Protecting the human body against injury dates as far back as recorded history, when ancient warriors wore bronze, copper and iron breastplates during battle. Today, for the army, police officers, security personnel and those that compete in the world of sport, the use of equipment to protect the body is almost commonplace. From sports shin pads and ballistic vests to thorn-proof gardening gloves, wearers rely on body armour to protect them against severe injury, and at the same time require it to be lightweight and comfortable to wear.
Protective Equipment from a Design Perspective
The range of incidents that can cause injury is immense, and while there are various body armour solutions for particular problems, the issues of impact, man-made protection versus natural body protection, and damage to vital organs are often not looked at in an integrated manner when designing equipment for body protection. Key questions must be answered as part of this design process. What is it important to guard against - impact velocity, mass, force, impulse, energy, or momentum? Which of these factors is attenuated most on passage through a protective layer, and do different protector designs/materials do this to different degrees? The ability to stop a bullet or knife blade may decide success in some applications, but what of blunt trauma, such as being hit on the shin? How good is the body’s own protective system?
Protective Equipment from Technological Perspective
‘From a technological viewpoint, body armour is now well developed, with a variety of competing materials systems and manufacturers, all offering good all-round solutions,’ says Ian Horsfall from Cranfield University RMCS Shrivenham, UK. ‘The current challenge is to engineer a better balance of properties to improve ergonomics and the integration with other equipment and clothing.’ As he points out, the military market is relatively mature and its main challenge is to increase the level of protection and coverage while keeping weight low. The police and other civilian markets, such as public transport workers, cab drivers and ambulance paramedic crews, are relatively less mature with significant challenges in providing wearable and often discreet systems for everyday use.
Since the advent of modern, body armour in the 1970s, ballistic fibres have become stronger, enabling manufacturers to produce lighter, more flexible body armour. ‘Improvements in fibre technology are providing a steady increase in armour efficiency,’ says Horsfall. ‘It is not unusual for modern systems to incorporate two or three types of fibre in three or more types of weave.’
Fibres Used in Armour Applications
The two main fibres used in soft body armour are aramid and polyethylene. Kevlar, Twaron and Technora are all aramids, while Dyneema and Spectra are part of the polyethylene group. Designed to stop bullets that soften on impact, soft body armour offers little protection against bursts of fire from submachine guns. This level of protection is afforded in hard armour bullet-proof vests, which traditionally have rigid ceramic plates in the front and in the back, making them heavy and restricting to movement.
Combining Hard and Soft Armours
In a move to overcome the problems of wearing hard armour vests, a new type of material has been designed that not only protects the body from soft bullets but is also designed to withstand a burst of fire from an automatic firearm. The Lorica Armour Vest, recently demonstrated on the BBC’s Tomorrow’s World programme, is the brainchild of Digby Dyke From Lorica Research. The vest uses the new material, which is made from three types of synthetic polymer fibre that have very high impact and penetration resistance. These fibres are already used in the manufacture of body armour, but the key to the new fabric’s strength lies in deeply impregnating the fibre textile with an ethylene-based thermoplastic resin to form a strong net. In the finished vest, 22 layers of the fabric work independently to absorb and dissipate the energy from the bullet.
Soft Armour and Blunt Traumas
Stopping the weapon is only part of the problem. In soft armour vests, which use fabrics such as Kevlar that don’t stretch, when the bullet is contained, the energy is absorbed and dispersed from the struck fibres to other fibres in the weave of the fabric and ultimately to the body. This shock to the body is known as blunt trauma and must be kept at a level in which injury from it does not occur. The human body can withstand a certain amount of blunt trauma and this tolerance is defined in millimetres of ‘back face signature’ during body armour testing.
Some vests have to protect the wearer from knives as well as bullets, and minimising blunt trauma in such cases needs to be addressed when testing body armour against being stabbed.
Challenges in Armour Design
The first challenge is characterising a ‘real attack’, which involves, for example, deciding on the force, energy, momentum and velocity, and how these vary in the time domain. Current methods for testing body armour are based on the use of a ballistic or drop-weight driven missile with a knife blade as the source of the stab. The specimen is mounted against a support medium, which mimics the compliance of a human torso. ‘There are two parts to the testing system,’ says Professor Thomas Gray from the Department of Mechanical Engineering at the University of Strathclyde (UK), another speaker lined up for the seminar, ‘the delivery part (maybe the attacker) and the target, which can vary substantially depending on the part of the body that is struck and what kind of armour there is. Ideally you need different kinds of armour for different places.’
Characterising ‘Real’ Attacks
The University of Strathclyde was commissioned by Strathclyde Police to set up a facility to test body armour according to the then-current standards. Concerned that the test machine was doing something different from real attacks, which could have meant the whole thrust of materials development was aimed in the wrong direction, the Bioengineering Department set up a project, funded mainly by the Scottish Office and Police Union contributions, to measure what actually happens when a range of volunteers were invited to stab a dummy target. ‘This showed that our fears had not been unfounded and, in conjunction with findings elsewhere, this led to the development of a new standard, using a different form of test system,’ says Gray.
Refining the Test Simulation
While the new system allows the basic criteria in terms of force, energy etc to be in the right ballpark when used in the standard drop weight test, the machines are somewhat stiffer than the relatively floppy limbs of an attacker, and correct time profiles of these quantities are not necessarily being reproduced. ‘I don't think we have by any means reached the end of the road in terms of realistic simulation,’ says Gray. At Strathclyde we are trying to find ways of fine tuning the drop weight systems to deliver something nearing what we measured in earlier studies.’
Body Armour for Males and Females
Body armour is a highly competitive field with a large number of manufacturers selling armour. However, one market and challenge that remains is the provision of the same levels of comfort for female wearers as is already afforded to most male wearers. ‘There are both technological, medical and ergonomic problems still to be solved in order to provide comfortable and effective protection to female users,’ says Horsfall.
A New Type of Cushioning Material
A new material developed by Cheshire Innovation could provide ergonomic advantages to help solve the problems of designing comfortable personal protective equipment for the upper female body-form. SALi is a new type of cushioning material for use in helmets, protective clothing and other items in which protection against violent impact is required. SALi consists of lots of resilient elastomeric capsules immersed in a matrix liquid and stored in a strong flexible package. During an impact, the capsules shrink in size as they are compressed on all sides by the matrix fluid. The capsules, lubricated by the fluid, rearrange themselves inside the package, so that the front face of the package takes up the shape of the impacting body, figure 1.
Figure 1. SALi could provide ergonomic advantages to help solve the problems of designing comfortable personal protective equipment for the upper female body-form.
Elastomeric capsules investigated include expanded polystyrene beads, polymeric microspheres, bubbles cut from bubble packing and narrow diameter, open ended, hollow tubes, with pockets of air trapped inside them. ‘All of the blends tested to date display useful impact absorbing characteristics,’ says Bill Courtney Engineering Consultant at Cheshire Innovation, UK. ‘The diverse choice of materials available for making up blends of SALi should allow us to develop formulations to provide different elastic and viscous damping characteristics, to solve a broad range of impact absorbing problems.’
Research at Queen Mary, University of London has also centred on designing body protection for female wearers in the form of the women’s bobsleigh team. The work, has focused on designing appropriate body protection and support for female bobsleigh athletes, after a high number of hip and lower back injuries were reported. The women have to compete using essentially men’s sleds that are 20kg lighter, which are not designed with the female body shape in mind. The research team at the university has developed a number of design improvements for customising the existing men’s sled for women, which have mainly centred on improving the interior design of the sleds using new materials, and designing appropriate clothing for limiting the damage caused by skin burns.
Whether the body protection is for male or female wearers, it needs to be practical to wear, light, flexible and must cover the vulnerable areas for its particular application. ‘Although manufacturers have made great strides, there is still plenty of room for improvement,’ says Gray. In the next few years, Horsfall from Cranfield University believes it should be possible to apply powerful modelling capabilities to body armour to allow much better optimisation of current systems. ‘Improvement in fibres and other technologies are likely to provide further incremental gains but there may be other technologies that can be applied, such as reactive and active systems,’ he says. With such optimism, many could soon be wearing body armour with enhanced features such as active cooling.