The ability of a double-walled heat exchanger pipe to resist liquid leakage is an important feature of the pipe. Even if the pipe is cracked, the crack must be stopped from penetrating the double-walled pipe and causing a liquid leakage accident, instead leaving enough opportunity for repair. Double-walled heat exchanger pipes work under high pressure from both internal and external liquid with both ends fixed. The rapid expansion of an initial crack in such conditions should be avoided. For example, after the crack on the outer pipe expands to the interface, stress concentration may occur there, which may cause cracks to grow on the inner pipe and then penetrate under the action of internal pressure. As previously mentioned, the main residual stress in the inner pipe of the formed double-walled pipe is the circumferential compressive stress, which helps to counteract the tendency for the inner pipe to crack, while the circumferential tensile stress in the outer pipe makes the cracks more likely to grow. However, in practice, the cracking conditions that may happen in double-walled pipes are more complex and require numerical simulations and experimental verification to ensure their ability to stop cracking. As can be expected, the pipe wall will buckle under compression , and the effect of buckling on crack expansion is one focus of the present study’s experiment. If the two pipes do not buckle in the same way, the interface of the two pipes will separate, which is conducive to hindering the expansion of cracks.
2.2 Simulation and Experimentation
Because the pipe is fixed at both ends during operation and subjected to axial compression at high temperatures, axial compression was chosen as the loading means for the crack expansion test. The pre-crack was set on the surface of the outer pipe and driven to expand by loading to confirm if it would cross the interface between the two pipes. In ABAQUS software, the residual stresses generated during the tensile simulation were used as pre-set stress values, and pre-cracking was set in the outer pipe of the model and axial compression was applied. The results were compared with the corresponding experimental results.
2.2.1 Longitudinal cracks
A 67.5-mm-long section of the prepared sample was cut, and axial cracks of 0.2-mm width and depths of 1 mm and 1.8 mm were prefabricated in the body of the pipe. Axial pressurization experiments and corresponding numerical simulations were performed, and the results were as follows.
The experimentally obtained longitudinal crack extension results can be seen in Figure 4. Figure 4(a) shows the results obtained for three pipes prepared by the tensile method with 1-mm deep vertical cracks and axial pressure. It is apparent that all three pipes buckled near the ends, and the cracks also opened in the buckling. This crack expansion at the location with residual stresses will cause the tensile residual stresses to be released , making the cracking of the outer pipe more obvious (compared with oblique cracks). The pipes from left to right in Figure 4(a) were subjected to compressions of 4 mm, 4.4 mm, and 4.8 mm, respectively, and as can be seen, the amount of compression significantly affected the degree of crack opening. Figure 4(b) shows the compression curves corresponding to the different compressions, where the horizontal axis shows the amount of the pipe that continued to be stretched after the outer pipe contacted the inner pipe during the preparation process. The difference in stiffness between the three pipes can be seen in this plot, as well as how the pressure tends to be consistent and smooth after the pipe yields. Figure 4(d) shows the crack extension results obtained from the simulation. Figure 4(c) shows the experimental results obtained under a 1.8-mm deep pre-set crack, in which the crack penetrated through the outer layer as seen in Figure 4(f), where the fracture is visible under the microscope.
Figure 5 examines the section obtained after cutting the sample horizontally, which shows that the buckling of the outer pipe is more obvious than that of the inner pipe due to the pre-cracking, and the inner and outer pipes are shown to be separated at the buckling. In Figure 4(a)(c)(f), there is no obvious crack extension at the unbent area. Therefore, it can be concluded that when the structure of the double-walled pipe is subjected to axial pressure that causes wall buckling and obvious crack expansion, a gap between the inner and outer pipe will be produced, making it difficult for crack expansion through the inner and outer pipe interface to occur. However, the clearly visible buckling phenomenon alone does not lead to the separation of the two pipes, but instead, the combined effect of both buckling and crack extension leads to the separation and prevents further crack extension. In the case of longer pipe lengths, the pipes may show overall buckling phenomena  that are not closely related to the crack extension discussed here.
2.2.2 Oblique cracking
Simulation results obtained during compression for a model with an axial length of 65 mm are shown in Figures 6 and 7. The crack penetrates the outer layer at pressures above 800 kN, shown in Figure 7(c) and (d), after which it begins to expand toward the ends of the pipe. The inner pipe showed no obvious stress concentration or signs of cracking, indicating that no crack penetration occurs in this state. The later cracking turns into axial development, which is consistent with the use of axial residual stress to prevent it as mentioned before, indicating that the axial residual stress obtained by the tensile method does counteract the cracking.
The results obtained from the experiment are shown in Figure 8. The pipe length was 65 mm long with a 45°-inclined, 1-mm-deep, and 0.2-mm-wide pre-set crack, and subjected to a downward pressure causing a 3-mm length deformation. The change in buckling of the pipe can be seen in Figure 8(a).
As shown in Figure 8(b) and (c), the pre-crack has been extruded and dislocated under extrusion. This is consistent with the dislocation pattern from the simulation shown in Figures 6 and 7. In contrast to the vertical cracks, the oblique cracks become tighter rather than opening due to the axial pressure, causing the residual stresses to be released in a dislocated manner. This also directs the crack expansion in the direction that intersects the crack, developing it inside the outer pipe, as seen in Figures 6 and 7. However, given the limited range of available pressures in the equipment, it was not possible to pressurize to the extent that crack expansion occurred.