Sled tests are generally not as well-known as crash tests. Everyone is immediately familiar with crash tests, where a car with test dummies is crashed. But what does the term ‘sled test’ mean? In principle, these are simplified crash tests that make it possible to test various combinations of restraint systems to increase vehicle safety. Let’s take a look at the most modern technologies used in sled tests.
Sled tests allow engineers to reproduce the dynamic conditions of various full-scale crash scenarios and they are non-destructive. Usually, only a few vehicle components are needed to perform them. As the side sled tests are performed, the catapult accelerates vehicle components and dummies along a rail. The sled acceleration is several times higher than the gravitational acceleration. These tests are based on the simple energy conservation principle. This means that in a crash test the vehicle is usually accelerated to a prescribed speed and then crashed into a fixed item. As a result of the collision, the occupants move by inertia in the direction the vehicle was travelling in before the collision. The occupants are then caught by restraint systems such as airbags or safety belts. Several principles can be applied when simulating crash test via sled test. They are divided into two main categories, according to the principle on which they are based - acceleration or deceleration. The most modern sled tests usually using the acceleration approach, mainly because this allows greater conformity to crash tests. In acceleration sled tests, in contrast to crash tests, everything happens in reverse. The test starts at zero speed and the test vehicle is accelerated backwards using a dynamic catapult. During this movement, the loads on the occupants are equivalent to a crash test. The side sled tests enable the restraint systems performance evaluation and also the assessment of their effect on the occupant’s injuries without destroying the body-in-white. The potential of the sled tests lies mainly in a laboratory environment, where the input conditions are always the same. Additionally, one sled test costs a fraction of the cost of producing and destroying an entire prototype vehicle.
According to the regulations, every new car must fulfil two main requirements relating to side impacts. These relate to pole impacts, which simulate an accident where the vehicle collides with a tree or possibly lamp pole, and barrier impacts, which simulate an accident where the vehicle collides with a stationary vehicle on the driver’s side. In addition to the prescribed legislation, both of these types of the test appear in the NCAP consumer tests. The NCAP consumer tests challenge the vehicle manufacturers to produce ever safer cars and to fulfil the strict criteria for achieving the maximum number of stars. The NCAP’s requirements are regularly amended in order to increase the safety of occupants. Therefore, from 2020, the protection of the drivers in impacts on the passenger side of the car has been also included in NCAP side crash tests. This test is known as the “far side” test. Within the far side test, the interaction between the heads of the driver and the passenger is evaluated, as the secondary contact of the heads of two passengers can cause serious injuries. In order to prevent the mutual contact of the driver´s and passenger´s heads a new type of airbag known as the centre airbag has been developed in recent years. This airbag inflates between the occupants from inside the seat or from the roof trim.
The occupant kinematic in the side impact is quite complex as it is determined by the acceleration of the vehicle as well as by intruding structure. In both impact configurations (pole/barrier) the structure of the vehicle is significantly deformed. This fact is double true for soft cars, where the intrusion into the interior can easily be up to 400 mm. The deformation, furthermore, takes place very rapidly. The entire collision lasts about 0.2 seconds. The bio-mechanic load on the test dummy peaks about 0.06 seconds after the impact. In the meantime, the vehicle must perform many tasks. It must identify the fact that a collision is happening, assess what kind of collision it is (front, side, rear, rollover) and active the passive safety elements. In the case of side impacts, this typically involves pretensioners of the safety belts and the side, head and centre airbags. These airbags usually inflate within 0.03 seconds from the start of the impact to provide the occupants with a soft cushioning and restraint. It is this very rapid sequence combined with the major intrusion into the interior that makes side impacts one of the most complex of sled tests. The performance of these tests requires special systems, which are quite rare in Europe. One of them, which is called ALIS (Active Lateral Intrusion Simulation) from the Spanish firm ENCOPIM is being used by TÜV SÜD Czech in Mladá Boleslav.
The side-impact testing equipment is part of the modern DYCOT testing facility, which was brought into operation in 2016. It took almost three years to build this laboratory and it cost almost 4 million EUR. The result is a laboratory with a giant INSTRON catapult at its heart, which can accelerate a one-tonne load at up to 90G. Achieving such a load requires a force of about 2 500 kN. The catapult works on a hydraulic-pneumatic principle with a working pressure of 300 bar. For side impacts, there is the unique ALIS system located on the rails in front of the catapult, generating the intrusion into the interior in side impacts and creating, together with the catapult, the correct load of the test dummies. The ALIS has three pneumatic actuators, which together generate a force of up to 240 kN. The system has very complex control logic, as it must synchronise all three actuators and the main catapult pulse. The ALIS actuators are controlled using hydraulic brakes, which work on a similar principle as the brakes in a car. But simply, it is as if you were stepping on the brake pedal several times in 0.06 seconds. These brakes are controlled by feedback function, which means that the system itself can compensate for unexpected internal forces to achieve a required actuator extension.
Lateral sled tests are always the main challenge for the entire development team, especially pole sled tests. Due to the high accuracy requirements, the preparations for these tests can take several months, as 10 millimetres of deviation from the crash test can lead to different airbag behaviour, which results in a poor bio-mechanical dummies response. Accuracy is thus the essential precondition for success. The side sled test preparations start before the first full-scale crash test of the vehicle. The inputs into the sled test are gained from a virtual FE (Finite-element method) simulation of the full-scale crash. This full-scale virtual crash simulation is reduced to the components required for the sled test – the seats, safety belts, interior trim and airbags. This reduced model is further used for intrusion dept analysis. The mutual dummy and trim interaction are also investigated. Based on these findings the intrusion mechanism under door trim is further designed. It consists of multiple moving parts in order to accurately represent the deformation of the interior. Subsequently, the FE simulation of the movable intrusion mechanism is prepared. The door trim and pillar trims are fastened to the intrusion mechanism. The dummy and seat are included. The simulation prepared in such a way further allows to adjust the intrusions and tune the ALIS actuator pulses in order to achieve the biomechanical response of the dummies as equivalent to their response in full-scale crash simulation as possible. Usually, the accurate loading of the dummy chest and shoulder is the most important. This part of the side sled test preparation process usually takes the most time, as a matrix of about two hundred simulations is created to tune up optimal dummy response. The computing time of the side sled test simulation is 4 to 10 hours, depending on the complexity of the model.
As the virtual part of the side sled test preparation process is finished, the data from the full-scale crash test are usually received. Based on the analysis of crash test data, the necessary adjustments of intrusion mechanisms and pulses are made in order to increase the accuracy of the simulation. As soon as all these preparation processes are finished, the side sled test can be performed. As you can see, the side sled test performance is just the tip of the iceberg in the context of the long preparation process.
Tuning the ALIS system takes almost five test shots, in which the speed of the individual actuators and, if necessary, the interior parts, are compared with the crash test. The basis of the test is the movement of interior parts. These must be in a precisely given place at a certain time so that the airbag will inflate correctly. Their movement is controlled by the ALIS actuators and as soon as the airbag has been correctly deployed, the catapult takes over the main activity, giving the dummies the correct movement. When everything is well synchronised, the bio-mechanical response of the dummies gives comparable values to a crash test. After the whole system has been tuned, a repeatability test is carried out, checking whether two tests give the same results and then comes development, which is the most important part. Different types of airbag and different restraint system configurations are tested. The bio-mechanical loading of the dummies is the resulting criterion. The monitoring focuses mainly on the seriousness of the injuries to the occupants.
The first lateral sled tests at TÜV SÜD Czech laboratory were performed in 2017 and many well-known cars have been tested on ALIS since then. So far, ALIS has completed about one hundred tests in which many airbags imperfections were discovered, and restraint systems settings were adjusted in order to achieve better occupant´s protection. The greatest advantage of ALIS is undoubtedly its variability. Thanks to the experience in the area of side impacts gained over many years , the development team can prepare the test exactly according to the customer´s requirements. At present, the system enables to test front and rear dummies together in barrier impacts. The occupant to occupant tests (test focused on mutual interaction between the driver and front passenger) are currently under development.
To fulfil the regulations and satisfy the requirements for vehicle safety, however, it is crucial to understand the sled tests as one of the pillars of a comprehensive development system. The vehicle development cycle starts with virtual simulations determining the direction of development, the sled tests then confirm this direction and finally, everything is validated in a crash test. This process is often repeated during the vehicle development cycle, and if one of these development pillars were to fall out, it would increase the risks in real accidents and mean additional costs for the vehicle manufacturer. TÜV SÜD is a proud partner of vehicle manufacturers in this development cycle, and help them to reduce the development cost as well as speed up the development.
Author: Petr Zaruba
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