Ezio Cadoni

Study of the mechanical properties of plain concrete under dynamic loading

A new testing methodology based on the Hopkinson bar principle is proposed for studying globally and locally the mechanical properties of plain concrete at a high strain rate. A Hopkinson bar bundle measures the local mechanical characteristics over the cross section of a large specimen of plain concrete subjected to impact loading. With this method, more accurate measurements of the stress-strain diagram are obtained, especially during the fracturing phase of the concrete specimen.

Experimental Analysis on Tensile Dynamic Behavior of Existing Concrete under High Strain Rates

The presented research is part of a wider research project involving the study of the dynamic behavior under extreme loads of the Tenza Bridge, a concrete arch bridge located in southern Italy. The dynamic behavior of the concrete of the bridge under tensile loads is herein investigated. Several dynamic tensile tests under different strain rates were performed on concrete specimens at the DynaMat laboratory of the University of Applied Sciences of Southern Switzerland using modified Hopkinson bars. The results were then processed in terms of strength dynamic increase factor-strain rate relationships. These are fundamental to assess constitutive laws of concrete to be implemented in analytical models of the bridge under dynamic loads. The results are compared with existing analytical formulations that attempt to predict the dynamic tensile strength of concrete. The comparisons show that, even though tested concrete was taken from an existing structure, the relationships found in the literature accurately describe its tensile dynamic behavior.

Tensile High Strain-Rate Behavior of Reinforcing Steel from an Existing Bridge

Events such accidental or deliberate explosions need to be considered in load design, but this can be difficult due to the uncertainty related to load definition for blast actions. This paper highlights a dynamic characterization that was carried out on reinforcing steel belonging to an existing structure. The steel was from the Tenza Bridge, a reinforced concrete arch bridge in southern Italy. The behavior of both concrete and reinforcing steel under dynamic loading rates at high strain-rate levels was investigated. Tensile failure tests were performed on steel specimens at different strain rates using a modified Hopkinson bar device. Data from the tests were processed to obtain stress-strain relationships under different strain-rate conditions, and the results were compared with existing formulations, providing the dynamic increase factor of yield and ultimate stresses for reinforcing steel. Finding show that the reinforcing steel was strain-rate sensitive in terms of yield stress, ultimate stress and ultimate strain. As the strain rate increased, yield stress increased more than ultimate stress.

Strain-Rate Sensitivity of a Pultruded E-Glass/Polyester Composite

Structural analysis of composite structures subjected to dynamic loads requires detailed knowledge of the mechanical behavior of component materials under high strain-rates. This paper presents the results of tests to investigate the tensile dynamic behavior of a pultruded E-glass/polyester composite used in a steel-less blast protection barrier. The described activity is part of the Security of Airport Structures research project, focusing on structural protection of airport infrastructures against disruptive action. Modified Hopkinson bars and hydropneumatic machine devices were used to conduct strain-rate controlled tensile failure tests on glass fiber-reinforced polymer specimens. The results are discussed and then implemented within a viscoplasticity constitutive model and a strain-rate-dependent failure criterion in order to simulate the exhibited mechanical behavior.

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.

Influence of Aggregate Size on Strain-Rate Tensile Behavior of Concrete

A study of concrete behavior over a large range of strain rates has been the focus of research conducted at the Joint Research Center-Ispra (JRC). The program was developed to produce new data concerning a few basic but debated subjects of industrial importance. In this paper, the influence of increasing aggregate size on the strain-rate tensile behavior of plain concrete is described. This study is a first step toward calibrating the constitutive law of concrete in the high-strain-rate field.

Dynamic Tensile and Compressive Behaviors of Mild Steel at Wide Range of Strain Rates

AbstractThe purpose of the present paper is to investigate the mechanical behavior of mild steel at quasi-static (0.001 s−1) and different rates of dynamic tensile (5–750 s−1) and compressive (125–2,350 s−1) strain rates. Quasi-static experiments are conducted on a universal testing machine to study the stress-strain behavior of mild steel. A hydropneumatic machine and a modified Hopkinson bar are used to investigate the dynamic tensile behavior of mild steel specimens at medium and high strain rates, respectively, whereas the specimens are tested on a split Hopkinson pressure bar to acquire understanding of the strain rate sensitivity of mild steel under dynamic compression. The effects of a pulse shaper and gauge length of the specimen in the dynamic compression tests are investigated. High-speed photography has been used to monitor the deformation of the specimen at high strain rate experiments. The applicability of the existing Cowper-Symonds and Johnson-Cook material models to represent the mechanica...

Quasi-Static and Dynamic Tensile Behavior of CP800 Steel

An experimental investigation on the tensile behavior of Complex Phase 800 (CP800) steel at different strain rates (0.001 to 750 s−1) is reported here. The material is tested on a ZWICK universal testing machine to obtain the stress-strain relationship under quasi-static condition, and on a Hydro-Pneumatic machine and modified Hopkinson bar to study the mechanical behavior at medium and high strain rates, respectively. The failure surfaces of the tested specimens at different strain rates are studied from their fractographs. Finally, the applicability of the existing Cowper-Symonds and Johnson-Cook material models to represent the mechanical behavior of CP800 is examined.

Advances in the Hopkinson bar testing of irradiated/non-irradiated nuclear materials and large specimens

A brief review of the technological advances of the Hopkinson bar technique in tension for the study of irradiated/non-irradiated nuclear materials and the development of this technology for large specimens is presented. Comparisons are made of the dynamic behaviour of non-irradiated and irradiated materials previously subjected to creep, low cycle fatigue and irradiation (2, 10 and 30 displacements per atom). In particular, complete results of the effect of irradiation on the dynamic mechanical properties of AISI304L steel, tested at 20, 400 and 550°C are presented. These high strain rate tests have been performed with a modified Hopkinson bar (MHB), installed inside a hot cell. Examples of testing large nuclear steel specimens with a very large Hopkinson bar are also shown. The results overall demonstrate the capability of the MHB to efficiently reproduce the material stress conditions in case of accidental internal and external dynamic loadings in nuclear reactors, thus contributing to the important process of their structural assessment.

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.