Validation and application of a finite element model simulating failure thresholds of skin during blunt puncture with varying impactor geometries

Injuries caused by knives or other sharp tools such as scissors and screwdrivers are common in violent crimes and self-defense acts. The force thresholds of skin have been quantified based on the puncture instrument to assess degree of force in forensic cases, but limited studies have investigated blunt instruments and the effect of skin thickness. A finite element (FE) computational model was developed to simulate blunt puncture of skin. Curve fitting and manual optimization were performed to obtain Ogden material coefficients. The model was validated with experimental force-time curves for spherical impactors of diameter 3, 5, and 8 mm into thin, average, and thick skin at slow and fast loading rates (n = 18 total conditions), resulting in an average CORA score of 0.725. The average maximum principal stress at the time of experimental failure was 57.3 MPa with a coefficient of variance of 0.18, and the median value of 54.8 MPa was selected as the failure criterion. The validated model was applied to load seven spherical impactors, five Hex screwdrivers, and three Torx screwdrivers into skin with thicknesses ranging from 2 to 3 mm. Increased skin thickness resulted in greater force, displacement, and strain energy at failure. Cross-sectional area of the impactor and failure thresholds of skin expressed a linear relationship for normalized force (R² ≥ 0.88), displacement (R² ≥ 0.77), and normalized strain energy (R² ≥ 0.92). The validated FE model may be used to determine the force required to penetrate skin with a case-specific blunt instrument.