Effect of imperfect cure on tensile bond measurement

•Pull-off tensile tests are the most common field test for FRP bonded materials.•Tests show pull-off tests cannot achieve bulk tensile strength values.•Static mixing nozzles are not a reliable method to prepare pull-off test fixtures.•For high strength bond applications, a seven-day dolly cure time is recommended. This paper quantifies variability in tensile bond measurement due to incomplete mixing within the nozzles of manual gun epoxy dispensers. Controlled pull-off tests were conducted using an MTS machine and steel substrates were tested to minimize variations in bond line thickness. The pull-off tensile force was evaluated after 1, 2, 3, 7, 14 and 28 days. In each of these six series, five identically prepared specimens were tested. The average coefficient of variation was below 0.10; where it was higher, investigations showed that the epoxy had not mixed thoroughly inside the nozzle. The average 7-day cure tensile bond from identically tested hand mixed epoxy specimens was over 19% higher with lower variability. Hand mixing is recommended in preparation and bonding of pull-off dollies.