As you enter a building on the northern rim of the campus at the University of Leicester in central England, the colorful signboard that catches your eye suggests science at the cutting edge. Whether or not the Space Research Center deserves this tag, cutting-edge would be absolutely the right description for the work being carried out by one of the building’s other inhabitants.
Sarah Hainsworth is an engineer who, for the past few years, has been investigating the sharpness of knives — and not just knives, but screwdrivers, scissors and even ballpoint pens. Any implement, in fact, that has featured as a murder weapon.
“If you give somebody a knife and ask them if it’s sharp, how do you measure that? How do you quantify sharpness?” Hainsworth said. “I suppose that’s what we’ve been trying to do.”
The cutlery industry has measured the effectiveness of the slicing edges of blades designed for chopping up vegetables or carving meat, and how best to maintain that sharpness, she says.
“But what nobody has really done is to investigate the sharpness of points,” she said.
They certainly have now. Hainsworth sometimes spends whole afternoons dropping knives from various heights on to foam or legs of pork — a close substitute for human skin — and recording the results in finest detail.
The ease with which a blade penetrates the human body has become an important consideration in court cases.
“When there’s been a stabbing, one of the common defenses is: I didn’t mean to kill; the knife was extremely sharp,” Hainsworth said.
The implication is that the accused is less guilty because they had not used extreme force.
Hainsworth began looking at knife sharpness in 1999 when she was leading an undergraduate project about materials that might improve their ability to retain a keen edge. Because of this work she was approached five years ago by a solicitor.
“A knife had been used in a murder,” she said. “It had made a wound of significant depth through the chest and into the spine. The accused had said he always kept his knives extremely sharp. I was asked whether I could verify this so they could use it as a defense.”
After testing the knife, she established that it was not sharp.
“I imagine the solicitor rejected that line of defense,” she said.
It was while working on that case that Hainsworth realized there was a gap in knowledge. The existing research had been short-term and was out of date.
“What we really needed to do was look at this with some of the modern tools that are available to engineers,” she said.
This lack of convincing research was posing a problem for the criminal justice system, she said.
“The courts in the first instance will ask the forensic pathologist for an indication of the degree of force required in a stabbing offense and the pathologist might assess the sharpness by testing the blade on their fingers. When you’re trying to communicate to a jury how sharp something is, I don’t think that is a particularly convincing assessment. And it’s open to criticism from the barrister on the opposing side that the method is unscientific,” she said.
Is Hainsworth’s work likely to help more people mount successful defenses against the most serious charges of violence involving knives? She cites two cases in response.
“In one, the weapon was relatively blunt and the prosecution barrister argued that because it was blunt, considerable force was used, and that influenced the sentence. I had another case where I found the weapon to be sharp, and the barrister argued that the accused had knowingly kept the knife very sharp and had taken it to use as a weapon in that knowledge,” she said.