The short answer
Flat crush presses on the face of the board and collapses the flutes. The result reports in psi or kPa, and it describes how well the corrugator formed those flutes.
It is a process check, not a stacking number. The method says it does not simulate crushing on a container in use, and it ignores liner stiffness.
Flat crush is the test most often mistaken for a strength rating. The number sits on a mill certificate beside the others, reports in familiar units, and rises when the board improves.
What it measures is narrower. Flat crush loads the face of the board and collapses the flutes, so it reports on the medium and the machine that shaped it. The liners barely participate.
That makes it a manufacturing diagnosis rather than a performance prediction. Read as the first, it is one of the most useful numbers on the sheet. Read as the second, it answers a question nobody asked.
This reference covers what the test measures, the two methods and which reads lower, what a low result tells you, and why it does not predict stacking.
What Flat Crush Measures
A specimen lies flat between platens, and the force runs perpendicular to the surface rather than along the flutes.
So the flutes take it. The method defines the property as the resistance of the flutes to a crushing force applied perpendicular to the surface.
Failure has a precise definition: the machine records the maximum load before the corrugation side walls collapse completely, and that peak becomes the result.
Units follow from the geometry. Flat crush loads an area and reports pressure, in kilopascals or pounds per square inch to three significant figures.
Two Methods, and One Reads Lower
Two flat crush methods are in print, and they do not sit level.
| Method | Support | Status | Reads |
|---|---|---|---|
| TAPPI T 825 om-24 | Rigid support | ANSI approved | Higher |
| TAPPI T 808 cm-25 | Flexible beam | Classical method | Significantly lower |
The flexible beam method states the gap itself, saying it reads significantly lower than the rigid support method. It names no figure, so arithmetic cannot reconcile the two.
Status differs too: the published index marks rigid support as an approved American National Standard, while flexible beam remains a classical method without that approval.
Scope separates them further: the flexible beam method covers single-faced and single-wall board only, so double and triple wall fall outside it.
One test loads the face and one loads the edge, which is why neither converts into the other.
What a Low Number Is Telling You
A low result points at the fluting, and the method lists the candidates: weak medium, leaning flutes, crushed flutes, or flutes never formed properly.
Every item is something the corrugator did or the medium failed to do, which is why the number reads as a process signal.
One cause carries a condition worth knowing. Leaning flutes read low only when the liners move laterally during the test, so that defect can slip through a specimen unseen.
The report exists to expose exactly that. Beside the average it carries the standard deviation, the count tested, the machine, the flute identity, and how many specimens failed by rolling.
Specimen rules guard the result. Each piece covers 5 or 10 square inches, preferably circular, cut at least an inch and a half clear of print, scores and die-cuts, with ten the minimum count.
Loading is set as a rate of force, not of travel: 25 pounds per second, give or take 5. Conditioning uses the same atmospheres as the edge crush test.
Flute geometry decides what counts as normal, so the corrugated flute types behind a figure matter as much as the figure. Medium strength is the other half, which is where corrugated basis weight comes in.
Why It Does Not Predict Stacking
The method is candid about its limits, and names two that matter. It does not simulate the crushing forces a container meets in use, and it does not account for linerboard stiffness.
One federal specification shows how this plays out. The National Archives specification for archival board sets flat crush minimums of 48 psi for B-flute and 85 psi for E-flute.
It sets burst minimums as well, 170 psi for B-flute and 150 for E-flute, while edge crush appears nowhere in the document.
The wording adds a last signal: the board “should” meet that flat crush figure, while most other requirements in the same specification say “shall”.
Which fairly summarizes the number: treat flat crush as evidence the flutes came out right, and ask the box compression test whether a stack holds.
Common Questions
Is flat crush the same as edge crush?
No, and neither converts into the other. Flat crush loads the face and reports pressure over an area, while edge crush loads the edge and reports force per inch of width.
Why do two labs report different flat crush values?
Most likely they ran different methods. Two are in print, and the flexible beam standard states that it reads significantly lower than the rigid support one. It names the direction and no figure, so arithmetic cannot reconcile them.
What does a low flat crush result mean?
Something went wrong in the fluting. The method lists weak medium, leaning flutes, crushed flutes and poorly formed flutes. Leaning flutes read low only when the liners move laterally, so that defect can pass unseen.
How to weigh this page. The method figures come from the flexible beam standard as its publisher issues it, and the designations from that body’s own index. The rigid support method sits behind a paywall, so every specimen and rate figure here belongs to the flexible beam method. The minimums quoted are what one federal archival specification requires.
Key takeaways
- Flat crush loads the board face and collapses the flutes, reporting the peak before the side walls give way.
- Two methods are in print: a rigid support method approved as a national standard, and a classical flexible beam method that reads significantly lower.
- The flexible beam method covers single-faced and single-wall board only, so double and triple wall fall outside it.
- A low result points at the fluting, and leaning flutes read low only when the liners move laterally.
- The method states it ignores liner stiffness and does not simulate in-use crushing, which is why one federal specification sets no edge crush minimum at all.