  In medicolegal situations, the consequences of a stabbing incident are
described in terms that are qualitative without being quantitative. Here, the
mechanical variables involved in knife-tissue penetration events are used to
determine the parameters needed to be controlled in a measurement device. They
include knife geometry, in-plane mechanical stress state of skin, angle and
speed of knife penetration, and underlying fascia. Four household knives with
different geometries were used. Synthetic materials were used to simulate the
response of skin, fat and cartilage: polyurethane, foam, and ballistic soap,
respectively. The force and energy applied by the blade and the skin
displacement were used to identify skin penetration. The skin tension is shown
to have a direct effect on the force and energy for knife penetration and on
the depth of displacement of the 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 required for puncture
when the blade is parallel to a direction of greater skin tension than when
perpendicular. Surprisingly, evidence suggests that the quality control
processes used to manufacture knives fail to produce consistently uniform blade
points in nominally identical knives, leading to penetration forces which can
vary widely.
