Forensic Physics: Glass Fracture Analysis and 3R Rule

Medium-band UGC-NET Forensic Science Paper II Unit VII drill on the forensic examination of fractured glass, covering radial and concentric fractures, the 3R rule, Wallner lines, Hertzian cone mechanics, bullet bevel direction, the ALER (Mahatma) sequencing rule, hackle marks, tempered glass failure, and multiple-impact stop-line interpretation. The set opens with the fundamental distinction between radial fractures, which radiate outward from the impact point and form first, and concentric fractures, which encircle the impact point and form second as the glass pane elastically rebounds. Stress marks (conchoidal fractures, rib marks) on the cross-sections of these fractures are the primary data for the 3R rule: on Radial fractures the rib marks meet the surface at Right angles on the Reverse side from the applied force, giving the examiner a reliable compass pointing back to the impact surface without knowing which side of the glass faced the shooter or the stone. Wallner lines, also called rib marks, are the curved stress-wave interference patterns on radial fracture surfaces that confirm the fracture propagation direction. Hertzian cone mechanics govern point-impact fractures: a cone of compression forms beneath the contact point and propagates into the glass, producing the characteristic Hertzian or cone fracture ring around the point of impact. Bullet holes produce a distinctive inward bevel at the entry surface (smaller hole, glass excavated toward the interior) and an outward bevel at the exit surface (larger crater, glass missing on the far side), a key direction-of-fire indicator used by CFSL Chandigarh examiners and documented in ASTM E1492. The ALER mnemonic encodes the Mahatma rule for fracture sequence: A Lateral cracks form Earlier than Radial cracks when two impacts are present, so a radial from one impact that terminates at the concentric of another establishes which impact came first.

Targeted at UGC-NET Forensic Science Paper II aspirants (Unit VII, Physical Evidence), NFSU MSc Forensic Chemistry students, FACT aptitude candidates, and CFSL and state FSL trainees rotating through the glass and paint examination section. Questions are calibrated at medium difficulty with near-neighbour distractors drawn from common student confusions: radial vs concentric timing, inward vs outward bevel, Wallner lines vs hackle marks, and the 3R rule vs the ALER rule.

Topics covered:

• Radial and concentric fracture formation sequence and identification
• 3R rule: Right angles on Reverse of Radial stress marks to determine impact side
• Wallner lines (rib marks): stress-wave interference patterns on radial surfaces
• Hertzian cone mechanics: compression cone beneath point of impact
• Bullet hole bevel: inward bevel at entry, outward bevel at exit
• ALER / Mahatma rule: fracture sequence from stop-line termination
• Hackle marks: terminal velocity fracture propagation zone
• Tempered glass: dice fracture pattern, absence of radial and concentric fractures

Work through each question before checking the explanation, and revisit every wrong answer against the cited Caddy, Curran, Saferstein, and ASTM E1492 references. Allow 30 minutes.