430 JOURNAL OF COSMETIC SCIENCE
There are several parameters that can be extracted from such curves to provide
quantification—the most obvious being the amount of force required to snap the fiber.
However, this parameter has a strong dependence on the fiber dimensions (all things being
equal, a thicker fiber requires more force to break than a thin fiber),18 and so this factor
is usually normalized out by first measuring fiber dimensions and then reporting a break
stress (where stress =force/unit area). Figure 8 shows how the wet state break stress of hair
declines as a result of bleaching treatments of differing severity.
Similar outcomes arise from other chemical treatments (e.g. perms, permanent coloring,
relaxing, Brazilian Keratin Treatments, etc.), exposing hair to the high temperatures
encountered with curling and straightening irons,19 and exposing hair to UV radiation.20
Accordingly, there is ample evidence to support deleterious alteration of hair’s internal
structure.
Yet consumers do not assess their hair’s strength by pulling on individual fibers and
attempting to assess the forces involved. Instead, consumer perception would seem to
involve some self-assessment of the tendency for breaking. This may include noticing
broken fibers in a brush or comb during grooming or in the base of the shower after
bathing. Moreover, on a consumer’s head, it seems likely that fiber breakage ultimately
occurs as the result of a culmination of various manipulations, rather than one single
catastrophic deformation. From a mechanical testing perspective, the resistance of a
material to repeated external stimuli represents a fatiguing experiment and these have been
becoming ever more popular in the hair care world over the past decade.21,22 Commercial
equipment for performing single fiber fatigue experiments is available (Diastron Corp,
Andover, UK) and the outcomes from this testing have gradually been superseding those
from the traditional approach. For example, these newer experiments often show much
bigger differences between samples than seen by the usual tensile approach. To illustrate,
an insult that produces a 10% decrease in the dry state break stress might lead to a 10-fold
Figure 7. Typical tensile testing curves for wet and dry hair.

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.