S. Sivaprasad

True stress-controlled ratcheting behavior of 304LN stainless steel

Cyclic plastic deformation response of materials under asymmetric stress cycling is known as ratcheting. Combined effect of fatigue and permanent tensile strain accumulation results in early failure of materials during ratcheting. For this reason, ratcheting should be emphasized in the safety assessment and life estimation of engineering structures. Engineering and true stress-controlled ratcheting behavior of 304LN stainless steel has been carried out at room temperature. Effects of stress amplitude, mean stress, and their histories (i.e., step loading) on the ratcheting behavior are analyzed in this investigation. It is noticed that under true stress-controlled ratcheting experiments, ratcheting life increases in presence of mean stress, and hysteresis loop area and plastic strain energy decreases with the increasing mean stress. A comparison has also been drawn in between the true and engineering stress-controlled tests, and massive differences in ratcheting life and strain accumulation were found. Ratcheting strain accumulation ceases in descending step loading, is noticed in this work.

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.

Local Ratcheting Response in Dissimilar Metal Weld Joint: Characterization Through Digital Image Correlation Technique

Ratcheting fatigue behavior of different alloys and their weld joints has been a topic of interest in the past few decades. However, little information is available about the ratcheting response in different zones, when a composite weld section is subjected to asymmetric loading cycle. This work aims at understanding the local ratcheting response in various zones of a dissimilar metal weld (DMW) joint. Digital image correlation technique has been used to measure local ratcheting strain components of a DMW joint. Accumulation of ratcheting strain in various zones of a DMW joint was found to be different, harder region accumulating negligible and softer region accumulating larger local ratcheting strain. Fatigue crack initiation or neck formation occurred in soft region owing to high ratcheting strain accumulation.

A Simplified Procedure to Determine Post-necking True Stress–Strain Curve from Uniaxial Tensile Test of Round Metallic Specimen Using DIC

Triaxial state of stress is usually generated in the necked zone because of neck geometry, and as a consequence proper correction of true tensile stress–strain curve after necking is mandatory. Various correction factors like Bridgman, Davidenkov and Spiridonova, Siebel and Schwaigere are available in the published literature to calculate true stress from mean axial stress. Similarly true strains can be calculated from the minimum diameters of the round specimen in the necked region for various true stress levels. But experimental determination of correction factors and minimum diameters of the round specimen in the necked region is a cumbersome task. This investigation shows a simplified procedure to determine true strains and the correction factors from digital image correlation-based local strain measurement in the necked region. The present procedure is validated by experimental results of rail steel.

Identification of Post-necking Tensile Stress–Strain Behavior of Steel Sheet: An Experimental Investigation Using Digital Image Correlation Technique

The stress–strain behavior of sheet metal is commonly evaluated by tensile test. However, the true stress–strain curve is restricted up to uniform elongation of the material. Usually, after the uniform elongation of the material the true stress–strain is obtained by extrapolation. The present work demonstrates a procedure to find out the true tensile stress–strain curve of the steel sheet after necking using digital image correlation (DIC) technique. Hill’s normal anisotropic yield criteria and local strains measured by DIC technique are used to correct the local stress and strain states at the diffuse necked area. The proposed procedure is shown to successfully determine the true tensile stress–strain curve of ferritic and dual-phase steel sheets after necking/uniform elongation.