Quantitative evaluation of the effect of fibers on the fracture resistance of oil well cement based on subcritical crack growth

Fibers have been widely incorporated into oil well cement to enhance mechanical performance and maintain wellbore integrity. However, the lack of quantitative evaluations of fracture resistance has hindered the systematic design. This study focuses on the oil well cement reinforced with different types of fibers (i.e., micrometer calcium carbonate whiskers (CW), millimeter basalt fibers (B), and carbon fibers (C)), both individually and in hybrid configurations. A semi-circular bend (SCB) fracture test combined with digital image correlation (DIC) enables detailed analysis of subcritical crack growth, delineation of the fracture process zone (FPZ), and the traction-free crack lengths. This approach allows for accurate calculation of true unstable fracture toughness, incorporating effective crack lengths often overlooked in conventional assessment. The assessment of initiation and cohesive fracture toughness provides a quantitative framework to distinguish matrix toughening prior to crack initiation from the resistance to crack propagation thereafter. The results reveal that while both CW and B improve cohesive fracture toughness comparably (up to 114.3 %), C fiber exhibits a remarkable increase of 1585.7 %, highlighting their exceptional resistance to crack propagation post-initiation. Furthermore, the hybridization strategy reveals significant variability in unstable fracture toughness efficiency: B-CW combinations yield a modest 23.8 % improvement, whereas C-CW hybrids achieve a substantial 278.6 % increase, benefiting from synergistic reinforcement both pre- and post-cracking. This work offers a first-of-its-kind quantitative framework for evaluating fiber-induced toughening in oil well cement, providing critical insights for the rational design of hybrid fiber-reinforced cementitious systems aimed at enhancing fracture resistance in energy infrastructure.