Structural tests on a 54 in. diameter epoxy-phenolic adhesive bonded titanium honeycomb sandwich tank resulted in failure substantially below design expectations. Company sponsored efforts were undertaken to determine the cause of the discrepancy and to arrive at realistic design parameters for future bonded construction. Of specific interest were the effect of geometric variables, including scaling effects, and elastic properties of adherends and adhesives. The investigations utilized titanium, stainless steel, aluminum, and beryllium adherends and HT-424 and FM-1000 adhesives, the former comparatively brittle, the latter quite tough.
Analysis of the results, in addition to confirming the generally known relations between bond strength and elastic properties of joint constituents, indicated surprisingly large scaling effects (25-30%) not hitherto taken into account in design practices, and revealed that the anomalously high strength-to-modulus ratio of titanium alloys is probably responsible for the often encountered and reported difficulty to obtain expected bond strength levels in titanium structures. The attained insight has led to the postulation of a “critical strain parameter” concept for brittle adhesives which appears to be worthy of further exploration.