Tensile Testing Human Hair

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Tensile Testing Human Hair

There is large demand in the cosmetics and health industries for mechanical testing data on human hair. Tensile test data on hair can be useful in designing hair combs and brushes, as well as jewelry for hair. Friction data is needed for design verification for hair conditioners and gels. Yet little data is available; and anyone who has performed mechanical tests on human hair knows there can be great variability in test results.

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.

Gripping very thin, very strong materials for tensile testing can be challenging. If you simply clamp them, you can create large stress concentrations at the edges of the grip jaws. These stress concentrations can cause premature failures right at the grip edges. One technique we can recommend is the use of capstan or horn grips:  

The hair can be wrapped several times around the capstan horn or cylinder before clamping. This mitigates the stress concentration at the edge of the jaws almost entirely. However, a simpler and lower cost gripping alternative technique is to use vice grips with rubber lined jaws. Tape can be used on the specimen to minimize slippage within the rubber lined jaws.

Pneumatic grips can also be used to help with the grip edge stress concentration problem because you can adjust the air pressure to find the right gripping force that does not slip but is as gentle on the specimen as possible. Plus the gripping force is completely repeatable based on your air pressure setting.

If you have any doubt about gripping your human hair, textile fiber or silk fiber, contact one of TestResources’ application engineers. We can provide experienced advice, guidance and we may also be able to perform some trial testing on your specimens, at no charge to you, to ensure your testing success. 

TestResources’ model 100-250 test machine can be equipped with a 500 gf load cell for very accurate measurement of the tensile strength of a single fiber of hair. Although our 500 gf load cell has mechanical overload protection, you still need to treat it with care. The challenge in working with a very small load cell is avoiding damage to the cell during test setup and specimen changes. For example, it is not difficult to accidentally overload a 500 gf load cell when you are tightening a manual vice grip tight enough to avoid slippage of the hair specimen. TestResources can recommend two specimen mounting techniques which have been very successful for our customers in testing single-strand hair, textile fibers and silk strands. One method involves mounting the specimen in a cardboard frame prior to mounting it in the test machine. The last step before initiating the test is to gently cut both sides of the cardboard frame:  

Another method involves having a rigid test fixture into which the hair fiber specimen is glued on both ends. The fixture has a pin that is gently removed to expose the load cell to the specimen assembly, as the last step before initiating the test. 

Both of these techniques are designed to minimize the disturbance on the load cell as the specimen is mounted in order to minimize the risk of damaging the load cell as you are mounting and dismounting the specimen.

Learn more about these techniques  

Other popular mechanical tests of hair involve friction testing. Developers of hair conditioners and gels need this information. A popular test for hair friction involves measuring the force needed to pull a comb or similar object through a bundle of hair. The hair is tested prior to treatment and after treatment to compare. The characteristic of reducing friction is a key perception attribute to customers for hair conditioners and gels. 

TestResources offers all of the testing equipment you need for successful single-fiber tensile testing of human hair and other materials, as well as testing bundles of hair. Contact one of TestResources’ knowledgeable and experienced application engineers today. You will be pleased with the value of our testing equipment and your testing success. 

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