Low Cycle Fatigue Deformation Research Articles

Deformation and wear of pyramidal, silicon-nitride AFM tips scanning micrometre-size features in contact mode

An experimental study was carried out, in order to investigate the deformation and wear taking place on pyramidal silicon-nitride AFM tips. The study focuses on the contact mode scanning of silicon features of micrometre-size. First the deformation and the mechanisms of wear of the tip during scanning are discussed. After that the results of an experiment showing both phenomena on a used AFM tip are presented. Both the damaged and the unused tip are shown on AFM and SEM images. Using these images the actual mechanisms of wear are determined. It is shown that adhesive wear, low cycle fatigue and plastic deformation take place on the tip.

Dynamic strain ageing evidences during low cycle fatigue deformation in ferritic–martensitic stainless steels

The influence of dynamic strain ageing (DSA) on the strain cyclic behaviour of ferrite–martensite stainless steels was investigated at temperatures ranging from room temperature to 823 K. For fully annealed AISI 420 initial hardening followed by a saturation stage was observed at each test temperature. This steel was found to be susceptible to DSA as evidenced by the temperature independent stress saturation observed between 523 and 723 K. Normalized and tempered MANET II and F82H mod. softens during cyclic loading at all temperatures. In this steel DSA manifestations were observed on plotting the peak tensile stress difference between hysteresis loops obtained at different strain rates. Strongly abnormal behaviour with higher peak tensile stresses corresponding to slower strain rates was observed in the temperature range between 500 and 700 K. It is proposed that DSA mechanisms caused by the drag of solution carbon atoms is responsible for this unusual behaviour.

On the low cycle fatigue deformation of haynes 188 superalloy in the dynamic strain aging regime

A solid solution strengthened wrought cobalt-base superalloy, Haynes 188, which exhibits an excellent combination of high temperature strength, oxidation resistance and tensile ductility is currently used for a variety of components in gas turbine engines. This alloy is particularly effective for long-term applications at temperatures to {approx} 650 C. Although the temperature ranges of its applications encompasses the domain in which dynamic strain aging (DSA) is active, to date there has been little to no information available on the effects of DSA on cyclic deformation and accompanying substructure. The purpose of this paper is to report some recent observations pertaining to the temperature dependence of cyclic stress response and evolving deformation substructure of Haynes 188 between 25 and 1,000 C. Given that this alloy exhibits planar slip and stacking fault fringes and ribbons in the DSA domain, an attempt has been made to discuss the origin of planar slip in this alloy.

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.

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In this study, the low cycle fatigue deformation properties and the surface crack propagation behaviour of α/β-rolled plates of Ti-6Al-4V alloy with conventionally annealed equiaxial microstructure were investigated under the constant strain amplitude and the single over-straining conditions. Two types of the single over-straining were performed; that is, tension only (T waveform) and tension-compression (T-C waveform) over-straining, and the effects of the overstraining ratio λ on the cyclic deformation and the crack acceleration or the crack retardation behavior were also studied from the stand-point of the changes in crack opening displacement Φ and tensile peak stress σpt. As the results, in the T waveform test in the range of λ≥1.62, the crack ratardation occurred when compared with the crack propagation of the constant amplitude test, and the retardation effect increased with the ratio λ. On the other hand, in the T-C waveform test, the crack propagation curve after over-straining was almost coincident with the curve of the constant amplitude test. It became clear that the retardation behaviour of surface crack propagation after the over-straing of the T waveform resulted from both of the changes in Φ and σpt after the over-straining. The relation between the crack propagation rate da/dN and the J integral range ΔJ under the over-straining conditions was in good agreement with the da/dN and ΔJ relation of the constant strain amplitude test.

Determination of the room-temperature cyclic stress—strain curve of AISI 304LN austenitic stainless steel by two different methods

A comparison is made between the companion specimen test (CST) and the incremental step test (IST) methods for determining the cyclic stress—strain curve of AISI 304LN stainless steel at room temperature. Only seven blocks were needed for obtaining a stable pattern in the IST. Also, Masing behaviour was seen only when this testing procedure was used. In both types of test, during room-temperature low-cycle fatigue deformation the metastable AISI 304LN stainless steel underwent a martensitic transformation. In the CST secondary hardening was observed in specimens tested at and above a strain amplitude of ±0.85% and this could be attributed to martensite formation. Two martensite phases formed: an ε-martensite (h.c.p) and an α′-martensite (b.c.c). The dislocation arrangement in the IST resembled that present in the specimens tested at higher strain amplitudes in the CST.

Influence of high temperature low cycle fatigue deformation on the microstructure of a CrMoV rotor steel subjected to long-term service exposure at 425°C and retempering at 677°C

A study has been made of the influence on the microstructure of a CrMoV rotor steel of low cycle fatigue (LCF) deformation at 538°C with or without a 1-hour hold time applied at the peak tensile load of the fatigue cycle, following service exposure at 425°C for 23 years and retempering at 677°C for 24 hours. Samples for transmission electron microscopy were taken from the shoulder and gage sections of the broken fatigue specimens. The shoulder section of this steel revealed massive M 23C 6 along the grain boundaries, M 3C in globular or ellipsoidal form, fine spheroidal MC type carbide, and long needles of M 2C. LCF deformation at 538°C led to a tendency toward spheroidization of M 3C, promoted the nucleation of M 2C in ferrite, and slightly coarsened the MC carbide. Superimposing a 1-hour hold period at the peak tensile load resulted in nucleating M 2C preferentially at cementite-ferrite interfaces.

Strain-controlled low cycle fatigue behaviour of type 304 stainless steel base material, type 308 stainless stell weld metal and 304–308 stainless steel weldments

Total-axial-strain-controlled low cycle fatigue (LCF) tests have been conducted in air at 823 and 923 K on type 304 stainless steel (SS) base material, type 308 SS weld metal and 304–308 weldments prepared by the manual metal arc welding process. A symmetrical triangular waveform and a constant frequency of 0.1 Hz were employed for all the tests performed over strain amplitudes in the range from ±0.25% to ±0.80%. Crack initiation and propagation modes were studied. Microstructural features prior to and during LCF deformation were evaluated. Deformation and damage mechanisms which influence the stress response and endurance have been identified. The results indicated that the LCF resistance of base material was superior to that of the weldment at both the temperatures investigated. A reduction in fatigue life was noticed for all the conditions when the temperature was raised from 823 to 923 K. The δ-ferrite in welds and weldments transformed to M 23C 6 and σ phase duringk LCF testing. The transformed portion of δ-ferrite increased with increasing number of cycles to failure and increasing temperature. Solution annealing (at 1173 K for 3 h) the weldment prior to LCF testing caused an improvement in the fatigue life.

The low cycle fatigue deformation response of a single-crystal superalloy at 650 °C

The purpose of this study was to characterize the cyclic stress-strain response, the corresponding deformation structure and their relationships in the single crystal nickel-base superalloy PWA 1480. The isothermal low cycle fatigue response and deformation structures of specimens oriented near the [001], [2 5 20], [3 6 10], [011], [234] and [111] crystallographic orientations were characterized at an intermediate temperature (650 °C). The initial yield strength of all these specimens was controlled by the shearing of the γ′ precipitates by dislocation pairs. The low cycle fatigue tests exhibited cyclic hardening, which was associated with dislocation interactions in the γ matrix. In specimens deforming by slip on a single slip system, dislocations of the primary slip system accumulated in the γ matrix and formed sessile entanglements. In specimens deforming by slip on several slip systems, the dislocations of the different operative slip systems intersected in the γ matrix and formed sessile arrangements.

On the mechanism of uniform matrix cavitation in high temperature fatigue

Evidence for the occurrence of uniformly distributed voids within the matrix of a nuclear grade 304 stainless steel during high temperature low cycle fatigue (LCF) has been presented previously by the authors. Based on the detailed investigations carried out employing various microstructural conditions, it was inferred that either quenching the alloy from a very high solutioning temperature or prior cold working of the material is a necessary prerequisite to cause homogeneous cavitation in the grain interiors during LCF deformation. In order to determine the stage at which the uniform matrix voids form and to understand the underlying mechanism in the formation of these voids. TEM studies were undertaken on samples cycled to a predetermined fraction of the fatigue life (N/sub f/). The results of this study should help to clarify the mechanism and lead to a better mechanistic basis for modelling uniform matrix cavitation in high temperature fatigue.

Strain-rate effect on high temperature low-cycle fatigue deformation of AISI 304L stainless steel

The effect of strain rate on the behaviour of high temperature low-cycle fatigue is investigated for AISI 304L stainless steel. Regardless of the test temperature of 873 or 973 K, the fatigue life is saturated in the strain-rate range of slower than 4 × 10−3 sec−1. Also it is interesting to note that serrated flow, which is evidence of the occurrence of dynamic strain ageing, is clearly observed in the load-elongation hysteresis loops for strain rates that are slower (at 873 K) and faster (at 973 K) than 4 × 10−3 sec−1. Since the combination of temperature and strain rate is concerned with the phenomenon of dynamic strain ageing, it is considered that the above-mentioned saturated fatigue life at 873 K is caused by dynamic strain ageing and that the hardening effect due to dynamic strain ageing abnormally increases the fatigue life. However, even though the behaviour of fatigue life under strain rates slower than 4 × 10−3 sec−1 at 973 K has nothing to do with the dynamic strain ageing, it has been found that the failure life is also saturated in this slower strain-rate range. This behaviour is considered to be caused by the effect of creep, because the deformation under the low strain-rate activates the recovery process and as a result it causes saturation of the inelastic strain range.

Vacancy winds in cyclic deformation

From these calculations it is shown that if a deformation induced vacancy flux should occur as hypothesized, it will result in a Zn concentration differrence between the center of a dislocation cell and the dislocation cell walls of at least 10 −3 atom fraction, or possibly more in cylically deformed CuZn alloy. The zinc concentration should decrease near the vacancy sinks and increase near the sources. Hence, if such concentration gradients can be shown to develop during steady state fatigue, they will provide evidence as to the dominant direction of vacancy flow during fatigue. Such measurements will also provide insight into the magnitude of the vacancy generation rate and should lend support to one of the theories mentioned in the introduction. Such measurements require concentration determinations at the sensitivity of 10 −4 over a spatial dimension approximately 1/10 of the cell size, or approximately 0.2 μm. A step counting energy dispersive x-ray technique such as available on modern electron microprobes with measurement of Cu and Zn peaks or the use of STEM with energy dispersive x-ray capability should provide the answers. We are in the process of developing collaborative efforts to carry out these experiments. These calculations also estimate the vacancy flux at the cell wall due to the creation of 1 ppm vacancies per second [7] during cyclic deformation as sufficient to generate one unit climb of every dislocation in the cell wall during 35 cycles under typical low cycle fatigue deformation conditions. The rapid dislocation rearrangement and recovery in the cell walls produced by this flux of vacancies lends additional credibility to the cell shuttling model.