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Holmquist Johnson Cook (HJC) model has been extensively adopted to simulate the projectile penetration of concrete targets. In this study, based on the available experimental data of uniaxial compression, triaxial compression, split Hopkinson pressure bar (SHPB) and Hugoniot tests, HJC model parameters in terms of the strength surface, strain rate effect, damage evolution and equation of state (EOS) were systematically calibrated for a newly fabricated ultra-high performance concrete termed as geopolymer-based ultra-high performance concrete (G-UHPC). Using the HJC model with calibrated model parameters, numerical simulations of projectile penetration into plain and fibre reinforced G-UHPC targets were performed in a commercial finite element program LS-DYNA. The numerical results for the depth of penetration (DOP) exhibited fair agreement with the test data. The numerical projectile velocity and displacement evolutions were also validated through comparing to the semi-analytical model. These observations demonstrated the applicability and validity of the calibrated HJC model to estimate DOP of G-UHPC targets subjected to projectile impact. With the calibrated and validated HJC model, parametric studies were further conducted to explore the effect of uniaxial compressive strength of G-UHPC, projectile impact velocity, mass, diameter and nose shape on the final DOP values. Based on the numerical results from the parametric studies, an empirical equation concerning the aforementioned variables was proposed, which could help design G-UHPC protective barriers against projectile penetration.

Geopolymer-based ultra-high performance concrete (G-UHPC) is a new form of UHPC, which has been developed to meet the demand for ultra-high strength, cost-effective and eco-friendly construction materials. This paper preliminarily investigated local damage of fibre reinforced G-UHPC slabs subjected to contact explosions. Under 0.4 kg TNT, three 150 mm thick slabs including one control specimen made of steel rebar reinforced normal strength concrete (NSC) and two G-UHPC slabs, i.e. plain G-UHPC and 1.5% steel fibre reinforced G-UHPC, were tested. Under 1.0 kg TNT, two 200 mm thick slabs including one control specimen made of steel rebar reinforced NSC and one 2% basalt fibre reinforced G-UHPC slab were tested. Comparing with steel rebar reinforced NSC, inferior performance of plain and 2% basalt fibre reinforced G-UHPCs was observed owing to their own brittle characteristics, whereas superior performance of 1.5% steel fibre reinforced G-UHPC was achieved. Further, numerical investigations were conducted on the steel rebar reinforced NSC and 1.5% steel fibre reinforced G-UHPC slabs to reproduce their local damage induced by contact explosions through the explicit finite element code LS-DYNA. With the validated numerical model, parametric studies were performed to explore the effect of slab thickness and TNT charge weight on the local damage of 1.5% steel fibre reinforced G-UHPC slabs under contact explosions, and then the local damages were identified and classified via the support vector machine (SVM) method. Based on the machine learning results, empirical equations were derived for the fast assessment of local damage levels of 1.5% steel fibre reinforced G-UHPC slabs under contact explosions.

In this work, a novel method is presented to track site-specific microstructure evolution in metallic materials deformed biaxially along proportional and complex strain paths. A miniaturized bulge test setup featuring a removable sample holder was designed to enable incremental measurements to be performed in a scanning electron microscope, by probing the same position on the sample at different deformation levels, with electron backscatter diffraction (EBSD), electron channeling contrast imaging (ECCI) and other imaging modes. Validation experiments were performed at room temperature on samples prepared from commercial sheet metal (dual-phase steel) and foils (stainless steel). Local strain measurements with the digital image correlation technique confirmed that proportional strain paths with a strain ratio up to 5 can be investigated using elliptical dies in the bulge test holder. It is also shown how complex strain paths can be obtained using a combination of overlapping elliptical dies. Incremental EBSD and ECCI were conducted in configurations relevant for the multi-scale investigation of localized plasticity and damage mechanisms in dual-phase steel. Quantitative information regarding microstructure evolution (phase fractions, orientation fields, dislocation structures, etc.) and regarding local strain distributions could be successfully obtained. This type of data sheds light on underlying deformation mechanisms and provides opportunities to calibrate crystal plasticity models.