An Improved Method of Projected Area Determination in Nanoindentation Using Image Processing With Sub-Pixel Edge Location

Abstract

We propose a novel approach to determine the precise indent area using image processing with sub-pixel edge location in nanoindentation hardness testing. The impressions made by nanoindentation on copper and mild steel specimens were captured using a high-resolution optical microscope. Each indent area was cropped out and subjected to pixel-level edge detection using the Canny operator. Based on the detected edge the invariant moment sub-pixel edge location method was applied to determine the precise location of the edges of the indent using combined vertical and diagonal scanning. The projected area was then determined directly from the enclosed contour to sub-pixel accuracy. The hardness value was determined by dividing the load with the area determined using the proposed approach. Comparison of the hardness values was made using the area determined by sub-pixel edge location and that determined from the nanoindentation tests showed a maximum difference of about 3.74% for the copper specimen and 4.17% for mild steel specimen. Unlike in the actual nanoindentation test, the proposed method of determining hardness does not assume that the indentation area is triangular, thus resulting in higher accuracy hardness values.