bluuebunny/crossref_metadata_2025 huggingface dataset:
THE SIGNIFICANCE OF MEAN STRESS ON THE FATIGUE CRACK GROWTH THRESHOLD FOR MIXED MODE I + II LOADING
Authors: TONG J. , YATES J. R. , BROWN M. W.
Published: April 2007

Abstract— The effect of mean stress on near threshold fatigue crack growth behaviour under mixed mode I+II loading has been studied in a structural steel BS4360 50D in laboratory air at room temperature. It was found that the branch crack threshold decreased significantly as R ratio (Pmin/Pmax) increased from 0.1 to 0.7. A simple model is proposed to predict the branch crack threshold behaviour for R‐ratio sensitive materials. Further investigation is required to model the mode II dominant situations where the branch crack thresholds tend to converge on a high value.
FINITE ELEMENT ANALYSIS OF SPECIMEN GEOMETRY EFFECTS ON FATIGUE CRACK CLOSURE
Authors: McClung R. C.
Published: April 2007

Abstract— Elastic‐plastic finite element analysis is used to study fatigue crack closure at three different crack length to width ratios for three plane stress specimen geometries: center‐cracked plate, single‐edge‐cracked plate (tension), and single‐edge‐cracked plate (bend). The maximum stress to flow stress ratio, Smax/σO, which successfully describes closure results in many center‐cracked plate configurations, does not correlate the effect of different geometries on the normalized opening stress, Sopen/Smax. Crack opening stresses for different geometries and crack lengths are successfully correlated by a normalized stress intensity parameter, Kmax/K0, where K0=σ0φa. The quality of the correlation is very high at small Kmax/K0, and gradually deteriorates as Kmax/K0 increases beyond the small‐scale yielding regime.
DYNAMIC FRACTURE STUDY ON CrNiMoV ALLOY STEEL UNDER DIFFERENT HEAT TREATMENTS AND IMPACT RATES
Authors: Sun Y. B. , Lee K. H. , Tay T. E.
Published: April 2007

Abstract— Two methods are employed, namely the Charpy impact test and a strain gauge technique, for the determination of the dynamic fracture toughnesses of CrNiMoV27 and CrNiMoV45 alloy steels under various impact loading rates and heat treatments. The results show that the dynamic fracture toughness KId is significantly affected by impact velocity and that tempering has a greater influence on KId than quenching. It is also shown that the alloy containing a greater amount of vanadium yields consistently a higher value of Kid.
DUCTILE CRACK GROWTH PREDICTIONS FOR LARGE CENTRE CRACKED PANELS BY DAMAGE MODELLING USING 3‐D FINITE ELEMENT ANALYSIS
Authors: Howard I. C. , Li Z. H. , Bilby B. A.
Published: April 2007

Abstract— This paper reports on a three‐dimensional numerical prediction that involves continuum damage theory for the deformation of centre cracked panels under uniaxial and biaxial tensile loading. The predicted differences of crack tip blunting movement, crack tunnelling behaviour and the changing shape of the plane ahead of the crack tip for the two loadings are close to those observed in tests. The material parameters that encapsulate the damage response of the A533 B class 1 steel tested, were selected from a detailed parametric study on laboratory crack‐growth resistance tests on the material.
FRACTURE BEHAVIOUR OF A HIGHLY ALLOYED HIGH SPEED STEEL
Authors: Gomes M. A. , Wronski A. S. , Wright C. S.
Published: April 2007

A high speed steel, processed by two powder metallurgy routes and heat treated to give a range of microstructures, was investigated in 4‐point bending at room temperature using smooth and precracked specimens. The finer microstructures were in the material from gas atomised powder which was hot isostatically‐pressed, commercial ASP60 alloy, while the coarser microstructures derived from laboratory vacuum sintering of water‐atomised powder. The resultant hardness values Hv50 were in the rage 780 to 1050, prior autenite grain sizes, 5 to 25 pm and maximum carbide sizes, 6 to 32 μm.

Only some of the uncracked samples exhibited macroscopic yielding, at stresses in the range 1.64 to 2.59 GPa; the finer microstructures being asSociated with the higher strengths. Macroscopic plastic deformation never exceeded 0.33%; fracture strengths were in the range 1.46 to 2.75 GPa. Fracture toughness, Klc, varied from 12 to 17 MPa√m in the Hy50 range 920 to 800 for the directly sintered steel and only from 10 to 12 MPa√m in ASP60. The insensitivity of K1c to macroscopic hardness in ASP60 is asSociated with the plastic zone size of 1.5 μm which approximates to the average carbide spacing.

Nucleation and growth of natural, i.e. stress‐induced, microcracks in un notched specimens was studied by surface replica microscopy. Crack nucleation took place at stresses between 0.5 and 1.5 GPa, i.e. below those for yielding and for fracture, σF, and was by debonding of inclusions (alumina and calcium‐alumino silicates) or cracking of carbides. In the coarsest microstructure monotonic stepwise subcritical crack growth was observed from stress levels of ∼1.3 GPa, i.e. ∼0.8 to 0.9σF. Similarities to the behaviour of short fatigue cracks in metallic materials and the R‐curve behaviour of ceramics are referred to.
CORRELATION OF FATIGUE CRACK GROWTH BY CRACK TIP DEFORMATION BEHAVIOUR
Authors: Hammouda M. M. I. , Ahmad S. S. E. , Sallam H. E. M.
Published: April 2007

Growth of a long mode I crack under variable fatigue loading was experimentally investigated on mild steel specimens. A dynamic elastic‐plastic two‐dimensional finite element program, purposely developed for the simulation of cyclic crack tip deformation, was utilised to model the transient effects on crack tip advance. The model accommodated crack tip opening displacement and both crack tip and crack edge closure. Fifty one different cycle patterns were analysed to include the application of a single overload, a single underload, a single cycle having a combined overload and underload and finally loading blocks of different sequences. Correlations of experimental fatigue crack growth rates were made from knowledge of crack tip deformation behaviour, including the use of data found in the literature. Specimens of eight materials and different geometries were analysed to determine the validity of the present approach.
ANALYSIS OF FATIGUE CRACK GROWTH IN ROTARY BEND SPECIMENS AND RAILWAY AXLES
Authors: DE Freitas M. , François D.
Published: April 2007

Fatigue crack growth tests have been carried out on pre‐cracked specimens of steel AFNOR XC 38 under rotary bending conditions. The semi‐elliptical surface crack shape evolution was determined and the results were analyzed by da/dN =f(ΔK) using a stress intensity factor solution for semi‐elliptical surface cracks in round bars subjected to a bending stress modified to take into account the rotatary bending conditions. Results show a very good correlation with fatigue crack growth data obtained in standard centre cracked tension specimens of the same steel. An application to the sensitivity of design parameters to an analysis of cracked railway axles is proposed, allowing an improvement in maintenance procedures.
ELASTIC‐PLASTIC BEHAVIOUR AND UNIAXIAL LOW CYCLE FATIGUE LIFE OF NOTCHED SPECIMENS
Authors: Shatil G. , Ellison E. G. , Smith D. J.
Published: April 2007

Experimental and numerical analyses were carried out to examine the elastic‐plastic behaviour and low cycle fatigue (LCF) life of EN15R steel notched specimens. Notch root strains were measured and compared with estimates obtained from three methods: Neuber, Glinka and hite element (FE) analyses. All methods provided fairly accurate estimates of cyclic strain up to net section yield, from which point the Neuber and Glinka predictions were greater than measured. The finite element results compared well with measured results. The estimated notch root strains were used to predict the life of the notched specimens based on LCF results from unnotched specimens. Uniaxial Coffom‐Manson and multiaxial Lohr‐Ellison approaches were used. Improved fatigue life predictions were obtained when the FE predictions of the multiaxial strains were combined with a multiaxial strain parameter. The possible influence of strain gradient is inferred by comparing LCF lives for hollow thin‐walled and solid bar specimens.
NUMERICAL PREDICTION OF FATIGUE CRACK GROWTH OF A SURFACE DEFECT
Authors: Lin X. B. , Smith R. A.
Published: April 2007

Fatigue crack growth is modelled by an improved step‐by‐step finite element technique for a shallow and long surface crack in a finite thickness plate under different combinations of tension and bending loads. The predicted crack shape development is presented for growth through the plate thickness and the shape compared with the widely assumed semi‐elliptical profile. The predictions of the present simulation technique for the aspect ratio and the number of crack growth cycles are compared with those obtained both by Newman and Raju's method, and experimental results for aspect ratio changes taken from the literature.
ROUTINE FE DETERMINATION OF STRESS INTENSITY FACTORS USING A MÜLLER‐BRESLAU INFLUENCE FUNCTION TECHNIQUE
Authors: ÅKesson B. Å. , Bjarnehed H. L. , Anderson H. O. , Josefson B. L.
Published: April 2007

Abstract A remarkably simple and accurate one‐step application of the finite element (FE) method is suggested as a means for the designer's routine determination of stress intensity factors in linear fracture mechanics for complicated non‐symmetric geometries. The vector‐valued influence functions (Green functions) introduced here can be seen as a special kind of weight functions. Each of them is numerically found as the displacement field resulting from a certain unit deformation singularity being implanted at the crack tip through a prescribed set of mutual nodal displacements between the crack surfaces. Mode separation is inherent to the procedure. Plane model, mode II and mixed mode I and II numerical examples demonstrate the ease and accuracy of the method. Detailed guidance to the design of the FE mesh at the crack tip is given and is related to accuracy. Any standard FE code can be used. The literature in the field of computational fracture mechanics is surveyed, and some suggestions for further work are made. The present method draws on a classical technique for the calculation of influence lines in structural mechanics. The method is believed to have an added value in that it promotes an overview and understanding of how different load combinations on a given cracked body contribute to a stress intensity factor. Field plots of a calculated influence function are given in one of the examples.
A PROCEDURE FOR ANALYSIS OF LEAK BEFORE BREAK IN PIPES SUBJECTED TO FATIGUE OR IGSCC ACCOUNTING FOR COMPLEX CRACK SHAPES
Authors: Bergman M. , Brickstad B.
Published: April 2007

AbstractA procedure is described which predicts the growth of an initial circumferential surface crack through a pipe wall and further on to final failure of the pipe. The crack growth mechanism can either be fatigue or stress corrosion. Consideration is taken to complex crack shapes, since especially the last growth mechanism often results in a substantially longer crack length on the inside of the pipe than on the outside for the initial leaking crack due to the distribution of weld residual stresses. The procedure has been implemented in a computer program that in an integrated fashion calculates crack sizes and mass leak rates as a function of time and also predicts when leakage and final failure occur. The information obtained makes it possible to judge if the concept of leak before break is fulfilled.
FRACTURE MODELLING OF BRITTLE‐MATRIX COMPOSITES WITH SPATIALLY DEPENDENT CRACK BRIDGING
Authors: ÖStlund S.
Published: April 2007

Abstract In brittle‐matrix composites cracking of the matrix is often accompanied by bridging of the crack surfaces. The bridging will reduce the net stress intensity factor at the crack tip and consequently increase the toughness of the composite material. The bridging mechanism is due to for example unbroken whiskers, fibres, ductile particles or interlocking grains.

Analysis of the bridging mechanism in cracked structures is conveniently carried out using the concept of cohesive zone modelling. In this case the action of the bridging elements is replaced by a distribution of forces, so called cohesive forces trying to close the crack. The commonly used approach in such modelling has been to replace the action from individual bridging elements by a continuous spatially independent distribution of closing tractions whose magnitude is a function of the crack opening displacement only.

In this paper the influence of the spatial distribution of bridging elements is considered for plane crack problems. The cross section of the bridging elements is assumed to be circular and the distance between the different bridging elements is determined by the volume fraction, the radius and the geometrical distribution of the bridging elements.

Damage resistance curves have been calculated for typical whiskers‐reinforced ceramic composites, and the results from the present spatially dependent models are compared with results from calculations with spatially independent models. The influence of the radius of the bridging element, the volume fraction of whiskers and the material properties are illustrated and the use of spatially independent models is discussed.
THE EFFECT OF GRAIN SIZE, STRAIN AND TEMPERATURE ON THE MONOTONIC STRESS‐STRAIN BEHAVIOUR OF POLYCRYSTALLINE ALUMINIUM AND AL ALLOYS
Authors: Turnbull A. , De Los Rios E. R.
Published: April 2007

Abstract The tensile yield and flow stresses of aluminium, A1‐2.63Mg alloy and A1‐2.07Li alloy at room temperature are shown to depend on the inverse square root of the polycrystal grain size and are described empirically by the Hall‐Petch relation. The same relation describes the flow stress‐grain size dependence for A1‐2.07Li alloy at temperatures ranging from ‐ 196°C to 400°C. The strain hardening in the friction stress of each material at 20°C is independent of the grain size, is approximately parabolic and is greatest for the precipitation strengthened A1‐2.07Li alloy. The grain size contribution to the tensile flow stress is dependent on both the tensile strain and composition. The friction stress, σ0, and slip band stress intensity parameter, kε, at yield, ky, are both dependent on temperature. Low temperature suppresses dislocation annihilation and recovery processes, leading to planar pile‐ups at grain boundaries and a hardening that is linear with strain. Weak hardening is observed at 250°C and 400°C due to extensive annihilation and recovery. The value of kε, at all temperatures falls following initial yielding with the generation of freshly unlocked sources.
THE EFFECT OF CATHODIC POLARIZATION ON CORROSION FATIGUE OF A HIGH STRENGTH STEEL IN SALT WATER
Authors: Wang Y.‐Z. , Akid R. , Miller K. J.
Published: April 2007

Abstract— Corrosion fatigue crack growth rates in high strength steel are often increased when a large cathodic polarization is applied. The corrosion fatigue mechanism in this case is generally considered to be due to hydrogen embrittlement. In the present study the crack growth process was carefully monitored by taking replicas from initially smooth specimens of a high strength steel under fully reversed push‐pull loading while: (1) exposed to laboratory air, (2) immersed in a 0.6 M sodium chloride (NaCl) solution at open circuit potential (OCP) and (3) with an applied cathodic potential of —1250 mV (SCE). It is shown that the effect of cathodic polarization is dependent on the applied stress level and the nature of the cracking process, which in turn, is related to the sue of the crack. For stress levels at or below the in‐air fatigue limit, failure did not occur for cathodically polarised specimens despite the number of loading cycles being 10 times that of the lifetime of identical tests in solution at the open circuit potential. At stress levels above the in‐air fatigue limit the reduction in fatigue endurance caused by the presence of the corrosive environment can be partially recovered through cathodic polarization. The role of non‐metallic inclusions in the cracking process under various exposure conditions is discussed, and a cracking mechanism is proposed.
SCATTER OF FRACTURE TOUGHNESS IN AN INHOMOGENEOUS MATERIAL
Authors: Mimura H. , Ando K. , Tsuji K. , Kobayashi T.
Published: April 2007

Abstract— Usually, scatter in fracture toughness values is studied by assuming the employed steel plate to be macroscopically homogeneous in toughness, but it is probable that inhomogeneity contributes to scatter. This paper proposes a method which can discriminate scatter due to inhomogeneity from scatter measured in tests by using small size test pieces, cut out of previously fractured test specimens used for fracture toughness tests. By this method, the observed scatter of Kc(J) in a previous paper by some of the present authors was shown to be affected considerably by inhomogeneity, although the observed scatter was fairly well described by a Weibull distribution analysis.
INELASTIC LINE SPRINGS IN NON‐LINEAR ANALYSIS OF CRACKED TUBULAR JOINTS
Authors: Skallerud B.
Published: April 2007

Abstract— In the present investigation the ultimate capacity of cracked tubular T‐joints, loaded in tension or out‐of‐plane bending, is computed by means of non‐linear finite shell element analyses. The cracks are accounted for using inelastic line springs. The calculated results are compared to corresponding test data and other published case computations. Global load‐displacement behaviour and local behaviour by means of the J integral are utilised for the cracked joints. The analyses demonstrate the feasibility of the FE analyses in assessing the joint capacity, but some deficiencies in the modelling are pointed out.
FATIGUE CRACK GROWTH UNDER AN EQUIVALENT FALSTAFF SPECTRUM
Authors: Mitchenko E. I. , Prakash Raghu v. , Sunder R.
Published: April 2007

Abstract—Optical fractography was used to estimate growth of small cracks at notches under programmed FALSTAFF loading in an Al‐Cu alloy. Crack sizes as low as 25 microns and growth rates over two orders of magnitude could be resolved using this technique. Randomized MiniFALSTAFF load sequence was modified into a programmed load equivalent with major loads either preceding or following marker loads. Crack growth rate under programmed FALSTAFF spectrum as estimated by optical fractography conformed to compliance based estimates on a SE(T) specimen. Long crack growth rates under programmed and randomized MiniFALSTAFF spectrum were essentially similar. Spectrum load fatigue crack growth was studied in central hole coupons under notch inelastic conditions. Scatter in growth rates for small notch cracks was found to be of the same magnitude as that of long cracks. Multiple fatigue cracks are observed at the notch root, and they appear to influence each other.
CLEAVAGE TOUGHNESS OF HETEROGENEOUS MATERIALS AND APPLICATION TO WELDED JOINTS
Authors: Catherine C. Sainte , Carius H. , Fant M. Di
Published: April 2007

Abstract—Toughness is generally considered as a property of homogeneous materials. In this paper, the main interest is devoted to the cleavage toughness of multi‐materials. Weldments are a special case of these heterogeneous materials and are of great importance because of their wide applications. The case of a crack front intercepting more than one material is investigated experimentally and numerically. The agreement is generally very good. The special case of a brittle core zone located all along the crack‐tip is investigated only with numerical tools. However, a special feature arising from this study is that a critical HAZ size can be defined. Below this critical size, the failure probability should never exceed a selected value (e.g. 10%). This could be used as a design rule for multipass weldments.
RETROSPECTIVE VIEW ON THE ROLE OF THE PLASTIC ZONE AT A FATIGUE CRACK TIP
Authors: Bathias C.
Published: April 2007

A brief review is presented concerning the history of the development of links between the cyclic plastic zone at a fatigue crack tip, the fracture process zone and the occurrence of fatigue striations; a development in which Regis Pelloux played a pivotal role.
APPLICATION OF THE VILLARI EFFECT IN A FATIGUE EXAMINATION OF NICKEL
Authors: Kaleta J. , ŽEbracki J.
Published: April 2007

This paper presents the possibility of using the reverse magnetostriction (Villari) effect in fatigue testing of ferromagnetic materials. The tests were conducted on cyclically loaded nickel with no auxiliary external magnetic field. The following properties were determined: magnetic induction B, magnetic field strength H, energy of the magnetic hysteresis loop ΔM (in the B–H coordinate system), plus such mechanical quantities as stress σ, strain ɛ, plastic strain ɛp and energy of the mechanical hysteresis loop ΔW (in the σ–ɛ coordinate system). A variety of magneto‐mechanical characteristics are presented and their susceptibility to loading parameters of the fatigue process are discussed. A relationship between ΔW and ΔM is demonstrated. The Villari effect is shown to be especially useful in determining the cyclic yield limit.
FATIGUE PROPAGATION OF SURFACE FLAWS IN ROUND BARS: A THREE‐PARAMETER THEORETICAL MODEL
Authors: Carpinteri Andrea , Brighenti Roberto
Published: April 2007

A three‐parameter fracture mechanics model is proposed to theoretically analyse the propagation of an elliptical‐arc part‐through flaw in a round bar subjected to constant cyclic amplitude axial or bending loads. The edge flaw presents an aspect ratio α=ael/bel (ael, bel= ellipse semi‐axes) and a relative crack depth ζ=a/D, where a and D are the depth of the deepest point on the crack front and the bar diameter, respectively. Additionally a parameter s=ael/a (ellipse shifting) defines the distance of the ellipse centre from the bar circumference. The surface flaw growth occurs according to preferred patterns which tend to converge to an inclined asymptotic plane in the diagram of α against s and ζ.
EVALUATION OF LOW CYCLE FATIGUE UNDER NON‐PROPORTIONAL LOADING
Authors: Chen X. , Gao Q. , Abel A. , Wu S.
Published: April 2007

Abstract— A series of low cycle fatigue experiments have been conducted on a 42CrMo steel under tension‐torsion loading. Thin‐walled tube specimens were used. Low cycle fatigue under various loading paths, including circular and square paths, have been investigated.

The plastic work criterion for low cycle fatigue failure has previously been generally accepted, but it is difficult to calculate stress and strain for complex loading paths, especially for non‐proportional loading. This present study suggests a simple method for the calculation of the stable cyclic stress and strain values based on a Modified Endochronic Constitutive Theory (MECT) that redefines an intrinsic time scale. The loading path effect under non‐proportional loading is also considered when evaluating fatigue life.

The results show that the plastic work approach using the MECT method in multiaxial fatigue calculations correlates reasonably well the data and is a reflection of loading path dependence.
EFFECT OF NON‐PROPORTIONAL OVERLOADING ON FATIGUE LIFE
Authors: Hua Gao. , Fernando Upul. S.
Published: April 2007

Abstract— The effect of non‐proportional overloading on both low cycle and high cycle fatigue life has been studied. Low cycle multiaxial fatigue tests were performed on EN 15R (a low alloy steel) using sequential loading blocks which comprised uniaxial “ordinary” cycles and torsion “overload” cycles, and vice versa. In high cycle fatigue, the behaviour of mode I crack growth in a medium carbon steel subjected to mixed (I and II) mode overloading was examined.

Under tension‐torsion sequential overloading, crack growth behaviour shows an earlier transition from Stage I to Stage II with a pronounced reduction in accumulated fatigue life. Tensile overloading on torsion cycles was found to be more damaging compared to torsion overloading on repeated tensile cycles. The crack‐load interaction in sequential overloading and its influence on crack growth and fatigue life is discussed. In low strain fatigue, Stage II crack growth retardation closely relates to the overload plastic zone size, crack tip blunting and crack surface shielding. Mixed mode overloading is shown to have a significant effect only if the mode I component of overloading is large enough to keep the crack open. Under both low cycle and high cycle fatigue conditions non‐proportional overloading is shown to be more damaging than proportional overloading.
CRACK PROPAGATION UNDER MIXED MODE (I + III) LOADING
Authors: Yates J. R. , Mohammed R. A.
Published: April 2007

Abstract— In this paper are presented the results of fatigue crack propagation tests on angled‐slit, three point bend mixed‐mode (I + III) specimens manufactured from a low pressure steam turbine rotor forging. The path of crack propagation has been studied for two mixed mode (I + III) loading conditions. It has been observed that crack growth occurs by a mode I mechanism and a model has been developed to correlate crack growth rates in mixed mode (I + III) specimens with data from pure mode I fatigue tests.
FATIGUE DAMAGE IN TWO STEP LOADING OF 316L STEEL I. EVOLUTION OF PERSISTENT SLIP BANDS
Authors: Poláak J. , Vašek A. , Obrtlíak K.
Published: April 2007

Abstract— An experimental study of the surface evolution during controlled plastic strain amplitude single‐step and two‐step loading tests reveals the important damage mechanisms for 316L stainless steel. In the first stage, the cyclic plastic strain is concentrated into persistent slip bands (PSBs) and a surface relief is formed consisting of extrusions and intrusions. The frequency of occurrence and the total density of PSBs has been assessed using systematic observations in a scanning electron microscope. The relative volume occupied by PSBs determines the fatigue damage in this stage. Two‐step loading has only a small effect on the PSB damage evolution and nearly equal saturated values (corresponding to the applied plastic strain amplitude) were achieved in the single‐step and the two‐step loading investigations.
A STUDY OF CUMULATIVE FATIGUE DAMAGE UNDER VARIABLE LOADING‐MODE CONDITIONS
Authors: Zhang W. , Miller K. J.
Published: April 2007

Abstract— The accumulation of fatigue damage was investigated by studying deformation processes and the kinetics of short crack initiation and propagation under variable loading‐mode conditions. Fatigue tests were carried out under push‐pull and torsion, both separately and sequentially, that is specimens were subjected to push‐pull loading prior to torsion. The results show that specimens tested in push‐pull followed by torsion have a much longer fatigue life than virgin specimens. The mechanisms of this phenomenon were explored by experimental observations and theoretical analyses of crack initiation and propagation. The effect of stress state on cumulative damage is described in some detail.
SHORT CRACKS AND LIFETIME OF A NORMALIZED CARBON STEEL UNDER CYCLIC BLOCK LOADING
Authors: Bomas H. , Linkewitz T. , Mayr P.
Published: April 2007

Abstract— The behaviour of short cracks and the lifetime of unnotched specimens of a normalized medium carbon steel is observed under constant cyclic loading and under cyclic block loading with changing mean stresses. The greatest part of the life is covered by the growth of short fatigue cracks. During cyclic block loading, the length of the loading blocks has a marked influence on the crack density and the lifetime of the specimens. Short blocks lead to high crack densities and low lifetimes. This effect can be explained by a short crack growth model which takes into account the crack arrest before microstructural barriers and crack linking in the case of high crack densities. The lifetimes for cyclic block loading are calculated on the base of the constant level data and found to be in good agreement with the measured lifetimes.
AN ANALYSIS OF MEAN STRESS IN MULTIAXIAL RANDOM FATIGUE
Authors: Brown M. W. , Surer D. K. , Wang C. H.
Published: April 2007

Abstract— Generally engineering components are subjected to multiaxial variable amplitude loading, which may be non‐proportional. One of the best known low cycle fatigue design codes that addresses non‐proportional loading problems is the ASME Boiler and Pressure Vessel Code, but this does not include a cycle counting method and it does not allow for the presence of a mean stress. Traditional rainflow methods, incorporating a strain range and a mean stress, are applicable only where there is a single load varying with respect to time, or possibly where multiaxial loads are proportional. So a multiaxial non‐proportional cycle counting method and a fatigue damage calculation procedure are proposed here, based on plastic deformation response and a critical plane hypothesis that incorporates the effect of mean stress. A cyclic deformation model, based on the Ramberg‐Osgood equation and a multiaxial memory rule, is used to calculate the mean stress response under variable amplitude loading.

The proposed procedure is assessed with combined tension/torsion tests on En15R steel under variable amplitude loading. Fatigue life predictions are compared for analysis with and without mean stress corrections, to assess the sensitivity to mean stresses of non‐proportional fatigue endurance in the low cycle regime.
CURVILINEAR RANDOM FATIGUE CRACK GROWTH: EFFECTS OF OVERLOADS
Authors: Trębicki J. , Sobczyk K.
Published: April 2007

Abstract— In the paper a previously proposed stochastic description for curvilinear fatigue crack growth is extended and adopted to the characterization of crack retardation due to overloads. The model has the form of a cumulative process with random elementary crack increments and random angles of the crack deflection. The basic statistical characteristics of the model‐process are related to fracture mechanics and to the parameters known from traditional experimental predictions.
CREEP‐FATIGUE CRACK GROWTH BEHAVIOR IN 1Cr‐1Mo‐0.25V STEELS. PART I: ESTIMATION OF CRACK TIP PARAMETERS
Authors: Adefris N. , Saxena A. , McDowell D. L.
Published: April 2007

Abstract— Non‐linear finite element (FEM) analyses, involving various creep deformation laws, as well as experiments with hold times of 100 s and 15 min were performed on compact type specimens with stationary cracks. The work was aimed at developing accurate expressions for estimating the small‐scale creep parameters, (Ct)avg, for 1Cr‐1Mo‐0.25V steel at 538°C (1000°F). Here (Ct)avg is a representative value for the small‐scale creep parameter Ct, averaged over a hold period under the conditions of creep‐fatigue loading. These expressions were then applied to crack growth data obtained from specimens tested under the various hold times. When an elastic‐plastic‐primary creep‐secondary creep constitutive model was used in the FEM analyses, the calculated values of Ct compared well with the measured values. The FEM results also showed that the accumulated creep deformation during the hold time was not significantly reversed during the unloading portion of the creep‐fatigue cycle for this material. Therefore, a new method of estimating (Ct)avg is proposed on the basis of these numerical and experimental results.
EVALUATION OF KII USING A COPPER FOIL WHEN ASSESSING FRICTIONAL FORCES ON CRACK SURFACES
Authors: Egami N. , Kitaoka S.
Published: April 2007

Abstract— The relation is examined between the strain sensitivity and the grain size of a grown copper foil gauge which has a preferred orientation to slip. We adopted this gauge to evaluate the mode II stress intensity factor KII of a through‐thickness crack in a plate subjected to cyclic torsion. The KII values obtained are lower than the calculated values because of the frictional force on the crack surfaces. By using the experimentally determined KII value for torsion and the calculated KI values for plane bending, the threshold condition for crack propagation under the combination of bending and torsion is predicted on the basis of the strain energy density criterion.
THE SIGNIFICANCE OF CRACK TIP DEFORMATION FOR SHORT AND LONG FATIGUE CRACKS
Authors: Hammouda M. M. I. , El‐Sehily B. M. , De Los Rios E. R.
Published: April 2007

Abstract— The behaviour of physical short mode I cracks under constant amplitude cyclic loading was investigated both numerically and experimentally. A dynamic two‐dimensional elastic‐plastic finite element technique was utilised to simulate cyclic crack tip plastic deformation. Different idealisations were investigated. Both stationary and artificially advanced long and short cracks were analysed. A parameter which characterises the plastically deformed crack tip zone, the strain field generated within that zone and the opening and closure of the crack tip were considered. The growth of physically short mode I cracks under constant amplitude fully reversed fatigue loading was investigated experimentally using conventional cast steel EN‐9 specimens. Based on a numerical analysis, a crack tip deformation parameter was devised to correlate fatigue crack propagation rates.
INVESTIGATION OF THE ROLE OF RESIDUAL STRESSES IN THE WARM PRESTRESS (WPS) EFFECT. PART I—EXPERIMENTAL
Authors: Reed P. A. S. , Knott J. F.
Published: April 2007

Abstract— The role of residual stresses in the warm prestress (WPS) effect has been investigated. Three types of specimen have been tested in this investigation: smooth uniaxial tensile specimens, blunt notched single edge notched bend (SENB) specimens and sharply precracked SENB specimens. Room temperature prestraining of uniaxial tensile specimens leads to a dramatic decrease in the measured nominal fracture stress at — 196°C. Such an embrittling effect may be expected to reduce the beneficial increase in subsequent fracture toughness commonly observed in WPS sequences. The blunt‐notched specimens were prestressed in tension and compression. Compressive prestressing was found to lead to a decrease in subsequent fracture load whereas tensile prestressing leads to an increase. The load decrease following a compressive WPS was greater than the load increase following a tensile WPS. Various sequences of loading, unloading and cooling have been investigated and the differences in the subsequent fracture behaviour of specimens have been explained qualitatively by superposition arguments. The theories of Chell and Curry have been supported by the general trend of results.
INVESTIGATION OF THE ROLE OF RESIDUAL STRESSES IN THE WARM PRESTRESS (WPS) EFFECT. PART II—ANALYSIS
Authors: Reed P. A. S. , Knott J. F.
Published: April 2007

Abstract—Curry's model of the WPS effect has been applied to the results of a previous paper, and is extended to treat warm prestressing in blunt notched test‐pieces. The effect of more complex prestress cycles is also predicted by an extrapolation of the model. The effects of the load‐cool‐fracture, LCF, cycle can be reasonably predicted for both sharply precracked and blunt notched specimens. For the sharply precracked specimens the effects of the load‐unload‐cool‐fracture, LUCF, cycle at — 196°C are consistently overpredicted and this may be due to a decrease in the cleavage fracture stress at — 196°C of the material at the crack tip which has been subjected to repeated plastic straining by the combination of loading cycles. Modifications to the model are suggested which reduce the overproduction but a wide degree of scatter is observed in the experimental observations. Blunt notched specimens show a reasonable correlation between prediction and theory for the tensile LUCF cycle. Problems have been found in predicting the effect of various prestress cycles in different specimens due to the inherent variability in baseline fracture behaviour of the weld metal. It is concluded that the general trend of results is adequately explained by superposition models but that a greater understanding of local flow properties at a crack tip is required to achieve reasonable predictive success for weld metals such as A533BW.
THE RANDOMNESS OF FATIGUE CRACK GROWTH UNDER CONSTANT‐AMPLITUDE LOADS
Authors: Lapetra C. , Mayo J. , Dominguez J.
Published: April 2007

Abstract—

An analysis of fatigue crack growth from an statistical point of view has been carried out. Eighteen pre‐cracked specimens obtained from the same sheet of aluminium alloy were subjected to identical load and environmental conditions. Tests were conducted under constant‐amplitude loads in order to isolate the effect of material randomness. The experimental results exhibited two different types of dispersion: one showed as a change in the mean growth rate between tests and the other as nonuniform growth in the same specimen.

At a subsequent stage, the statistical distribution of the experimental results was studied and a theoretical model was developed to account for the growth pattern observed. The proposed model uses a growth law comprising random parameters to account for the low‐frequency component (slow changes). The comparison of the performance of two different laws, viz. the Paris‐Erdogan law and the cubic law, is presented. Both were tested on the assumption of randomness in two of their fitting parameters. In addition to the above‐mentioned law, the model uses a stochastic log‐normal process to model the high‐frequency component (rapid changes). The parameters for this process were determined by time series analysis of fatigue crack growth rate data.
FRACTURE TOUGHNESS TESTING OF WELD METAL: RESULTS OF A EUROPEAN ROUND ROBIN
Authors: Hadley I. , Dawes M. G.
Published: April 2007

Abstract— This paper presents the results of a collaborative project on fracture mechanics toughness testing of weldments, carried out by twelve European laboratories. Each collaborator was asked to measure the fracture toughness of a steel weldment, notched in the weld metal. Tests were carried out at two temperatures, using three test geometries, in accordance with a standard procedure. Results are reported in terms of the effect of temperature and specimen geometry on fracture toughness (mean value and standard deviation), the extent of inter‐laboratory reproducibility of results and the effect of “invalid” results on the measured value of fracture toughness. As a result of the study, it is recommended that a number of the current validity criteria in BS7448:Part 1 should be relaxed in Part 2 when applied to tests on weld metal.
CONSTRAINT BASED EVALUATION OF CRACK GROWTH EXPERIMENTS IN THE TRANSITION REGION
Authors: Shehu S. , Nilsson F.
Published: April 2007

Abstract—A large number of experiments has been performed over a wide temperature range in the transition region of a ferritic steel with the aim of studying its fracture behaviour and assessing the applicability of a two parameter fracture mechanics approach for such situations.

The experimental results clearly show a constraint effect, reflected in a decrease of the transition temperature with a decrease in specimen size and in a narrower transition region for shallow cracks. The results also show that theJ‐Qmodel, in conjunction with a fracture criterion (like the RKR criterion for cleavage fracture) is a promising approach for predicting the cleavage fracture event even in the transition region. No constraint effect could be detected for ductile initiation. The cleavage stress has been measured directly by testing. This reduces the number of fracture tests needed for the construction of the criticalJ‐Qlocus and improves the accuracy of the predictions.
AN EXPERIMENTAL PARAMETER Jmax IN ELASTIC‐PLASTIC FATIGUE CRACK GROWTH
Authors: Lu Y.‐L. , Kobayashi H.
Published: April 2007

Abstract— Imitating Garwood's 3‐parameter technique, an experimental parameter Jmax was introduced to predict fatigue crack growth rate (da/dN) over a wide range including small scale yielding and large scale yielding. It was found that for a ΔK‐increasing fatigue test condition, Jmax is a valid parameter. A significant crack growth acceleration, caused by a transition of fracture mechanism, occurs when Jmax=JIC The fracture mechanism involving striation formation when Jmax < JIC becomes ductile tearing when Jmax > JIC Equations to predict the effect of stress‐ratio on Jmax as well as on da/dN are given.
CRACK CLOSURE EFFECT ON CRACK GROWTH RATE AT 650°C IN DOUBLE NOTCHED SPECIMENS OF A NICKEL BASE SUPERALLOY
Authors: Pomnier S. , Prioul C. , Lautridou J. C. , Bompard P.
Published: April 2007

Abstract— Creep‐fatigue tests were performed at 650°C in air on a N18 nickel base superalloy, using double notched and smooth specimens. The deformation mechanisms observed by TEM at the notch root are shown to be compatible with the constitutive set of equations used in the finite element analysis which is presented. For a given Kmax at the notch root, the crack growth rate is much higher in a notched specimen than in a smooth one. This effect can be explained by a variation of the crack closure stress level with the local R ratio and the local stress. A strong accelerating effect of the R ratio, especially for negative values, is found in smooth specimens. Introducing a Kop correction in the experimental results leads to a good agreement between the measured crack growth rate plotted versus Keff in notched and smooth samples.
THE INFLUENCE OF THE HARDENING STATE ON TIME DEPENDENT DAMAGE AND ITS CONSIDERATION IN A UNIFIED DAMAGE MODEL
Authors: Aktaa J. , Schinke B.
Published: April 2007

Abstract— A model is presented for the prediction of the lifetime of metals in the high‐temperature range under arbitrary variable multiaxial load. The definition of an internal variable for damage in continuum damage mechanics is adopted, which allows indirect measurement of damage via the deformation behaviour. To acquire some knowledge of damage evolution, damage is measured in two ways during uniaxial strain controlled cyclic tests: (a) a change of the modulus of elasticity and (b) a decrease of the peak stress. Surprisingly, both methods lead to results which are in good agreement. A new damage law is then developed (with reference to known models and lifetime rules) which is a modification of the creep damage law of Rabotnov that is extended by a dependence on the inelastic strain rate instead of the dependence on internal variables to take into account the hardening state. Uniaxial as well as multiaxial formulations of the new damage model (Inelastic Strain Rate Modified (ISRM) model) are presented.

The parameters of the ISRM model are determined with a view to applying them to AISI 316 L(N) austenitic steel. Some of the parameters are derived from standard creep experiments. To determine further parameters, the ISRM model is applied to uniaxial cyclic tests. Both failure behaviour and damage evolution are well described.
CONFOCAL SCANNING LASER MICROSCOPY MEASUREMENTS OF THE GROWTH AND MORPHOLOGY OF MICROSTRUCTURALLY SHORT FATIGUE CRACKS IN A1 2024‐T351 ALLOY
Authors: Varvani‐Farahani A. , Topper T. H. , Plumtree A.
Published: April 2007

Abstract— A new technique for studying the growth and morphology of microstructurally short fatigue cracks is described. The technique allows short crack growth, crack depth, and crack front configuration to be measured using Confocal Scanning Laser Microscopy (CSLM). Good agreement is shown between CSLM measurements of the initial stage of crack growth (along an inclined shear plane), crack depth and aspect ratio, together with the measurements obtained using a surface layer removal technique. It is also found that non‐propagating short fatigue cracks have approximately the same aspect ratios in different specimen orientations and that the aspect ratio is independent of stress amplitude.
SURFACE MICRO‐CRACKS IN BLOCK AND RANDOM FATIGUE CYCLING OF 500 MPa STRENGTH STEEL AND WELD METAL
Authors: Iida K. , Shimamoto H. , Itoh N.
Published: April 2007

Abstract— Crack density, main crack length and the sum total length of micro‐cracks, which initiated and extended on the surface of a 500 MPa strength steel and its weld metal, were investigated by a replica technique applied during fatigue tests. The base and weld metal specimens were subjected to constant amplitude, random and block strain cycling. The base metal specimens were further tested under 4 kinds of block cycling and 2 kinds of incremental strain cycling. All the cumulative cycling patterns in random and block modes followed the so‐called p‐distribution.

As a result of an analysis of micro‐cracks, it was shown that the most useful parameter to estimate the accumulated fatigue damage was the sum total length of micro‐cracks in a unit area, which increased exponentially with cycle ratio. Empirical formulae were obtained expressing the increasing tendency as a function of the cycle ratio for three groups of the equivalent strain amplitude. The formulae were applicable to all strain cycling patterns investigated in the present study.
MICROSTRUCTURE AND FATIGUE PROPERTIES OF A WELDED DUPLEX STAINLESS STEEL
Authors: Beretta S. , Boniardi M.
Published: April 2007

Abstract— In response to the increasing structural applications in duplex steels for welded structures, fatigue behaviour of a SAF 2304 grade duplex stainless steel was investigated, considering both the base metal and GTAW welded joints. Fatigue curves and fatigue limits under rotary bending fatigue were obtained. The study focused attention on the microstructural features of fatigue crack propagation of the two series of experiments, thereby permitting an evaluation of the tortuous crack path of welded joints and the mechanisms related to threshold microstructural barriers.
FATIGUE DESIGN CRITERION FOR WELDED STRUCTURES
Authors: Fayard J.‐L. , Bignonnet A. , Van K. Dang
Published: April 2007

Abstract— For continuously welded structures subjected to cyclic loading, the highly stressed zones where cracks initiate and lead to failure are usually located at weld toes. At these critical points, called hot‐spots, the very local stress states are difficult to determine so that standard fatigue criteria are very difficult to apply for fatigue life prediction.

This work presents a fatigue design criterion for continuously welded thin sheet structures, based on a unique S‐N curve. The approach, which refers to the hot‐spot stress concept, defines the design stress S as the geometrical stress amplitude at the hot‐spot.

In practice, the geometrical stress state is calculated by means of the finite element method (FEM) using thin shell theory. Meshing rules for the welded connection, which can be applied methodically to any welding situation, allow the hot‐spot location, and therefore the design stress of any structure, to be determined.

Experimental data and FEM calculations show that a unique S‐N curve can be obtained whatever the geometry of the welded structure and the loading mode.
THE EFFECT OF UNDERCUT, MISALIGNMENT AND RESIDUAL STRESSES ON THE FATIGUE BEHAVIOUR OF BUTT WELDED JOINTS
Authors: Nguyen N. T. , Wahab M. A.
Published: April 2007

Abstract— A mathematical model is developed to predict the effect of weld toe undercut, misalignment and residual stresses on the fatigue behaviour of butt welded joints subjected to zero‐to‐tensile loading. Linear Elastic Fracture Mechanics (LEFM), Finite Element Analysis (FEA) and superposition approaches have been used for the modelling. It has been found that an undercut at the toes of welded joints is one of the most important weld geometry parameters. The reduction of fatigue strength of welded joints with a weld toe undercut is at least twice that of joints without an undercut in comparison with flush‐ground welded plate. A misalignment of 5% of plate thickness and an undercut of 2% of plate thickness are fairly representative for the lower boundary of S‐N curves of butt joints. The improvement of fatigue limits by means of surface treatments is shown to be effective for both undercut and misaligned joints. This approach is practical for a “Fitness‐for‐Purpose” assessment of welded joints subject to fatigue conditions.