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Three different ductile damage criteria, Ayada, normalized Cockcroft and Latham and a new shear stress based criterion taking into account hydrostatic tension, are utilized for predicting the onset of cracks in various deformation processes. It is found that the Ayada criterion predicts well the onset of cracks when they originate from hydrostatic tension. The shear based criterion predicts cracks triggered by shear and the normalized Cockcroft and Latham criterion indicates the overall area of onset of cracks caused by either hydrostatic or shear stresses. However the prediction is not as accurate as the Ayada criterion for cracks caused by hydrostatic tension. [ABSTRACT FROM AUTHOR]

The mechanical properties are impacted by the surface condition of metals. Titanium alloy is oxidized and diffused oxygen into substrate, leading to phase transition in an atmosphere with oxygen. The alpha-case kinetics and tensile behavior of Ti-6Al-4V alloy by high temperature oxidized in different phase regions were investigated by static oxidation test over the temperature range of 850-1100 °C and tensile tests over the temperature range of 830-950 °C. The relationship between high temperature plasticity, oxidation and microstructural transformation of titanium alloy was analyzed by metallographic method, hardness measurements and scanning electron microscopy. The results showed that the inward diffusion of oxygen was the reason of alpha-case generation, which modified the different effects at the surfaces of titanium in different phase regions. The cracks were observed at the surface of the specimens whose depths were greater at higher temperatures. The thickness of this alpha-case was increasing with elevated temperature. Under the high strain rate, the lower stress value of the thermally oxidized Ti-6Al-4V alloy in the vicinity of yielding point was observed compared with that unoxidized. The fractographic appearances of specimens oxidized were brittle and sharp type, while the fracture mode of specimens was ductile fracture without alpha-case. The higher amount of oxygen in α phase both stabilized α phase forming a microstructure and strengthened the α phase via solid solution strengthening. The existence of alpha-case led to easier surface crack initiation and propagation under the tension stress state. [ABSTRACT FROM AUTHOR]

The objective of this work is the tune of a methodology for the determination of the fracture toughness in ductile materials at high strain-rate by means of dynamic 3 Point Bending (3PB) tests. The specimens made in high strength steel have been pre-cracked in fatigue with a single edge notch. The dynamic loading conditions have been reached by means of a modified Split Hopkinson pressure Bar setup, in which two small diameter bars are used as transmitted bars instead of the standard single one. In order to obtain the maximum amount of information from each test, different measuring systems have been adopted, in addition to the strain measurements on the bars performed using semiconductor strain-gages. On the specimen surface, one or two crack-gages (one for each side for specimens) have been bonded for obtaining data about the crack propagation and the crack tip position vs. time curve. Moreover, a high speed camera has been used for obtaining the measurements of the Crack Opening Displacement (COD) and the opening velocity: digital images have been processed using tracking techniques following some points on the specimen notch and input bar tip. Starting from the experimental measurements in term of force, displacement and crack-gage data, a protocol for fracture toughness calculation at the initiation stage of the crack growth in dynamic condition has been defined and a hybrid experimental – numerical methodology has been used. One of the key variables for the determination of dynamic initiation fracture toughness is the crack time to initiation. Many efforts have been done in order to better determine a precise method to experimentally measure this entity: this datum has been obtained from the analysis of both the force and crack-gage vs. time signals and the COD measurement on high speed video. Good agreement has been found between the two methodologies. At this stage, a 3D model, which simulates the SE(B) specimens during the Hopkinson 3PB test, has been done in order to determine the toughness (J integral) as a function of time. Knowing the initiation time, the fracture toughness at crack initiation can directly be determined. [ABSTRACT FROM AUTHOR]

The indirect (Brazilian) tensile strength test is commonly used in rock engineering and rock mechanics. Most of the research on how to interpret Brazilian test is only valid for isotropic rock. The load may be applied to disc shaped samples through flat platens or curved jaws and the different load configurations give rise to subtle but important differences to stress distributions throughout samples during testing and at the onset of failure. To reliably obtain the tensile strength accurate stress distributions, especially at the location of the initial crack, must be determined. However, most real rocks exhibit a certain type of anisotropy known as transverse isotropy due to the presence of preferred directions of grains, beddings, microcracks or pores. This study focuses on the failure modes when transverse isotropy is present. Previous studies for transverse isotropy indicate that the exact location of the initial crack is at the disc centre. These studies, however, ignore the effect of the load contact configuration on the stress distribution. This paper overcomes this limitation and applies Amadei's analytical solution to assess the effects of the load contact area on the stress distribution. The stress distribution is coupled with a transverse isotropy failure criterion in a mechanically consistent way to find the exact location of initial crack. It is shown that the location of the initial crack depends on the transverse isotropy orientation and load configuration. [ABSTRACT FROM AUTHOR]

Mineralogical composition and microstructural features influence rock strength and their failure mechanisms. Crack initiation (CI) and crack damage (CD) are important processes of the failure mechanisms of rocks. Since CI is the lower limit of the long term strength of rocks at low confinement, this study is mainly concerned with triaxial tests and the determination of CI and CD using different methods. A triaxial cell has been designed to examine spalling behavior of magmatic rocks under low confinement, at minor principal stresses up to 2.5 MPa. Two granites with different grain sizes were tested. Methods to determine CI and CD include the calculation of circumferential, volumetric and crack volumetric strain. In addition, acoustic emission monitoring provides the onset of CI and CD. Uniaxial compressive tests including primary wave velocity measurements as well as acoustic emission were used to supplement the dataset. The wave velocities also give an indirect determination of CI and CD as the initiation of cracks reduces the speed of the waves. This study aims to compare the different methods determining CI and CD and to connect the results with mineralogical and microstructural features. Orientated thin sections were prepared to identify the mineralogical composition. Furthermore, the growing influence of confining pressure from 0.1 to 2.5 MPa on rock strength was analyzed. [ABSTRACT FROM AUTHOR]

Behavior of intact brittle rocks during loading is characterized by a micro-cracking process, which occurs due the presence of flaws in their microscopic structure, propagating through the intact rock leading to shear fracture. This fracturing process is of fundamental importance as it affects the mechanical properties of the rock. The purpose of the present study is to investigate the fracture mechanism and specifically detect the crack initiation and crack damage stresses in a granite rock. For this purpose, stress-strain data were recorded and the acoustic emission method was used in uniaxial compression tests. The results from both methods correlate well and show that the crack initiation and crack damage thresholds are about 46% and 78% of the uniaxial compressive strength of the intact rock respectively, slightly higher than the results reported in literature from previous studies in granite. [ABSTRACT FROM AUTHOR]

Dimensioning of high-strength steels relies on the knowledge of Wöhler-type S/N data and the assumption of a fatigue limit for applications where the number of load cycles exceeds 10 7 . Very high cycle fatigue (VHCF) experiments applied to a 0.5C-1.25Cr-Mo tempered steel (German designation: 50CrMo4) revealed surface crack initiation at prior austenite grain boundaries in medium strength condition (37HRC) and internal crack initiation at non-metallic inclusions at high strength condition (48HRC). Despite the formation of small cracks during cycling up to 10 9 cycles, it seems that the medium strength condition exhibits a real fatigue limit. Application of automated electron back-scattered diffraction (EBSD) within the shallow-notched area of electro-polished fatigue specimens had shown that prior austenite grain boundaries act as effective obstacles to crack propagation. High resolution thermography during cycling of the specimens allowed the identification of local plasticity, which led to crack initiation at a later stage of the fatigue life. It was found that Cr segregation rows play a decisive role in the crack initiation process. By means of high-resolution electron microscopy in combination with focused ion beam milling (FIB), evolution of cyclic plasticity and crack initiation was correlated with the material's microstructure. The results are discussed in terms of the completely different crack initiation mechanisms of medium and high strength variants of the same steel. EBSD and crack propagation data are used to adapt numerical modeling tools to predict crack initiation and short crack propagation. [ABSTRACT FROM AUTHOR]

The third edition of API 579-1/ASME FFS-1 Fitness-For-Service will include a new Part covering fatigue assessment procedures for in-service components. Fitness-For-Service (FFS) assessments are quantitative engineering evaluations that are performed to demonstrate the structural integrity of an in-service component that may contain a flaw or damage, or that may be operating under a specific condition that might cause a failure. The API 579-1/ASME FFS-1 Standard was specifically written to cover in-service pressurized equipment typically found in the refining and petrochemical industries as well as the fossil utility industry. The new Part will cover methods used to estimate the time to crack initiation using a strain-life approach and will be written as a multi-tiered approach covering screening, current design code methods, and advanced methods that take into account the latest in technology. The screening and design code assessment methods in the new Part will be based on an updated version of the procedures in the ASME B&PV Code, Section VIII, Division 2. The advanced methods will include next generation versions for the fatigue assessment of welded joints, the Master S-N Curve Method as described in WRC 523, and the Verity Fatigue Assessment Method developed by Battelle. The advanced methods will also include a new smooth-bar fatigue assessment method that incorporates a multi-axial fatigue criterion with a critical plane approach. Cycle counting methods for both welded joint and smooth-bar fatigue methods will be provided. Methods to evaluate fatigue in the subcritical crack-growth regime in API 579-1/ASME FFS-1 using a fracture mechanics approach are also covered. [ABSTRACT FROM AUTHOR]

Rotors of electric machines are built from stacks of thin steel sheets. The fabrication process of these components usually involves punching operations that generate defects on the steel sheet edges. In this study, high cycle fatigue tests are performed on punched and polished edges specimens to investigate the effect of the punching process on the fatigue behaviour of these thin sheets. Results show a significant decrease of the fatigue strength for punched specimens. SEM observations of fracture surfaces reveal that crack initiation always occurs on a punching defect. Residual stresses on punched edges are analysed using X-Ray diffraction techniques. High tensile residual stresses along the loading direction are found. Some specimens edges were scanned using 3D topography prior to the fatigue tests. This allows for identifying the real geometry of the most critical defect. Murakami criterion was then evaluated in order to take into account the effect of defects. The best trend of the experimental results is given when residual stresses are taken into account. Local elastic stresses for 3 defects geometries have been calculated using FEA. Crossland fatigue criterion has been evaluated to try accounting for the local stress state around defects. Results show that the assessed fatigue strength is overestimated using this criterion. [ABSTRACT FROM AUTHOR]

Boiler tube to tubeplate welds are often seen in the power generation industry. In the nuclear power industry, especially in Advanced Gas-Cooled Reactors, the welds are exposed to long term creep temperature and are subjected to fatigue cycles due to start-up and shutdown. The creep-fatigue crack initiation life assessment for the weld is required for the purposes of ensuring of safe operation and life extension. This paper presents the detailed assessment methodologies to perform a creep-fatigue crack initiation assessment using the latest R5 volume 2/3 (Issue 3 Revision 002, 2014) for the boiler superheater header tubeplate to tube Type 316 welds. The key changes of the revised R5 procedure are also outlined in the paper. [ABSTRACT FROM AUTHOR]

The paper presents a systematic investigation of the effects of a corrosive condition on grain boundary corrosion and other deleterious phenomena in a nickel alloys commonly used in nuclear waste containers, chemical refining plants and steam generator tubes. Grain boundary corrosion is investigated with the aid of microscopy. The results show that a concentrated corrosive condition greatly increases the susceptibility to crack initiation and crack propagation along grain boundaries, i.e. intergranular fracture. [ABSTRACT FROM AUTHOR]

The basic regularities of the 17MnSi pipeline steel impact fracture without and after the ultrasonic impact surface treatment were identified. The diagrams of the material dynamic deformation which allowed determining quantitative characteristics of the crack initiation and crack propagation energy consumption in the Charpy specimens of the pipeline steel were analyzed. The crack was found out to correspond approximately to 25 % of the total energy consumption of the material fracture. The fractographic analysis of the specimen failure surfaces in the initial state and after the ultrasonic treatment was carried out. [ABSTRACT FROM AUTHOR]

Early fatigue damage has been studied in a number of model and structural polycrystalline materials using modern experimental techniques. Early initiation of fatigue cracks was found to be related to the localization of the cyclic plastic strain into persistent slip bands during cyclic loading. Internal dislocation structure of the persistent slip bands and also the evolution of the surface relief as surface persistent slip markings are documented. The model of the surface relief formation describing the formation of extrusions and intrusions is briefly described. The intrusions representing sharp surface crack-like defects play the principal role in the initiation of fatigue cracks. [ABSTRACT FROM AUTHOR]

Fatigue crack growth behavior of a series of aluminum alloy under single overload tests, repeated overload tests and a variable amplitude loading spectrum are analyzed. It is shown that the material behavior is governed by micro-mechanisms. For crack initiation at a notch, a short crack model presented here gives acceptable results. [ABSTRACT FROM AUTHOR]

Abstract: Fatigue failure of high-strength steels still occurs beyond 107 cycles in the very high cycle fatigue (VHCF) regime [1]. The reason for this late failure is that the fatigue properties in the long life region are strongly affected by flaws like non-meta llic inclusions inside the materia l [2]. In the case of VHCF failure a characteristic fine granular area (FGA) which is responsible for subsequent crack init iation can be observed in the vicinity of the inclusion on the fracture surface [3]. It is still unclear how diffe rent inclusion types affect the initiation mechanis m. Our study aims to clarify the influence of different inclusion types on the crack initiat ion in the very high cycle fatigue regime. Additionally, the threshold value for crack in itiation as a result of FGA formation shall be revealed. For this purpose ultrasonic fatigue tests (R = -1) of the high-strength steel 100Cr6 were carried out until an ultimate number of cycles of 109. Additionally, runout specimens were tested repeatedly with higher stress amplitudes until failure occurred. By this method an inclusion/flaw type dependent threshold value for the formation of the FGA was derived from fracture surfaces and stress amplitudes. The lowest threshold value indicates an absolute threshold value for VHCF crack initiation due to FGA formation. [Copyright &y& Elsevier]

Abstract: In order to demonstrate fulfillment of LBB design criterion and have better understanding under cyclic loading of the fatigue crack growth behaviour of piping components, experimental investigations were carried out on four numbers of 168mm Outer Diameter (OD) elbows made of SA403 Type 304LN stainless steel. The elbows had a machined part-through notch on the outer surface at different locations, i.e., crown, extrados or intrados. Fatigue crack growth during the experiments was continuously monitored using ultrasonic and alternate current potential drop techniques. Number of cycles to crack initiation and through-wall crack under in-plane bending moment were determined. Multiple cracks were observed in the elbow at various locations during fatigue crack growth studies. The two elbows with axial notch at crown location have shown less fatigue life compared to that of one elbow with circumferential notch at extrados and one elbow with circumferential notch at intrados. [Copyright &y& Elsevier]

Abstract: The mechanisms of the fatigue damage in nickel superalloys and TiAl intermetallics used for the design of components working at elevated temperatures are studied. The characteristic surface relief and the accompanying internal dislocation structures of these materials subjected to strain controlled cycling are documented using high resolution SEM, AFM and TEM. Typical structures corresponding to cyclic strain localization are shown in each class of materials and they are discussed in relation to the characteristic internal structure of a particular material and the types of dislocations participating in cyclic plastic deformation. The relation between surface persistent slip markings (extrusions and intrusions) and persistent slip bands in the interior of the material is presented. The mechanisms leading to fatigue crack initiation and early crystallographic crack growth are discussed. [Copyright &y& Elsevier]

Abstract: Rolling contact fatigue initiation life of 1070 steel was predicted by the FE method with an advanced cyclic stress-strain plasticity model and a general multiaxial fatigue criterion. The 2D repeated rolling contact processes were carried out by translating the normal pressure and the tangential traction across the contact surface step by step. The normal pressure and the tangential traction were applied to the nodes through the time-dependant amplitude functions as the concentrated nodal forces. With the detailed stress-strain responses outputted from the FE analysis after 20 rolling passes, the multiaxial fatigue criterion was used in the prediction of the initiation life. Several cases were simulated in this work. The P 0 /k ratio ranges from 6.0, 7.0 to 8.0, where P 0 the maximum Hertzian pressure, k is the yield stress in shear of the material. Comparing these cases, the initiation life was reduced with the increase of the P 0 /k ratio, and crack initiation tends to subsurface. [Copyright &y& Elsevier]

Abstract: Biaxial tension-tension cyclic loading tests were conducted with cruciform shaped specimens using AZ31 Mg alloy sheet, which produced by warm rolling. The residual stress of the sample surface can be obtained by in-situ measurement of x-ray under biaxial low cyclic loading. The surface fracture morphology of the cyclic load test specimens was examined by scan electron microscopy (SEM) and optical microscope. From the results, it was found that the compressive residual stress occurs at crack tip in Y-axis direction. Crack nucleation is earliest in the load ratio r=1. In addition, the crack initiation angle is different due to the coupling effect of KI and KII in mixed modes I and II for 45° case. The crack propagation rate was significantly influenced by the non-singular stress parallel to the crack when it was correlated to the stress intensity range.The surface of the case of 90° is similar to that of the case of 45°, and striation-like-patterns and twin boundaries were observed in the specimens. [Copyright &y& Elsevier]

Abstract: In order to evaluate the fatigue limit of lamellar pearlitic steel used for railroad rails, tensile tests and fatigue tests are performed. Although the fatigue ratio of the lamellar pearlitic steel is lower than that of general steels, the reason for this has not been clarified. The fatigue cracks of the pearlitic steel initiate at a very early stage during the fatigue test. It is speculated that the steel should be treated as a steel with initial defects. In order to determine the initial defect size of the ultra-low cycle fatigue test, tensile tests are performed. Based on the test results, it was clarified that the crack initiation size depends on the crystal structure. In order to predict the fatigue limit of the pearlitic steel, Murakami''s prediction method is applied to the steel. The measured defect sizes are applied to the method, and the fatigue tests are performed. The predicted fatigue limits and the test results have good agreement. In addition, from the SEM observations, the initial crack causing the fatigue failure was found to be a pearlite block. We then concluded that the fatigue limit of the pearlitic steel can be predicted by Murakami''s method and the defect size is the pearlite block size. If the pearlite block sizes then become small, the fatigue limit of the pearlitic steel will increase. [Copyright &y& Elsevier]

Abstract: Recent work on microstructure-scale crack propagation from corrosion damage on a 7000-series alloy in realistic airframe environments is reviewed and its pertinence to airframe prognosis established. The impact of corrosion damage can be accurately modeled by assuming formation life fraction is nil, long crack growth rates, and linear elastic fracture mechanics. An empirically validated corrosion modified-equivalent initial flaw size distribution was established by “back calculation” from measured-total fatigue life; serving as a quantitative descriptor of the deleterious effect of corrosion on fatigue and an initial flaw size input to fracture mechanics modeling. High fidelity characterization of the corrosion pit and fracture surface, coupled with finite element analysis, validated the crack formation and micro-structurally small crack propagation assumptions inherent to fracture mechanics modeling. A large-beneficial effect of low-temperature loading environment was observed; incorporation of this behavior into airframe prognosis could substantially reduce the inspection burden. [Copyright &y& Elsevier]

Abstract: Fatigue tests of Zr-based bulk metallic glass plates were conducted under cyclic-torsion, and the results were compared with those obtained under cyclic-bending and axial-loading. The fatigue strength of axial-loading was lower than that under cyclic-bending because the fatigue strength depends on the cooling rate in the casting process of the material, which is different in the thickness direction. The fatigue strength of plane-bending reflects the strength at the specimen surface while that of axial-loading is determined by the weakest strength in the thickness direction. For cyclic torsion of specimens with rectangular cross-section, however, the fatigue crack initiation site must be determined by the ratio of specimen width to specimen thickness, and fatigue strength depends on the ratio. Therefore, the criteria for fatigue limit under combined stress for metallic glass should be constructed considering the crack initiation site. [Copyright &y& Elsevier]

Abstract: The present study aims at analyzing the crack initiation in an austenitic stainless steel in low-cycle fatigue. A fatigue test was carried out using a polished specimen. The surface of the specimen was observed in-situ during the fatigue test, in order to establish the time of slip activity or crack initiation. After a number of cycles sufficient to initiate small crack, the test was stopped and the surface observed in Scanning Electron Microscope. Electron backscattered diffraction (EBSD) was used to identify the orientations of surface grains in the central zone of the fatigue specimen. Crack-initiation sites and slip systems associated to the initiated microcracks have been identified. [Copyright &y& Elsevier]

Abstract: Fatigue-life curves are used in order to estimate crack-initiation, and also to prevent water leakage on Pressure Water Reactor pipes. Such curves are built exclusively from push-pull tests performed under constant and uniaxial strain or stress-amplitude. However, thermal fatigue corresponds to a nearly perfect biaxial stress state and severe loading fluctuations are observed in operating conditions. In this frame, these two aspects have been successively investigated in this paper: [•] In order to investigate on potential difference between thermal fatigue and mechanical fatigue, tests have been carried out at CEA using thermal fatigue devices. They show that for an identical level of strain-amplitude, the number of cycles required to achieve crack-initiation is significantly lower under thermal fatigue. This enhanced damage results probably from a perfect biaxial state under thermal fatigue. In this frame, application of the multiaxial Zamrik’s criterion seems to be very promising. [•] In order to investigate on cumulative damage effect in fatigue, multi-level strain controlled fatigue tests have been performed. Experimental results show that linear Miner’s rule is not verified. A loading sequence effect is clearly evidenced. The double linear damage rule (“DLDR”) improves significantly predictions of fatigue-life. [Copyright &y& Elsevier]

Abstract: In the present study, the influence of the deformation-induced non-diffusional phase transformation from austenite into martensite of a metastable stainless steel on the short crack growth behaviour in the high cycle fatigue regime was investigated. For this purpose in situ as well as ex situ cyclic deformation experiments in combination with scanning electron microscopy and supplementing imaging and analytical techniques (i.e., electron backscatter diffraction for orientation and phase analysis) were employed. Cyclic loading of the AISI304L stainless steel investigated at stress amplitudes slightly above the classical fatigue limit was found to lead to a local transformation from the face centred cubic (fcc) austenite into the body centred cubic (bcc) -martensite. The nucleation takes place in selected areas of the microstructure in form of lamellae even already after few initial cycles near active slip systems of high Schmid factors of almost 0.5. In the majority of cases cracks initiate at twin boundaries in the absence of any martensite formation. After a first stage of shear-stress-controlled propagation along the respective twin boundary, the cracks change growth mode and direction and continue to grow in a transcrystalline manner perpendicular to the loading axis. In this stage, an alternating growth on two adjacent {111} slip systems results in low-indexed crack planes such as {100} or {110}. As a consequence of the plastic deformation at the tip of the propagating short crack a transformation into -martensite occurs. This martensite formation results in an increase of the specific volume and gives rise to compressive stresses leading to premature contact of the crack surfaces and hence to a retardation of the short crack growth. [Copyright &y& Elsevier]

Abstract: Corrosion fatigue crack initiation behavior of various kinds of stainless steels is reviewed mainly on the basis of the author’s experimental results. The role of corrosion pit in the corrosion fatigue crack initiation process of martensitic, ferritic, austenitic, duplex and precipitation-hardening stainless steels is briefly summarized. The recent investigation of an electrochemical noise measurement method is demonstrated for 12%Cr martensitic stainless steel and 2.5%Mo containing high strength austenitic stainless steels. Finally a couple of future problems to be solved in corrosion fatigue crack initiation are touched on briefly. [Copyright &y& Elsevier]

Abstract: This paper presents low-cycle fatigue data derived for a sample of puddle iron removed from an ancient Portuguese riveted road bridge - the Fão bridge. Constant amplitude strain-life data is presented for strain ratios equal to 0 and −1. This data was correlated using the classical strain-life model, known as Morrow’s model. Additionally, overloading strain-controlled fatigue tests were carried out to identify the net effective strain-life model, as proposed by D.L. DuQuesnay. Finally, models are applied to assess variable amplitude (variable amplitude blocks) strain-life data derived for smooth specimens. [Copyright &y& Elsevier]

Abstract: Forged Ti6Al4V alloy in two different microstructures was used for investigations on fatigue behaviour with special focus on crack initiation and short crack propagation characteristics. Both microstructures are in the bi-modal condition containing different amounts and sizes of primary alpha grains. Interrupted fatigue experiments were carried out using a servohydraulic test facility. Different stress levels were imposed at a constant R ratio of −1 and a frequency of 20Hz with a sinusoidal command signal. SEM together with the EBSD technique was applied for the crack observation as well as for the determination of local crystallographic orientation data with the objective of linking initiation sites and crack paths to microstructural features. It was found that most of the cracks initiate on boundaries between two lamellae in favourably oriented colonies. These cracks propagate on prismatic glide planes with high Schmid factors until they reach another boundary. In some cases crack splitting was observed leading to crack propagation in different directions on different slip planes resulting in a reduction of crack growth rate. The crack paths can be attributed either to prismatic slip planes or the basal slip plane, whereas basal slip was found inside the lamellae or in primary alpha grains. Crack deflection at boundaries is also a common feature, and is probably related to high tilt and twist angles between the grains involved. [Copyright &y& Elsevier]

Abstract: The influence of crystallographic texture on fatigue failure mechanisms was studied by comparing the fatigue behaviour of two extruded magnesium alloys, AZ31 and ZN11. The microstructures and fracture surfaces of specimens were examined using scanning electron microscopy (SEM) to reveal the micromechanisms of fatigue crack initiation. The AZ31 alloy has an inhomogeneous grain structure and strong fibre texture, which cause strong asymmetry in the tensile and compressive yield strengths. This yield stress asymmetry is related to the high twinning activity under compressive loading. The metallographic investigation reveals that the cracks are mainly initiated at twin boundaries. On the other hand, a weak texture and fully recrystallised, homogeneous grain structure are found in the experimental alloy ZN11 after extrusion. As a consequence, twinning is suppressed and no yield stress asymmetry is observed. The fatigue failure of ZN11 is initiated by cyclic slip deformation. [Copyright &y& Elsevier]

Abstract: The effects of temperature as well as the initial hardness in the range 42 to 50HRC on the fatigue behaviour and life of a 5%Cr tool steel are investigated. Total strain controlled isothermal fatigue experiments between 200 °C and 600 °C are carried out under 1Hz frequency. The significant softening induced by tempering and/or cyclic loading is related to a strong reduction of the dislocation density clearly observed by transmission electron microscopy and x-ray diffraction. The coalescence of secondary carbides is also involved in the yield strength decrease above 550 °C and during cyclic loading. Increasing the initial hardness reduces the cyclic softening rate and increases the fatigue life. Moreover, several isothermal fatigue experiments with regular interruptions at different fractions of life are performed in order to investigate the crack initiation mechanisms. Three main crack initiation sites were observed depending on testing temperatures: non-metallic inclusions, prior austenitic grain boundary and lath boundary. [Copyright &y& Elsevier]

Abstract: When ferritic stainless steels are exposed to the temperature range of 700–800 °C, embrittlement takes place, which is known as the -phase embrittlement. In the present study, axial fatigue tests were conducted using a high-chromium ferritic stainless steel, type 447 with the chromium content of 30.7%, which was aged at 750 °C for 150h and 300h, and the effect of -phase embrittlement on the fatigue behaviour was discussed. Vickers hardness increased significantly by aging due to the precipitation of hard -phase. The aged specimens exhibited much lower fatigue strengths than the un-aged one, where fatigue strength decreased with increasing aging time. The precipitation of brittle -phase reduced the crack initiation and growth resistances, resulting in the lower fatigue strengths of the aged specimens. [Copyright &y& Elsevier]

Abstract: In order to investigate the fatigue properties of extruded magnesium alloy in very high-cycle regime, rotary bending fatigue test was performed in ambient atmosphere at room temperature using the hourglass shaped specimens of AZ80 alloys extruded (Fspecimen) and treated by an artificial aging after extrusion (T5-specimen). From the experimental results, both specimens show a clear step-wise – curve on which two knees exists. Specific stress amplitude formed the knee corresponded to the 0.2% offset proof stress of 160 MPa in compression. From the detailed observation of fracture surface, small facet-like smooth area appeared at crack initiation site on the specimen failed at low-stress amplitude level and in a high-cycle regime. On the other hand, lamella structure appeared at crack origin in case of the test at high-stress amplitude level. It was concluded that the step-wise – curve was induced by the crack initiation mechanism changing from the twin deformation in high-stress amplitude level to slip deformation in low-stress amplitudes, because of a mechanical anisotropy caused by the extrusion and the particular deformation characteristics of the hexagonal crystal structure in a magnesium alloy. [Copyright &y& Elsevier]

Abstract: The plastic stretching within a yield surface has to be described to simulate mechanical fatigue phenomena under macroscopically elastic condition, while the plastic strain is induced remarkably as the stress approaches the dominant yielding state. In this study, an unconventional plasticity model is proposed for the description of the cyclic loading behavior observed during the so-called high-cycle fatigue subjected to the cyclic stresses lower than the yield stress. The extended elasto-plastic model is applied for materials obeying not only isotropic but also kinematic hardening law and its mechanical responses under cyclic loading conditions are examined briefly. [Copyright &y& Elsevier]

Abstract: Systematic experimental investigation was carried out to examine the phenomena of crack initiation under pure compression fatigue condition with samples having different pre-prepared notches. Finite element analysis (FEA) simulation was also conducted to determine the stress concentration and stress distribution for the tested samples. Although a smooth bar tested under the pure compression fatigue condition with a peak compressive load equal to the yield strength did not show any crack formation, crack was indeed detected under compressive fatigue load in those samples whereby a critical stress concentration was reached. Such critical condition may be expressed through a critical von Mises stress level. In addition, experimental observation indicates that such crack initiation process was indeed always associated with localized plastic deformation [Copyright &y& Elsevier]

Abstract: The present paper deals with two specific subjects related to the fatigue strength and life of notched bars under combined torsional and axial loading. The first subject is the fatigue thresholds of materials with small defects. The fatigue threshold of materials with small defects or sharp notches is not controlled by the initiation of fatigue cracks, but by their propagation. The R-curve method is very useful to predict the fatigue thresholds of notched components. Small cracks nucleated at the notch root becomes nonpropagating when the applied stress intensity factor drops below the resistance of the material. It is important that the R curve is independent of loading conditions and only the applied stress intensity factor depends on loading conditions. In the present paper, the R-curve method was successfully applied to predict the fatigue thresholds of holed tubes made of carbon steels under in-phase and outof-phase combinations of cyclic torsion and axial loading. The second subject is an anomalous phenomenon of notch strengthening found in torsional fatigue of circumferentially notched round bars of austenitic stainless steel. In torsional fatigue of circumferentially notched bars of austenitic stainless steel, the fatigue life of notched bars was found to be longer than that of smooth bars and to increase with increasing stress concentration under the same amplitude of the nominal torsional stress. On the basis of the electrical potential monitoring of the initiation and propagation of small cracks at the notch root, the crack initiation life decreased with increasing stress concentration, while the crack propagation life increased. The anomalous behavior of the notch-strengthening effect was ascribed to the larger retardation of fatigue crack propagation by crack surface contact for the cases of sharper notches. The superposition of static tension reduced the retardation due to the smaller amount of crack surface contact, which gave rise well-known notch-weakening of the fatigue strength. The crack initiation life is controlled not by the peak strain amplitude at the notch root, but by the strain amplitude about 0.1mm distant from the notch root where the strain distribution was calculated by the elasticplastic finite element method using cyclic stress-strain relation. The propagation rate of cracks can be predicted using the J-integral range when the static tension is superposed on cyclic torsion. The shielding by crack face sliding contact greatly reduced the crack propagation rate under cyclic torsion without static tension. [Copyright &y& Elsevier]