Experimental Study on Tensile Behavior of Cement Paste, Mortar and Concrete under High Strain Rates

Effects of the strain rate on cement paste, mortar and concrete were studied. A modified SHPB testing technique with fl attened Brazilian disc（FBD） specimen was developed to measure the dynamic tensile stress-strain curve of materials. A pulse-shaped split Hopkinson pressure bar（SHPB） was employed to determine the dynamic tensile mechanical responses and failure behavior of materials under valid dynamic testing conditions. Quasi-static experiments were conducted to study material strain rate sensitivity. Strain rate sensitivity of the materials was measured in terms of the stress-strain curve, elastic modulus, tensile strength and critical strain at peak stress. Empirical relations between dynamic increase factor（DIF） and the material properties were derived and presented.

Influence of notch volume on mechanical properties of a novel high-strength and high-toughness steel

The effect of notch volume on the mechanical behavior of a novel high-strength and high-toughness steel with negative Poisson’s ratio effect(called NPR steel)was studied.First,the quasi-static tensile test of NPR steel with different notch volumes was carried out,and its failure characteristics and mechanical properties parameters were studied.Then,the modified Johnson-Cook(J-C)constitutive model with coupled notch volume ratio was proposed.The model was validated based on three-dimensional finite element numerical simulations.The results show that the engineering stress–strain curve of NPR steel has no yield platform,and has the mechanical properties of high strength,high elongation,and high energy absorption.Notch volume significantly affects the mechanical properties of NPR steel.The elongation,yield strength,tensile strength,and energy absorption characteristics of steel bar gradually decrease with the increase in notch volume.The notch volume ratio V^{*},a characteristic parameter describing the notch volume of reinforcement,is defined,and the quantitative relationship between this parameter and mechanical parameters is established,which can accurately characterize the mechanical properties of specimens with different notch volume ratios.Based on the true stress–strain curves with different notch volume ratios,a constitutive model with modified Johnson-Cook model parameters is proposed.The finite element results show that the modified J-C model can accurately fit the quasi-static tensile mechanical behavior of NPR steel.

Developing Mechanical Metamaterials Under an Adaptable Topology Optimization Design Framework

Developing mechanical metamaterials through topology optimization attracts high attention in both computational design and engineering applications.However,most of the studies in the literature are of quite limited applicability and poor extensibility.Hence,this work originally established an adaptable metamaterial topology optimization framework through integrating a commercial finite element analysis(FEA)platform.Particularly,the sensitivity analysis was derived and simplified to avoid the complex extraction of internal FEA information according to the strain-energy-based homogenization method.A series of two-and three-dimensional metamaterials with different properties,i.e.,bulk and shear moduli,negative Poisson’s ratio,were subsequently devised.These optimized metamaterials were fabricated and experimentally tested based on the additive manufacturing,firmly demonstrating the effectiveness of the developed design framework.This well-structured design framework can be conveniently extended to the systematic design of metamaterials with various other exclusive performances,fulfilling the urgent need for metamaterial design methods.