AN EXPERIMENTAL AND THEORETICAL-STUDY OF THE FATIGUE PROPERTIES OF HOT-DIP-GALVANIZED HIGH-STRENGTH SHEET STEEL

The mechanisms behind the reduction of the fatigue strength of high-strength steel owing to hot-dip galvanizing have been studied. Three coating thicknesses were considered, ranging from 80 to 220-mu-m. The steel has a proof stress of 680 MPa. Fatigue loading was performed at an amplitude below the fatigue limit of the uncoated steel. The coated steels gave lives of between 5 x 10(4) and 8 x 10(4) cycles. The thickest coating gave the shortest life. Metallographic observation of material not exposed to fatigue loading showed there was a crack network in the delta- and zeta-layers of the coatings. Some initial cracks penetrated through half or more of the coating depending on the coating thickness. Short cracks were found in the steel at the interface to the coating. The crack pattern after fatigue loading followed the original pattern in the coating. Additional cracks were created in the thickest coating. A higher frequency of cracks inside the steel was observed for the steels with thicker coatings. These cracks started at cracks in the coating. Finite-element calculations for the driving forces of the original cracks showed that the material with the thinnest coating was fatigue loaded close to the threshold for crack propagation. The two materials with thicker coatings had driving forces for their original cracks that exceeded the threshold. These results were in good agreement with the lives observed experimentally.