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Crack Growth (crack + growth)
Kinds of Crack Growth Terms modified by Crack Growth Selected AbstractsInfluence of Kmax and R on Fatigue Crack Growth,A 3D-ModelADVANCED ENGINEERING MATERIALS, Issue 4 2010Hans-Joachim Gudladt To study the influence of the stress intensity factor and the R-ratio on the fatigue crack growth rate different kind of crack propagation experiment have been carried out on the steels X5CrNi18-10 and C45E. The experiments show that both parameters the maximum stress intensity Kmax and the R-ratio affect the crack growth rate. Consequently, the authors developed a 3D-model, which describes the dependence of da/dN on Kmax and the R-ratio. Discussing the experiments, special attention has been paid to the threshold behavior of the investigated materials. Finally, a new model to describe the influence of the R-ratio on the threshold Kmax,th for R-ratios from ,, to 1 is presented. [source] Subcritical Crack Growth in Lead Zirconate TitanateJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 7 2004William S. Oates Subcritical crack growth in terms of velocity,stress intensity factor (v,K) curves in lead zirconate titanate (PZT) were experimentally characterized on poled and unpoled compact tension specimens. The poled specimens were tested under open- and short-circuit electrical boundary conditions, which resulted in an increase in fracture toughness by 0.2 MPaˇm1/2 for the accessible velocity range (v= 10,9 to 10,4 m/s) in the open-circuit case. Subcritical crack growth of unpoled specimens was obtained under ambient (relative humidity = 35%) and dry (relative humidity , 0.02%) conditions over a regime in stress intensity factor of 0.5 MPaˇm1/2. [source] Crack Growth in Soda,Lime,Silicate Glass near the Static Fatigue LimitJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 9 2002Sheldon M. Wiederhorn The atomic force microscope (AFM) was used to explore the nature of features formed on the surfaces of cracks in soda,lime,silicate glass that were held at stress intensity factors below the crack growth threshold. All studies were conducted in water. Cracks were first propagated at a stress intensity factor above the crack growth threshold and then arrested for 16 h at a stress intensity factor below the threshold. The stress intensity factor was then raised to reinitiate crack growth. The cycle was repeated multiple times, varying the hold stress intensity factor, the hold time, and the propagation stress intensity factor. Examination of the fracture surface by optical microscopy showed surface features that marked the points of crack arrest during the hold time. These features were identical to those reported earlier by Michalske in a similar study of crack arrest. A study with the AFM showed these features to be a consequence of a bifurcation of the crack surface. During the hold period, waviness developed along the crack front so that parts of the front propagated out of the original fracture plane, while other parts propagated into the plane. Crack growth changed from the original flat plane to a bifurcated surface with directions of as much as 3° to 5° to the original plane. This modification of crack growth behavior cannot be explained by a variation in the far-field stresses applied to the crack. Nor can the crack growth features be explained by chemical fluctuations within the glass. We speculate that changes in crack growth direction are a consequence of an enhancement in the corrosion rate on the flank of the crack at stresses below the apparent crack growth threshold in a manner described recently by Chuang and Fuller. [source] Small crack growth in combined bending,torsion fatigue of A533B steelFATIGUE & FRACTURE OF ENGINEERING MATERIALS AND STRUCTURES, Issue 3 2001J. Park Crack growth rate data from bending, torsional and in-plane and 90° out-of-phase combined bending,torsional fatigue tests of A533B steel are presented. Crack growth was monitored from initial sizes generally in the range of 50,300 ,m to final sizes of several millimetres. Crack growth rate was found to vary linearly with crack size. Two approaches for correlating the A533B crack growth rate were evaluated, an effective strain-based intensity factor range and a method based on total cyclic strain energy density. The approaches were also evaluated using small crack growth data from the literature for SAE 1045 steel and Inconel 718 specimens tested under axial,torsional loadings. Predicted crack growth lives using both approaches were found to agree within a factor of two of observed lives for nearly all of the data examined. [source] A damage mechanics model for power-law creep and earthquake aftershock and foreshock sequencesGEOPHYSICAL JOURNAL INTERNATIONAL, Issue 1 2000Ian G. Main It is common practice to refer to three independent stages of creep under static loading conditions in the laboratory: namely transient, steady-state, and accelerating. Here we suggest a simple damage mechanics model for the apparently trimodal behaviour of the strain and event rate dependence, by invoking two local mechanisms of positive and negative feedback applied to constitutive rules for time-dependent subcritical crack growth. In both phases, the individual constitutive rule for measured strain , takes the form ,(t),=,,0,[1,+,t/m,]m, where , is the ratio of initial crack length to rupture velocity. For a local hardening mechanism (negative feedback), we find that transient creep dominates, with 0,<,m,<,1. Crack growth in this stage is stable and decelerating. For a local softening mechanism (positive feedback), m,<,0, and crack growth is unstable and accelerating. In this case a quasi-static instability criterion , , , can be defined at a finite failure time, resulting in the localization of damage and the formation of a throughgoing fracture. In the hybrid model, transient creep dominates in the early stages of damage and accelerating creep in the latter stages. At intermediate times the linear superposition of the two mechanisms spontaneously produces an apparent steady-state phase of relatively constant strain rate, with a power-law rheology, as observed in laboratory creep test data. The predicted acoustic emission event rates in the transient and accelerating phases are identical to the modified Omori laws for aftershocks and foreshocks, respectively, and provide a physical meaning for the empirical constants measured. At intermediate times, the event rate tends to a relatively constant background rate. The requirement for a finite event rate at the time of the main shock can be satisfied by modifying the instability criterion to having a finite crack velocity at the dynamic failure time, dx/dt , VR,, where VR is the dynamic rupture velocity. The same hybrid model can be modified to account for dynamic loading (constant stress rate) boundary conditions, and predicts the observed loading rate dependence of the breaking strength. The resulting scaling exponents imply systematically more non-linear behaviour for dynamic loading. [source] Crack Growth in Soda,Lime,Silicate Glass near the Static Fatigue LimitJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 9 2002Sheldon M. Wiederhorn The atomic force microscope (AFM) was used to explore the nature of features formed on the surfaces of cracks in soda,lime,silicate glass that were held at stress intensity factors below the crack growth threshold. All studies were conducted in water. Cracks were first propagated at a stress intensity factor above the crack growth threshold and then arrested for 16 h at a stress intensity factor below the threshold. The stress intensity factor was then raised to reinitiate crack growth. The cycle was repeated multiple times, varying the hold stress intensity factor, the hold time, and the propagation stress intensity factor. Examination of the fracture surface by optical microscopy showed surface features that marked the points of crack arrest during the hold time. These features were identical to those reported earlier by Michalske in a similar study of crack arrest. A study with the AFM showed these features to be a consequence of a bifurcation of the crack surface. During the hold period, waviness developed along the crack front so that parts of the front propagated out of the original fracture plane, while other parts propagated into the plane. Crack growth changed from the original flat plane to a bifurcated surface with directions of as much as 3° to 5° to the original plane. This modification of crack growth behavior cannot be explained by a variation in the far-field stresses applied to the crack. Nor can the crack growth features be explained by chemical fluctuations within the glass. We speculate that changes in crack growth direction are a consequence of an enhancement in the corrosion rate on the flank of the crack at stresses below the apparent crack growth threshold in a manner described recently by Chuang and Fuller. [source] Mechanical Properties of Compound Extruded Aircraft Stringer Profiles Under Cyclic Loading,ADVANCED ENGINEERING MATERIALS, Issue 7 2010Kay A. Weidenmann The worldwide competition in the field of aircraft structures leads to an increasing need for functionality and safety as well as for cost and weight reduction. For instance stringers could be directly welded on the aircraft's skin sheet. The requirements to be met are increased safety against crack initiation and crack growth as well as improved residual strength against failure after harmful impact of foreign objects. The application of continuously reinforced aluminium profiles which are manufactured by compound extrusion leads to increased strength and stiffness of the profiles by combining the aluminium matrix with high strength wires. Thus aircraft stringers of such profiles represent an innovative concept with improved properties. The characterisation of compound extrusions based on medium and high strength aircraft aluminium alloys EN AW-6056 and EN AW-2099 shows that a good embedding of the reinforcing high strength wires (Co-based and Fe-based) can be achieved. Furthermore the mechanical properties under cyclic loading of the profiles were measured and the S/N-curves for the different compound combinations were determined. Subsequently the crack initiation and propagation was analysed by using metallographic and SEM investigations. The fatigue resistance of reinforced specimens is increased compared to unreinforced ones. The fatigue cracks originate at the surface of unreinforced specimen while the cracks in reinforced specimens are initiated at the wire,matrix interface. [source] Fatigue Crack Propagation and History Effects Induced by Plasticity,ADVANCED ENGINEERING MATERIALS, Issue 9 2009Sylvie Pommier Abstract For security-relevant components, a fracture mechanics assessment has to be carried out. When complex loading conditions are encountered, various problems arise. Among them the prediction of history effects induced by plasticity remains a difficult task and is the object of this paper. After an overload, for instance, plasticity-induced crack closure is known to decelerate the crack growth. This effect is known to be related to residual stresses ahead of and behind the crack tip. Since residual stresses are related to the material stress,strain behavior, the overload effect may vary significantly from one material to another. Finite-element (FE) methods are commonly employed to model plasticity and were shown to give very satisfactory results. However, if millions of cycles need to be modeled to predict the fatigue behavior of an industrial component, the method becomes computationally too expensive. By employing a multiscale approach, very precise analyses computed by FE methods can be brought to a global scale. The data generated using the FE method enables the identification of a global cyclic elastic-plastic model for the crack tip region. Once this model is identified, it can be employed directly with no need of additional FE computations, resulting in fast computations. This method was employed so as to predict fatigue crack growth under variable amplitude fatigue in steels at room temperatures and correlates well with experimental data. It was also extended so as to model fatigue crack growth in a nickel base superalloy under non-isothermal fatigue-dwell conditions. At present, the method is being extended to mixed-mode variable-amplitude loading conditions. [source] Fatigue crack growth and life prediction of a single interference fitted holed plateFATIGUE & FRACTURE OF ENGINEERING MATERIALS AND STRUCTURES, Issue 10 2010T. N. CHAKHERLOU ABSTRACT To understand the different aspects of fatigue behaviour of complex structural joints it will be much helpful if the effects of different parameters are studied separately. In this article, to study the isolated effect of interference fit on fatigue life a pined hole specimen is investigated. This specimen is a single-holed plate with an oversized pin which force fitted to the hole. The investigation was carried out both experimentally and numerically. In the experimental part, interference fitted specimens along with open hole specimens were fatigue tested to study the experimental effect of the interference fit. In the numerical part, three-dimensional finite element (FE) simulations have been performed in order to obtain the created stresses due to interference fit and subsequent applied longitudinal load at the holed plate. The stress distribution obtained from FE simulation around the hole was used to predict crack initiation life using Smith,Watson,Topper method and fatigue crack growth life using the NASGRO equation with applying the AFGROW computer code. The predicted fatigue life obtained from the numerical methods show a good agreement with the experimental fatigue life. [source] Sensitivity analysis of creep crack growth prediction using the statistical distribution of uniaxial dataFATIGUE & FRACTURE OF ENGINEERING MATERIALS AND STRUCTURES, Issue 9 2010M. YATOMI ABSTRACT Due to the variables and unknowns in both material properties and predictive models in creep crack growth (CCG) rates, it is difficult to predict failure of a component precisely. A failure strain constraint based transient and steady state CCG model (called NSW) modified using probabilistic techniques, has been employed to predict CCG using uniaxial data as basic material property. In this paper the influence of scatter in the creep uniaxial properties, the parameter,C* and creep crack initiation and growth rate have been examined using probabilistic methods. Using uniaxial and CCG properties of C-Mn steel at 360 °C, a method is developed which takes into account the scatter of the data and its sensitivity to the correlating parameters employed. It is shown that for an improved prediction method in components containing cracks the NSW crack growth model employed would benefit from a probabilistic analysis. This should be performed by considering the experimental scatter in failure strain, the creep stress index and in estimating the,C* parameter. [source] Effects of spectrum modification on fatigue crack growthFATIGUE & FRACTURE OF ENGINEERING MATERIALS AND STRUCTURES, Issue 3 2010D. KUJAWSKI ABSTRACT The purpose of this study was to investigate experimentally the effects associated with modification of a loading spectrum recorded from P-3 a maritime aircraft on fatigue crack growth behaviour. The material is 2324-T39 Al alloy widely used in the aircraft industry. Experiments were conducted using the full spectrum and modified versions of it such as only ,positive' (no negative loads) or with reduced (clipped) high peaks. The results show that the compressive loads decrease fatigue life of the specimen by ,300%. Furthermore, by running tests with clipped peaks it was found that the fatigue life was shorten significantly due to reduction of crack growth retardation caused by highest tensile peaks. Multiple tests were conducted in order to establish a scatter in the experimental data under spectrum loads. [source] On the estimation of fatigue life in notches differentiating the phases of crack initiation and propagationFATIGUE & FRACTURE OF ENGINEERING MATERIALS AND STRUCTURES, Issue 1 2010J. VÁZQUEZ ABSTRACT Over the last three decades, a variety of models have been developed in order to predict the life of components under fatigue. Some of the models are based on the definition of the fatigue process as a combination of the phases of crack initiation and crack propagation, considering component life as the sum of the duration of each phase. Other models consider only one of the phases; some consider only initiation while others only propagation, though in this case, from cracks with lengths in the order of the microstructural dimensions. This article will carry out a comparative analysis of the methods that consider life as the sum of the duration of both phases. In this same line, it proposes yet another method, which simulates crack growth according to damage theories. In analysing the behaviour of each model, this paper will describe various elements: the prediction that each of them produces regarding notched specimens submitted to testing, the advantages and inconveniences of each, and lastly, the possibilities of applying each of the models to more realistic geometries. [source] Effect of test frequency on fatigue strength of low carbon steelFATIGUE & FRACTURE OF ENGINEERING MATERIALS AND STRUCTURES, Issue 6 2009N. TSUTSUMI ABSTRACT Ultrasonic fatigue tests (test frequency: 20 kHz) and conventional tension,compression fatigue tests (10 Hz) have been conducted on annealed and 10% pre-strained specimens of 0.13% carbon steel. Small holes were introduced on the specimen surface to investigate the effect of test frequency on small crack growth. The dynamic stress concentration factor and the stress intensity factor under ultrasonic fatigue tests were checked to be almost the same as those of conventional tension,compression fatigue tests. However, the fatigue properties were dependent on the test frequency. Ultrasonic fatigue tests showed longer fatigue life and lower fatigue crack growth rate for the annealed and 10% pre-strained specimens. Slip bands were scarce in the neighbourhood of cracks under ultrasonic fatigue tests, while many slip bands were observed in a wide area around the crack under conventional fatigue tests. In order to explain the effect of test frequency on fatigue strength, dynamic compression tests with Split Hopkinson bars were carried out. The stress level increases substantially with the strain rate. Thus, the increase in fatigue strength might be, to a large extent, due to a reduction in crack tip cyclic plasticity during ultrasonic fatigue tests. [source] Fracture path prediction of diamond segment in a marble cutting discFATIGUE & FRACTURE OF ENGINEERING MATERIALS AND STRUCTURES, Issue 7 2008. UCUN ABSTRACT This study investigates the fracture path behaviour of diamond segments that have been brazed on a marble cutting disc. The segments are braze-joined using the oxy-gas welding technique. The micro-structure of the brazing zone and the disc were investigated using standard metallographic techniques and scanning electron microscope (SEM). Additionally, we used numerical modelling to study crack growth at the welding zone. Two dimensional linear elastic fracture mechanics principles were used to analyze propagation behaviour of the crack. Stress intensity factors were calculated using displacement correlation method. It was deduced from the SEM analysis of the fractured segment surface that the fracture occurred in the diamond segment due to stress concentration near the sharp corners of the diamond particles that are embedded into the matrix. The existence of such sharp artefacts within the matrix leads to the formation of cracks. [source] Fatigue behaviour of friction stir welded AA2024-T3 alloy: longitudinal and transverse crack growthFATIGUE & FRACTURE OF ENGINEERING MATERIALS AND STRUCTURES, Issue 7 2008M. T. MILAN ABSTRACT The fatigue crack growth properties of friction stir welded joints of 2024-T3 aluminium alloy have been studied under constant load amplitude (increasing -,K), with special emphasis on the residual stress (inverse weight function) effects on longitudinal and transverse crack growth rate predictions (Glinka's method). In general, welded joints were more resistant to longitudinally growing fatigue cracks than the parent material at threshold ,K values, when beneficial thermal residual stresses decelerated crack growth rate, while the opposite behaviour was observed next to KC instability, basically due to monotonic fracture modes intercepting fatigue crack growth in weld microstructures. As a result, fatigue crack growth rate (FCGR) predictions were conservative at lower propagation rates and non-conservative for faster cracks. Regarding transverse cracks, intense compressive residual stresses rendered welded plates more fatigue resistant than neat parent plate. However, once the crack tip entered the more brittle weld region substantial acceleration of FCGR occurred due to operative monotonic tensile modes of fracture, leading to non-conservative crack growth rate predictions next to KC instability. At threshold ,K values non-conservative predictions values resulted from residual stress relaxation. Improvements on predicted FCGR values were strongly dependent on how the progressive plastic relaxation of the residual stress field was considered. [source] Influence of different load models on gear crack path shapes and fatigue livesFATIGUE & FRACTURE OF ENGINEERING MATERIALS AND STRUCTURES, Issue 5 2008S. PODRUG ABSTRACT A computational model for determination of the service life of gears with regard to bending fatigue at gear tooth root is presented. In conventional fatigue models of the gear tooth root, it is usual to approximate actual gear load with a pulsating force acting at the highest point of the single tooth contact. However, in actual gear operation, the magnitude as well as the position of the force changes as the gear rotates. A study to determine the effect of moving gear tooth load on the gear service life is performed. The fatigue process leading to tooth breakage is divided into crack-initiation and crack-propagation period. The critical plane damage model has been used to determine the number of stress cycles required for the fatigue crack initiation. The finite-element method and linear elastic fracture mechanics theories are then used for the further simulation of the fatigue crack growth. [source] Molecular dynamics simulation of crack tip blunting in opposing directions along a symmetrical tilt grain boundary of copper bicrystalFATIGUE & FRACTURE OF ENGINEERING MATERIALS AND STRUCTURES, Issue 11 2007A. LUQUE ABSTRACT Mode I crack growth along some grain boundaries of copper embrittled by solute segregation shows strong anisotropy. For instance, growth along the direction on the symmetrical tilt boundary has been reported to occur by intergranular brittle fracture, whereas growth along the opposite sense occurs in a ductile manner. In this paper, we simulate such crack configurations using molecular dynamics (embedded atom method [EAM]) in 3-dimensional perfect bicrystalline samples of pure copper of the aforementioned orientation at room temperature. In both cases the response is ductile, crack opening taking place by dislocation emission from the crack tip. The critical stress intensity factors (SIFs) for dislocation emission have been calculated by matching the displacement fields of the atoms in the tip neighbourhood with the continuum elastic fields. They are of the same order of magnitude for both growth senses despite the different morphology of their respective blunted crack tips and of the patterns of dislocations constituting their plastic zones. Thus, it seems that, in agreement with published results of continuum crystalline plasticity for the same problem, the plastic anisotropy associated with the different orientation of the slip systems with respect to the crack cannot in this case explain the experimental behaviour observed with solute embrittled bicrystals. [source] Elastoplastic modelling of subsurface crack growth in rail/wheel contact problemsFATIGUE & FRACTURE OF ENGINEERING MATERIALS AND STRUCTURES, Issue 10 2007R. LUNDÉN ABSTRACT Propagation of small subsurface cracks subjected to shear under repeated rolling contact load is studied. An analytical crack model (Dugdale) with plastic strips at the two crack tips is employed. Compressive stresses promoting crack closure and friction between crack faces are considered. The triaxial stress state is used in the yield criterion. A damage criterion is suggested based on experimental LCF data. In a numerical study, critical crack lengths are found below which propagation of an existing crack should be effectively suppressed. [source] Experimental and numerical studies on dynamic crack growth in layered slate rock under wedge impact loads: part II , non-plane strain problemFATIGUE & FRACTURE OF ENGINEERING MATERIALS AND STRUCTURES, Issue 10 2007M. R. ALAM ABSTRACT Dynamic crack propagation in non-plane strain (or 3D) slate blocks under wedge impact loads was investigated numerically in this part of the paper. A parabolic-shaped crack trajectory was taken into consideration to model the crack propagation in slate blocks for analyzing the impact splitting of layered slate rock. Major and minor axes of the parabola were determined from the condition of equal mode I stress intensity factors (SIFs) along the crack front. Mode I SIFs were determined for experimental breaking loads for each increment of crack growth in a manner similar to that mentioned in part I of this paper. These values were compared with the plane strain material fracture toughness value obtained from experimental studies and very good agreement was obtained between them, for the case of actual load applied on the specimen. Numerical analysis of a field problem, i.e., separation of a large-sized slate slab from the rock strata in a slate quarry using wedge impacting, was also carried out in this paper. It can be observed that a large magnitude of load is required to break large-sized slate blocks; but this load is applied through a number of smaller load-capacity actuators-in-parallel, requiring large power capacity for the hydraulic pumps. However, this required power could be reduced considerably if the load applied on the line of hydraulic actuators is cascaded across the (line of) actuators (starting from centrally placed actuators) with a small time delay (equal to the initial crushing time in slate rock). [source] A model of corrosion fatigue crack growth in ship and offshore steelsFATIGUE & FRACTURE OF ENGINEERING MATERIALS AND STRUCTURES, Issue 8 2007M. JAKUBOWSKI ABSTRACT A model describing corrosion fatigue crack growth rate da/dN has been proposed. The crack growth rate is assumed to be proportional to current flowing through the electrolyte within the crack during a loading cycle. The Shoji formula for the crack tip strain rate has been assumed in the model. The obtained formula for the corrosion fatigue crack growth rate is formally similar to the author's empirical formulae established previously. The different effects of ,K and the fatigue loading frequency f on da/dN, in region I as compared to region II of the corrosion fatigue crack growth rate characteristics can be described by a change of one parameter only: the crack tip repassivation rate exponent. [source] Incremental model for fatigue crack growth based on a displacement partitioning hypothesis of mode I elastic,plastic displacement fieldsFATIGUE & FRACTURE OF ENGINEERING MATERIALS AND STRUCTURES, Issue 7 2007S. POMMIER ABSTRACT The mode I displacement field in the near crack tip region is assumed to be depicted by its partition into an elastic field and a plastic field. Then, each part of the displacement field is also assumed to be the product of a reference field, a function of space coordinates only, and of an intensity factor, function of the loading conditions. This assumption, classical in fracture mechanics, enables one to work at the global scale since fracture criteria can be formulated as a function of the stress intensity factors only. In the present case, the intensity factor of the plastic part of the displacement field measures crack tip plastic flow rate at the global scale. On the basis of these hypotheses, the energy balance equation and the second law of thermodynamics are written at the global scale, i.e. the scale of the K-dominance area. This enables one to establish a yield criterion and a plastic flow rule for the crack tip region. Then, assuming a relation between plastic flow in the crack tip region and fatigue crack growth allows an incremental model for fatigue crack growth to be built. A few examples are given to show the versatility of the model and its ability to reproduce memory effects associated with crack tip plasticity. [source] Analytical and experimental studies on fatigue crack path under complex multi-axial loadingFATIGUE & FRACTURE OF ENGINEERING MATERIALS AND STRUCTURES, Issue 4 2006L. REIS ABSTRACT In real engineering components and structures, many accidental failures are due to unexpected or additional loadings, such as additional bending or torsion, etc. Fractographical analyses of the failure surface and the crack orientation are helpful for identifying the effects of the non-proportional multi-axial loading. There are many factors that influence fatigue crack paths. This paper studies the effects of multi-axial loading path on the crack path. Two kinds of materials were studied and compared in this paper: AISI 303 stainless steel and 42CrMo4 steel. Experiments were conducted in a biaxial testing machine INSTRON 8800. Six different biaxial loading paths were selected and applied in the tests to observe the effects of multi-axial loading paths on the additional hardening, fatigue life and the crack propagation orientation. Fractographic analyses of the plane orientations of crack initiation and propagation were carried out by optical microscope and SEM approaches. It was shown that the two materials studied had different crack orientations under the same loading path, due to their different cyclic plasticity behaviour and different sensitivity to non-proportional loading. Theoretical predictions of the damage plane were made using the critical plane approaches such as the Brown,Miller, the Findley, the Wang,Brown, the Fatemi,Socie, the Smith,Watson,Topper and the Liu's criteria. Comparisons of the predicted orientation of the damage plane with the experimental observations show that the critical plane models give satisfactory predictions for the orientations of early crack growth of the 42CrMo4 steel, but less accurate predictions were obtained for the AISI 303 stainless steel. This observation appears to show that the applicability of the fatigue models is dependent on the material type and multi-axial microstructure characteristics. [source] Fatigue crack growth under variable amplitude loading Part I: experimental investigationsFATIGUE & FRACTURE OF ENGINEERING MATERIALS AND STRUCTURES, Issue 4 2006M. SANDER ABSTRACT During use, a component or a structure is exposed to variable amplitude loading, which influences the lifetime. Within the scope of this work, systematic investigations of different loading situations are carried out by means of experimental studies (part I) as well as analytical and numerical studies (part II). The experimental investigations show that overloads lead to retardation effects, which are influenced by several factors, e.g. the overload ratio, baseline-level loading, number of overloads or the fraction of mixed mode. In a high,low,high block loading, both retarded and accelerated crack growth can be obtained, which is also influenced, e.g. by the block loading ratio and the length of the block. Moreover, experimental studies have been performed with load spectra, like FELIX/28, CARLOS vertical and WISPER. They have been applied in original form as well as in counted and reconstructed sequences. [source] Nanoscopic fatigue and stress corrosion crack growth behaviour in a high-strength stainless steel visualized in situ by atomic force microscopyFATIGUE & FRACTURE OF ENGINEERING MATERIALS AND STRUCTURES, Issue 11 2005K. MINOSHIMA ABSTRACT In situ atomic force microscope (AFM) imaging of the fatigue and stress corrosion (SC) crack in a high-strength stainless steel was performed, under both static and dynamic loading. The AFM systems used were (1) a newly developed AFM-based system for analysing the nanoscopic topographies of environmentally induced damage under dynamic loads in a controlled environment and (2) an AFM system having a large sample stage together with a static in-plane loading device. By using these systems, in situ serial clear AFM images of an environmentally induced crack under loading could be obtained in a controlled environment, such as in dry air for the fatigue and in an aqueous solution for the stress corrosion cracking (SCC). The intergranular static SC crack at the free corrosion had a sharp crack tip when it grew straight along a grain boundary. The in situ AFM observations showed that the fatigue crack grew in a steady manner on the order of sub-micrometre. The same result was obtained for the static SC crack under the free corrosion, growing straight along a grain boundary. In these cases, the crack tip opening displacement (CTOD) remained constant. However, as the static SC crack was approaching a triple grain junction, the growth rate became smaller, the CTOD value increased and the hollow ahead of the crack tip became larger. After the crack passed through the triple grain junction, it grew faster with a lower CTOD value; the changes in the CTOD value agreed with those of the crack growth rate. At the cathodic potential, the static SC crack grew in a zigzag path and in an unsteady manner, showing crack growth acceleration and retardation. This unsteady crack growth was considered to be due to the changes in the local hydrogen content near the crack tip. The changes in the CTOD value also agreed with those of the crack growth rate. The CTOD value in the corrosive environment was influenced by the microstructure of the material and the local hydrogen content, showing a larger scatter band, whereas the CTOD value of the fatigue crack in dry air was determined by the applied stress intensity factor, with a smaller scatter band. In addition, the CTOD value in the corrosive environment under both static and dynamic loading was smaller than that of the fatigue crack; the environmentally induced crack had a sharper crack tip than the fatigue crack in dry air. [source] A new method for assessing high-temperature crack growthFATIGUE & FRACTURE OF ENGINEERING MATERIALS AND STRUCTURES, Issue 9 2005A. J. FOOKES ABSTRACT Experimental creep crack growth (CCG) test data are obtained by following standards that characterize CCG rates using the C* parameter. Such data are then used in high-temperature failure assessment procedures. An alternative approach to defect assessment at high-temperature failure is an extension of the R6 failure assessment diagram (FAD). At high temperature, creep toughness, Kcmat, can be estimated from CCG tests and replaces low-temperature toughness in R6. This approach has the advantage that it is not necessary to establish a creep fracture regime, such as small-scale, primary or widespread creep. Also, a new strain-based FAD has been developed, potentially allowing variations of stress and temperature to be accommodated. In this paper, the results of a series of crack growth tests performed on ex-service 316H stainless steel at 550 °C are examined in the light of the limitations imposed by ASTM for CCG testing. The results are then explored in terms of toughness and presented in FADs. [source] Effects of non-proportional loading paths on the orientation of fatigue crack pathFATIGUE & FRACTURE OF ENGINEERING MATERIALS AND STRUCTURES, Issue 5 2005L. REIS ABSTRACT Fatigue crack path prediction and crack arrest are very important for structural safety. In real engineering structures, there are many factors influencing the fatigue crack paths, such as the material type (microstructure), structural geometry and loading path, etc. In this paper, both experimental and numerical methods are applied to study the effects of loading path on crack orientations. Experiments were conducted on a biaxial testing machine, using specimens made of two steels: 42CrMo4 and CK45 (equivalent to AISI 1045), with six different biaxial loading paths. Fractographical analyses of the plane of the stage I crack propagation were carried out and the crack orientations were measured using optical microscopy. The multiaxial fatigue models, such as the critical plane models and also the energy-based critical plane models, were applied for predicting the orientation of the critical plane. Comparisons of the predicted orientation of the damage plane with the experimental observations show that the shear-based multiaxial fatigue models provide good predictions for stage I crack growth for the ductile materials studied in this paper. [source] Fatigue as a process of cyclic brittle microfractureFATIGUE & FRACTURE OF ENGINEERING MATERIALS AND STRUCTURES, Issue 3 2005R. SUNDER ABSTRACT While fatigue crack growth in vacuum may occur by slip alone, environmental fatigue including crack growth in air is strongly influenced by crack-tip surface chemistry that adversely affects ductility. Cumulative diffusion, combined with adsorption and chemisorption in the loading half-cycle may promote instantaneous crack extension by brittle microfracture (BMF). Unlike slip, the BMF component will be sensitive to parameters that affect near-tip stresses, such as load history and constraint. BMF dominates near-threshold environmental fatigue. Being a surface phenomenon, it loses its significance with increasing growth rate, as slip-driven crack extension gains momentum and growth becomes less sensitive to environment. The BMF model provides for the first time, a scientific rationale for the residual stress effect as well as the related connection between stress,strain hysteresis and load-sequence sensitivity of metal fatigue including notch response. Experimental evidence obtained on a variety of materials under different loading conditions in air and vacuum appears to support the proposed model and its implications. [source] Theoretical crack path predictionFATIGUE & FRACTURE OF ENGINEERING MATERIALS AND STRUCTURES, Issue 1-2 2005H. A. RICHARD ABSTRACT In many practical cases, the crack growth leads to abrupt failure of components and structures. For reasons of a reliable quantification of the endangerment due to sudden fracture of a component, therefore, it is of enormous importance to know the threshold values, the crack paths and the growth rates for the fatigue crack growth as well as the limiting values for the beginning of unstable crack growth (fracture toughness). This contribution deals with the complex problem of a,however initiated,crack, that is subjected to a mixed-mode loading. It will present the hypotheses and concepts, which describe the superposition of Mode I and Mode II (plane mixed mode) as well as the superposition of all three modes (Mode I, II and III) for spatial loading conditions. Those concepts admit a quantitative appraisal of such crack situations and a characterization of possible crack paths. [source] A predictor,corrector scheme for the optimization of 3D crack front shapesFATIGUE & FRACTURE OF ENGINEERING MATERIALS AND STRUCTURES, Issue 1-2 2005K. KOLK ABSTRACT A predictor,corrector scheme is presented to improve the shape of 3D crack fronts within the 3D simulation of fatigue crack growth. This concept is fully functional for mode-I, and an extension for mixed-mode problems is presented. The whole procedure is embedded in an automatic incremental crack growth algorithm for arbitrary 3D problems with linear elastic material behaviour. The numerical simulation is based on the 3D dual boundary element method (Dual BEM) and on an optimized evaluation of very accurate stress intensity factors (SIFs) and T-stresses. As part of the proposed predictor,corrector scheme, 3D singularities along the crack front especially in the vicinity of the intersection of the crack front and the boundary are considered. The knowledge of these singularities allows the specification of crack front shapes with bounded energy release rate. Numerical examples with complex cross-sections are presented to show the efficiency of the proposed crack growth algorithm. The obtained results are in good agreement with recent experimental results. [source] Environmental and frequency effects on fatigue crack growth rate and paths in aluminium alloyFATIGUE & FRACTURE OF ENGINEERING MATERIALS AND STRUCTURES, Issue 1-2 2005S. A. MICHEL ABSTRACT The environmental and frequency effects on fatigue crack growth in aluminium alloys are studied theoretically and experimentally. 2024-T351 and 7075-T651 tested in corrosive environments (humid air or technically purified nitrogen) show a constant crack growth rate (da/dN) at low values of the effective stress intensity range (,Keff). Typical well-known fits of this curve (da/dN vs ,Keff) do not reflect the plateau-like region. A new model of crack growth is presented, which physically attributes this region to the formation and subsequent fracture of a crack tip oxide layer. The thickness of this layer is measured with X-ray photon electron spectroscopy. At higher loads, other mechanisms are understood to be active. The model parameters are determined from constant amplitude tests, and are valid for a given material and environment. In 7075-T651 tested in nitrogen, with R= 0.1 and 83 Hz, unexpected macroscopical crack branching is observed when ,Keff reaches approximately 3.0 MPa ,m. [source] | ||||||||||||||||