Materials
1. Adhesive
The adhesive selected for this study wasA two-component structural polyurethane adhesive SikaForce s 7888 L10, supplied by Sika (Portugal). It cures at room temperature (RT) and is used in the automotive industry. This adhesive is part of a new family of polyurethane adhesives which combines high strength with large ductility. Interest in this class of adhesives is on the rise because of various benefits that they can offer such as: high peel strength, impact resistance, and improved fatigue behaviour [16,17]. Another important advantage is the more stable crack propagation, which can prevent sudden catastrophic joint failure. This permits that damaged adhesive joints can be identified and repaired before total failure, which is a critical point in terms of safety in the automotive industry.
2. TEPs
Expancel 031 DU 40 particles supplied by Expancel Nobel Industries (Sweden) were selected. The diameter of these particles ranges mainly from 10 mm to 16 mm. The dimensions of the particles before and after expansion were verified through a SEM analysis (see Fig. 2). Data provided by the manufacturer is given in Table 1.
3. Substrates
Hard tool steel DIN 40CrMnMo7 substrates were used for the DCB specimens, in order to assure an elastic behaviour of the adherends. The mechanical properties of the tool steel DIN 40CrMnMo7 are given in Table 2 (data provided by supplier).
Specimens manufacture
The TEPs-modified adhesive was mixed with a Speed Mixer (DAC 150.1 FVZ Speedmixer, Hauschild, Germany) for 60 s at 2500 rev/min. This allows an efficient and homogeneous dispersion, creating visibly bubble-free mixing. The TEPs-modified adhesive cured matrix structure was checked through a SEM analysis (see Section 3.1.2) and a uniform dispersion of particles within the adhesive matrix was observed. Bulk “dogbone” and DCB specimens with different TEPs concentration by percentage weight (0 wt%, 5 wt%, 10 wt% and 15 wt% TEPs) were produced, while for the thermal analysis, specimens with six TEPs concentration by percentage weight (0 wt%, 5 wt%, 10 wt%, 15 wt%, 25 wt% and 50 wt%) were manufactured.
1. Bulk specimens
Thin sheets of SikaForce s 7888 L10 adhesive were produced by curing the adhesive between steel plates of a mould with a silicone rubber frame, which were pressed (2 MPa) for 24 h at RT (Fig. 3a) [18]. The silicone rubber frame stops the adhesive from flowing out and the pressure applied creates a good surface finish. The dimensions of the adhesive plate after cure were 150 mm 45 mm, with a thickness of 2 mm, which corresponds to the internal dimensions of the silicone rubber frame (Fig. 3b). Dogbone specimens 2 mm thick and 10 mm wide were machined from the bulk sheets plates. The geometry of the dogbone tensile specimens used (BS 2782 standard) is shown in Fig. 4. SikaForce s 7888 L10 dogbone tensile specimens before and after tests can be seen in Fig. 5.
2. DCB specimens
The DCB joint surfaces were grit blasted and degreased with acetone prior to the application of the adhesive. The specimen geometry and the loading are shown in Fig. 6. In a previous study by the authors [19], the effect of adhesive thickness on the mechanical behaviour of this polyurethane adhesive system was investigated and it was concluded that the recommended adhesive layer thickness to be used in practical application in automotive industry is 1 mm. Therefore, the bondline thickness used in this study was 1 mm. Spacers (calibrated steel bars of 1 mm) were inserted between the adherends before the application of the adhesive in order to control the bondline thickness. These spacers were removed after the adhesive was cured. A sharp pre-crack in the adhesive layer mid-thickness was assured using a razor blade. A mould with spacers for the correct alignment of the adherends was used [20]. The DCB joints were cured at RT.
3. Thermal analysis specimens
Prismatic samples of 4 mm 4 mm 40 mm were machined from the bulk sheets plates manufactured similar to procedure explained in Section 2.2.1. Thermal analysis specimens after tests can be seen in Fig. 7.