# 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 12.7 Mm (0.5 inch) Discontinuous Host Pipe
#### 3.1.3 Stress Concentration and Fatigue Life of IRP  
Figure 21 illustrates the distribution of normal stress in the IRP layer of a 12.7 mm (0.5 inch) discontinuous IRP system using an adhesive shear strength of 20 MPa (2.9 ksi) under a fatigue load of 21.4 kN (4.8 kip) [26; 27]. The case of the shear strength of 20 MPa (2.9 ksi) is chosen as representative for all three levels of shear strength. The results reveal a stress concentration phenomenon at the crack edge on the invert of the IRP, consistent with the findings of previous studies [19; 26].  
Figure 22 presents a visual representation of the normal stress (axial stress) distribution along the invert line of the IRP of the 12.7 mm (0.5 inch) discontinuous system under a fatigue load of 21.4 kN (4.8 kip) for three different levels of adhesive shear strengths.  
Figure 21: Normal stress results in the longitudinal direction of the IRP layer of a 12.7 mm (0.5 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 74 times).  
Figure 22: 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).  
The fatigue performance of the IRP system was evaluated by integrating the S-N curve for ALTRA10 with the FEA results. The process involved extracting the normal stress concentrations from critical regions, as shown in Figure 22, and mapping them onto the S-N curve presented in Figure 23[26], which correlates the stress amplitude to the number of cycles to failure for the ALTRA10 material. Table 9 summarizes the minimum life cycles to failure of a 12.7 mm (0.5 inch) discontinuous IRP system with varying levels of adhesive bonding shear strengths. The results indicate that a 12.7 mm (0.5 inch) discontinuous IRP system with 5 MPa (0.7 ksi) adhesive shear strength exhibits a lower stress concentration, consequently leading to a higher life cycle to failure. It is believed that the early debonding of the 5 MPa (0.7 ksi) adhesive at the fatigue load of 21.4 kN (4.8 kip) contributes to stress relief, thus positively affecting the system's durability and longevity. In practice, during fatigue cycling, the debonding near the discontinuity progressively grows larger than the predicted numerical results. These findings in the present numerical results reinforce the experimental findings presented in [15; 27; 28; 29], which demonstrated that debonding at the separation or discontinuity is a highly desirable characteristic of a well-installed IRP system. By allowing debonding, the IRP can accommodate a certain level of deformation by permitting extra deflection and extension. This ability to strain over a debonded length can prevent premature rupture of the IRP. Figure 24 shows 'before' and 'after' photos of the mechanical aging tests, including 50 years of vehicular traffic (1 million cycles) for the 150 mm (6 inch) nominal diameter cast iron IRP system. It is evident that there was substantial debonding of the IRP from the host pipe in the joint or discontinuity. The debonding occurs within a short distance on either side of the separation, totaling less than one pipe diameter in width.