Ring crush test (RCT)

ISO 12192 / TAPPI T822. A paperboard strip bent into a ring, then edge-compressed. Why RCT matters for both liner and medium, and how it relates to SCT as the industry convergence point.

Ring crush test (RCT) measures the edgewise compression strength of a paperboard strip bent into a cylinder. It is the standard method for characterising the in-plane compressive strength of liner and corrugating medium — the property that ultimately determines how well a corrugated box resists crushing under load.

The governing standards are ISO 12192:2011 and TAPPI T822. The two produce equivalent results on the same sample; the test geometry is essentially identical.

How the test works

A paper strip is cut to standardised dimensions (typically 152 mm × 12.7 mm for TAPPI T822; ISO 12192 uses a 152 mm × 12.5 mm strip). The strip is bent into a cylinder and placed in a cylindrical holder that prevents the ring from buckling outward during compression. The holder is placed between parallel compression platens on a testing machine, and load is applied at a constant crosshead speed until the ring fails by collapse.

The peak load in Newtons is the RCT result. It is reported as N/m (force per unit strip width, dividing by the 12.7 mm or 12.5 mm strip width) to allow comparison across different sample widths. The ring crush index normalises further by dividing by grammage:

RCT index (N·m/g) = RCT (N/m) / grammage (g/m²)

The ring crush index allows comparison across different weights of the same grade family.

Why RCT is the right metric for liner

Burst strength is the traditional liner specification, but burst measures the resistance to rupture under outward hydraulic pressure — a load mode that rarely governs corrugated box failure. Boxes fail under vertical compressive loads (stacking, warehouse storage), not internal pressure.

RCT and SCT (short span compression test) measure in-plane compression resistance, which is directly relevant to stacking loads. The ECT of finished corrugated board is well predicted by the combined compressive strengths of the two liners and the medium; that prediction uses RCT (or SCT) of the flat components, not burst index.

The testliner grade above (recycled OCC, 80–400 g/m²) illustrates the dominant competitive alternative to virgin kraft liner. In practice, the RCT difference between virgin kraft liner and recycled testliner at equal GSM is substantial — typically 25–40% — because virgin long-fibre softwood provides both more fibre surface area for bonding and better fibre alignment through the machine direction.

Typical RCT values

Conditioning: 23 °C / 50% RH, minimum 4 hours (ISO 187, T402). Tests run immediately after removal from conditioning environment.

| Grade | GSM | Typical RCT (N/m) | RCT index (N·m/g) | |---|---|---|---| | Virgin kraft liner (top grade) | 150 | 500–700 | 3.3–4.7 | | Virgin kraft liner (inner) | 125 | 380–520 | 3.0–4.2 | | Testliner (recycled) | 150 | 340–440 | 2.3–2.9 | | Testliner (recycled) | 125 | 270–360 | 2.2–2.9 | | Corrugating medium | 115 | 200–320 | 1.7–2.8 |

RCT vs SCT: the convergence

SCT (short span compression test, ISO 9895) has been gaining ground as the preferred compression metric because it addresses a known limitation of RCT: the ring geometry introduces buckling behaviour in the strip that depends on strip dimensions and cutting quality. Small variations in strip width, straightness, and edge condition affect RCT results more than SCT results.

SCT clamps the paper between two very closely spaced platens (0.7 mm span) and compresses it edge-on. At this span, buckling is mechanically prevented, so the test measures pure in-plane compression resistance — the property that models predict is the relevant input to ECT. SCT shows lower inter-lab variability than RCT and models ECT more accurately in regression studies.

The practical reality is that much of the global corrugated industry still specifies and trades on RCT because it is what decades of purchasing relationships are built around. SCT is increasingly specified by sophisticated buyers and in European markets, but RCT is not going away.

WPI editorial note: If you are writing a purchase specification for corrugated liner and your converter is willing, specify both RCT and SCT. This gives you the historical comparability of RCT and the better engineering predictor SCT. If you can only specify one and your boxes are high-value or high-stack applications, specify SCT.