F. Gálvez

Effect of Glass Fiber Hybridization on the Behavior Under Impact of Woven Carbon Fiber/Epoxy Laminates

The low-velocity impact behavior was studied in hybrid laminates manufactured by RTM with woven carbon and glass (S2) fabrics. Specimens with different thicknesses and glass fiber content (from 0 to 21 vol.%) were tested with impact energies in the range 30-245 J and the resulting deformation and fracture micromechanisms were studied using X-ray microtomography. The results of these analyses, together with those of the impact tests (maximum load and energy absorbed), were used to elucidate the role played by glass fiber hybridization on the fracture micromechanisms and on the overall laminate performance under low-velocity impact.

Blast Response Analysis of Reinforced Concrete Slabs: Experimental Procedure and Numerical Simulation

A series of blast loading experiments are performed with the aim of providing experimental data for the development and adjustment of numerical tools needed in the modeling of concrete elements subjected to blast. To this end, an experimental setup that allows testing up to four concrete elements simultaneously under the same blast load is developed. Altogether four detonation tests are conducted, in which 12 slabs of two different concrete types are subjected to the same blast load. Results of the experimental program are validated by numerical simulation using two different material models for the prediction of concrete behavior. Major assets of the experimental setup presented are the reduction of scattering on detonation tests and its cost effectiveness. Results from tests and numerical simulations suggest that the ability of reinforced concrete structures of withstanding blast loads is primarily governed by their tensile strength.

Modelling Explosions Using ALE Meshes: TheInfluence Of Mesh Refinement In Pressures AndIn Efforts Induced By Blast/structure Interaction

Hydrocodes are becoming a very important tool for simulation of explosive loads and the interaction between the shock wave and possible structures stroked. Landmine explosions and the damage induced when they explode under an armoured vehicle are good examples of this. However this type of simulation usually requires very complex meshes, which must also be able to reproduce a large number of elements involved in the simulation, such as the explosive, the air, the wheels, the transmission...etc. Furthermore, the problem worsens if we take into account that very small size element meshes are required if we are looking for a good accuracy of the calculations, especially when we want a good approximation for the sharp pressure peaks that appear during an explosion. In this work, numerical results are presented from a research focused on the study of the mesh refinement influence in this type of problem. The study has been focused specially in two fundamental variables: time-pressure history and the time-integral of pressure with respect to time history.

Impact Behavior of Hybrid Glass/Carbon Epoxy Composites

The high velocity impact performance in hybrid woven carbon and S2 and E glass fabric laminates manufactured by resin transfer molding (RTM) was studied. Specimens with different thicknesses and glass-fiber content were tested against 5.5 mm spherical projectiles with impact velocities ranging from 300 to 700 m/s to obtain the ballistic limit. The resulting deformation and fracture micromechanisms were studied. Several impacts were performed on the same specimens to identify the multihit behavior of such laminates. The results of the fracture analysis, in conjunction with those of the impact tests, were used to describe the role played by glass-fiber hybridization on the fracture micromechanisms and on the overall laminate performance under high velocity impact.

Texture Evolution of AZ31 Magnesium Alloy Sheet at High Strain Rates

In the current contribution the mechanical behaviour at high strain rates of AZ31 magnesium alloy sheet is studied. Uniaxial deformation properties were studied by means of tensile split Hopkinson pressure bar (SHPB) at different temperatures. The influence of the strain rate and temperature on the deformation mechanisms was investigated by means of electron backscatter diffraction (EBSD) and neutron diffraction. It is shown that twinning plays an important role on high strain rate deformation of this alloy, even at elevated temperatures. Significant evidence of prismatic slip as a deformation mechanism is observed, also at warm temperatures, leading to the alignment of directions with the tensile axis and to a spread of the intensities of the basal pole figure towards the in-plane direction perpendicular to the tensile axis. The rate of decrease of the CRSS of non-basal systems is observed to be slower than at quasi-static rates. Secondary twinning and pyramidal slip were also outlined for some conditions. At warm temperatures, in contrast to quasi-static range, a generalized dynamic recrystallization is not observed. Moreover, the activation of rotational recrystallization mechanisms is reported

Measurement of fracture properties of concrete at high strain rates

An analysis of the spalling technique of concrete bars using the modified Hopkinson bar was carried out. A new experimental configuration is proposed adding some variations to previous works. An increased length for concrete specimens was chosen and finite-element analysis was used for designing a conic projectile to obtain a suitable triangular impulse wave. The aim of this initial work is to establish an experimental framework which allows a simple and direct analysis of concrete subjected to high strain rates. The efforts and configuration of these primary tests, as well as the selected geometry and dimensions for the different elements, have been focused to achieve a simple way of identifying the fracture position and so the tensile strength of tested specimens. This dynamic tensile strength can be easily compared with previous values published in literature giving an idea of the accuracy of the method and technique proposed and the possibility to extend it in a near future to obtain other mechanical properties such as the fracture energy. The tests were instrumented with strain gauges, accelerometers and high-speed camera in order to validate the results by different ways. Results of the dynamic tensile strength of the tested concrete are presented. This article is part of the themed issue ‘Experimental testing and modelling of brittle materials at high strain rates’.

An Experimental and Numerical Study of Ballistic Impacts on a Turbine Casing Material at Varying Temperatures

An experimental and numerical study of ballistic impacts on steel plates at various temperatures (700°C, 400°C and room temperature) has been carried out. The motivation for this work is the blade-off event that may occur inside a jet engine turbine. However, as a first attempt to understand this complex loading process, a somewhat simpler approach is carried out in the present work. The material used in this study is the FV535 martensitic stainless steel, which is one of the most commonly used materials for turbine casings. Based on material test data, a Modified Johnson-Cook (MJC) model was calibrated for numerical simulations using the LS-DYNA explicit finite element code. To check the mesh size sensitivity, 2D axisymmetric finite element models with three different mesh sizes and configurations were used for the various temperatures. Two fixed meshes with 64 and 128 elements over the 2 mm thick plate and one mesh with 32 elements over the thickness with adaptive remeshing were used in the simulations. Both the formation of adiabatic shear bands in the perforation process and the modeling of the thermal softening effects at high temperatures have been found crucial in order to achieve good results.

Materials Behaviour And Numerical SimulationOf A Turbine Blade-off Containment Analysis

This paper analyses the impact phenomenon of a failed blade against the containment inside a turbine (called “blade off”) by using the Finite Element Method. In addition, the research has also focused on the secondary damage, which is the damage induced by the debris of the lost blade on the adjacent blades of the turbine. The paper shows how the model has been arranged from the beginning, pointing out aspects concerning the materials, initial and boundary conditions. The study shows the importance of having well known characterization of the materials involved, including the effects of high strain rate and high temperature. The results show how both blade off and secondary damage phenomena can be accurately modelled by FEM and provides useful information for the entire process.

Experimental Procedure for Testing Concrete Slabs Under Blast Loading

In this paper the pressure waves generated in a test rig previously presented by the authors for the experimental analysis of concrete slabs subjected to blast loading are analysed. To this aim, the concrete samples are replaced by one single aluminium slab instrumented with pressure gauges. In order to analyze the experimental scatter, the test set-up is repeated four times, with the only variation of the location of the aluminium slab within the four available positions in the test rig. The pressure histories registered show good agreement with the typical patterns expected in open-air explosions and demonstrates the quasi-planar shape of the pressure wave acting on the slabs. By comparing between the results obtained in different detonations, the explosive charge seems to be among the main sources of experimental scatter in this kind of tests.

Dynamic Fracture Behavior of Steel Fiber Reinforced Self-Compacting Concretes (SFRSCCs)

Three-point bending tests on notched beams of three types of steel fiber-reinforced self-compacting concrete (SFRSCC) have been performed by using both a servo-hydraulic machine and a drop-weight impact instrument. The lo ading rates had a range of six orders of magnitude from 2.20 × 10−3 mm/s (quasi-static) to 2.66 × 103 mm/s. These SFRSCCs had the same matrix, but various types of steel fiber (straight and hooked-end) and contents (volume ratios), 0.51%, 0.77% and 1.23%, respectively. The results demonstrate that the fracture energy and the flexural strength increase as the loading rate increases. Moreover, such tendency is relatively moderate at low rates. However, at high rates it is accentuated. For the 0.51% fiber content, the dynamic increase factors of the flexural strength and the fracture energy are approximately 6 and 3, while for the 1.23% fiber content, they are around 4 and 2, respectively. Thus, the higher the fiber content the less rate sensitivity there is.