431 SUSTAINABLE HAIR
decrease in the number of fatiguing cycles to break. Figure 9 shows a similar occurrence in
testing African hair and Caucasian hair by this approach.
The below graph is termed an S-N plot and shows the relationship between the magnitude
of the repeated fatiguing stress (S) and the subsequent number of such cycles to induce
breakage (N). Hair breakage is frequently mentioned as the single biggest hair-care issue
for those with very curly textured hair23 to the point that it can radically change habits
and practices. Yet, conventional tensile testing suggests relatively meager differences
between the two hair types, wherein the very curly hair typically has a break stress around
10% lower. In the following S-N plot experiment, the African hair is seen to break after
approximately 10 times fewer fatiguing cycles with this outcome seemingly being more in
line with consumer observations.
This method also illustrates the sizable contribution of factors that likely had not been
previously considered. For example, the regressions in Figure 9 have downhill slopes
illustrating that fibers will break faster when exposed to higher fatiguing forces. However,
the logarithmic nature of the y-axis indicates that this is an exponential relationship. Hair
will be subjected to fatiguing during everyday grooming wherein the graph in Figure 9
indicates that higher grooming forces will give rise to exponentially faster breakage. Further
to this point, the grooming forces of very curly, textured hair will be much higher than
Caucasian hair, which adds a further sizable contribution to the one already mentioned.
Further still, the African hair also has a much higher predilection for fibers to break after
only a few fatiguing cycles (so-called premature failures), and so it becomes eminently clear
as to why this hair type is so susceptible to breakage.24 Any of these three factors by
themselves would be cause for concern but, in concert, the outcome is dire.
Taking the above relationship in the opposite direction, it becomes evident that an ability
to lower grooming forces (and therefore fatiguing forces) should produce an exponentially
Figure 8. Decreasing wet state break stress of hair with bleaching.

432 JOURNAL OF COSMETIC SCIENCE
slower tendency for breakage. This is easily attained through use of conventional conditioner
products that coat the hair with a thin lubricating deposit comprised from fatty alcohols and
quaternium ammonium surfactants. In our industry, so-called repeated grooming experiments
are frequently used to demonstrate this occurrence whereby hair tresses are repeatedly
brushed with the periodic counting of broken fibers.25
In accordance with this theory, these experiments demonstrate sizable benefits as the result
of such treatments and represent tangible consumer benefits. As per a previous discourse,
if a consumer experiences breakage, there is the perception that their hair is “weak” and
“damaged ” however, if this situation is alleviated, the consumer perception is that the
hair has been “strengthened” and “repaired.” Accordingly, such language is commonplace
on products – but it should be remembered that these propositions relate to “consumer
language” and not the strict definitions associated with “technical language.”
To summarize, fatigue testing outcomes teach that “strength” and “breakage” are not the
same thing. Our industry has historically used standard tensile experiments to equate a
fundamental technical strength however, fatigue testing yields differing outcomes and
highlights the contribution of previously unrecognized factors. For example, a conventional
conditioner product has no effect on the tensile strength of individual hair fibers but can
produce sizable mediation of breakage.
SPLIT ENDS
The internal structure of hair is not unlike that of a rope and so, until recently, this author
had always presumed that after a hair fiber broke, the ends would fray (analogous to a rope)
Figure 9. S-N Plots for Caucasian and African hair as obtained from single fiber fatigue experiments.