Correlative light-electron fractography for fatigue striations characterization in metallic alloys.

The correlative light-electron fractography technique combines correlative microscopy concepts to the extended depth-from-focus reconstruction method, associating the reliable topographic information of 3-D maps from light microscopy ordered Z-stacks to the finest lateral resolution and large focus depth from scanning electron microscopy. Fatigue striations spacing analysis can be precisely measured, by correcting the mean surface tilting with the knowledge of local elevation data from elevation maps. This new technique aims to improve the accuracy of quantitative fractography in fatigue fracture investigations.

Correlative fractography: combining scanning electron microscopy and light microscopes for qualitative and quantitative analysis of fracture surfaces.

Correlative fractography is a new expression proposed here to describe a new method for the association between scanning electron microscopy (SEM) and light microscopy (LM) for the qualitative and quantitative analysis of fracture surfaces. This article presents a new method involving the fusion of one elevation map obtained by extended depth from focus reconstruction from LM with exactly the same area by SEM and associated techniques, as X-ray mapping. The true topographic information is perfectly associated to local fracture mechanisms with this new technique, presented here as an alternative to stereo-pair reconstruction for the investigation of fractured components. The great advantage of this technique resides in the possibility of combining any imaging methods associated with LM and SEM for the same observed field from fracture surface.

Extended depth from focus reconstruction using NIH ImageJ plugins: quality and resolution of elevation maps.

In this work, NIH ImageJ plugins for extended depth-from-focus reconstructions (EDFR) based on spatial domain operations were compared and tested for usage optimization. Also, some preprocessing solutions for light microscopy image stacks were evaluated, suggesting a general routine for the ImageJ user to get reliable elevation maps from grayscale image stacks. Two reflected light microscope image stacks were used to test the EDFR plugins: one bright-field image stack for the fracture of carbon-epoxy composite and its darkfield corresponding stack at same (x,y,z) spatial coordinates. Image quality analysis consisted of the comparison of signal-to-noise ratio and resolution parameters with the consistence of elevation maps, based on roughness and fractal measurements. Darkfield illumination contributed to enhance the homogeneity of images in stack and resulting height maps, reducing the influence of digital image processing choices on the dispersion of topographic measurements. The subtract background filter, as a preprocessing tool, contributed to produce sharper focused images. In general, the increasing of kernel size for EDFR spatial domain-based solutions will produce smooth height maps. Finally, this work has the main objective to establish suitable guidelines to generate elevation maps by light microscopy.

Low voltage and variable-pressure scanning electron microscopy of fractured composites.

Uncoated fracture surfaces of carbon-epoxy composites are investigated using a variable-pressure environmental scanning electron microscope (VP-ESEM), under optimized conditions for topographic description, image quality and sample preservation. Always using freeware or open source programs, parameters for low-voltage and low vacuum are stipulated with the support of Monte Carlo simulations combined to topographic measurements, tailoring the VP-ESEM setup for visualization of fine relief details. Based on topographic information from atomic force microscope (AFM) images, finest fracture steps were measured. These were the references to optimize and define boundaries for applied beam voltages and chamber pressures, restricted by the beam penetration depth and gas-electron interactions, guided by Monte Carlo simulations and signal-to-noise measurements. For VP mode, ideal chamber pressure was found around 30-40Pa at 3keV beam voltage and 6mm working distance. Lower pressures will cause noise due to electron charging and gas excess provokes resolution degradation and noise due to positive charging and electron beam scattering, raising the skirt radius. When a larger working distance is necessary, it can be compensated by adjusting the detector bias and the probe current, or even lowering chamber pressure, but the signal-to-noise ratio will certainly change. Monte Carlo simulations provided a good approach to optimize imaging conditions under low vacuum and low voltage for fractographic analysis of carbon-epoxy composites.

MRT letter: extended depth from focus reconstruction method for stretch zone measurement in 15-5PH steel.

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.