## 7 Discussion of Results
Internal pressure hinders the localization of wrinkling and bulging deformation, and also increases the tendency for buckling to be initiated further along the  
Figure 19: Model prediction vs. FEA results for RHT pipes for (**a**) LC1, (**b**) LC2, and (**c**) LC3. stress-strain curve. The results indicate a negligible influence of internal pressure variation on the CLS of RHT pipes at all levels of pressure. On the other hand, the CLS of YPT pipes tends to be unaffected by changes in internal pressure between 0% and 60% YS, whereas at higher pressure (\(f_{p}=0.8\)), there is a spike in the CLS depending on the \(D/t\) ratio. For YPT pipes, varying the length of the yield plateau is observed to have a higher effect on the CLS as the \(D/t\) ratio of the pipe decreases. For YPT pipes with \(D/t\) ratio = 41.15, the CLS is likely to be negatively affected by an increase in YPL at levels of internal pressure between 0% and 60% YS, whereas the YPL positively correlates with the CLS at a high level of internal pressure (\(f_{p}=0.8\)). Unlike YPT pipes, varying the PLUS ratio tends to have a less significant influence on the CLS of RHT pipes as the \(D/t\) ratio decreases. At DT1, however, the influence of the PLUS ratio on the CLS of RHT pipes is observed to be negligible, except at a level of pressure equal to 80% YS. It can be inferred from the results for YPT pipes that improvement of the CLS for pipes with lower \(D/t\) ratio or high internal pressure is not necessarily automatic, as the length of the yield plateau exhibits a secondary, but significant, influence on the deformational capacity. Figure 12b,c further illustrate that subjection of a pipe to bending considerably enhances the stiffness response of the pipe, even at zero-to-low levels of internal pressure. Enhanced stiffness is, therefore, accompanied by an increased tendency for buckling to be initiated in the nonlinear region of the material stress-strain curve. The higher stiffness of a pipe under bending, compared to a pipe under uniform axial compression, leads to a relatively greater positive effect of a longer YPL on the CLS of a pipe under bending, especially at internal pressures as high as 80% YS. However, additional benefits of a longer yield plateau tend to diminish once the YPL exceeds 1.75%. For levels of internal pressure between 0% and 60% YS, increasing the length of the yield plateau results in a reduced ability of the pipe to recover from initial softening of the mechanical response before reaching the actual peak stress or moment. A decrease in the PLUS ratio for RHT pipes implies a lower proportionality limit point, and invariably,'superior' strain hardening and vice versa. At higher \(D/t\) ratios, initiation of buckling tends to occur before the yield point of the material is reached, implying that materials with a lower proportionality limit earlier experience a softer inelastic response, which translates to higher deformability. As the \(D/t\) ratios become lower, initiation of buckling shifts towards the yield point of the material, where the stress-strain curves of materials with different PLUS ratios are likely to converge, hence the effect of PLUS ratio variation becomes negligible. However, beyond the yield point, the stress-strain curve of higher-PLUS ratio materials exhibits a softer post-YS response; hence, if the initiation of buckling is delayed beyond the yield point of the material (as is likely the case when internal pressure is as high as 80% YS), a higher PLUS ratio (i.e., inferior strain hardening) may have a positive effect on the deformational capacity of an RHT pipe, and vice versa.