Testing Kevlar material and stab-proof vests

Requirement

Kevlar® has many applications in the modern world ranging from bicycle tyres, racing sails through to protective gloves and body armour. It is one of the strongest materials available worldwide being five times stronger than steel on an equal-weight basis. Due to its high tensile strength-to-weight ratio Kevlar® is perhaps the most widely accepted material used in body armour and so-called ‘stab-proof’ clothing today.

Stab resistant vests incorporate Kevlar panels housed in carriers manufactured from softer fabrics, which are more comfortable when in contact with the skin over extended periods of time. These Kevlar panels need to protect the body by resisting penetration from edged and pointed weapons such as knives, spikes and broken bottles. 

Against a background of knife crime offences in the UK rising by 80% between 2014-2019, a prominent UK police force contacted Mecmesin to carry out a series of penetration tests on their overt and covert stab-proof vests, manufactured in the Far-East. The tests were performed using real weapons collected from knife amnesty bins. The objective was to obtain comparative data relating to the penetration forces at various parts of a single vest and across different aged vests, both new and used. Of particular interest were any differences in penetration resistance encountered in the shoulder, side, chest and back areas.

Solution

The Kevlar® fabric is intrinsically strong but when tightly woven together it creates an even more effective protective barrier. This tight weaving of the fibres makes it extremely difficult for a weapon to be able to penetrate the material because an amount of force is required. A knife becomes caught in the woven fabric, slipping through the material whereas a sharp, pointed object will cause the fibres to move around the point, slowing down the rate at which the item can pierce the vest, preventing a full-puncture condition from occurring.

A MultiTest 25 kN twin-column tensile tester was configured with a vice-grip to hold a variety of different knives and a penetration jig to support the stab vest specimens. Mecmesin’s Emperor software controlled the tensile tester to apply the high compression forces needed to completely penetrate the material. Calculations were set up to establish and report the maximum penetration force in newtons and also the energy in Joules, which is a significant comparative performance parameter.

Test equipment

• MultiTest 25-i twin-column software-controlled tensile/compression test system.
• 10 kN loadcell.
• Special upper fixture to grip a selection of knife blades and typical improvised stabbing weapons.
• Lower fixture to support the target area of interest the vest with an aperture to enable the blade to pass through completely.