Fatigue Crack Growth Kinetics Research Articles

Fatigue crack growth kinetics in titanium alloys at different interaction of cyclic loading amplitudes

Fatigue crack growth kinetics of various titanium alloys BT6 (α+β structure) and 3M (α-structure) under constant amplitude loading with different types of cyclic disturbances in the form of over- and underloading at a loading frequency of 25 Hz was investigated.The tests were conducted on a servohydraulic testing machine under controlled load with simultaneous automatic recording of loading parameters and readings from a crack opening gauge. Tests were carried out on compact specimens with an edge crack which were subjected to off-centre loading. The crack opening data was converted into crack length using a yield strength measurement method. Experimental results, namely fatigue crack growth curves and fatigue fracture kinetic diagrams, were analyzed. It is shown that crack growth kinetics depends mainly on the type of alternating loading amplitude sequences. Reliability estimation of crack growth on the basis of crack opening gauge and study of fractographic features of materials fracture has been carried out.

Incorporation of high‐altitude environmental effects in the linear elastic fracture mechanics‐based modeling of aluminum alloy 7075

AbstractAirframe structural components experience coupled load–environment spectra that are currently not captured in traditional damage‐tolerant management approaches. The effect of incorporating fatigue crack growth kinetics relevant to high‐altitude flight environments into LEFM‐based predictions of fatigue progression for aluminum alloy 7075‐T651 was quantitatively assessed for a variety of load–environment spectra using AFGROW. The results of this study demonstrated that in all conditions, the incorporation of high‐altitude‐relevant growth kinetics enhanced the fatigue life predictions. However, the magnitude of the impact on the overall life is dependent upon the subtleties of the coupled load–environment spectrum, despite the fact that environmental effects are most potent in the near‐threshold regime of the da/dN versus ΔK relationship. The potential impact to the structural integrity management community is discussed, and remaining challenges and opportunities are identified.

Temperature dependent fatigue crack growth in electrical steel laminates with μm-thick epoxy layers

The effect of adhesive layer thickness on monotonic roll peel strength and fatigue crack-growth kinetics was investigated for electrical steel laminates with bond line thickness values ranging from 7 to 48 μm. Roll peel strength was increasing with epoxy layer thickness up to 20 μm. In contrast, roll peel strength was leveling at higher thickness values. The fatigue fracture mechanics approach in mode I delivered an equivalent ranking such as roll peel testing. However, energy-based fracture mechanics analysis was much more sensitive than monotonic roll peel testing, also at thicker bond lines. The different failure modes were correlated with the plastic deformation zone size developing at the propagating crack. In case of a constrained plastic zone, cohesive failure mainly within the epoxy layer was ascertained. In contrast, interfacial failure and interestingly also a better fatigue crack growth resistance was deduced for a weakly or non-constrained plastic zone. Regarding the temperature dependency, an enhanced fatigue crack resistance was obtained at 60 °C. At this temperature about 50 °C below the maximum of the loss factor of the epoxy adhesive, the effect of epoxy layer thickness was much smaller than at 23 °C. At testing temperatures slightly below or above the glass transition temperature, the fatigue crack propagation rate was a factor of 10 to 100 higher.

Influence of electron‐beam irradiation on plasticity‐controlled and crack‐growth‐controlled failure in high‐density polyethylene

AbstractIn the present study, the influence of electron‐beam irradiation on plasticity‐controlled and crack‐growth‐controlled failure in high‐density polyethylene (HDPE) is investigated and the effect of both molecular weight distribution (MWD) and short chain branching (SCB) content are taken into account. Size exclusion chromatography (SEC) is used to study the evolution of the MWD of the sol fraction as a function of irradiation dose. Here, it is seen that chains shorter than the percolation threshold (5 kDa) are largely unaffected by electron beam radiation, while the fraction of longest chains (M > 300 kDa) is nearly entirely incorporated into the cross‐linked network. Both yield stress and Young's modulus increased with irradiation dose, where the magnitude of the increase appears to be connected to the gel fraction. The (fatigue) crack growth kinetics of the grades changed relatively little with irradiation dose, which is unexpected. Furthermore, convergence of the crack growth kinetics parameter to a narrow range of values could be observed for the investigated grades at relatively high gel fractions. This would imply that the crack growth kinetics become increasingly independent of the MWD upon irradiation cross‐linking, which could be attributed to a shift in the underlying crack growth mechanism from chain slip to chain scission.

MODELING AND CALCULATION OF FATIGUE CRACK GROWTH LIFE IN STEELS

In this work, we studied the kinetics of fatigue crack growth on compact steel tensile specimens (C (T)-type), in the middle section of crack growth diagram under regular and irregular loading with different asymmetries and maximum load. The crack growth kinetics was obtained by the authors experimentally on modern servo-hydraulic testing machine. Irregular loading was carried out using samples of standard loading spectra characteristic of various technical objects experiencing variable loading during operation. The values of the crack growth rate were obtained. Parameters that evaluate the character of irregular loading and crack closure, namely, irregularity factor and crack closure ratio were suggested. When calculating the effective value of the magnitude of the stress intensity factor (SIF) at the crack mouth, it is proposed to consider in addition to the closure coefficient and cracks also measure irregular loading. The fatigue crack growth life was predicted taking into account its “closure” and the nature of loading according to the approach proposed by the authors and the cyclic calculation method (cycle-by-cycle), all the data obtained are tabulated and distributed according to the type of loading. The results obtained showed good convergence of the calculated and experimental data, which confirms the high values of the correlation.

Влияние антикоррозионных пленкообразующих составов на усталость авиационных конструкций. Обзор исследований

Corrosion of aircraft structures is one of the main factors limiting the duration of their operation, affecting on the economic efficiency and safety of air transport. Corrosion protection is performed at the stage of manufacturing aircraft structures, during their repair and operation. One of the additional measures of anti-corrosion protection is the use of film-forming coatings, which displace moisture from the gaps of structural joints, form a protective film, and also slow down the corrosion process. The protective properties of film-forming coatings are investigated by standard corrosion tests. At the same time, the specifics of aircraft structures and the possible side effects of the use of anti-corrosion compounds require special studies. The article presents an analysis of works in which the effect of film-forming anticorrosive compounds on the fatigue of elements of aircraft structures is considered.The main type of joints in the structure of modern aircraft are riveted joints. The high penetrating ability of film-forming materials ensures their penetration into the gaps of riveted joints, which, in some cases, can affect the frictional forces between the connected elements, which ensure the integrity of the joints. In some experimental investigations, the influence of film-forming compositions on the kinetics of fatigue crack growth is noted as a side effect. A number of possible mechanisms of deceleration and acceleration of the rate of crack propagation are considered. Experimental data concerning the effect of anticorrosive compounds on the fatigue of riveted joints and the process of propagation of fatigue cracks in structural materials, indicate the need to develop a universal methodology for the selection and use of film-forming compositions for the protection of aircraft structures from corrosion, taking into account the minimization or prevention of negative side effects on the service life of aircraft.

Modeling and calculation of the fatigue crack growth life in structural steels

The kinetics of fatigue crack growth has been studied in tensile testing of compact steel tensile specimens (S(T)-type) in the middle section of the kinetic diagram of fatigue fracture (fatigue crack growth diagram) under regular and irregular loading with different asymmetry and maximum load values. The samples were tested on a BISS Nano-25kN servo-hydraulic machine. Standard loading spectra typical for different technical objects exposed to alternating loading during operation were used. The values of the crack growth rate per cycle in the loading block were obtained. Parameters for assessing the character of irregular loading and crack closure, namely, the irregularity factor and crack closure coefficient were proposed. When calculating the effective value of the range of the stress intensity factor (SIF) at the crack mouth, we propose also to take into account the loading irregularity in addition to the closure coefficient. With this approach, the obtained fatigue crack growth diagrams can be grouped into one equivalent curve, which is characteristic of regular loading with R = 0. Moreover, grouping of the fatigue crack growth diagrams provided the use of unified parameters when calculating the crack growth kinetics, regardless of the type and parameters of loading, which rather simplified the crack growth determination. The fatigue crack growth life was predicted taking into account the crack «closure» and the nature of loading according both to the approach developed by the authors and by cyclic calculation method (cycle-by-cycle). All the data obtained are tabulated and classed according to the type of loading. The calculation results and experimental data showed good convergence, which was confirmed by the high values of the correlation coefficient.

Methodical Approach to Studying Growth Kinetics for Short and Long Fatigue Cracks in Irradiated Reactor Materials: Part 1. Problem Definition. The Effect of Initial Notch Acuity on the Fatigue Crack Rate in Small-Sized Samples

The main methodological features of the studies on the kinetics of fatigue crack growth in the course of testing of irradiated small-sized samples are considered. The effect of the initial notch acuity on the long crack growth rate is studied. On the basis of experimental studies, the size of stress concentration zone is estimated depending on the notch type in the small-sized samples. A brief literature review of the main problems in the studies on the growth kinetics for short fatigue cracks is performed. The tasks for further research are formulated.

Application of optical methods in tests of new materials with gradient structure

The main idea of the article is to demonstrate the possibility of applying the laser dynamic speckle interferometry technique to control the processes of fatigue failure of specimens with a gradient structure. A distinctive feature of the described technique is the possibility of practical application on high-frequency machines as an experimental non-contact method for recording surface changes in the studying objects with a complex structure. Besides, the proposed technique can be used as an indicator characterizing irreversible processes during fatigue failure and for analyzing the fatigue crack growth kinetics.

Fatigue Crack Growth Kinetics in a 2024-T3 Aluminum Alloy during Alternating Cyclic Loading

The fatigue crack growth kinetics in a 2024-T3 aluminum alloy during constant- and variable-amplitude loading a various stress ratios in the medium-amplitude segment of a fatigue crack growth rate curve is studied. Variable-amplitude loading is represented by quasi-random samples of standard random loading spectra, which were preliminarily schematized using a rainflow algorithm. Parameters are proposed to estimate the character of random loading. The influence of the character of random loading using various standard spectra on the fatigue crack growth rate is analyzed. The fatigue crack growth time is estimated with allowance for fatigue crack closure and the character of random loading using a proposed formula and a cycle-by-cycle calculation method.

Influence of WC-Co hard metal microstructure on defect density, initiation and propagation kinetics of fatigue cracks starting at intrinsic and artificial defects under a negative stress ratio

The fracture behavior of WC-Co hard metals depends on the size of material inhomogeneities or defects from which cracks emanate. The size and frequency of these defects is of high significance to the quantitative understanding of the failure behavior of structural components and metalworking tools made of hard metal. Currently, the interdependence of a hard metal's microstructure, its defect density, its crack nucleation and growth kinetics and its observed fatigue behavior is not completely understood. The current work provides information on the defect density, fatigue limits, fatigue crack initiation thresholds and growth kinetics at a cyclic mean stress of zero for various hard metal grades as a function of their microstructure. The average WC grain size of the investigated materials varied from ultrafine to medium; the Co binder content ranged from 9 to 15 wt.%. The distribution of size and density of defects was determined by analytic relations based on Weibull theory applied to strength distributions determined in three-point bending experiments performed under monotonously increasing load. Fatigue limits were derived from stress amplitude-life curves determined in a six-point bending arrangement under a stress ratio R = σmin/ σmax = –1. Crack growth kinetics were determined for the same stress ratio for artificially cracks in eight-point bending, as well as for cracks emanating from intrinsic subsurface material defects in six-point bending. The observed differences in the fatigue behavior are discussed based on the determined defect densities and the influence of ambient air / vacuum conditions on crack initiation and growth.

Особливості росту втомних тріщин у псевдопружному NiTi сплаві

There have been analyzed the peculiarities of fatigue crack growth in pseudoelastic NiTi alloy at stress ratios R = 0,2 and 0,5. The fatigue crack growth resistance was determined on cylindrical specimens with a diameter of 8 mm with a unilateral segmental notch to a depth of 0,6 mm in their diametrical cross section at a temperature of 20°C in air. Using the servohydraulic testing machine STM-100, the specimens were preloaded by three-point bending in order to grow a fatigue crack from the notch, and the fatigue crack growth kinetics was determined on a cylindrical specimen with one lateral crack under uniaxial tension at a frequency of 25 Hz. The length of crack on the specimen surface was determined using the binocular microscope with the precision not less than 0,02 mm. To calculate the crack depth b in the deepest point of the crack front, the crack shape b/a of the initial and final cracks was determined using the linear interpolation. The fatigue crack growth rate was determined using this value in the deepest point of the crack front after certain number of loading cycles. It was found out that, unlike the traditional concepts of fracture mechanics, the mechanical driving force of fatigue crack growth, that unambiguously describes the failure kinetics despite of R is not the stress range, but a maximal value of stress intensity factor. According to the microfractographic studies, the main mechanism of crack propagation is connected with the brittle failure of martensite alloy structure elements. Therefore, the strain austenite–martensite transformation at the crack tip is responsible for its kinetics. The intensity of such transformation is determined mostly by the stress levels, and not by its range, that determines the mechanical driving force of fatigue crack growth.

Microstructural Control of Fatigue Behaviour in a Novel α + β Titanium Alloy

The novel titanium alloy TIMETAL® 407 (Ti-407) has been developed as an alternative to Ti-6Al-4V (Ti-6-4), for applications that demand relatively high ductility and energy absorption. Demonstrating a combination of lower strength and greater ductility, the alloy introduces a variety of cost reduction opportunities, including improved machinability. Thermo-mechanical processing and its effects on microstructure and subsequent mechanical performance are characterised, including a detailed assessment of the fatigue and crack propagation properties. Demonstrating relatively strong behaviour under high-cycle fatigue loading, Ti-407 is nevertheless susceptible to time-dependent fatigue effects. Its sensitivity to dwell loading is quantified, and the associated deformation and fracture mechanisms responsible for controlling fatigue life are explored. The intimate relationship between thermo-mechanical processing, micro-texture and fatigue crack initiation through the generation of quasi-cleavage facets is highlighted. Consistent fatigue crack growth kinetics are demonstrated, independent of local microstructure.

Methodical approach for studying kinetics of short and long fatigue cracks growth for irradiated reactor materials. Part 1. Statement of problem. The effect of the initial notch acuity on the fatigue crack rate on small-sized specimens

The paper considers methodical issues in the experimental research of fatigue crack growth kinetics when testing irradiated small-sized specimens. The effect of the initial notch acuity is studied on the long crack growth rate. The stress concentration zone sizes are estimated for notches of various types. A brif literature review of the main problems in the study of the growth kinetics of short fatigue cracks has been performed. The tasks of further research are formulated.

Effect of Temperature on the Tear Fracture and Fatigue Life of Carbon-Black-Filled Rubber.

The mechanical behaviour of carbon-black (CB)-filled rubber is temperature-dependent. It is assumed that temperature affects the fatigue life of rubber products by changing the tear energy of the material. The static tearing behaviour and fatigue crack propagation behavior of CB-filled rubber at different temperatures were investigated in this study. The critical tear energy of the material was measured through static tear fracture tests at different temperatures; it is shown that the critical tear energy decreases exponentially with increasing temperature. A fatigue crack growth test of a constrained precracked planar tension specimen was conducted at room temperature; the measurements verify that the fatigue crack growth follows a Paris–Erdogan power law. Considering the temperature dependence of the critical tear energy, the temperature dependent fatigue crack growth kinetics of CB-filled rubber was established, and the fatigue life of the material at high temperatures was predicted based on the kinetics. The predictions are in good agreement with experimental measurements.

Characterization of irradiation crosslinked polyamides for solar thermal applications – Fatigue properties

In this research, the influence of electronic beam irradiation on the fatigue crack growth (FCG) behavior of unreinforced polyamide 66 (PA 66) and two glass fiber reinforced PA 66 (30 w% glass fiber content, 35 w% glass fiber content) was investigated. In total, five different doses in the range of 0 kGy and 200 kGy were applied. While previous research covered the influence of irradiation on basic thermo-analytical characteristics and thermo-mechanical properties of these PA material grades, the present paper deals with the effect of irradiation crosslinking on the FCG performance. To cover the relevant range of service environmental conditions, the FCG investigations were performed for two different application near material states “as-received” and “water saturated” at 23 °C and 80 °C, respectively. For the water saturated material states, the tests were conducted with specimens under simultaneous water immersion. For this purpose, specific test devices and test arrangements were designed, built and implemented on an electro-dynamic test machine. In general, a change in the material state and test conditions from 23 °C in air with as-received specimens (fatigue testing occurs below the materials glass transition temperature TG) to 80 °C in water with water saturated specimens (fatigue testing occurs above the materials TG) led to a significant reduction in the FCG resistance. Furthermore, while for unreinforced PA 66 no significant effect of irradiation on FCG kinetics was detected for the as-received state at 23 °C (i.e., below TG), irradiation had a detrimental influence on the fatigue behavior at 80 °C for the water saturated material state (i.e., above TG). Interestingly, quite the opposite effect of irradiation on FCG resistance was found for glass fiber reinforced Pas. The irradiation caused a reduction in the FCG resistance for as-received material states at 23 °C, while hardly any effect of irradiation was detected for the water saturated materials at 80 °C.

Self-Similar Patterns of Damage Development and Reliability Assessment of AMg6 and D16T Aluminum Alloys under Consecutive Dynamic and Gigacycle Loading

In the paper, we study the kinetics of fatigue crack growth in MMg6 and D16T aluminum alloys in the gigacycle fatigue mode under dynamic preloading. The relevance of the problem statement is determined by the critical applications—life estimation of materials and structural elements of aircraft gas turbine engines experiencing random dynamic effects under flight cycle conditions. Specimens were preloaded by dynamic tension using the split Hopkinson (Kolsky) pressure bar at strain rates up to ~103 s−1, with consecutive gigacycle loading on the Shimadzu USF-2000 ultrasonic testing machine. Quantitative fractography of fracture surfaces was performed using profilometry and scanning electron microscopy data. We propose an original form of the kinetic equation, which relates the fatigue crack growth rate to a change in the stress intensity factor. The scale invariance of defect structures responsible for the formation of the fracture surface relief under gigacycle fatigue loading is found to be related to the power exponent of the Paris law. The fracture surface morphology of an aluminum-magnesium alloy under consecutive dynamic and gigacycle loading is studied by the multifractal detrended fluctuation analysis method. It is found that a transition from the stage of formation of a fish-eye zone of localized deformation is caused by the generation of fracture sites and accompanied by a qualitative change of the nonlinear dynamics of the system—a transition from monofractal to multifractal dynamics characterized by broadening of a multifractal spectrum at the final crack growth stage, which leads to macrofracture.

Mixed mode (I + II, I + III) fatigue crack growth rate description in P355NL1 and 18G2A steel using new energy parameter based on J-integral approach

In this paper, the problem of fatigue crack growth description under mixed mode loading condition is discussed. A new ΔS* crack driving force is proposed for mixed-mode loading conditions. For experimental verification, two types of low-carbon steels were used (P355NL1 and 18G2A /EU S355/). Mixed mode (I + II) fatigue crack growth rate experiment performed for R = 0.05 and R = 0.5 confirmed the usefulness of ΔS* parameter as a crack driving force. The similar effect was observed in the case of 18G2A (S355) steel and mixed mode (I + III) experiment for R = 0, −0.5, −1. On the contrary to the force factor ΔK or energy parameter ΔJ, the ΔS* approach allows to describe the kinetics of fatigue crack growth independently from stress ratio R. Based on experimental data it is worth to note that the usefulness of mentioned parameter can be reflected in further mathematical models of fatigue crack growth for both constant and variable amplitude loading conditions.

Fatigue Crack Growth Rate of the Long Term Operated Puddle Iron from the Eiffel Bridge

The paper summarises an experimental study on the fatigue crack propagation and cracks paths in ancient steel—19th-century puddle iron from the Eiffel bridge. The tests were performed with the load R-ratio equal to 0.05 and 0.5. All tests were performed under different notch inclinations (mode I + II). The fatigue crack growth rate in the tested material is significantly higher than its “modern” equivalent—low carbon mild steel. The crack closure phenomenon occurs in specimens during the process of crack growth. Understanding this aspect is crucial for the examination of a stress R-ratio influence on kinetic fatigue fracture diagram (KFFD) description. Both the experimental and numerical approach, using the HP VEE environment, has been applied to the crack closure as well as the crack opening forces’ estimation. These analyses are based on the deformation of the hysteresis loop. The algorithm that was implemented in the numerical environment is promising when it comes to describing the kinetics of fatigue crack growth (taking into consideration the crack closure effect) in old metallic materials.

Scale-invariant patterns of damage development in AMg6 alloy under consecutive dynamic and gigacycle loading

A study of fatigue crack growth kinetics in aluminum alloy AMg-6 in gigacycle fatigue mode at preliminary dynamic deformation has been carried out. The relevance of the statement is determined by the critical applications of estimating materials and structural elements resource of aircraft gas turbine engines in terms of flight cycle with random dynamic effects. Preliminary loading of samples was carried out by the dynamic tension on Hopkinson-Kolsky rod split at strain rates up to ~103 s-1 subsequent gigacycle loading on the ultrasonic testing machine Shimadzu USF-2000 quantitative analysis and fractography of fractures based on the data from profilometry and scanning electron microscopy. The authors propose an original form of kinetic equation which relates the rate of the fatigue crack growth with changes in the stress intensity factor. The scale invariance of defect structures responsible for the formation of the fracture surface relief under gigacycle fatigue loading was found out to be related to the power exponent of the Paris law.