Initiation fracture toughness of HSLA steel through automatic measurement of stretch zone

Abstract

Stretch zones are formed due to crack tip blunting in ductile materials before the initiation of a crack. Using special edge detection filters, an automatic measurement procedure based on image analysis (IA) for estimating stretch zone dimensions has been proposed. The in house developed software based on the proposed technique requires only fractographic images to automatically define stretch zone boundaries. Employing the proposed technique, stretch zone measurements were carried out on a high strength low alloy (HSLA) steel. The initiation fracture toughness of this HSLA steel has been estimated using these stretch zone dimensions. A comparison of measured stretch zone dimensions with the crack tip blunting region of the fracture resistance curve revealed that the blunting behaviour is non-linear and the data range normally considered for constructing the blunting line is inadequate to represent this non-linearity for these steels. Prediction of initiation fracture toughness (J i ) in high strength, high toughness materials therefore requires blunting line estimation from a modified data range. An upper bound blunting line data range is proposed for accurate prediction of Da cr (SZW) and J i . The results of this study indicate that the IA technique can be used effectively for accurate determination of SZW, SZD, J i , and J Q . It is also noted that for materials with non-linear blunting behaviour, SZW is a better parameter than SZD for predicting J i and J Q . MST/6138

Establishing a methodology to predict the crack initiation load in through-wall cracked components using tensile specimen test data

“…The SZW is measured experimentally from fractographs obtained by the scanning electron microscope (SEM). An essential difficulty encountered here is the demarcation between the fatigue pre-crack, blunted and stable crack growth regions [15]. Work in the past related to SZW was mainly experimentally based with few attempts to evaluate it numerically [16,17].…”

Evaluating a valid fracture specimen geometry for predicting geometry independent SZWc and JSZW value in SA333 Gr. 6 carbon steel material

“…Among several other definitions, the crack initiation fracture toughness (J i ) based on critical stretch zone width (SZWc), called J SZW , and has received much attention. The problem in SZWc experimental evaluation is in identifying the size of stretch zone on a blunted crack front, as this requires a high degree of precision and expertise in measuring the SZW [3].…”

Numerical characterization of compact pipe specimen for stretch zone width assessment

“…SZW can be experimentally evaluated on the fractured surface by using scanning electron microscope (SEM). But experimentally, it is difficult to identify the size of stretch zone on blunted crack tip . Assuming semi‐circular blunting, several times SZW has been evaluated numerically .…”

“…But experimentally, it is difficult to identify the size of stretch zone on blunted crack tip. 3 Assuming semicircular blunting, several times SZW has been evaluated numerically. 4,5 In order to evaluate numerically SZW and critical SZW, 6 proposed energy-based methodology without making any such assumption.…”