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Aluminum 2xxx alloys have been one of the primary materials for the structural applications of aerospace and transportation industry because of their performance, manufacturing and reliable inspection techniques. Welding is very important in the manufacturing process of structural parts and is now known as the most vital process in the manufacturing route. A relatively new process of joining of materials is friction stir welding process, which was invented by The Welding Institute (TWI) in the UK in 1991. The friction stir welding is mainly employed in aerospace, marine and transportation fields that have high safety requirements. The failure by fatigue is the dominant failure mode for structural weld joints. Since fatigue failure of parts accounts for 50 to 90% of all failures, it is of great significance to understand the fatigue properties of friction stir welded joints. The aim of this overview is to summarize the current research on fatigue crack growth behavior of friction stir welded AA2xxx alloys and critical attention is payed to the damage tolerance performance of friction stir welded aluminum joints that can be affected by welding process parameters, residual stress, stress ratio, environment and post weld treatments.

Based on the Paris model of fatigue crack growth theory, the fatigue crack growth behaviour of Center Crack Tension (CCT) specimens of laser melting deposited 12CrNi2 alloy steel is studied by extended finite element method (XFEM). The crack growth rules and fatigue life are analyzed by experiment and finite element simulation. The experimental results are in good agreement with the finite element results, which verifies the accuracy of XFEM method to simulate the fatigue crack growth behaviour of laser melting deposited 12CrNi2 alloy steel components. Based on this, the effects of initial crack direction and load amplitude on fatigue crack growth behaviour are discussed. The results indicate that even if the initial crack direction is different, the crack will finally propagate in a direction perpendicular to the load. With the increase of the load amplitude, the fatigue life of the specimen with initial crack decreases exponentially.

The limited data of fatigue crack growth (FCG) may cause an inaccuracy assessment of the fatigue crack growth rate (FCGR). For particular parts in aircraft such as fuselage skin, a high-reliability degree due to FCG must be determined accurately for the design and safety requirements. Generally, the 6xxx series of aluminum alloy is used as the material for the fuselage skin in the aircraft. In this study, reliability evaluation of FCGR of heat-treated TIG-welded Al 6013-t4 was investigated by two-parameter Weibull. The FCG tests were conducted by following the ASTM E647 under three different artificial aging time conditions of 6, 18, and 24 hours. The C and m constant values were obtained by drawing the regression line from FCG data following Paris’s equation and analyzed employing three methods; the least square fitting method (LSFM), a mean value method (MVM), and a probabilistic distribution method (PDM). The result showed that the PDM and MVM showed a better-fitted line to assess the C and m values than LSFM. From the reliability viewpoints, the two-parameter Weibull was proposed to be applied as the PDM. Furthermore, the MCM was successful in evaluating the probabilistic assessment of the FCGR with the 85% confidence interval.

The paper is aimed to the study of the corrosion-fatigue behavior of high strength steels for offshore pipelines. Tests have been performed in order to study fatigue crack growth in synthetic seawater under cathodic protection. The tests have been carried out on three different steel grades from 65 to 85 ksi with tempered martensite and ferrite-bainite microstructures. The effect of stress intensity factor, cathodic protection potential and cyclic loading frequency is shown.

A two-dimensional elastic-plastic finite element model was built to simulate the closure of a long fatigue crack with arbitrarily shaped crack faces. The model growth is simulated by the successive mesh splitting along the crack path defined by element edges. To obtain a realistic morphology of the fracture surface, fatigue crack growth experiments with CT specimen made from AISI 304 stainless steel were performed and fracture surface topology was determined using a single camera and a depth-from-focus method. Simulated closing loads and closure lengths for the cracks with rough and smooth faces and for plane-stress and plane-strain conditions are compared. A mismatch of rough crack faces, resulting in an additional contact, is visualized.

The fatigue crack growth rate is the most important factor in predicting the life of a product when applying the damage tolerance design concept. Studies related to pure mode I for structures under fatigue loading have been actively conducted, while not many studies are conducted on the mixed mode. In this study, therefore, mixed mode fatigue crack growth experiments were designed using the Compact-Tension-Shear (CTS) specimens and the loading devices, proposed by Richard. Furthermore, the finite element analysis was used in determining the stress intensity factors of CTS specimen. As the results, the fatigue crack growth rate using the equivalent stress intensity factors proposed by previous researchers was lower than that of pure mode I at the initial stage of crack growth when the load angle increases.

When fatigue crack growth rate da/dN = 0, the stress intensity factor range ΔKI is a threshold and it is expressed by ΔKI = ΔKth. The threshold ΔKth is important for design, repair/replacement, particularly, trouble shooting for cracked components. However, the thresholds were not well codified. Authorized and consensus ΔKth are required. This paper introduces the current thresholds provided by fitness-for-service codes and the definition of ΔKth under negative stress ratio R is discussed. Finally, the threshold ΔKth for aluminum alloys in air environment is proposed for fitness-for-service codes.

The crack growth can accelerate after applying unexpected overload or underload during usual operation of machine structure. An acceleration or deceleration of the crack growth was observed depending on the experimental conditions. In the present study, a method of detecting the application of an unexpected hazardous load, that is high levels of overload or underload, was investigated. It is expected that the acceleration of crack growth brings catastrophic failure in machine structure. Therefore, it is useful for machine maintenance to know whether an unexpected load was applied or not by doing daily inspection. In the present study by using center-crack specimens, a simple method of detecting application of overload or underload was investigated by using strain information.

The usual approaches for Fatigue Crack Growth (FCG) assessment are based on semi-empirical models that consider the stress intensity factor range of fracture mechanics, ΔK, as the governing driving force for crack propagation. However, FCG data incorporating compressive loads have brought discussions and controversies. The compressive loads are not taken into account in the usual life prediction codes and the negative portion of the loading cycle is neglected. In the present work, constant amplitude fatigue crack growth tests with load ratios-1 and 0 are performed, for two diferent stess levels, in AA 6005 alloy centre-notched middle tension M(T) specimens and the obtained results are discussed. It is shown that the crack closure concept, that usually work well for positive ratios, is not enough to satisfactorily explain the negative load ratio effects. The results also showed how the negative part of the load affected the propagation rate of the crack, mainly for larger cracks and smaller loads. Finally, it is shown that a previously proposed expression for the stress distribution ahead of the crack can shed some light on this question.

The paper is related to experiments on near-threshold fatigue cracks under shear modes II, III and II+III in bcc metals. Cylindrical bars with circumferential cracked notch were loaded by shear force. In-plane precracks with microtortuous geometry were created by compressive cyclic loading in mode I to measure the effective values of the remote crack driving force. Fatigue cracks in bcc metals loaded under remote shear modes II, III and II+III always grew by creation of local tongues loaded in mode II and their coalescence. Therefore, serrated precrack fronts of a linear roughness identical to those of the real fronts were modeled and the related local stress intensity factors k2 were calculated. Since such FEM calculation for various values of roughness were time consuming, a further task was to identify a lowest number of isolated teeth that produces k2 components identical with those created by the continuously serrated crack front. The results reported in this article reveal that this condition is fulfilled by only two isolated teeth.

The fatigue crack growth threshold ΔKth is an important characteristic of crack growth assessment for the integrity of structural components. However, the accurate threshold ΔKth values for austenitic stainless steels in air environment are lacking in many fitness-for-service (FFS) codes, although fatigue crack growth tests have been performed and many test data had been published. This paper focuses on fatigue crack growth threshold ΔKth values for austentic stainless steel in air environment. The paper introduces the current ΔKth values provided by four major FFS codes and summarizes the available test data based on the literature survey. The paper then discusses the applicability of the existing ΔKth for stainless steels and proposes a new relation as a function of the stress ratio (the R ratio) for use by FFS codes.

In this paper, fatigue crack growth simulation of interacting subsurface cracks using the s-version finite element method (SFEM) is presented. In order to evaluate the accuracy and reliability of the proximity rules published by the ASME, during the fatigue crack growth simulations, the subsurface cracks are approximated to either a single elliptical crack or semi-elliptical surface crack in accordance with the proximity rules. Then, the proximity rules are slightly modified for improving the accuracy and reliability. The results of crack depth evolution calculated by the SFEM with the use of the new proximity rules suggest that the approximation to deep cracks drastically improves the accuracy of the fatigue crack growth evaluation. Thus, the approximation to deep cracks must be a promising approach for having better evaluation of fatigue crack growth of subsurface cracks.

One of the important inputs while estimating the remaining life of critical components is the fatigue property of materials. Fatigue data, in the form of stress vs. cycles to failure (or) strain vs. cycles to failure (or) fatigue crack growth rate data is used to predict the residual life. Material’s fatigue property degrades with time and usage; hence, it is appropriate to use the current properties for remaining life assessment. Often the quantity of material available for generating fatigue data is limited, especially, if the material is scooped out of existing component of a power plant. Further, fatigue response being probabilistic in nature, requires multiple specimens to be tested at any given stress/strain levels. This has prompted us to develop test procedures to determine the fatigue data of materials from a limited volume of material. This paper presents the results of cyclic ball indentation test method as well as cyclic small punch test method that is used to generate the fatigue data at different stress levels. There are several fine details relating to these test techniques – viz., establishing a equivalent damage criteria for failure life with standard LCF/HCF test specimens. The influence of one of the variables, viz., friction at the specimen-tool interface of a small punch test is investigated through numerical simulation and the results are presented here.

Fatigue crack initiation usually starts from defects or inclusion (s) in the material in the zone of stress concentration. The stress concentration zone can be at the surface of the specimen or inside the material. The fatigue crack starts to propagate, depending on stress amplitude, if the range of the stress intensity factor is higher than the fatigue threshold. The aim of this paper is to present the procedure for determining the fatigue crack initial stage by using the Wöhler S-N curve and the fatigue crack propagation parameters obtained by experiment for the loading range R=-1. Determination of the fatigue initial stage has been determined by the analysis of tensile smooth specimens and notched bending specimens. Results show that linear elastic fracture mechanics is applicable in order to establish a model for fatigue crack propagation.

This paper analyses the role of cold drawing in the fatigue and fracture behaviour of pearlitic steels with distinct drawing degree (a hot rolled bar and a commercial prestressing steel wire). Fatigue crack growth develops globally in mode I and locally in mixed mode in both steels, the micro-crack deflection angle depending on the drawing degree. With regard to fracture behaviour, it takes place in mode I in the hot-rolled bar and in mixed mode (with a strong component of mode II) in the case of the cold-drawn wire, so that strength anisotropy appears in the drawn steel and a sort of directional toughness can be defined.

At room temperature, the low cycle fatigue tests for smooth specimens of TC25 titanium alloy under various stress ranges are operated at a CSS280I-20w Electro Hydraulic Servo Universal Testing Machine with a microscopic observation system, and the low cycle fatigue lifetimes are measured. Based upon the analysis of stress-strain hysteresis loop of low cycle fatigue of TC25 titanium alloy, a simplified Manson-Coffin formula is derived according to both the experimental characteristics and the stress-strain constitutive model, the fatigue lifetimes are plotted against stress ranges, and a stress-fatigue life curve for TC25 titanium alloy is obtained by the linear regression analysis method. Finally, the fracture surface morphologies of TC25 specimens are investigated using a JSM-6360 Scanning Electron Microscopy, and the fatigue fracture mechanisms of low cycle fatigue are studied. It shows that the plastic deformations are mainly formed at the accelerated fracture stage, and various shear lips can be observed on the fracture surfaces, which demonstrates that the shear stress results in the final rupture of TC25 titanium alloy. During the fracture of low cycle fatigue, the cleavage nucleation leads to the formation of fatigue crack initiation region, the fatigue crack growth exhibits a mixed transgranular and intergranular crack growth mode, and in the final rupture region, the fracture surface of low cycle fatigue of TC25 titanium alloy appears as a typical semi-brittle fracture mode.

The fatigue crack propagation tests for circular notched compact tension specimens of GH4133B superalloy used in a turbine disk of aero-engine are carried out at room temperature. The test data shows that in the transition region from short to long crack growth, the crack growth rate appears as a fluctuation form with acceleration following deceleration. A quasi harmonic function is constructed to model the fatigue crack growth in the transition region, and the result indicates that such function is suitable to describe the crack growth behavior. A metallographic analysis with respect to microscopic observation for the specimen’s surface suggests that the wave period, that is, a step length of a cycle fluctuation from acceleration to deceleration on the crack growth rate curve is agree well with the intrinsic scale of grain size, which indicates that in the transition region from short to long crack growth, grain boundary plays a dominant role in crack growth rate. Finally, the fracture surfaces of specimen are observed using a scanning electron microscopy. It can be found from the fracture surface morphologies that in the transition region from short to long crack growth, the stress intensity factor range is still lower than the fatigue crack growth threshold ΔKth, the effect of microstructure, such as grain size, grain boundary, secondary phase particle, and inclusion ahead of the crack tip, and the effect of crack closure on crack growth behavior should be considered, the local further plastic deformation at crack tip will meet larger resistance, so some twin bands are observed on the fracture surface. As crack length increases with increasing fatigue cycle, the crack propagation is noted to change from a transgranular crack propagation mode, a mixed transgranular and intergranular crack propagation nature, to intergranular crack propagation manner.

The effect of Friction Stir Welding joining process on the damage tolerance behaviour of friction stir welded aluminium alloys is experimentally investigated. Fatigue crack propagation tests for different aluminium alloys and welding configurations have been performed. The main conclusion arising from the experimental study is that the damage tolerance behaviour of the welded material is in most cases comparable to the base material. Furthermore, the most critical area of the joint was identified to be the weld interface, i.e. the area between the nugget and the thermo-mechanically affected zone

This paper deals with a numerical and experimental investigation on the influence of residual stresses on fatigue crack growth in AA2024-T3 friction stir welded butt joints. An integrated FEM-DBEM procedure for the simulation of crack propagation is proposed and discussed. A numerical FEM model of the welding process of precipitation hardenable AA2024-T3 aluminum alloy is employed to infer the process induced residual stress field. The reliability of the FEM simulations with respect to the induced residual stresses is assessed comparing numerical outcomes with experimental data obtained by means of the contour method. The computed stress field is transferred to a DBEM environment and superimposed to the stress field produced by a remote fatigue traction load applied on a friction stir welded cracked specimen. Numerical results are compared with experimental data showing good agreement and highlighting the predictive capability of the proposed method. Furthermore, the influence of the residual stress distribution on crack growth is evidenced.

This work presents a set of experiments devoted to studying the crack initiation stage under different combined tension-compression and torsion loads. Two different load levels were applied, producing very different fatigue lives. Lower strains generated lives approximately 10 times longer than higher strains. Results allowed retardation and acceleration effects due to microstructure to be clearly visualised.

Mini-samples technique was utilized to determine mechanical properties of technically pure titanium produced by means of selective laser melting (SLM). Full-field digital image correlation (DIC) measurements and inverse method were applied for crack tip position and stress intensity factors calculations in the case of fatigue crack growth rate tests. DIC was also used for strain measurement during tensile tests on sub sized samples. There was studied the influence of samples orientation on the mechanical properties of mini-samples. Samples were cut out from rectangular cubes and were oriented with 0°, 45° or 90° angle to the direction of laser beam travel. There were also tested samples directly produced via SLM. Additionally microstructure observations were performed to verify the quality of SLM processed materials and explain mechanical properties variations.

This paper deals with fatigue crack propagation under 4 bar hydrogen atmosphere in low alloyed steel (3.5Ni-1.5Cr-0.5Mo-V) used for turbine generator of nuclear plant. The tests are conducted in the same way in ambient air and high vacuum on CT specimens and the fatigue crack growth rate specially investigated in the near threshold range is plotted with respect to the applied stress intensity factor. It is shown that the propagation under hydrogen atmosphere is similar to that obtained in air up to a Kmaxvalue of 16,5 MPam1/2with increased growth rate compared to that in high vacuum leading to a threshold value lower that in vacuum, this effect being related to residual water vapor. For Kmaxhigher than 16,5 MPam1/2, much faster growth rates under hydrogen atmosphere becomes are associated to an intergranular propagation mechanism induced by an hydrogen effect. The results are discussed on the basis of available models.

The drill pipe near the surface stands the largest tension and torsion load for the full hole during drilling operation. And fatigue crack growth is always the major cause of failure of drill string. As an example, 5ʺ drill pipe that was near the well head of an ultra-deep straight well and made of 30CrMo, whose constitutional relation was fitted by experiment, was analyzed here. Simplifying the initial crack of the drill pipe as circumferential semi-elliptical surface crack, we simulated the elastic-plastic fracture feature of the drill string with surface crack, partly through-wall crack and fully through-wall crack under combined loading of axial force and torsion. Crack front geometry evolvement is simulated for the different stages of crack propagation. This work would provide a basis for the full-range analysis of fatigue crack growth.

The short fatigue crack growth tests for circular notched compact tension specimens of GH4133B superalloy used in turbine disk of aero-engine are carried out at ambient temperature and atmospheric pressure. The stress intensity factor ranges and the fatigue crack growth rates at various stress ratios are measured, and the corresponding effective stress intensity factor ranges considering the crack closure effect are calculated. It is shown that the effective stress intensity factor range ΔKeff, can be applied to describe the deceleration and acceleration of crack growth rate during the short crack propagation. The fatigue fracture surface morphologies in the short crack growth region are investigated using a scanning electron microscopy. It is found that there is a cleavage step between two adjacent radial striations, a series of early fatigue striations exist on the cleavage step, and some secondary cracks perpendicular to the direction of main crack propagation emerge on the fracture surface, the superalloy exhibits a mixed fracture mode in the short crack growth region, which reveals the microscopic mechanism of short crack propagation that the fatigue crack growth rate is primarily higher, and then gradually decreases with the propagation of short crack.

In this paper the results of experimental tests of the fatigue cracks propagation in Ti-6Al-4V titanium alloy subjected to cyclic bending are presented. The tests were performed on plane specimens with the stress concentrators in form of one-sided sharp notches. The tested specimens were made of the Ti-6Al-4V alloy subjected to oxygenating, and hardening and ageing. In the specimens after heat treatment, crack in the core has quite a different course than in the hardened oxygenated top layers.

In this paper, the possibilities of detection and measurement of fatigue crack length in welded joints made of steel of duplex type, have been presented. In the investigations there has been used an original research system FatigueVIEW employing advanced optoelectronic techniques and numerical methods of image analysis. The proposed approach enables analysis of the fatigue crack growth during fatigue tests and after their finishing in off-line mode.

One of the approach to develop a system of continues, automated monitoring of the health of the structures is to use elastic waves excited in a given medium by piezoelectric transducers network. Elastic waves depending on their source and the geometry of the structure under consideration can propagate over significant distance. They are also sensitive to local structure discontinuities and deformations providing a tool to detect local damage of large aerospace structures. In the paper the issue of fatigue crack growth monitoring by means of elastic guided waves actuated by a sparse array of sensors will be presented. In particular we propose signal characteristics, robust enough to detect different kinds of damages: Barely Visible Impact Damages (BVIDs) in composite materials and fatigue cracks of metallic structures. The model description and the results of specimen tests verifying damage detection capabilities of the proposed signal characteristics are delivered in the paper. Some issues concerning the proposed damage indices and its application to damage detection and its monitoring are also discussed.

Fatigue crack growth (FCG) rates in an EA4T railway axle steel heat treated by two different methods in near threshold and stable regions of growth were evaluated. Quite significant differences were observed, when the obtained results were compared with those published in the literature. Participation of the laboratory in an Exova (GE Aviation) FCG measurement qualification round robin programme with very good results practically excluded errors in the experimental methodology used. Strength of the two different evaluated series of the experimental material was equivalent. Nevertheless, there were substantial differences in fatigue crack growth rates, about 5-times in stable FCG region and even more than 10-times in the near threshold region, when oil quenching and air hardening treatments were compared. The differences were explained by different microstructures of the two groups of materials. Some minor differences between the character of the FCG curve in the threshold region evaluated using SEN(B) and M(T) specimens, published in the literature, are discussed considering crack closure phenomenon.

Aluminum alloys, widely used for constitutive parts of aircrafts are confronted to a wide range of temperature depending on altitude and climate, from room temperature on the ground down to some 223K at high altitude. The fatigue crack growth behavior of two new generation aluminum alloys, 2024A in T351 temper and 2022 in T351 and T851 tempers, have been investigated at both temperatures. It is shown that temperature and air humidity do not affect the crack growth resistance of the peak aged 2022 while these two parameters widely influence the crack growth in the under-aged alloys which exhibit in cold air a crystallographic retarded propagation similar to that in vacuum. The respective role of microstructure, temperature, atmosphere residual humidity and crack closure is discussed on the basis of a preexisting modeling framework for environmentally assisted fatigue.

The microscope mechanism of hydrogen embrittlement (HE) is overviewed from the viewpoint of Mechanics-Microstructure-Environment Interactions. The plastic deformation (Mechanics) at crack tip for low strength steel is controlled by hydrogen concentration (Environment) to crack tip, eventually resulting in very strong time dependent phenomenon in static fracture and fatigue crack growth. Various typical phenomena in low strength steels which can be understood from the viewpoint of Mechanics-Environment Interactions will be presented. Fracture and fatigue of high strength steels (Microstructure) are strongly influenced by hydrogen. Especially, fatigue crack growth is remarkably accelerated by hydrogen-induced deformation twins. The HE phenomemon of the high-strength steels was applied to a newly inclusion rating method. Hydrogen trapped by nonmetalliec inclusions causes the elimination of fatigue limit at very high cycle fatigue. The values of threshold stress intensity factor KTH in hydrogen for small cracks are much smaller than those for long cracks measured by the standard WOL or CT specimens, which are eventually unconservative for the design of hydrogen components. This phenomenon is similar to the small crack problem in fatigue.

The paper contains the results of tests on the fatigue crack growth in titanium Gr.1 and Ti-6Al-4V as well as Ti-3Cu titanium alloys. The chosen materials contained titanium but their structure was different. Plane specimens with the stress concentrators in form of one-sided sharp notches were tested. Influence of the microstructure on the crack paths in plane specimens was observed.

The study is focused on finding and describing microscopic paths of modes II and III cracks in commercially pure α-titanium (hcp structure). Experiments were done on cylindrical bars with a circumferential cracked notch loaded in shear (modes II, III and II + III) and torsion (mode III). Fractography analysis showed that the crack growth was crystallography-controlled and the deflection and twisting angles of crack propagation direction (with respect to the shear plane) were relatively high. This behavior is a mixture of previously observed morphology in ARMCO iron (bcc) and austenitic steel (fcc). It can be understood in terms of a number of available slip systems in bcc, fcc and hcp lattices.

The paper presents the results of fatigue crack growth rate tests performed in magnesium AZ91 alloy cast of different wall thicknesses. Due to the limited size of investigated cast the tests were carried on mini-samples of 4x4x1 mm gauge section dimensions. The samples were cut from the 5, 10 and 30 mm thick walls. The optical displacement measurement technique, namely Digital Image Correlation, and inverse method were applied for determination of the stress intensity factors and the crack tip coordinates during tests. There were also performed uniaxial tensile tests in mini-samples for the determination of the mechanical properties changes related to different cast thicknesses. The relation between the microstructure and the results of mechanical tests was discussed.

The paper deals with the influence of order of cycles in the loading block on the fatigue crack growth rate in railway axle. The railway axle can include some cracks from manufacturing process or initiated fatigue cracks from previous operation. It is advantageous to know how the crack will behave during further service of the train to ensure its safe operation. The most common approaches describing the fatigue crack growth do not take into account the effects of overload cycles, which enlarge the plastic zone ahead of the crack tip. The enlarged plastic zone generates residual compressive stresses, which cause a retardation of the fatigue crack growth. Finite element numerical calculations were used together with the generalized Willenborg model to determine influence of overload cycles on the increment of fatigue crack growing in railway axles. Real geometry of the axle, the crack front shape and typical loading spectrum were taken into account.

This paper presents the results of durability tests of AW7075 aluminium alloy, which is widely used in producing high-durable and particularly reliable constructions in aeronautical and automotive industries. The plain-strain fracture toughness, depending on the orientation of fracture direction with respect to the rolling direction of the material, and the fatigue crack growth rate were determined. Based on scanning electron microscope (SEM) studies the two stages of fatigue fracture are shown: the first is plastic striations and the second is phenomenon of connecting micropores that were formed around intermetallic second phase particles. As a results of research a model of fatigue crack for the second stage is presented.

One of the major issues from a structural integrity point of view of the aircraft structure is an appropriate health monitoring technology delivery for the damage tolerant philosophy. This paper presents a development of a system for fatigue crack growth monitoring and early damage detection in the PZL 130 ORLIK TC II turbo-prop military trainer aft structure. The maintenance system of the aircraft shifts from the safe-life to the hard-time. The aircraft started Full Scale Fatigue Test (FSFT) which will continue up to 2013. In the article a built block approach for the system design, signal modeling, sensing and signal processing as well as damage detection is presented. Taking into the consideration a previous experience of AGH as well as AFIT, a network of PZT transducers was deployed in the aircraft structure hot-spots. The system components are: remote monitoring unit, signal analysis, graphical user interface, sensor self-diagnostic tools, and data classification model. Description of damage detection capabilities are delivered in the paper. In particular some issues concerning the proposed damage indices and its application to crack growth estimation models are discussed. Fishers Linear Discriminant is used as a method to obtain effective crack growth predictors and one of the self-diagnostic tools used in the system. The results of the data collected from specimen fatigue tests are delivered and cross-validation technique is used to evaluate a classification model based on the damage indices derived.

For the components working in high temperature and enduring fatigue loading, the fatigue fracture properties will be reduced remarkably when the working temperature is higher than the critical temperature of creep-fatigue interactionTcof the material. In consequence, the damage mechanism from creep-fatigue interaction becomes more complex. A method is presented in this paper to determine theTcof a nickel-based powder metallurgy superalloy. Pure fatigue crack growth and creep-fatigue crack growth tests were conducted in several different elevated temperatures. The fracture mechanism was investigated via observing the fractographic characteristics using scanning electron microscope (SEM). The test results show that theTcof this superalloy is a little bit lower than a half of the melting temperatureTm.

Fatigue Property of an Extruded AZ61 Magnesium Alloy with the Processing Layer Introduced by Machining was Investigated. Rotating Bending Fatigue Tests were Carried out with the Specimen with and without the Processing Layer. According to Results of the Fatigue Tests, Fatigue Life Significantly Increased by Introducing the Processing Layer to the Specimen Surface. Fatigue Crack Initiation and Propagation Behaviors were Observed by Replication Technique during the Fatigue Test. Fatigue Crack Initiation Life of the Specimen with the Processing Layer was Slightly Longer than that of the Specimen without the Processing Layer. Higher Fatigue Crack Growth Resistance was also Observed when the Fatigue Crack was Growing in the Processing Layer in the Specimen with the Processing Layer. the Longer Fatigue Life Observed in the Fatigue Test in the Specimen with the Processing Layer could be Mainly due to the Higher Crack Growth Resistance. it is Speculated that the Fatigue Strength can be Controlled by Change in Condition of Machining Process. it could be Effective way in Industry to Improved Fatigue Strength only by the Cutting Process without Additional Surface Treatment Process.

Evaluating fracture and fatigue life properties of structural components involves tests that are costly and time consuming. To estimate total life of engineering parts, high cycle fatigue data (S-N) for the material under study is needed. In many cases the S-N data is not available to the analyst and both the time and budget required for testing prevent engineers to meet the deadline imposed on the program. An analytical combined Progressive Damage and Fracture Mechanics based approach is proposed that estimates the S-N data for components that have stress concentrations. The proposed methodology starts from a full engineering tensile stress-strain curve of the material under study and ends up with the estimation of fracture toughness, fatigue crack growth and fatigue S-N curves.

Different experimental methods are being used in laboratories for automatic measurement of fatigue crack growth rates and threshold values. Such data belong to mechanical properties essential for an assessment of residual life of components and structures containing cracks of length more than several millimetres. However, if the material contains small crack-like defects like inclusions or pores, knowledge about local resistance against fatigue growth of physically short cracks becomes very important. Damage mechanisms are even more complicated in case of a presence of subsurface residual stress field, e.g. due to technological effects. Experimental difficulties connected with investigation of short fatigue crack growth (FCG) can be reduced in case of use of automatic indirect methods. The aim of the work described in this paper was to explore possibilities and limits of the use of DCPD (direct current potential drop) method for physically short crack measurement and to use the optimised method for an evaluation of local damage process of initiation and early growth of short fatigue crack in residual stress field induced by shot peening.

Al 7075 alloy is a high strength material usually used for highly stressed components in lightweight structures, typically in aircraft, aerospace and defence applications. It can be applied in different heat treatment conditions, but the T7351 temper state is most widely used because of improved stress-corrosion cracking resistance. An investigation of effects of overloads on fatigue crack growth (FCG) and retardation in Al 7075-T7351 alloy was carried out. FCG rates were measured at load asymmetry R = Fmin / Fmax = 0.1, in quite wide region of growth between 10-8 and 10-5 m/cycle (stress intensity factor range ΔK between 6 and 40 MPa m1/2). Retardation effects of overloads of the magnitudes 2.7-times and 3.0-times of the maximum load in the constant range fatigue loading were significant. Crack mouth opening displacement was evaluated at numerous stages of crack growth including pre-cracking with so called load shedding method. The overloads resulted in substantial crack closure effects, which, however, did not occur immediately after the overloading, but after further fatigue crack extension. Results are discussed considering both theoretically and experimentally estimated plastic zone size and considering crack closure issues recently published in the literature

This letter describes the work conducted at our laboratory for the implementation of an automated vision system for fatigue crack growth measurement. The system relies on a dedicated illumination system with grazing incidence and optimized feature extraction by morphological image processing and continuous calculation of the crack growth, for adjustment of the optimal time interval for image registration.

The paper deals with the effects of variable loading on residual fatigue life of the railway wheelset. The railway wheelsets can include some cracks created during manufacturing process or during previous operation. Therefore, it is important to know how the existing cracks will behave during further service of the train. The experiments show that the fatigue crack growth rate depends not only on size of the load amplitudes in a loading spectrum, but it depends also on the sequence of load amplitudes. Taking into account interaction effects of overloading cycles requires use a method that calculates increments of crack length in each cycle, i.e. cycle-by-cycle. One of such methods represents generalized Willenborg model. This model was used for residual fatigue life time estimations of railway wheelset and results obtained were compared with classical approach, which did not take into account interaction effects. Results obtained can be used for establishing of service intervals of railway wheelsets.

Results of a fairly comprehensive experimental programme aimed at evaluation of fatigue crack growth (FCG) rates in an aircraft Al 2124 alloy in T851 state, after hot rolling, solution annealing, small plastic strain and artificial ageing are presented and discussed. Measurement was performed not only in the region of stable crack growth, but also in the threshold region enabling to estimate threshold values of stress intensity factor range using method of regression analyses. FCG rates were evaluated in both L-T and T-S directions to evaluate sensitivity of FCG resistance on different microstructure orientation. Different load asymmetry conditions R = Fmin / Fmax were used, namely R = 0.1 and 0.6. Repeated measurements in more than one specimen at each condition enabled to evaluate reproducibility, scatter of measurement and to perform eventually probabilistic assessment. The reproducibility of measurement was particularly good for L-T orientation. In this case, threshold values were somewhat higher and FCG rates lower in comparison with the T-S orientation. In the contrary, cyclic fracture toughness in T-S orientation was slightly higher. The results are discussed form the viewpoint of residual fatigue life in aircraft components considering probabilistic aspects.

The paper presents the results of fatigue crack growth rate test of iron sinters. The samples were produced by means of the impact sintering. Applied production method allowed to obtain dense sinters with fine grain size resulted from large shear strains. Due to the limited size of the final products the mechanical tests were carried out in mini-samples. Optical, non-contact method of displacement measurement, namely Digital Image Correlation (DIC), was applied for determination of the displacement fields near the crack tip at the maximal force of selected loading cycles. The results of DIC measurement were utilized in the calculations of stress intensity factors and crack tip coordinates by means of the iterative procedure based on inverse method. These parameters were used for measuring crack development rate. There were investigated two types of materials produced by the consolidation of two different kinds of Fe powders and sintered in different temperatures. The results of crack growth rate tests were correlated with the microstructure changes, as well as yield and ultimate strength of the materials.

This article aims at evaluating the extent of the surface region in notched Middle Cracked Tension specimens. Firstly, a fully automatic fatigue crack growth technique is developed to obtain stable crack shapes. After that, the stress triaxiality along the crack front is evaluated for different notch shapes. Then, objective criteria are defined to quantify the extent of the surface region from the stress triaxiality data collected. Next, the extent of the surface region is related to the elastic stress concentration factor of the uncracked geometry by a linear relationship. Finally, empirical two-constant equations able to evaluate the extent of the surface region from the thickness, notch radius, notch depth and elastic stress concentration factor are formulated.

Two methods for the extraction of Stress Intensity Factors (SIFs) from three-dimensional (3-D)problems are presented: the Contour Integral Method and the Cutoff Function Method. The formula-tions are tailored for the Generalized Finite ElementMethod andmixed-mode 3-D propagating cracks.The case of crack faces loaded by prescribed tractions is also considered. Another contribution of thispaper is a procedure to control the noise of extracted SIFs based on theMoving Least SquaresMethod.The proposed approach provides a continuous and smooth approximation of 3-D SIF functions foreach fracture mode. Numerical experiments demonstrating the accuracy and robustness of the pro-posed methodology are presented. They include 3-D mixed-mode fatigue crack growth simulationsand the case of a pressurized crack.

Fatigue crack growth in metallic plates was monitored using Lamb waves which were generated and captured by surface-mounted piezoelectric wafers in a pitch-catch configuration. Instead of directly pinpointing signal segments to quantify wave scattering caused by the existence of crack damage and related severity, principal component analysis (PCA), as an efficient approach for information compression and classification, was undertaken to distinguish different structural conditions due to fatigue crack growth. For this purpose, a variety of statistical parameters in the time domain as damage indices were extracted from the wave signals. A series of contaminated counterparts with different signal-to-noise ratios were also simulated to increase the statistical size of the data set. It was concluded that PCA is capable of reducing the dimensions of a complex set of original data, whose information can be represented and highlighted by the first few principal components. With the assistance of PCA, the different structural conditions attributable to crack growth can be classified.

This paper analyzes the propagation of fatigue cracks in pearlitic steel in two forms, hot rolled bar and cold drawn wire. The experimental procedure consisted of fatigue tests on bars under tensile loading, using steps with decreasing amplitude of stress and constant stress range during each step. The curves plotting cyclic crack growth rate versus stress intensity factor range show a main steady-state regime preceded by transient paths. The steady-state regime is associated with the curves of the Paris regime. The cold drawing process improves the fatigue behaviour of steel by retarding the cyclic crack growth rate, and the propagation rate is not dependent on theR-ratio. The transient branches allow one to calculate the plastic zone size, considering that they are a consequence of the overload retardation effect at each step change, and a unique expression is fitted as a function ofKmaxΔKproduct and of the conventional mechanical properties.

Experiments have been conducted to investigate the crack growth characteristics of 7050-T7451 aluminium plate in L-S orientation. Two loading conditions are selected, i.e. constant amplitude and constant stress intensity factor range (ΔK). The effects of ΔK-levels and stress ratios (R) on crack splitting are studied. Test data shows that crack splitting could result in the reverse of crack growth rate trend with the increasing R ratio at high ΔK-level. The appearance of crack splitting depends on both ΔK and R.