Blown bulb

Researchers are trying to shed some light on why tungsten filaments in light bulbs fail they way they do.

Ideally, incandescent light bulbs last for 42 days in continuous operation. But in reality, some do not burn out for years, while others last only a few days.

Fine cracks in the tungsten filament, which eventually cause it to break, are one reason that manufacturers are unable to produce a more uniform quality product  – a problem faced by both Osram and Philips, the world’s biggest light bulb manufacturers.

So far, the industry has relied on trial and error to improve the drawing process for the filament. But those production processes can be enhanced by simulating the behaviour of the filament material on a computer.

And supported by researchers from the Fraunhofer Institute for Mechanics of Materials IWM, that’s just what the manufacturers are doing – they are investigating the cracks and the resultant difficulties when spiralling the wire.

Osram project manager Bernd Eberhard is confident that,  “once we have more insight into the composition and behaviour of the filament, we will be able to optimise and standardise our production processes.”

With an average diameter of 40 micrometres depending on the type of lamp, the tungsten filament is only about half as thick as a human hair. To reach this diameter, the wire has to be pulled repeatedly through a wire-drawing die that stretches it lengthwise and makes it progressively thinner.

Depending how often the process is repeated, it may acquire a varying number of longitudinal cracks.

Splits of this kind form primarily during the first stages of the drawing process, when the wire is thinned from almost 4mm to only 0.3mm.

The fine cracks grow longer when the wire is stretched further to a diameter of as little as five micrometers because of the tension that remains in the wire after drawing out, as IWM project manager Holger Brehm and his predecessor Sabine Weygand have discovered.

“We have already succeeded in mathematically describing the behaviour of the wire and the cracks that form during and after the drawing-out process.”

And for the first time ever, the tungsten filament can now be monitored on the screen during the entire thinning-out process.

Crack formation is being further investigated, and other factors are now being integrated into the computer model. One such factor is the friction between the wire and the wire-drawing die. High friction makes the metal hotter.

So the researchers are now building the temperature change during and after drawing into their simulation.

“The drawn wire cools faster on its surface than on the inside,” Brehm said. “Unfortunately, splits can occur during this process as well.”