A. Nagesha

Effect of mean stress and stress amplitude on the ratcheting behaviour of 316LN stainless steel under dynamic strain aging regime

Abstract The influence of dynamic strain aging (DSA) on the ratcheting behaviour of 316LN stainless steel was investigated at 823K as a function of mean stress (σm) and stress amplitude (σa). Test results obtained under different combinations of σm – σa were analysed in order to arrive at a map delineating different deformation regimes viz. ratcheting, strain burst and elastic shakedown. It was shown that the synergistic effect of σm and σa can be described by the ratio of mean stress and stress amplitude (σm/σa). A critical value of this ratio was identified to mark the transition between ratcheting and elastic shakedown and the same was used to predict deformation behaviour of the material in DSA under asymmetrical loading.

On the Dynamic Strain Aging Effects during Elevated Temperature Ratcheting of Type 316LN Stainless Steel

Abstract Ratcheting experiments were carried out on type 316LN stainless steel (SS) at different temperatures in the range, 300–923 K using different mean stress (σm) – stress amplitude (σa) combinations. Occurrence of dynamic strain aging (DSA) in the intermediate temperature range led to an anomalous temperature dependence of ratcheting strain accumulation. While the peak DSA temperature was found to be 823 K for all mean stress-stress amplitude combinations, the temperature regime of occurrence of DSA was found to depend on the σm–σa combinations used.

A Perspective on Fatigue Damage by Decoupling LCF and HCF Loads in a Type 316LN Stainless Steel

Abstract Prior low cycle fatigue (LCF) deformation in a 316LN austenitic stainless steel reduced the remnant high cycle fatigue (HCF) life as a function of the amount of LCF exposure and the applied strain amplitude. A critical LCF pre-damage was found necessary for an effective LCF-HCF interaction to take place.

Generation of Constant Life Diagram under Elevated Temperature Ratcheting of 316LN Stainless Steel

Abstract Combined influence of mean stress and stress amplitude on the cyclic life under elevated temperature (823–923 K) ratcheting of 316LN austenitic stainless steel is discussed. Constant life Haigh diagrams have been generated, using different combinations of stress amplitude and mean stress. In the plastic domain, the allowable stress was found to increase or decrease with mean stress depending on the temperature and combination of mean stress – stress amplitude employed. Strong influence of dynamic strain aging (DSA) was found at 823 K which affected the mode of deformation of the material in comparison with 923 K. Failure mode expressed through a fracture mechanism map was found to change from fatigue to necking depending on the test temperature as well as combinations of mean stress and stress amplitude. Occurrence of DSA at 823 K proved to be beneficial by way of extending the safe zone of operation to higher R-ratios in comparison with 923 K.

Manifestations of dynamic strain aging under low and high cycle fatigue in a type 316LN stainless steel

Abstract Influence of Dynamic strain aging (DSA) under low cycle fatigue (LCF) and high cycle fatigue (HCF) loading was investigated by conducting LCF and HCF tests on specimens over a wide range of temperature from 573 to 973 K. DSA was found to be highly pronounced in the temperature range of 823–873 K. DSA was seen to have contrasting implications under LCF and HCF deformation. The cyclic hardening owing to DSA caused an increase in the cyclic stress response under LCF, leading to decrease in cyclic life. On the other hand, the DSA-induced strengthening suppressed the crack initiation phase under HCF where the applied stress remains fixed, leading to an increase in the cyclic life.

Assessment of surface relief and short cracks under cyclic creep in a type 316LN austenitic stainless steel

Formation of surface relief and short cracks under cyclic creep (stress-controlled fatigue) in type 316LN stainless steel was studied at temperatures ranging from ambient to 923 K using scanning electron microscopy technique. The surface topography and crack distribution behaviour under cyclic creep were found to be strong functions of testing temperature due to the difference in strain accumulation. At 823 K, surface relief mainly consisted of fine slip markings due to negligible accumulation of strain as a consequence of dynamic strain ageing (DSA) which led to an increase in the cyclic life. Persistent slip markings (PSM) with distinct extrusions containing minute cracks were seen to prevail in the temperature range 873–923 K, indicating a higher slip activity causing higher strain accumulation in the absence of DSA. Besides, a large number of secondary cracks (both transgranular and intergranular) which were partially accentuated by severe oxidation, were observed. Extensive cavitation-induced grain boundary cracking took place at 923 K, which coalesced with PSM-induced transgranular cracks resulting in failure dominated by creep that in turn led to a drastic reduction in cyclic life. Investigations on the influence of stress rate were also carried out which underlined the presence of DSA at 823 K. At 923 K, lowering the stress rate caused further strengthening of the contribution from creep damage marked by a shift in the damage mechanism from cyclic slip to diffusion.

Influence of Casting Defects on S–N Fatigue Behavior of Ni-Al Bronze

Abstract Nickel-aluminum bronze (NAB) alloys have been used extensively in marine applications such as propellers, couplings, pump casings, and pump impellers due to their good mechanical properties such as tensile strength, creep resistance, and corrosion resistance. However, there have been several instances of in-service failure of the alloy due to high cycle fatigue (HCF). The present paper aims at characterizing the casting defects in this alloy through X-ray radiography and X-ray computed tomography into distinct defect groups having particular defect size and location. HCF tests were carried out on each defect group of as-cast NAB at room temperature by varying the mean stress. A significant decrease in the HCF life was observed with an increase in the tensile mean stress, irrespective of the defect size. Further, a considerable drop in the HCF life was observed with an increase in the size of defects and proximity of the defects to the surface. However, the surface proximity indicated by location of the defect in the sample was seen to override the influence of defect size and maximum cyclic stress. This leads to huge scatter in S–N curve. For a detailed quantitative analysis of defect size and location, an empirical model is developed which was able to minimize the scatter to a significant extent. Further, a concept of critical distance is proposed, beyond which the defect would not have a deleterious consequence on the fatigue behavior. Such an approach was found to be suitable for generating S–N curves for cast NAB.

On the Anomalous Coffin-Manson Behavior Observed under Elevated Temperature Ratcheting in Type 316LN SS

Ratcheting is the progressive directional accumulation of deformation due to asymmetric loading in structures. Coffin-Manson plots derived from ratcheting experiments conducted at temperatures over the range, 823-923 K showed anomalous behavior at 873 K and 923 K in the form of dual slope and positive slope respectively, which was attributed to a change in the deformation mechanism during ratcheting in the above temperature domain. This was also reflected in the transition in the fracture mode from fatigue to creep at 873 and 923 K.