Microstructural influence on the scatter in the fatigue life of steel reinforcement bars

• Surface martensite contributes to the scatter reduction in the fatigue crack initiation phase.
• The scatter increases as the area fractions of ferrite–pearlite tend to be similar.
• The microstructure is responsible for about half of the scatter in crack initiation phase.

Fatigue damage accumulation and failure of steel reinforcement bars (rebars) is a stochastic process. Scatter can be influenced by the sensitivity of the short crack growth to the microstructural features, especially near the fatigue limit. This work investigates the scatter inherent to the microscopic conditions near the fatigue limit of ferrite–pearlite and martensite microstructures found in the outer layer of rebars. An adapted Navarro–De Los Rios model within a Monte-Carlo framework is used to simulate the short crack growth in material grains. Grain size variation, grain orientation factor and multiple phases i.e., ferrite–pearlite and martensite were considered in the model. The results are compared with the scatter found in fatigue tests on hot-rolled-cold worked (HR-CW) as well as quenched and tempered (QST) rebars. It is shown that microstructural effects explains part of the observed scatter in the fatigue tests.