# The Static Frictional Behaviors of Rubber for Pipe-Laying Operation
## 4 Discussion
### Tangential Load Condition
#### 4.2.2 Increasing Rate of the Tangential Load
Therefore, even the intense deformation at the rear edge of the contact (Figure 11d 6), which is stronger than that for the higher increasing rate of \(F_{T}\) condition can be seen, the broken adhesive junctions facilitate to opening of the boundary of the adhesive zone, and make it easier for the macroscopic slip to take place (Figure 11d 5). Hence, the creep deformation can be regarded as a function that decreases the maximum static friction. Figure 12: Creep deformation of the rubber surface at the rear edge of the contact area; (**a**) The tangential load starts to affect; (**b**) Adhesive junctions reach limitation of elastic deformation; (**c**) some of adhesive junctions have been further creep deformed, whereas the other adhesive junctions do not have enough time to creep. It is hard to detect the creep deformation directly in this study. However, we can examine the wear surface within the adhesive zone to identify the existence of the creep deformation. As far as we know, the creep deformation is beyond the extent of the elastic deformation and it cannot recover to its original shape [32]. As shown in Figure 13a, the rubber surface is full of wrinkles which are perpendicular to the direction of sliding when the increasing rate of \(F_{T}\) is 0.062 N/s. It is deduced that the wrinkles can be seen as the permanent deformation due to the creep. When the increasing rate of \(F_{T}\) is 1.5 N/s, fewer wrinkles are observed on the rubber surface (Figure 13b), this phenomenon may indicate that the creep deformation is weaker distinctly than that for the low increasing rate condition. Therefore, it is concluded that the instability of the static frictional behaviors of rubber contact that are observed in this research can be seen as consequences of the interaction between normal load and increasing tangential load. Apparently, the increasing rate of \(F_{T}\) takes the primary effort to cause  
Figure 13: Wear morphology of the rubber surface within the adhesive zone; (**a**) Increasing rate of \(F_{T}\): 0.062 N/s; (**b**) Increasing rate of \(F_{T}\): 1.5 N/s. Normal load: 30 N. The black arrow line: direction of the tangential load; The white straight line: inherent texture of the rubber surface; The red broken line: distinct wrinkle at the rubber surface; The red dotted line: fine wrinkle at the rubber surface. fluctuation in the maximum static friction. For the low increasing rate, the creep deformation acts as a function of decreasing the static friction, whereas for the high increasing rate, the creep deformation cannot behave active due to short loading time. On the contrary, the propagation of the folds on the rubber surface becomes distinct with the increasing rate of the tangential load and makes the critical value of the tangential load that initiates the macroscopic slip between interfaces increase. It is inevitable to note that the instability of contact stays in a certain rage of normal load. As is mentioned in Section 3.1, the fluctuation of the \(F\) is getting less intense as the normal load increases for the stiff rubber sample (rubber B). The reason for this phenomenon is mainly attributed to the high compression stress, which is concentrated in the contact area and the high elastic modulus of the rubber, both of them restricting the deformation of the rubber surface. For the soft rubber (rubber A), deformation of the rubber surface can still occur even the normal load is increasing. The above discussions may not completely analyze the precise interactions between rubber and pipe samples. However, the results in this research propose that the maximum static friction fluctuates under different increasing tangential loads. The instability of \(F\) results from the combination of propagation of folds and creep deformation. These results may provide experimental knowledge for further systematic tensioner design and more accurate tribological behavior analysis of static contact.