In an explosion, fragments, and debris can be ejected at great speed and strike the surroundings. Then comes the shock wave. It's a scary combination.

Combined ballistic impacts pose a major challenge for engineers who build structures that must withstand extreme stresses. The combination of blast pressure and impact at high speed increases the chances of greater damage. Ph.D. candidate at the Norwegian University of Science and Technology (NTNU), Benjamin Stavnar Elveli describes it as the scariest stress there is.

“These combined impacts work in the same way as shrapnel bombs,” he says.

Infrastructure shift from massive and military to light and civilian

In the past, protective structures have involved massive concrete military buildings. In recent decades, new threats have emerged, and the need to protect civilian buildings and structures in urban areas has increased.

This has fuelled interest in lighter, thin-walled solutions that can withstand large deformations without cracking and collapsing.

The regulations have not followed this same development. No standards address this type of load yet, and research in the field is very limited.

Elveli has investigated how different types of thin steel plates behave when exposed to such extreme stress loads. His work can help to establish guidelines for how resistant, lightweight structures should be designed.

Initial projectiles do the most damage

Whether they occur in accidents or on purpose: explosions can cause massive damage. Debris and fragments can be torn loose from parts of buildings, cars, gravel, or stones. When they hit, they can act like projectiles.

Elveli says that any buildings, cars, or other objects in the vicinity would be exposed to a load that is more serious than if either stress load occurred alone. The damage is believed to be greatest when fragments hit first.

“That’s because the structure already has a defect or weakness from the projectile and then has to withstand the shock wave itself,” he says. Most often, cracking and destruction starts in the weak spots.”

Safer structures, safer society

Elveli's Ph.D. is based on more than 80 small-scale explosion tests on three different types of steel. By combining physical experiments with theory and mathematical modeling, he has recreated explosive loads in supercomputer simulations. The aim is to gain as much control as possible over how structures react to such loads.

The more scientists understand the actual physics of these loads, the more accurate, safe, and sustainable solutions the construction engineers of the future can deliver.

The danger of overestimating the 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 of 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 than 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 a machine to mimic damage from real fragments.

Overestimating resilience

Elveli has compared the behavior 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 supercomputer simulations

Understanding the need to develop accurate supercomputer 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 researchers in the military and civilian arenas. For industrial use, precise and reliable simulations 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. Elveli obtained his doctorate at NTNU’s SIMLab/Department of Structural Engineering