Contents lists available at ScienceDirect Theoretical and Applied Fracture Mechanics journal homepage: www.elsevier.com/locate/tafmec The fracture load analysis of different support patches in adhesively bonded single-lap joints Kübra Demir, Simay Bayramoglu, Salih Akpinar ⁎ Dept. of Mechanical Eng., Technical University of Erzurum, 25050 Erzurum, Turkey ARTICLE INFO Keywords: Adhesively bonded joint Fracture Peel stress Residual stress Mechanical testing Numerical analysis ABSTRACT Adhesively bonded joints are frequently used in the aviation and automotive industries. Bending appears in the bonded materials as a result of eccentric loading when tensile loading is applied to an adhesively bonded single- lap joint (SLJ) geometry. As a result of this bending, moment effect occurs in the overlap region in the joints joined with the adhesive. This moment effect causes peel stresses at the edges of the overlap region of the joint such that these stresses are the main reasons of the damage in the joint. Obviously, reductions in these stresses should result in higher joint strength and increased load capacity. In this study, support patches were added at a varying distances from the overlap region to decrease the bending effect occurring in adhesively bonded single- lap joints. In the study, adhesive bonded single-lap joints were produced by using DP460 liquid structural epoxy as the adhesive, AA2024-T3 aluminum alloy as the adherend and flat or curved AA2024-T3 aluminum alloy and AISI 304 steel with different thickness as the support patches. As a result, when the failure load obtained from the experiments was examined, the use of support patches in single-lap joints increased the damage load of the joint between approximately 16% and 94%. However, such increases in the damage load are highly dependent on the type, thickness, geometry (flat or curved) and positioning of the support patches. 1. Introduction Adhesively bonded joints are preferred due to their advantages such as formation of uniform stress distribution upon loading, ability to join different materials, high fatigue resistance and impermeability. There are different connected joints such as single-turn joint, double-turn joint, forehead joint, T joint, taper and weft joint etc. Due to its simple geometry, the joint type that is frequently used in the industry is the single-lap joint type (SLJ). However, when this joint type is subjected to tensile loading, a bending moment occurs in its overlap region due to eccentric loading. This bending moment causes the formation of peel stresses (those resulting in damage) at both edges of the overlap region of the joint. In adhesively bonded joints, there are many approaches to reduce these large stress concentration that affect the bond strength. One of the techniques used to increase the joint strength is the use of bi-adhesive in the overlap area of the joint [1–5]. In a study performed by da Silva and Lopes [5], the performance of the SLJ types obtained by using only flexible, only rigid and bi-adhesive (flexible and rigid) was examined under the tensile load. According to the damage load per- formances of the joints obtained using only brittle or ductile adhesive, the damage load performance of the joint obtained using bi-adhesive has increased. The spew fillet method is used in the overlap region to increase the strength of the bonded joints [6–13]. Crocombe and Adams [6] ex- amined the effect of additional parameters as well as material and geometric properties. They concluded that the triangular spew filling at the ends of the overlap regions of the bonding joints reduces the stress value formed in the joint. The effect of the change in the geometry of the adhesion corners on the stress distribution in SLJs and thus the effect on the joint force has been studied numerically and experimen- tally by Zhao. [11,12]. Meanwhile, it has been observed that the use of a step in the overlap length region of the joints bonded with the adhesive also increases the failure load of the joints. [14–20]. In the study by da Silva [16], the relationship between increasing the overlap length in the step-lap joint and the change in the damage load of the joint was examined. Three different adhesives, flexible, medium hard and hard, were used in the study and the study was carried out experimentally and numerically. A slow break in flexible adhesives and a sudden break in hard adhesives have been observed. Additionally, it has been found that flexible ad- hesive is better for connections with short tapping lengths, while hard adhesive is better for connections with increased tapping length. In a study by Akpınar [19], damage loads under the tensile load of the single lap joint, the single-step lap joint and the three-step lap joint were https://doi.org/10.1016/j.tafmec.2020.102653 Received 13 February 2020; Received in revised form 14 May 2020; Accepted 15 May 2020 ⁎ Corresponding author. E-mail address: salih.akpinar@erzurum.edu.tr (S. Akpinar). Theoretical and Applied Fracture Mechanics 108 (2020) 102653 Available online 19 May 2020 0167-8442/ © 2020 Elsevier Ltd. All rights reserved. T