Mixed-Mode Fatigue Failure in Structural Adhesives

Abstract

Fatigue data obtained under biaxial loading conditions for adhesively bonded joints are used to plot S-N type diagrams to assess the effects of biaxiality in loading. Independently Loaded Mixed-Mode Specimens (ILMMS) are used for data collection purposes. These specimens are basically two (steel) beams bonded together to be fatigue loaded under cantilever (opening) mode while a simultaneous but physically separate inplane (static) shear load is also induced with the aid of a small hydraulic piston embedded in the specimen. Application of such static shear loads on the bonded joint results in S-N behavior different than that obtained with opening load only. The model adhesives used are Metlbond 1113-2 and Metlbond 1113 solid film thermoset adhesives similar to those commonly used in aircraft and aerospace industries. The former is an elastomer-modified epoxy adhesive and the latter is identical except that it contains a synthetic carrier cloth. Thus, the effect of carrier cloth in adhesive's S-N behavior is also assessed. Analytically, the classical linear log-log representation of the adhesive S-N data in log PImax versus log Nc or log PII versus log Nc formats is explored. The modifications necessary to reflect the effects of biaxiality in loading and also the presence of a carrier cloth are assessed. The fatigue failure results are also compared with the results obtained under monotonic biaxial loading conditions.