HAIR TENSILE STRENGTH

Tensile strength is the largest amount of force that can be applied to an object being stretched or pulled until it breaks.

Tensile testing human hair is a common practice among manufacturers of various hair products, such as shampoos, dyes, and gels. It is important to be able to characterize the strength of hair to determine whether these products weaken or strengthen it. Similar to single strand textiles, there are many challenges associated with tensile testing of hair. Hair is normally very fine and often not long enough to be used in a cord and yarn style grip. In addition, hair is typically inconsistent, so it is important to test large sample sizes.

Human hair is made of a protein called keratin; a polyamide molecule with many disulfide bridges between the chains which gives the hair is strength and stiffness

The colour is due to eumelanin [fair hair if in low concentrations or brown and black if higher] or pheomelanin [a yellow red colour giving rise to redheads and freckles]. Grey hair lacks either colouring pigment. In keeping with other

man-made fibres the strength of hair will depend to a large extent on the nature of the intermolecular bonds, the more there are the stiffer the hair will be, and the higher its tensile strength.

Method I.
Materials
Strand of hair
Spring scale gauge


Procedure
Tie a short length of your test material in a secure knot around the hook of the spring scale.
Secure the top of the spring scale to something steady, like a table or wall.
Pull on the test material until it breaks. Keep an eye on the spring scale reading. (Depending on your spring scale, some of your test materials may not break before the spring scale breaks. Be careful!)
Record the force measured by the spring scale right before the test material breaks.
Compare results.


Method II
Therefore, if you are developing hair care products of any kind, you should plan to perform intelligent tests to determine your own results. Human hair consists almost entirely of a protein called Keratin. Keratin is extremely strong – about half the strength of steel. But a single strand of hair is very small – about 0.025 mm (0.001 in) to 0.076 mm (0.003 in) in diameter. Therefore, if you are tensile testing a single strand, you will only measure about 100 gf.
Very small materials that are very strong, such as hair, pose some challenges to test accurately. You will need a small and sensitive load cell and you will need special specimen grips or a special technique for gripping. The TestResources model 100 series tabletop test machine is an excellent machine for tensile testing human hair and other single fiber tests, particularly because it is a very configurable test machine.


Method III
Procedure: 1 A card is cut as shown in figure 1 with holes reinforced with strips of tape. A single hair is fixed as shown, with ends curled round to ensure that it breaks instead of simply being pulled out from under the tape.


2 Once assembled the cord and hair is suspended from a pencil held horizontally in a clamp. The card is then cut as shown in figure 1. It is then loaded to breaking point. At that point both weights and the lower piece of residual card is weighed and recorded.