Materials 2022 , 15 , 663
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explain the differences in the bending stiffness of asymmetrical corrugated boards when the layers on the B or E flute side are compressed.
2. Materials and Methods 2.1. The Four-Point Bending Test of a Sample with an Asymmetric Cross-Section
In the case of four-point bending of a sample with an asymmetric cross-section, es- pecially when its cross-section consists of thin-walled faces (an example of such board is the corrugated cardboard), the dependence of the bending stiffness on the direction of the moment can be noticed. Using theoretical models as well as linear numerical models, this effect cannot be capture. This phenomenon belongs to the imperfection class of problems. Since in the four-point bending test, the mid-segment is bent with a constant moment M (see Figure 1) and all other section forces are not present, therefore, the problem is greatly simplified. In the case of asymmetrical sections in such test, the sample can be placed and, consequently, examined in two positions, which results in different values of the determined bending stiffness.
Figure1. 4-point bending test.
Thin-walled structures, when they undergo bending (i.e., one part of the cross-section is compressed and other part is in tension), have higher bending stiffness if the “stronger” part of the cross-section is compressed (see Figure 2a). On the other hand, when the “weaker” part of the cross-section is compressed (see Figure 2b), the BS is lower—this is due to the preliminary buckling of the compressed fragments of the thin-walled cross-section. More information and a short discussion on this phenomenon can be found in the following subsections.
( a ) ( b ) Figure 2. Two possibilities of placing the corrugated board in the 4-point bending test: ( a ) B-flute upwards (BE); ( b ) E-flute upwards (EB). In our case, where a five-layer corrugated cardboard sample is tested, the stronger part of the cross-section is on the E-wave side. Therefore, from now on, the following description is used to distinguish two cases: (a) EB—compression of a part of the cross-section on the B wave side (see Figure 2b) and (b) BE—compression of a part of the cross-section on the E wave side (see Figure 2a). In the BE configuration, higher BS values are obtained. 2.2. Corrugated Cardboard-Samples This study uses the results of the research presented in the work by Czechowski et al. [29]. The authors presented the mechanical parameters of the component papers (cor-
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