Energies 2021 , 14 , 1095
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3. Results In this section, the results of boxes are presented for both physical tests and numerical simulations. The boxes selected in the study represent a common flap box design of FEFCO 201, although with perforations included on the front and sidewalls. One box design was considered with dimensions of 300 × 200 × 300 mm. All boxes considered here had the same overall design. However, it was considered with three types of different perforations of the SRP type, see Figure 3 (cf. see Figure 1b). The perforations were on three walls, in which, on two opposite walls they were diagonally inclined (30 mm from both corners), and on one wall, in-between wall, the perforation was horizontally positioned (30 mm from the bottom).
Figure 3. Schematic designs of FEFCO 201 box with different perforations considered in the study: ( a ) 25 × 75, ( b ) 50 × 50 and ( c ) 75 × 25.
In the study, the perforations were described by two values, namely, A × B,where A means the percentage of the length of 10 mm, in which the knife cut the corrugated cardboard, and B means the percentage of the length of 10 mm, in which the corrugated cardboard was intact. Thus, the following perforations were considered in the study: 25 × 75, 50 × 50 and75 × 25 (in decreasing order of perforation/box strength). The boxes selected in the study were subjected to compression in a strength testing machine, namely BCT-19T10 from FEM at [37]; see Figure 1c. According to the specification, the testing machine has a 0.1 N resolution of force control and may induce the force up to 10 kN. The displacement may be controlled with 0.001 mm accuracy. For each type of perforation, five box samples were tested to acquire a representative result. For 50 × 50 perforation, one of the test results was rejected due to unusual deformation mode. Thus, finally, 14 measurements of ultimate loads of boxes with perforations were used in the study, see Table 1. In Table 1, the mean values computed with two standard deviations, which, for the normal distribution, accounts for 95.5% of cases for each box design, are also presented. The samples were produced from the same corrugated cardboard, namely single-side three-layer quality with E flute of 450 g/m 2 (marked as 3E450-1). Cardboard thickness measured was from 1.58 mm to 1.60 mm, with the mean equal to 1.59 mm. Each box sample was cut by plotter and manually folded; at the top and bottom, the flaps were taped. Since the plotter was used, no converted corrugated cardboard, i.e., raw cardboard material without printing, lamination, etc., was taken into consideration in this study. Before and after the test, the cardboard/box samples were visually inspected for any damages or unusual issues. Boxes were subjected to a displacement control test. The resultant force was registered by the testing machine (no additional strain gauges were used). Cardboard was laboratory conditioned with a temperature of 23 ◦ C ± 1 ◦ C and relative humidity
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