Breaking glass to make it safer
Our craving for daylight has accelerated the use of glass in modern buildings. However, for people concerned about safety, our passion for the light comes with a dark side.
Karoline Osnes, who recently defended her PhD at NTNU’s Centre for Advanced Structural Analysis (CASA), knows more than most of us about how glass behaves under an extreme impact.
“Glass is becoming more and more of stand-alone building material. The use of it has advanced from simple windows to full façade claddings and load-bearing components. Still, the knowledge on how to calculate its ability to withstand extreme loads has not kept up with these trends,” she says.
The biggest threat
In the years to come, we expect more severe weather events and the risks of sabotage and terror attacks are expected to continue. When the unthinkable happens and whether we sit in our car, in a railway coach or in an office building, we must be as well protected as possible.
The problem is, even as the use of glass increases, it still remains the weakest point in a building. This means less protection for vulnerable human bodies.
During an explosion, the shockwave propagates against the façade. In a split second, glass, concrete, aluminium or steel can change into thousands of potentially deadly fragments. When incidents like this happen, glass usually poses the biggest threat to people nearby. In the event of an urban explosion, 80 per cent of the injuries are caused by pieces of glass that are forcefully ejected when the glass shatters.
Need for predictive models
The most common approach to reducing this threat is to use laminated window glass. This type of glass consists of two or more plates of monolithic glass bonded together by a polymeric interlayer. When it breaks, the polymer keeps the dangerous fragments in place. But how does this type of glass behave under impact? How can we use this knowledge to facilitate predictive glass design?
Osnes says there is a strong need to develop models and methods that can predict the response before, during and after a glass fracture. Triggered by these questions, she embarked on her PhD-journey 4 years ago.
Where, when, why and how does glass break?
Since then, the PhD candidate has subjected more than 100 glass panes to blast loads and ballistic impact. The experiments were performed in the Shock Tube and the Gas Gun at NTNU’s SIMLab.
Her dissertation, Monolithic and laminated glass under extreme loading: Experiments, modelling and simulations, describes the characteristics of the two types of glass as precisely as possible. That makes it possible to calculate how strong a specific kind of glass should be so it is protective.
BMW has already put Osnes’ model to use in their standard roof strength test. Another CASA partner, Audi, has also show ninterest in using her model to describe what happens in glass microstructures.
Reference: Fracture and fragmentation of blast-loaded laminated glass: An experimental and numerical study. Karoline Osnes, Jens KristianHolmen, Odd Sture Hopperstad, Tore Børvik. International Journal of Impact Engineering Volume 132, October 2019, 103334