Magnetic characterization of HSLA steel by power-law decay exponents of Barkhausen emission signal

The general trend of magnetic behaviour of materials is that the mechanically hard materials are also magnetically hard. However for the high strength low alloy (HSLA) steel tempered at various aging temperatures, the correlation was reported as negative. The anomaly could not be explained by the magnetic parameters like RMS voltage calculated from the Barkhausen emission signal and the coercivity from the magnetic hysteresis loop. This paper reports another magnetic parameter known as power-law decay exponent which shows excellent correlation with the mechanical properties and thus explains the progressive evolution of the microstructural constituents in HSLA steel

Self-similar and self-affine characteristics of microstructural images of HSLA steel

This paper reports a procedure based on discrete Fourier transformation adopted for verifying statistical self-affinity and self-similarity in microstructural images of high strength low alloy steel. The procedure is first applied on synthetic images based on fractional Brownian motion to show the degree of self-similarity in them and finally, applied on microstructural images of three different magnifications. Effects of length scales on power spectral estimates of the microstructural images captured by scanning electron microscopy have been discussed. Importance of appropriate image resolutions and characteristic length for fractal based quantification of microstructural images is highlighte

Hydrogen induced brittle crack growth in Cu-strengthened HSLA-100 steels

The annular brittle cracking in notched samples of an HSLA-100 steel has been studied by crack interrupts. The average crack growth rate was found to be geometry and strain rate dependent. Suitable normalisation led to a generalised expression for crack growth rates that was strain rate and geometry independent. The crack growth rates were found to be directly proportional to a ‘critical product’ namely {KIcrack-tip·CHcrack-tip}. The crack initiation threshold for notched specimens showed an inverse exponential relationship with the notch tip hydrogen concentration. The crack tip hydrogen concentration was best represented by a hydrostatic stress equilibrated concentration for shallow notched specimens. Hydrogen deficiency in deep notches necessitated the use of a ‘frozen-in’ concentration approach for determining the crack tip hydrogen

Fractal-based quantification of crack paths for determination of effective microstructural length scales and fracture toughness

A model to compute an effective microstructural length scale by fractal analysis of self-affine tensile fractographic images has been proposed. Good correlation between the characteristic microstructural length scale and the average circular diameter of voids as well as yield strength is demonstrated for a Cu-strengthened HSLA steel. Using the fractal-based model, fracture toughness is computed, which agrees well with that determined experimentally

Factors influencing the extent of hydrogenenhanced brittle cracking in a Cu-strengthened HSLA steel during monotonic loading

The extent of brittle crack as a function of hydrogen charging conditions was studied for a HSLA steel using circumferentially notched cylindrical tensile samples. Two different notch depths were used. The effect of hydrogen could be well represented by an effective hydrogen potential which was defined using a representative hydrogen concentration and a diffusive time parameter, for relatively faster strain rates. The high triaxiality in deep-notched samples led to the initiation of ductile failure mechanisms overwhelming the brittle cracking process

Fractal Characterisation of Ductile Fracture Surface

In this investigation we report the fractal characteri-stics of the fracture surfaces and microstructure of HSLA steel using the fractal dimensional D computed from the fractographs and microstructural images of HSLA-100 steel at variously tempered conditions. Digital images of fract-ographs and microstructure obtained by the Scanning Electron Microscope (SEM) were analysed to compute fractal dimensions

Fractal Analysis of Microstructural Images For Evaluation of HSLA Steel

Investigation of materials invariably requires usages of high resolution images of material structures for quantification of microstructural features, and subsequent development of correlations with the material properties. Often microstructural images of materials exhibit self similar structures or patterns formed due to network of grain boundaries, presence of precipitates, uneven surface elevations, etc. Microstructural images containing self similar fractal patterns at different length scales or resolutions remain unaffected by translations, rotations, projections and many other operations with regard to images. This makes the fractal analysis a useful technique for quantifying microstructural images by fractal dimension the non-integer dimensional exponent used for representing non-linear complex phenomena. This paper presents fractal analysis of a set of microstructural images of high strength low alloy (HSLA) steel at various aging conditions and reports that the variation of fractal dimensions identifies the morphological changes occurring in nano-scale due to copper precipitation