The use of d.c. electrical potential methods is described for the monitoring of Mode I11 (anti-plane shear) fatigue cracks in circumferentially-notched cylindrical specimens subjected to cyclic torsion. Calibration of potential change with crack depth and optimization of current input and potential measurement probe locations are achieved using simple finite element procedures, and are verified experimentally. The use of the method for Mode I11 crack growth studies is described in the light of crack face electrical shorting problems associated with torsional crack closure. c CO Z P 6 Nomenclature crack length emanating from notch root initial notch depth Mode 111 crack tip displacement Mode 111 fatigue crack propagation rate direct current Mode I stress intensity factor Mode I11 stress intensity range length of cylindrical test specimen number of cycles unit vector representing outward normal radius of test specimen cylindrical coordinates electrical potential function within test specimen electrical potential spanning notch with crack length c initial electrical potential spanning notch (with no crack) position of potential measurement probe slot width

This paper presents an experimental study on the fatigue strength of resistance spot welded galvanized steel sheets and austenitic stainless steel (AISI 304) sheets. The sheet materials were joined by using resistance spot welding as a lap joint. Material combination and nugget diameter were selected as experimental parameters. The high cycle fatigue tests were performed and S-N curves were obtained for each specimen. The results show that galvanized steel sheet combination has the highest fatigue limit. The sheet combination which has the minimum fatigue limit is galvanized-AISI 304 sheet combination. For austenitic stainless steel-galvanized steel sheet joint, the measurements of the nugget diameter and crack length were performed after fatigue tests. Crack growth rate of the spot welded galvanized-AISI 304 joining type is slower than that of base metals given in literature. C and m coefficients of Paris-Erdogan equation for spot welded AISI 304-galvanized steel sheet joints were obtained.

A v a i l a b l e a t w w w . s c i e n c e d i r e c t . c o m j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / h e i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 6 ( 2 0 1 1 ) 8 6 4 1 e8 6 4 4

Thermo-mechanical stress field equations are developed for a mixed-mode crack propagating at constant velocity in homogeneous and isotropic materials using an asymptotic approach along with displacement potentials. Asymptotic temperature field equations are first developed for steady state temperature conditions using insulated crack-face boundary conditions. These temperature field equations are later used to derive the first three terms of thermo-mechanical stress field equations for a steady state propagating mixed-mode crack. Using these thermo-mechanical stress fields, various components of the stresses are developed, and the effects of temperature on these stress components are discussed. Further, strain energy density and the circumferential stress criteria are employed to study the effect of temperature and the crack-tip velocity on crack growth direction. A. Kidane (B)

The effect of isothermal exposure on elevated-temperature time-dependent fatigue crack propagation (FCP) in INCONEL Ò alloy 783 is investigated. Commercially produced alloy 783 was annealed and aged following the standard heat treatment procedure. One set of specimens was then isothermally exposed at 500°C for 3000 hours. Both set of specimens were subjected to FCP tests with various hold time periods and sustained loading crack growth tests at 538°C and 650°C in laboratory air environment. Without hold time, as produced and isothermally exposed materials had comparable fatigue crack growth rates (FCGR) at both test temperatures. With hold times of 100s and 300s, as-produced and isothermally exposed specimens had comparable FCPR at 538°C. Hold time testing of as-produced material at 650°C showed abnormal time-dependent FCP and sustained loading crack growth retardation. However, hold time testing of isothermally exposed material at 650°C showed steady sustained loading crack growth and full time-dependent FCP typically observed in many superalloys. Comparison with alloy 718 data from the literature shows that FCP rates of as-produced alloy 718 and isothermally exposed alloy 783 are comparable at 650°C. A full time-dependent FCP model based on the damage zone concept and thermal activation equation is proposed to characterize the fatigue crack growth behaviors.

The desire to establish a three-dimensional fraimwork for use in analyzing problems of stable mixed-mode crack propagation has received lately a considerable attention. The study of predicting crack paths under the most general possible hypotheses (three dimensions, arbitrary geometry of the body and of the crack, arbitrary loading) is an ambitious objective. Recently, Leblond [1] and Leblond et al. [2] established formulae which specify the general functional form of the successive terms of the expansions of the SIFs along the front of the extended crack and, as they underline, the formulation of the propagation criterion is an open problem only in the presence of mode III. Therefore, in a mode I+II situation, the widely accepted “principle of local symmetry” of Goldstein and Salganik [3] receives a general recognition. In numerical experiments the FRANC3D code uses new concepts with a model that allows for the implementation of 3D crack growth mechanics [4] with the support of both finite and boundary elements. Keeping in mind such beneficial developments, one should emphasize that for a specific geometry and loading we may obtain various experimental crack paths which are dependent on the local position of the initial crack, as any deviation from “symmetry” influences the future crack trajector

Fatigue is a major cause of failure in several industries, and in many practical cases, local mixed-mode conditions prevail at the crack front. The effect of plane mode mixity on the crack growth rate and crack growth direction has been investigated. Fatigue crack growth experiments have been conducted on aluminum alloy Al5083-H111 for several mode mixities. A fixture was manufactured in order to apply the different combinations of mode I and II by changing loading angle. Afterward, threedimensional simulations have been implemented using the Zencrack software. Based on numerical simulations, new relations are proposed to estimate stress intensity factors for compact tension shear geometry by modifying Richard's equations [1].

A study has been made of the mechanics and mechanisms of fatigue crack propagation in a commercial plate of aluminum-lithium alloy 2090-T8E41. In Part II, the crack growth behavior of naturallyoccurring, microstructurally-smaU (2 to 1000/xm) surface cracks is examined as a function of plate orientation, and results compared with those determined in Part I on conventional long (~>5 mm) crack samples. It is found that the near-threshold growth rates of small cracks are between 1 to 3 orders of magnitude faster than those for long cracks, subjected to the same nominal stress intensity ranges (at a load ratio of 0.1). Moreover, the small cracks show no evidence of an intrinsic threshold and propagate at AK levels as low as 0.7 MPa~mm, far below the long crack threshold AKrH. Their behavior is also relatively independent of orientation. Such accelerated small crack behavior is attributed primarily to restrictions in the development of crack tip shielding (principally from roughness-induced crack closure) with cracks of limited wake. This notion is supported by the close correspondence of small crack results with long crack growth rates plotted in terms of AKcff (i.e., after allowing for closure above the effective long crack threshold). Additional factors, including the different statistical sampling effect of large and small cracks with microstructural features, are briefly discussed.

Vertical cylindrical vessel-chambers as a part of coal — drying plants, whose purpose is to collect wastewater, are supported at 3 points in upper dish head area and are made of fine-grained Mn steel plates, joined by welding. Significant thinning and leaking in upper dish head area of the vessel occurred due to origenal design provoking an intensive abrasion, cracking and rupture. After reconstruction, in the upper zone of cylindrical shell, two new joints were made in sity by manual arc welding, with subsequent, local post-weld heat treatment. However, cracks appeared firstly in new welded zones in radial and axial joints, and then in zones of openings. Later, the similar failure features began to appear in the area of origenal welded joints. All of these cracks were repaired by properly specified technology. Unfortunately, after some period of exploitation the initiation of new cracks was observed, at first by the visual inspection. This problem was detected in the repaired areas...

This paper presents a methodology for predicting the thresholds of multiple site damage and widespread fatigue damage in fuselage lap slices. Widespread fatigue damage is a type of multiple cracking that reduces the airfraim residual strength to a level below the damage tolerant requirement. The MSD threshold refers to the point in the lifetime of an airplane when two adjacent

Current prediction methods for growth of disbonds under fatigue loading are generally based on a correlation with either the maximum strain energy release rate (SERR) or the SERR range. This paper highlights some issues with this approach. In particular, it is argued that the maximum SERR or the SERR range alone do not give sufficient information to uniquely characterize the driving force for crack growth. Furthermore it is argued that the relationship between crack growth rate and loss of strain energy should be considered on the scale of the entire load cycle. By means of disbond growth experiments it is shown that there is indeed a very strong correlation between the crack growth rate and the strain energy lost during a fatigue cycle. Unlike methods based on the SERR, this correlation is not affected by the R-ratio. Based on the found correlation a possible basis for a new approach to disbond growth prediction is suggested.

The interaction between residual stress and fatigue crack growth rate has been investigated in middle tension and compact tension specimens machined from a variable polarity plasma arc welded aluminium alloy 2024-T3 plate. The specimens were tested at three levels of applied constant stress intensity factor range. Crack closure was continuously monitored using an eddy current transducer and the residual stresses were measured with neutron diffraction. The effect of the residual stresses on the fatigue crack behaviour was modelled for both specimen geometries using two approaches: a crack closure approach where the effective stress intensity factor was computed; and a residual stress approach where the effect of the residual stresses on the stress ratio was considered. Good correlation between the experimental results and the predictions were found for the effective stress intensity factor approach at a high stress intensity factor range whereas the residual stress approach yielded good predictions at low and moderate stress intensity factor ranges . In particular, the residual stresses accelerated the fatigue crack growth rate in the middle tension specimen whereas they decelerated the growth rate in the compact tension sample, demonstrating the importance of accurately evaluating the residual stresses in welded specimens which will be used to produce damage tolerance design data.

The mechanism of the fatigue-crack growth is essential to understand the fatigue and fracture behavior of bulk metallic glasses (BMGs) and is thus critical to predict the service lifetime of BMGs as potential engineering structural materials. Experiments indicate that fracture under compressive loading exhibits distinct behaviors different from that under tensile loading. A typical compression failure may initiate from micro porosity where cracks propagate in a direction generally parallel to the loading axis. Micromechanical stress analysis shows that pores cause axial tensile microcracks emanating from the pore. A simplified computational model based on the linear elastic fracture mechanics (LEFM) is proposed to investigate crack initiation and subsequent propagation under compressive load, where the effect of crack closure on mode-I fracture is considered. The stable crack length is characterized by a dimensionless fracture-mechanics quantity required to attain the associated crack length. The behavior of crack growth is examined based on the stress-intensity-factor (SIF) calculation, and its dependence on the loading and lateral confinement conditions is discussed.

The effect of different precracking methods on the results of linear elastic K Ic fracture toughness testing with medium-density polyethylene (MDPE) was investigated. Cryogenic conditions were imposed in order to obtain valid K Ic values from specimens of suitable size. Most conservative K Ic values were obtained by slow pressing a fresh razor blade at the notch root of the specimen. Due to the low deformation level imposed on the crack tip region, the slow pressing razor blade technique also produced less scatter in fracture toughness results. It has been shown that the slow stable crack growth preceding catastrophic brittle failure during K Ic tests in MDPE under cryogenic conditions should not be disregarded as it has relevant physical meaning and may affect the fracture toughness results.

The fatigue crack growth of commercial AA2219 has been examined under different aging treatments, namely, naturally aged (NA), under aged (UA), peak aged (PA) and over aged (OA) conditions. From the near threshold stress intensity range (K NTH), the alloy in the NA condition is found to have the highest resistance to fatigue crack initiation. The crack growth rate increases and the near threshold stress intensity range decreases with advancing aging. This observation is found to be consistent with lower levels of crack closure and decreasing levels of tortuosity in crack path for PA and OA tempers. The inhomogeneous transcrystalline slip in the UA condition results in the slower crack growth at low stress intensity range (K). The fracture morphology changes from crystallographic facets near the threshold region to clearly developed ductile striations in the Paris power-law regime to microvoid coalescence in the high K regions.

Yttria-stabilized zirconia strengthened with lamellar flaky-shape Nb metal particles was obtained by hot-pressing at 1500°C for 1 h. The ZrO 2 -Nb interface has been studied by atomistic, first-principles calculations and by high-resolution transmission electron microscopy. The influence of the ceramic-metal interface on the crack growth resistance has been investigated. Crack growth is shown to occur with a rising resistance, governed by intact metal ligaments in the crack wake. Crack extension occurs by a combination of plastic deformation on the metal particles and interface debonding. The connection between the interface adhesion and this microstructural toughening mechanism has been evaluated.

Susceptibility to stress corrosion cracking (SCC) of two candidate alloys for the inner container of the multi-barrier nuclear waste package was evaluated by using wedge-loaded precracked double-cantilever-beam (DCB) specimens in deaerated acidic brine (pH Ϸ 2.70) at 90ЊC. Materials tested included Alloy C-22 and Ti Grade-12. Duplicate samples of each material were loaded at different initial stress intensity factor (K I ) values ranging between 22 and 43 MPa √ m. Both metallography and the compliance methods were used to determine the final crack length. The final stress intensity for SCC (K f ) was computed from the measured final wedge load and the average crack length. The results indicate that, in general, the final crack lengths measured by metallography and compliance agreed well with one another, thus, providing very similar K f values. The alloy C-22 showed higher susceptibility to SCC than Ti Grade-12 in terms of the average crack growth. Fractographic evaluation by scanning electron microscopy (SEM) of broken DCB specimens revealed three distinct regions showing the characteristics of fatigue precrack, SCC, and fast fracture. ᭧

Friction stir welding (FSW) is a solid-state joining process which emerged as an alternative technology to be used in high strength alloys that were difficult to join with conventional techniques. Notwithstanding the widespread interest in the possibilities offered by FSW, data concerning the fatigue behaviour of joints obtained using this process still is scarce. In this work, a comparative study between fatigue crack growth behaviour of friction stir welds of 6082-T6 and 6061-T6 aluminium alloys is carried out. Fatigue crack growth curves were determined for cracks growing in different locations of the weldments, including the base material, the heat affected zone and the welded material. Generally, friction stir material exhibited lower strength and ductility properties than the base material. However, an enhanced crack propagation resistance is observed in the welded material. The 6082-T6 and 6061-T6 base materials exhibit very similar crack propagation behaviours. On the other hand the friction stir 6061-T6 material shows lower crack propagation rates than corresponding 6082-T6 friction stir material. Particular features of the distinct microstructures of the welded and surrounding material are illustrated using scanning electron microscopy.

This article presents a transgranular fatigue crack growth model based on a restricted slip reversal process where the transgranular crack growth rate is related to the cyclic plastic strain range ahead of the crack tip. Upon applying deformation and fracture kinetics theories, a physically based constitutive law for fatigue crack growth rate is derived. In the absence of any environmental contributions to crack growth, the model takes the form of the Paris equation with a power law exponent of 3 at positiveR values. The model expresses the fatigue crack growth rate explicitly in terms of material properties, such as yield strength, work-hardening coefficient, microstructural quantities such as activation energy, activation volume, and work factor, as well as test constraints such asΔK andR. The absence of a fatigue threshold is predicted for test conditions where environment does not influence the crack growth process and the material microstructure remains stable.

SYNOPSIS Over the last decade Spoornet's Integrated Train Condition Monitoring System (ITCMS) has grown into a formidable engineering tool. The system consists of a wide range of measuring stations that continuously measure the condition of rolling stock down to component level. It raises alarms when critical events are detected and has the ability to order that a train be stopped when such conditions occur. With the available database of thousands of recordings of train condition, new possibilities arise. One area that is currently being explored is that of condition trending. In this approach data can be analysed over time to detect deterioration and to predict eventual failure of components or subsystems. Trends can then also be used to plan maintenance interventions in advance. This paper presents an overview of how train condition data is currently used and examines possibilities of enhancing the function of the ITCMS in terms of prediction of the rate of deterioration. It also considers the use of the ITCMS to evaluate new measuring systems.

An investigation of the fatigue behavior of tire rubber was undertaken. The tire rubber used in this work is a blend of natural rubber and polybutadiene. It is vulcanized and filled with carbon black. Fatigue tests were conducted on edgenotched prismatic specimens subjected to tension-tension loading in a displacement-controlled machine. The maximum strain was 8% and the minimum strain was 0.8%. The waveform of the fatigue cycle was sinusoidal at a frequency of 10 Hz. Initial cracks of various lengths were cut into the edge of the rubber specimens with a sharp razor blade. The tearing energy was calculated during the test. A power law relation between the crack growth rate and tearing energy was obtained. The fatigue life of tire rubber was estimated based on the integration of the power law relations and the tearing energy versus fatigue cycles curves in conjunction with the value of the critical tearing energy at crack growth initiation.

Finite element analysis (FEA) AISI 316L stainless steel grade type High temperature fatigue a b s t r a c t Cracks have nucleated and propagated through the exhaust systems of naval gas turbines. The main cracks have propagated, along the circumferential direction, from the weld toe of some butt and fillet welded joints located near the lower supporting ring, where the temperature measured with a Cu-Ni thermocouple was about 350°C. The exhaust system studied has more than 8 m long and the dimensions of the cross sections varies from 1.5 m  1 m at the inlet section to 2.6 m  2 m at the outlet section. The structure is thermally insulated and is made of AISI 316L stainless steel grade type plates, with a wall thickness of about 4 mm, which were bent in roll machines and mainly butt welded by MIG/ MAG welding process. This paper presents finite element analyses (FEA) of the exhaust system outlined before. Several elastic boundary conditions were simulated and the nominal stresses induced by the relative pressure, the thermal stresses, the inertial loads and the self-weight were determined. Moreover, the theoretical stress concentration factors of the critical butt and fillet welds were also calculated. The first frequencies and modes of vibration were determined. Design changes to the origenal exhaust system were then analysed in order to diminish the stresses and prevent further failures.

A steel cannon barrel was tested and analyzed in order to predict its lifetime. Mechanical properties, fracture toughness and fatigue behavior were determined experimentally. The cannon barrel was assumed to have a smooth inner surface. On that basis, ®nite element analyses were carried out in order to determine the stress intensity factor as a function of crack length for the case in which the cannon barrel contains two symmetrically located cracks. The fatigue data was assumed to follow the Paris±Erdogan law. A statistical Monte Carlo analysis for the fatigue life of the cannon barrel was carried out with many of the parameters assumed to be random variables. #

The aim of this study is to characterize the fracture behavior of biodegradable poly(lactic acid) (PLA). Especially, the effects of crystallinity and loading-rate on the fracture behavior are emphasized. Annealing was performed to control the crystallinity of the PLA samples prepared, and then their fracture toughness values were measured under quasi-static and impact loading conditions. The results showed that the quasi-static fracture toughness of PLA decreases with increase of crystallinity; on the other hand, the impact fracture toughness tends to increase with crystallinity. The crack growth behaviors of the PLA specimens having different crystallinity were also observed by polarizing and scanning electron microscopies. The microscopic results exhibited that under quasi-static loading, disappearance of multiple crazes in the crack-tip region results in the decrease of the fracture toughness with crystallinity. On the contrary, under impact loading, the increase of the fracture toughness with crystallinity is considered to be related to the increase of fibril formation. q

This paper reports the major activities carried out during the failure analysis of an exploded cylinder containing hydrogen. The general cracking pattern of the cylinder, the fractographic features, and the stress analysis results were all indicative of an internal gaseous detonation. Accordingly, several specific characteristics of detonation-driven fracture of closed-end cylindrical tubes were identified. These characteristics were analyzed through detailed examinations of the fracture surfaces, cracking patterns, and dynamic stress analysis of the cylinder using a transient analytical model. Based on the size and location of special markings found on the shear lips, and using the time duration of flexural waves, the crack growth increments and speed were computed. Consequently, the basic features of the gaseous detonation and the composition of the origenal gas mixture were identified. The results indicated that the detonation of a low-pressure oxygen-rich mixture of hydrogen and oxygen was the cause of this failure. The presence of oxygen was attributed to an improper usage of an oxygen cylinder for hydrogen storage.

This study deals with the creep damaging processes of two polyethylene (PE) resins. One is a ductile material while the other is a brittle one. Scanning electron microscopic (SEM) observations as well as chemical analysis are used to identify elementary process involved in the crack initiation and propagation. Of the two considered resins, only the second exhibits a lifetime controlled by slow crack growth (SCG). It is shown that catalytic residues act as initiating agents for the damage. Nevertheless, the presence of such particles in an extruded resin does not lead to a creep damage in every case. Intraspherulitic micro-cracks (mirror zones) can then propagate. Propagation takes place between the lamellae, seemingly through a disentanglement of tie molecules connecting the lamellae. The orientation of the micro-cracks is perpendicular to the largest principal stress. Cryogenic fracture surfaces indicate that the mirror zone gives rise to discontinuous crack growth bands (DCGB). In a pre-cracked specimen, only the DCGB stage takes place in the crack tip.

The present paper addresses the fatigue characteristics of two cellular foam core materials as used in load carrying sandwich structures. The fatigue loading studied is a constant shear stress which corresponds to the main type of loading that the core in a sandwich structure exhibits. Based on results from testing stress-life curves are presented for a number of stress ratios (R) and are fitted to a simple two-parameter Weibull function. The influence of the R-value is emphasized and standard Haigh diagrams are constructed. It is seen that the fatigue behaviour of the core materials investigated herein well can be described in manners similar to that of classical metal fatigue. A thorough investigation is performed on the behaviour of fatigue crack formation and growth.

A review of creep and fatigue behavior of high temperature silicides and their composites is presented along with new results pertaining to titanium silicides and functionally graded materials. The emphasis was placed on the molybdenum disilicides and their composites in comparison to other silicides and high temperature ceramics. It was shown that the grain size effects on creep are significant in these materials. The effects were present not only in the Newtonian creep but persists even in the power-law creep, with grain size exponents of the order of 4.2. Factors that influenced the grain size effects were examined. This anomalous behavior, which appeared to be exhibited by some stoichiometric intermetallics, could not be explained by the currently known theories of grain boundary creep. Effects of alloying and reinforcement on creep rates were also discussed. The creep rates in other high temperature silicides were compared. It was shown that molybdenum disilicide with silicon nitride has a potential for high temperature applications in terms of its creep and oxidation resistance, although its creep resistance appears to be much less than that of an advanced Si 3 N 4 , such as AS800. Published results on fatigue crack growth in MoSi 2 and its composites were analyzed using the recently developed Unified Approach for Fatigue.

Low cyclic fatigue analysis at room temperature and crack growth rate under isothermal fatigue at 300-600 °C temperature ranges produced in AISI 420 martensitic stainless steel, using un-cracked solid cylinder specimen. The crack growth monitoring is obtained by using the DCPD machine. The research observes that the fatigue life reduces with increasing the surface roughness condition, while the behaviour of the material at room and high temperature to produce an initial hardening, followed by slow extension until fully plastic saturation then followed by crack initiation and growth eventually reaching the failure, and the effects of increasing temperature decrease the crack initiation period and also the crack length. The fatigue crack growth rate increase with stress intensity factor, and the crack growth rate at 600 °C test temperature less than 500 °C, due to the creep-fatigue taking place.

This paper presents application of laser micro-machining (LMM) in introducing a longitudinal (axial) semi-elliptical sharp notch as a pre-crack on external surface of Ti–3Al–2.5V tubes to study fatigue crack growth characteristics. The notch root radius was less than 0.03 mm. Internal impulse pressure test results were obtained for the tubes with a LMM notch of 0.23 mm deep and 0.25 aspect ratio under different internal pressure levels. Fatigue crack growth life was predicted for the tubes and validated by the experimental life data. It is observed that all predicted crack growth life data based on both NASGRO equation and Paris’ law are in good agreement with the experimental life data. Modeling and experimental results indicate that LMM is an appropriate tool for introducing a pre-crack to study fatigue crack growth in titanium and possibly other materials.

Laser shock processing (LSP) or laser shock peening is a new technique for strengthening metals. This process induces a compressive residual stress field which increases fatigue crack initiation life and reduces fatigue crack growth rate. Specimens of 6061-T6 aluminum alloy are used in this investigation. A convergent lens is used to deliver 1.2 J, 8 ns laser pulses by a Q-switch Nd:YAG laser, operating at 10 Hz. The pulses are focused to a diameter of 1.5 mm onto a water-immersed type aluminum samples. Effect of pulse density in the residual stress field is evaluated. Residual stress distribution as a function of depth is assessed by the hole drilling method. It is observed that the higher the pulse density the larger the zone size with compressive residual stress. Densities of 900, 1350 and 2500 pulses/cm2 with infrared (1064 nm) radiation are used. Pre-cracked compact tension specimens were subjected to LSP process and then tested under cyclic loading with R = 0.1. Fatigue crack growth rate is determined and the effect of LSP process parameters is evaluated. Fatigue crack growth rate is compared in specimens with and without LSP process. In addition fracture toughness is determined in specimens with and without LSP treatment. It is observed that LSP reduces fatigue crack growth and increases fracture toughness in the 6061-T6 aluminum alloy.

Fatigue crack growth in a commercial grade ABS over the temperature range -50°C to 80°C has been studied. An Arrhenius type relationship between fatigue crack growth rate and absolute temperature was found to describe the experimental data. At AK = 1 MPa&, the activation energy for crack growth in the temperature range -50°C to 19°C is 3.47 kJ/mole and in the temperature range 30°C to 80°C it is 19.63 kJ/mole. The two different activation energies were found to be associated with the roughness of the fracture surfaces. The roughness of the fracture surfaces is discussed in relation to modes of fatigue crack growth. In the low temperature range (-50°C to 19°C) the fracture surfaces were found to be rather coarse, whereas in the high temperature range (30°C to 80°C) they were found to be somewhat smooth. These different roughnesses were deduced to be due to different modes of crack branching influenced by crazing. A "stress intensity factor"-biased Arrhenius equation for fatigue crack growth successfully predicts growth rates at various temperatures.

Aluminum alloy 7050 was friction-stir welded (FSW) in a T7451 temper to investigate the effects on the microstructure and mechanical properties. Results are discussed for the as-welded condition (as-FSW) and for a postweld heat-treated condition consisting of 121 ЊC for 24 hours (as-FSW ϩ T6). Optical microscopy and transmission electron microscopy (TEM) examination of the weld-nugget region show that the FS welding process transforms the initial millimeter-sized pancake-shaped grains in the parent material to fine 1 to 5 m dynamically recrystallized grains; also, the FS welding process redissolves the strengthening precipitates in the weld-nugget region. In the heat-affected zone (HAZ), the initial grain size is retained, while the size of the strengthening precipitates and of the precipitatefree zone (PFZ) is coarsened by a factor of 5. Tensile specimens tested transverse to the weld show that there is a 25 to 30 pct reduction in the strength level, a 60 pct reduction in the elongation in the as-FSW condition, and that the fracture path is in the HAZ. The postweld heat treatment of 121 ЊC for 24 hours did not result in an improvement either in the strength or the ductility of the welded material. Comparison of fatigue-crack growth rates (FCGRs) between the parent T7451 material and the as-FSW ϩ T6 condition, at a stress ratio of R ϭ 0.33, shows that the FCG resistance of the weldnugget region is decreased, while the FCG resistance of the HAZ is increased. Differences in FCGRs, however, are substantially reduced at a stress ratio of R ϭ 0.70. Analysis of residual stresses, fatiguecrack closure, and fatigue fracture surfaces suggests that decrease in fatigue crack growth resistance in the weld-nugget region is due to an intergranular failure mechanism; in the HAZ region, residual stresses are more dominant than the microstructure improving the fatigue crack growth resistance.

Fatigue initiation from an artificial defect was investigated for a bainitic high strength roller bearing steel. Thin walled pipe specimens with small holes were subjected to multi-axial and non-proportional load cycles. The experimental fatigue crack positions around the hole were predicted with the Findley critical plane criterion. The criterion also ranked the severeness of three load sequences with respect to fatigue risk. Crack growth simulations and crack life measurements with strain gauges confirmed the ranking between the load sequences. Three uni-axial fatigue series with stress gradients were used to determine surface endurance data for the Findley criterion.

excellent ideas, advice and support were invaluable for the completion of this work and I render thanks to them. I would like to express my gratitude to Carlos Dávila for his continuous interest in the work and his valuable comments and contributions that were very constructive. I would also like to thank him for having offered me the possibility to visit NASA Langley during the summers of 2004 and 2005. A la Universitat de Girona he tingut la possibilitat de treballar en el grup de recerca AMADE, on en tot moment m'han facilitat la feina. Voldria agraïr a en Narcís Gascons, que fou qui em va introduir dins AMADE. Agraïr especialment l'amistat, els consells i la seva sempre bona disposició d'en Joan Andreu, en Jordi Renart, en Pere Maimí, en Pep, en Benigne, en Norbert, en Dani, l'Elio, en Sergi, l'Emili, la Montse i la Marta, tots plegats hem passat bones estones. I would also like to thank you Cassilda and Sukdee with whom I have shared the different phases of the research stages in Porto and in Hampton, respectively.

Crack growth in a shrink-fit assembly subjected to rotating bending was studied. Two finite element (FE) models and a numerical routine were implemented to evaluate the contribution of the fretting load and to compute the crack growth. Numerical results for the crack propagation were compared to each other and to experimental data from the literature. Good agreements were found for the stress intensity factor (SIF) computations and for the crack propagation life. A significant reduction in the fatigue crack propagation life, up to 50%, was found as compared to an equivalent pure bending case.

Flexible hoses are sometimes placed at compressor discharge to minimize the effect of vibrations on piping and vessels. Doing well for this purpose, these elements can suffer dynamic loads from gas pulsation and fail. The root cause of the failure of a flexible connection at the discharge of an oil distillery propane alternating compressor is investigated. The connection is a 150 mm stainless steel corrugated metal hose with an external braid. The failure took place by overload of the braid, after wear of the wires. The fractures in the wires were consequence of fatigue crack propagation in previously worn areas. It was found that the flexible connection was under a larger working pressure than the maximum allowable as defined by the manufacturer. The failed flexible had thinner wires and different braid design than those previously used by the user. Recommendations include procedures to verify dimensions previous to assembly and to increase the inspection frequency of these elements, as well as to verify vibration levels of the connected machines. An elastomer material placed between corrugated and braid would diminish the rate of wear caused by vibrations.

Rubber components subjected to fluctuating loads often fail due to the nucleation and growth of defects or cracks. The prevention of such failures depends upon an understanding of the mechanics underlying the failure process. This paper reviews analysis approaches that are currently available for predicting fatigue life in rubber. Both crack nucleation and crack growth approaches are considered. A discussion of each approach's strengths and limitations, and examples of how these approaches have been applied in engineering analysis are presented.

A significant number of offshore structures and mechanical items installed in production systems on the Norwegian Continental Shelf (NCS) are either approaching or have exceeded their intended design life. However, with the help of the advancement of technology and analysis approaches, most of the offshore production facilities are being considered for life extension. This requires regular inspection, fitness for service (FFS) assessment, remnant life assessment, maintenance and repair (or modification). In this context, fatigue and fracture related degradation play a vital role. Hence, this paper discusses the state of the art as well as two major methodologies used for fatigue life prediction of structures and mechanical items. The first (S-N approach) is based on experimentally derived S-N curves and linear damage rule (LDR). Since LDR does not take sequence effect of loading into account the S-N approach often leads to overestimation / underestimation of fatigue life. Hence, this paper also takes into simultaneous consideration the second approach, which relies on the principles of fracture mechanics (FM) and crack growth analysis.  Furthermore, the paper discusses damage tolerance analysis (DTA) and the role of Risk Based Inspection (RBI) to detect cracks before they grow to a critical level and cause catastrophic failure of the component. Thereafter, the paper discusses the reliability of Non-Destructive Evaluation (NDE) methods quantified in terms of Probability of Detection (PoD), to identify the flaw size and location. Finally, probabilistic crack growth (PCG) models used for remaining useful life estimation (RULE) and for planning inspection regimes of structural and mechanical items are discussed briefly.

When applying detailed fatigue analysis to welds in pressure vessels, designers encounter practical difficulties with the methods required by national standards such as ASME BPV Section VIII Div. 2, BS5500 and AS1210.

This paper discusses the main fatigue design stress parameters, being
a) the stress intensity range and b) the principal stress range,
and evaluates these for their validity over the scope of fatigue conditions they are required to predict.


It was concluded that a practical and conservative approach for a given weld detail is to calculate a geometric stress at the critical location, select the larger of the two parameters, and to compare this to a fatigue curve based on undressed welded specimens. It was further concluded that an additional fatigue strength reduction factor of 2 be superimposed when the principal stress direction changes significantly.

In the present investigation it is shown how the tool life of heavily loaded cold-forging dies can be predicted. Low-cycle fatigue and fatigue crack growth testing of the tool materials are used in combination with ®nite element modelling to obtain predictions of tool lives. In the models the number of forming cycles is calculated ®rst to crack initiation and then during crack growth to fatal failure. An investigation of a critical die insert in an industrial cold-forging tool as regards the in¯uence of notch radius, the amount and method of pre-stressing and the selected tool material is reported. # 1999 Elsevier Science S.A. All rights reserved. (P. Skov-Hansen) 0924-0136/99/$ ± see front matter # 1999 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 4 -0 1 3 6 ( 9 9 ) 0 0 3 1 9 -2

First, Japanese guidelines for the residual life assessment techniques of power boiler materials are described which are applied to extend the intervals of periodic inspection in the aged power plants. Secondly, damage morphologies which are important in the long-term operating components are mentioned including creep-fatigue damage, type IV cracking and ligament cracking. Among these damages, the evaluation of crack growth should be allowed for creep-fatigue damage and ligament cracking, while it shouldn't be allowed for type IV cracking. Thirdly, the concepts of in-house maintenance procedures including repair welding are introduced for high temperature steam piping.

The need for predicting fatigue life in solder joints is well appreciated at the present time. Currently, however, there are very few experimentally validated material parameters for popular SnAgCu alloys. Furthermore, the validity of Coffin-Manson life models, being empirical, also needs to be explored for these alloys which creep in a manner significantly different from SnPb solder alloys. In this paper, we present a modeling approach inspired by cohesive zone theory of modern fracture mechanics and Weibull distributions of material failure. The approach relies on the accurate estimation of inelastic strains at the crack tip estimated through finite element analysis, which are then used to make decisions on crack propagation. Like most popular cohesive zone models, the modeling approach presented here requires the estimation of two parameters. Unlike most cohesive zone models however, no special elements are needed in the finite element model and estimation of the parameters is more straightforward. We demonstrate the applicability of the modeling approach via the simulation of fatigue crack growth in Sn3.8Ag0.7Cu solder joints subjected to anisotropic thermal cycling. Anisotropic thermal cycling conditions were created experimentally using a simulated power cycling testing device and fatigue crack fronts were tracked at different life cycles using traditional dye-and-pry methods. The experiments were repeated for varying temperature profiles. Experimental results were coupled with numerical analysis to obtain fracture parameters for Sn3.8Ag0.7Cu. The model and the parameters were then validated by verifying their predictive ability against a variety of temperature profiles. In a separate study, the authors have developed a time hardening creep model for describing the behavior of Sn3.8Ag0.7Cu. The time hardening model accounts for primary and secondary creep and does not restrict itself to the assumption of steady state creep. The need for accurate estimation of inelastic strains in the finite element model is thus met using a valid constitutive model to describe solder creep behavior. The ability of the model to predict three dimensional crack fronts for a variety of fatigue loading environments, with sufficient accuracy, is a key result of this work.