Qualitative assessment on fold cracking of coated substrates: a digital image-based approach

Abstract This study introduces two novel image-based metrics to evaluate fold cracking in coated papers: the Crack Perimeter and Area ratio (CPA ratio) and Fold Crack Distribution (FCD). These metrics aim to supplement traditional white pixel percentage (WPP) measurements by describing crack patterns and their distribution. The results show that both metrics effectively characterise the qualitative aspects of fold cracking. The CPA ratio distinguishes between fine, segmented cracks in lighter papers and larger, cohesive damage in heavier ones. The FCD metric reveals irregular crack distributions in lower basis weight samples and more uniform patterns in heavier papers. Folding in the cross-direction consistently caused more severe and evenly spread damage compared to machine-direction folding. Strong negative correlations were observed between both metrics and the WPP values (R 2 up to 0.9952), confirming their effectiveness in characterising surface damage patterns. Results revealed that flat sample positioning (180°) fails to accurately represent physical damage, while scanning offered superior results compared to photography and microscopy for automated image analysis of fold cracking. These findings highlight the importance of standardised digitisation protocols and methodological transparency in fold crack analysis, while providing new tools for a more comprehensive characterisation of surface damage in coated papers.