3R Rule
Definition
The fracture-analysis mnemonic: Radial cracks form at Right angles to the Reverse (far, non-impact) side of the glass. Applied to the fracture surface edge of a fragment to determine which face of the glass received the force.
- Applies to
- Radial fracture patterns in glass
- Purpose
- Determine direction of impact force on glass
- Key indicator
- Rib marks on the reverse (non-impact) side form at right angles to that glass face
Common questions
What do the 3Rs stand for in glass fracture analysis?+
The 3Rs stand for Radial, Right angle, and Reverse. It is a mnemonic that helps examiners remember that radial fracture lines show right-angle rib marks on the reverse (non-impact) side of broken glass.
How do you use the 3R rule to find which side of glass was hit?+
Look at a radial fracture on a glass fragment and examine the edge of the fracture surface. The rib marks on the reverse side of the glass (the side that did not receive the force) form at right angles to that glass face. This geometry tells you which face received the impact.
Why is the 3R rule useful for crime scene work?+
Glass fragments at a crime scene can seem random, but the 3R rule lets you recover the direction of impact from the fracture patterns. This helps establish how a window or glass object was broken, which can support a timeline or suspect narrative.
Related terms
- Bayesian Likelihood Ratio (LR)
- The statistical framework that evaluates glass-fragment evidence by comparing the probability of the observed fragment population assuming contact with the source glass...
- Concentric Cracks
- Approximately circular cracks centred on the impact point; they form second, on the near (impact) face, and terminate when they encounter pre-existing...
- Concentric Fracture
- An arc-shaped fracture encircling the impact zone, produced by outer-table bending tension at a distance from the focal impact; always post-dates the...
- Cone Fracture
- A conical crack produced by concentrated contact loading (Hertzian contact), opening on the far (exit) face of the glass; the cone geometry...
- Entrance Bevelling
- The widening of a bullet-penetration hole toward the exit side, analogous to bevelling in bone wounds; the narrower opening is on the...
- Fracture Termination Rule
- The principle that a propagating crack terminates when it encounters a free surface created by a pre-existing crack; at any T-junction, the...
- GRIM3
- Glass Refractive Index Measurement instrument, third generation. Hot-stage microscopy with sodium D-line illumination; reports RI to ±0.00002.
- Hackle Lines
- Fine secondary cracks on the fracture surface that fan from the origin, perpendicular to the moving crack front. Used to track crack...
- Hackle Marks
- Ridges on the fracture surface oriented perpendicular to the crack propagation direction; they appear in the mist and hackle zones at higher...
- Mirror Region
- The smooth, flat area of a fracture surface nearest the crack origin, where the crack was propagating slowly and stably before accelerating...
- Mist Region
- The transitional fracture surface zone between the smooth mirror region and the rough hackle region, where the crack begins to bifurcate at...
- PDQ
- Paint Data Query database (RCMP-maintained). Cross-referenced from glass work when a vehicle is identified through paired glass and paint evidence.
Explained in these topics
- Glass Evidence: Fracture Patterns and Refractive IndexRadial fractures show Right-angle Rib marks on the Reverse side of the force. The mnemonic Indian SFSL examiners use to recover direction of impact.
- Glass Fragmentation and Direction-of-Force AnalysisThe fracture-analysis mnemonic: Radial cracks form at Right angles to the Reverse (far, non-impact) side of the glass. Applied to the fracture surface edge of...