1. Introduction
A bonded joint is a permanent connection formed by connecting two or more parts together with an adhesive layer [
1,
2]. Metallic adhesives are used to bond two or more metal surfaces strongly and flexibly enough to resist separation, without the need for traditional methods such as nails or screws. These adhesives are natural or synthetic substances [
3,
4,
5,
6]. Adhesion has made progress with the development of material technology [
7]. The use of adhesive in carrier structures, time and cost savings, high corrosion and fatigue resistance, cracking delay, excellent damping properties and other properties has made it an important player in the aerospace and automotive, wood, and plastic industries [
8,
9,
10,
11]. The usage needs of different materials together has also improved joining techniques and versatility [
12]. However, in many applications that require structural durability, combining with adhesive is preferred. Such joints are especially common in the aerospace and automotive industries [
13,
14,
15].
Adhesive joints have a great impact on making designed systems lighter, stronger, and more economical. Adhesivebonding has more advantages than mechanical bonding [
16,
17]. These include the ability to combine different thicknesses and types of materials, create joints with equal load distribution, and include the ability to function as a seal [
18]. The holes, notches, etc., can also be applied to the surface. It is a cost-effective, simple and fast solution because no joint details or precision machining tolerances are required [
19,
20,
21]. Despite all these factors, adhesive bonds also have disadvantages. Good adhesion requires proper surface preparation, proper adhesive selection and a suitable environment [
8,
22]. The strength of such joints is affected by factors such as the surface treatment, material type, adhesive thickness, ambient temperature, and type of load. However, exposure to environmental conditions causes aging over time due to changes in its chemical structure [
23]. Due to the properties above, the bond forces and the factors affecting them have been discussed by many researchers. Adhesive bonding is inexpensive, does not cause crystal structure changes on melting, such as welding, riveting and other types of bonding, does not generate voltage concentrations, and bonds under melting temperatures, resulting in smoother bonding [
24,
25,
26].
The types of adhesives developed with the use of adhesives have been increasing. In addition, adhesive research continues and people want to benefit from it in every field.
Due to the lightness of magnesium alloys, the specific tensile strength is higher than that of aluminum and steel. Due to the low density of magnesium, it appears to have higher specific strength than aluminum, plastic composite materials, and steels [
27,
28,
29]. Therefore, magnesium alloys are widely used in the automotive, electronics, defense, and aerospace industries today [
30]. In recent years, the use of magnesium alloys in the automotive and aircraft industries has been predicted to increase significantly. Mg-Al-Zn (AZ91) alloy is mostly used for casting automotive and aircraft parts due to its light weight [
27,
31,
32,
33].
AZ91, a magnesium alloy is better at casting properties and better at yielding strength properties than other magnesium alloys. From engines to chassis, the magnesium alloys are used in every part of cars [
28,
34,
35,
36].
In Kelly’s 2005 study, he examined using a three-dimensional finite element model that included the effects of load distribution, bolt hole contact, and nonlinear material behavior of the hybrid composite single lap joints. He examined the derail and finite element analysis comparatively and it showed that the hybrid joint showed structurally more advanced performance than adhesive bonding and he produced some harmonious results [
37]. Colombi and Poggi used the finite element method in 2006 to eliminate the damage to the adhesive-reinforced gear joint and to ensure this. They investigated in which area the acting force acts on. They showed that one of the two adhesives compared has a fragile structure and the other has a more stable structure. Given these circumstances, it has been concluded that bonding errors can be minimized [
38]. In 2011, Reis, et al. compared the shear strength of single lap joint rods from different materials. They combined three different materials. These materials are composite with carbon/epoxy layer, high modulus steel, and 6082-T6 aluminum alloyed materials. The material hardness affects the shear strength, and they stated that the use of the hardest material would provide the highest shear strength. And they showed that the length of the lapping would affect the shear strength which varies according to the material, while the numerical analysis showed that the rotation of the sample decreased with increasing material hardness and resulted in a smoother stress distribution for the six adhesives. They stated that this contributed to the development of shear strength; it was also confirmed by experimental results. They produced high shear strength for steel/steel bonding, but stated that the composite/composite shear strength was lower. They concluded that increasing the hardness of the material would provide a stronger connection [
39]. In a 2012 study, Sayman also conducted an analytical elasto-plastic stress analysis used to calculate the shear stress of the single-lap ductile adhesive joint. In this study, the adhesive Dp 460 was used. Sayman used the von Mises criterion to check the flow state of the adhesive. During the analysis, he assumed that the shear stress was constant along the adhesive thickness and ignored the bending moment. The analysis results were confirmed by using finite element analysis. Analytical and numerical results were coherent with each other [
40]. In their 2013 study, Liao, et al. experimentally and numerically examined geometric parameters such as overlap length and material thickness for low strength steels (medium and low carbon steels), and material parameters such as adhesive stress–strain. They observed injury patterns in different situations and determined the first place of injury. Unlike the joints made of high-strength steel, they stated that the damage mechanism depended on the leakage of the bonded material. They concluded that the localized tensions at the edges of the winding caused the joining errors in low and medium carbon steels [
41]. In 2014, Rahman and Sun determined an accurate damage prediction criterion usable to predict the stress of tear damage to fibers by using a field-based approach. They experimentally determined the type of damage by using carbon fiber composites, epoxy adhesive and splices of various sizes. They tested several voltage/strain-based failure criteria for composites. They calculated the size of the critical fields with the help of finite element analysis by using the known collapse loads of single lap joints. As a result, they found that the Azzi–Tsai criterion (Norris) was appropriate for estimating the stress of fiber tear damage in single rots [
42]. In a paper in 2018, Selahi made an error analysis on an ANSYS Workbench by applying shear and bending loads to double lap joints with hybrid bonded and single bolted laminated composite adhesions exposed to axial effects by using three-dimensional finite element simulation. Simulation analysis was compared with literature studies. It was concluded that there was a significant increase in bonded joints at the rate of 56% when compared to bonded joints [
43]. In their study in 2014, Silva and Nunes tested nine aluminum-epoxy single lap joint samples with different adhesive thicknesses and outer bonded segment ratios under tensile loading. They estimated the deviations of single lap joint samples by using the digital image correlation method. They also examined the Hart-Smith, Goland and Reissner models and modified the Goland and Reissner models to compare the theoretical prediction by using experimental data. Improvement in the Goland and Reissner model proposed by Tsai-Morton was examined. The improvement in the Goland and Reissner model proposed by Tsai-Morton concluded that the single lap joints studied would be more appropriate to explain their mechanical behavior. As a result, they determined that the proposed alternative methodology would be a good way to predict the moment of bending [
44]. In their study, Li et al. (2020) conducted an experimental examination of the tensile property of single-lap joints in carbon-fiber-reinforced bismaleimide (BMI) resin composite laminates. The effect on the tensile feature of single lap joints of the stacking order and width/diameter ratio of composite laminates with three joint configurations, which are bolted, bonded with adhesive and hybrid bolted/bonded single lap joints, was discussed. They concluded that the stacking order and W/D ratio are important in the design of the joints, and the bolts in hybrid joints can increase the tensile property after adhesive failure [
45]. In their study, Ye et al. (2018) conducted a comprehensive study of the composite single-lap joints with adhesive-resistant strength. They created 3D open models of single-lap joints with different overlap lengths which were subjected to uniaxial tensile load. They compared the efficiency and accuracy of the models, load-displacement curves, and damage morphology with numerical and experimental results. They concluded that the load displacement curves were consistent with the experimental results and that the fault morphology could be predicted according to the model types [
46]. In their study, Belardi et al. (2021) examined the capabilities of the single lap multi-bolted composite joints and composite bolted joint elements (CBJEs). In particular, it was shown that a simplified FE model, which includes shell elements to simulate plates and CBJEs for the bolted zone, can reliably determine the bolt load distribution and secondary bending of the joint. They compared experimental data of single lap three-bolted composite joints with FE theoretical data. They concluded that the obtained data could be used profitably for the design of the joint [
47]. In their study, Chen et al. (2019) investigated both the mechanical behavior and damage mechanism of riveted, joined and hybrid carbon fiber-reinforced plastic (CFRP)-aluminum joints with various overlap lengths. They analyzed the load-displacement curves, peak load, energy absorption, and joint stiffness. They used high-resolution micrographs of the broken surface to understand various failure modes in the CFRP-aluminum composite by taking typical failure development photos with a high-speed camera for progressive failure analysis. According to the experimental results, they showed that hybrid joints offer superior performance in terms of peak load and energy absorption, increasing up to ~380%, 434% and ~19%, 56%, respectively, only when compared to riveted and joined joints [
48].
In the light of the literature studies, in this study bonded, bolted and bolted-bonded single lap joint elements were mechanically analyzed under the static tensile and bending loads. Acrytron 1E1, a high-performance, acrylic-based metal adhesive, was preferred as an adhesive. In recent years, AZ91 magnesium alloy and carbon fiber plate materials used in the aerospace and automotive industries due to their mechanical properties such as high specific strength and light weight have been deployed as bonding surfaces [
49]. This study is unique because the bonded, bolted and bolted bonded single lap joint made with Acryton 1E1 adhesive, which is especially an AZ91 plate material and an Acrylic adhesive type used in the study, is not found in any other study.