Mita Tarafder

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.

Micromechanical Aspects of Transgranular and Intergranular Failure Competition

Quantification of characteristics that govern intergranular fracture initiation and propagation of this fracture micromechanism in competition with cleavage one is main aim of the paper. A NiCr steel of commercial quality and the same steel with an increased content of impurity elements, Sn and Sb, were used. Step cooling ageing was applied in order to induce intergranular embrittlement. Standard and pre-cracked Charpy type specimens were both tested in three-point bending to determine fracture toughness characteristics. In order to characterise the quantitative differences in fracture surfaces roughness a fractal analysis was applied. A boundary level of fractal dimension has been determined to be 1.12: fracture surface roughness with a higher value reflects high level of intergranular embrittlement and thus fracture resistance degradation.

Stretch-zone analysis by image processing for the evaluation of initiation fracture toughness of a HSLA steel

In this work, stretch zone width and stretch zone depth of ductile fracture surfaces of a high-strength low-alloy steel at various prestrained conditions are computed using digital image processing techniques, where the images are taken by scanning electron microscopy. Histogram equalisation is applied on the input image for enhancement of the image quality. Various edge detection filters, such as Laplacian, Sobel, and Kirsch are separately applied to the digital image to detect the edges of the stretch-zone. The images obtained after processing are binarized through thresholding for identifying the stretch zone boundaries and to compute the stretch-zone width. It is observed that the Laplacian filter produced better results compared to the other filters. The measured values of stretch-zone width and stretch-zone depth by image analysis agreed well with those obtained manually; however they showed discrepancy with the same obtained indirectly from the fracture-resistance curve. The reasons for such a discrepancy are highlighted in this paper. From the estimated stretch-zone width, initiation fracture toughness of the HSLA steel has been obtained.

A Novel Technique for Extracting Stretch Zone Features from Fractographs

The crack tip blunting process leaves an imprint in the form of stretch zone on fracture surfaces during the event of ductile fracture. A schematic representation of the stretch zone in a fractured specimen and a corresponding fractographs is shown in Fig. 1. A typical stretch zone has two components, stretch zone width (SZW) and stretch zone depth (SZD). The SZW is basically the virtual crack extension Δa and SZD is half of the CTOD,. Stretch zones can be easily identified, when observed under the SEM, since they have visually identifiable boundaries in between the fatigue precracked region and the ductile fracture region Open image in new window FIGURE 1. (a) Elevation profile of crack tip zone (b) SEM image of fracture surface

Factors influencing the extent of hydrogen-enhanced 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.