## 3 The Shear Sliding Test of the Composite Timber Column's Interface
### Test Results and Discussion  
#### 3.2.1 Failure Mechanism  
The failure mechanism of the interface was analyzed by the shear stress-displacement curves that were obtained from the sliding characteristic test. The shear stress within the interface was obtained by dividing the shear load, as measured by the universal press, by the sum of the areas of the shear surface. The shear stress-displacement curves of all of the test groups had similar variation trends in this test. The test groups with different numbers of iron hoops were taken as examples for the analysis of the change in the shear stress-displacement curves during the loading process, as shown in Figure 9.  
Figure 8: Schematic diagram of the test. (**a**) Test loading setup; (**b**) the arrangement of strain gauges on the iron hoop, unit: mm.  
Figure 9: Shear stress–displacement curves of test groups with different numbers of iron hoops. (**a**) G1-1; (**b**) G1-2; (**c**) G1-3; (**d**) G1-4.  
From the Figure 9, it can be seen that the shear stress-displacement curves of the interface increased monotonically and almost linearly during the initial loading stage. The large slope of the curves indicates that the shear stress of the interface increased rapidly, but the growth rate of the displacement was low. At this stage, the interface was in the sticky stage with the trend of sliding. The static friction and shear stress of the interface were balanced and their values gradually and synchronously increased. With the continuation of the loading process, the curves increased monotonically in the convex direction. The slope of the curves decreased, showing that the growth rate of the displacement sped up, and the shear stress increased slowly until reached its peak value, which is equal to the maximum sliding friction of the interface. That value is exactly the boundary between the partially sticky stage and the sliding stage.  
Following this, uniform sliding occurred at the interface. The shear stress of the interface should, in theory, have remained unchanged and been in balance with the maximum sliding friction. However, by observing the shear stress-displacement curves of the specimen in the test, it can be found that the shear stress dropped after reaching its peak. The main reasons for this are as follows: first, the friction coefficient of the interface does not remain constant due to the size error (i.e., the surfaces of the interface are not always in close contact, as is shown in Figure 10a), operation error and material defects (i.e., there were initial shrinkage cracks and knots in the wood, as are shown in Figure 10b). Second, when the interface slid, the iron hoops were no longer parallel to the interface (Figure 10c). There is a certain included angle between them that reduces the positive pressure of the iron hoops on the interface. Based on the above reasons, the sliding friction (i.e., the shear stress) of the interface decreased after reaching its peak.  
Figure 10: Test errors. (**a**) Initial gaps between the interface surfaces; (**b**) initial shrinkage cracks and knots in the wood; (**c**) inclined iron hoop at end of loading.