Cyclic combing of untreated and bleached human hair: Analysis of the time-dependent breakage of hair through recording the formation of fibre fragments.

Objectives: Machine-based cyclic combing of hair tresses under dry conditions is a proven method for evaluating hair strength and the impact of treatments. Recent advancements in image analysis allow for a detailed review of hair fragment lengths and quantities produced after specific combing cycles. Our aim is to provide an in-depth analysis of the kinetics of hair fragment formation.

Methods: We analysed the combing performance of untreated and bleached European straight hair, assessing two conditioning treatments. Hair tresses underwent 5000 combing cycles, with the increasing number of fragments recorded. Results were fitted using a three-parameter Voigt-Kelvin model.

Results: The fragment counts were log-normally distributed in all cases. The model uses ln(N₀) to indicate early fragment release, showing higher numbers for bleached hair compared to untreated hair, which significantly drop with conditioner treatments. The ln(N_∞) parameter estimates the maximum expected fragments. The Failure Cycle Index (FCI) remains largely unchanged across materials according to its 95% confidence limits.

Conclusions: The selected function's characteristics and the invariance of FCI indicate that friction primarily controls fragment formation in straight hair during combing. This suggest that there is no direct link between combing performance and tensile fatigue failure for this hair type. In contrast, textured hair will likely show more complex combing performance. Our analysis shows that the total number of fragments after many combing cycles can predict early failures and assess conditioning agents, thereby supporting product claims about 'hair strength'.