# Investigation of Debonding Effect in Internal Replacement Pipe System Under Lateral Loading
## 4 Conclusions  
A comprehensive numerical investigation was conducted to evaluate the performance of internal replacement pipes (IRPs) under deflection loading, considering various configurations and inside surface preparations of the host pipe. Through the development and validation of an advanced numerical method using the full-scale four-point bending physical test results and shear strength criteria of the adhesive layer, this study yields valuable insights into the behavior of the IRP system. The key findings of the investigation are as follows:  
Figure 39: Equivalent stress results in the IRP layer of a continuous IRP system employing the adhesive shear strength of 20 MPa (2.9 ksi) at a maximum load of 71.2 kN (16 kip) (magnified 98 times). Figure 38: Normal stress results in the IRP layer of a continuous IRP system employing the adhesive shear strength of 20 MPa (2.9 ksi) at a fatigue load of 71.2 kN (magnified 270 times). * The adhesive shear strength should be considered in numerical analyses of the nonlinear behavior of the IRP system under lateral deformation. A deviation of only 1% was observed between the results of the numerical analysis and the full-scale experiment. This deviation can be as high as 20% when the IRP is assumed to be fully bonded to the host pipe. * Debonding of the IRP in a host pipe with a 12.7 mm (0.5 inch) circumferential discontinuity can lead to increased deflection but reduced stress concentration near the discontinuity edge, thus improving the life cycle of the repair system. * For systems featuring a 12.7 mm (0.5 inch) crack width, the critical point of potential failure was observed at the invert midspan of the IRP layer, while large discontinuous systems with a 152.4 mm (6 inch) discontinuity width exhibited potential local deformation at the crown midspan of the IRP layer, particularly under excessively large loads. * The strength of the adhesive or surface preparation methods does not significantly impact the deflection or fatigue performance of the IRP system in a host pipe with a 152.4 mm (6 inch) or half discontinuity or half the host pipe diameter. This can be due to the stress absorption capability of such a wide discontinuity. * The adhesive shear strength and surface preparation have a negligible effect on the performance of the continuous IRP system in terms of load-bearing capacity, fatigue, and catastrophic failure, attributable to the higher level of overall stiffness. * It is crucial to select adhesives with a minimum level of shear strength while also considering the necessary debonding length. This ensures that small discontinuities are managed effectively, promoting improved fatigue life for the system. * A balance of adhesive strength and debonding length is necessary to maintain the fatigue life of the IRP system without adverse effects. Excessive bonding might be detrimental. Typically, the maximum shear strength for adhesive or debonding length is determined through analysis, ensuring it does not compromise fatigue life. The successful validation of the numerical approach against analytical solutions and experimental data with high accuracy reinforces confidence in the fidelity of the findings. This enhanced numerical method using maximum stress failure criteria, along with the insights gained from this study, can serve as a valuable resource for future research in the field of pipe-in-pipe studies. While the primary focus of this work is on lateral deflection, the current approach can also be applied to other performance objectives, such as axial deformation and parallel compression. Moreover, the guidelines derived from this investigation can aid in the design and maintenance phases of trenchless IRP technology, contributing to more efficient and robust pipeline repair systems. Although the FEA results align well with both analytical and experimental data, further refinement of the FEA model is an ongoing effort. One potential improvement is the calibration of the adhesive layer thickness, which is currently limited by the element aspect ratio.