On the mechanism of fatigue failure in the superlong life regime (N>107 cycles). Part II: influence of hydrogen trapped by inclusions

High cycle fatigue fracture surfaces of specimens in which failure was initiated at a subsurface inclusion were investigated by atomic force microscopy and by scanning electron microscopy. The surface roughness R _a increased with radial distance from the fracture origin (inclusion) under constant amplitude tension–compression fatigue, and the approximate relationship: R _a ≅ C Δ K ²_I holds. At the border of a fish-eye there is a stretched zone. Dimple patterns and intergranular fracture morphologies are present outside the border of the fish-eye. The height of the stretch zone is approximately a constant value around the periphery of the fish-eye. If we assume that a fatigue crack grows cycle-by-cycle from the edge of the optically dark area (ODA) outside the inclusion at the fracture origin to the border of the fish-eye, we can correlate the crack growth rate d a/ d N , stress intensity factor range Δ K _I and R _a for SCM435 steel by the equation
   
and by d a/ d N proportional to the parameter R _a .
Integrating the crack growth rate equation, the crack propagation period N _p2 consumed from the edge of the ODA to the border of the fish-eye can be estimated for the specimens which failed at N _f > 10⁷. Values of N _p2 were estimated to be ∼1.0 × 10⁶ for the specimens which failed at N _f ≅ 5 × 10⁸. It follows that the fatigue life in the regime of N _f >10⁷ is mostly spent in crack initiation and discrete crack growth inside the ODA.     

The influence of modified intermetallics and Si particles on fatigue crack paths in a cast A356 Al alloy

Mechanical fatigue tests were conducted on uniaxial specimens machined from a cast A356-T6 aluminium alloy plate at total strain amplitudes ranging from 0.1 to 0.8% ( R = − 1). The cast alloy contains strontium-modified silicon particles (vol. fract. ~6%) within an Al–Si eutectic, dispersed α intermetallic particles, Al₁₅ (Fe,Mn)₃ Si₂ (vol. fract. ~1%), and an extremely low overall volume fraction of porosity (0.01%). During the initial stages of the fatigue process, we observed that a small semicircular fatigue crack propagated almost exclusively through the Al–1% Si dendrite cells. The small crack avoided the modified silicon particles in the Al–Si eutectic and only propagated along the α intermetallics if they were directly in line with the crack plane. These growth characteristics were observed up to a maximum stress intensity factor of ~ K trmax = 7.0 MPa m^1/2 (maximum plastic zone size of 96 μm). When the fatigue crack propagated with a maximum crack tip driving force above 7.0 MPa m^1/2 the larger fatigue crack tip process zone fractured an increased number of silicon particles and α intermetallics ahead of the crack tip, and the crack subsequently propagated preferentially through the damaged regions. As the crack tip driving force further increased, the area fraction of damaged α intermetallics and silicon particles on the fatigue fracture surfaces also increased. The final stage of failure (fast fracture) was observed to occur almost exclusively through the Al–Si eutectic regions and the α intermetallics.     

再结晶对DD6单晶高温合金轴向高周疲劳性能的影响

为了研究再结晶对二代单晶高温合金DD6高周疲劳性能的影响,对标准热处理的DD6合金进行表面吹砂处理,然后分别在1120℃和1315℃保温4h,以获得不同类型的再结晶组织。在疲劳试验机上分别测试了光滑和含再结晶的DD6合金试样在1070℃的轴向高周疲劳寿命。采用SEM观察DD6合金再结晶组织及疲劳断口。结果表明:胞状再结晶和等轴再结晶降低了DD6合金的轴向高周疲劳性能,胞状再结晶作用小于等轴再结晶;含再结晶的DD6合金试样的轴向高周疲劳断裂机制为类解理断裂和枝晶间的局部韧窝断裂共存的混合断裂;再结晶使DD6合金试样变为多源疲劳断裂。高温条件下,再结晶晶界的存在加快合金试样的氧化损伤,显著缩短早期疲劳裂纹的萌生和扩展时间,降低合金的轴向高周疲劳性能。     

Stability analysis and experimental verification for optimum notch configuration in chevron-notched round bar specimens

In some earlier communications [Ray and Poddar, FFEMS, 27 (2004), Sarkar and Ray, FFEMS, 31 (2008)], a methodology to estimate the minimum normalized stress intensity factor for chevron-notched round bar (CVNRB) specimens has been outlined. The primary aim of this report is to theoretically analyse stable crack propagation in CVNRB specimens in order to estimate conservative fracture toughness value associated with this specimen geometry. The theoretical analyses have been substantiated using fracture toughness tests on CVNRB specimens of steel having initial normalized notch lengths (α₀) between 0.2 and 0.5. The major inferences drawn from this investigation are: (1) the optimum notch geometry in CVNRB specimens corresponds to 0.2 < α₀ < 0.3 for the maximum stable crack extension, and (2) the estimated fracture toughness of the steel using CVNRB specimens indicates minimum K_ICV at α₀= 0.29 in good agreement with the theoretical analyses.     

DYNAMIC FATIGUE AND COMPUTER SIMULATION OF THE BEHAVIOUR OF Y2O3-ZrO2 CERAMICS

Abstract— There is a critical stress rate K _c for each of the three ZrO₂ ceramics tested. Fracture is controlled by the materials polycrystal fracture toughness, K ^c_p, if the stress rate is less than K _c. Otherwise, fracture is controlled by the single crystal fracture toughness, K ^c_s. The crack growth parameters determined by dynamic fatigue experiments can only represent macrocrack growth behaviour although the fracture of specimens in experiments originates from small surface flaws.     

Mechanical properties and microstructure of Al/Al laminated structure produced via ultrasonic consolidation process

Mechanical properties of a 2024 aluminium alloy laminated structure produced by the ultrasonic consolidation were investigated. In comparison with the monolithic aluminium alloy, the existence of laminated structure gave different fatigue and fracture mechanisms that associated with the layer interfaces. The Al/Al laminated specimens had the lower tensile strength but much higher fracture toughness than the monolithic Al specimens due to the exit of interface delaminations around the crack tip. The fatigue life of the laminated specimens was comparable to that of the monolithic Al specimens, though the initiation and propagation of the crack in the laminated specimens depended strongly on the microstructure of each material. The interface between layers could arrest the fatigue crack and impede the further propagation.     

The influence of the environment and corrosion on the structural integrity of aircraft materials

Fatigue results of 1¹/₂ dog-bone jointed specimens manufactured from 7075-T6 aluminium alloy indicated that the application of corrosion preventative compounds (CPCs) at the faying surfaces slightly decreased the log mean fatigue life at 144 MPa, while the effect was not statistically significant at the higher stress level (210 MPa). The addition of the CPC also reduced fretting corrosion at the faying surfaces and shifted the fatigue initiation sites closer to the edge of fastener holes. Scatter in fatigue life was found to be associated with the location and size of the intermetallics at the initiation site. The presence of corrosion in the bores of the countersunk fastener holes reduced the fatigue life by up to one order of magnitude. Fatigue test results for dog-bone specimens manufactured from 7075-T651 and 2024-T351 aluminium alloys indicated that the presence of exfoliation corrosion reduced the fatigue life under dry conditions, with a greater reduction under humid conditions. The application of a CPC to the corroded region eliminated the influence humidity had on fatigue life.     

THE SIZE OF PLASTIC ZONES AND FATIGUE CRACK GROWTH BEHAVIOUR OF THREE FORMS OF A Ti–6Al–2.5Mo–1.5Cr ALLOY

The effect of microstructure on the fatigue properties of Ti–6Al–2.5Mo–1.5Cr alloy was investigated. The experimental results for both the fatigue crack initiation and propagation behaviour, as well as the dynamic fracture toughness ( K _Id ) showed clearly that a lamellar microstructure is superior to two other structures. It was found that, as in the case of steels, the initiation and subsequent growth of cracks in the titanium specimens with a sharp notch may also occur on loading levels below the threshold values of the K factor (Δ K _th ) determined for long fatigue cracks. In addition, measurements by interferential-contrast of the plastic zone size on the surface of specimens revealed that the different rate of crack growth at identical values of Δ K in individual structural states can roughly be correlated with the size of the plastic zone. A general relationship between the fatigue crack growth rate and plastic zone size, the modulus of elasticity and the role of crack tip shielding is discussed.     

Gigacycle fatigue of ferrous alloys

The objective of this paper is to determine the very long fatigue life of ferrous alloys up to 1 × 10¹⁰ cycles at an ultrasonic frequency of 20 kHz. A good agreement is found with the results from conventional tests at a frequency of 25 Hz by Renault between 10⁵ and 10⁷ cycles for a spheroidal graphite cast iron. The experimental results show that fatigue failure can occur over 10⁷ cycles, and the fatigue endurance stress S _max continues to decrease with increasing number of cycles to failure between 10⁶ and 10⁹ cycles. The evolution of the temperature of the specimen caused by the absorption of ultrasonic energy is studied. The temperature increases rapidly with increasing stress amplitudes. There is a maximum temperature between 10⁶ and 10⁷ cycles which may be related to the crack nucleation phase. Observations of fracture surfaces were also made by scanning electron microscopy (SEM). Subsurface cracking has been established as the initiation mechanism in ultra-high-cycle fatigue (>10⁷ cycles). A surface–subsurface transition in crack initiation location is described for the four low-alloy high-strength steels and a SG cast iron.     

Crack tip damage development and crack growth resistance in particulate reinforced metal matrix composites

A study of crack tip damage development and crack growth resistance of aluminium 359/20% V_f silicon carbide and aluminium 6061/20% V_f Micral^Tm particulate reinforced metal matrix composites has been conducted. Observations of crack tip process zone development at the specimen surface have been compared with the results of fractographic examination of the centre of the specimen. Both materials were found to fracture by a process of void nucleation, growth and coalescence. Void nucleation was found to be by fracture or debonding of reinforcement particles, and/or fracture or debonding of secondary matrix particles. The preferred mode of void nucleation was found to vary depending on the constituents of the PR MMC and even the heat treatment state of the material. It was found that in these materials fractured particles identified on the fracture surface fractured during loading rather than being pre-cracked during fabrication. It was further found that observations of damage development from the specimen surface did not necessarily reflect the mechanisms prevailing in the specimen bulk. Under plane strain conditions, both materials were found to exhibit decreasing crack growth resistance as crack extension proceeded, due to the “anti-shielding” effect of damage accumulated in the process zone ahead of the crack tip. In thin specimens of the Comral-85 composite, however, dramatically improved toughness was obtained, and K_R curves have been obtained for such specimens. The method of measuring crack length was found to have a profound effect on the K_R curve; it was concluded that the K_R curve determined using the crack length measured at the specimen surface best reflected the true crack growth resistance of these materials.     

NEAR-THRESHOLD FATIGUE CRACK GROWTH IN A12O3 PARTICLE REINFORCED 6061 ALUMINIUM ALLOY

Abstract— The influence of Al₂O₃ particle reinforcement on the fatigue crack growth properties of 6061-T6 aluminium alloy in the near threshold regime has been investigated at a load ratio of R =– 1 using an alloy with 15 vol.% fine particles (6061/Al₂O₃/15p) and one with 21 vol.% coarser particles (6061/Al₂O₃/21p). The Al₂O₃ particles act as obstacles for fatigue crack growth and are especially effective at very low cyclic loads. For the reinforced alloy with fine particles the threshold of the stress intensity amplitude is higher than that for the alloy containing coarse particles, and the lowest threshold value of K _max was obtained for pure 6061-T6. Fracture of ceramic particles and interfaces between matrix and Al₂O₃ particles, both more frequent for coarser particles, may serve as an explanation for the more effective improvement of fatigue crack growth properties by fine particles. At maximum stress intensity factors above 6.5 MPa√m, fatigue crack growth in the particle reinforced alloys is faster than in the unreinforced alloy 6061-T6, which is attributed to more frequent particle and interface fracturing.     

CYCLIC FATIGUE OF Al2O3 AND Al2O3-Al2TiO5 COMPOSITES IN DIRECT PUSH-PULL

Abstract— A reaction sintering route is developed to produce, "in situ", composites of alumina-aluminium titanate using alumina and titania as starting powders. Aluminium titanate, can be formed by a solid state reaction between Al₂O₃ and TiO₂ at temperatures above the eutectoid temperature of 1280°C. These composites have different grain sizes of alumina matrix and a different quantity and distribution of aluminium titanate according to the heating cycle used.
In the present work direct push-pull tests under cyclic loads have been carried out with both monolithic alumina and alumina-aluminium titanate composites. It has been found that all the samples show a decrease in tensile strength with the number of applied cycles of loading when plotted in graphical form but the slopes of these graphs for both Al₂O₃-Al₂TiO₅ composites are lower than for the alumina specimens. The role of aluminium titanate and the alumina matrix grain size in fatigue crack growth resistance has been studied during push-pull tests, where failure occurs by catastrophic propagation of small surface cracks after a very short regime of subcritical crack growth. These results have been compared with measurements of slow stable fatigue crack growth rates in Al₂O3-Al₂TiO₅ composites carried out elsewhere with pre-notched specimens of the compact tension type. These latter tests provide information about the behaviour of significantly long cracks, i.e. cracks that are several millimetres long.     

FATIGUE STRENGTH OF 7075-T6 ALUMINIUM ALLOY UNDER COMBINED AXIAL LOADING AND TORSION

The fatigue strength of 7075-T6 aluminium alloy under combined axial loading and torsion was examined. The S-N relations were correlated with the von Mises criterion for the high cycle region ( N _f≥ 10⁴ cycles) and with the Tresca criterion for the low cycle region ( N _f < 10⁴ cycles), where N _f is the cycles to failure. This transition at N _r= 10⁴ cycles was related to the occurrence of macroscopic plastic straining and a change in fracture modes. The results are discussed in comparison with those for a high strength steel (SNCM8) in a previous study. Particular attention is given to differences in cyclic deformation behaviour, fracture modes and fatigue crack growth rates between the two materials.     

Crack growth in the gigacycle fatigue regime for helicopter gears

Investigations were performed on helicopters' gears that had failed in service, using quantitative fractographic analyses. It was shown that a combination of LCF and HCF creates damage in gears on a flight-by-flight basis which produces beach marks on the fatigue surface of the gears that correlate one-to-one with flight cycles. The crack increment between beach marks occurs under HCF at 7000–8000  r.p.m. The ratio of the crack growth period to durability is smaller in the gigacycle fatigue area (more than 10⁸ cycles) than in the high cycle fatigue area (10⁶–10⁷ cycles) for those gears investigated that have stress raisers. This relationship for damaged gears depends not only on durability but also on the type of stress raiser and its location.     

A NEW METHOD FOR THE MEASUREMENT OF MODE II FATIGUE THRESHOLD STRESS INTENSITY FACTOR RANGE ΔKτth

Abstract— In order to evaluate the threshold value Δ K _τth for mode II fatigue crack growth, a new measurement method of mode II fatigue crack growth has been developed. This method uses a conventional closed-loop tension—compression fatigue testing machine without additional loading attachments. Mode II fatigue tests for structural steel and rail steel have been carried out. This method has proved successful and has reproduced mode II fatigue fracture surfaces similar to those found in the spalling of industrial steel-making rolls. The crack length during testing was measured by an AC potential method. The relationships between d a /d N and Δ K _τ and AK _τth for several materials have been obtained.     

A TEST METHOD FOR MODE II FATIGUE CRACK GROWTH RELATING TO A MODEL FOR ROLLING CONTACT FATIGUE

Abstract— From fractographic observations of specimens that have failed due to rolling contact fatigue, it has been concluded that the first stage of damage is the formation of mode II fatigue cracks parallel to the contact surface due to the cyclic shear stress component of the contact stress. Although these initial subsurface cracks, in both metals and ceramics, are produced in a direction parallel to the cyclic shear stress, cracks eventually grow in a direction close to the plane of the maximum tensile stress if we apply a simple mode II loading to them. The difference between crack growth in simple mode II loading and crack growth due to rolling contact fatigue is, we suppose, whether or not there is a superimposed compressive stress. Based on this hypothesis, we developed an apparatus to obtain the intrinsic characteristics of mode II fatigue crack growth, and developed a simplified model of subsurface crack growth due to rolling contact fatigue.
Some results in terms of da/dN versus ΔK_II relations have been obtained using this apparatus on specimens of steel and aluminum alloys. Fractographs of the mode II fatigue fracture surfaces of the various materials are also provided.     

Reduction of hydrogen embrittlement in an ultra-high-strength steel by laser surface annealing

This paper shows that laser beam irradiation improves the resistance to hydrogen embrittlement (HE) in an ultra-high-strength maraging steel. Localized laser irradiation of a peak-aged steel plate resulted in the formation of a soft surface layer called the laser-annealed zone (LAZ). A composite region (CR) was formed when both the top and bottom surfaces of a peak-aged specimen were laser-annealed (LA) to leave an interior layer of untransformed base metal (BM) sandwiched between the two LAZs. Slow strain rate tensile tests showed that LA specimens had lower strength and ductility than the peak-aged specimens when tested in air, but in a H₂ S solution, the soft LAZs showed less susceptibility to HE than the BM. The fatigue crack growth rates (FCGRs) in the CR were lower than those in the BM regardless of testing environment and stress ratio ( R ). The retarded crack growth in the CR was attributed to the combination of residual compressive stresses and the soft microstructures in the LAZs. The tensile fracture appearance of LA specimens tested in a H₂ S solution exhibited intergranular fracture in the BM. Fractographs of the fatigue specimens tested in gaseous hydrogen revealed transgranular fracture in the LAZs and mainly quasi-cleavage fracture in the BM.     

Microcrack nucleation, growth, coalescence and propagation in the fatigue failure of a powder metallurgy steel

Detailed quantitative micrographic data are presented for Stages I and II of a Powder Metallurgy Fe-1.5Cr-0.2Mo-0.7C steel specimen fatigued in bending with R  =−1 at 24 Hz and a stress amplitude of 312 MPa. The fatigue limit was ∼240 MPa, at which stress level no microcracks were detected in static loading. Testing was interrupted at 100 cycles and at further 29 intervals until failure after 49 900 cycles. For each arrest, surface replicas were made in the two regions where maximum stress was applied. Microcracks could nucleate below 100 cycles, when their sizes ranged from <5 to ∼20 μm. Fractographic examination identified the failure-originating site, which was then associated with the crack system observed on the 'last' pre-failure micrograph. Detailed examination of the eventual failure region showed nucleation, at various cycle intervals, of 18 microcracks, their subcritical growths, arrests and coalescences with continuing cycling to form a critical crack 2.25 mm deep. Stepwise microcrack growth was probably rapid – to the next arrest or coalescence. For each (micro)crack size stress intensity factors, K _a s, were estimated and, at the end of Stage II, for the coalesced crack, K _a reached K _1C, independently estimated to be ∼36 MPa m^1/2.     

Study of fatigue behaviour of 7475 aluminium alloy

Fatigue properties of a thermomechanically treated 7475 aluminium alloy have been studied in the present investigation. The alloy exhibited superior fatigue life compared to conventional structural aluminium alloys and comparable stage II crack growth rate. It was also noticed that the fatigue crack initiated from a surface grain and the crack extension was dominated by ductile striations. Analysis also revealed that this alloy possessed fracture toughness and tensile properties superior to that noticed with other structural aluminium alloys. Therefore the use of this alloy can safely reduce the overall weight of the aircraft.     

Fatigue strength of crack-healed Si3N4/SiC composite ceramics

Si₃ N₄ /SiC composite ceramics were sintered and subjected to three-point bending on specimens made according to the appropriate JIS standard. A semi-circular surface crack of 110 μm in diameter was made on each specimen. By using three kinds of specimen (smooth, cracked and crack healed), crack-healing behaviour, cyclic and static fatigue strengths were determined systematically at room temperature and 1000 °C. The main conclusions are as follows: (i) Si₃ N₄ /SiC composite ceramics have the ability to heal after cracking; (ii) crack-healed specimens showed similar cyclic and static fatigue strengths as smooth specimens, this being caused by crack healing; (iii) crack-healed zones had a sufficient fatigue strength and most fractures occurred outside the pre-cracked zone in those crack-healed specimens.