The Ring Crush Test (RCT) is similar to the Short-Span Compressive Test (SCT) in that the strength of the liner or the fluting is measured in machine and cross directions. However, a universal formula to convert SCT to RCT has failed because there is no simple relationship between the two values, as they are dependent on furnish and the type of forming equipment used. Thus, the conversion between SCT and RCT needs to be done for each specific supplier.
Furthermore, the conversion will also change with basis weight. It is speculated that this dependency of basis weight is caused by a tendency of samples to buckle in the RCT because of a longer span length than the SCT, viz. 12 mm and 0.7 mm respectively. The RCT also depends very much on the quality of specimen preparation. Only an exact parallelism of the specimen and guides guarantees an exact determination of the Ring Crush Strength. Thus, the SCT frequently replaces it.
The material characteristic "compressive strength" can be determined with high accuracy using SCT because of the buckling stability provided by short test lengths over traditional testing methods (Ring Crush Test, Corrugated Crush Test or Linear Crush Test). Due to these technical advantages, the SCT is being used more and more worldwide. Thus qualified paper testing laboratories around the world recommend using the SCT to predict the final characteristics of boxes.
For those users still not very familiar with this property, it is possible to calculate back the RCT value knowing the value of the SCT. There are some very important limits to this practice, pointed out above, which should be taken into serious consideration each time the formula needs to be applied.
The formula, in general, is the equation of a linear regression of the RCT and SCT values of samples with different basis weights. Lorentzen & Wettre provide a general equation which should be considered as a standard example and not used directly without a careful check of the parameters.
RCT CD= 120 x SCT CD - 97
or approximately
RCT CD= 95 x SCT CD (N)
The equation is specific for each paper machine. This means that the equation should be agreed upon by producer and users based on a series of dedicated tests.