The mechanics of stabbing: assessing stab-penetration forces
12 July 2016
During a recent high-profile criminal trial, headlines appeared in the national newspapers stating that the victim had ‘walked into the knife’. While this scenario was surprising for some, for those working in forensic medicine, this situation poses a well-known difficult challenge.
When a stabbing is fatal, the amount of force required to inflict a stab wound is often the source of much debate in court. The forensic pathologist usually assesses the force exerted based on the condition of the blade, the extent of the tissue damage, the presence of clothing and other factors, and describes the level of force as ‘mild’, ‘moderate’ or ‘severe’. This information is then used by the jury to assess the degree of intent of an assailant. In certain specific situations, it can be difficult, if not impossible, to distinguish between a walk-on or run-on scenario and a thrust action.
This situation prompted Dr Mike Curtis and Prof Marie Cassidy in the Office of the State Pathologist to team up with Prof Michael Gilchrist and Prof Michel Destrade (now at NUI Galway) in the School of Mechanical & Materials Engineering at UCD in order to develop a quantitative measure of the force involved in puncturing skin. The objectives of the research, completed by Dr Aisling Ní Annaidh as part of her PhD thesis, at first appear to be simple:
- Determine the minimum force required for a blade to puncture human skin;
- Examine the feasibility of the ‘walk on’ scenario; and
- Replace the qualitative descriptors used by forensic pathologists in court with a quantitative scale.
However, this work required a widely collaborative approach encompassing the fields of forensic medicine, mechanical engineering and biomechanics, and posed many experimental and computational challenges. A laboratory test rig (shown in Figs 1a and 1b) was built to investigate the effect of the key variables in stabbing in a controlled and repeatable manner. This apparatus was compatible with standard mechanical test machines, so that experiments could be performed at a variety of speeds and angles. Additionally, the tension of the skin could be altered and monitored and various tissue and clothing layers could also be tested. In each experiment, the force required to puncture and penetrate the skin and tissue was recorded.
It was found, based on these results, that the minimum force required to puncture the skin with a sharp knife was between 10-20N. It was also found that the level of force required to puncture the skin varied depending of the speed of approach, varying from 15N at quasi-static speeds to between 9-12N at speeds of up to 9m/s. The results of these experiments confirmed that even at slow speeds of approach, comparable to those seen in the ‘walk on’ scenario, the level of force required to puncture the skin is extremely low.
Finite element model of blade penetration
Based on these results, Dr Ní Annaidh developed a finite element model of blade penetration. The model, shown in Fig. 2, replicates the conditions of the stab-penetration test and uses a sophisticated failure criterion to model the puncturing of the skin. In this model, once the stress on a single element exceeds a certain level, known as the failure stress, it is deemed to have failed and is deleted. This allows the blade to move through the skin when force is increased, as it would in a real-life situation. The failure threshold was determined separately through experiments performed on human skin harvested from cadavers.
This study chose to characterise a blade by three major geometric measurements: the blade tip radius, the tip angle and the cutting angle. The development of the stab metric has been described further in a 2015 paper in The American Journal of Forensic Medicine and Pathology.
While the stab metric can provide a quick estimate of the minimum stabbing force for a given blade, there are a number of simplifying assumptions of which users should be aware. The first is that the presence of clothing is ignored in this model. This assumption can have a significant effect on the result as the same team of researchers has previously shown that a single layer of denim can increase the penetration force by up to 50%. Another simplification of the model is the representation of the flesh as skin alone, without underlying cartilage or bone. While this is the case in 47% of stabbing incidents, in the remaining cases, bone or cartilage is damaged which would require a greater penetration force.
In reality, while the stab metric is the first attempt at replacing qualitative descriptors in forensic pathology with a quantitative stabbing scale, it still underestimates the penetration force in a number of scenarios. A current study at UCD seeks to address these shortcomings before it could be presented as reliable evidence in court and to make the stab metric and model more useful for real-life situations.
According to State Pathologist Prof Marie Cassidy, however: “In its current form, it could be used to perform a relative comparison between two blades which have been identified as potential weapons in a particular incident.”
Dr Aisling Ní Annaidh is Lecturer of Mechanical Engineering Design in the School of Mechanical and Materials Engineering at UCD and Programme Director of the MEngSc in Engineering Management. She is leading a Tissue Biomechanics research group at UCD and is Co-Lead Investigator on the Marie Curie Innovative Training Network, HEADS (Head Protection: A European Training Network for Advanced for Advanced Designs in Safety). Her research interests include tissue biomechanics, medical device design and assistive technologies. This research was co-funded between 2008-2012 by the Irish Research Council and the Department of Justice and Equality.http://www.engineersjournal.ie/2016/07/12/stab-penetration-forces-marie-cassidy/http://www.engineersjournal.ie/wp-content/uploads/2016/07/Stab-penetration-1024x793.jpghttp://www.engineersjournal.ie/wp-content/uploads/2016/07/Stab-penetration-300x300.jpgMechDepartment of Justice and Equality,mechanical,UCD