Forensic scientists are trying to understand what tears and distortions in the fabric around a stab wound can say about the knife type, angle of attack, and stabbing technique that caused the wound. But the patterns have been difficult to work out, partly because researchers have had to do most of their laboratory experiments by hand, manually stabbing different fabric swatches. But inconsistencies and human error are unavoidable. So a team of forensic scientists and engineers invented a stabbing machine to help standardize this type of research.
While there have been a few other stabbing machines made for science, this new one—built by researchers at the University of Technology Sydney and the University of Santa Cruz do Sul in Brazil—takes the cake. Its pneumatic arm can grip just about any kind of knife, and stab at fabric samples from a variety of angles and forces.
"The machine offers at least 60 possible stabbing positions and the knife holder allows for a range of stabbing implements to be analysed," the authors write.
To put the machine to the test, they used four knife designs and some blue cotton fabric. After jabbing the knives at the fabric from a variety of angles, the authors were able to discern some preliminary patterns.
The first knife had a straight spine, a sharp tip, and a smooth cutting edge, so it left behind a clean-cut mark in the fabric.
The second knife was similar to the first except that its spine, which gradually thickened from the tip to the base, could be identified via the widened hole and extra distortion near the top of the rip.
Knife three had a serrated blade, which explains the heavy fraying in the tear it created. The knife's straight spine pushed the fabric into the foam behind it, which created some secondary tears as other parts of the fabric snagged on the serrated blade.
Because knife four had a rounded spine, the tear it creates is relatively clean cut from the center down, but fraying at the top, above where the point entered the fabric. Because of their shapes, knives three and four also left behind hints for the angle of attack.
Future versions of the machine will probably need to work on accuracy, consistency, and power—the device currently jabs at a pressure of 1 megapascal, which is about the force of a human bite.
But eventually, a device like this could help to turn the analysis of textile damage into a science.