IMPACT LOADING AND HAIR BREAKAGE 253 Figure 5. Snag of slightly wavy Caucasian hair on a white plastic fork showing wrapped hair ends. Note hair ends wrapped around teeth and crossed over other hairs. Figure 6. Slightly wavy to straight Caucasian hair in a snag. Note hair ends wrapped around comb teeth and crossed over other hairs. hairs, causing damage to the fiber ends by abrasion as these hair ends grip the comb and other hairs. Combing or pulling on the snag induces extension and compression on the damaged weakened site and, if weakened sufficiently, produces short-fragment breakage. The longer fragments (longer than 6.4 cm or longer) most likely break by impact

254 JOURNAL OF COSMETIC SCIENCE Comb Tooth · · ··· · ······ ···· -....... Scale eversal & Inter ockinq of Scales Fi g ure 7. Wrapping of hair ends around comb teeth reverses the scales. loading of one hair fiber against another hair in a snag, as described previously. On impact loading a hair fiber loop over another hair, severe bending of the fiber loop on impact (sometimes with abrasion) initiates crack formation in the highly bent and extended surfa�e layers. If the crack extends deep enough into the hair, catastrophic failure results, i.e., complete breakage occurs in one of several ways, producing a smooth fracture, a step fracture, or a split or fibrillated end. The mechanisms for the different types of fractures have been described very well and in detail in the paper on fractog­ raphy of hair fibers by Karnath and W eigmann (3 ). In this paper on fractography, the primary route for crack initiation involves bending and thus extension in the outer layers of the surface, and it is likely that this is the most common type of crack initiation. However, there is evidence that some hairs may initiate fracturing by impact loading primarily involving compression rather than extension, e.g., straight sections of hair fibers rather than loops appear also to break readily on impact loading, as in the one case described above where the top and bottom hairs of the attached-loop experiment broke equally and where the bottom hair is a straight section of a hair and not a loop. Therefore, another experiment was conducted where a straight section of a hair fiber in a special holder was impacted onto another straight section of another hair held in a vise (Figure 8), providing the results summarized in Table V. These results confirm that straight sections of hair fibers can break on impact loading. It may be argued that some bending occurs in this experimental setup, but not nearly as much bending as by impacting hair loops. Furthermore, eight bottom hairs and 16 top hairs broke. It can be argued that crack initiation still occurs on the side opposite to the impact site where bending and extension will occur, but the simple fact that breakage occurs almost as readily for straight sections of hairs as for severely bent hair loops suggests that another mechanism for breakage may be involved. These details