The Mechanics of Stabbing: A Combined Experimental and Numerical Study of Sharp Force Injury

Abstract

While the domain of ballistics has been widely investigated by interested parties such as the Military and the Policing services, the area of stab mechanics has not been investigated so thoroughly. This may be partly due to the prevalence of gun crime in many countries rather than knife crime. In Ireland and the UK however, stabbing remains the most common form of homicide. The magnitude of force used in a stabbing incident can be difficult to quantify, although the estimate given by forensic pathologists is often seen as ‘critical’ evidence in medico-legal situations. The main objective of this study is to develop a quantitative measure of the force associated with a knife stabbing biological tissue, using a combined experimental, theoretical and numerical technique.

Skin is the soft tissue which provides the most resistance to puncture, and therefore the mechanical properties of skin are of the utmost importance in this study. A series of in vitro uniaxial tensile tests and a histological investigation were performed to characterise the mechanical behaviour of human skin. An algorithm was developed in MATLAB for the automated estimation of collagen fibre distribution within the dermis. This data provided sufficient information for the anisotropic modelling of human skin using the Gasser-Holzapfel-Ogden structural constitutive model.

A series of quasi-static and dynamic stab-penetration tests were performed to quantify the force required for a sharp instrument to penetrate skin or skin substitutes. These experiments allowed for the investigation of the effect of a number of key variables in stabbings, including the thickness and tension of the skin, the angle of attack, the underlying substrate, the presence of clothing, the speed of the attack and the type of instrument used.

A computational model of blade penetration was developed using ABAQUS/EXPLICIT 6.9, a non-linear FEA commercial package which employs an explicit integration scheme. This model, which incorporated element deletion along with a suitable failure criterion implemented in a user-defined subroutine, is capable of systematically quantifying the effect of the many variables affecting a stab event. This quantitative data led to the development of a stab metric that can help indicate the level of force used in a stabbing incident, and could, in time, replace the qualitative indicators used at present in medico-legal situations.

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Cited By (2)

Year	Entry
2015	Ní Annaidh A, Cassidy M, Curtis M, Destrade M, Gilchrist MD. Toward a predictive assessment of stab-penetration forces. Am J Forensic Med Pathol. September 2015;36(3):162-166.
2013	Ní Annaidh A, Cassidy M, Curtis M, Destrade M, Gilchrist MD. A combined experimental and numerical study of stab-penetration forces. Forensic Sci Int. December 10, 2013;233(1-3):7-13.