Piercing the surface: A mechanical analysis of stabbing with household tools

Estimating the applied power during a stabbing incident, or estimating the minimal force necessary to penetrate the body with a certain weapon is a challenging task in forensic practice. A thorough forensic evaluation of stabbing forces needs objective numerical experimental data. Stabbing tests of 12 different weapons – including knives, a pair of scissors, a fork, screwdrivers, a rasp, a corkscrew, and a utility knife blade – were performed with a Mecmesin MultiTest‐dV material tester on pork loin and ballistic gel to estimate the stabbing forces and dynamics. Penetration force (Fp) and maximal force (Fmax) were recorded, and the registered force curves were analyzed. Fmax was 159.8–212.07 Newton (N), 30.56–30.58 N, 168.9–185.48 N for various knives; 171.39–190.43 N for the pair of scissors, 233.6 N for the fork; 532.65–562.65 N, 370.31–367.19 N and 314.51–432.89 N for various screwdrivers, 44.14–56.62 N for utility knife during pork loin stabbing. The butter knife, corkscrew and rasp were not able to penetrate the pork loin, and the curved fork bent during stabbing. The results prove that weapon characteristics greatly influence the force necessary for penetration. Maximal stabbing force depends mostly on tip sharpness, and the force sharply decreases after penetration occurs, which indicates that edge sharpness is not as important as tip characteristics during stabbing perpendicular to skin surface. The penetration force during stabbing with a pair of scissors is comparable to the penetration force of knives. Stabbing with screwdrivers generally needs larger force than average knives but depends greatly on screwdriver size.


| INTRODUC TI ON
Sharp force injuries were involved in a minimum of 97.183 victims of homicide in 2017 worldwide, and injuries by sharp objects are a dominant form of homicide in many countries (e.g. Sweden, Hungary, Poland, and Canada) [1]. Most stabbing injuries are caused by knives, however, any tool with a pointed end can be used as a weapon [2]. In a few percent of the cases, indeed stabbings are committed by other tools like scissors, and screwdrivers [3][4][5].
Forensic pathologists are frequently asked to estimate the stabbing power, or the minimal force necessary for skin penetration with a certain weapon [2,6]. Typically, a subjective rating of "mild", "moderate", and "severe" is applied based on the features of the weapon and/or the wound [7,8]. This estimation is however considered inappropriate by some authors, and objective numerical data based on experiments should be used for the estimation of stabbing forces [8,9]. It is a challenging task to determine whether a certain weapon can penetrate the skin or not, and how large of a force is necessary for skin penetration. The sharpness of the knife tip is the principal factor in penetration, but the speed of the knife on impact, and the stabbing force are also important. The targeted skin characteristics (thickness, anatomical location differences), stretching, tension lines and body cover (clothes; quality and quantity) are to be considered as well. An interaction of these active and passive elements must be considered in the final judgment [2,6,10,11].
The first objective stabbing tests resulting in numerical data were performed by Knight with a purpose-built knife in 1975 [9], but technological evolution allowed a more sophisticated approach to stabbing tests since the 2000's. Previous biomechanical studies compared the forces during stabbing with various knives or screwdrivers [8,[12][13][14], but the results of these studies are hard to compare to each other due to the distinct methods and different type of tools applied. There is even more limited scientific forensic data available about instruments other than knives and screwdrivers; furthermore, a systematic approach is missing. The goal of our study is to create objective and replicable methods for weapon comparison and to accurately measure the penetration forces during stabbing with various household tools.

| MATERIAL S AND ME THODS
The forces involved in stabbing with 12 common household tools were examined. Weapons were chosen to represent various tools available in households, including knives, screwdrivers, a pair of scissors, a circular rasp, a corkscrew, and a utility knife blade ( Figure 1).
• Stabbing with screwdrivers required greater force than stabbing with knives.
• Further studies are necessary to extrapolate the results to real-life stabbing.
Sharpness of the tip of knife No. 1 was considered subjectively "average", No. 2 was considered subjectively "extremely sharp", and No.
3 was considered subjectively "moderately sharp" by the participants of the experiments with forensic medical experience. Objective weapon characteristics (Tables 1-4) were measured as shown in Figure 2. Weapon dimensions were measured using a Workzone digital caliper (accuracy: 0.01 mm), the angle was determined with a Brüder Mannesmann M81220 Digital Protractor (accuracy: 0.1°).
After scanning the blades with a Cannon MG5750 scanner (600 dpi, grayscale), the tip radius was determined by measuring an overlapping dot placed on the tip with the pencil tool of Gimp Software (version 2.8.22).
Stabbing tests were performed with a Mecmesin MultiTest-dV (automatic, low-force material tester (2.5 kilonewtons (kN)) maximal force, 1200 mm maximum crosshead travel, accuracy: ±0.13 mm per 300 mm travel, resolution: 0.001 mm). Blades were fixed to the tester with a Mecmesin Vice Grip (Mec240k-S30 grip, Mec240g-BG Pyramid Jaws) and it was stabbed into the target with 1000 mm/s speed (90 mm downward displacement). Stabbing force was registered by a loadcell (ELS 500 N, accuracy: ±0.5% of reading from 5% to 100% of the loadcell capacity). Ballistic gel blocks (Defensible Ballistics, 10% gel powder, 90% water) and pieces of pork loin with skin (purchased from a local butcher) were used as stabbing targets ( Figure 2). Ballistic gel -as a homogenous material -was used as a target for examining the reliability and reproducibility of the method.
Blades were positioned perpendicular to the surface of the target before stabbing (Figure 3). The load (Force) and displacement curve was registered with the Mecmesin VectorPro MT software (version: 6.15.0.0). Penetration force (F p ) and maximal force (F max ) were determined by the curves: the first drop on the curve represented F p , the maximal achieved load represented F max (Figure 4). Two series of stabs were pork loin, and three series in ballistic gel were performed.

| DISCUSS ION
Measuring stabbing forces and their reliable reconstruction is a challenging task. Stab tests by human participants on targets with force measurements (e.g., dynamometer) can simulate stabbings [8]; however, variations in stabbing force, speed, and angle may have an F I G U R E 4 Explanation of the registered curves. Direction of the tool movements are indicated with arrows (direction of pushing is the stabbing phase), change of direction is indicated with a circle (at maximal downward displacement). F p marks the penetration force (at the first drop on the curve), F max represents the maximal achieved force. Load represents force in Newton (N).

F I G U R E 3
Test set-up with pork loin (1) and ballistic gel block (2).
impact on the outcome [12,15,16]. It is also challenging to record and reconstruct the blade displacement-force curve. While these types of experiments are particularly helpful for simulating stabbing incidents, they have severe limitations when comparing different weapons. Drop tests could be able to address some of these problems, as they can estimate stabbing force under controlled and reproducible conditions [7][8][9][10][11][12][13][14][15][16][17]. The disadvantage of this method is the lack of registering force curves, which are required for understanding stab dynamics. Electronically instrumented blades can get beyond the aforementioned limitation, but variations in stabbing mechanism by human participants are influencing the results [6]. Force or tension test machines with a constant speed can record force curves, and therefore these can also be used for examining stab biomechanics [14,15,18]. The main limitations are that stabbing speed is higher, and changes dynamically in a real stabbing incident, which could not be usually achieved with these machines.
Additionally, choosing proper test targets for stabbing studies is also challenging. Multiple targets with different biomechanical properties can be used for stabbing tests, all with shortcomings. Human samples (skin or torso) are used rarely [14], because of ethical considerations and availability, so porcine products, and skin simulants -like foam [7,19], gelatin, modeling clay [20] can be used. The advantage of porcine is its high structural similarity to human skin, however the biomechanical properties -regarding numerical data -are not the same [21,22], and individual differences between samples decrease reproducibility and comparability. The advantage of using artificial skin simulants is reproducibility, but the question always arises, how precisely can they simulate the biomechanics of human skin (e.g., they are completely missing the complex fibrous structure). In the case of forensic assessment of stabbing weapons, the use of sharpness testers [23] used in industry can be also considered.
A real-life stabbing can be characterized by speed, strength, and angle. Mean stabbing velocity during stabbing is usually between 5 and 10 m/s: 5.01-5.63 m/s [24], 9.2-9.6 m/s [19], and 6.6-12.3 m/s [25] according to the literature. A typical stabbing force is a few hundred Newtons [10] but can be higher according to some authors [19]. The angle of stabbing can be longitudinal, or slightly angled [12].
Skin needs the largest penetration force among soft tissues [6], but bone penetration needs a much larger force: rib penetration with a knife needs a minimum of 906-1198 N according to the drop tests by Bolliger et al. [17] and 52.7-228.9 N according to Gitto et al. [14] using material tester.  [26,27].
Our results showed that screwdrivers need multiple times greater force for skin penetration, than knives, and -compared to the average stabbing forces previously reported [10] -the force necessary for skin penetration is at the limit which can be achieved by an average adult. Our findings suggest that the penetration force of a screwdriver is affected by its size and shape, with size being more important than shape, which is consistent with the findings of Nolan et al. [10]. O'Callaghan et al. [6] used electronically augmented knives for stabbing human tissues, and found, that penetration of skin creates the largest (initial) peak force (55 N average), that the force decreases to 36% of the initial peak and increases to 73% of the initial Parmar et al. [13] examined the penetration forces during stabbing with screwdrivers on synthetic skin simulant with a material testing device (stabbing speed was not included in their report).
The penetration force was between 10 and 30 N for flat-headed (cross section of head between around 1 and 16 mm 2 ); 20-120 N for Phillips (cross section between around 1 and 22 mm 2 ); 25-120 N for pozidriv (cross section between around 2.5 and 18 mm 2 ); and 16-55 N for torx screwdrivers (cross section between around 1 and 12 mm 2 ). The exact registered forces and screwdriver sizes were not disclosed by the authors. They also used knives for reference and found that the maximum force at penetration was 35 N for a "blunt", 27 N for a "sharp", and 12 N for an "extremely sharp" knife. However, they did not describe the characteristics of the knives. The overall level of their registered force contradicts our results and the results of Nolan et al. [10], where penetration forces were also far greater in case of screwdrivers than in case of knifes. The discrepancies that studies [6,8], which can be explained by methodological differences.
They used a biaxial tension device where they registered the forces laterally (the lateral stretching of the skin). They also did not specify the screwdriver or scissors' characteristics [15]. Gilchrist et al. [12] also used a biaxial tension device to measure the penetration of various knives with skin simulant penetration (blade characteristics were not specified) using 50 and 500 mm/min stabbing speed. The penetration force ranged from 18 to 36 N, and they found that stabbing speed did not influence the force in the examined range and stabbing angle had only a modest influence on forces.
Gitto et al. [14] used material tester machines on human cadavers to compare forces during stabbing with three blades (steak knife, butcher knife, and lock-blade knife) and found that the maximum force during skin penetration was between 23 and 25 N (stabbing speed was 1000 mm/s). The applied method was similar and stabbing speed was the same as in our experiment, and the penetration force was also like weapon No. 2 in our study. However, they did Standardization of procedures is advised to analyze and compare stabbing dynamics in stab tests in scientific studies or at a minimum all the data (especially blade characteristics, stabbing speed, numerical data of forces, and registered curves) should be included in the associated publications.
Our results indicate that skin penetration can be easily achieved with standard knives. The penetration force during stabbing with a scissor is comparable to the penetration force of knives, stabbing with screwdrivers requires a greater force compared to stabbing with knives. The greatest force during stabbing is achieved at penetration, so wound depth is not indicative of stabbing strength. The penetration force is affected mainly by tip characteristics (sharpness of the tip: apex area and angle and geometry of wedges behind) of the knife, and sharpness of the blade seems to be a less important characteristics in the penetrability of a certain knife. It can be suggested therefore, that tip characteristics (tip radius, tip angle) should be recorded in assessed cases and scientific papers.

CO N FLI C T O F I NTER E S T S TATEM ENT
The authors declare no conflict of interest.