PSI - Issue 28

Vinicius Carrillo Beber et al. / Procedia Structural Integrity 28 (2020) 1950–1962 V.C. Beber and M. Brede / Structural Integrity Procedia 00 (2019) 000–000

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4. Results and Discussion

4.1. Static Testing

The force-strain plots from static tests are presented in Figure 6 for the butt joints (black), scarf joints (red) and the TAST joints (blue): (a) elastic adhesive, (b) structural adhesives . For structural adhesive joints, all joints failed close to 1% of nominal strain. The bonding surface of the TAST joint is smaller; therefore, lower values of force were obtained. Nonetheless, all joints showed a similar stiffness. On the other hand, in static tests on the elastic adhesive, failure of joints took place with much higher values of nominal strain (>200%), and the stiffness of TAST joints was smaller than that of the butt and scarf joints.

(a)

(b)

Figure 6 - Force-strain plots for the butt, scarf and TAST joints at quasi-static monotonic load of the: (a) elastic adhesive, (b) structural adhesive

By taking the maximum value of the nominal force in the force-strain plots and dividing by the original bonding surface it is possible to obtain a reference for the static strength for each joint. The average nominal maximum stress values and their respective deviations are presented for both adhesives in Figure 7a. As expected the maximum values of stress for the structural adhesive joints is higher than the ones for the elastic adhesives. In order to understand the effect of stress multiaxiality on the static strength of both adhesives, a Finite Element Analysis (FEA) was carried out using the commercial software Abaqus 6-13 © from Dassault Systèmes. A 3D model of the joints (geometry from Figure 3) was used along with a consideration of linear-elastic material behaviour (data from Table 1). Stresses were taken from the middle of the adhesive layer thickness. The results of FEA simulations are presented in Figure 8 in the form of stress multiaxiality distributions for butt, scarf, and TAST joints for both adhesives types. These distribution were obtained considering a nominal stress of 1 MPa. These values of stress multiaxiality could support the understanding of the static results regarding the strength of the elastic and structural adhesives. By comparing the ratio of the maximum stress between the structural adhesive and the elastic adhesive as shown in Figure 7b the butt joint presents the highest values, whereas the TAST joint the lowest. This could be explained by the difference in the stress multiaxiality of the types of joints. The multiaxiality is much higher in the butt and scarf joints of the elastic adhesive.