Room Temperature Low Cycle Fatigue Research Articles

Room-temperature low-cycle fatigue and fracture behaviour of asymmetrically rolled high-strength 7050 aluminium alloy plates

The asymmetrical rolling (ASR) process with shear deformation is considered as a promising technology to adjust/improve through-thickness microstructure homogeneity and integrated properties of high-strength aluminium alloy plates. But the advantages may come with caveats that are the subject of our research. In this paper, the room-temperature low cycle fatigue properties and fracture behaviour of the ASR-ed AA7050 aluminium alloy plates are compared with the symmetrical rolling (SR) one. It is shown that after either type of rolling the plates exhibit similar low-cycle fatigue lives but the SR-ed one displays a better cyclic deformation ability and slightly higher fatigue lives at high strain amplitudes. It is demonstrated that the severe surface-localized deformation contributes to the formation of slip relief on the surface and subsequently initiates micro-cracks that are propagated via transgranular and/or intergranular fracture modes along with obvious fatigue striations. Recrystallised grains with coarse grain boundary precipitates and wide precipitate-free zones near the upper/bottom layers as well as numerous and larger secondary particles in the ASR-ed plates may cause early crack initiation for a short crack initiation life. However, the SR-ed plate with more frequent subgrains near surface layers, numerous fine subgrains and less indissoluble particles could possess better crack initiation/propagation resistance and cyclic loading behaviour. Fine subgrains with higher microhardness/strength can facilitate passing of dislocations or slip bands into adjacent grains so as to delay crack propagation such as via energy-intensified transgranular fracture for extending fatigue life. Properly balancing the through-thickness strain/deformation distribution and the formation of recrystallization/indissoluble particles via implementing a feasible ASR process becomes a critical issue to achieve fracture-resistant microstructures for high-strength aluminium alloy plates. The underlying causes/mechanisms regarding the differences of microstructures and mechanical behaviour are revealed and discussed based on modelling through-thickness temperature/strain distribution and detailed microstructure characterization.

Creep and high temperature fatigue performance of as build selective laser melted Ti-based 6Al-4V titanium alloy

The present work focuses on the room and high temperature low-cycle fatigue and creep testing of Ti-6Al-4V manufactured by selective laser melting (SLM) or laser-based powder bed fusion process (LBPF). The fatigue specimens were tested in a strain control mode with the as-built and machined surfaces, evaluating the influence of the surface roughness on the fatigue performance. This helps in understanding the potential negative influence of a surface defect/roughness on the fatigue performance of the complex components produced by SLM (in as-built state), where surface machining may not be possible due to geometrical constraints. The fatigue fractures at room and high temperature were investigated with the help of a scanning electron microscope. The creep tests were performed at three different temperatures with as-built SLM samples, demonstrating an equivalent or better performance when compared to their counterparts produced by hot-forging.

Effect of cold expansion on room-temperature low cycle fatigue life of GH4169 superalloy with hole structure

Effect of cold expansion on room-temperature low cycle fatigue life of GH4169 superalloy with hole structure

Fatigue Performance of the Novel Titanium Alloy TIMETAL®407

TIMETAL®407 (Ti-407) is a novel titanium alloy formulated as a medium strength, highly ductile alloy offering a range of manufacturing cost reduction opportunities. It can be used as a direct replacement for Ti-6Al-4V or Ti-3Al-2.5V alloys, particularly in applications where energy absorption during fracture or HCF endurance are the key design criteria. The effect of thermo-mechanical processing on microstructure has been characterised and the room temperature high cycle, low cycle and dwell cycle fatigue properties of Ti-407 containing 30-40% primary alpha volume fraction are presented and discussed. These results are compared with data generated from Ti-6Al-4V processed to provide a similar but non-standard microstructure and demonstrate that Ti-407 shows superior HCF endurance strength, despite having a significantly lower tensile and yield strength.

Short crack propagation from cracked non-metallic inclusions in a Ni-based polycrystalline superalloy

Fatigue cracks initiating from surface and sub-surface non-metallic inclusions (NMIs) have recently been demonstrated to be a necessary but not sufficient explanation for atypically short low-cycle fatigue life in Inconel 718 alloy at intermediate temperature. Therefore, the early stages of short crack propagation from surface NMIs were investigated in a crystallographic and two-dimensional versus three-dimensional morphological manner after room temperature low cycle fatigue (LCF) testing. In the present investigation, NMIs were purposely pre-cracked using different techniques to suppress the natural crack initiation period and thus the incubation period prior to the early stages of crack propagation. Under such fatigue testing conditions, different mechanisms of crack transmission from pre-cracked NMIs were identified: (i) no propagation, (ii) NMI/adjacent metallic grain interfacial debonding, (iii) transgranular crack propagation within the adjacent metallic grain. Focused-ion-beam cross-section observations of numerous fatigue tested NMIs aimed to define a morphological criterion for non-propagating NMIs. Large cracked NMIs at the surface (2c) with limited extension into the depth (a) did not propagate under such fatigue conditions for 2c/a ratio higher than 3. Furthermore, specific crystallographic relationships between NMIs and the adjacent metallic grain explained different crack propagation configurations from pre-cracked NMIs, i.e. interfacial debonding and transgranular crack propagation involving single or multiple-slip activity.

Microstructure evolution and mechanical properties changes in the weld zone of a structural steel during low-cycle fatigue studied using instrumented indentation testing

Room-temperature low-cycle fatigue (LCF), instrumented indentation, tensile tests, finite element (FE) analysis, and optical microscopy examinations were carried out to investigate the mechanical properties and microstructures in the weld zone of a structural steel, SS400, in cyclic loading conditions. Five indentation arrases were performed across the weld zone of five specimens, including one virgin specimen and four specimens subjected to LCF loading at different strain levels. The mechanical properties, such as elastic modulus E, yield strength σy, and strain hardening exponent n, were then estimated from indentation testing and FE analysis results. The obtained results and optical microscopy examinations show that the mechanical properties and microstructures in the individual region of the weld zone are not only decided by the metallurgical conditions resulting from the welding but also were highly influenced by the strain level applied during cyclic loading. In the as-welded condition, the values of elastic modulus, yield strength, and strain hardening exponent in the weld metal (WM) are higher than those in the base metal (BM), while materials in heat affected zone (HAZ) reveal a gradual decrease of elastic modulus, yield strength, and strain hardening exponent values from the WM to BM region. It is also shown that with increasing strain amplitudes during cyclic loading, the elastic modulus E displays decrease in all regions of the weld zone, both yield strength σy and strain hardening exponent n exhibit decreasing characteristic in WM and increasing characteristic in BM region, while the yield strength in HAZ region increases, the strain hardening exponent in this region keeps almost unchanged. The dependence of the mechanical properties on the applied strain level is also discussed in terms of microstructural changes in fatigue failure specimens.

Microstructure evolution in CLAM steel under low cycle fatigue

In the process of room-temperature low cycle fatigue, the China Low Activation Martensitic steel exhibits at the beginning cyclic hardening and then continuous cyclic softening. The grain size decreased and the martensitic lath transformed to cells/subgrains after the tests. The subgrains increase in size with increasing strain amplitude.

Improvement of high temperature fatigue lifetime in AZ91 magnesium alloy by heat treatment

In the present paper, an improvement in high temperature fatigue properties of the AZ91 magnesium alloy with rare earth elements has been obtained by a typical heat treatment, denoted by T6. For this objective, out-of-phase thermo-mechanical fatigue, room temperature and high temperature low cycle fatigue tests are performed to compare lifetimes. Several rare earth elements are initially added to the AZ91 alloy during a gravity casting process in permanent molds. Also, the type of the heat treatment is examined. Results of specimens with only the solution (the T4 heat treatment) and the solution with the ageing process (the T6 heat treatment) are compared under isothermal fatigue loadings. Microstructural investigations are carried out, before and after fatigue experiments to demonstrate the heat treatment effect. Results showed that both low cycle fatigue and thermo-mechanical fatigue of the alloy at high temperatures increases tremendously after the T6 heat treatment. This behavior attributes to the variation of the ductility, which was a result of microstructural changes during the heat treatment and the varying temperature in fatigue tests.

Use of instrumented indentation testing to study local mechanical properties of 304L SS welded joints subjected to low-cycle fatigue loadings

In this paper, a series of experiments including room-temperature low-cycle fatigue tests, instrumented indentation measurements, optical microscopy examinations, etc. were carried out to study the local mechanical properties and microstructures in particular zones of 304L SS welded joints in both the as-welded and cyclic straining conditions. An analytical algorithm proposed by Dao et al. [12] was utilized to estimate the basic mechanical properties (E, σy and n) from the indentation load-depth data. It is shown that in the as-welded condition, the weld metal has a higher strength and strain hardening capacity but a lower stiffness than does the base metal, while the material within HAZ exhibits a significant gradient of the mechanical properties. A strain amplitude-dependent variation in the local mechanical properties in particular joint regions was also obtained. Optical microscopy examinations showed that the microstructure in particular zones of 304L SS welded joints was not only decided by the metallurgical condition resulting from the welding process, but also determined by the strain amplitude applied during cyclic straining. In this study, an empirical criterion for judging the fracture locations of a weldment based on the yield stress mismatch ratio was also introduced to assess the integrity of the present 304L SS welded joints subjected to low-cycle fatigue loadings. The research work performed in this paper would further expand the applications of instrumented indentation technique in fatigue strength analysis of welded components.

Effect of grain size on austenite stability and room temperature low cycle fatigue behaviour of solution annealed AISI 316LN austenitic stainless steel

The present paper investigates completely reversed room temperature low cycle fatigue (LCF) behaviour of solution annealed austenitic stainless steel AISI 316L with two different grain sizes of 90 and 139 μm developed by solution annealing treatment at 1050 and 1150°C respectively and at six strain amplitudes ranging between ± 0·375 and ± 1·00%. Complete cyclic hardening has been observed for both the grain sizes. While fine grained steel shows an improvement in cyclic life compared with that of coarse grained steel for strain amplitudes ± 0·375 and ± 0·50%, and perfectly follows the Coffin–Manson (C–M) behaviour within the experimental domain, higher cyclic life with bilinear C–M behaviour is observed in the case of coarse grained steel at ± 0·625% strain amplitude and above. Optical microscopy of fatigue fracture surfaces reveals the formation of martensite on cyclic straining predominantly at higher strain amplitudes.

Microstructural response on the room temperature low cycle fatigue behaviour of two high strength duplex ferrite–martensite steels and a normalised ferrite–pearlite steel

Strain controlled low cycle fatigue (LCF) behaviour of two high strength duplex ferrite–martensite (DFM) steels with continuous martensite developed from a commercial grade vanadium bearing microalloyed steel through two different intercritical annealing routes has been studied at room temperature. The results have been compared with that of normalised ferrite–pearlite structure of the same steel. A mixed cyclic stress-strain behaviour and much higher cyclic life were observed in steel containing ∼50% martensite as compared to that containing ∼80% martensite exhibiting complete cyclic softening behaviour. Much higher cyclic life of the steel containing ∼50% martensite, particularly at low strain amplitude, has been discussed in terms of crack growth retardation phenomena. Bilinear Coffin–Manson (C-M) behaviour has been observed in case of the steel with 80% martensite. Under total strain control mode, better LCF resistance has been observed for DFM steels as compared to the normalised ferrite–pearlite structure.

Room temperature low cycle fatigue behaviour of two high strength lamellar duplex ferrite–martensite (DFM) steels

Room temperarure low cycle fatigue (LCF) behaviour of two high strength Duplex ferrite-martnsite (DFM) steels with lamellar morphology developed from a commercial V-bearing low carbon microalloyed steel has been studied. Cyclic hardening and better LCF performance were observed in case of the steel containing ∼45% martensite as compared to the steel with 60% martensite exhibiting cyclic softening behaviour. Cyclic hardening/softening behaviour of the steels has been qualitatively explained in terms of dislocation phenomena involved in the cyclic plastic deformation process. Both the steels followed Coffin–Manson (C–M) and Basquin behavior. The reason for poor fatigue performance with higher amount of martensite has been attributed to the plastic strain magnification at ferrite–martensite interfaces.

High-Temperature Fatigue Properties of Single Crystal Superalloys in Air and Hydrogen

Hot section components in high-performance aircraft and rocket engines are increasingly being made of single crystal nickel superalloys such as PWA1480, PWA1484, CMSX-4, and Rene N-4 as these materials provide superior creep, stress rupture, melt resistance, and thermomechanical fatigue capabilities over their polycrystalline counterparts. Fatigue failures in PWA1480 single crystal nickel-base superalloy turbine blades used in the space shuttle main engine fuel turbopump are discussed. During testing many turbine blades experienced stage II noncrystallographic fatigue cracks with multiple origins at the core leading edge radius and extending down the airfoil span along the core surface. The longer cracks transitioned from stage II fatigue to crystallographic stage I fatigue propagation, on octahedral planes. An investigation of crack depths on the population of blades as a function of secondary crystallographic orientation (β) revealed that for β=45+/−15 deg tip cracks arrested after some growth or did not initiate at all. Finite element analysis of stress response at the blade tip, as a function of primary and secondary crystal orientation, revealed that there are preferential β orientations for which crack growth is minimized at the blade tip. To assess blade fatigue life and durability extensive testing of uniaxial single crystal specimens with different orientations has been tested over a wide temperature range in air and hydrogen. A detailed analysis of the experimentally determined low cycle fatigue properties for PWA1480 and SC 7-14-6 single crystal materials as a function of specimen crystallographic orientation is presented at high temperature (75°F–1800°F) in high-pressure hydrogen and air. Fatigue failure parameters are investigated for low cycle fatigue data of single crystal material based on the shear stress amplitudes on the 24 octahedral and 6 cube slip systems for FCC single crystals. The max shear stress amplitude [Δτmax] on the slip planes reduces the scatter in the low cycle fatigue data and is found to be a good fatigue damage parameter, especially at elevated temperatures. The parameter Δτmax did not characterize the room temperature low cycle fatigue data in high-pressure hydrogen well because of the noncrystallographic eutectic failure mechanism activated by hydrogen at room temperature. Fatigue life equations are developed for various temperature ranges and environmental conditions based on power-law curve fits of the failure parameter with low cycle fatigue test data. These curve fits can be used for assessing blade fatigue life.

A new method for improving the resistance of high strength steel wires to room temperature creep and low cycle fatigue

The effects of warm deformation treatments on the room temperature creep and the low cycle fatigue resistance of high carbon patented steel wires and high strength low carbon low alloy (HSLCLA) steel wires were studied. The low temperature creep strains of high carbon patented steel wires and high strength low carbon low alloy steel were decreased 85 and 65%, respectively, by the warm working treatments for the optimum warm deformation parameters (3% axial tensile plastic deformation at 300°C for 5 min). The low cycle fatigue lives of both steels were increased by 30–35% after warm deformation at the optimum deformation parameters. It is shown that the warm deformation treatment only affected the micro states such as lattice distortion, internal stress, dislocation density, sub-grain size and the amount of solute atoms in solid solution but did not change the overall microstructure or strength of the steel wires. Internal friction studies at different temperatures showed that the amount of solute atoms re-dissolved is different for different warm deformation temperatures and reaches a maximum at 300°C. It is considered that the improvement in the resistance of high strength steel wires to room temperature creep and low cycle fatigue can be attributed to the reduction of the amount of mobile dislocations through the rearrangement of dislocations and strengthening of matrix by the re-dissolution of solute atoms and dislocation pinning.

Effect of γ′ size on room temperature low cycle fatigue behaviour of a nickel base superalloy

The results of a study on the effect of γ′ particle size on the room temperature (23°C) low cycle fatigue (LCF) behaviour of a Ni base superalloy, Nimonic 90, is reported. The γ′ particle sizes were estimated from transmission electron micrographs. Ranges of particle sizes corresponding to underaged, peak aged, and overaged conditions were identified by examining the age hardening response curve. The solutionised samples had longer LCF lives compared with the aged alloys. Coffin-Manson and cyclic stress–strain plots showed bilinearity at a plastic strain amplitude of around 0.4% in the solutionised, underaged, and peak aged conditions. The observed bilinearity could be attributed to a change in the deformation mode from single slip to multiple slip. The cyclic stress response showed relatively stable behaviour for the peak aged and the overaged specimens compared with the underaged and the solution treated specimens.

Low cycle fatigue behaviour of a medium carbon microalloyed steel

Room temperature low cycle fatigue (LCF) and fracture behaviour of a medium carbon V-bearing microalloyed steel has been studied. The experimentally determined LCF life is compared with the life predicted by the ‘modified universal slopes’ equation. Cyclic stress response showed softening at lower plastic strain amplitudes (≤0.3%) and hardening at higher plastic strain amplitudes (>0.3%). The possible reasons for the above behaviour are discussed. An analysis of the fracture surfaces revealed a mixed mode of fracture that involved different fracture mechanisms. Cracks did not initiate at the inclusions.

Effect of prior cold work on the room-temperature low-cycle fatigue behaviour of AISI 304LN stainless steel

Room-temperature total strain-controlled low-cycle fatigue tests were carried out on AISI 304LN austenitic stainless steel specimens that were cold worked by swaging to different levels (10, 20 and 30% reduction in area) prior to testing. Cyclic softening was mostly noticed. A crossover in the total strain-life plots for the material with different degrees of prior cold work is explained in terms of the differences in strength and ductility. Prior cold work increased the total strain fatigue resistance at total strain amplitudes less than about 0.50%. A reduction in the transition fatigue life with an increase in the percentage of prior cold work was observed. Masing behavior was observed only in the 30% prior cold worked material.

A comparison of the room-temperature behaviour of AISI 304LN stainless steel and Nimonic 90 under strain cycling

Abstract The influence of room-temperature low-cycle fatigue (LCF) deformation on the microstructure and the consequent modification of the LCF behaviour were examined in the case of AISI 304LN stainless steel and the superalloy Nimonic 90. Secondary hardening due to martensite formation in AISI 304LN enhanced its resistance to plastic flow. On the other hand, in Nimonic 90 shearing of γ′ particles led to cyclic softening. A change in the number of operating slip systems as well as the fracture mode was responsible for the observed two-slope behaviour in the Coffin-Manson, the cyclic stress-strain and the energy-life plots in Nimonic 90. While Nimonic 90 resisted the applied strain elastically on the basis of its strength, AISI 304LN resisted the strain plastically on the basis of its ductility. Nimonic 90 had a much higher plastic strain energy absorption capacity than AISI 304LN.

Effect of electropolishing on the room-temperature low-cycle fatigue behaviour of AISl 304LN stainless steel

The surfaces of specimens of AISI 304LN stainless steel were given different levels of finish by means of electropolishing and mechanical polishing with emery papers of different grit sizes. They were subjected to total strain-controlled low-cycle fatigue (LCF) deformation at room temperature. The cyclic stress response and the strain-life plots were recorded. As opposed to the mechanically polished specimens, the electropolished specimens displayed secondary hardening in the later stages of deformation even at low strain amplitudes. This behaviour could be attributed to increased martensite formation in the latter case because of the enhanced life. The effect of electropolishing in enhancing the LCF life was more pronounced at the lower strain amplitudes.

Influence of martensite formation and grain size on room temperature low cycle fatigue behaviour of AISI 304LN austenitic stainless steel

Abstract The effects of strain induced martensite formation and grain size on the room temperature low cycle fatigue behaviour of AISI 304LN austenitic stainless steel were considered. Two grain sizes, namely, 60 and 350 μm, were developed via suitable solution annealing treatments. Microstructural changes before and after low cycle fatigue testing were identified. The martensitic transformation was studied using aferritescope, X-ray diffractometry, and optical microscopy. The mechanical response was correlated with the microstructural changes. Secondary hardening as well as a crossover in the strain–life plots for the two grain sizes resulted from martensite formation. Dislocation configurations depended on the strains imposed.MST/1902