Measuring knife stab penetration into skin simulant using a novel biaxial tension device

Abstract

This paper describes the development and use of a biaxial measurement device to analyse the mechanics of knife stabbings. In medicolegal situations it is typical to describe the consequences of a stabbing incident in relative terms that are qualitative and descriptive without being numerically quantitative. Here, the mechanical variables involved in the possible range of knife-tissue penetration events are considered so as to determine the necessary parameters that would need to be controlled in a measurement device. These include knife geometry, in-plane mechanical stress state of skin, angle and speed of knife penetration, and underlying fascia such as muscle or cartilage. Four commonly available household knives with different geometries were used: the blade tips in all cases were single-edged, double-sided and without serrations. Appropriate synthetic materials were used to simulate the response of skin, fat and cartilage, namely polyurethane, compliant foam and ballistic soap, respectively. The force and energy applied by the blade of the knife and the out of plane displacement of the skin were all used successfully to identify the occurrence of skin penetration. The skin tension is shown to have a direct effect on both the force and energy for knife penetration and the depth of out of plane displacement of the skin simulant prior to penetration: larger levels of in-plane tension in the skin are associated with lower penetration forces, energies and displacements. Less force and energy are also required to puncture the skin when the plane of the blade is parallel to a direction of greater skin tension than when perpendicular. This is consistent with the observed behaviour when cutting biological skin: less force is required to cut parallel to the Langer lines than perpendicularly and less force is required to cut when the skin is under a greater level of tension. Finally, and perhaps somewhat surprisingly, evidence is shown to suggest that the quality control processes used to manufacture knives fail to produce consistently uniform blade points in knives that are nominally identical. The consequences of this are that the penetration forces associated with nominally identical knives can vary by as much as 100%.

A combined experimental and numerical study of stab-penetration forces

“…While at quasi-static speeds the influence of velocity was found to be insignificant in [8], here, where the test speeds were comparable to the maximum and mean velocities attainable during a stabbing incident [12,10], a clear effect can be seen when increasing the velocity. The decrease in penetration force between the quasi-static case and 1 m/s is due to the effects of viscoelasticity whereby the skin behaves stiffer at higher velocities.…”

“…Open-cell polyethylene foam of density 35kg/m 3 was placed below the target material to ensure that it did not deform nor vibrate excessively, which would not be representative of a reallife scenario. Open-cell polyethylene foam has previously been found to be a suitable surrogate material [10,11,8] and this density is close to that used by [10] and by [11].…”

“…The biaxial tension device and blade holder are illustrated in Fig. 1 [8]. The device is designed in such a way that the test material can be held in biaxial tension by adjusting the lead screws of the clamping mechanism.…”

“…in particular those of [8] who found that of the three blades used (cook's, carving and utility knife), the cook's knife had the highest penetration force and the utility knife had the lowest. Comparing the penetration force of the blades, to the non-blade implements, we see that these much blunter objects can exceed 300% the penetration force of a sharp blade.…”

“…Typically these studies use customized instrumented knives, where the stab-penetration test is performed by a volunteer holding the blade [3,4,5,6]. An alternative technique, which is used here, is to attach an instrumented blade to a machine which drives the stabbing motion [7,8], offering greater control over a range of test variables. In this study, the data obtained through experimentation has been used to develop a finite element model of stab penetration.…”