01Energy is the total; stress is what the wall meets
Log 20–200 N·m and you have described an event, not a load. An 80 mm face spreads that energy across a broad patch of wall; a 10 mm face concentrates it into something closer to a fingertip. Area scales with the square of diameter, so the ratio between the two exceeds 60. Identical energy, two entirely different insults, and the panel knows which one it got.
That is why an energy figure alone cannot be compared across runs. Pair it with the face dimension, or the record means nothing six months later when nobody remembers which tool was fitted. The 8–60 mm edge length belongs in the same row as the energy, because it is the denominator that turns a total into a stress.
02Impulse over contact area is the only stress that counts
Speed sets impulse. Drive the striker at 1 m/s and it arrives with little momentum; at 8 m/s the same mass carries eight times as much, and if it stops over a comparable distance the average force rises with it. Divide that force by the contact patch and you have the only number the wall responds to, so log the speed.
A narrow face drives material past yield on contact. A wide face lets the same energy bow the wall and hand most of it back as springback. Two samples can report identical energy and differ by an order of magnitude in damage. Neither report is wrong; they simply describe different contact geometry, and geometry is not a rounding error.
03A 0.2–2.0 mm dent holds two different stories
Depth is measured after unloading, and it mixes two contributions: the part that springs back and the part that does not. Local strain of 0.5–6% describes the ring of material at the indenter edge, not the average across the whole panel. Small readings confined to a very small zone, and that zone is exactly where cracking quietly starts.
Read depth without springback and the conclusion can invert. A 0.6 mm dent that returns 0.5 mm leaves 0.1 mm behind, which is elastic and harmless. A dent of the same 0.6 mm that returns only 0.15 mm leaves 0.45 mm, and the material has been pushed through yield. Same depth, opposite verdict, and only one of them recovers.
04Hold for 0.5–5 s and the dent keeps growing
The strike is not the end of the event. While the indenter rests for 0.5–5 s, material creeps under constant stress: depth inches up, springback inches down. Run the same peak with a 0.5 s hold and a 5 s hold and measured depth can differ by 0.2–0.6 mm, and every bit of that difference lands in the irreversible column.
An unspecified hold makes two runs incomparable. Pick a value, 2 s is common on a bench, and apply it to every sample without exception. Change it midway and the earlier data is spent, because time has been folded into depth and you can no longer separate the effect of geometry from the effect of duration at all.
05Verdict on the irreversible part, not the crater
Recheck runs in a fixed order: read total depth right after release, read again once springback settles, subtract. Springback of 0.1–0.6 mm is the normal band; anything outside it means the point has lost elastic reserve even if total depth still sits inside 0.2–2.0 mm. Footage captures the strike, while release rides on four figures logged separately at the bench.
The pass line is the irreversible figure. A sample whose residual stays at or under 0.3 mm, with no visible cracking at the indenter edge, counts as clean for that round. Anything above gets flagged and re-run at the original hold time, because a second reading taken under different conditions measures the schedule rather than the case itself.
| Impact energy | 20–200 N·m per strike |
|---|---|
| Indenter diameter | 10–80 mm |
| Face edge length | 8–60 mm |
| Impact speed | 1–8 m/s at contact |
| Dent depth | 0.2–2.0 mm after unload |
| Local strain | 0.5–6% at the indenter edge |
| Hold duration | 0.5–5 s |
| Recoverable springback | 0.1–0.6 mm |
FAQ
Same energy, why do two cases end up so different?
Energy is a total; the indenter face is the denominator. Going from an 80 mm face to a 10 mm one shrinks the patch by more than sixty times, and stress rises by the same factor. Confirm both runs used the same face before comparing anything.
Does a shallow dent mean nothing was damaged?
Not on its own. A 0.6 mm dent that springs back 0.5 mm leaves 0.1 mm of damage. The same depth returning only 0.15 mm leaves 0.45 mm, which means the material yielded. Depth alone cannot separate the two.
Why fix the hold time?
Material keeps deforming while the indenter rests. A 0.5 s hold and a 5 s hold can differ by 0.2–0.6 mm in measured depth, and that difference is indistinguishable from a geometry effect. Change the hold and earlier runs stop being comparable.
What can the video actually show?
Only the seconds around the strike. Springback, hold timing and the irreversible figure all happen off camera. The four numbers a verdict needs, energy, face size, hold and residual, sit in the record, and a missing column means no recheck.
