PSI - Issue 82

Celalettin Baykara et al. / Procedia Structural Integrity 82 (2026) 206–212 C. Baykara et al./ Structural Integrity Procedia 00 (2026) 000–000

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1. Introduction Adhesively bonded joints, particularly single-lap joints (SLJs), have become indispensable in structural applications demanding lightweight construction, dissimilar material joining, and efficient load transfer. These joints are commonly employed in transportation, aerospace, and renewable energy systems, where mechanical fasteners can create stress concentrations, contribute to corrosion, or increase weight (Zhang et al., 2024; Baykara, 2023; Baykara et al. 2023). Among the many factors affecting the mechanical performance of SLJs, adhesive layer thickness plays a pivotal role in defining stress distribution, energy absorption, and fatigue life. Studies such as Guo et al. (2022) demonstrated that tensile strength reaches an optimal point around 30 μm in precision joints, beyond which increased thickness leads to excessive compliance and strength degradation. Beber et al. (2017) further revealed that elastoplastic adhesive modeling is essential for fatigue life prediction, especially in the presence of multiaxial stress states near the overlap ends. Kumar and Pandey (2011) emphasized that fatigue failures in SLJs predominantly originate from crack initiation zones near the joint edges, often consuming the majority of the total fatigue life. Their computational approach based on the Coffin–Manson model confirmed that adhesive thickness and geometry influence both initiation and propagation phases. Supporting this, Heidarpour et al. (2018) showed that defects—whether 2D or 3D—in thicker bondlines substantially reduce strength, especially when located near stress concentration zones. Defect sensitivity is further highlighted by Biscaia et al. (2024), who modeled the effect of localized and distributed debonding in CFRP-to-steel joints. Their results showed that localized defects in thick adhesive layers produce greater strength loss and unstable crack propagation. These results underline the importance of thickness uniformity in bonded interfaces. The surface condition of adherents is equally important. Saleema et al. (2012) showed that NaOH treatment on aluminum surfaces enhances roughness and chemical compatibility, producing cohesive failure modes and improved bond durability. This is particularly relevant in hybrid joints such as DC01 steel to unprocessed aluminum, where dissimilar oxide films and surface energies can hinder adhesion. Geometrical features such as fillets, tapering, and notches also interact with adhesive thickness. Metehri et al. (2024) used 3D finite element analysis to demonstrate that modifying adhesive fillets and adherent geometries can reduce peel stresses and improve joint strength when appropriately balanced with adhesive thickness and overlap length. Damage monitoring methods like Digital Image Correlation (DIC) provide further insight. Abbasi et al. (2024) introduced the Backface Strain (BFS) technique to detect the zero-strain point (ZSP) during fatigue. They observed that adhesive thickness affects strain localization and damage visibility, with polyurethane and epoxy adhesives showing distinct ZSP shifts depending on bond-line geometry. Surface activation via laser is another promising strategy. Liu et al. (2025) demonstrated that CFRTP joints subjected to infrared laser treatment showed enhanced surface energy, better resin interaction, and higher bond strength, particularly for thicker adhesives. Similar strategies may be applicable to hybrid metallic joints with variable bond-line dimensions. Zhang et al. (2024) confirmed the utility of combined acoustic emission (AE) and DIC methods in detecting damage progression modes such as adhesive de-bonding and delamination in CFRP/steel joints. Their study underscores the need for real-time monitoring in fatigue-sensitive structures where adhesive thickness varies across the interface. Venugopal and Sudhagar (2023) showed that tailoring the joint geometry, even with a constant adhesive area, substantially impacts joint strength. Their results highlighted how lap length, width, and bond-line thickness combine to define stress flow and failure location. Other researchers, such as Azari et al. (2021), have studied surface roughness effects and found that both static and cyclic strength improve with moderate roughness, especially when the adhesive thickness is controlled. This again emphasizes the synergy between surface topography and adhesive thickness in determining joint durability. Despite extensive research on adhesive joints, the specific influence of varying adhesive thickness on the fatigue performance of DC01 steel and aluminum joints remains underexplored. Hence, in this study, we experimentally investigate the fatigue performance of DC01–aluminum single-lap bonded joints using three adhesive thicknesses (1 mm, 2 mm, and 3 mm). Through a combination of fatigue testing and failure mode analysis, we aim to elucidate how adhesive thickness governs fatigue durability, crack initiation patterns, and overall joint behavior. The findings provide critical guidance for optimized design of bonded joints in hybrid structures subject to cyclic mechanical loading.