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

Abstract

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.

Investigation of dental materials as skin simulants for forensic skin/skull/brain model impact testing

“…Parmar et al [9] found that porcine skin, despite being slightly thicker due to its increased collagen density, has similar mechanical properties to human skin. When compared to other animal models (dogs and rats) the response of the porcine skin was the closest to that of human skin [10].…”

“…Several studies have identified that human skin's mechanical behavior is dependent on a variety of factors, such as age, gender, health and weight of the donor, as well as the location of the site and its orientation [1,2,[4][5][6][7][8]. The use of human skin in test situations may potentially make it difficult to analyze any experimental results and repeating of the experiment would be problematic [9]. A small number of studies [9][10][11] have used animal skin as an alternative but similarly its organic properties make results difficult to replicate.…”

“…Non-organic materials have also been proposed for skin simulant research, including industrial silicones and polyurethanes [2,9,[12][13][14][15][16]. Being non-human or animal sourced, the use of these materials obviates the need for ethics and their mechanical behavior and properties can be studied in a more controlled manner.…”

“…The use of human skin in test situations may potentially make it difficult to analyze any experimental results and repeating of the experiment would be problematic [9]. A small number of studies [9][10][11] have used animal skin as an alternative but similarly its organic properties make results difficult to replicate.…”

A combined experimental and numerical study of stab-penetration forces

“…This places serious doubt over the validity of 'walk-on' style defenses where such non-blade implements have been used. Recently, [25] have quantified the penetration force for other implements such as screwdrivers using silicone gel and related the cross-sectional area of the screwdriver head to the penetration force. Here, we have identified three characteristic dimensions of a blade which affect the penetration force.…”