Creating Buildings That Can Withstand the Most Extreme Stress Loads
Danger of Overestimating Strength
A shock wave can last for several milliseconds and cause great destruction over a large area. A fragment moves even faster and produces concentrated damage. Simulating this combined effect means that you have to describe two completely different phenomena in one and the same model. It’s complicated.
“Often you’ll end up with some sort of trade-off. In order to capture the local weaknesses that occur during the explosion, we need to determine how accurate the descriptions of the impact of the fragments should be. If we don’t achieve full control over this, the model could overestimate the strength of the building to withstand the stress,” says Elveli.
Need Solutions That Can Be Trusted
Overestimating strength can have fatal consequences. The solutions that construction engineers deliver have to be dependable. A large part of Elveli’s doctoral work has been to investigate how accurate the models need to be to ensure reliable buildings.
A common approach has been to assume that the fragments hit before the shock wave happens. The physical experiments then have to be divided into two different sequences that follow each other. Often such studies use a simplified approach, where the test pieces have holes milled out by machine to mimic damage from real fragments.
Overestimating Resilience
Elveli has compared the behaviour of machined plates with plates hit with real projectiles. Real projectiles created small petal-like cracks and deformation around the points of impact, whereas the pre-formed defects had perfectly even edges.
The explosion tests showed that the destruction started in the cracks and spread outwards. The researcher thus shows that the simplified approach has weaknesses.
“Idealized defects, like in the machined plates, are easier to test and simulate. But because they lack the deformations and damage that occur in real explosions, there’s a risk of exaggerating the strength of the materials in these models,” he says.
Great need for Computer Simulations
Understanding the need to develop accurate computer simulations is easy enough. Researchers who work with strength calculations cannot blow up actual buildings to test their resilience.
Elveli has put a lot of work into designing controlled and reliable small-scale explosion tests. He believes that his research will be useful for other military and civilian researchers in the military. Precise and reliable simulations for industrial use are currently expensive and time consuming.
The many tests have produced large amounts of data that may interest the research and development departments of large companies. Elveli’s work makes it possible to simulate how structures behave when they are bent, stretched or otherwise deformed.
In total, he has carried out 110 tests, of which 82 were explosion experiments. High-speed cameras filming 37 000 frames per second have captured the details as the steel plates are damaged.
Sølvi Normannsen is info employee at SFI CASA, Department of Construction Engineering NTNU.The article is published courtesy of NTNU: Norwegian University of Science and Technology and SINTEF.
