Issue 71

M. Abdulla et alii, Fracture and Structural Integrity, 71 (2025) 124-150; DOI: 10.3221/IGF-ESIS.71.10

In stark contrast, the introduction of thermo-mechanical loading at an elevated temperature of 110°C results in a pronounced deformation characterized by the swelling of the composite patch. This swelling is evident from the significant departure of the deformed contour from the original shape as illustrated in Fig. 26. The swelling is a clear manifestation of the oil canning effect, which arises from the differential thermal expansion between the composite patch and the underlying host plate material. Given the distinct thermal expansion coefficients of the composite material and the host structure, exposure to elevated temperatures results in uneven expansion within the patch. The patch's material properties, influenced by the high temperature, cause it to expand more than the host material, leading to localized out-of plane deformation, or swelling, commonly referred to as oil canning.

(a) (b) Figure 26: Oil canning in the composite patch under thermo-mechanical loading (a) side view (b) top view.

This oil canning effect under thermo-mechanical loading is significant because it alters the stress distribution across the composite patch. The localized bulging seen in the deformed shape is not just a surface anomaly but can potentially influence the overall mechanical performance of the repair. Such deformation might introduce areas of increased stress concentration, particularly around the swollen regions, which could affect the load-bearing capability of the patch. This change in stress distribution, if not properly accounted for during the design phase, might lead to compromised repair effectiveness, especially when the structure is subjected to cyclic loads over time. Additionally, it is important to note that the oil canning effect is not a uniform deformation across the patch but rather a localized phenomenon that may vary in intensity depending on the specific geometry, material properties, and thermal history of the composite and adhesive layers. In the context of this study, the bulging observed in the composite patch under thermo-mechanical loading highlights the potential for significant deviations from expected performance if thermal effects are not adequately accounted for. The findings suggest that the interaction between thermal expansion and mechanical loading, coupled with the presence of adhesive defects, can lead to complex deformation patterns that must be understood and mitigated to ensure the long-term success of composite patch repairs in aerospace and other critical applications. he study demonstrates that the efficiency of repaired plates under thermo-mechanical loading diminishes due to the generation of thermal stresses at elevated temperatures. Analysis of positive & negative temperature variations indicates a consistent increase in Stress Intensity Factor (SIF) with rising temperatures, driven by differential thermal expansion between aluminium and the composite patch. Consequently, the consideration of thermal stresses becomes paramount in the repair process. Contrary to purely mechanical loading, a consistent increase in the SIF with T C ONCLUSION

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