In a traffic accident, a car hits an obstacle and slows down. According to the law of inertia, the car and its passengers continue to move until something brings them to a stop. At first glance, the sled test does exactly the opposite. Starting from a standstill, the simplified part of the car with the test dummies is accelerated backwards by using the dynamic pulse of the catapult to achieve the same acceleration/deceleration as during the crash, according to the law of conservation of energy. The simplified part of the car, as well as the test dummies, are subjected to the equivalent energy of a real traffic accident.
Euro NCAP’s legal regulations and requirements assess passenger protection in different types of frontal impacts – for example, a collision between two cars travelling at 50 km/h or a collision with a solid wall at 50 km/h. The catapult can accelerate the reinforced body to a gravity force of 90 G (a force equivalent to 90 times the passenger’s weight) with a payload of 1,000 kg (the combined weight of body and passengers).
Very complex side sled tests are also carried out in the laboratory. “During development, we try to take into account the most common types of side impact. The first represent the situation of a skidding car heading into a tree at 32 km/h. Another typical side impact represents a collision with another car at 60 km/h. In both cases, there is a significant deformation of the car’s body structure and interior”, explains Radek Urbiš, Head of EKS – Car Safety.
In addition to the interior deformation, the developers are also up against the speed of the action. For this task, the ALIS (Active Lateral Intrusion Simulation) advanced test rig is used, which consists of several pneumatic pistons with a force equivalent to up to 12,000 kg. They are used to create lining deformation and accelerated by the catapult piston.
Sled tests cannot capture all the crash test phenomena (e.g. dynamic body roll). This makes simulation support all the more important. Thanks to advanced FEM (finite element method) simulations, it is possible to predetermine the appropriate restraint configuration or the appropriate deceleration/acceleration pulse so that the result is as faithful a replica of the car crash test as possible. FEM simulation is also used in ALIS synchronisation.