Dosage-dependent non-linear fracture mechanics of graphene-enhanced ECC beams

Graphene nanoplatelets (GnPs) offer a promising approach for improving the fracture performance of engineered cementitious composites (ECC); however, their dosage-dependent effects on crack propagation and energy dissipation remain insufficiently understood. This study investigates the fracture behaviour of GnP-modified ECC beams through three-point bending tests on notched specimens and discrete finite element modelling. Four GnP dosages, 0, 0.03, 0.06, and 0.09% by cement weight, were evaluated using the load–crack mouth opening displacement (CMOD) response. The experimental results showed a clear transition from crack initiation to post-peak softening, governed by fibre-bridging mechanisms. GnP dosage significantly influenced peak load, stiffness, residual load capacity, and fracture resistance. The 0.03% GnP mixture exhibited the most favourable performance, with enhanced peak load, stable post-peak softening, and improved fracture process zone stability. In contrast, higher dosages reduced ductility due to changes in crack propagation behaviour. X-ray diffraction and field-emission scanning electron microscopy showed that the 0.03% GnP mixture promoted calcium silicate hydrate formation and produced a denser matrix with well-dispersed nanoplatelets, whereas higher dosages caused localized heterogeneity associated with GnP agglomeration. A two-dimensional finite element model with discrete crack interfaces successfully reproduced the experimental load–CMOD response, including peak load, crack initiation, and post-peak trends. Numerical fracture energy values were consistent with experimental results. Interface opening analysis showed that 0.03% GnP reduced the crack localization amplification ratio from 12.9 to 3.0, indicating suppressed crack localization and improved fracture control.