Monitoring residual strains in oven- And induction-bonded joints

Differential thermal expansion of bonded joint constituents results in residual stresses within the adhesive and at the bi-material interfaces, which can significantly reduce the strength of the resulting joints. In this work, experimental strains were recorded during both the heating and cooling cycles of electromagnetic (EM) induction bonded and oven bonded joints. The fiber optic sensors that use distributed sensing technology and provide strain measurements at every 1.2 mm along the length of the sensor were placed with in the adhesive during the bonding process. A parabolic strain distribution was observed between the edges and midpoint of the adhesive bondline at the edges of joints in joints manufactured from oven bonding technique. Furthermore, the magnitude of strains developed in the geometrical center of adhesive bondline through EM bonding was three times that of the oven bonded joints due to the difference in the thermal boundary conditions of the two processes. The study showed that the EM bonding results in increased thermal residual strains despite of its other processing advantages such as rapid heating and lower energy consumption. Further studies are necessary to fully quantify the residual strains developed during the processing thereby aid in better design of both the processing and structural parameters.