Fatigue fracture surface analysis in C45 steel specimens using X-Ray fractography

X-Ray fractography is a useful technique to analyse the mechanisms operating in fracture and involves an examination of the fracture surface. In the present investigation, this technique has been employed to study the fatigue fracture behaviour of a medium carbon steel of C45 grade in different heat treated conditions. The different trends observed in the residual stress (σr) and diffraction profile full width at half maximum intensity (B) relationships with the maximum stress intensity factor (Kmax) on the fracture surface have been correlated to the differences in flow characteristics of these materials. The root mean square value of microstrain , and the coherent domain (particle) size, D, were determined through single line diffraction profile (Voigt's) analysis. It has been observed that contribution of microstrain to profile broadening is more significant than that due to domain size. However, at higher Kmax values an influence of D was found on line broadening, to a small extent. Results of sub-surface stress measurements were compared in two conditions. A good correlation was noticed between the depths below the fracture surface, designated as ymax, at which the measured σr, reaches the base material value and the corresponding monotonic plastic zone size (rp) obtained by calculation. The observed depths below the fracture surface (ypeak) at which an increase in σr or a decrease in B was noticed, appear to be related to the conditions of the near-tip regions where the material has undergone severe fatigue damage and cyclic softening

Crystal structure changes in Ni3Al and its anomalous temperature dependence of strength

The existence of a structural transformation in Ni3Al alloys established earlier through X-ray diffraction, dilatometry and TEM investigations are summarised. The results obtained are discussed through a model proposed. The L12 structure appears to transform to another L12 or to a DO22 structure during heating. Such a transformation starts at around 700°C and seems to complete around 1100°C. In the temperature range 700-1100°C both phases coexist. This causes a tetragonal distortion of the L12 lattice giving rise to a tweed morphology in TEM observations. The flow stress studies indicate that the anomalous strengthening behaviour is not only due to the dislocation kind of mechanism as proposed in the literature but also due to the structural changes noticed in this work

Thermal evidence for the structural instability in Ni3 Al alloys

The thermal expansion coefficient (¿) and calorimetric data were obtained as a function of temperature in order to clarify some of the significant variations reported with regard to these values in the literature. Stoichiometric and off-stoichiometric compositions of Ni3Al alloys (with and without boron addition) were investigated. Dilatometric experiments were performed on all the alloys and the ¿ values were estimated over the temperature range from ambient to 1000°C. Two runs were made on each sample under different initial conditions and differences in ¿ values were noticed. The results were analysed based on our earlier X-ray diffraction results. Additional isothermal dilatometric tests were also carried out and a significant volume change was noticed (0.45% contraction) when the alloy quenched from 1000°C was heated to 600°C and held for a long duration. The calorimetric data were obtaindd over the same range of temperature and enthalpy changes, though less distinct, were noticed at around 360, 660 and 900°C. The variations seen further augment our earlier results on the instability of the L12 structure and the existence of a structural transformatio

Some studies on the influence of stress ratio and test temperature on X-ray fractography observations in C45 steel specimens

This paper deals with additional ¿X-ray fractography¿ observations made on C45 steel, subsequent to the earlier work [K. Rajanna, B. Pathiraj and B. H. Kolster, Fatigue fracture surface analysis in C45 steel specimens using X-ray fractography. Engng Fracture Mech. 39, 147¿157 (1991)]. Fatigue crack: growth tests were carried out at 20°C on CT samples prepared from this steel using different stress ratios (0.1R, 0.5R and 0.7R). Tests were also carried out at different temperatures (¿20°C, 20°C and 115°C) using a stress ratio of 0.1. The fractured surfaces were analysed for their residual stress state. The influence of the test conditions on the residual stresses developed is discussed

Ultra fast laser machined hydrophobic stainless steel surface for drag reduction in laminar flows

Hydrophobic surfaces have attracted much attention due to their potential in microfluidics, lab on chip devices and as functional surfaces for the automotive and aerospace industry. The combination of a dual scale roughness with an inherent low-surface-energy coating material is the pre-requisite factor for the development of an artificial superhydrophobic surfaces. Ultra short pulse laser (USPL) machining/structuring is a promising technique to obtain the dual scale roughness. Moreover, ultra short laser pulses allow machining without or with limited thermal effects. Flat stainless steel (AISI 304L) were laser machined with ultraviolet laser pulses of 6.7ps, at different laser processing parameters. Next, the samples were coated with a monolayer of\ud perfluorinated octyltrichlorosilane (FOTS) to get a superhydrophobic surface. The degree of hydrophobicity was accessed by static contact angle measurement. Laser patterned surface has longitudinal micro channels. Drag reduction in liquid flow can be obtained due to the shear free boundary condition at air-liquid menisci. The geometry of the patterns was analyzed with optical and scanning electron microscopy. Micro-Particle Image Velocimetry (μPIV) has been employed to measure and visualize the flow over such pattern

Ultra fast laser machined hydrophobic stainless steel surface for drag reduction in laminar flows

Hydrophobic surfaces have attracted much attention due to their potential in microfluidics, lab on chip devices and as functional surfaces for the automotive and aerospace industry. The combination of a dual scale roughness with an inherent low-surface-energy coating material is the pre-requisite factor for the development of an artificial superhydrophobic surfaces. Ultra short pulse laser (USPL) machining/structuring is a promising technique to obtain the dual scale roughness. Moreover, ultra short laser pulses allow machining without or with limited thermal effects. Flat stainless steel (AISI 304L) were laser machined with ultraviolet laser pulses of 6.7ps, at different laser processing parameters. Next, the samples were coated with a monolayer of perfluorinated octyltrichlorosilane (FOTS) to get a superhydrophobic surface. The degree of hydrophobicity was accessed by static contact angle measurement. Laser patterned surface has longitudinal micro channels. Drag reduction in liquid flow can be obtained due to the shear free boundary condition at air-liquid menisci. The geometry of the patterns was analyzed with optical and scanning electron microscopy. Micro-Particle Image Velocimetry (μPIV) has been employed to measure and visualize the flow over such pattern