Role of microstructure on initiation and propagation of fatigue cracks in precipitate strengthened Cu–Ni–Si alloy

Fatigue tests were conducted on round-bar specimens to understand the fatigue behavior of precipitate-strengthened Cu–6Ni–1.5Si alloy. Aging at 500 °C for 0.5 h produced δ-Ni₂Si precipitates in the matrix, homogeneously and heterogeneously precipitated δ-Ni₂Si particles, and a precipitate-free zone around the grain boundaries. The cracks were initiated at the grain boundaries, followed by growth along the crystallographic slip planes in the adjacent grains. Crack propagation from the crack origin along the grain boundaries was occasionally observed. The physical background of fatigue damage is discussed in light of the role of microstructure on the behavior of fatigue cracks.     

Experimental evidence of a common local mode II growth mechanism of fatigue cracks loaded in modes II,III and II + III in niobium and titanium

The paper is focused on an identification of the local mode II mechanism of fatigue cracks loaded under the remote mode III and the mixed mode II + III and presents a convincing experimental evidence of such a mechanism in materials with a nearly coplanar crack growth. Closure-free data were obtained by applying fatigue experiments in modes II, III and II + III in commercially pure titanium and niobium. The results revealed that the micromechanism of propagation of all kinds of shear-mode cracks can be described by a common model of advances of local mode II crack segments nearly in the direction of applied shear stress. These segments nucleated at spatial geometrical irregularities of the precrack front generating fibrous patterns at fracture surfaces.     

Fatigue propagation of short and long cracks in gaseous hydrogen environment in 3.5NiCrMoV steel

Turbo generators for nuclear plants are mostly equipped with hydrogen cooling systems. Current practice of characterizing the growth of fatigue cracks on the basis of fracture mechanics primarily relies on fatigue tests for long cracks which are typically of several millimeters in length. However, in view of extended life for the plants, the damage tolerance evaluation of such fatigue-critical engineering components requires understanding of the propagation of cracks of significantly smaller dimensions. Then the near threshold of short cracks is investigated and compared to the behavior of long crack by experiments under 4 bar hydrogen atmosphere. The short crack fatigue propagation in hydrogen atmosphere is shown similar to that in air, growing faster than the long crack and at ΔK ranging below the long crack threshold; this effect is related to a reduced crack closure shielding. The propagation behavior of long crack under hydrogen atmosphere is shown similar to that obtained in air in the low rate range, i.e. when the maximum of the stress intensity factor K_max is lower than a critical level of about 16 MPa m^1/2 with higher crack growth rate than in high vacuum. This environment effect is related to the presence of residual water vapor in both gases. For higher K_max, much faster growth rates under hydrogen atmosphere in comparison to air and vacuum are observed and related to hydrogen assisted intergranular propagation combining fatigue and sustained loading damage. The results are discussed on the basis of micrographic observations supporting the involved mechanisms.     

Investigating environmental effects on small fatigue crack growth in Ti–6242S using combined ultrasonic fatigue and scanning electron microscopy

In situ ultrasonic fatigue with a cyclic frequency of 20 kHz was employed in an environmental scanning electron microscope (ESEM) to characterize fatigue crack formation and growth in the near alpha titanium alloy Ti–6242S. The role of environment on small fatigue crack initiation and growth was investigated in vacuum and in variable pressures of saturated water vapor, as well as in laboratory air. Small crack growth behavior from cracks initiated at FIB-produced micro-notches indicated a significant environmental dependence, with fatigue crack growth rates increasing with increasing partial pressures of water vapor. Environment also influenced crack initiation lifetime in that cracks initiated earlier in laboratory air than in vacuum or saturated water vapor environments. Transgranular, crystallographic crack growth was observed in each environment, with the crack path in primary α grains producing facets parallel to basal planes when crack size was small. Small crack growth resistance had a marked sensitivity to microstructural features, such as α/α grain boundaries with high misorientation and α/α + β boundaries. These initial investigations demonstrate the usefulness of in situ ultrasonic fatigue instrumentation (UF-SEM) as a new tool for the characterization of environmental and microstructural influences on very high cycle fatigue (VHCF) behavior.     

Short cracks initiated in Al 6013-T6 with the focused ion beam (FIB)-technology

The fatigue crack growth behaviour of short corner cracks in the Aluminium alloy Al 6013-T6 was investigated. The aim was to determine the crack growth rates of small corner cracks at a stress ratio of R = 0.1, R = 0.7 and R = 0.8 and to find a possible way to predict these crack growth rates from fatigue crack growth curves determined for long cracks. Corner cracks were introduced into short crack specimens, similar to M(T) – specimens, at one side of a hole (Ø = 4.8 mm) by cyclic compression (R = 20). The precracks were smaller than 100 μm (notch + precrack). A completely new method was used to cut very small notches (10–50 μm) into the specimens with a focussed ion beam. The results of the fatigue crack growth tests with short corner cracks were compared with the long fatigue crack growth test data. The short cracks grew at ΔK-values below the threshold for long cracks at the same stress ratio. They also grew faster than long cracks at the same ΔK-values and the same stress ratios. A model was created on the basis of constant K_max-tests with long cracks that gives a good and conservative estimation of the short crack growth rates.     

The effects of pre- and post-weld heat treatment variables on the strain-age cracking in welded Rene 41 components

Rene 41, a precipitation-hardened nickel-base superalloy, is mostly used for hot section parts of jet aircraft engines. When cracks occur in these parts, welding is frequently utilized for crack repair to extend their service life. Rene 41 is very susceptible to strain-age cracking during welding and post-weld heat treatment because of its strengthening mechanism. The strengthening mechanism depends on the existence of gamma prime (γ′) phase and the morphology of carbides both of which are affected by heat treatment. In this study, the effects of pre- and post-weld heat treatment variables such as heating rate, holding temperature and time, and cooling rate on the strain-age cracking of Rene 41 during welding and post-weld heat treatment were investigated. An optimum combination of heat treatment variables was investigated to obtain a microstructure that would be less susceptible to strain-age cracking during welding and post-weld heat treatment. Pre- and post-weld solution heat treatments at 1080 °C for 30 min with 20 °C/min heating and 34 °C/min cooling rates produced optimum microstructures, and no cracks were observed in the Rene 41 samples after welding. MC carbides were dissolved lightly and distributed homogenously within the grains. M₆C carbides precipitated as globules within the grains and in the grain boundaries. M₂₃C₆ carbides precipitated as chains of globules in the grain boundaries.     

Numerical simulation of fatigue crack propagation in friction stir welded joint made of Al 2024-T351 alloy

In this work, fatigue crack propagation in thin-walled aluminium alloy structure with two friction stir welded T joints has been simulated numerically. Crack propagation in stiffened part of the structure between two friction stir welded T joints is analysed by using the eXtended Finite Element Method (XFEM), including software ABAQUS, as well as MORFEO, for modelling and results display. Tensile fatigue loading is applied, with stress ratio R = 0, and maximum stress σ_max = 10 MPa. Material properties (Al 2024-T351, as used in aeronautical industry) in different welded joints zones are adopted from available literature data. Following results are obtained by numerical analysis: stress–strain and displacement state in the structure, position of the crack tip and value of stress intensity factor for every crack propagation step, as well as the structural life estimation, i.e. number of load cycles, N, also for each crack propagation step. Using these results the number of cycles at which the crack starts to propagate in an unstable manner is predicted.     

The effect of braking energy on the fatigue crack propagation in railway brake discs

Thermal fatigue cracks can often be found on the friction surface of brake discs used in railway vehicles after a period of usage and include crackle, radial and circumferential patterns. These cracks typically exhibit different initiation and propagation behavior under different braking conditions. In this paper, the effect of braking energy on fatigue crack evolution is analyzed by using experimental testing and numerical simulations. Macro observations show that a significant number of radial cracks appear on the surface of brake discs which operate at 300 km/h, while crackles typically appear after repeated emergency braking (EB) at 200 km/h. No crack growth was observed on disc surfaces after routine braking. The cyclic load that leads to the fatigue crack propagation consists of compressive stress during braking and residual tensile stress after cooling. Simulation results show that the depth of cracks correlates well with the residual tensile stress distribution in brake discs. Breaking tests exposed that the fracture surface of fatigue cracks which were covered by oxides shows nearly elliptic-type. Higher braking energy leads to a hardened layer on the friction surface and oxide generation near the crack edges, which are also important factors that contribute to accelerating crack propagation.     

Study of the fatigue failure of an anti-return valve of a high pressure machine

This paper presents a study of the fatigue failure of an anti-return valve, designed to work in the high pressure system (500 MPa) of a high pressure processing machine. To do this, the crack propagation has been simulated by means of the linear-elastic fracture mechanics approach under mixed-mode loading conditions. From an initial crack, which size is related with the microstructure and superficial finish, the crack growth has been simulated using the stress intensity factors K_I and K_II of the cracked valve axisymmetric geometry. The crack propagation path has been obtained step by step, applying the criterion of the maximum circumferential stress at the crack tip. The experimental and simulated crack propagation paths have been compared and, as a consequence of the reliable results obtained, the fatigue life of the valve has been calculated using the Paris law of the material with an effective stress intensity factor K_eff. The good agreement with experimental fatigue life allows to perform new improved designs using the methodology presented.     

Acoustic emission study of fatigue crack closure of physical short and long cracks for aluminum alloy LY12CZ

Crack closure of physical short and long cracks of LY12CZ aluminum alloy during fatigue process was investigated using acoustic emission (AE) technique. Results showed that the effective fatigue crack growth curve (da/dN vs. ΔK_eff) of physical short and long cracks obtained by the AE technique was consistent with the effective fatigue crack growth curve at high stress ratio (R = 0.8), which implied that the AE technique could measure the crack closure level, especially for physical short crack. The growth rate of physical short crack was much higher than that of long crack at the same ΔK, and the lower crack closure level of short crack was the main reason.     

Very high cycle fatigue properties of bainitic high carbon–chromium steel

Fatigue properties of bainitic 100Cr6 (SAE 52100, JIS SUJ2) steel are investigated in the high cycle and very high cycle fatigue (VHCF) regime. Fully reversed tension–compression fatigue tests are performed with ultrasonic fatigue testing equipment. Specimens are grinded which leads to surface compression stresses and increased surface roughness. About 1/3 of the specimens failed after crack initiation at interior Al₂O₃? or TiN-inclusions and 2/3 failed after surface crack initiation at scratches or cavities. When inclusions are considered as cracks, failures can occur at minimum stress intensity range of 2.8 MPa m^1/2, and maximum stress intensity range without failure is 3.3 MPa m^1/2. Facets are visible close to the inclusion in some specimens, and the stress intensity range at the border of the facet is approximately 4.5 MPa m^1/2. Murakami’s model can well predict the endurance limit at 10⁹ cycles for internal failures considering the area of the inclusion in the evaluation. Surface fatigue crack initiation can lead to failure above 10⁸ cycles. When scratches are considered as cracks, minimum stress intensity range of 2.5 MPa m^1/2 can propagate surface cracks to failure. Fracture mechanics approach showed several similarities to literature results of the same material tested in tempered martensite condition.     

Experimental study on the fatigue failure mechanism of QP-16 type ball-eye under asymmetric load

The ball eye (BE) is a key connecting component between the insulator and transmission tower, whose fatigue characteristics concern the safety of transmission lines. To understand the fatigue mechanism and characteristics of it, the fatigue test was conducted based on the following data: r = 0.25, S = 500 MPa,then plotting of SN and Δε_axis − N, to analyze the fatigue failure of the test specimen from the macro and micro point of views. The research results show that: the life of BE significantly reduces with the increase of the stress amplitude, but the relative reduction in life is not the same; softening and strain amplitude of the specimen change differently before and after the stress amplitude of 300 MPa; when S ≤ 300 MPa, the fracture is more smooth, the fatigue crack propagation is slow; when S > 300 MPa, the rate of fatigue crack growth is faster, and the fatigue crack growth zones are not obvious. The cracks are easily detectable appear at the joint of the BE and insulator cap, and the cracks along the fracture cross section are constantly expanding, showing multiple fatigue sources and fatigue steps. The number of fatigue steps increases as the magnitude of the tensile stress increases. When S = 500 MPa, the yield strength decreases during the lifetime, the decrease rate of the tensile strength and microstructure strength in each stage are different. Axial lengthening and section shrinkage ratio decrease with the development of fatigue, fatigue evolution process is accompanied by phenomenon of crystalline slip, deformation, dislocation, at the same time, dissipation and decomposition of pearlite occur, and carbide precipitates from the matrix, growing and moving to the grain boundaries, the specific phenomenon of grain growth appears.     

The investigation of crack growth in specimens with rectangular cross-sections under out-of-phase bending and torsional loading

The paper presents the fatigue test results of rectangular cross-section specimens made of 10HNAP (S355J2G1W) steel. The specimen height to width ratio was 1.5. The tests under bending with torsion were performed for the following ratios of bending to torsional moments M_aB/M_aT = 0.47, 0.94, 1.87 and the loading frequency 26.5 Hz. Nominal stresses were chosen for the equivalent stress according to the Huber-Mises hypothesis equal to 360 MPa. The tests were performed in the high cycle fatigue regime for the stress ratio R = −1 and phase shift between bending and torsion loading equal to ϕ = 0 and 90°. Crack initiation and propagation phases were observed on the specimen surface using the optical microscope (magnification 20×) with an integrated digital camera. The test results for the fatigue crack growth rate versus the stress intensity factor range for mode I and mode III have been described with the Paris equation.     

Fatigue design of wire-wound pressure vessels using ASME-API 579 procedure

The wire winding of high pressure vessels is a technique usually applied to introduce initial compressive stresses in the inner core of the vessel, with the aim to improve the fatigue life under cyclic pressure conditions. In this work, the procedure followed to calculate the number of design cycles is presented, using the fracture mechanics approach and the structural integrity concepts. In particular, the API 579-1/ASME FFS-1 procedure has been used to analyse the structural integrity of the vessel through the crack propagation stage. Starting from a postulated internal semi-elliptical crack the number of design cycles is determined, the flaw aspect ratio is updated and the structural integrity of the cracked vessel is evaluated using the Failure Assessment Diagram (FAD). Different propagation laws, which take into account for negative stress intensity ratio factors R = K_min/K_max < 0, are reviewed, because of their high influence on the fatigue life of wire-wound vessels. In addition, this paper presents a number of useful expressions to calculate the stress intensity factor (SIF) for internal semi-elliptical cracks in wire-wound pressure vessels, in order to carry out the numerical integration of the number of cycles, updating the flaw aspect ratio, during the fatigue crack growth.     

Very high cycle fatigue of nitrided 18Ni maraging steel sheet

Thin sheets of nitrided 18Ni maraging steel are tested under cyclic tension (load ratio R = 0.1) in the very high cycle fatigue (VHCF) regime. The ultrasonic fatigue testing method with a cycling frequency of about 20 kHz has been further developed for these experiments. Sheet specimens with 0.35 mm thickness are mounted on a carrier specimen, they are pre-stressed and are forced to vibrate jointly. Between 10⁷ and 10⁹ cycles, fatigue cracks are initiated exclusively at internal TiN inclusions. The areas of the crack initiating inclusions projected perpendicular to the applied tensile stress are evaluated. The square root of inclusion areas, (area_INC)^1/2 lies between 2.5 μm and 5.3 μm. Considering inclusions as cracks, their stress intensity range is between ΔK_INC = 1.3 MPa m^1/2 and 2.4 MPa m^1/2. The sizes of crack initiating inclusions influence fatigue lifetimes. This is considered in a crack propagation model and by presenting lifetimes versus the stress amplitudes multiplied by (area_INC)^1/12. A mean lifetime of 10⁹ cycles is found at a stress amplitude of 22% of the tensile strength, which is comparable to other high strength steels tested under cyclic tension.     

Fatigue behavior of surface cracked filament wound pipes with high tangential strength in corrosive environment

The aim of this study is to examine the corrosion fatigue behavior of filament wound composite pipes with a surface crack under alternating internal pressure. The filament wound pipes are composed of multi-layered E-glass/epoxy composites with a [±75°]₃ lay-up. The surface notches were formed on the outer surface of the pipe along the pipe axis. Dilute (0.6 M) HCl acid was applied to the surface crack region by a corrosion cell mounted on the outer surface of the pipe. The results of an experimental investigation into the corrosion fatigue tests are conducted to observe the oil leakage failure and the crack propagation of the composite pipe subjected internal pressure loading with an open ended condition in which the pipe can be deformed freely in the axial direction. The internal pressure was generated by conventional hydraulic oil for fatigue loading. The fatigue tests are performed at 0.42 Hz frequency and a stress ratio of R = 0.05 in accordance with ASTM D-2992 standard. The oil leakage from the crack tip was observed after the crack propagation reached to the critical stress intensity level. The fatigue crack propagation behavior with the environment exposure was strongly dependent on the crack parameters such as crack-depth ratio and crack-aspect ratio. The micro structure of the fracture surface with the effect of environment and the fatigue loading were also observed.     

On the mechanism of squat formation on train rails – Part II: Growth

Longitudinal cross-sectioning of squats reveals characteristic features of internal crack front propagation. Leading crack planes propagate over longer lengths and greater depths as compared to more superficial trailing crack planes. A favourite depth of crack propagation occurs in the subsurface (2–3 mm), is related to the residual longitudinal stress profile, and may lead to an internal crack ‘terrace’. Especially during deeper crack propagation and branching oxidation processes are found to be metallurgical drivers of crack growth. Contact surface modification during squat growth can be distinguished between phases of transient local stress redistribution and of dynamic wheel–rail contact. If the hypothesized shearing wedge in the failure mechanism loses its load bearing capacity, this gives rise to a redistribution of normal stresses within the actual contact ellipse and the formation of a hardness envelope along the crack pattern. This may partially explain why maturing squats show decoloured and hardened surface areas bordering the surface-breaking cracks. A second effect occurs for contact patches not matching the failure ‘envelope’: due to the Poisson effect the surface overlying the crack planes settles slightly, experiences reduced contact, and corrosive products, ‘pumped’ from inside the cracks, may accumulate on the surface (as confirmed by SEM-EDX analysis). During progressive growth of the defect the harder and decoloured envelope as well as the original wedge is pressed into the deeper elastic material, accompanied by a gradual expansion of the contact band and a bilateral bridging of the defect. This may cause high-frequency impact, resulting into progressive internal crack growth affecting the global stress response and rail fracture.     

Oxidation and fatigue crack propagation in the range of low stress intensity factor in relation to the microstructure in P122 Cr–Mo steel

Fatigue strength and life of weldment at high temperatures are important for the materials in power plants. The fatigue crack growth rate is accelerated by oxidation. Similarly, the high-temperature fatigue life is influenced by oxidation. The base metal, the weld metal and the heat-affected zone (HAZ) of the P122 (Cr–Mo steel) weldment were oxidized between 600 °C and 700 °C for up to 500 h in air, and their oxidation behavior was examined. The oxidation resistance increased in the order of HAZ, base metal and weld metal. The scales were mainly Fe₂O₃. Fatigue tests were performed to measure the fatigue crack growth rate in the range of low stress intensity factor, and the results are discussed from the viewpoint of different microstructures and oxidation.     

Crack growth rate under cyclic bending in the explosively welded steel/titanium bimetals

The paper presents the results of the tests on fatigue crack growth in a steel/titanium composite under oscillatory bending. Two kinds of specimens of rectangular cross sections were tested. In the tested specimens, the ratio of heights of basic and overlaid materials was h₁:h₂ = 2.5:1 and 1:1. In the specimens, the fatigue crack growth was parallel to the applied loading and its direction changed at the interface line. Next, the crack growth along the interface line or the crack growth passing through the interface line were observed. When the crack growth passed along the interface line, decrease of the crack growth rate took place. The specimens have the uniform crack growth at both sides of lateral surfaces. At the composite fractures in the steel and titanium, transcrystalline cracks are dominating.     

Role of microstructural condition on fatigue damage development of AISI 316L at 20 and 300 °C

At 300 °C, when dynamic strain ageing takes place, the fatigue life of AISI 316L for lower strain amplitudes is lower than under equivalent conditions at 20 °C. Exhaustive examination of the changes in: (1) apparent elastic modulus, (2) microstructural condition, and (3) fractographic features has been performed to reveal the reason for the life reduction. The analysis of apparent elastic modulus variations and the results of fractographic observations show that the propagation rates for fatigue cracks at 20 °C are faster than for 300 °C. Crack initiation however occurs earlier at 300 °C, in particular for lower strain amplitude tests, due to the activity of localised deformation bands as a consequence of cyclic loading. In addition to persistent slip bands, a form of ladder-free deformation bands is also present at 300 °C, in particular at low strain amplitudes. When the fatigue life is rather short, the influence of the ladder-free deformation bands on cyclic endurance is negligible. The ladder-free type of localised bands have a strong influence on crack initiation once the material endurance increases with lowering strain amplitude, leading to the relative life reduction at the elevated temperature. In addition, the incidence of secondary cyclic hardening for lower strain amplitude tests at 300 °C partly contributes to the more evident life reduction. The influence of dislocation walls on the propagation of microstructurally short fatigue cracks is also examined.