Thickness measurement of cell walls of closed-cell foams in micro-CT images.

Characterising the microstructure of foams is an important task for improving foam manufacturing processes and building foam numerical models. This study proposed a method for measuring the thickness of individual cell walls of closed-cell foams in micro-CT images. It comprises a distance transform on CT images to obtain thickness information of cell walls, a watershed transform on the distance matrix to locate the midlines of cell walls, identifying the intersections of midlines of cell walls by examining how many regions each pixel on the midlines of cell walls connects with, disconnecting and numbering the midlines of cell walls, extracting the distance values of the pixels on the midlines (or midplanes) of cell walls, and calculating the thickness of individual cell walls by multiplying the extracted distance values by two. Using this method, the thickness of cell walls of a polymeric closed-cell foam was measured. It was found that cell wall thickness measured in 2D images shows larger average values (around 1.5 times) and dispersion compared to that measured in volumetric images.

Fatigue Life Prediction of Machined Specimens with the Consideration of Surface Roughness.

The fatigue strength and fatigue life of high-strength steels are greatly affected by their surface roughness. This study investigates the underlying mechanisms responsible for fatigue failure of the high-strength steel 42CrMo. Bending fatigue tests of stepped shafts with different levels of surface roughness were conducted to observe the fatigue live reduction affected by surface topography. Besides, the mechanical properties of 42CrMo and its strain-life relationship were established. Moreover, the analytical formulas to describe the stress concentration factor (SCF) and fatigue notch factor (FNF) induced by surface topography were introduced. To estimate the fatigue life of machined specimens with the consideration of surface roughness, the elastic portion of the total strain-life curve of the material was revised with the proposed analytical FNF imposed by surface topography. Comparisons between the estimated fatigue lives and experimentally obtained fatigue lives show that the effect of surface roughness on fatigue lives could be estimated effectively and conveniently by the proposed procedure.