Shear test analysis
Figure 3 shows the fracture topography of bamboo samples after compression shear test. The three types of glued bamboo are all destroyed at the bamboo region, thus the bonding strength results of bamboo samples actually reflect the shear strength of bamboo itself. In particular, for the sample glued by outer side to inner side, the broken part is always the outer side. And the fracture topography on inner–outer side is very smooth may cause by lack of glue (just for this sample). In addition, the fracture surface of the inner side is much rougher than the fracture surface of outer side. The failure position and fracture surface topography seem to imply that the inner side is the stronger side under the shear load.
The shear strength results are shown in Fig. 4a. Among the three types, the inner–inner sample has the greatest shear strength value, which is 18.35 MPa. The other two kinds of samples have similar shear strength values, approximately 14 MPa, which are about 20% lower than that of the inner–inner sample. The shear strength results of the three types are all slightly higher than results of other researchers (11.33–12.84 MPa) [28]. And the reason for the difference may be microwave was employed as the heating method in this study. Figure 4b shows the load–displacement curves of the three types of samples. In particular, the curves of the three types of samples can be divided into two regions. When the load reaches around 2200 N, there are points of inflection on the load–displacement curves, and separate the curves into two regions. We will discuss this in more detail later in this paper.
Morphology analysis
Bamboo materials are mainly composed by bamboo fiber cells and parenchyma cells. On the outer side fiber content is much higher than that of the inner side, and this is in agreement with the morphology of fracture bamboo surface, as shown in Fig. 5.
Figure 6 shows the fracture morphology of bamboo fiber cells and parenchyma cells. Figure 6a, b indicates that the fiber cell itself is little broken in compress shear test. Under the action of shear force, there are some fibrils that warp and drop on bamboo fiber surface. And dominant destruction occurs at the interface between bamboo fibers. There are two types of cells in parenchyma in bamboo materials: long cells and short cells [14]. The long cells arrange vertically, and the short cells are scattered in the longer cells. In comparison with the long cells, the short cells have thinner cell walls and are not woody. Figure 6c–f shows the fracture morphology of long cells and short cells. It is clear from the SEM images that the shear force will break the parenchyma cells, whether they are long cells or short cells. Fracture occurs on the wall of parenchyma cell and finally chops up the parenchyma cell.
Mechanism analysis
The bamboo has a consistent structure from top to bottom. So along the glued line, the percentage of fibers (or parenchymata) can be calculated by the ratio of fibers (or parenchymata) length to the total length of glued line, as shown in the following graphic (Fig. 7).
According to this method, ten inner–outer samples were observed and calculated. For the outer side, the percentage of fiber along the glued line is 50.28%. And for the inner side, the percentage of fiber along the glued line is 18.49%, which means 71.51% are parenchymata.
Figure 8 is a schematic drawing that illustrates the different functions from different elements in bamboo. The bamboo fiber cells are connected by intercellular layer [18, 29]. The intercellular is naturally free of cellulose, whose dominant component is pectin and lignin. Cellulose is the skeleton element in biological cell wall, and cellulose is the key factor in bamboo mechanical performance.
The chemical components of intercellular layer result in its low mechanical performance, thus the interface between bamboo fiber cells is very weak. When the bamboo is broken by shear force, the weak interface will be the priority for cracks growth direction. So the fiber bundles have less contribution to shear strength of bamboo, as shown in Fig. 8a. Comparing with bamboo fiber cell, parenchyma cell is hollow and its cell wall is very thin. The size of a parenchyma cell is about 50 μm to 100 μm, and the cell wall is only about 3 μm to 5 μm. Thus the crack will easily break the parenchyma cell wall. Although the parenchyma cell wall is thin, its strength is much higher than that of bamboo fibers interface. Thus we consider that the parenchyma cell have greater contribution to shear strength of bamboo, as shown in Fig. 8b. In addition, the parenchyma cells are hollow structure. This means PF adhesive can infiltrate into parenchyma cells more deeply, and this is another important reason to the difference of shear strength. Since in the inner side, 71.51% cells are parenchymata, the inner-inner sample has the greatest shear strength value.
In this research, sample is broken at the bamboo part where is nearest to the glued interface. Figure 9a shows morphology of the glued interface. It is indicated from the SEM image that there is a deformable layer near the glued interface. This deformable layer includes about two to three layers of parenchyma cells, which have deformed under the action of pressure. We consider that there will be a decline of mechanical performance of the deformed parenchyma cells. Thus this deformable layer is the broken area in the shearing process. Figure 9b is a schematic drawing that illustrates the shearing process. If the shearing displacement is small, the pressed parenchyma cells will be still in the state of relaxation. Thus in the early period of shearing process, the parenchyma cells are not yet fully effective, for it is in relax status. In the later period of shearing process, the interface of bamboo fibers have been destroyed, and the parenchyma cells are tensioning with the increase of shearing displacement. At this period, parenchyma cells are the main contributor to shear strength. We consider that this is the main primary reason for the inflection on the load–displacement curves, which is shown in Fig. 4b.