MRT letter: Extended depth from focus reconstruction method for stretch zone measurement in 15‐5PH steel

Abstract

The stretch zone width (SZW) data for 15-5PH steel CTOD specimens fractured at -150°C to + 23°C temperature were measured based on focused images and 3D maps obtained by extended depth-of-field reconstruction from light microscopy (LM) image stacks. This LM-based method, with a larger lateral resolution, seems to be as effective for quantitative analysis of SZW as scanning electron microscopy (SEM) or confocal scanning laser microscopy (CSLM), permitting to clearly identify stretch zone boundaries. Despite the worst sharpness of focused images, a robust linear correlation was established to fracture toughness (K(C) ) and SZW data for the 15-5PH steel tested specimens, measured at their center region. The method is an alternative to evaluate the boundaries of stretched zones, at a lower cost of implementation and training, since topographic data from elevation maps can be associated with reconstructed image, which summarizes the original contrast and brightness information. Finally, the extended depth-of-field method is presented here as a valuable tool for failure analysis, as a cheaper alternative to investigate rough surfaces or fracture, compared to scanning electron or confocal light microscopes.

Fracture mechanics as a tool in failure analysis — Prospects and limitations

“…[43]. In [44] the authors obtained the relationship given in Figure 35 for 15-5PH steel tested at different temperatures from -150 o C to +23 o C. Unfortunately, the cited paper gives no information on how the toughness value K c was defined. If this is really based on the stress intensity factor concept and not subsequently derived from critical J integral or CTOD values, this might possibly be problematic.…”