Issue 52

H. Ghahramanzadeh Asl et alii, Frattura ed Integrità Strutturale, 52 (2020) 9-24; DOI: 10.3221/IGF-ESIS.52.02 10 I NTRODUCTION erospace, naval and automotive industries have been using composite materials along with steel and aluminum for structure lately. Due to the difficulties of joining of two or more different materials, adhesive joints become more suitable in the application. While joining technique should enable the ability to bond different materials, it also should be strong enough regarding strength. Although the performance of the joint under static and dynamic conditions is essential, crashworthiness of joint is also considered as crucial in case of accidents. As for crashworthiness, it highly depends on the strain rate of impact [1,2]. Single lap joint (SLJ) is widely used for adhesive joints due to its easy application, simple geometry and explicit results in order to understand joint strain rate dependency [3]. There are some inconsistent factors that affect adhesive SLJs strength. For instance, a structural adhesive DP460, manufactured by 3M, technic sheet suggests aluminum adherend’s overlap shear strength as around 30 MPa when epoxy fully cured. Some researchers report that it could be 13.6 MPa curing conditions at 140 °C for 60 min [4], 23.6 MPa at 60 °C for 120 min [5] and 13.9 MPa 23 °C for 2 days [6]. Along with this, adhesive strength enhancement studies have been proposed by others [7,8] and these pure adhesive strengths were used as a reference of base adhesive strength. These studies focused on strain rate, adhesive thickness or surface quality of adhesive joints but few of them considered all of these together in one work. However, these parameters are interconnected between and impose bond strength crucially [9,10]. Although some of the studies report different adhesive strengths with high fluctuation [5,11–13] experiments need to be performed by considering all factors into account to clarify results. From this perspective of view, separate works have been conducted by researchers by taking thickness, overlap length, adherend type, adherend thickness, strain rate, and surface quality into consideration. Bamberg et al. [6] conducted SLJ tests by changing adhesive thickness, overlap length and adherents using DP460 (3M) adhesive. They concluded that increasing overlap length from 7 to 25 improved failure load of joints. Aydin et al.[11] compared different adherend thicknesses and overlap lengths by using the same adhesive at 0.28 MPa curing pressure and 0.12 mm adhesive thickness. They reported that increasing adherend thickness also distributed stress concentrations from edges to the middle of the adhesive layer. They have not supplied the effects of different curing pressure and adhesive thickness while other factors change. By using the same adhesive, similar thickness and aluminum adherents; failure load of SLJ was reported by Bamberg et al. [6] as 8.7 kN and Gültekin et al.[8] as 14.7 kN. This difference of 68% needs to be explained by further studies. Adams and Peppiatt[14] investigated the effects of adhesive thickness on failure load by adopting five approaches then predicted an intersection point at 0.13 mm as an ideal adhesive thickness. Niranjan[15] reported three independent factors that were affected by adhesive thickness; adhesive defects, stresses, and strain rate. Adams and Peppiatt landed up the presence of voids and microcracks in the adhesive layer that affects adhesive strength which was the first consideration of Niranjan. Further study by Grant et al. [16] concluded that while adhesive thickness increased, the bending moment also amplified. This caused a decrease of joint strength which is related to Niranjan's second consideration. Although this adhesive thickness is widely accepted as optimum adhesive thickness for SLJ, it needs to be investigated more due to the reliance of stress on the single lap joint, its dependence of strain rate and adherend material. Along with previous studies, a study that considers both the effects of overlap length and strain rate under impact and quasi-static conditions conducted by Araújo et al. [17]. They concluded that 25mm overlap length provides better damping than 12.5 and 50mm overlap lengths. Blackman et al.’s study [18] showed crack propagation with regards to the test rate for adhesively bonded joints. According to their results, while the test rate increased, crack formation velocity accelerated, thus adhesive fracture energy lowered. Lißner et al. [19] investigated the rate dependency of adhesive joints. They used 3 different surface treatments, adhesive thicknesses and loading rates for this purpose. They concluded that for higher loading rates; while stress in adhesive increased, the energy that disappears during fracture decreased independently of adhesive thickness. When adhesive thickness set from 0.3 to 1.0, stress in joints tended to reduce. Trimino et al. [12] employed epoxy adhesives (include 3M DP460NS) to determine strain rate dependency. They applied quasi-static and impact tests on the adhesive samples. They found that stress at failure augmented by increasing the strain rate. Avendaño et al. [20] conducted tests on SLJ’s under two crosshead speeds. They observed that while speed rose from 1mm/min to 100mm/min, failure loads amplified by 14%. Boutar et al. [21] investigated the effect of surface roughness on adhesive joint failure. They used sandpaper from 50- to 1000-grid afterward measured surface roughnesses of adherends. They reported that surface roughness decreased by using fine-grained sandpaper consequently bond strength of joint improved. Among approaches to model adhesively bonded joints, the Cohesive Zone Model (CZM) is widely used because of its relatively easy application and simulation capability of joints [22]. Campilho et al. [23] inspected triangular, trapezoidal and exponential CZMs and concluded that although triangular CZM has been used most, all of these models predict mechanical A