Characterization of sequential fatigue behavior in fiber-reinforced concrete under bending

This study characterizes the sequential flexural fatigue behavior of fiber-reinforced concrete (FRC), in which two damage mechanisms occur in sequence: (i) matrix fatigue up to cracking, followed by (ii) fiber-matrix bond fatigue. A simple criterion is proposed that, based on the relationship between static strengths ( f LOP and f R ) and the maximum cyclic stress level ( σ fat,max ), allows anticipation of this sequential behavior. When σ fat,max is below both first-crack strength f LOP and residual strength f R , the cyclic creep curves (defined as the maximum CMOD per cycle versus the number of cycles) display two concatenated inverted-S segments, reflecting the two damage processes. The method is validated through an experimental program on three FRCs with fiber contents of 0.3%, 0.6%, and 1.0%, subjected to flexural fatigue under constant stress levels. The results confirm that the proposed criterion correctly discriminates between the general case and particular cases in which only one of the two mechanisms operates. In addition, the Sparks–Menzies relation between the slope of the central segment of the cyclic creep curve ( dCMOD/dn ) and the fatigue life ( N ) is extended to derive closed-form expressions that estimate, with high accuracy, the number of cycles to matrix fracture ( N m ) and the remaining cycles to final failure ( N f–m ). This tool provides a practical basis for assessing accumulated damage and optimizing the design and maintenance of FRC structural elements subjected to cyclic loading.