PATHWAYS OF HAIR BREAKAGE 241 load cell because the entire compression cell surface provides compression readings and the button could be used to gauge the actual amount of contact between the compression cell and the hair. It is also assumed that the compression loads are distributed uniformly over these hairs. These experiments demonstrate that compression loads between hair fibers and comb surfaces during combing can be very high. One can assume that the average compression load for a hair-on-hair interaction in a difficult snag would be of the same order per fiber or in some cases even higher, leading to very high compression forces per unit area in actual contact for one hair fiber against another. Therefore, for hair breakage during combing or brushing, compression forces are in­ volved in addition to extension forces, and this fact is consistent with all three of the previously suggested pathways for breakage. Furthermore, these compression forces can be very high, and with rubbing can lead to extensive cuticle disruption (3 ). Yet com­ pression forces are not considered in ordinary tensile testing of human hair fibers. IMPACT LOADING EXPERIMENT Most tensile testing conditions employ very slow strain rates, of the order of 0.25 cm per minute. Under these conditions, hairs generally stretch about 40% to 60% of their length before breaking. However, during combing, hairs do not stretch to such lengths before breaking therefore, another important variable is likely to be strain rate differ­ ences in combing versus those in tensile testing. During combing, strain rates can be very rapid, at least an order of magnitude faster than in normal tensile testing, and combing more closely simulates impact loading than slow stretching. Therefore, to more closely simulate the action of combing, experiments on hair fiber breakage using impact loading conditions were examined. BREAKAGE OF HAIR OVER HAIR VS HAIR OVER COMB TOOTH Hair loop over comb tooth vs hair loop over hair loop. Hair fiber loops were made as described in the Experimental section and the weighted loop (51-gm load) was impact loaded over a large thick-comb tooth (varying from 1800 to 1400 microns thickness). Hair breakage generally did not occur on the first impact (see Table III). However, when the comb tooth was thinner (varying from 1245 to 940 microns thickness), using one of the fine teeth on this same comb, breakage occurred with fewer impacts using the same 51-gm load (Table IV). Yet, when one hair fiber loop was impacted over another hair fiber loop Table III Impact Loading a Hair Fiber Loop Over a Thick-Comb Tooth (-1800 µ) Using a 51-Gram Load 4 Fibers broke on impact 1, near contact site 2 Fibers broke on impact 2, at contact site 2 Fibers broke on impact 3, not at contact site 3 Fibers broke on impact 4, at contact site 1 Fiber did not break after 10 impacts (36 Total impacts and 11 broken hairs)

242 JOURNAL OF COSMETIC SCIENCE Table IV Impact Loading a Hair Fiber Loop Over a Thinner-Comb Tooth (-1245 to 940 µ) Using a 5 1-Gram Load 8 Fibers broke on impact 1, 2 at contact site 2 fibers broke on impact 2 near contact site 2 fibers broke on impact 5, 1 near contact site (22 Total impacts and 12 broken hairs) Table V Impact Loading a Hair Fiber Loop Over a Single Hair (Loop) 10 of 10 hairs broke on impact 1 for 51-gm load, all near contact site (10 Total impacts and 10 broken hairs) 12 of 12 hairs broke on impact 1 for 31-gm load, all near contact site (12 Total impacts and 12 broken hairs) Figure 6. Crease in hair loop after impact over horizontal taut hair. where the approximate average fiber thickness was 70 microns, breakage occurred with fewer impacts with either a 51-gram load or a 31-gram load (Table V, Figure 5). In all but five cases, the bottom hair broke however, no attempt was made to control fiber diameter in this experiment. In all cases, the unbroken loop was left with a small crease in it (see Figure 6). These simple experiments show that hair breakage occurs more readily when a hair fiber is impacted against another hair as compared to a hair against a comb tooth. Therefore, the force per unit area in the impacted region of the hair is