N. Narasaiah

The load dependence of hardness in alumina–silver composites

The room temperature Knoop micro-hardness properties of three alumina-silver composites vis-a-vis a monolithic alumina have been investigated at different indentation loads. The composites have been developed using conventional pressureless sintering at 1550°C and these consisted of 1.6, 3.38 and 5.01% by volume of silver particles. It has been shown that the measured Knoop hardness values of these materials sensitively depend on the applied load of indentation. But following an emerging analytical technique, the composites can be assigned some unique true hardness values, which are functions of their silver content.

INFLUENCE OF MICROSTRUCTURE ON WEAR RESISTANCE PARAMETER OF CERAMIC CUTTING TOOLS

This report examines the role of microstructure of a new type of cutting tool material on an existing relationship between its abrasion wear resistance, fracture toughness (K IC), and hardness (H). Three alumina–silver composites with different amounts of metal particles have been prepared, and their hardness and fracture toughness properties have been determined together with the assessment of their microstructural features such as volume fraction of the second phase, porosity, etc. The mechanical wear on the flanks of cutting tool inserts, made from the developed composites, has also been estimated by machining experiments against 0.45% carbon steel. The results indicate that flank wear resistance of these silver toughened ceramic cutting tool inserts is not proportional to an existing wear resistance parameter K IC 3/4 H 1/2. A modified relation between flank wear resistance, hardness, and fracture toughness has been suggested here for these cutting tool materials. The modification incorporates consideration of the volume fraction of the second phase and the porosity in the developed metal toughened ceramics.

Low Cycle and Ratchetting Fatigue Behavior of High UTS/YS Ratio Reinforcing Steel Bars

Cyclic deformation behavior of high UTS/YS rebars has been studied employing both symmetric strain-controlled and asymmetric stress-controlled cycles in an attempt to understand the influence of UTS/YS ratio on fatigue life. While strain-controlled cyclic deformation did not exhibit a pronounced influence of UTS/YS ratio, a substantial life enhancement is noted for the asymmetric stress-controlled cycle. Reasons for life enhancement were found to be due to the ratchetting strain development and the associated hardening behavior. An equivalent stress-based model has been used to predict both the symmetric and asymmetric fatigue lives of rebars.

Effect of aging on fatigue crack growth behaviour of Cu bearing HSLA-100 steel

Abstract The effect of aging on fatigue crack growth rate (FCGR) of Cu bearing HSLA-100 steel has been studied. The steel was solution treated, water quenched and aged at various temperatures in the range of 350–700°C. The fatigue crack growth resistance of the steel decreased for the initial stages of aging from 350–500°C. Further aging up to 650°C resulted in an improvement in the crack growth resistance. Beyond 650°C, once again an inferior crack growth resistance was observed. This nature of variation of FCGR behaviour was similar to the trend portrayed by the strength properties with aging treatment. The results are related to the changes in the microstructural constituents owing to the aging treatment.

Novel optimization technique to determine polarization characteristics of a corroding metal

Abstract Polarization data characterize the corrosion behavior of a metal giving a quick estimate of corrosion current density icorr and corrosion potential Ecorr. These two characteristics determine the corrosion rate and position of the metal in galvanic series. The chosen system for the study is steel (SS304) in NaCl solution. In these studies, icorr and Ecorr of Butler-Volmer equation are obtained by fitting the full expression to experimental current vs potential data unlike the graphical method using Tafelslopes. MATLAB optimization tool box is utilized for this purpose. The novel optimization technique is explained for determining Ecorr and icorr

Technique for direct measurement of effective fatigue threshold of materials

Abstract A new method for studying crack growth behaviour in the fatigue threshold regime is reported here. The method uses a simple rotating bending test set-up, instead of servohydraulic or electromagnetic resonance test systems, to determine the fatigue threshold of structural materials. The reliability of the new technique is demonstrated by determining fatigue threshold of a 0·62%C steel by this technique and comparing the results with those obtained from conventional tests following the procedure of ASTM standard E647. The suggested technique provides higher sensitivity of measurement for crack growth rate and directly gives the fatigue threshold value for the steel that is in good agreement with the effective fatigue threshold determined by conventional tests.

Effect of Flyash Treatment on the Properties of Al-6061 Alloy Reinforced with SiC–Al2O3–C Mixture

Abstract Studies are conducted on an Al-6061 alloy reinforced with in situ ceramic mixture (Al2O3–SiC–C). The mixture is prepared by carbothermal reduction of flyash in a plasma reactor, where SiO2 is converted to SiC. The strength values for Al-6061-flyash (FA) and Al-6061-plasma synthesized flyash (PSFA) composites are 35% and 63% higher than that of the Al-6061 alloy. Wear resistance of the developed alloy composite prepared with treated flyash (Al-6061-PSFA) has increased two-fold in comparison to virgin alloy (Al-6061) and 1.5 fold higher in comparison to a similar alloy composites prepared with untreated flyash (Al-6061-FA). The unique property of the developed alloy composite is due to harder SiC particle reinforcement. Excess carbon in the form of graphite and the unique morphological feature of microstructure have enhanced the properties such as strength and wear resistance of the developed alloy composite.

Comparative Assessment of Fatigue-Thresholds Estimated by Short and Long Cracks

The safe life design philosophy of structural components is based on the knowledge of the fatigue threshold for long cracks (ΔKth). The emergence of the concept of short cracks, its numerous experimental verification and the continued studies on its growth behaviour over the past years have brought forward challenges to the basis of the safe life design philosophy. However, short cracks interestingly exhibit either single or multiple-thresholds. But prior investigations related to the determination and examinations of fatigue threshold for short cracks (ΔKthsc) in structural materials are limited in number. In an earlier communication [1], the largest value amongst the detected thresholds associated with short cracks has been referred as “near long crack fatigue threshold” (NLFTH) and the magnitude of NLFTH has been hypothesized as close to the fatigue threshold obtained from long crack experiments as illustrated using Fig.1. This investigation aims to explore the magnitudes of ΔKthsc and ΔKth for a few steels, to determine the NLFTH values for these materials and finally to make a comparative assessment of NLFTH with their corresponding ΔKth values.