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IIT Jodhpur

Department of Metallurgical & Materials Engineering

International Journal of Fatigue

A Back-stress deconvolution method towards modelling of low cycle fatigue behavior of type 316L(N) stainless steel at room temperature

Controlling the elastic modulus of simple synthetic hydrogels like polyacrylamide is essential to their use in many areas of biotechnology, including tissue engineering, medical device development, and drug delivery applications. Indentation-based methods for measuring hydrogel elastic moduli are preferred over measurements in shear rheometers or tensile testing instruments when the freedom to choose sample volume and shape are restricted; contact lenses represent such an example. It is often believed that the local application of indentation loads will volumetrically compress hydrogels, increasing the sample's polymer concentration even when the applied pressure is less than the hydrogel's osmotic pressure. Here, we test this idea by volumetrically compressing polyacrylamide hydrogels of different compositions while measuring the degree of compression with increasing applied pressure. Our results reveal that at applied pressures below the hydrogel osmotic pressure, the gels exhibit only marginal compression, while above the osmotic pressure the gels compress as predicted by classical polymer physics theory. Combining measurements of osmotic pressure and polymer mesh size, we determine the scaling relationships between hydrogel composition, mesh size, and osmotic pressure. By demonstrating agreement between experiment and theory, we use our measurements to determine the Kuhn length of the individual polymer chains constituting the hydrogels. © 2020

World University root node

Biotribology

Hydrogel compression and polymer osmotic pressure

The simple shear response of soft polymers under large deformation (&gt;50%) and strain rates spanning 10 −3 – 10 3 s −1 is characterized by developing quasi-static and split-Hopkinson pressure bar based single-pulse dynamic simple shear experiments rooted in continuum mechanics fundamentals. Cross-linked polydimethylsiloxane (PDMS) is chosen as a model material. By examining the evolution of stress, strain and strain rate, the latter two parameters measured using two-dimensional digital image correlation (DIC), it is demonstrated that dynamic simple shear deformation consists of four distinct stages: momentum diffusion, inertia effect, steady-state material response, and strain rate decay. By isolating the unsteady and steady-state deformation stages, inertia-free material response is captured under a uniform strain rate. It is shown that the shear response of PDMS is nearly linear with a weakly rate-sensitive shear modulus in the investigated strain rate range. Further, by analyzing the DIC strain-field and comparing the kinematic experimental results with those predicted by classical continuum mechanics, it is demonstrated that the proposed experiments not only achieve a nearly theoretical simple shear state that is uniform across the specimen, but also allow for post-test validation of individual experiments based on these criteria. © 2019, Society for Experimental Mechanics.

Experimental Mechanics

Quasi-Static and High Strain Rate Simple Shear Characterization of Soft Polymers

Functionally graded materials offer potential as a solution to the premature failure of thermal barrier coatings due to thermal residual stresses, however this solution has not been fully evaluated to date. An analytical methodology based on the bending of plates has been used to model the residual stress distribution through the layers of a functionally graded (FG) cermet top coat-bond layer-substrate system after cooling from a high deposition temperature. The model is then applied to two material systems with different trends in variation in the coefficient of thermal expansion from the substrate to the top coat layer. Increasing the number of layers reduces the discrete stress variations across the interfaces through the FG layers and reduces the propensity of interfacial cracking. While the stress in the top coat is always compressive for the Inconel substrate, the stress in the top coat is tensile in nature for the steel substrate when the ceramic volume fraction in the functionally graded layers is low. The stress in the top coat gradually becomes compressive with an increasing volume fraction of ceramic in the functionally graded top coat. The curvature of the TBC system follows a similar trend and changes its sign with increasing ceramic content for the steel substrate. The effect of the number of layers on the curvature gradually diminishes and the curvature becomes constant beyond only a few layers. © 2018

Fulltext

Mechanics of Materials

Residual stresses in functionally graded thermal barrier coatings

The principle of elastic strain energy minimization is used to predict the possible ranges of allowable lattice misfit for the formation of coherent and semi-coherent interfaces between a Ni superalloy substrate and a cubic yttria stabilized zirconia (c-YSZ) ceramic coating. Depending upon the elastic modulus, failure strength in compression and tension, and the Burgers vector of the film material, the magnitude of allowable lattice misfit can vary over a wide range. It is shown that compressive stresses as high as several gigapascals may exist at the coherent YSZ films, and a higher positive lattice misfit can sometimes be more favorable for the formation of a semi-coherent interface than is a lower negative lattice misfit. Both coherency and the effect of lattice misfit on the interfacial stress decrease with an increase in the thickness of the film. The effect of lattice misfit on the evolution of interfacial stresses during film deposition must be considered during the design of dissimilar materials interfaces and for the prediction of interfacial delamination and the fracture of films. © 2018

Surfaces and Interfaces

On the evolution of stresses due to lattice misfit at a Ni-superalloy and YSZ interface

An analytical model was developed to determine the stress distribution over thickness for a multilayered thermal barrier coating (TBC) system deposited within a cylindrical reaction vessel. Temperature dependent material properties were used to estimate the stress values. It was found that, even for small lattice misfits, very high compressive elastic stresses could exist at the ceramic-bond coat interface immediately after deposition. Furthermore, this interfacial stress could ultimately relax to a lower value with increasing film thickness by dislocation nucleation. In the presence of a Thermally Grown Oxide (TGO), however, a tensile stress was generated within the oxide layer and discrete changes in stress profile were predicted at ceramic-TGO and TGO-bond coat interfaces. While the stress-change was higher at the ceramic-TGO interface for a high deposition temperature, the change was greater at the TGO-bond coat interface at a lower deposition temperature. A high compressive stress was predicted within the TGO layer upon cooling down the TBC system to room temperature and the stress-change was highest at the TGO-bond coat interface. Finally, when the TGO layer was subjected to fatigue loading under compressive mean stress during thermal cycling, the model predicted that the internal pressure of the cylindrical vessel reduces the magnitude of mean stress and increases the stress-range in the thermal stress cycle. The effects of evolved stresses on the context of interfacial failure of TBCs should provide fundamental insight into material selection and component design. © 2018

Modeling of thermal and lattice misfit stresses within a thermal barrier coating

The failure of a bearing-raceway assembly is governed by the spatial distribution of subsurface stresses at the vicinity of a bearing-raceway contact and the evolution of these stresses during rolling contact fatigue (RCF) loading. In this paper, we propose an experimental methodology that allows one to accurately measure the location and magnitude of the cyclically evolving elastoplastic von Mises stresses in terms of microhardness numbers. An M50NiL steel rod is subjected to RCF by three silicon nitride (Si3N4) balls for over several hundred million cycles at 5.5 GPa contact stress level. Microindentation hardness measurements within the subsurface RCF-affected regions of the rod revealed significant material hardening. A mechanistic methodology to construct a stress-life (S-N) diagram for RCF loading is proposed. S-N diagrams are constructed based on maximum von Mises stress amplitude and volume average von Mises stress amplitude. The effects of elastic modulus and yield strength gradient on stress fields are also considered in this analysis. Comparison of S-N diagrams based on both stress amplitudes indicates that the maximum von Mises stress amplitude overpredicts the fatigue strength of material in S-N diagrams. The experimental results obtained by following this methodology can help construct material hardening models for RCF, which may lead to an improved estimate of bearing fatigue life. Copyright © 2017 by ASTM International

Materials Performance and Characterization

A new approach towards life prediction of case hardened bearing steels subjected to rolling contact fatigue

Microstructural and mechanical characterization investigations on three variants of a through-hardened M50 bearing steel are presented to compare and contrast their performances under rolling contact fatigue (RCF) loading. Baseline (BL) variant of M50 steel bearing balls is subjected to: (i) a surface nitriding treatment and (ii) a surface mechanical processing treatment, to obtain distinct microstructures and mechanical properties. These balls are subjected to RCF loading for several hundred million cycles at two different test temperatures, and the subsequent changes in subsurface hardness and compressive stress-strain response are measured. It was found that the RCF-affected subsurface regions grow larger in size at higher temperature. Micro-indentation hardness measurements within the RCF-affected regions revealed an increase in hardness in all the three variants. The size of the RCF-affected region and intensity of hardening were the largest in the BL material and smallest in the mechanically processed (MP) material. Based on Goodman's diagram, it is shown that the compressive residual stress reduces the effective fully reversed alternating stress amplitude and thereby retards the initiation and evolution of subsurface plasticity within the material during RCF loading. It is quantitatively shown that high material hardness and compressive residual stress are greatly beneficial for enhancing the RCF life of bearings. Copyright © 2016 by ASME.

Journal of Engineering Materials and Technology, Transactions of the ASME

Influence of Residual Stress and Temperature on the Cyclic Hardening Response of M50 High-Strength Bearing Steel Subjected to Rolling Contact Fatigue

A combined experimental and numerical method is developed to estimate the continuously evolving cyclic plastic strain amplitudes in plastically deformed subsurface regions of a case-hardened M50 NiL steel rod subjected to rolling contact fatigue (RCF) over several hundred million cycles. The subsurface hardness values measured over the entire plastically deformed regions and the elastoplastic von Mises stresses determined from the three-dimensional (3D) Hertzian contact finite element (FE) model have been used in conjunction with Neuber's rule to estimate the evolved cyclic plastic strain amplitudes at various points within the RCF-affected zone. The cyclic stress-strain plots developed as a function of case depth revealed that cyclic hardening exponent of the material is greater than the monotonic strain-hardening exponent. Effective S-N diagram for the RCF loading of the case-hardened steel has been presented and the effect of compressive mean stress on its fatigue strength has been explained using Haigh diagram. The compressive mean stress correction according to Haigh diagram predicts that the allowable fatigue strength of the steel increases by a factor of two compared to its fatigue limit before mean stress correction, thus potentially allowing the rolling element bearings to operate over several hundred billion cycles. The methodology presented here is generalized and can be adopted to obtain the constitutive response and S-N diagrams of both through- and case-hardened steels subjected to RCF. Copyright © 2015 by ASME.

Journal of Tribology

Cyclic constitutive response and effective S-N diagram of M50 NiL case-hardened bearing steel subjected to rolling contact fatigue

The nanoindentation technique was employed to examine the strain rate sensitivity, m, and its dependence on the structural state of a Zr-based bulk metallic glass (BMG). The free volume content in the BMG was varied by examining samples in the as-cast (AC), shot-peened (SP), and structurally relaxed (SR) states. Hardness values measured at different loading rates and over a temperature range of 300-423. K as well as the strain-rate jump tests conducted in the quasi-static regime at room temperature, show that m is always negative. All the load-displacement (. P-. h) curves in this temperature regime exhibit serrated load-displacement responses, indicating that the shear band mediated inhomogeneous plastic flow governs deformation. Such localization of flow and associated softening is the raison d'être for the negative m. Significant levels of pile-up around the indents were also noted. The order in the average values of hardness, pile-up heights, and the displacement bursts on the P-. h curves was always such that SR>AC>SP, which is also the order of increasing free volume content. These observations were utilized to discuss the reasons for the negative strain rate sensitivity, and its dependence on the structural state of metallic glasses. It is suggested that the positive values of m reported in the literature for them are possibly experimental artefacts that arise due to large pile ups around the indents which lead to erroneous estimation in hardness values. © 2014 Elsevier B.V.

Materials Science and Engineering A

On the strain rate sensitivity of plastic flow in metallic glasses

Accelerated rolling contact fatigue (RCF) experiments on M-50 NiL case carburized bearing steel were conducted using a three-ball-on-rod tester at high contact stress levels (5.5 GPa) and over several hundred million contact cycles to investigate the evolution of plastic deformation zone characteristics. A microindentation technique was used to measure the changes in the material hardening and softening characteristics within the RCF affected zone and represent the changes in the form of microhardness maps. These maps revealed regions of considerable material hardening inside the plastic zone and softening of material outside the hardened region. The results indicate large amount of plastic strain accumulation beneath the contact surface via ratcheting due to RCF cycling. The hardened and softened regions continue to evolve in severity and size with increasing number of RCF cycles. Using the Hertzian contact theory, a rationale for the evolution of hardened and softened regions during RCF loading is provided. The entire plastic zone is shown to be dominated by four different principal stress states which contribute to the evolution of these regions during RCF. © 2013 Elsevier Ltd. All rights reserved.