Progressive fracturing of concrete under biaxial confinement and repetitive dynamic loadings: From damage to catastrophic failure

• Strength degradation under repetitive impacts and biaxial confinement is studied with a Triaxial Hopkinson bar . • Damage evolution and progressive fracturing are quantified by ultrasonic and micro-CT techniques. • Digital volume correlation (DVC) is firstly applied into dynamic tests to reconstruct the 3D deformation fields . • Relationships among mechanical, fracturing and deformation properties are further explored. Concrete materials are frequently exposed to extreme environments, such as high confining pressures (e.g., deep underground support), dynamic loadings (e.g., natural phenomena and human-induced events), and coupled confinement and dynamic loadings. The behaviours of concrete materials under such conditions result in challenges for the diagnosis and prognosis of structural changes from local damage to catastrophic failure, which is critical to the safety and sustainability of civil infrastructures. This paper aims to explore mechanical properties and progressive fracturing of concrete materials subjected to biaxial confinement and repetitive dynamic loadings. A triaxial Hopkinson bar (Tri-HB) system is used to apply the coupled loading conditions, and obtain the dynamic stress-strain information by interpreting recorded stress-wave signals. Non-destructive evaluation (NDE) techniques, including ultrasonic measurement and synchrotron-based micro-computed tomography (micro-CT), are utilised to quantify progressively damage evolution and fracture characteristics. The digital volume correlation (DVC) and imaging processing techniques are further applied to compute volume deformation fields and to classify microcrack types (i.e., matrix crack, interfacial crack and transgranular cracks). Results show that, with increasing the number of impacts, dynamic peak stress decreases along the impact direction but increases along the lateral direction while the peak strain values increase in both directions. The microcracks firstly initiate at the middle of rear-end of the specimen, continuously propagate along the impact direction, then develop at the top and bottom of the specimen, and eventually coalesce with the occurrence of shear sliding. The observation of microcracks are well validated by ultrasonic measurement. The formation of shear bands was highly dependent on the propagation and coalescence of interfacial and matrix cracks, while transgranular cracks induced by compressive strain localization as displayed in DVC deformation fields play an essential role in the fracture energy under repetitive dynamic loadings.