Krishna Dutta

Influence of Heat Treatment on Ratcheting Fatigue Behavior and Post Ratcheting Tensile Properties of Commercial Aluminum

The objective of this work is the study of ratcheting fatigue behavior and post fatigue tensile properties of the commercial aluminum via heat treatment like annealing and normalizing. In view of this, stress controlled fatigue tests carried out at room temperature up to 150 cycles under different combinations of mean stress and stress amplitude using a servo-hydraulic testing machine. It is found that in both annealed and normalized conditions, accumulation of ratcheting strain increases with increasing stress parameters. Strain accumulation in normalized condition is less due to increased dislocation density during asymmetric cyclic loading. In normalized, the saturation in strain accumulation was also achieved earlier as compared to annealed due to attainment of stable dislocation configuration. It has been noticed that the Post fatigue tensile strength enhances while the ductility decreases with increasing stress parameters because of cyclic hardening. Fractographic studies have demonstrated that dimple size decreases with increasing strain accumulation.

Fatigue life estimation in presence of ratcheting phenomenon for AISI 304LN stainless steel tested under uniaxial cyclic loading

This investigation aims to describe the experimental ratcheting life of AISI 304LN stainless steel with a proposed model. A series of stress-controlled low cycle fatigue experiments have been carried out at room temperature under uniaxial loading on the steel up to failure of specimens, under three sets of stress amplitude and mean stress values. Comparison of the theoretically predicted results with the experimental ones is found to be reasonably satisfactory in the fatigue life range of 102–105 cycles. Additionally the proposed stress-based model for predicting ratcheting fatigue life has been critically discussed with reference to some previous models.

Evolution of Dislocation Density During Tensile Deformation of BH220 Steel at Different Pre-strain Conditions

Tensile behavior of BH220 steel with different pre-strain conditions (2 and 8%) followed by bake hardening was studied at different strain rates (0.001 and 0.1/s). Dislocation densities of the deformed specimens were successfully estimated from x-ray diffraction profile analysis using the modified Williamson-Hall equation. The results indicate that other than 2% pre-strain the dislocation density increases with increase in pre-strain level as well as with strain rate. The decrease in the dislocation density in 2% pre-strain condition without any drop in strength value is attributed to the characteristic dislocation feature formed during pre-straining.

Ratcheting Behavior of a Non-conventional Stainless Steel and Associated Microstructural Variations

Ratcheting fatigue behavior of a non-conventional stainless steel X12CrMnNiN17-7-5 has been investigated with varying combinations of mean stress (σm) and stress amplitude (σa) at room temperature using a servo-hydraulic universal testing machine. X-ray diffraction profile analysis has been carried out for assessing possible martensitic phase transformation in the steel subjected to ratcheting deformation. The results indicate that ratcheting strain as well as volume fraction of martensite increases with increasing σm and/or σa; the phenomenon of strain accumulation is considered to be governed by the associated mechanics of cyclic loading, increased plastic damage as well as martensitic transformation. A correlation between strain produced by ratcheting deformation and martensitic transformation has been established.

Deformation Fatigue and Fracture vis-a-vis Deformation Induced Martensite in AISI 304LN Stainless Steel

Understanding of deformation, fracture or fatigue behaviour of AISI 304LN grade stainless steel with reference to in-situ evolution of deformation induced martensite (DIM) is important for the structural integrity of numerous critical engineering components made of this steel. The primary objective of this report is to present a concise overview on the state-of-the-art of these aspects based on a series of investigations by the authors and their co-workers through over more than a decade. The major experiments involved are determination of tensile, fatigue and fracture behaviour of the steel using standard testing procedures. The associated structural and sub-structural changes in the deformation volume or at local regions such as fracture surfaces or crack tips are characterized. The nature and amount of DIM have been detected through microstructural analysis, X-ray diffraction, hardness measurement, ferrofluid based technique, ferritoscope assessment and TEM, in addition to extensive fractographic analysis by SEM. The major highlights of the investigations centre on revelations of the role of DIM on tensile deformation of 304LN stainless steel at various strain rates and temperatures, illustrating the association of DIM with constrained and unconstrained deformation ahead of crack tips in monotonic and cyclic fracture tests, and examination of the extent of DIM transformation during stress controlled and strain controlled cyclic loading and fatigue crack growth, with an underlying theme of continuously emphasizing the nature, location and amount of DIM formed.

Static and Dynamic Behavior of Fibrous Polymeric Composite Materials at Different Environmental Conditions

The study of static and dynamic behavior of environmentally conditioned fibre reinforced polymeric (FRP) composites is necessary and crucial to examine the durability, reliability and sustainability of these noble materials. FRP composites are being used all around the globe and substituting the conventional materials, starting from mini toys to large aerospace components. Present review has introduced to accumulate and understand the disseminate literature in concentrating the significance of understanding the static and dynamic behavior of FRPs with changing environmental conditionings (hygrothermal, low and high temperature, salt solution, freeze thaw, UV light) and with the interaction of different nano-fillers. Their stability and integrity in diverse service environments may be reformed by their reactions against different nature of loadings i.e. static or dynamic and the components such as fibre, matrix and fibre/matrix interfaces in those environments. The static and dynamic states of loading may come with a possible weaker region to encounter the durability and integrity of the composites. To understand the exact failure modes that correlates the position of environmentally conditioned interfaces and dynamic state of loadings, thus confusing the estimation of its overall performance and mechanical behavior. Interface reliability and durability is vital since in-service environments the degradation in the interfacial region leads to complete composite failure. Therefore, the study of combined effects of various in-service environmental conditions and the role of static and dynamic behavior on the interface will be a crucial part related to the multiaxial dynamic states of failures occurring in FRP’s.

Low Cycle Fatigue Life Prediction of Al–Si–Mg Alloy Using Artificial Neural Network Approach

The aim of this investigation is to develop a model to predict low cycle fatigue (LCF) life of Al–Si–Mg based alloys and establish a correlation between some important processing parameters and LCF life of the investigated alloy. A most popular statistical analysis tool known as artificial neural network model based on multilayer feedforward neural network has been used in this prediction approach. For accurate prediction of fatigue life, a large dataset has been created by collecting the input–output pairs of the experimental results from existing literature. The effects of various processing parameters such as Si content, Mg content, heat treatments, etc. on LCF life have also been predicted using the created network. The predicted results indicate that the fatigue life increases with increase in both Si and Mg content in the alloy; the results are in accordance with some experimental observations available in literature. It is also predicted that fatigue life, which increases with decreasing strain amplitude, was shifted towards the higher number of cycles to failure under T6 heat treatment condition than under both T5 and some modified T6 heat treatment conditions. Similar conclusions are also drawn for experimental results as reported in some literature. The life predictive capability of the created network shows a good acceptability as most of the predicted results lies within a factor of 2.

Ratcheting fatigue behaviour of Al-7075 T6 alloy: Influence of stress parameters

The use of aluminium and aluminium based alloys are increasing rapidly on account of its high formability, good thermal and electrical conductivity, high strength and lightness. Aluminium alloys are extensively used in aerospace, automobile, marine and space research industries and are also put into structural applications where chances of fatigue damage cannot be ruled out. In the current work, it is intended to study the ratcheting fatigue behavior of 7075-T6 aluminium alloy at room temperature. This Al alloy is potentially used in aviation, marine and automotive components as well as in bicycle parts, rock mounting equipment and parts of ammunition where there is every chance of failure of the parts due to deformation caused by ratcheting. Ratcheting is the process of accruement of plastic stain produced when a component is subjected to asymmetric cyclic loading under the influence of low cycle fatigue. To accomplish the requirements of the projected research, stress-controlled cyclic loading experiments were done using a ±250 kN servo-hydraulic universal testing machine (Instron: 8800R). The effect of stress parameters such as mean stress and stress amplitude were investigated on the ratcheting behavior of the selected aluminium alloy. It was observed that, ratcheting strain increased with increase in the value of stress amplitude at any constant mean stress while a saturation in strain accumulation attained in the investigated material after around 10-20 cycles, under all test conditions. The analyses of hysteresis loop generated during cyclic loading indicate that the material exhibits cyclic hardening in the initial fifty cycles which gets softened in further loading up to about 70-80 cycles and finally attains a steady state. The increase in the ratcheting strain value with stress parameters happens owing to increased deformation domain during cycling. The cyclic hardening accompanied by softening is correlated with characteristic precipitation features of the investigated Al 7075 alloy.

Stress Ratio Effect on Ratcheting Behavior of AISI 4340 Steel

Ratcheting is known as accumulation of plastic strain during asymmetric cyclic loading of metallic materials under non-zero mean stress. This phenomenon reduces fatigue life of engineering materials and thus limits the life prediction capacity of Coffin-Manson relationship. This study intends to investigate the ratcheting behavior in AISI 4340 steel which is mainly used for designing of railway wheel sets, axles, shafts, aircraft components and other machinery parts. The effect of stress ratio on the ratcheting behaviour in both annealed and normalised conditions were investigated for investigated steel. Ratcheting tests were done at different stress ratios of -0.4, -0.6 and -0.8. The results showed that the material responds to hardening behavior and nature of strain accumulation is dependent on the magnitude of stress ratio. The post ratcheted samples showed increase in tensile strength and hardness which increases with increasing stress ratio and these variations in tensile properties are correlated with the induced cyclic hardening.

On the comparative assessment of ratcheting-induced dislocation density in 42CrMo4 steel by X-ray diffraction profile analysis and hardness measurement

ABSTRACT The phenomenon of ratcheting occurs under the influence of non-zero mean stress during cyclic loading; it singificantly reduces the low cycle fatigue life of engineering structures. The present investigation deals with a detailed comparison on the estimation of dislocation densities in the 42CrMo4 steel induced by ratcheting using two different methods, i.e. X-ray diffraction (XRD) profile analysis and hardness. The dislocation densities in the ratcheted specimens were assessed using XRD profile analysis following the modified Williamson–Hall method as well as hardness measurements following the modified Nix and Gao model. The results showed that dislocation density increased in the ratcheted specimens as compared to the unratcheted ones and increase in accumulation of ratcheting strain was correlated with the increase in dislocation densities in the ratcheted specimens. It was established that both hardness and X-ray diffraction profile analysis methods can very effectively be used to assess the dislocation densities in the ratcheted specimens.