Quantification of forces required for stabbing with screwdrivers and other blunter instruments

In the UK, stabbing is the most common cause of homicide. The weapons used include knives, swords, screwdrivers and glass shards. Quantifying the exact force used in a stabbing incident is considered to be a difficult area due to the large number of variables present, such as sharpness of weapon, angle of attack and relative movements of the people involved. Having quantifiable data would allow a forensic pathologist to make a more informed decision when it comes to answering the commonly posed question in court “what was the degree of force involved in the stabbing incident?” The answer to this question is considered significant in determining an alleged assailant’s intent to cause harm. This paper presents results of the first detailed study relating geometry of screwdrivers to the forces required for penetration. Additionally, a range of other blunt weapons such as pens and chisels have also been studied. A silicone rubber–foam analogue has been used as the main skin simulant owing to it having similar mechanical properties to that of human skin and giving highly repeatable results. Different screwdrivers of varying shape and size have been tested (i.e. slotted, Phillips, posidriv and Torx), along with other implements including chisels and pens. The weapon geometry was characterised and related to the peak force required for penetration. Our results show that there is a direct correlation between the cross-sectional area of a screwdriver head and the amount of force required for penetration. Screwdrivers with larger cross-sectional areas require a significantly greater force to penetrate (forces in the region of 100–120 N) but “sharper” slotted screwdrivers penetrate with much lower forces (∼30 N). The forces required for penetrating the rubber–foam analogue with screwdrivers are higher than for “sharp” knives, but in some cases similar to the forces required for stabbing with “blunt” knives. For the other weapons such as chisels and biros, the force required for penetration was again high and there was found to be a good relationship between area of the implement making contact and penetration force.