Low cycle fatigue of an extruded Mg–3Nd–0.2Zn–0.5Zr magnesium alloy

The purpose of this study was to evaluate strain-controlled cyclic deformation behavior of an extruded Mg–3Nd–0.2Zn–0.5Zr (NZ30K) magnesium alloy. The microstructure of this alloy consisted of a bimodal microstructure with equiaxed recrystallized grains and unrecrystallized coarse grains along with a large number of smaller second-phase particles present inside the grains and larger particles along the grain boundaries alongside a characteristic precipitate free zone (PFZ). The average grain size was about approximately 5–7 μm. It was observed that unlike the higher RE-containing Mg–10Gd–3Y–0.5Zr (GW103K) magnesium alloy, the NZ30K alloy exhibited asymmetrical hysteresis loops in tension and compression in the fully reversed strain-control tests at a strain ratio of R_ε = −1. This was mainly due to the presence of relatively stronger crystallographic texture, PFZ, and the resultant twinning–detwinning activities during cyclic deformation. While this alloy exhibited cyclic softening at lower strain amplitudes and cyclic hardening at higher strain amplitudes, it had an equivalent fatigue life to that of other extruded Mg alloys. Fatigue crack was observed to initiate from the specimen surface with some isolated facets of the cleavage-like planes near the initiation site. Crack propagation was basically characterized by serrated fatigue striations.     

Finite element modelling of fatigue crack initiation in SiC-fibre reinforced titanium alloys

In the present investigation, fatigue crack initiation in SiC fibre (SCS-6) reinforced titanium has been analysed on the basis of a finite element (FE) model. In this composite material after processing a complicated interfacial zone exists, consisting of the remains of the carbon coating and the reaction zone. This reaction zone usually causes the initiation of a fatigue crack as it fails at a low stress. The growth of a fatigue starting at a reaction layer crack is analysed for different thick reaction layers (from 0.5 to 3 μm). The conditions under which a fatigue crack can be arrested and the influence of additional fibre failure on fatigue crack growth have been analysed. The results show that the formation of the matrix crack largely depends on the applied stress and reaction layer thickness. Under an applied stress, σ_max<800MPa, a crack in 1-μm-thick reaction layer cannot extend into the matrix. For higher applied stress a matrix crack can grow form the cracked reaction layer but after an extension of several microns it can be arrested. A mechanism of ΔK-reduction is found to be responsible for the crack arrest. The thickness of the reaction layer up to 3 μm has no significant influence on fatigue crack growth rate for larger fatigue cracks (>10 μm).     

Characterisation of fatigue fracture surfaces of friction stir channelling specimens tested at different temperatures

The fatigue fracture surfaces of friction stir channelling specimens tested at room temperature, 120 °C and 200 °C were observed in a scanning electron microscope (SEM) in order to analyse their morphology and the crack propagation mechanisms. Three different friction stir channelling conditions were tested and analysed. For all specimens tested the developing fatigue-crack has always initiated at the advancing side, namely on the boundary between the nugget and the thermo mechanically affected zone (TMAZ) into the interior of the specimen. The crack has propagated through the channel nugget with a path tangential to the advancing side. After the crack has reached the processed surface, a second crack initiated at the channel bottom. The fracture surfaces have shown a semi-elliptical shape crack front. This second crack has propagated uniformly through the base material. Fatigue crack propagation on the TMAZ was mainly characterised by fatigue striations. It was found, on most of the surfaces observed, a clear coexistence of the intergranular fracture mode and the transgranular fracture mode. A relationship between the fatigue testing temperature and the roughness of the fracture surfaces was found. The fracture surfaces roughness was considerably lower at a testing temperature of 200 °C for the three friction stir channelling conditions analysed.     

EBSD study on microstructure and texture of friction stir welded AA5052-O and AA6061-T6 dissimilar joint

Friction stir welded AA5052-O and AA6061-T6 dissimilar joint has a more obvious impact on microstructure and texture evolution compared to single material welding due to differences in physical and chemical parameters between two aluminum alloys. Microstructure, texture evolution and grain structure of AA5052-O and AA6061-T6 dissimilar joint were investigated by means of OM,EBSD and TEM measurements. Experimental results showed that FS weld was generalized in four regions–nugget zone (NZ),thermomechanically affected zone (TMAZ),heat affected zone (HAZ) and base metals (BM), using standard nomenclatures. NZ exhibited the complex structure of the two materials with flowing shape and mainly composed of the advancing side material Subgrain boundaries in weld nugget zone gradually transformed into high angle grain boundaries by absorbing dislocation and accumulating misorientations. Grain refinement of weld nugget zone was achieved by dynamic recrystallization. In the friction stir welding process, the presence of the shear deformation in weld made {001} < 100 > C cube texture, {123} < 634 > S texture in BM gradually transformed into {111} < 1(−)12(−) > A1¹ shear texture. HABs distribution were most significant in nugget followed by RS and then by AS. In TMAZ and NZ, numerous precipitates and lots of dislocations were observed.     

The fatigue crack growth rate and crack opening displacement in 18G2A-steel under tension

In the paper, the results of crack tip opening displacement (CTOD) and crack opening displacement (COD) in place of crack initiation as well as the fatigue crack growth rate in higher strength steel are presented. The investigation were carried out on flat specimens with central notch under constant amplitude tensile fatigue loading at stress ratio R = 0.2 and different value of the stress σ_max. The test results showed that with growth of crack length l grew values of the CTOD and COD. In the work, it was proposed calculation of the CTOD value on basis various dependence of plastic zone radius on crack tip.     

Low cycle fatigue properties of friction stir welded joints of a semi-solid processed AZ91D magnesium alloy

A semi-solid processed (thixomolded) Mg–9Al–1Zn magnesium alloy (AZ91D) was subjected to friction stir welding (FSW), aiming at evaluating the weldability and fatigue property of the FSW joint. Microstructure analysis showed that a recystallized fine-grained microstructure was generated in the nugget zone (NZ) after FSW. The yield strength, ultimate tensile strength, and elongation of the FSW joint were obtained to be 192 MPa, 245 MPa, and 7.6%, respectively. Low-cycle fatigue tests showed that the FSW joint had a fatigue life fairly close to that of the BM, which could be well described by the Basquin and Coffin-Manson equations. Unlike the extruded magnesium alloys, the hysteresis loops of FSW joint of the thixomolded AZ91D alloy were basically symmetrical, while the non-linear or pseudoelastic behavior was still present. The FSW joint was observed to fail in the BM section rather than in the NZ. Fatigue crack initiated basically from the pores at or near the specimen surface, and crack propagation was mainly characterized by fatigue striations along with the presence of secondary cracks.     

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.     

Effect of stress ratio on very high cycle fatigue properties of Ti-10V-2Fe-3Al alloy with duplex microstructure

In fatigue critical applications, Ti-10 V-2 Fe-3 Al alloy components are expected to endure cyclic loading with cycles above 109. To assess their operating safety, S-N relations of Ti-10 V-2 Fe-3 Al alloy in very high cycle fatigue(VHCF) regime are of concern and have been investigated in this work. Fatigue behavior including S-N curves and crack initiation mechanisms is reported. Two transitions of fatigue crack initiation mechanism, from internal crack initiation to surface crack initiation and from α_p cleavage to α_s/βdecohesion, occur when the stress ratio(R) and stress level are reduced. Fatigue limits exist at N_f = 6 × 10~7 cycles for all stress ratios except for 0.5. In the VHCF regime two kinds of internal crack initiation mechanisms exist, i.e., coalescence of cluster of α_p facets and α_s/β decohesion. Their mutual competition depends on the stress ratio and can be interpreted in terms of different stress character required for promotion on different internal crack initiation mechanism. Small crack propagation is discussed to be life controlling process under the stress ratio range from-0.5 to 0.1 during VHCF regime while under the stress ratio 0.5 VHCF, life almost refers to the life required for crack initiation.     

Fatigue failure investigation on anti-vibration springs

Fatigue failure investigation on anti-vibration springs, involving both metal and rubber materials, is presented. Rubber-to-metal bonded springs are widely used in industry as anti-vibration components giving many years of service. Recently a need to improve time and cost efficiencies has caused an unexpected early fatigue failure of the component with no immediate explanation. The required total fatigue life was 1.25 million cycles but only 0.7 million cycles achieved. There was an urgent need to investigate the causes of the fatigue failure and to modify the component design accordingly to meet the customer requirement and the supply schedule.The investigation, based on the actual fatigue loads, is carried out on these failed and modified products using a method of continuum mechanics. To simplify the simulation, a non-linear quasi-static analysis is carried out and then the residual stresses are superimposed to obtain the effective stress range to predict the metal crack initiation. For the rubber parts a three-dimensional effective stress criterion is employed to predict the fatigue crack initiation. The fatigue failure is taken as visual crack observation (normally 1–2 mm).The fatigue crack initiation for the metal parts of the failed component is predicted at 225 K cycles under specified fatigue load against total metal broken at 700 K cycles from the test. For the modified part the minimum total fatigue life for the metal parts of the component, estimated conservatively, is 2.1 million cycles against 1.75 million cycles from the test without any crack observed. The rubber fatigue crack initiation is predicted at 90 K cycles against crack onset around 79 K cycles and crack length 40 mm at 145 K cycles from the test. From design point of view it is important to optimize the rubber profile under this very tight allowable space to provide the maximum support of the metal interleaves and at the same time to meet the minimum requirements of the manufacture process. It is shown that this approach can be employed at a design stage for both metal and rubber fatigue evaluations on anti-vibration springs.     

Microstructure evolution in a single crystal nickel-based superalloy joint by linear friction welding

Linear friction welding (LFW) was used with a single crystal nickel-based superalloy to produce sound welds. Microstructural examination shows that the joint has a distinct weld zone (WZ) and thermomechanically affected zone (TMAZ). In the WZ, the microstructure has recrystallized polycrystals instead of a single crystal. In the TMAZ, the amount of γ′ phase increased from the TMAZ/WZ interface to the parent material (PM) and decreased from the periphery to the centre along the weld creating a U-shaped microhardness profile in the TMAZ. The microhardness is however lower than that in the WZ. The average tensile strength was found to be 837.5 ± 50 MPa, comparable to the PM (880 MPa [26]).     

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.     

A methodology for predicting creep/fatigue crack growth rates in Ti 6246

A methodology has been developed which is capable of predicting creep/fatigue crack growth rates at ambient and elevated temperatures in Ti 6246. Predictions are based on finite element analysis and strain-control testing of plain specimens. The prediction of fatigue crack growth rates for a given crack configuration and cyclic plastic zone size is assumed to be consistent with the processes leading to crack initiation in plain specimens. Such an assumption leads to the conclusion that a similar stress–strain profile will lead to similar lives in both the plain specimens and in the cyclic plastic zone ahead of a crack in a notched specimen. Therefore, fatigue crack growth results from the accumulation of damage in the cyclic plastic zone ahead of the crack tip. Once the damage accumulated in this element of material becomes critical, the crack propagates through the damaged region into a new region of virgin material where the process of damage accumulation begins again. The creep/fatigue model is described and assessed with reference to measured fatigue crack growth rate data for Ti 6246 at 20 °C and 500 °C.     

Effect of welding parameters on microstructure and mechanical properties of AA6061-T6 butt welded joints by stationary shoulder friction stir welding

Stationary shoulder friction stir welding (SSFSW) butt welded joints were fabricated successfully for AA6061-T6 sheets with 5.0 mm thickness. The welding experiments were performed using 750–1500 rpm tool rotation speeds and 100–300 mm/min welding speeds. The effects of welding parameters on microstructure and mechanical properties for the obtained welds were discussed and analyzed in detail. It is verified that the defect-free SSFSW welds with fine and smooth surface were obtained for all the selected welding parameters, and the weld transverse sections are obviously different from that of conventional FSW joint. The SSFSW nugget zone (NZ) has “bowl-like” shapes with fairly narrow thermal mechanically affected zone (TMAZ) and heat affected zone (HAZ) and the microstructures of weld region are rather symmetrical and homogeneous. The 750–1500 rpm rotation speeds apparently increase the widths of NZ, TMAZ and HAZ, while the influences of 100–300 mm/min welding speeds on their widths are weak. The softening regions with the average hardness equivalent 60% of the base metal are produced on both advancing side and retreating side. The tensile properties of AA6061-T6 SSFSW joints are almost unaffected by the 750–1500 rpm rotation speeds for given 100 mm/min, while the changing of welding speed from 100–300 mm/min for given 1500 rpm obviously increased the tensile strength of the joint and the maximum value for welding parameter 1500 rpm and 300 mm/min reached 77.3% of the base metal strength. The tensile fracture sites always locate in HAZ either on the advancing side or retreating side of the joints.     

Effect of stress ratio on fatigue lifetime of high Nb containing TiAl alloy at elevated temperature

The effect of stress ratio (R) on fatigue lifetime of a cast Ti–45Al–8.0Nb–0.2W–0.2B–0.1Y (at.%) alloy was investigated at 750 °C. Fatigue tests with various stress ratios ranging from 0.1 to 1 were performed using a mini servo-hydraulic fatigue machine inside a chamber of scanning electron microscope (SEM). Fatigue crack initiation and propagation behavior was studied by in situ SEM observation and fatigue fracture mode was examined by fracture surface analysis. It is found that fatigue lifetime shows a reversed S-type curve with the increase of stress ratio. At R ranging from 0.1 to 0.4, creep–fatigue interaction dominates the fatigue lifetime and the fatigue lifetime reaches its minimum value at R = 0.3. At R ranging from 0.4 to 1, creep damage dominates the fatigue lifetime and the fatigue lifetime exhibits inverse proportional relation with R. Meanwhile, with the increase of stress ratio, the fatigue crack initiation sites transform from lamellar interface at R = 0.1, to lamellar interface and colony boundary at R = 0.3, and to lamellar colony boundary at R = 0.5. Accordingly, the fatigue fracture mode transforms from transgranular cracking, to transgranular and intergranular cracking, and to intergranular cracking.     

Plain–fretting fatigue competition and prediction in spline shaft-hub connection

The dynamically loaded spline shaft-hub connection that without macro relative movement between shaft and hub are exposed to the danger of fretting fatigue in the contact zone of teeth flank and plain fatigue at teeth fillet at the same time. The competition of fretting fatigue and plain fatigue determines that which one dominates the failure of it and therefore the fatigue performance of it. In order to deal with this plain–fretting fatigue coexisted situation, a plain–fretting fatigue unified prediction model is introduced in this paper and implemented in the representative spline teeth pair. Predicted by this model, the failure of involute spline shaft-hub connection teeth DIN 5480 45 × 2 × 21 is plain fatigue at teeth fillet dominated. Corresponding to the theoretical modeling efforts, a representative teeth pair fatigue test apparatus was developed. With this test apparatus, the initiation and propagation of fatigue cracks can be detected on line by monitoring the change of resonant frequency. The test results also showed that the crack occurs at teeth fillet at first.     

Influence of second phases on fatigue crack growth behavior of nickel aluminum bronze

The influence of phases with different morphology and mechanical properties on fatigue crack growth behavior in nickel aluminum bronze (NAB) has been investigated. Annealing at 675 °C and normalizing at 920 °C heat treatments were used to produce different morphologies and fractions of second phases. This analysis shows that the coarse dendritic κ_II particles and κ_III lamellae as hard brittle phases in as-cast and annealed NAB have an accelerative effect on the fatigue crack propagation where by cracks propagate through α and κ_II/κ_III interface. Fatigue cracks in normalized NAB prefer to propagate through the ductile α grains, form fatigue striations and have the lowest crack growth rate. The uniformly distributed, fine κ_IV precipitates in the α grains improves fatigue crack growth resistance. This work identifies the role of NAB second phases on propagation of fatigue cracks, and provides suitable heat treatment for improving fatigue crack resistance in terms of controlling second phase type, distribution and percentage.     

Rotating bending fatigue properties of two case hardening steels after nitriding treatment

Fatigue properties of two Al-containing steels have been investigated by rotating bending fatigue tests. Results show that the fatigue limits (the fatigue strength at 10⁷ cycles) were improved remarkably by plasma nitriding due to the high hardness of 1000 Hv and compressive stress of 400 MPa in the nitrided layer. Scanning electron microscopic observations show that after nitriding the fatigue crack initiation sites moved from the surface flaws or near-surface matrix into the AlN inclusions at around the case-core interface. Degassing treatment can increase the fatigue limit because it prevented fatigue crack initiation at AlN inclusions due to the reduced [N] contents and refined inclusion size.     

Crack initiation in VHCF regime on forged titanium alloy under tensile and torsion loading modes

This paper is focused on the VHCF behavior of aeronautical titanium alloy under tensile and torsion fatigue loadings. Tensile tests were carried out with two different stress ratios: R = −1 and R = 0.1. Both surface and subsurface crack initiations were observed. In the case of subsurface crack initiation several fatigue life controlling mechanisms of crack initiation were found under fully-reversed loading conditions: initiation from (1) strong defects; (2) ‘macro-zone’ borders; (3) quasi-smooth facets and (4) smooth facets. Tests with stress ratio R = 0.1, have shown that initiation from the borders of ‘macro-zones’ becomes the dominant crack initiation mechanism in presence of positive mean stress. Like for the tensile results, surface and subsurface crack initiations were observed under ultrasonic torsion in spite of the maximum shear stress location on the specimen surface. But the real reason for the subsurface crack initiation under torsion was not found.     

Effects of graphite shape on thermal fatigue resistance of cast iron with biomimetic non-smooth surface

In order to improve thermal fatigue resistance of cast iron and further study the effects of graphite shape on it, samples with different graphite shapes were processed by laser to create a biomimetic non-smooth surface. The results indicated that laser processed zone caused the longer crack initiation life and the slower crack propagation rate, which endowed biomimetic non-smooth surface with a beneficial effect on improving thermal fatigue behavior of iron with flake, vermiform or nodular graphite. Thermal fatigue resistance of samples with the same kind of surface (smooth or non-smooth) all were sorted as nodular graphite iron > vermicular graphite iron > flake graphite iron.     

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.