Journal:National University of Singapore

Key Words:Projectile impact; Deformable; Rigid; Penetration depth; Hardness; Bauxite; Ultra-high performance concrete (UHPC); K&C material model; Dynamic increase factor (DIF)

Abstract:This thesis presents an in-depth investigation on the resistance of cement-based materials, across a wide range of compositions and material properties, subjected to high-velocity projectile impact (HPVI). Specifically, aim of the present work is to (a) identify critical parameters for the penetration depth in cement-based materials against small caliber non-deformable HVPI; (b) identify critical relative characteristic parameters for the penetration depth in cement-based materials against small caliber deformable HVPI; (c) develop advanced cement-based materials with superior mechanical properties and resistance against HVPI, and (d) provide a systematic implementation of K&C model for ultra-high performance concrete (UHPC) against HVPI.
The compressive strength alone cannot accurately characterize the penetration depth in cement-based materials and their compositions also play an important role. To determine critical effective parameters for the penetration depth against small caliber non-deformable HVPI, a series of effective parameters is identified for investigation, including compressive strength, elastic modulus, effective hardness index, density, splitting tensile strength, and flexural toughness. The cement pastes, mortars, concretes, UHPCs, and engineered cementitious composites (ECCs) with 28-day compressive strengths from 34.2 to 220.2 MPa are included. The impact tests were conducted using 300×170×150 mm3 specimens against 8.0-mm-diameter, 7.8-g-mass, conical-nosed ASSAB XW-42 steel projectile impact at velocities of approximately 400 m/s. It was found that the effective hardness index and elastic modulus are the most critical parameters governing the penetration depth.
For the same range of cement-based materials, we further investigate their resistance against small caliber deformable HVPI with an aim to determine critical relative characteristic parameters for the penetration depth. A series of relative characteristic properties between projectile and target is investigated, including relative compressive strength (RCS), relative elastic modulus (REM), relative effective hardness index (REH), and relative density (RD). To facilitate the identification of critical relative characteristic parameters for different combinations of projectile and target, projectiles fabricated from two different materials (ASSAB XW-42 and copper) are included. It was found that the REH is a good characterization of the penetration depth and projectile deformation.
Motivated by the insights obtained previously, calcined bauxite aggregate is utilized to develop advanced cement-based materials with superior mechanical properties and resistance against HVPI, considering its high hardness. The effect of bauxite sand or / and coarse aggregate in cement-based materials on the mechanical properties and ASSAB XW-42 steel projectile impact resistance is investigated, compared to that of siliceous sand or / and granite coarse aggregate. It was fond that the usage of bauxite sand or / and coarse aggregate is beneficial for improving mechanical properties and impact resistance. The concrete with both bauxite sand and coarse aggregate exhibits compressive strength of about 200 MPa and elastic modulus of about 105 GPa, and better impact resistance than a granite rock specimen.
The Karagozian & Case (K&C) material model is widely used to numerically predict the response of concrete structures against impact or blast. Most of K&C model parameters are calibrated for normal strength concrete (NSC), and are thus not directly applicable for UHPC. Moreover, many existing work on UHPCs involve ad-hoc calibrations of K&C parameters, which may not be applicable for situations beyond the calibration conditions. To address these limitations, we carry out a systematic calibration of K&C failure surfaces and dynamic increase factors (DIFs) for UHPCs, based on available experimental data collected from literature. General guidelines are also provided for the determination of other model parameters. Furthermore, the predictive capability of K&C model with this systematic determination of material parameters is demonstrated by considering several sets of experimental data on HVPI against UHPCs, across a wide range of UHPC properties and mix designs, geometrical details, and loading conditions.

First Author:Fengling Zhang

Discipline:Engineering

First-Level Discipline:Civil Engineering

Document Type:D

Translation or Not:no

Date of Publication:2020-12-01

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