S. Ganesh Sundara Raman

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.

Room-temperature low-cycle fatigue behaviour of a Ni-base superalloy

Abstract A nickel-base superalloy, Nimonic 90, was subjected to room-temperature low-cycle fatigue under constant total strain control. Cyclic stress response was investigated as a function of the total strain range. Hardening was attributed to slip band formation and softening to the shearing of γ′ precipitates Slip band spacing decreased with increasing strain range. Two-slope behaviour or bilinearity seen in Coffin-Manson and cyclic stress-strain plots has been attributed to a change in the deformation mode with increasing strain range.

On cyclic stress–strain behaviour and low cycle fatigue life

Abstract In low cycle fatigue of metallic materials, the Manson–Coffin and Basquin relations are considered to be in conjunction with the cyclic stress–strain curve. Design considerations under constant load range or constant total displacement range are discussed based on the above relations.

Influence of current pulsing on microstructure and mechanical properties of Ti–6Al–4V TIG weldments

Abstract The present study has investigated the influence of current pulsing on the structure and mechanical properties of Ti–6Al–4V tungsten inert gas (TIG) weldments. Autogenous full penetration bead on plate TIG welds were made with and without direct current pulsing. Microstructural observations and evaluation of hardness, tensile and fatigue properties of the weldments were made in different conditions: as welded and post-weld heat treatment (PWHT) conditions (PWHT was done at 700 and 900°C). Current pulsing resulted in slight refinement of prior β grains, leading to better tensile strength, ductility and fatigue lives of weldments in the as welded condition. In the case of weldments subjected to PWHTs, the pulsed weldments exhibited inferior fatigue lives compared with the unpulsed weldments. The PWHT done at 900°C resulted in inferior fatigue lives of weldments compared with that done at 700°C.

Fretting Wear Behavior of Laser Peened Ti-6Al-4V

This work deals with the influence of laser peening on the fretting wear behavior of Ti-6Al-4V. Laser peening was carried out on Ti-6Al-4V. The laser-peened surface was characterized by transmission electron microscopy. Surface roughness, nanoindentation hardness, residual stress, and tensile properties of the material in both laser-peened and unpeened conditions were determined. Fretting wear tests were conducted at different normal loads using a ball-on-flat contact geometry. Laser peening resulted in the formation of nanocrystallites on the surface and near-surface regions, increased hardness, and compressive residual stress. Laser peening did not affect the tensile properties and surface roughness significantly. There was no considerable difference between the values of the tangential force coefficient of laser-peened and unpeened samples. The fretting scar size, wear volume, and wear rate of laser-peened specimens were lower than those of unpeened samples. This may be attributed to an increase in surface hardness due to strain hardening and grain refinement at the surface and near-surface regions, higher compressive residual stress, and higher resistance to plastic deformation of laser-peened samples.

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 laser peening on microstructure and fatigue lives of Ti–6Al–4V

Abstract The influence of low energy laser peening on fatigue lives of Ti–6Al–4V was investigated. Laser peening was carried out on Ti–6Al–4V samples. Laser peened samples were characterized by residual stress analysis, surface roughness measurements, X-ray diffraction, optical microscopy, nanoindentation hardness tests, scanning and transmission electron microscopy and fatigue testing. Laser peening resulted in the formation of nanocrystallites on the surface and near surface regions with associated increase in hardness and introduction of compressive residual stress. Owing to positive influence of nanostructured surface and compressive residual stress, fatigue lives of the laser peened samples were significantly increased compared to the unpeened samples.

Influence of Surface Mechanical Attrition Treatment on Fretting Wear Behaviour of Ti-6Al-4V

Ti-6Al-4V alloy was subjected to surface mechanical attrition treatment (SMAT) by using SAE 52100 steel balls of 5 mm diameter for two treatment durations (30 and 60 min). SMAT resulted in the formation of nanostructured material on the surface and near surface regions, increased hardness, increased surface roughness and compressive residual stress on the surface. Treated samples exhibited lower tangential force coefficient (TFC) compared to untreated samples. Samples treated for 60 min exhibited higher grain refinement, higher hardness, lower surface roughness and higher TFC compared to the samples treated for 30 min. Fretting wear resistance of the samples treated for 30 min was higher than that of untreated samples and the samples treated for 60 min. Due to very high hardness and presumably reduced ductility, the fretting wear resistance of the samples treated for 60 min was lower than that of the untreated samples and samples treated for 30 min.

Performance of alumina coatings prepared by hard anodizing, micro arc oxidation and detonation spray processes on Al–Mg–Si alloy under fretting wear loading

Hard anodizing, micro arc oxidation and detonation spraying of alumina were done on AA 6063 substrate. Fretting wear tests were done at different normal loads and a constant displacement of 55 µm for 50,000 cycles at 5 Hz. The micro arc oxidation and detonation spray coated samples exhibited lower coefficient of friction compared to hard anodizing coated and uncoated samples. Both micro arc oxidation and detonation sprayed samples showed superior fretting wear resistance compared to hard anodizing and uncoated samples. However, hard anodizing coated samples exhibited inferior fretting wear resistance compared to uncoated samples due to cracking and fragmentation of brittle and hard anodized layer.