Smash hit

Novel test equipment is being used by US researchers at Tennessee’s Oak Ridge National Laboratory to provide previously unavailable data on the crashworthiness of new car materials.

The servo-hydraulic machine, called TMAC (Test Machine for Automotive Crashworthiness), is designed to help the automotive industry accelerate the design process.

The system is basically a hi-tech crushing machine that can reach a speed of 18mph and sustain forces of more than 60,000lb over approximately 10in of crush length.The research, part of the US Department of Energy’s FreedomCAR programme – a collaborative initiative focused on the development of emission-free and petrol-free vehicles — is concerned with the crashworthiness of polymer composites, high-strength steels, aluminium and other materials that show promise in absorbing and dissipating the kinetic energy of a moving vehicle in a collision.

Raymond G Boeman, one of the chief researchers on the ORNL team, explained that traditional methods of analysing the crashworthiness of materials are fraught with problems.

The ORNL team defined the specifications for TMAC following discussions with Ari Caliskan, a Ford Motor Company engineer. Boeman said Caliskan was frustrated because ‘there are standard test machines available that will travel at higher velocities than TMAC and there are test machines that will test at higher forces. However, there aren’t any that can provide as high a force within the same velocity range or as high a velocity within the same force range.’

There are two types of machine used for this kind of material testing. They are either electro-mechanical, which are basically gear-screw-type mechanisms and therefore don’t offer high-velocity testing, or servo-hydraulic systems, which can be operated faster.

Servo-hydraulic systems usually use a feedback signal to control the position of the actuator and rate of loading. However, at very high velocities response time is not fast enough and feedback control is out of the question. Such systems typically use a poppet valve which is open or closed. This means there isn’t any control, and as the specimen is crushed, the system is robbed of energy.

‘Commercial systems of this type are usually low-force because it doesn’t make sense to have high force and high velocity if you don’t have any control over the velocity,’ Boeman said.

He claimed the TMAC system overcomes these issues in two ways. It uses software that dispenses with the need for a feedback mode by simulating the machine’s response based on expected or measured specimen performance.

However, the system is also equipped with an ‘energy capacitor’. This is basically a 450kg mass on the end of the actuator that ‘stores’ energy prior to crushing the specimen.

When impact occurs, energy is supplied by the moving mass (just as in a drop tower) and also by the hydraulic flow. This, said Boeman, is why TMAC can control velocity better.