Glass Fractures
Glass fractures occur when lines or patterns within the glass occur as a result of stress or force. These fracture patterns are essential in forensic investigations, crime scene reconstruction, and evidence analysis. Glass fractures can be classified based on the features of the cracks into many types.
- Types of Glass Fractures based on the features of the cracks
- Types of Glass Fractures based on the force causing the break
- Short Notes on Glass Fractures
Glass (Physical Evidence)
Types of Glass Fractures based on the features of the cracks
Some common types include:
Radial Fractures
These break outward from the impact site, making lines that look like the spokes of a wheel. For instance, a bullet hit or a point strike from a hard item can cause these breaks.
Concentric Fractures
These break types look like circles around the impact site. When something hits the glass in a straight line, like with a punch or hammer, it leaves marks.
Hinge Fractures
Laminated glass is made up of pieces of glass with a plastic layer between them, often gets hinge fractures. When fused glass breaks, the plastic layer between the layers may hold the pieces together, giving the glass a split look.
Combinational Fractures
Most glass breaks have a mix of radial, circular, and other types of fractures, which makes the study more difficult.
Stress fractures
You can’t see where the blow happened when you get a stress fracture. They result from the glass being heated or cooled unevenly, manufacturing flaws, or other internal stresses.
Types of Glass Fractures based on the force causing the break
Types of Glass Fractures based on the type of stress or the source of the force causing the breakage:
Low Velocity Fractures
Fractures caused by low-velocity projectiles lead to the formation of cracks in glass, radiating outward from the impact point known as radial cracks. When a pane is securely held on all sides, concentric cracks can encircle the impact point. Multiple impacts can be inferred by examining cracks from subsequent impacts that terminate at previously formed ones.
The Wallner lines on radial cracks can aid in determining the direction of the breaking force. To do so, focus on observing Wallner lines on radial cracks closest to the impact point. If the impact site is not preserved, glass reconstruction is necessary, requiring knowledge of the original glass orientation. Note that the 4R rule (Ridges on Radial cracks are at Right angle to the Rear) is not reliable for laminated glass, tempered glass, and small windows tightly held in a frame.
Chemical Examination of Glass Fragments
High Velocity Fractures
When a high-speed projectile strikes glass, it produces a cone or crater. If the projectile passes through the glass, the exit opening will be larger than the entry opening. Glass reconstruction may be needed if the impact site is not preserved, especially to observe coning effects. However, reconstructing a sufficient portion of the object to display coning effects may be challenging due to the small size of shattered fragments.
The size of the hole and crater diameter cannot reliably predict the projectile size. Projectiles passing through glass at an angle produce elongated holes. Radial cracks may also emerge from high-velocity impacts, with the sequence of impacts deducible by cracks from subsequent impacts terminating at previously formed cracks.
Glass Fracture Patterns
Thermal Fractures
In non-tempered glass, heat-induced cracks typically exhibit a curved shape, smooth edges, and no indication of the crack’s point of origin. Localized heating of thick glass pieces can result in cracks with a feathered appearance. Fracture edges do not provide information about the side to which heat was applied.
Short Notes on Glass Fractures
- Concentric cracks manifest as fractures arranged in an approximately circular fashion around the impact point, typically appearing as straight segments that conclude at an existing radial crack.
- The cone or crater, often referred to as the Hertzian cone, takes on a funnel-shaped form, resulting from damage caused by a high-velocity impact.
- Hackle denotes a line present on the crack surface, aligning parallelly with the local direction of crack propagation.
- Radial cracks, on the other hand, are fractures that extend outward from the point of impact.
- Ream signifies an imperfection, characterized by nonhomogeneous layers within flat glass.
- Wallner lines, also known as ridges, are rib-shaped marks exhibiting a wave-like pattern. Described as rib marks or ridges due to their shape, they are typically concave in the direction of crack propagation.
