How much force does it take to stab somebody to death? Strangely enough, forensic scientists do not know.

A number of groups have attempted to measure the forces necessary to penetrate skin but the results are difficult to apply to murder cases because of the sheer range of factors at work. The type and sharpness of the knife; the angle and speed at which it strikes; the strength of skin which varies with the age of the victim and the area of body involved; these are just a few of parameters that need to be taken into account.

So when giving evidence, forensic scientists have to resort to relative assessments of force.

“A mild level of force would typically be associated with penetration of skin and soft tissue whereas moderate force would be required to penetrate cartilage or rib bone. Severe force, on the other hand, would be typical of a knife impacting dense bone such as spine and sustaining visible damage to the blade,” says Michael Gilchrist at University College Dublin and pals who are hoping to change this state of affairs.

They’ve developed a machine that measures the force required to penetrate skin–either the animal kind or an artificial human skin made of polyurethane, foam and soap.

The surprise they’ve found is that the same knives from the same manufacturer can differ wildly in sharpness. And the force required for these knives to penetrate the skin can differ by more than 100 per cent.

That could have a significant bearing in some murder cases. And that’s important because in many European countries such as the UK, stabbing is the most common form of homicide.

Gilchrist and co say their work could even help tease apart what has happened in that most common of defences: “he ran onto the knife, your honour”.

The key thing here is the speed and angle of penetration. The angle can be measured easily enough but the speed is another matter altogether. Gilchrist and co say future work may throw some light on this.

Ref: arxiv.org/abs/0811.3955: Mechanics of Stabbing: Biaxial Measurement of Knife Stab Penetration of Skin Simulant