Multiple cracking of galvannealed coating layer on steel substrates

The fracture behavior of the Fe-Zn intermetallic compound coating layer on ductile steel substrate sheets was studied experimentally and analytically. When tensile strain was applied, the brittle Fe-Zn intermetallic compound coating layer showed multiple cracking perpendicular to the tensile axis. The average crack spacing of the coating layer decreased with increasing applied strain. The crack spacing for a given strain was dependent on the coating thickness and species of the steel substrate. The calculation of the exerted stress of the coating layer with the finite element method revealed that, when the steel substrate deformed plastically and the crack spacing was narrow, the maximum tensile stress of the coating layer was approximately proportional to the crack spacing and tensile stress of substrate at the corresponding strain, and inversely proportional to the coating thickness. From the comparison of the measured crack spacing versus strain with the calculated one, the average strength of the coating layer was estimated to be around 250 MPa. From these results, an empirical equation was proposed, which can be used for rough prediction of the crack spacing as a function of applied strain for any substrate steels and thickness of the coating layer