Adhesive strength improvement by providing steps in joints and differentiating initial and final debonding stresses

Step adhesive joints have a special characteristic quite different from other joints. When initial delamination occurs in other joints such as butt, lap and scarf joints, final failure always occurs. In these cases, the external stress causing initial delamination ${\sigma }_c^{\mathrm{Initial}}$ is equal to the final failure stress as ${\sigma }_c^{\mathrm{Final}}$ = ${\sigma }_c^{\mathrm{Initial}}$. However, in step joints, the final failure stress ${\sigma }_c^{\mathrm{Final}}$ can be greater than the initial delamination stress ${\sigma }_c^{\mathrm{Initial}}$ < ${\sigma }_c^{\mathrm{Final}}$. To clarify the adhesive improvement mechanism, first, this paper discusses the ISSF (Intensity of Singular Stress Fields) for fully bonded step joint by varying the number of steps $N_S$. Second, the singular stress field causing 2nd debonding is discussed by analyzing partially delaminated step joint. The results show that 2nd debonding requires larger external load than the initial debonding as ${\sigma }_c^{\mathrm{Initial}}$ < ${\sigma }_c^{\mathrm{Final}}$. This is because under the same external load the singular stress causing the 2nd deboning is smaller than the one causing the initial debonding. When $N_S\ge 6$ and suitable overlap length, the final bond strength ${\sigma }_c^{\mathrm{Final}}$ can be more than 3.6 ∼ 4.4 times larger than the initial delamination stress ${\sigma }_c^{\mathrm{Initial}}$ $\ll {\sigma }_c^{\mathrm{Final}}$ resulting in much larger bond strength.