Impact of Intact Length and Initial Debond on the Mode I Fracture Toughness of Sandwich Structures

ABSTRACT The single cantilever beam (SCB) configuration, mounted on a roller base or equipped with a flexible long loading rod, has been widely adopted as a dominant Mode I skin/core fracture test for sandwich structures. However, the influence of SCB geometry in determining the accuracy of Mode I characterization remains underexplored. This paper outlines a systematic experimental parametric analysis on the influence of final intact length ( L _b ) and initial debond ( a ₀ ) length on Mode I fracture toughness using a SCB with a roller base setup. The study employs a mono‐polymer thermoplastic sandwich structure, featuring commingled E‐glass/polypropylene (PP) skin and a closed‐cell PP foam core, manufactured using one‐stage non‐isothermal compression molding without any aid of adhesives. The results demonstrated that the SCB designed according to the specified sizing limitations, particularly a ₀ and L _b , effectively reduces the global shear components, promoting dominant Mode I conditions. Among various sizing limitations, the configuration following the dimensional criteria proposed by Ratcliffe et al. ( a ₀ and L _b as 100 mm) achieved dominant Mode I fracture toughness (≈3.76 kJ/m ² ) with minimal shear at crack tips throughout the crack length. Despite lowering the shear, sizing limitations do not guarantee that crack propagation will occur through the skin/core interface. The specimens with L _b or/and a ₀ (i.e., L _b as 40 mm, or both L _b and a ₀ as 40 mm) that fall outside the SCB limitations show deviation from the dominant Mode I behavior. In contrast, changes in a ₀ at a constant L _b that align with the sizing limitations have no significant impact on the average fracture toughness. Nonetheless, fracture toughness reduction at shorter crack lengths was evident due to the mixed‐Mode conditions associated with smaller a ₀ , even when L _b conforms to the sizing limitations. These findings establish specimen sizing as a critical, yet previously underexplored, factor in achieving reliable Mode I characterization of sandwich structures.