Impact strength of thermally aged double lap adhesively bonded joints

In this paper deep investigations and full characterization of SA 70 epoxy's properties as a bulk material and as an adhesive in a bonded assembly, are performed. Those properties are essential for an industrial or a supplier and whose determination is not at all obvious. Double lap bonded steel-SA 70 epoxy assemblies were prepared and then thermally aged within a wide interval of temperatures up to 370 degrees C. The thickness of the central adherend was kept always equal to twice the thickness of each of the upper and lower adherend. Ethanol and sand blasting were the two surface treatments adopted for the substrates while three different central adherend thicknesses from each surface treatment were prepared. Preliminary investigations were performed through thermogravimetric analysis (TGA) tests applied on cured SA70 epoxy specimens and this for the whole ageing temperatures range; showing that the onset temperature for this adhesive is around 380 degrees C and a major mass loss occurs at 420 degrees C. Isothermal TGAs were performed at 300 degrees C, 350 degrees C and 370 degrees C in order to choose ageing duration. The aged specimens were then tested under impact at 10 m/s using the Split Hopkinson Pressure Bar (SHPB) technique. It was found that the sand blasted specimens exhibit higher shear strength. No clear effect of substrates' thicknesses on the joint shear strength was noticed. In addition, the evolution of the shear strength with the ageing temperature was found to be in line with the TGA results: the strength increases with the ageing temperature up to 200 degrees C; after 225 degrees C the strength decreases. From 300 degrees C and above the strength drops drastically.