Initiation and Growth of Cracks Near Fatigue Threshold in Plain Carbon Steels

Fatigue crack initiation, short crack growth behaviour and fatigue thresholds of short and long cracks of a few plain carbon steels (0.003%C, 0.14%C, 0.25%C, 0.47%C and 0.62%C steels) have been studied. The phenomena of the formation of small cracks in 0.003%C (single phase) and 0.14%C (multiphase) steels have been studied on dumble-shaped plate type specimens under varied cyclic stress amplitudes at the load ratio of R = 0. Simple experimental techniques have been developed with new specimen configurations to examine short crack growth behaviour vis-à-vis microstructural features of materials and to determine the fatigue thresholds using a rotating bending machine. The fatigue thresholds have been estimated by load drop procedure and each crack profile is recorded together with the associated microstructures during these studies. The microstructures of the investigated steels have been characterized using optical and scanning electron microscopes. The influence of the microstructural features of these steels on the near threshold crack growth has been examined. Conventional mechanical properties like hardness and tensile parameters have been determined following standard procedures. Studies on crack initiation revealed that ferrite-pearlite interfaces and/or ferrite-ferrite grain boundaries are preferred crack initiation sites in the investigated steels. The lengths of the initiated cracks at ferrite-pearlite interfaces or at grain boundaries are usually larger in size compared to those of grain body cracks. The formation of slip band inside the grain body, slip band impingement at grain boundary and elastic-plastic incompatibility are attributed as the main causes for crack initiation. The formation of irregular voids inside slip bands, initiation and growth of small voids at grain boundaries and subsequent joining of these are some interesting observations for fatigue crack initiation in 0.003%C steel. It is observed here for the first time that the angle between the direction of banding and the loading axis, has pronounced effect on the orientation and on the size of initiated cracks. Studies on short crack growth have demonstrated that the developed technique (with new specimen configuration) can be used satisfactorily to understand short crack growth behaviour and to determine the transition length from short to long xviii cracks. The estimated critical crack lengths can be considered as function of the nature of the short cracks. Short crack growth rate decreases or gets arrested with increasing crack length under the same applied crack driving force. The fatigue threshold values obtained from short crack growth experiments are found to lie in a narrow range and are closer to long crack thresholds. The cracks were found to have affinity to grow through interfaces, but the length of a crack passing through a specific phase was found to be approximately proportional to its amount. The developed technique to determine fatigue thresholds using a rotating bending machine yielded reliable results. The fatigue thresholds of four steels with carbon contents 0.003, 0.25, 0.47 and 0.62% were found to be 3.8, 4.2, 5.8 and 6.0 MPam1/2 respectively. The developed procedure is based on the measurement of da/dN with high sensitivity of the order of 10-12 m/cycle and the obtained threshold values appear to represent effective fatigue thresholds determined by conventional technique. Analysis of the crack growth behaviour near the notch indicates occurrence of non-propagating cracks only in 0.25%C and 0.47%C steels. The occurrence of nonpropagating cracks has been attributed to the presence of some secondary stress field at the crack tip. Fatigue threshold at R = -1 for plain carbon steels increases with increasing carbon content or the harder pearlitic constituent. A new model for short fatigue crack growth has been proposed to overcome some of the limitations of the earlier models. The model accounts the effect of mixed mode loading and the surface characteristics of a specimen, and describes both short and long crack growth. A series of computations have been carried out to examine the influence of different parameters considered in the model to describe short crack growth rate. The generated experimental results as well as some reported results have been satisfactorily described using the proposed model

Small crack formation in a low carbon steel with banded ferrite–pearlite structure

The phenomenon of the formation of small cracks in a banded plain carbon steel has been studied on dumble-shaped plate type specimens under varied cyclic stress amplitudes at the load ratio of R = 0. The locations at which the cracks were found to nucleate could be classified as: (i) ferrite–pearlite interface (FPI), (ii) ferrite–ferrite grain boundary (FFGB), (iii) ferrite grain body, and (iv) inclusion–matrix interface. The most preferred site for such crack nucleation in the investigated steel was found to be the ferrite–pearlite interface. The orientation of the initiated small cracks was found to vary widely between 0° and 90° with respect to the loading direction unlike some earlier reported results. It is reported here for the first time that the angle between the direction of banding and the loading axis has pronounced effect on the orientation of such small cracks. The lengths of these cracks at FPI and FFGB are found to be larger than the ones nucleated inside ferrite grain body. The preferred site of crack nucleation and the influence of the banding on the size and the orientation of the small cracks have been explained using inhomogeneous distribution of stress/strain in the microstructure and incompatible strains along the interfaces

Effect of Plastic Deformation on the Magnetic Properties 304 Stainless Steel During Tensile Loading

The present investigation addresses effect of tensile deformation on the magnetic properties of virgin 304SS as well as cold rolled samples containing a low volume fraction of 12% and 17% martensite. In-situ Non-destructive evaluation (NDE) techniques by magnetic Barkhausen emission (MBE) and magnetic hysteresis loop (MHL) measurement were used for evaluation of plastic deformation during tensile loading. Both the techniques indicated different stages of variation in magnetic properties with progressive plastic deformation. The trend of coercivity and Barkhausen measurements also throw light on the ductile and brittle fracture occurring in virgin and cold worked samples with validation using SEM fractography

Analysis of microstructure effects on edge crack of thin strip during cold rolling

Edge cracks in cold rolling of the thin strip affect the strip quality and productivity significantly. In this study, an experimental and mechanical investigation on microstructures has been carried out to study the edge crack formation during cold rolling of the thin strip. The effects of the feed material microstructures on the edge crack evolution were studied employing optical microscopy and scanning electron microscopy (SEM). Experimental observation indicates that fine grain occurs in hot-rolled microstructure and coarse grain is produced in ferritic rolled microstructure. Different grain sizes affect significantly the formation mechanics of the microcrack, crack initiation, and orientation of crack extension. The grain size and grain boundaries effects on crack retardation are discussed also during edge crack initiation. During the crack growth in coarse grain, most edge crack tips will blunt, which improves the crack toughness by causing less stress concentration. Overall, the fine microstructure shows a good crack initiation resistance, whereas the coarse microstructure has a better resistance to crack propagation. This research provides additional understanding of the mechanism of microstructure influence on edge crack evolution of cold strip rolling, which could be helpful for developing defect-free thin strip

Enhancer of Zeste Homolog 2 Induces Pulmonary Artery Smooth Muscle Cell Proliferation

Pulmonary Arterial Hypertension (PAH) is a progressively devastating disease characterized by excessive proliferation of the Pulmonary Arterial Smooth Muscle Cells (PASMCs). Studies suggest that PAH and cancers share an apoptosis-resistant state featuring excessive cell proliferation. The proliferation of cancer cells is mediated by increased expression of Enhancer of Zeste Homolog 2 (EZH2), a mammalian histone methyltransferase that contributes to the epigenetic silencing of target genes. However, the role of EZH2 in PAH has not been studied. In this study, it is hypothesized that EZH2 could play a role in the proliferation of PASMCs.In the present study, the expression patterns of EZH2 were investigated in normal and hypertensive mouse PASMCs. The effects of EZH2 overexpression on the proliferation of human PASMCs were tested. PASMCs were transfected with EZH2 or GFP using nucleofector system. After transfection, the cells were incubated for 48 hours at 37°C. Proliferation and cell cycle analysis were performed using flow cytometry. Apoptosis of PASMCs was determined using annexin V staining and cell migration was tested by wound healing assay.EZH2 protein expression in mouse PASMCs were correlated with an increase in right ventricular systolic pressure and Right Ventricular Hypertrophy (RVH). The overexpression of EZH2 in human PASMCs enhances proliferation, migration, and decrease in the rate of apoptosis when compared to GFP-transfected cells. In the G2/M phase of the EZH2 transfected cells, there was a 3.5 fold increase in proliferation, while there was a significant decrease in the rate of apoptosis of PASMCs, when compared to control.These findings suggest that EZH2 plays a role in the migration and proliferation of PASMCs, which is a major hallmark in PAH. It also suggests that EZH2 could play a role in the development of PAH and can serve as a potential target for new therapies for PAH

Initiation and growth of micro-cracks under cyclic loading

The early stages of fatigue damage causing nucleation of micro-cracks, followed by their growth and coalescence to macro-cracks have been studied in single and multi-phase steels. The effect of microstructure on initiation and growth of micro-cracks has been examined. The inter-relation between mechanics, mechanism, microstructure, and the phenomenology associated with the early stages of anomalous crack growth is the primary content of this investigation. The initiation of micro-cracks in single and multi-phase materials during cyclic loading, the growth of the short cracks, and the transition from short to long cracks in relation to the microstructures of a few commercial (304LN stainless, SA333, 0.003, 0.25 and 0.47% plain carbon) steels are discussed. It is found that formation of irregular voids inside slip bands, initiation and growth of small voids at the grain boundary and subsequent joining of these with other voids and splits, slip band impingement at grain boundary and elastic–plastic incompatibility synergistically influence the process of crack initiation. The fatigue threshold values obtained from short crack growth experiments are found to lie in a narrow range and are closer to long crack thresholds. The cracks were found to have affinity to grow through interfaces, but the length of a crack passing through a specific phase was found to be approximately proportional to its amount

Effect of crack depth on fracture toughness of 20MnMoNi55 pressure vessel steel

Ductile fracture behaviour of 20MnMoNi55 steel under quasi-static loading has been studied using single-edge notched bend specimens. To understand the influence of crack depth on fracture behaviour, J–R curves were obtained from specimens pre-cracked to various a/W in the range of 0.25–0.75, in steps of approximately 0.1. Stretch zone width dimensions were measured on the fractured surfaces of the broken specimens. The stretch zone dimensions that were determined have been used in conjunction with the experimentally derived J–R curves to obtain a value of the ductile fracture toughness parameter JSZW. The initiation toughness, Ji, at the intersection of the blunting line and the power-law fit to the J–R curve, and the critical toughness, Jc, following the ASTM standard, were estimated. Comparisons of JSZW with Ji and Jc have been made for 20MnMoNi55 steel as a function of the a/W ratio. The Ji and Jc values are found to be higher at lower a/W, i.e. under low constraint, and decrease with increasing a/W. It is observed that JSZW is not affected by a/W, and that its value is lower bound to Ji and Jc values of the material investigated

Report on failure analysis of connecting rod bolt

Failure analysis of connecting rod bolts has been carried out. The assessment as made through visual observation, scanning electron microscopic examination of the as-failed /impact fracture surfaces and microstructure. It has been found that the fracture surfaces of failed bolts reveal inter-granular cracking. The failure of bolts can be attributed to embrittlement

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

Synthesis and Characterization of nano-sized Al2O3 particle reinforced ZA-27 metal matrix composites

Metal matrix composite (MMC) focuses primarily on improving specific strength, high temperature and wear resistance applications. In this article, the present study is concentrate on the synthesis and characterization of ZA-27 metal matrix composite (MMC) reinforced with nano-sized aluminium oxide (Al2O3) particle in different weight percentage of 1, 3 and 5%. A stir casting followed by squeeze casting technique is used for the fabrication of these composite materials. Wear volume loss of the composites is measured using Pin-on-Disc testing machine at different operating conditions and different weight percentage of Al2O3 Nanoparticles. Further the microstructures of worn samples were characterized with the help of Scanning Electron Microscope (SEM). The results indicated that the wear volume loss of the composites is decreased with the increase in Al2O3 nanoparticles reinforcement. (C) 2015 The Authors, Published by Elsevier Ltd