Strength of single lap joints with artificial defects

Adhesive joints encounter in-service defects that may have an impact on the joint strength. The objective of the present study was to understand in which situations defects can affect the joint strength. Two different types of adhesive were studied with different degrees of ductility as the stress distribution along the overlap depends on the adhesive's capacity to deform plastically. Three different steel adherends were used, from low strength and high ductility to very high strength, since the adherend yield strength is known to influence the joint mechanics. Rectangular and circular defects located in the middle of the overlap and of increasing size were studied. When a toughened structural adhesive is used with a high strength steel, there is an almost linear decrease in joint strength as the defect area increases. In the case of the brittle adhesive, the reduction in strength, as the defect size increases, is not proportional for small defect sizes, indicating that the end of the joint becomes more important due to local strains exceeding limiting values. When medium carbon or mild steel adherends are used, a non-linear decrease in strength is observed. For small defect sizes, there is no significant effect. But, as the size of the defect gets bigger, then a more rapid decrease in the strength of the joints is apparent. Failure is dominated by end effects for the case of mild and medium carbon steels (when the defect size is small).