer morphology in the deterioration of the mechanical properties of thin creep specimens.

Fatigue growth rate of inclined surface cracks in aluminum and titanium alloys

In this paper the fatigue crack growth tests were carried out on surface-crack tension (SCT) specimens, made of 7050 and Ti6Al4V alloys, with initial semi-elliptical surface cracks. Pure Mode I conditions were realized on SCT specimens with crack plane located orthogonal to the loading direction, while Mixed-mode conditions were observed on SCT specimens with inclined crack. Optical microscope measurements and the crack mouth opening displacement (CMOD) method were respectively used to monitor crack length and calculate crack depth. Current crack shape during the tests was highlighted by alternation of loading spectrum with baseline load block and a marker load block. The stress strain field along the crack front of semi-elliptical cracks in the SCT specimens was assessed by Finite Element Method (FEM) analysis. The stress intensity factors (SIFs) were calculated along crack fronts and equivalent elastic SIF formulation was used for crack growth rate assessment under mixed mode conditions. As a result, the fracture resistance parameters of aluminum and titanium alloys were obtained for two crack propagation directions under Mode I and Mixed-mode loading. The benefits of using the computational and experimental results of SCT specimen for the assessment of the surface crack growth rate in aluminum and titanium alloys under Mixed-mode loading conditions were stated.

Development and Numerical Justification of the Specimen for the Surface Crack Growth Rate Tests

Development and Numerical Justification of the Specimen for the Surface Crack Growth Rate Tests

Stress intensity factors in the specimen with a surface semi-elliptical defect

In this work, equations for stress intensity factors calculating in the specimen with a surface semi-elliptical defect in order to interpret experimental data of the surface crack growth rate tests at elevated temperatures were obtained. In the commercial finite element code ANSYS 3D finite element models with surface flaws for the range of the relative crack depth b/t = 0.1…0.7 and the aspect ratio b/a = 0.3…1 were generated. For these ratios stress intensity factors were calculated based on the finite element method. For the same configurations of semi-elliptical crack fronts using Murakami's stress intensity factors handbook stress intensity factors were calculated. It has been established that the results of numerical calculations and the Murakami's handbook have a similar pattern of the stress intensity factors distribution. For the stress intensity factors calculating in the specimen with a surface semi-elliptical crack to the Murakami's handbook specimen geometry-dependent stress intensity factor functions a correction function was added. The results allow to interpret the experimental data of the surface crack growth rate tests of the specimen at elevated temperatures.

Fatigue and creep-fatigue crack growth in aviation turbine disk simulation models under variable amplitude loading

This study analyzes the simulation model configuration and its loading conditions in fatigue and subsequent crack growth tests. Following the simulation principles, a full-size 3D finite element analysis of a representative GTE turbine disc was performed. With the aim of reproducing the simulation model, the in-service critical zone of both the stress–strain state and damage accumulation is studied. Based on the parametric computations, the block type loading conditions of the simulation models are determined and verified. Experimental study of the surface crack growth rate in the simulation model under harmonic and block loading at elevated temperature is performed for the fatigue and creep-fatigue interaction conditions. To interpret the experimental data, we calculate the nonlinear stress intensity factor distributions along the semielliptical crack front for flaws growing on the inner surface of the hole in the simulation model using numerical procedures. It is found that in terms of the nonlinear fracture resistance parameters, the sections of the fatigue fracture diagrams form one common curve representing the previous and subsequent increasing stages of block loading in pure fatigue and creep-fatigue interaction loading conditions. The possibility of using the proposed simulation model, tested under variable amplitude loading, to assess the structural integrity of GTE turbine disks is discussed.

An investigation on the circumferential surface crack growth in steel pipes subjected to fatigue bending

In the present paper, we propose an analytical method to calculate the Stress Intensity Factor (SIF) of circumferential surface cracks in steel pipes subjected to bending. In light of pipe geometry and bending load case, the analytical formula is raised by introducing new bending correction factors and new geometry correction factors on the basis of the Newman-Raju’s method. The bending correction factors are deduced based on the bending stress gradient, while the geometry correction factors are determined by parametric studies for internal surface cracks and external surface cracks respectively. Owing to a large data set requirement by the parametric studies, three-dimensional finite element (FE) models of evaluating SIFs of circumferential surface cracks are developed. The FE method is validated to ensure that it could provide accurate SIF estimations. Analytical verification is conducted which shows that the SIF evaluated by the proposed analytical method match well with the results evaluated by the recommended analytical method. Then experimental investigations of external surface crack growth in offshore steel pipe subjected to fatigue bending are implemented to further validate the analytical method of predicting surface crack growth rate. The analytical results match well with the test results and the available experimental data from literature, indicating that the analytical method can be used for practical purposes and facilitate the crack growth evaluation and residual fatigue life prediction of cracked steel pipes.

Coverage and Peening Angle Effects in Shot Peening on HCF Performance of Ti-6Al-4V

Shot peening of titanium alloys is known to enhance the HCF strength by inducing in near-surface regions residual compressive stresses which can drastically retard the growth rates of surface cracks. In addition, the induced high dislocation densities may increase the resistance to fatigue crack initiation while the typically accompanied high surface roughness has the opposite effect.The present work aimed at studying the effect of coverage and peening angle effects in shot peening on fatigue crack initiation and micro-crack growth on Ti-6Al-4V.The coverage was varied to a wide extent ranging from 20 % to 1200 % and the peening angle from 90 to 30 degrees. Fatigue life of shot peened rotating bending hour-glass shaped and flat bending fatigue specimens was studied and compared to an electrolytically polished reference.The results indicate that low (20%) coverage peening leads to a loss in HCF strength, presumably caused by insufficient residual compressive stress fields which cannot compensate the early crack initiation caused by the high roughness. In contrast, full (100 %) up to a high (1200 %) coverage was found to result in a marked increase in HCF strength.The variation of peening angles resulted in a significant beneficial effect on the fatigue strength of the flat bending fatigue samples in direction to increasingly flat angles and high coverages (1200 %) at constant Almen intensities of 0,20 mmA. Beside the deeper compressive stress layer at flat impact angles especially the increased dislocation density and the creation of a material texture orientated 90 degrees to the cracking direction seems to be responsible for the increased fatigue strength.These results are even more interesting as those peened surfaces resulted in the highest roughness values and a topography with a starting waviness on the surface.

Surface flaws behavior under tension, bending and biaxial cyclic loading

Fatigue surface crack growth and in-plane and out-of-plane constraint effects are studied through experiments and computations for the aluminum alloy D16T. A tension/bending central notched plate and cruciform specimens under different biaxial loadings with external semi-elliptical surface cracks are studied. The variation of the fatigue crack growth rate and surface crack paths is studied under cyclic tension, bending and biaxial tension–compression loading. For the experimental surface crack paths in the tested specimens, the T-stress, out-of-plane Tz factor, local triaxiality parameter h and the governing parameter for the 3D-fields of the stresses and strains at the crack tip in the form of the In-integral are calculated as a function of the aspect ratio by finite element analysis to characterize the constraint effects along the semi-elliptical crack front. The plastic stress intensity factor approach is applied to the fatigue crack growth on the free surface, as well as at the deepest point of the semi-elliptical surface crack front, of the tested tension/bending plate and cruciform specimens. From the results, characteristics of the fatigue surface crack growth rate as a function of the loading conditions are established.

Surface Crack Growth Rate under Tension and Bending in Aluminum Alloys and Steel

Fatigue surface crack growth is studied using FE-analysis and experiments for aluminum alloy D16 and low carbon steel under tension and bending. The subject for the study is central notched specimens with external semi-elliptical surface crack.Crack growth rate was determined on precracked specimens under tension and three point bend loadings. As experimental result the crack length increment on the specimen surface and crack edges opening displacement were obtained. A relationship between the crack form geometry and a number of loading cycles was obtained during tests by beach marks produced on each specimen.For the experimental surface crack paths in tested specimens the governing parameter for the 3D-fields of the stresses and strains at the crack tip in the form of In-integral was calculated by finite element analysis along semi-elliptical crack front. The governing parameter of the elastic-plastic stress fields In-integral was used as the foundation of the elastic-plastic stress intensity factor. The plastic stress intensity factor approach was applied to the fatigue crack growth on the free surface of the specimens and in the deepest point of the semi-elliptical surface crack front.A significant influence of material properties and loading type on crack growth rate characteristics was established in the present study.The experimental and numerical results of the present study shows that the elastic-plastic stress intensity factor, which is sensitive to elastic-plastic material properties, is attractive for fatigue crack growth rate characterization.

Fatigue Surface Crack Growth in Aluminum Alloys under Different Temperatures

The variation of crack growth behavior is studied under cyclic axial tension fatigue loading for the different temperatures conditions. The subject for studies is cylindrical hollow specimens of B95 and D16 aluminum alloys with semi-elliptical surface cracks. By experimental studies for considered temperature conditions the relations between the crack sizes on the free surface of specimen, crack opening displacements (COD), crack growth rate and aspect ratio were obtained. These relationships are useful for automation of experimental studies of surface crack growth. For the same specimen configuration and the different crack front position as a function of cyclic tension loading and temperatures conditions, the following constraint parameters were analyzed, namely, the non-singular T-stress, Tz -factor and the stress triaxiality parameter h in the 3D series of elastic-plastic computations. The governing parameter of the elastic-plastic stress fields In-factor distributions along various crack fronts was also determined from numerical calculations. The plastic stress intensity factor (SIF) approach was applied to the fatigue crack growth on the free surface of the hollow specimens as well as the deepest point of the semi-elliptical surface crack front. As result principal particularities of the fatigue surface crack growth rate as a function of temperature conditions are established.

Surface and through thickness crack growth in cruciform specimens subjected to biaxial loading

Fatigue surface crack growth are studied through experiments and computations for aluminum alloy D16T. Subjects for studies are cruciform specimens under different biaxial loading with external semi-elliptical surface crack. The variation of fatigue crack growth rate and surface crack paths behaviour was studied under cyclic uniaxial tension, equal biaxial tension and biaxial tension-compression loading. For the experimental surface crack paths in tested specimens the T-stress, the out-of-plane TZ factor, the local triaxiality parameter h and the governing parameter for the 3D-fields of the stresses and strains at the crack tip in the form of In-integral were calculated as a function of aspect ratio by finite element analysis to characterization of the constraint effects along semi-elliptical crack front. Both elastic and plastic stress intensity factor approach was applied to the fatigue crack growth on the free surface of the cruciform specimens as well as in the deepest point of the semi-elliptical surface crack front. As result principal particularities of the fatigue surface crack growth rate as a function of loading conditions were established. The experimental and numerical results of the present study provide an opportunity to explore the suggestion that crack growth rate may be represented by the plastic stress intensity factor, rather than the magnitude of the elastic SIF alone.

균열성장 지연현상에 대한 유효 소성역 개념을 사용한 파괴역학적 연구

In this study, the growth rate of surface cracks (da/dN) during the retardation period was analyzed in terms of effective stress intensity factor range(<TEX>${\Delta}K_{eff}^*$</TEX>) obtained by using the proposed effective plastic zone concept. Effective stress intensity factors obtained by using the effective plastic zone concept were smaller than those obtained by using Willenborg analysis. On the growth rate of surface cracks analyzed by <TEX>${\Delta}K$</TEX>, the dependence of overload stress levels appears. On the growth rate by <TEX>${\Delta}K_{eff}$</TEX> obtained by Willenborg analysis, there is a linear relationship with two different slops between da/dN and <TEX>${\Delta}K_{eff}$</TEX>. However, on the growth rate by <TEX>${\Delta}K_{eff}^*$</TEX> obtained by the proposed effective plastic zone concept, there is a linear relationship between da/dN and <TEX>${\Delta}K_{eff}^*$</TEX> that coincides with the results of constant amplitude loading.

Mechanical properties and surface morphology of photodegraded polyoxymethylene modified by a core–shell acrylate elastomer with UV stabilization

AbstractIn order to improve the photostability of polyoxymethylene (POM), a core‐shell acrylate elastomer with UV stabilization, i.e. poly[(methyl methacrylate)‐(butyl acrylate)‐2‐hydroxy‐4‐(3‐methacryloxy‐2‐hydroxypropoxy)benzophenone] (core‐shell poly(MMA‐BA‐BPMA)), was added into the POM matrix using a melt‐mixing method. The effect of the modification with core‐shell poly(MMA‐BA‐BPMA) on POM was compared with that of poly(MMA‐co‐BA‐co‐BPMA) copolymer. Scanning electron microscopy, metallographic microscopy, Fourier transform infrared spectroscopy, differential scanning calorimetry, X‐ray diffraction and X‐ray photoelectron spectroscopy were employed to characterize POM blends before and after UV irradiation, and the mechanical properties of the POM blends were investigated. The results showed that core‐shell poly(MMA‐BA‐BPMA) improved well the compatibility with and toughness of the POM matrix, and its light‐stable functional groups could increase the UV resistance of POM blends. During UV aging, the impact strength and elongation at break of POM/core‐shell poly(MMA‐BA‐BPMA) blends were retained, the growth rate of surface cracks of POM was inhibited effectively by core‐shell poly(MMA‐BA‐BPMA) and the degree of photo‐oxidation of POM blend surfaces was improved to a certain extent. Compared with poly(MMA‐co‐BA‐co‐BPMA), core‐shell poly(MMA‐BA‐BPMA) had a better UV stabilization effect on the POM matrix. Our results indicate that the core‐shell acrylate elastomer with toughening and UV stabilization functions can significantly improve the long‐term UV stability of POM. Copyright © 2012 Society of Chemical Industry

Study on the surface crack growth behavior in 14MnNbq bridge steel

Three-dimensional crack closure correction methods are investigated in this paper. The fatigue crack growth tests of surface cracks in 14MnNbq steel for bridge plate subjected to tensile and bending loadings are systematically conducted. The experimentally measured fatigue crack growth rates of surface cracks are compared with those of through-thickness cracks in detail. It is found that the crack growth rates of surface cracks are lower than those of through-thickness cracks. In order to correct their differences in fatigue crack growth rates, a dimensionless crack closure correction model is proposed. Although this correction model is determined only by the experimental data of surface cracks under tensile loading with a constant ratio R = 0.05, it can correlate the surface crack growth rates with reasonable accuracy under tensile and bending loadings with various stress ratios ranging from 0 to 0.5. Furthermore, predictions of fatigue life and crack aspect ratio for surface cracks are discussed, and the predicted results are also compared with those obtained from other prediction approaches. Comparison results show that the proposed crack closure correction model gives better prediction of fatigue life than other models.

RESEARCH ON SURFACE-CRACK GROWTH BEHAVIOR UNDER CYCLIC HIGH STRESSES USED STRUCTURAL STEEL

The surface crack growth rate and characteristics under high stresses cyclic loading were investigated using SN490B structural steel. It has been found that the crack growth rate under the elastic-plastic and general yielding condition was expressible as the power function of the range of J-integralΔ J. The main results obtained were as follows. (1) The shape of the cracks changed the semi-circular one into the semi-elliptical one according to the cracks propagated under the load and/or deformation controlled condition respectively. (2) TheΔ J values given by three estimation methods of J-integralΔJ-ΔJaeq, ΔJw, ΔJφ-were almost equivalent respectively. (3) The surface crack growth rate was able to estimate by the through-thickness crack growth one by mean of convert the surface crack into the through-thickness one. In this case, there were limitations of the applicability which were σnet<sf, c/t<0.8, a/W<0.5, and da/dN<2×10-4m/cycle.

Effects of laser peening treatment on high cycle fatigue properties of degassing-processed cast aluminum alloy

It is well known that the degassing (DG) process is very useful for reducing cast defects such as porosities, micro-shrinkages, and inclusions in metallic alloy casting processes. The authors had previously clarified that DG processing is an effective method for improving the fatigue property of cast aluminum alloy. In this study, a treatment involving laser peening without coating (LPwC) was applied to the DG process cast aluminum alloy with an aim to further improve the fatigue property of the alloy. Fatigue tests under rotating bending were carried out on the LPwC-treated DG process cast aluminum alloy in order to investigate the effects of the LPwC treatment on fatigue properties such as fatigue strength, crack initiation, and propagation behavior. From the results, it was found that the fatigue life and fatigue strength at 10 7 cycles of the DG process cast aluminum alloy improved by the LPwC treatment. The possible reason for this improvement is the deceleration of the surface crack growth rate due to the compressive residual stress in the surface layer induced by the LPwC treatment.

Surface crack and cracks networks in biaxial fatigue

Semi-elliptical fatigue crack growth in 304 L stainless steel, under biaxial loading, was investigated. Compared to those of through-cracks under uniaxial loading, the growth rate of surface cracks is increased by a non-singular compressive stress and reduced by a tensile stress, when R = 0. Plasticity-induced crack closure under biaxial loading was investigated through 3D finite element simulations with node release. Roughness and phase-transformation-induced closure effects were also discussed. The interactions in two-directional crack networks under biaxial tension were investigated numerically. It appears that the presence of orthogonal cracks should not be ignored. The beneficial influence of interaction-induced mode-mixities was highlighted.

Fatigue crack growth for a surface crack in a round bar under multi‐axial loading condition

ABSTRACTIn this paper, the fatigue life, surface crack extension direction and crack growth rate in an elastic bar with a circular cross section are determined through experiments under cyclic torsion with axial static and cyclic tension/compression loading. The effects of the loading type, loading value and stress ratio on the crack growth behaviour are discussed. The results show that, under pure fatigue torsion loading, the crack extension direction is almost the same whatever the value of torsion loading. Under fatigue torsion with cyclic tension loading, it is found that the crack extension direction is mainly determined by the alternating parts of the stresses and is almost independent of the average parts of the stresses, whereas the fatigue life is obviously dependent on the average stress.

An Investigation of the Effects of Microstructure on Fatigue Crack Growth in Ti-6242

Surface and subsurface crack nucleation and growth mechanisms are elucidated for equiaxed (microstructure 1), elongated (microstructure 2), and colony (microstructure 3) microstructures of Ti6242. Prominent cleavage facets, indicative of a Stroh-type dislocation-pile phenomenon characterize the nucleation sites. Beachmarking and scanning electron microscopy (SEM) techniques are used to study fatigue crack growth rates and crack shape evolution in the short and long crack regimes. The studies reveal that surface crack growth rate data are generally comparable to the through-crack growth rate data in the long crack growth regime. However, the depth crack growth rates are somewhat slower than the through-crack growth rates. Surface crack evolution profiles are shown to exhibit a tendency towards “Preferred Propagation Paths” (PPPs). However, the magnitudes of the aspect ratios along the PPPs are different from those reported for square or rectangular cross sections subjected to cyclic tension or bending loads. Finally, the measured crack lengths and aspect ratios are compared with predictions obtained from a fracture mechanics model.

Deaerator feedwater vessel weld cracking monitored over a service time of around 80,000 hours

This paper describes the fatigue crack growth data which were recorded during a 12-year study on weld cracking in a small deaerator feedwater storage vessel. The sub-critical crack growth mechanism was corrosion fatigue in nature and varied from 5×10 −9 to 5×10 −8 mm/s. Also, crack extension was tunnelled in nature and resulted in a thumbnail shape with the crack depth growth outstripping the surface crack growth rate. A detailed fractographic study indicated that flat fan-shaped facets were common and typified corrosion fatigue in high temperature aqueous conditions. The reported fatigue crack growth data were compared with published results for low alloy steels in high temperature waters in terms of frequency, average crack tip strain rate and stress intensity factor range and favourable agreement was observed in all instances.