Evaluation of high-performance pressure-sensitive adhesives and VHB™ acrylic foam tapes bonded aluminum joints subjected to environmental aging

A range of 3M™ high performance pressure-sensitive adhesives (PSAs) and 3M™ VHBTM™ acrylic foam tapes were used to bond 25.4 mm × 3.175 mm aluminum 2024 T-4 adherends in both single-lap joint (SLJ) and three-point end-notch flexure (ENF) configurations. The samples were subjected to two types of aggressive environments to simulate extreme service conditions: freeze–thaw and heat–cool cycling, both for 21 days. Impulse-frequency response vibration and electrochemical impedance spectroscopy (EIS) were used for monitoring bond quality non-destructively. Data were first obtained on a set of specimens at room conditions (i.e., before being subjected to freeze–thaw, or heat–cool cycling), referred to as "baseline" in this paper. After obtaining baseline data, several batch sets were subjected to quasi-static lap-shear and dynamic impact loadings to compare the mechanical and electrochemical properties before and after environmental cycling. EIS results show that moisture absorption caused a reduction in low-frequency impedance, whereas decrease in adhesive thickness caused a reduction of impedance over the entire frequency range. The impulse-frequency response vibration NDE technique was able to detect the changes in loss factor (damping) of adhesive joints after environmental aging. Quasi-static lap-shear loading of SLJs showed that acrylic foams took less failure load compared to PSA tapes, and SLJs subjected to dynamic impact showed all PSAs and acrylic foams taking about the same impact load to failure, except the softer acrylic foam.