When fatal stabbing becomes a scientific experiment

25 April 2019

In 2017, Stefan Axmann came to the Institute of Forensic Medicine at the Mainz University Medical Center to establish a forensic physics department. It attracted a lot of attention from the media at the time, which was keen to report on the new facility, the first of its kind in Germany. The physicist himself likes to talk about his enthusiasm for his work and explains how he ended up in Mainz.


Stefan Axmann sits at a conference table in his large office, a solid leather writing case in front of him, accompanied by a cup of coffee and a photograph of four differently shaped knives – potential murder weapons. Behind him on the bookshelf, one book immediately catches the eye: From X-Rays to Quarks: Modern Physicists and their Discoveries by the Nobel prize-winner Emilio Segrè. Close to it sits a human skull, together various textbooks on forensics.

“When I first got here, there was nothing but a folding stool and a small table,” the 36 year-old recalls. “Since then it has become a proper office.” Axmann took up his role at the Institute of Forensic Medicine at the University Medical Center of Johannes Gutenberg University Mainz (JGU) on May 1 2017. He arrived to set up a forensic physics department, which would become the only establishment of its kind in Germany. “I do a lot of the work,” he says. “I’m basically responsible for everything, even the administrative side of things. But that also means I have the opportunity and the freedom to decide how I do things. Even the equipment in the lab next-door for example – I design all that myself. They're all prototypes."

A bonus for the Institute of Forensic Medicine

Axmann runs his department all by himself, but that doesn’t mean he feels alone. “I’ve managed to find a suitable place here in the Institute of Forensi Medicine. I’ve not replaced anything that was already here, I’m just an added bonus if you like. We work together hand in hand, the interdisciplinary aspect is quite important. The things I learn from my colleagues are very valuable. Our work thrives on the exchange of ideas and information. It’s mutually beneficial, not only in terms of our respective expertise but also due to the variety of the methods and approaches that we use.”

He often assists with autopsies performed on-site. The physicist had to get used to working with dead bodies, recognizing that: “What I can gain from seeing for myself is worth more than a thousand photos.”

Assisting in legal cases, teaching and research – Axmann has to juggle all of these activities. However, he appreciates this diversity. His writing of expert reports for a wide range of legal cases is perhaps his most prominent role. “At the moment I only get requests from the local region. My work is getting gradually more and more well known though, and will become much more sought-after. So of course, I will have to work out how to manage that.” The media have also shown interest in Axmann on many occasions, as the subject of news articles in the papers and on television. “I enjoy that, because I like telling people about what I do.”

Axmann leads us into his laboratory. He is not allowed to talk about ongoing investigations, but he can talk a bit about a typical case he previously worked on. It concerned a death from a knife wound. The alleged perpetrator claimed that the victim ran onto the knife. The physicist now had the task of proving if this was possible, or if it was a deliberate stabbing. “When you stab someone the whole body is involved, your arm and your shoulder are exerting kinetic energy.” That can be measured experimentally.

Stabbings in the lab

Of course, to ensure that what happens in an experiment is reproducible, the procedure must be designed in such a way that it is possible to record objective measurements. “Every body is different, and even if I have to reconstruct an incident with half a pig’s body, for example, I have to take its individual characteristics into account.” This is why Axmann prefers to work with so-called ‘validated simulants’. These are synthetic materials that have properties that are as similar as possible to those of human tissue.

“For example, there are artificial bones made from polyurethane and ballistic soap that has roughly the same density as human tissue.” He points to a block. “This is covered in chamois leather, just like the material we use to clean windows. When moistened or impregnated with wax, it is very similar to human skin.” In an experiment, a block like this can be penetrated over and over again with the knife in question or better – because it is generally necessary to preserve forensic evidence – a replica of the murder weapon that reproduces the essential details of the original employed. However, in cases where another person is not involved, other individual factors can also come into play. Axmann uses a drop tower which he designed himself. He clamps the replica in, and it rushes down from a previously calculated height. “This is the only way I can get scientifically accurate and meaningful results.”

Back in his office, the physicist returns to the theme of knives. He now turns to the photo of the four different knives. “I would be interested to know what effect the different geometric characteristics of a knife blade have on the penetration behavior. Each of these knives is designed for a different purpose, hence it will also cut differently. This would be a rich topic for a research project. “Unfortunately, I just don’t have the time at the moment,” he says with a touch of regret.

A few years ago, the physicist came into contact with aspects like this and other similar forensic issues quite by accident. He was studying technical and applied physics at Bremen University of Applied Sciences, and urgently looking for an internship abroad. “In the fall of 2008 I got one single offer,” he says, smiling. Dr. Beat P. Kneubuehl had just established a forensic physics and ballistics center at the University of Bern, and he was looking for an intern.

With the great ballistics expert in Bern

"In the following five months I had the honor of being there for the opening of the center and making my contribution." It also inspired his Bachelor’s thesis on the effect of blunt force impact on structures of the human body. “Kneubuehl offered me the chance to stay in Bern, but I wanted to learn more about the theoretical side of physics first.” Axmann completed a corresponding Master’s degree program at Bremen University of Applied Sciences before returning to Bern. It was there he wrote his graduation thesis on blast trauma and the effect on the human ear of exposure to gunshot noise .

Axmann does not explicitly say so, but it is clear that Kneubuehl, the internationally-renowned ballistics expert, was an influential mentor to him for several years. He speaks of him with admiration. His books, acclaimed standard texts, are on the bookshelf next to the human skull.

Axmann was approached by Prof. Tanja Germerott, the head of the Institute of Forensic Medicine at Mainz University Medical Center. “She had worked in Bern herself for a time and asked me to think about setting up a similar forensic physics department in Mainz.”

And now here he is, sitting in his brand new renovated office. However, he’s not someone who likes to rest on his laurels for very long. He is always ready to talk about his work, medicine and physics or his research ideas. He set aside two hours for this interview, despite having a very busy schedule. When asked about this, he replies: “I can get back to work quite quickly and these sorts of discussions are important to me.”