## 7 Discussion of Results  
The results obtained for YPT and RHT pipes indicate that the CLS varies inversely with the \(D/t\) ratio for all combinations of parameters. For the CLS of RHT pipes, however, the effect of varying the \(D/t\) ratio is observed to be mild for all cases considered. The observed deformation behavior, with respect to the \(D/t\) ratio, may be attributed to the somewhat dichotomous phenomenon associated with the buckling response of cylindrical shell structures as the initiation of buckling transitions from the elastic region of the material stress-strain curve (elastic buckling) to the nonlinear region of the material stress-strain curve (inelastic or plastic buckling). RHT pipes exhibit continuous strain-hardening, and invariably a gradual reduction in the tangential modulus of the stress-strain curve beyond the proportionality limit point, and therefore do not experience significant changes in deformational capacity due to the delay of buckling initiation. The effect of \(D/t\) variation on the CLS of RHT pipes can therefore be deduced to be predominantly influenced by the increase in axial or bending rigidity of the pipe due to decrease in slenderness of the pipe wall, and vice versa. YPT pipes, on the other hand, feature a sudden zeroing of the slope of the stress-strain curve at the proportionality limit, and subsequent strain hardening that commences toward the end of a significantly-extended yield plateau. The existence of a distinct yield point in the stress-strain curve tends to cause an automatic trigger of buckling instability in YPT pipes once the initiation of buckling extends beyond the elastic region of the pipe material. However, there exists the possibility of recovery from initial softening of the load-deformation or moment-curvature response if intrinsic properties (e.g., \(D/t\) ratio) or loading conditions (e.g., internal pressure) provide additional stiffness that sufficiently counteracts the detrimental impact of the yield plateau on the deformational response of the pipe. The deformational response of a pipe to uniform axial compression is characteristically different from the response to bending, even with the additional application of a compressive force. Under uniform axial compression, the entire cross-section of the pipe is susceptible to buckling, and the pipe relies on geometric, material, loading, and boundary conditions for sustenance against induced deformation. Under bending, however, the pipe gains additional stiffness due to the counteractive interaction between tensile and compressive stresses in the pipe's cross-section. This can, therefore, be considered to be responsible for the more apparent effect of \(D/t\) variation on the CLS of YPT pipes subjected to uniform bending and combined bending, and axial compression at pressure levels between 0% and 60% YS. The deformational response of a YPT pipe subjected to all three loading conditions at a high level of internal pressure (\(f_{p}=0.8\)) is analogous to the deformational response of RHT pipes, as an almost-linear relationship is observed between the CLS and the \(D/t\) ratio. This is attributable to the tendency that a pressure level of \(f_{p}=0.8\) significantly increases the stability of the pipe's response, such that catastrophic collapse leading to failure of the pipe inevitably takes advantage of the strain-hardening portion of the stress-strain curve at the end of the yield plateau. This also implies that the influence of \(D/t\) ratio variation on the CLS will be predominantly impelled by the stiffness of the pipe response associated with pipe wall slenderness factors. The results presented for the influence of internal pressure on the CLS further substantiate the assertion that increase of the internal pressure in a pipe results in a corresponding increase in the stability of the pipe's response to loading.