Mechanics of fatigue crack initiation in submicron-thick freestanding copper films

• Fatigue experiments and stress analyses were conducted for 500-nm-thick Cu films.
• An intrusion/extrusion was formed on the slip plane parallel to a Σ3 twin boundary.
• Intrusions/extrusions formed on slip systems where the slip deformation penetrates.
• The number of cycles increased as the resolved shear stress field decreased.
• The fatigue crack initiation strength was ∼10× higher than that of a bulk Cu.

The mechanics of fatigue crack initiation in roughly 500-nm-thick freestanding copper (Cu) films were investigated by using single-side-edge-notched specimens and elastic finite element method (FEM) analyses. By applying cyclic loading at a stress ratio R = 0, an intrusion/extrusion was formed near a notch root, and a fatigue crack then initiated at the intrusion/extrusion, which penetrated the film in the thickness direction on a slip plane parallel to a Σ3 twin boundary. Local stress distributions were evaluated by three-dimensional FEM analyses, taking account of the individual grain shape and crystal orientation around the fatigue crack initiation sites. The results revealed that a fatigue crack was initiated on slip systems where (i) the slip deformation penetrated the film in the thickness direction without being blocked by grain boundaries or twin boundaries, and (ii) a high resolved shear stress occurred under the conditions in (i). The number of cycles to fatigue crack initiation increased as the intensity of the resolved shear stress field at the fatigue crack initiation site decreased. The resolved shear stress required for fatigue crack initiation in the Cu films is roughly one order of magnitude higher than that in a bicrystal or single-crystal bulk Cu of 30–35 MPa.