Daniele Forni

Mechanical Characterization of Cement Composites Reinforced with Fiberglass, Carbon Nanotubes or Glass Reinforced Plastic (GRP) at High Strain Rates

Advanced researches on concrete are directed toward investigating the behavior of reinforced concrete structures in severe conditions such as those promoted by impact loads. Some particular structures (protective shelters, nuclear reactor containment, offshore structures, military structures, chemical or Energy production plant) may be subjected to loading at very high rate of stress or strain caused by impact of missiles or flying objects, also by vehicle collisions or impulses due to explosions and earthquakes. Resistance to impact loads is guaranteed by using cementitious materials having both high strength and ductility. In order to improve ductility cementitious mortars with Glass Reinforced Plastics (GRP) replacing partially the natural sand were manufactured. Moreover, glass fiber (GF) reinforced mortars were produced to enhance toughness. For this scope two types of glass fibers were used different in length and diameter. Since the use of GRP and GF don’t produce any increase in strength of the mortars Carbon Nanotubes were added in the cement matrix to enhance tensile strength of the cementitious composite. Flexural, compressive and Hopkinson bar tests were carried out to evaluate the role of the different materials used. Replacing partially the natural sand with Glass Reinforced Plastics (GRP), compressive and flexural strength decrease (about 20%) with respect those of the reference mortar both on static and dynamic condition as a consequence of an anomalous air entrapment. Adding glass fibers (GF), GRP or/and Carbon Nanotubes (CNTs) no substantial improvement in terms of mechanical properties under static condition was occurred. The Dynamic Increase Factor of the reference mortar was higher than that of the reinforced mixtures, but fracture energy was lower. In particular, combined addition of carbon nanotubes and GRP determines an increase in the energy fracture. The higher the carbon nanotubes content, the higher both fracture energy and tensile strength because nanoparticles oppose to wave and crack propagation, increasing the high strain rate strength. GRP and CNTs reinforced mortars need more fracture energy to failure at 150 s-1 strain rate.

Dynamic Behaviour of Reinforcing Steel Bars in Tension

In this paper the preliminary results of the tensile behavior of reinforced steel in a large range of strain rates are presented. Tensile testing at several strain rates, using different experimental set-ups, was carried out. For the quasi-static tests a universal electromechanical testing machine with the maximum load-bearing capacity of 50 kN was used, while for the intermediate and high-strain rate regimes a hydro-pneumatic apparatus and a JRC-Split Hopkinson Tensile Bar respectively were used. The target strain rates were set at the following five levels: 10-3, 30, 250, 500, and 1000 1/s. The specimens used in this research were round samples having 3mm in diameter and 5mm of gauge length obtained from reinforcing bars. Finally, the material parameters for Cowper-Symonds and Johnson-Cook models were determined.

Strain-Rate Effect on the Tensile Behaviour of High Strength Alloys

In this paper the first results of the mechanical characterization in tension of two high strength alloys in a wide range of strain rates are presented. Different experimental techniques were used for different strain rates: a universal machine, a Hydro-Pneumatic Machine and a JRC-Split Hopkinson Tensile Bar. The experimental research was developed in the DynaMat laboratory of the University of Applied Sciences of Southern Switzerland. An increase of the stress at a given strain increasing the strain-rate from 10-3 to 103 s-1, a moderate strain-rate sensitivity of the uniform and fracture strain, a poor reduction of the cross-sectional area at fracture with increasing the strain-rate were shown. Based on these experimental results the parameters required by the Johnson-Cook constitutive law were determined.

Tensile behaviour of reinforcing steels at high strain rate and high temperature

The performance of reinforced concrete structures under combined effects of blast and fire is growing in interest of the research and engineering communities specially after the recent terrorist attacks as well as severe accidents (i.e. Gotthard tunnel, etc.). The mechanical behaviour of concrete and reinforcing steel when are subjected to extreme temperatures, impacts or blast has still many aspects open to investigation. In this paper the behaviour of AISI304, B500B and B500A reinforcing steel at high strain rate (500 s-1) and at three levels of temperature (200, 400 and 600°C) is presented. The results were obtained by using a Split Hopkinson Tensile Bar (SHTB) equipped with a heating system.

Influence of Fiber Type on the Tensile Behavior of Strain-Hardening Cement-Based Composites (SHCC) Under Impact Loading

Two different types of strain-hardening cement-based composites (SHCC) were investigated under uniaxial quasi-static and impact tensile loading. The normal-strength matrix was combined with polyvinyl-alcohol (PVA) fiber in one composite and with high-density polyethylene (HDPE) fiber in another. A modified Hopkinson bar was used to assess the impact resistance of SHCC in terms of stress–strain relationships at strain rates of up to 120 s−1.

Identification Methods of Parameters for Johnson-Cook Constitutive Equation - Comparison

The paper presents two identification methods of parameters for the Johnson-Cook constitutive equation. The Johnson-Cook equation is one of the most popular semi-empirical constitutive models to describe the equivalent of plastic stress-strain curves. The first presented method is the approximation method of plastic hardening stress-strain curves, obtained during split tension Hopkinson bar tests. The second proposed method to determine parameters of the Johnson-Cook equation is based on solutions of the inverse problem. This means that during optimization and identity calculations, physical phenomenon is simulated as, for example, an axially symmetric deformation, i. e., the Taylor impact test.