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Forensics of knife crime

Figure 2: Schematic diagram showing the different parameters to characterise the features of the different knives Figures drawn by Simon Roulstone.

Research centre

At the University of Leicester, effective collaboration between the Department of Engineering and the Forensic Pathology Unit (recognised to be the principal research establishment for forensic pathology in the UK) has been providing new insights into how modern analytical techniques and engineering approaches can help us to understand how crimes were committed and assist the legal system to interpret the complex evidence before the court. We have conducted a range of studies associated with cutting as used in criminal acts, even down to how saws are used to dismember bodies (see box labelled ‘Saw Points’).

Implements all too commonly used against victims include kitchen knives, folding knives, sheath knives, scissors, chisels, bayonets, kukri, shards of glass, and even sail-makers’ awls and samurai swords.

Understanding the engineering factors associated with cutting or stabbing crimes can prove important both during investigations and when cases come to court. A forensic pathologist is often asked what degree of force would have been necessary to inflict a particular wound and the answer can help the court decide between a tragic accident and a deliberate crime.

It is the engineering disciplines – knowing how hardware is made and operates, and the analysis of forces involved – that can help get at the truth or at least minimise subjectivity (which, in court, may benefit either the defence or prosecution case).

Starting point

I first looked at knife sharpness in 1999 when leading an undergraduate project into materials used to improve edge retention on knives but the subject came into the realms of court work when I was approached by a solicitor to apply this knowledge to stabbing. This was the first time I’d been asked to act as an expert witness in this area, though I’ve served in that capacity a dozen more times over the last several years.

In knife crime, a number of arguments are often used in court to show that the person stabbing did not intend to inflict a severe wound. One of the legal defences, for example, may be that the knife was particularly sharp and therefore easily (and by implication accidentally) penetrated the skin. Our work, using more quantitative information from drop testing (see below) along with measurement of the blade tip sharpness and edge profile, can say with some accuracy whether or not this defence is reasonable. If a blade is proven to be blunt, then the court has the necessary information to judge the degree of violence in a particular case, and if the person is found guilty, sentence appropriately.

By applying our techniques and building on existing knowledge and capabilities, our work is increasingly being used to assist the court. While studies of knife crime to date at Leicester (and elsewhere) have been through small projects, I’m hopeful that our work will lead to more funding and bigger projects with broader scope.

Cutting edge

Our research so far has covered which factors influence the effectiveness of a particular knife as a stabbing weapon, the aim being to move from a qualitative to quantitative assessment of how effective a particular knife would be for stabbing.

Pathologists usually have categories for the force required to produce stab wounds – slight pressure, moderate force, or severe force. Since this depends on a range of factors – like the type and sharpness of the weapon, the area of the body concerned and alignment of the blade with the cleavage lines of the skin, the angle of attack, and the relative movements of the people involved – the estimation of the actual force employed in a stabbing is highly complex.

One area where quantification is more tractable is in assessing the sharpness of an implement or weapon. Carving knives are the most commonly used in stabbing incidents, particularly during domestic crimes, but also for fights in public places as they are easy both to obtain and dispose of.

How sharp is sharp?

In looking at how to quantify sharpness in a reproducible way and relate this to the effectiveness of a knife for stabbing, we have utilised two major techniques: the use of high-speed video to understand the mechanism by which knives penetrate skin; and a drop testing rig to quantify a knife’s ability to penetrate foam, relating this to the characteristics of the knife. Foam or pork are used as a substitute for human skin, although pig skin is slightly tougher than our own.

High-speed video testing for sharp knives shows there are a number of stages by which the knife penetrates the skin (see figure 2). The first contact sees the skin initially deflect elastically. Then, at a critical contact stress, the knife penetrates the skin; the critical contact stress for penetration is related to the sharpness of the tip of the knife. Once the knife has penetrated the skin, the sharpness of both the tip and the blade edge become important in determining how much further the knife will penetrate.

When blunt knives contact the skin, the video records a much greater degree of elastic deflection of the skin than for a sharp knife – and the critical contact stress for skin penetration may or may not be reached, depending on the degree of force applied to the knife.

Forensic engineering

Blade variations

The shape of the knife blade is also relevant; bread knives with a ‘sheep’s-foot’ geometry show large elastic deflections of the skin. In fact, it is often very difficult to get these to penetrate the skin, yet bread knives have been used to stab people to death. The geometry and characteristics of each individual knife need to be determined before drawing conclusions about their ability to penetrate the skin.

Our drop tests have demonstrated a good correlation between the blunt edge tip radius of a knife and the depth of penetration into foam or pork. While we cannot simulate exactly the combination of factors in an attack, the controlled conditions of drop tests do record impact velocity, energy and momentum, and so deliver a faithful, robust and reproducible comparison of knife sharpness.

We have tested a wide range of knives with differing blade thickness, geometry, and edge types (ie plain or serrated) and the following factors are important for determining a knife’s ability to penetrate skin:

  • Blunt edge tip radius: gives a good indication of its ability to penetrate into foam or the body (smaller tip radii giving better penetration).
  • Blade thickness: slender filleting knives, for example, penetrate more easily than thicker blades.
  • Blade geometry, in particular the shape of the blade’s tip: knives that taper to a point are more effective at penetrating than profile 1, even if the blunt and sharp edge tip radii are identical for both profiles.
  • Edge sharpness: helps continued penetration once the skin has been breached.
  • We have established a good database with a register of quantified sharpness and penetrability of various knives – and can usually place a knife within this scale, which takes some of the subjectivity out of the quantification of force in stabbing incidents.

    Saw points

    ESEM – a key forensic tool

    Blunting knives’ impact

    We are hopeful that this and future work will be useful in informing future knife design. We’ve focused on kitchen knives, as these are often used in stabbing, particularly in crimes of passion where a knife may be readily to hand. Since the knife tip is fundamental to stabbing, more could be done to fabricate rounded ends for use in the kitchen, since the tips are usually unimportant for food preparation, with the exception of filleting fish and meat. In delicatessens and carveries for example, employers often use blunt-ended knives for health and safety reasons; while the blade needs to be sharp to carve meat, the tip does not need to be sharp or pointed, so blunt-ended knives are used to minimise the risk of injury to employees.

    People may need to be re-educated in their perceptions of sharpness, but it is fairly certain that rounded tips on kitchen knives would inevitably save lives by reducing the number of both fatal accidents and intentional stabbings.

    Professor H V Atkinson of the University of Leicester, Professor M E Fitzpatrick of The Open University and Gary Atkins are thanked for their helpful and constructive comments on the drafts of this article.

    BIOGRAPHY – Dr Sarah Hainsworth CEng CSci FIMMM

    Dr Sarah Hainsworth is a Reader in Materials Engineering at the University of Leicester and Director of the University’s Advanced Microscopy Centre. Her research interests are in forensic engineering, automotive tribology, microstructural evolution in power plant materials (steel and Ni-based superalloys) and materials characterisation. She was the 2008 recipient of the Institute of Materials, Minerals and Mining Rosenhain Medal.

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