The following is an excerpt.

Within a few days this spring, 30 blast loads detonated inside one of the Norwegian Defence Agency’s test facilities. Behind the destruction lies CASA and NDEA’s shared goal to understand more of what happens during close-range blasts in confined spaces.  

 

Researcheras at work in a mechanical lab
More trustworthy simulations. The group hopes to provide future users with more guidelines on using the software tools. The challenge is to get the model to recreate what the researchers measure and observe in the experiments. Here (from left), Benjamin Stavnar Elveli, Vegard Aune (both SFI CASA), Knut Ove Hauge and Ole Vestrum (both NDEA) work on the test setup.

The experiments are part of SFI CASA’s and partner NDEA’s joint effort to address the complexity of close-range detonations. They have overlapping interests in the physical tests. CASA’s main objective is to increase our understanding of the actual loading and how such blasts affect the dynamic response of thin steel plates.

THE MAIN THREAT IN HISTORICAL EXPLOSIVE EVENTS

According to Associate professor Vegard Aune, several studies show how near- and far-field blast events affect blast-loaded structures. Thus, we know quite a bit about the loading and what happens when significant explosive charges detonate on the ground, some distance away from critical infrastructure. «This is because such incidents have been the main threat in historical explosive events,” says Aune.

UNDERSTANDING THE PHYSICS OF BLAST LOADS

Man rigs a test setup for blast load experiments

Close-range detonations imply complex interactions between the blast overpressure and expanding detonation products such as fireballs. However, there is less knowledge on the actual loading resulting from confined and close-in explosive detonations. Today’s computational tools enable advanced simulations.
«We must challenge the existing methods for near- and far-field events and see how they work for modelling effects of blasts in small, confined spaces», Aune says.

Simulation on blast load in a confined space

The figure (right) shows a preliminary simulation using the Particle Blast Method in LS-DYNA. It has not been very much tested before. It leads to a lot of trial and error, without anyone having the correct answers. 

WHAT IS THE LOAD?

Associate professor Aune underlines that the actual loading environment is not yet fully understood. The core question is, «what is the load»? This is an essential element of uncertainty because it affects the material’s and the structure’s behaviour. We want to explore this, to provide models with reliable predictions».
Experimental validation is ideal as it represents the actual physics of the problem. Thus, the idea is to use controlled small-scale experiments to evaluate current methods’ robustness, efficiency, and accuracy.

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BUILDING THE BEST BASIS FOR ADVICE

Researchers in a test facility for blast loads

It was Dr Ole Vestrum, a senior engineer and researcher at NDEA. He approached CASA and proposed the topic for an MSc thesis. Vestrum earned his PhD at SFI CASA in the spring of 2021.«For us, the purpose is to become better acquainted with methods for studying and calculating explosion loads. The work is part of NDEA’s continuous research and development activities to form the best possible basis for our advisory assignments», says Vestrum.

IMPRESSED BY THE STUDENT’S WORK

To understand more, Vegard Aune, PhD candidate Benjamin Stavnar Elveli and MSc students Marie Bacher (to the right in the photo) and Anne Myran Larsen brought 30 + steel plates to NDEA’s test facilities. NDEA-researchers Vestrum and Knut Ove Hauge are also the students’ supervisors. Dr Vestrum says master’s theses are an excellent arena for NDEA to become better acquainted with new and existing methods together with the students. He underlines that the students have put in impressive efforts. «They have developed and converted a lot of knowledge in a short time to solve the task, which is very valuable to us. With further good assistance from the students and their supervisors, the outcome looks very promising», Vestrum says.

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PRAISE THE OPPORTUNITY – AND THE COLLABORATION

Bacher and Larsen´s MSc thesis is titled «Thin steel plates subjected to confined, close-range blast loading». The students chose the topic as it was a challenging opportunity to learn something new. Also, they found it exciting to work with explosives and issues a bit out of the ordinary.
The students praise the collaboration and opportunity to be surrounded by skilled and helpful discussion partners. In total, they have performed around 30 experiments with explosives.
«Everyone was hands-on, and we worked as a team. Also, NDEA shows great commitment in our work, which makes it extra motivating and fun».

SIMLAB AND CASA FORM A MEETING POINT

The test set-up at NDEA’s test facilities builds further upon the set-up originally made by Researcher Martin Kristoffersen at CASA. He developed a test rig for confined detonations during his postdoc in the large E-39 project of the Norwegian Public Roads Administration, also a partner in the Centre.

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Two researchers mounting a test set up for blast load tests

Now, PhD candidate Benjamin Stavnar Elveli (to the left in the photo) takes the opportunity to spend the last months of his doctoral work on issues related to NDEA’s initiative.
In Vegard Aune’s words, this MSc project exemplifies SIMLab and CASA forming a meeting point for joint research efforts with its partners.
Ole Vestrum says that collaboration is vital to NDEA for several reasons.
«It is useful to get a different perspective and approach to the problem and the solution».

FRESH THOUGHTS, NEW KNOWLEDGE

«The students and supervisors contribute with fresh thoughts and new knowledge that we might not otherwise have developed ourselves. For us, it is crucial to be visible and contribute to an awareness of our work. In turn, this can contribute to the recruitment of resourceful new colleagues. In addition, it may give a general understanding of our social mission in industry and society», Vestrum says.
«Beyond this, it is rewarding to work with the skilled and motivated students and employees at SIMLab and CASA».
When asked how NDEA and CASA complement each other, Dr Vestrum says that the group in Trondheim has unique knowledge of several working methods essential for NDEA’s activities.
«NDEA focuses on using this knowledge in our work».

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Steel plate with painted speckles and a blast hole
Cold-rolled sheets. The steel plates are manufactured from cold-rolled sheets of type Docol 600DL/DP. Material tests have been performed at SIMLab to obtain their mechanical properties.  Digital Image Correlation is used to measure the plates’ 3D transverse displacement fields.  Blast measurement diagnostics will be used to quantify the loading.
A meeting point. PhD candidate Benjamin Stavnar Elveli (left) takes the opportunity to spend the last months of his doctoral work on issues related to NDEA’s initiative. The involved researchers point out that this MSc project exemplifies that SIMLab and CASA form a meeting point for joint research efforts with its partners.
Female students mounting a rig for blast loads
Previous summer students. Anne Myran Larsen (right) and Marie Bacher did quasi-static and dynamic experiments last summer, but no experiments with explosive loads. «We worked with wooden constructions, concrete, and aluminium in the lab. That made us want a master’s thesis that included lab work». They admit that the learning curve has been steep. «Since everything is new to us, we think it has been demanding. However, as we have had good supervisors, we have enjoyed it too».
Researchers mounting a test set up for blast load experiments
Understanding the physics of the blast load. Vegard Aune, PhD candidate Benjamin Stavnar Elveli, and two MSc students brought 30 + steel plates to NDEA’s test facilities.
Two researchers discussing the design of a test setup
For multiple uses. The challenge is to get the model to recreate what the researchers measure and observe in the experiments. At the same time, the set-up must be as generic and straightforward as possible. Ole Vestrum, NDEA (left), and Vegard Aune, SFI CASA.