EN 1078: 2012
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 cycle helmets, the headform is dropped from a height of approximately 1.5m (up to 89J) onto a flat anvil, and approximately 1.06m (up to 62J) onto a kerb anvil, with a maximum allowable acceleration of 250g (2453m/s2). Testing is carried out following conditioning to high temperature, low temperature or UV ageing.
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).
Helmets can only protect the head when they are being worn and therefore the means for retaining the helmet on the user’s head requires as much attention as the rest of the head protection, and so is subject to a series of tests. The specific test carried out is dependent on the type of helmet, but two main tests are carried out:
Retention system strength: The retention system (in particular, the chin strap) is subjected to a force, applied either statically or dynamically, to ensure the strap is unlikely to fail at the point where it is most necessary. In the case of industrial helmets, it is however desirable that the chin strap will not cause a strangulation hazard, and so cannot be too strong, and therefore straps need to include a break-away element at the anchorages, intended to fail within a specific load range. Typically, the helmet, including chin strap, is fitted to a suitably-sized headform, with the chin strap either fitted to an artificial chin (consisting of two rollers mounted on a frame), where the headform remains static, or to the chin of the headform itself, where the headform is used to dynamically apply the force. The chin strap is then subjected to either a static force (where the artificial chin is slowly loaded until failure) or a dynamic (shock) load, applied using a falling mass, and the amount of stretch in the chin strap is measured.
Retention system effectiveness: Helmets are subjected to a shock load, applied to the rear or front of the helmet in an attempt to pull the helmet off the headform. This is intended to consider the risk of the helmet catching on an obstacle and being unintentionally pulled off the user’s head. The test load (applied using a 10kg falling mass) is applied, via a system of pulleys, to the rear of the helmet when mounted on a suitable headform, with the direction of loading following a direction approximately 45° from the horizontal towards the front of the headform (this is occasionally repeated on the front of the helmet). In order to meet the requirements of most protective helmet standards, the helmet must remain on the headform.