Insights from fracture and cohesive stress transfer in fiber-reinforced ductile 3D printed assemblies

The cohesive constitutive response of fiber-reinforced 3D-printed and cast concrete of comparable material flowability is established. The fracture response of concrete containing short straight steel fibers (0, 0.5, 0.75, and 1% fiber volume) is investigated. Concrete matrix and interfacial characteristics, and their influence on overall fracture response, are evaluated as a function of fiber volume fraction. The cohesive stress-crack opening (σ-w) relationship was established through inverse analysis of experimental fracture test responses using a fictitious-crack hinge model. Fiber orientation and matrix porosity in the fracture specimens were quantified using X-ray computed tomography. The extrusion process increased the fiber orientation factor by 22%. However, increasing fiber volume to 1% progressively increased matrix porosity to 733% and 710% for cast and printed concrete, respectively. The tensile strength of printed concrete exceeded that of cast concrete by 16%, but this difference diminished to 5% as the fiber volume increased to 1%. The cohesive response is governed by multiple mechanisms, including fiber pullout and fiber-matrix interactions arising from extrusion-induced fiber alignment, material compaction, and porosity. The effectiveness of fibers in printed concrete decreases with increasing fiber volume due to increased matrix porosity. Highly aligned fibers and higher extrusion-induced compaction in printing result in earlier fiber engagement and higher cohesive stresses at crack openings up to 0.2 mm. At larger crack openings, printed specimens exhibit continuously decreasing cohesive stresses owing to direct pullout of aligned fibers. Random fiber orientation in cast concrete increases pullout resistance by enhancing frictional resistance, thereby improving cohesive stress retention.