Papers by Author: Ezio Cadoni

Abstract: 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.

Abstract: 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.

Abstract: An experimental investigation on the strain rate sensitivity of die steel (D3) has been presented in this paper at different rates (0.001-2500s-1) of uni-axial compression. Quasi-static tests (0.001s-1) of the material are conducted on universal testing machine (UTM), whereas, the experiments at high strain rates are performed on split Hopkinson pressure bar (SHPB) apparatus. The effects of gauge length of the specimen on the material properties of the material are studied at different strain rates. The material parameters of existing Cowper-Symonds and Johnson-Cook material models are determined and the suitability of the models is examined.

Abstract: An experimental investigation on the dynamic compressive behaviour of the aluminium alloy, AA6063-T6 in the strain rate range from 0.001s^-1 to 850s^-1 is reported here. Cylindrical specimens of AA6063-T6 are tested under universal testing machine at quasi-static (0.001s-1) condition, whereas, experiments at high strain rates (110s-1,400s^-1,550s^-1,700s^-1 and 850s^-1) are conducted on the traditional split Hopkinson pressure bar setup. The strain hardening in the material is found to increase with increasing strain rate. It is observed that the existing Johnson-Cook material model with appropriate material parameters predicts the dynamic compressive flow stress of AA6063-T3 aluminium alloy precisely.

Abstract: The purpose of the present paper is to investigate the mechanical properties of multi phase 800 high yield strength (MP800HY) steel under compressive loading at different strain rates (-4700s^-1 to-0.001s^-1). Specimens of MP800HY steel are tested on universal testing machine to study their stress-strain behavior under quasi-static (-0.001s^-1) condition. Then, the specimens are tested under split Hopkinson pressure bar (SHPB) to study the strain rate sensitivity of the material under different rates of compressive loading (-4700s^-1, -4300 1/s, -3800 1/s, -2900s^-1 and-1600s^-1). The effect of pulse shaper in SHPB experiments has been studied. Thereafter, the applicability of the existing Johnson-Cook material model to represent the flow stress of MP800HY is examined.

Abstract: The promise of bre reinforced cementitious composites for dynamic loading ap-plication stems from their observed good response under static loading. However, very littleresearch has been carried out to investigate if their good static response corresponds to animproved dynamic response. Current understanding of the dynamic response and impact resis-tance of cementitious composites, and especially of SFRC, is very limited. In the framework ofthe ACCIDENT project, an experimental research aimed at contributing to the understandingof the behaviour of steel bre reinforced concrete subjected to low and high displacement rateswas carried out. The material investigated is a self compacting steel bre reinforced concretewith compressive strength equal to 70 MPa. Hooked steel bres 35 mm long were used. The brecontent was 50 kg/m3. The material behaviour was investigated at high displacement rates (1.2m/s) by exploiting a modi ed Hopkinson bar (MHB) and the tests results were compared withthe results obtained in static tests. A comparison between static and dynamic tests highlightedseveral relevant aspects regarding the material behaviour at high displacement rates.

Abstract: Fiber reinforced inorganic materials, such as concrete or mortars, are expected to present good mechanical properties in case of high dynamic loading conditions, as those induced by impact or blast actions. Furthermore, basalt fibers, recently widely investigated in structural applications, are also expected to present good performance in case of high strain-rate dynamic conditions. The present paper presents the results of a dynamic characterization of basalt fiber reinforced mortar, carried out at DynaMat laboratory of the University of Applied Sciences of Southern Switzerland. Dynamic tensile failure tests have been conducted on mortar samples at selected strain rates, using a Modified Hopkinson bar apparatus.

Abstract: 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.

Abstract: This paper presents the mechanical behavior of advanced high strength steel, Dual Phase 1200 steel (DP1200) at high strain rates (250s^-1 - 750s^-1) under tensile loading. The mechanical behavior of materials depends on the loading rates. The accurate knowledge of the mechanical behavior of materials at high strain rates is essential in order to improve the safety against crash, impacts and blast loads. High strain rate experiments are performed on modified Hopkinson bar (MHB) apparatus; however, some quasi-static (0.001s^-1) tests are also conducted on electromechanical universal testing machine at tensile loads. Based on the experimental results, the material parameters of the existing Cowper-Symonds and Johnson-Cook models are determined. These models fit the experimental data well and hence can be recommended for the numerical simulation of the problems involving this material at high strain rates.

Abstract: In this paper is described the mechanical characterization at high strain rate of the high strength steel usually adopted for strands. The experimental set-up used for high strain rates testing: in tension and compression was the Split Hopkinson Pressure Bar installed in the Laboratory of Dynamic Investigation of Materials in Nizhny Novgorod. The high strain rate data in tension was obtained with dog-bone shaped specimens of 3mm in diameter and 5mm of gauge length. The specimens were screwed between incident and transmitter bars. The specimens used in compression was a cylinder of 3mm in diameter and 5mm in length. The enhancement of the mechanical properties is quite limited compared the usual reinforcing steels.