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Glass Fracture Sequence Analyzer

Run the Glass Fracture Sequence Analyzer Fullscreen

About This MicroSim

When a window is broken by more than one impact, investigators can often work out which blow came first. The key is a simple physical rule: a radial crack stops the moment it runs into a crack that is already there. Glass cannot crack across a gap that has already opened, so the later impact's cracks end abruptly when they meet the earlier impact's cracks.

This MicroSim shows a glass pane with two impact points. Each impact sends out radial cracks and concentric rings. The cracks from the impact that came first run uninterrupted to the edge of the glass. The cracks from the impact that came second — the ones pointing toward the first impact — stop short, ending at a termination point. Your job is to read the pattern and decide which impact cracked first.

How to Use It

  1. Study the two impact points and their radial cracks. Look for radial lines that seem to stop partway instead of reaching the glass edge.
  2. Click the impact point you think cracked first.
  3. The answer is revealed: the first impact is ringed in green and labelled #1; the later impact is labelled #2.
  4. Red ✕ termination marks appear where impact #2's radial cracks were stopped by the cracks already laid down by impact #1.
  5. Read the feedback to confirm whether your choice was correct and how many radials terminated.
  6. Press New Pattern for a fresh randomized scenario, or Reveal Answer to see the solution without guessing.

What You Can Learn

  • Apply the sequencing rule — a radial crack stops at a pre-existing crack — to decide which of two impacts happened first.
  • Recognize termination points: the tell-tale spots where one set of cracks ends against another.
  • Explain how fracture sequencing lets investigators reconstruct the order of events when glass is broken more than once.

You can embed this MicroSim on your own web page with this iframe:

<iframe src="https://dmccreary.github.io/forensic-science/sims/glass-fracture-sequence/main.html"
        width="100%" height="502" scrolling="no"></iframe>

Lesson Plan

Audience: High-school forensic science (grades 9–12) Time: 10–15 minutes Bloom level: Apply (L3) — apply a rule to reach a conclusion.

Worked example. Generate a pattern and trace one radial crack from each impact with a finger. Find a radial from one impact that stops before reaching the edge — that impact came later, because its crack ran into an existing one. Predict which impact is #1, then Reveal Answer to check.

Guided questions:

  • What does it mean when a radial crack stops partway instead of reaching the glass edge?
  • Why can a crack from the first impact run all the way to the edge while a crack from the second cannot cross the first impact's cracks?
  • How many of impact #2's radials terminate in this pattern, and how do those termination points prove the order?

Extension. Have students sketch a two-impact pattern of their own and mark where the terminations must occur, then check their reasoning against several New Pattern runs.

References

Specification

This MicroSim was generated from a specification in Chapter 5: Glass, Soil, and Physical Trace Evidence.

Design note: the original specification allowed free clicking to add up to three impacts. This implementation instead presents randomized pre-built two-impact patterns (via New Pattern) so the terminations are always physically valid and unambiguous — a freely placed third impact could produce a pattern with no clear sequence. The fracture geometry is a simplified illustration of the sequencing rule, not a physics simulation.