Five miles outside of St Albans, in a humdrum facility obscured behind a chain-link fence topped with barbed wire, a small group of researchers stab, shoot and slash their way through the day. Their goal? To make sure that the body armour worn by the UK police protects them from knives and bullets if the worst happens.
Inside one of the squat buildings, in a room crammed with mannequins kitted out in police gear, Graham Smith, an expert in firearms and protective equipment at the Defence Science and Technology Laboratory (DSTL), is describing the physics of a stabbing. “A human stabbing deposits the energy over a long time period and you get this double-peak as the knife goes in and there’s a follow-through from the arm,” he says, miming as he talks.
On the desk next to Smith is a particularly mean looking stainless steel knife with a curved blade, but instead of a handle, this blade is wedged into a large chunk of plastic. Originally, Smith explains, this was designed so the researchers at the DSTL could fire knives at high speeds into a test piece of armour.
But after initial testing rounds Smith and his colleagues realised that an air cannon wasn’t the most accurate way of mimicking a stabbing. A knife thrown by an air cannon hits its target with a quick burst of energy and then that’s it – it bounces away to nothing. After inviting a series of volunteers to stab a dummy wired up with telemetric trackers to measure the force of the attacks, the researchers at the St Albans laboratory devised a new contraption to test body armour to its limits.
This new device – a clear tube extending through the ceiling and into the loft of the single-story building – looks a lot like the pneumatic tubes that are sometimes still used to propel documents or cash through banks and hospitals. But this length of tubing has a far deadlier cargo. Inside is a finely-honed steel blade wedged into another hunk of plastic that backs onto a couple of foam inserts. As the blade drops from the top of the tube it falls with between 24 and 33 joules of energy – the upper limit of most people’s stabbing thrusts. And as the foam contracts then expands again, the knife drives back into the armour it falls upon with a second burst of energy.
But this is no match for the square of stab-resistant armour. After the test Tom Payne, principal scientists at the DSTL holds up the blade. The tip, razor sharp seconds earlier, is almost completely blunted.
For Payne and Smith, the blunt blade is a sign that their work is going as planned. The DSTL, which is part-funded by the Home Office, sets the testing standards for body armour and protective equipment worn by police in the UK. In July 2017 it published its latest body armour standards for bullet and stab resistant armour, the latest in a long line of standards that stretches back to 1993. Police forces then use these standards to help them decide on what armour to buy from private manufacturers.
The square of body armour being tested at the St Albans lab is mostly made up of aramid – a class of strong synthetic fibres that includes Kevlar and are tightly-woven to stop a blade from getting through. “This type of armour would be looking to blunt the blade and arrest the knife, absorbing the energy and stopping it penetrating too far,” says Payne. “There's a really high density of fibre so the blade won't slip through them.”
Early versions of the stab-resistant armour were made up of interlocking metal circles designed to stop the blade from penetrating, but officers ended up leaving the bulky and hot vests on the back seat of their police cars rather than wearing them. And a stab-resistant vest that no one wants to wear, Smith points out, doesn’t provide much protection at all.
Modern stab-resistant vests are now made out of the relatively lightweight aramid and are either combined with a thin layer of chainmail-style metal or resin-coated fibres to catch and deflect the blade. “It's about matching the threat we're seeing with the comfort and performance of the body armour,” Smith says. Slash-resistant armour is slightly different again, and is designed to make the blade skip across areas like the neck and arms that are less likely to be subjected to a penetrating knife blow.
In order to pass the DSTL’s latest set of standards, stab-resistant body armour must stop a blade penetrating further than eight millimetres out of the back of the armour – a measurement decided upon because that is the shallowest possible depth that a knife may penetrate through body and still hit a major organ. On the floor there are an array of mannequins with the outlines of the five major organs – lungs, heart, liver, spleen and kidneys – drawn on the surface. “You physically can’t cover every part of a person,” says Sarah O’Rourke, a principal scientist at the DSTL, but the main aim is to make sure body armour covers as much of those five organs as possible.
But every now and then, the DSTL hears whispers of a new weapon that means it has to go back to the testing room and check its armour standards are up to the new threat. From the prison service, for example, Smith started hearing that prisoners were using spikes instead of knives. “What you tend to find is that prisoners will fabricate things and that tend to be along the lines of a spike,” he says. To defend against spikes, armour must have an ever higher density of fibres than stab-resistant vests to stop the weapon sneaking between fibres and penetrating the armour altogether.
In the early 2000s a message filtered up from the National Police Chiefs Council that criminals were buying antique weapons and modifying them so they’re capable of firing bullets again. “The ammunition is difficult to get because they tend to be obsolete calibers, but various methods of manufacturing this ammunition are emerging,” Smith explains. When it came to the antique weapons, testing existing armour standards against the new threat showed that body armour was already capable of defending against the relatively low-power weapons – but to stay ahead of threats the DSTL always has its ear to the ground.
Antique weapons rank relatively low on the DSTL’s list of possible threats. On the other side of the facility, past a display case containing half a dozen blades and a corrosives lab, there’s a firing range where the researchers test whether body armour is capable of withstanding attacks from pistols and rifles.
At one end of the firing range, an armour vest is strapped around a human torso modelled from Plastilina – a brand of non-hardening modelling clay commonly used in ballistics testing. While the pass-fail standard for stab-resistant armour is whether the blade penetrates more than eight millimetres past the armour, for bullets it is the depth of the crater left in the Plastilina model after the bullet has struck.
When a bullet travelling at upwards of 720 metres per second hits the ceramic or silicone carbide plates commonly used to protect against high-velocity rounds, the force of the impact causes the rear of the plate to deform into a dome-like shape. As long as that indent in the Plastilina is less than 25 millimetres deep, then the armour will do its job of protecting the person – who might still be knocked off their feet or severely bruised – from any serious harm.
When it comes to ballistics protection, the main goal is for the armour to completely stop the bullet says O’Rourke, pointing to a pair of bullets on the workbench in front of her. One of them is still intact but the other, having been fired into an armour plate, looks nothing like a bullet. The metal slug appears as though it has been split open and peeled apart so it resembles the petals of a flower. This ‘petaling’, O’Rourke says, is a sign that the armour has done its job, reducing the bullet’s aerodynamic properties and stopping it penetrating the armour.
However, O’Rourke admits, there are limitations to using modelling clay as a way of estimating the impact of a bullet on a human body. The DSTL is considering moving to silicone or gelatine models instead, which are closer to mimicking the physical properties of humans but don’t have such a long history of being used in test standards as current models do. But if it can shift to more human-like models, the DSTL wouldn’t just be able to make armour that is better at protecting bodies, it might also make it easier to wear. “If you had a validated method that was more like the body you might be able to drive that change to make the armour less burdensome,” O’Rourke says.
This article was originally published by WIRED UK
