CFS - Technical Information Sheets - Chemistry - Physical Match
Technical Information Sheets
If objects are subjected to a force that causes them to be broken, cut or torn the result can be separate pieces that once formed part of the same object.
A scientific examination of the broken, cut or torn material can be conducted in order to determine whether or not they once formed part of the same object.
This is known as a physical match examination and it is used in the examination of materials such as paint, glass, fabric, tape, metal and plastic.
The examination is based on the theory that many materials have a structure imparted during manufacture such that when they are cut, torn or fractured it will happen in a random way.
The submitted items are first examined visually to determine if the pieces are large enough to attempt a physical match and to see whether or not there are any incidental features on the surface of the object. The fracture surfaces are also examined using an appropriate level of magnification to see if corresponding features exist on each of the fracture surfaces.
If there are random features that correspond between the examined items this may be due to a physical match.
If there is insufficient detail between the fracture surfaces of the pieces being fit back together then a physical match cannot be determined. To determine whether or not the pieces can be excluded as having originated from the same source a microscopic and chemical analysis can be performed.
If the known and questioned items share sufficient corresponding random characteristics and there are no contraindicating features observed the examiner will conclude that the items once formed a single item.
This conclusion is based on objective observations made by an examiner who has received specific training to be able to recognise incidental surface features and corresponding features on fracture surfaces that are of significance.
An opinion that is expressed as a conclusion of a physical match is the highest degree of confidence expressed by trace evidence examiners in physical comparisons.
It is not possible to:
- set a numerical or statistical threshold to define what might be a sufficient number/type of corresponding features.
- reproduce the exact forces that were used to break, tear or cut the object in question.
- break/tear/cut and examine all objects in existence that are the same as the one in question.
- determine with absolute certainty that a physical match between two items is due to them having once formed part of the same object
The interpretations and conclusions in the report take into consideration these scientific limitations.
Physical Match Glossary
Fracture: the separation of an object or material into two or more pieces under the action of stress.
Fracture surfaces: These are the edges along the fracture or the surfaces facing each other when the fracture is reconstructed. When glass breaks, the edge through the thickness of the glass, displays markings called ridges.
Physical Match: Two or more items that share sufficient corresponding random characteristics that demonstrate that the items once formed a single item. This includes items that exhibit corresponding fractured surfaces.
Stereoscopic Microscopy: A stereo microscope is used to view the item at various levels of magnification. A typical stereo microscope will range from 4 times magnification up to 100 times magnification.
Scanning Electron Microscopy (SEM): The SEM produces a magnified image based on the interaction of an electron beam with the sample’s surface. Magnifications in the range of 1,000 times are routinely used with the SEM, though the SEM may be operated at much higher and lower magnification depending on the features being examined.