Deformation Behavior of Polyurethane Adhesive in the Single-Lap Joint Based on the Microbeam X-ray Scattering Method

Single-lap joint (SLJ) specimens were prepared using two stainless steel substrates as adherends and polyurethanes (PU) as an adhesive and then employed for adhesion testing. To investigate the internal structure of the PU adhesive during the shear deformation process, the in situ synchrotron radiation microbeam small-angle and wide-angle X-ray scattering (SAXS and WAXS) measurements were carried out. A lab-made portable tensile tester was developed for the SAXS/WAXS measurement during lap shear deformation. The structure of adhesive bulk was also investigated during the simple uniaxial deformation for the comparison. An isotropic scattering ring─which was formed from the periodic structure of hard segment domains of the PU adhesive─was clearly observed in the SAXS pattern at the initial state. This isotropic ring changed to an ellipsoid shape whose minor and major axes were tilted from the stretching direction. The spacing of the hard segment domains of PU adhesives increased, and the tilt angle of the stress direction was merged with the stretching direction with an increase in the applied deformation. The orientation direction of the hard segment domains and molecular chain of the soft segment changed with a similar trend. Furthermore, the degree of change in these values was larger at the edge of the adhesive of the SLJ specimens, indicating the existence of spatial distribution of direction and the value of stress in the adhesive during lap shear deformation. These findings are expected to be quite useful for practical design of adhesives.