How to measure the physical and dimensional properties of hair – Tensile and Bend tests

Hair mono filament testing

Tensile testing is the simplest method for measuring the strength of a hair fibre. Hair strength correlates with the amount of damage that it has undergone, whether from UV, bleach or mechanical treatments. 

If the diameter of the hair is accurately measured using a micrometer, fundamental measurements of breaking stress, breaking strain, energy to failure and elastic modulus can be calculated. Hair fibres can be variable, so a large number of repeats will aid with data interpretation.

This type of test is generally performed at a constant strain rate (constant loading arm velocity), but fatigue testing can also provide useful information, as it is more imitative of the trials and tribulations undergone by a head of hair over its lifetime. In a fatigue test, a large number of small displacement cycles are performed. The sample is stretched a small distance, then a fluctuating movement applied for a given number of cycles, or until breakage.

Hair bend testing

The bending force of a hair bundle can be used as an indicator of softness. The effect of various hair treatments on softness can be determined by measuring the tress’s bending properties before and after treatment. For a treatment such as bleaching, an average bending force from a cycle of ten three-point bend tests is calculated for each sample. 

This is also applicable to measuring the efficacy of conditioner formulations containing new softening ingredients. However, to prove the holding power of fixatives such as hair spray, the drop in bending force is calculated from the first to last cycle, as the polymer holding cast is broken after the first deformation. The hair bundle is clamped at one end and the remaining length placed on two supports. The Texture Analyser probe descends vertically on the sample a set number of times, applying pressure to the central section of the hair bundle.

Hair suppleness

The cosmetics industry frequently promises softer, suppler hair after the use of their products, or promises that certain traditionally harmful methods (such as bleaching) will have no adverse effect. The difficulty comes in testing these properties in an imitative, repeatable way. The use of a Hair Suppleness Rig can measure the resistance of a hair sample to being run through a set of smooth bars, representative of running fingers through the hair on a human head. 

The force detected by the loadcell when a hair sample is pulled through this rig is made up of several factors: 

• The stiffness of the hair fibres (as they have to constantly bend on their way up through the rods)
• The friction between hair strands (as they rub against each other constantly when the tress is being pulled upward)
• The friction between the hair strands and the rig (as they are pulled against the surface of the rig itself)

An alteration to any of these factors will have an effect on the measured force. A higher force represents a higher resistance to motion through the rods of the rig. A suppler hair sample will have a lower resistance to motion, and in turn, suppler hair on a person’s head feels flexible and smooth when fingers are run through it. 

This is similar to subjecting a sample to a three-point bend test, but the bending configuration is slid along the length of the tress – i.e., continuous three-point bending. Bending tests can be challenging to set up in a reproducible way, particularly for suppler samples that cannot support their own weight without drooping, so this test is a simpler alternative that can be directly related to the consumer’s perception of their own hair texture.

Part 3 in a 4 part series on how to measure the physical and dimensional properties of hair. Read Part 1, Part 2 and Part 4.