# Investigation of Debonding Effect in Internal Replacement Pipe System Under Lateral Loading
## 3 Surface Preparation and Its Impact on Debonding
### Behaviour of IRP System in 152.4 Mm (6 inch) Discontinuous Host Pipe
#### 3.2.3 Stress Concentration and Fatigue Life of IRP  
Similar to a 12.7 mm (0.5 inch) discontinuous IRP system, a 152.4 mm (6 inch) discontinuous IRP system exhibits a stress concentration at the crack edge on the invert of the IRP, as shown in Figure 30. However, the ratio between the concentrated stress at the crack and the midspan stress is higher for the 12.7 mm (0.5 inch) discontinuous case, i.e., 1.39, in comparison with 1.12 for the 152.4 mm (6 inch) discontinuous system as shown in Figure 22 and Figure 31, respectively. These findings indicate that the 12.7 mm (0.5 inch) IRP system  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Adhesive Strength MPa (ksi)** & **Fatigue Load 21.4 kN (4.8 kip)** & **Maximum Load 56.9 kN (12.8 kip)** \\ \hline \hline \end{tabular}
\end{table}
Table 11: Debonding modes of the various levels of adhesive strengths at a fatigue load of 21.4 kN (4.8 kip) and a maximum load of 56.9 kN (12.8 kip) for a 152.4 mm (6 inch) discontinuous IRP system (true scale).  
experiences a more pronounced stress concentration at the crack edge than the 152.4 mm (6 inch) IRP system. This behavior can be explained by the relatively wide discontinuity of 152.4 mm (6 inches), which allows for better absorption of stress concentration compared with the shorter discontinuity of 12.7 mm (0.5 inches). It is also observed that the shear strength levels of the adhesive layer have little effect on the magnitude of normal stress for the 152.4 mm (6 inch) discontinuous system.  
Under fatigue, the 152.4 mm (6 inch) discontinuous system exhibits similar concentration stresses in all three cases: 27.308 MPa (3.961 ksi) for a 5 MPa (0.7 ksi) shear strength and 26.924 MPa (3.095 ksi) for both 20 MPa (2.9 ksi) and 40 MPa (5.8 ksi) shear strengths. These  
Figure 31: Normal stress distribution in the longitudinal direction of the bottom line of the 12.7 mm (0.5 inch) discontinuous IRP system utilizing the debonding mechanism under a fatigue load of 21.4 kN (4.8 kip).  
Figure 30: Normal stress results in the longitudinal direction of the IRP layer of a 152.4 mm (6 inch) discontinuous IRP system employing the adhesive shear strength of 20 MPa (2.9 ksi) at a fatigue load of 21.4 kN (4.8 kip) (magnified 23 times).  
levels of concentration stresses are higher than those observed in the 12.7 mm (0.5 inch) discontinuous IRP system, resulting in lower life cycles, as shown in Table 12: 432,419 cycles for 5 MPa (0.7 ksi) shear strength and 510,812 cycles for both 20 MPa (2.9 ksi) and 40 MPa (5.8 ksi) shear strength. These fatigue cycles fall short of 1 million cycles, which is required to demonstrate the 50-year design life of the pipeline repair system. To enhance the fatigue life of the 152.4 mm (6 inch) discontinuous IRP system while maintaining the material property, a thicker IRP layer should be employed. It can be explained that the 152.4 mm (6 inch) discontinuous system experiences a higher magnitude of stress concentration compared with the 12.7 mm (0.5 inch) discontinuous IRP system due to the elevated level of deformation.