When an object has been torn, broken, or separated, one piece of it has the potential to match another piece of it when they are placed next to one another. In forensic investigations this is called fracture matching. Because both the composition of an object and the stress applied to break it are always unique, when something is broken, torn, or separated, the edges of the pieces will always have characteristics that identify them with each other. When the pieces fit together, an investigator can conclude they were originally part of the same object. For example, when a piece of paper is ripped in half the tear will never happen in exactly the same pattern twice. This is because each piece of paper has slightly different imperfections and the forces applied to the paper in order to rip it are never repeated identically. When the two halves of paper are put next to each other, it is obvious that they were originally part of the same object. Fracture match is such an important concept in collecting and presenting evidence that it is considered scientific evidence in courts of law.
Anything that can be torn, broken, or separated can fracture matched. Items commonly used for fracture match analysis include plastics, glass , metal, wood, metal, car parts, paper, currency, tape, cloth, and paint chips. Experts in fracture match examine the objects that are a potential match in either two or three dimensions, depending on the object. Paper, tape, and cloth are generally compared in two dimensions. Glass, metal, wood, and plastics are examined in three dimensions. The entire surface of the fracture, as well as the surfaces of the object, will be analyzed. There are four different fracture match criteria:
- The pieces have been broken apart.
- The pieces can be realigned.
- The pieces fit together along the fracture and the fit is verified by markings on the surface or within the three-dimensional structure of the fracture.
- The pieces contain unique shapes.
In order to fulfill these criteria, inspectors examine the shape of the break, any irregularities in the surfaces of the two pieces, and any striations that might have occurred during the break. They examine the composition of the pieces for similarities in age, texture, and deformation. They may also analyze the chemical composition of the pieces.
When working with glass, investigators can recover a considerable amount of information from reconstruction using fracture match. Glass pieces resist movement when they are placed next to pieces to which they were originally adjacent. Special ridges, called Wallner lines, are almost always aligned so that they curve in a concave manner towards the point of impact. When the impact is from a low-velocity object, the cracks in the glass will radiate out from the point of impact. If the object that breaks the glass is moving at higher velocities, the point of impact will be cone-shaped and the larger end of the cone indicates the exit side of the glass. Cracks that are smooth and curved and show no indication of a point of contact indicate that the crack was generated from thermal stress.
A variety of examples of the use of fracture match demonstrate its importance in solving crimes. An Iowa detective used fracture match with paper to identify the person who had made a bomb threat at a warehouse. The man claimed to have discovered a note on the windshield of his car that stated that a bomb was hidden in the warehouse. The note was on a piece of notebook paper from a spiral binder. The detective searched the man's car and found a spiral binder. When the note was ripped out of the spiral binder, pieces of paper were caught inside the metal spiral. The detective was able to make a fracture match to a piece of paper found inside the metal spiral and he arrested the suspect.
Tools are also often involved in fracture match. In Virginia, robbers stole the contents from night-deposit boxes in a series of crimes. In each case the boxes were forced open. At one crime scene, the police found a small piece of metal, which they saved as evidence. Eventually, the police identified suspects and searched their possessions. They found a variety of broken tools. One of these was a screwdriver that was a perfect fracture match to the piece of metal collected from the crime scene. The police were able to convict the criminals based on this evidence.
Fracture match of the ends of tape can also provide key information to criminal investigators. In 2003 in Florida, a woman was sexually assaulted and then murdered. Her body was wrapped in bed sheets, a shower curtain, and masking tape and dropped in the ocean. A man fishing off a bridge hooked the body on a line and pulled it up. After locating a suspect, police investigators fracture matched the end of the masking tape on the body to the end of masking tape on a roll in the suspect's house. The man was convicted of the assault and murder based on this fracture match as well as other corroborating evidence.
see also Paint analysis; Tape analysis; Toolmarks.