Bi-adhesive (or mixed-adhesive) joint is an alternative stress-reduction technique for adhesively bonded lap joints. In this work, an experimental study was performed to assess the effect of bi-adhesive bondline on the failure load of mono-adhesive and bi-adhesive single-lap joints. The first step in the experimental study related to mono-adhesive joint, the overlap length was checked using the Global yielding criteria. For the selected overlap length, failure loads of AV138, Araldite 2015 and DP-8005 mono-adhesive joints were determined. It was seen that the highest failure load occurred in the Araldite 2015 mono-adhesive joint. Secondly, the bi-adhesive joints were considered as two different adhesive combinations (AV138+2015, AV138+DP-8005). An interesting result was obtained that the considered bi-adhesive joints with two different adhesive combinations (AV138+2015, AV138+DP-8005) give joint strengths higher than the joint strengths of the adhesives used individually, even if ductile adhesive has a joint strength higher than that of the stiff adhesive. In the last step of this study, experimental investigation was then focused on the determination of optimum bond-length ratio for the bi-adhesive joint. Optimum bond-length ratio for bi-adhesive joint was then investigated using three ratios (0.3, 0.5, 0.7). It is concluded that the adhesive type and the bond-length ratio play an important role in the bi-adhesive bondline. Comparisons of the experimentally determined failure loads and shear strengths with available closed-form solutions were also carried out. Experimental failure loads were in good agreement with those obtained from the Volkersen method and Global yielding criteria.
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