PSI - Issue 77

Sunny O. Uguzo et al. / Procedia Structural Integrity 77 (2026) 521–528

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plastic yielding was the universal failure mode for all TCP configurations occurring below both buckling and fibre failure limit, confirming the liner as the weak link at high T i . The controlling failure load and mode across the T i values are highlighted in Table 4. In terms of failure mode transition, further investigation revealed that the T i at which failure transitioned from buckling to material failure depends on the TCP laminate architecture. In particular, [±55] 4 laminate changed from buckling to material failure (inner liner yielding) at approximately 70 o C, while its counterpart with [±65 2 /±75 2 ] laminate transitioned in the same way at 90 o C (see Fig. 4). This can be attributed to the more hoop dominated laminate of the latter TCP which increases circumferential stiffness that resists brazier ovalization and external pressure effects, so that buckling capacity falls more slowly with T i . In contrast the [±55] 4 laminate TCP has less hoop-oriented plies causing ovalization to grow faster as T i increases, so P cr drops comparatively faster. Moreso, hoop dominated [±65 2 /±75 2 ] plies carry more hoop demand from pressure and bending induced ovalization, so the PEEK liner experiences less combined von-Mises stress at a given T i , hence its effective material failure capacity decays slower. On the other hand, the [±55] 4 laminate TCP exhibits weaker circumferential restraint as T i rises, hence a significant amount of the hoop-axial load is transferred to the liner while its yield strength degrades, causing the liner capacity to decay faster. Thermally induced ovalization is also reduced in the [±65 2 /±75 2 ] laminate TCP due to the outer hoop oriented plies which stay cooler and retain stiffness and section roundness, thereby preserving stability more at higher T i .

Fig. 4. Failure loads vs. T i for TCP with [±55] 4 laminate (left) and [±65 2 /±75 2 ] laminate (right)

In summary, ply angle influence axial vs. hoop stiffness. Low angles (±30) align with the longitudinal axis, enhancing axial stiffness and buckling strength but lowering hoop support, thereby raising liner hoop stress under pressure and thermal expansion, prompting liner yield. High angles (±65, ±75) increase hoop rigidity and pressure capacity but reduces axial modulus, amplifying susceptibility to axial compression. This is worsened by thermal expansion mismatch as the laminate low/negative fibre and higher transverse coefficient of thermal expansion (CTE) conflicts with the PE EK liner’s large CTE, thereby causing interface stresses. High hoop laminates constrain liner hoop expansion and increase thermal hoop stress, whereas axial dominant laminates constrain axial expansion and promote axial mismatch. Moreso, thermal gradient due to high T i , pre-stress the liner in compression, accelerating yield and reducing stability. Consequently, TCPs with hybrid laminate lay ups combining low and high angles could provide better thermal resilience by distributing axial/hoop load resistance and moderating thermal mismatch effects. 3.3. Effect of geometry change In this section two geometric variants were assessed as detailed in Table 5. TCP 1 with larger radius, thicker liners and laminate significantly increased bending stiffness, axial rigidity and thus P cr at all T i values compared to the base TCP (see Figs. 4 and 5). At 4 o C P cr was 8910kN compared to 1680kN for the base TCP. Material failure loads also rose, even though less than P cr . Therefore, material failure always preceded buckling. At low T i (4 o C - 30 o C) matrix compression in the laminate governed failure, whereas inner liner yield became critical as PEEK softened around