Test procedures, bonded joints peel tests

Aluminum panels, which had a thickness of 0.61 and 1.63 mm, were etched with chromic acid. ASTM procedure D3167—76 (reapproved 1981) was followed for 135° peel tests. The adhesive film was placed between the aluminum panels and press-laminated at a temperature of 177 °C for 1 h at a pressure of approximately 5 psi (34.5 x 103 Pa). The temperature was then increased to 220 °C, and the joints were kept under pressure for another hour. The heaters in the hydraulic press were then switched off and the platens air-cooled and then water-cooled until the platen temperature was down to 100 °C. The bonded panels were cut into 12.7-mm-wide joints and tested at a rate of 20 mm/min and at a peel angle of 135°. 

No account has been taken so far of the thickness of the adhesive within the joint line, but conventional wisdom, based on standard test procedures, dictates a bond line that is as thin as possible. While, to a degree, this is equally true for both the older types of conventional high-strength - though brittle - adhesives and the new toughened , non-brittle adhesives, the peel strength of the latter increases with bond thickness, as shown in Figure 2.10. And, it should be borne in mind that real joints are not usually stressed like standard test joints. 

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