BS 7928: 2013
Where helmets are intended to provide protection for the users’ head in the case where the user themselves provide the movement, or where the user is likely to come under impact from items other than from above, helmets are usually tested using the falling headform method. Instead of using a fixed headform impacted with a falling mass, the headform itself, with the helmet fitted, is raised above a fixed anvil and dropped to generate the impact. Headforms, which are typically made from aluminium alloy, are made in several sizes so as to allow a reasonable fit to the helmet, and contain a tri-axial accelerometer (three single accelerometers in the x, y and z planes). On impact, these accelerometers will record the acceleration (or in this case, deceleration) of the headform in all three directions, and record a resultant value. In addition, the acceleration plotted over time can be used to calculate the head injury criterion (HIC), which gives a measure of the expected likelihood of serious injury to the user. It is calculated based on an integration of the acceleration against time between two points on in time.
Helmets can be dropped onto different types of anvil, including flat, kerbstone (corner) and specific-shaped anvils, such as balls. Drop heights will vary from each standard, depending on the perceived hazards in use. In the case of cricket helmets, the headform is dropped from a height of up to approximately 0.5m (15J) onto a ball-shaped anvil, with a maximum allowable acceleration of 250g (2453m/s2). Testing is carried out following conditioning to high temperature, low temperature or UV ageing.
BS 7928:2013 introduced a test to assess whether or not the ball is able to make contact with a specified ‘no contact zone’ of the face if it strikes between the peak and the top of the faceguard. The same test is also able to assess whether or not the faceguard is likely to make contact with the ‘no contact zone’ in the event that the ball strikes the faceguard. Testing is carried out using an instrument capable of firing a ball in turn at five different specified sites – one of which is the gap between faceguard and peak. The standard defines the position of the helmet and headform with respect to the direction of the ball. Depending on the size of the helmet tested either a junior ball (with a mass of 115-122g and a diameter of 67-69mm) or a senior training ball (with a mass of 140-150g and a diameter of 70-73mm) is fired towards the helmet. The senior ball is fired at a velocity of 28m/s while the junior ball is fired at a velocity of 23m/s. The helmet under test is mounted on a standard headform that is of a suitable size. The face of the headform is covered with a suitable witness material designed to highlight any contacts made between the ball or faceguard and the front of the headform. The standard requires that no contact is made between the ball or the faceguard and the headform in the specified zone.
Most specifications for protective helmets include a number of requirements for the design of a helmet in addition to the specific performance requirements. These typically encompass the area of coverage provided by the helmet, as well as the field of vision afforded to the user when worn. They can also cover a number of ergonomics and safety-based requirements, such as clearance between the head and the shell of the helmet (particularly in the case of industrial helmets).