Measuring Hair Strength, Part I: Stress-Strain Curves

Aug 1, 2013 | Contact Author | By: Trefor Evans, PhD, TA Evans LLC
Your message has been sent.
(click to close)
Contact the Author
Save
This item has been saved to your library.
View My Library
(click to close)
Save to My Library
Title: Measuring Hair Strength, Part I: Stress-Strain Curves
stress-strain curvex hair strengthx tensile strengthx break forcex break stressx
  • Article
  • Media
  • Keywords/Abstract
  • Related Material

Keywords: stress-strain curve | hair strength | tensile strength | break force | break stress

Abstract: This article is the first in a series that will address the approaches of measuring the “strength” of hair and quantifying the manner by which this property may be altered. Specifically, it will begin with the generation of stress-strain curves through the use of constant rate extension experiments.

View citation for this article

T Evans, Measuring Hair Strength, Part I: Stress-Strain Curves, Cosm & Toil 128(8) 590 (2013)

Market Data

  • Consumers are looking toward hair for anti-aging benefits.
  • Anti-aging hair care products address concerns such as thinning, coloring, breakage and drying, with emphasis on particular ingredients that target specific hair issues.
  • Although trends in anti-aging hair care are currently focused primarily in North America and Europe, hair care brands are seeing opportunities globally, including emerging markets.
view full article

Excerpt Only This is a shortened version or summary of the article you requested. To view the complete article, please log in or create an account. Registration is Free!

When talking to consumers about their hair care wants and needs, it generally does not take long until the word “strength” is uttered. It is especially common to hear the desire for “strong, healthy hair,” which illustrates the inextricable link between these two attributes in the mind of consumers. This, of course, is recognized by marketers of hair care products, who accordingly desire to craft communication messages on product packaging and in advertisements that speak to an ability to improve hair strength. As a result, it becomes necessary to establish a means for measuring and quantifying the “strength” of hair.

To the scientist, this immediately conjures up an assortment of instrument-based mechanical testing approaches that can be used to probe the physical properties of individual fibers. A variety of methodologies can be (and have been) adopted, but it is important to recognize that the properties of hair are a function of its highly complex structure, and different techniques may specifically probe select regions of this remarkable substrate. Therefore, it is not uncommon for different measurement approaches to yield different outcomes.

With this said, it is also recognized that consumer assessment of their own hair strength likely does not involve tugging and manipulating individual fibers in a manner comparable to these laboratory methods. Instead, it would appear that consumer judgment is based on observing the number of broken strands in a brush or comb after grooming, by noticing the number of fibers on the shower floor after bathing, or simply by observing the presence of split ends when looking in the mirror. This highlights a common issue in in vitro testing, wherein there is the need to consider whether a measurement approach is intended to simulate real-life conditions or to provide a convenient means of technical characterization. Both of these approaches have merit and provide distinctly different useful information. By means of illustration, it will be shown that the tensile properties of hair can be compromised by certain treatments and conditions, but it will also be demonstrated that the best means of addressing this issue is not necessarily the obvious one.

This article is the first in a series that will address the approaches of measuring the “strength” of hair and quantifying the manner by which this property may be altered. Specifically, it will begin with the generation of stress-strain curves through the use of constant rate extension experiments. This is a useful fundamental approach, and yields a great deal of information about the physical structure of hair. In most instances, results from this methodology will suffice as a means of characterization; however, follow-up articles will build on this foundation and show how other methods probe different regions of the hair structure, which in turn may be affected differently by a given treatment. In addition, alternative approaches will also be discussed that appear to better simulate consumer-usage conditions. In all instances, attempts will be made to provide guidance in properly performing such experiments to allow production of quality data.

Before beginning, it is worth noting that the mechanical properties of hair are a direct reflection of its complex structure. These two topics should be discussed jointly, but the length of this article precludes the ability to fully describe this remarkable substrate. Instead, the reader is referred to additional reading that provides this background information.1

Excerpt Only This is a shortened version or summary of the article you requested. To view the complete article, please log in or create an account. Registration is Free!

This content is adapted from an article in GCI Magazine. The original version can be found here.

 

Close

Table 1. Typical tensile parameters for hair in the wet and dry states

Table 1. Typical tensile parameters for hair in the wet and  dry states

However, this parameter is dependent on two variables (the break force and break extension) that often move in opposite directions as conditions change (see Table 1).

Figure 1. A typical stress-strain curve for a hair fiber as obtained from a constant rate extension experiment

Figure 1. A typical stress-strain curve for a hair fiber as obtained from a constant rate extension experiment

If one were to progressively stretch (strain) an individual hair fiber at a given extension rate (for example, 40 mm/min) and measure the resulting internal force, the outcome would resemble the graph shown in Figure 1.

Figure 2. The dependence of break force on hair fiber dimensions

Figure 2. The dependence of break force on hair fiber dimensions

Figure 2 shows experimental data that highlights this relationship.

Figure 3. The effect of relative humidity on the break stress of hair

Figure 3. The effect of relative humidity on the break stress of hair

Figure 3 shows the break stress for hair fibers that were equilibrated and tested over a range of different humidity conditions.

Figure 4. The reduction in break stress after exposing hair to simulated sunlight for progressively longer durations

Figure 4. The reduction in break stress after exposing hair to simulated sunlight for progressively longer durations

For example, Figure 4 shows the decrease in break stress for hair as a function of prolonged exposure to simulated sunlight through the use of an accelerated weathering chamber.

Footnotes (CT1308 Evans)

a Q-Sun Model 3100 is manufactured by Q-Lab Corp., www.q-lab.com.
b
Dia-stron MTT675 is an automated tensile tester manufactured by Dia-stron Ltd., www.diastron.com.

.

Next image >

 
 

Close

It's Free...

Register or Log in to get full access to this content

Registration includes:

  • Access to all premium content
  • One click ingredient sample requests
  • Save articles in the My Library tool

Create an Account or Log In