The 4-point bending test Compared to bending instrumentswhich use dead weights for specimen With the use of the McKee formula,bending stiffness is important for the prediction of the box crush resistance. With the 4-point bending method, dependable stiffness values are measured in the machine and cross direction. This is particularly important for miniature flute corrugated board, wherestiffness plays an increasing role, compared to standard flute heights.With the knowledge gained fromthe bending units for structural materials, (aerospace, ceramics and high performance metals), Zwick developed a 4-point bending unit for carton and corrugated board (Fig.21). The result is a tool that iscomparable to other existing solutions at a moderate cost. Another important point is thatthe bending unit can be used on the smallest machine, the zwicki. loading, handling is uncomplicated and fast: the specimen is simply placed on the bending unit and the test is started. The stiffness is calculated automatically, taking in consideration of the elasticity limits.Because of rotatable and swivablesupports, shrinked or warped specimens can be tested within wide limits. Testing specimens with excessive shrink or warp, the validity of the test can be decided with unambiguous criteria. Standard ISO 5628 Material corrugated board,cardboard Zwick-Item-No bending test rig 070290.00.00*load cell TC-LC0500N.F02 fftest program069002.20.ff for specimens widthmax. 100 mm lengthmax. 400 mm to machines from zwicki
Fig. 21: 4-point bending test rig with high resolution extensometer Fig. 22: Stiffness determination on corrugated board including monitoring of start phase.Recognition of excessively shrunk or warped specimens by limits. The advantages:•Test time below 60 s•The test graph clearly shows ifa test is valid or not (Fig. 22).•Supports of the bending unitare optimized for the characteristics of carton or corrugated board.•Measured results are mostlyindependent of warp or shrink of the specimens caused by production.•The test is performed within theelastic range of the specimen.
Force in N Deflection in mm 15