Patricia Verleysen

Monitoring of fibre reinforced composites with embedded optical fibre Bragg sensors, with application to filament wound pressure vessels

Tests carried out on bare optical fibres with a Bragg sensor show the feasibility of using these sensors for strain sensing. They have been embedded into simple composite laminates and have been subjected to static loading in bending tests. The measured strain from the Bragg sensor is perfectly linear with the applied force. Optical fibres with a Bragg sensor have also been embedded into filament wound pressure vessels. Tests carried out on such a pressure vessel include both static and slowly varying load schemes. The Bragg signal is nearly perfectly linear with the applied pressure. The results demonstrate the applicability of Bragg sensors for continuous monitoring of composite materials.

Experimental investigation of the deformation of Hopkinson bar specimens

Split Hopkinson bar (SHB) experiments are often used to study the strain rate dependent mechanical properties of materials. During a SHB experiment a small sample of the material under study is subjected to a high strain rate, uni-axial, tensile, compressive or torsion load. From the classical measurements the time history of the mean stress, strain rate and strain in the specimen can be derived. For some applications, more detailed information concerning the variation of the deformation in the specimen is necessary. In this contribution a technique is presented which makes it possible to obtain the deformation along the length of the specimen. The deformation of a line grid attached to the specimen is recorded during an experiment by means of a streak camera. An advanced and innovative numerical technique, based on a combination of geometric moire and phase shifting, is developed to extract the time history of the deformation along the axis of the specimen from the picture of the deforming grid automatically. Large specimen deformations are allowed, and the technique proved to give highly accurate results. In this contribution results are presented of a SHB experiment on a steel sheet specimen. Some remarks are formulated concerning the generally assumed homogeneity of the deformation in the specimen, and the deformation obtained with the classical measurement techniques.

Optical measurement of target displacement and velocity in bird strike simulation experiments

Thousands of bird strikes on aircraft occur annually with subsequent consequences for repair costs but also for public safety. In bird strike simulation experiments a bird or birdlike object is usually accelerated towards a target. Knowledge of the parameters of the impact, such as the velocities involved, the target displacement and the energy transferred from the projectile to the target, lead to a better understanding of the phenomenon, and eventually to better protection against damage caused by bird strikes. However, due to the extremely high velocities and energies involved in the experiments, methods for registration of the impact parameters are not obvious. Non-contact measurement techniques have a number of advantages over the more common mechanical contact methods. In this paper, a relatively simple optical technique is presented for recording the history of target displacement, from which the target velocity and energy can be readily obtained. The technique is based on the relative displacement of two moire line gratings: one grating attached to the target and the other serving as a stationary reference grating. The proposed technique has proved to be useful. Results of a representative bird strike experiment are presented.

Microstructure of adiabatic shear bands in Ti6Al4V

Abstract Microstructural deformation mechanisms in adiabatic shear bands in Ti6Al4V are studied using traditional TEM and selected area diffraction, and more advanced microstructural characterisation techniques such as energy dispersive X-ray spectroscopy, high angle annular dark field STEM and conical dark field TEM. The shear bands under investigation are induced in Ti6Al4V samples by high strain rate compression of cylindrical and hat-shaped specimens in a split Hopkinson pressure bar setup. Samples from experiments interrupted at different levels of deformation are used to study the evolution of the microstructure in and nearby the shear bands. From the early stages of adiabatic shear band formation, TEM revealed strongly elongated equiaxed grains in the shear band. These band-like grains become narrower towards the centre of the band and start to fraction even further along their elongated direction to finally result in a nano-crystalline region in the core. In fully developed shear bands, twins and a needle-like martensite morphology are observed near the shear band.

DC Electrical Breakdown in a Metal Pin–Water Electrode System

Electrical breakdown between a metal pin and a water-surface electrode is studied in this paper. The physics of discharges in this electrode geometry are still largely unknown, particularly the breakdown mechanism. Images of the sparks are presented, and the different features are discussed. The formation of a Taylor cone prior to breakdown is studied, and no significant polarity dependence is observed. When the water-surface deformation is small, a glow-to-spark transition is observed when the pin is cathode. When the metal pin is anode, a streamer-to-spark-like transition occurs.

Optical measurement of the specimen deformation at high strain rate

During recent years, the investigation of the strain-rate-dependent properties of materials has become more and more important. The experimental techniques used to establish the specific dynamic behavior of materials all have in common that the acquisition of information concerning the deformation of the specimen is cumbersone and often questionable. Mostly, only limited information on the spatial distribution and time evolution of the deformation in specimen can be obtained. In this paper, a non-contact, optical technique is presented, providing the time evolution and spatial distribution of the axial deformation in specimens during a high strain rate test. The deformation of a line grid attached to the specimen is recorded during an experiment by means of a rotating drum camera. The time history of the axial displacements is subsequently derived by an advanced technique based on digital geometric moiré combined with a phase-shift method, specially developed to this purpose. The technique can be applied to a wide range of materials and high strain rate tests, and is illustrated by means of a split Hopkinson tensile bar experiment on a steel sheet specimen.

The effect of silicon, aluminium and phosphor on the dynamic behavior and phenomenological modelling of multiphase TRIP steels

Multiphase TRansformation Induced Plasticity (TRIP) steels combine excellent ductility and high strength, making them ideally suited for shock absorbing parts in the automotive industry. When designing structures for impact, an understanding of the mechanical properties of materials under high strain rate conditions is essential. An extensive experimental program using a split Hopkinson tensile bar set-up was established in an effort to investigate the dynamic properties of various TRIP steel grades. Four different TRIP steels are described with varying contents of the alloying elements silicon, aluminium and phosphor. Moreover, several phenomenological models describing the strain rate and temperature-dependent mechanical behaviour are validated. TRIP steel grades in which aluminium is the main alloying element show high elongation values, whereas a high silicon content results in an increase in strength. The widely used Johnson-Cook model can describe the behaviour of TRIP steels and provides the opportunity to study its material and structural response.

Crashworthiness characterization and modelling of high-strength steels for automotive applications:

Abstract In recent years significant advances have been made in the field of multiphase steels for automotive applications. Complex steel grades have been developed with exceptional mechanical properties: they combine high strength values with an excellent ductility. Transformation-induced plasticity (TRIP) steels show these properties pre-eminently. The high ductility makes TRIP steels well suited for use in energy-absorbing devices. To guarantee a controlled dissipation of the energy released during a crash, knowledge and understanding of the impact-dynamic material properties are essential. An extensive experimental programme to investigate the strain rate dependent mechanical properties was set up, and the results for two CMnAl TRIP steels and a CMnSi TRIP steel are presented in this paper. A split Hopkinson tensile bar set-up was used for the experiments. Microstructural observation techniques were used to reveal the mechanisms governing the observed high strain rate behaviour. From the results it is clear that the excellent mechanical properties not only are preserved at higher strain rates but even improve. Several phenomenological material models were investigated to describe the strain rate and temperature dependent behaviour of TRIP steels. Both the Johnson—Cook model and an extended version of the Ludwig model were found to give good agreement with the experimental data.

High Strain Rate Torsion and Bauschinger Tests on Ti6Al4V

An accurate isotropic and kinematic hardening model and description of the strain rate dependent material behaviour is necessary for simulation of fast forming processes. Consequently, the material model parameter identification requires experiments where large strains, high strain rates and strain path changes can be attained. Usually, quasi-static tension-compression Bauschinger tests are used to assess the materials kinematic hardening. Hereby it’s important to have the same specimen geometry and boundary conditions in the forward and reverse loading step which is not easily achieved in high strain rate testing techniques. In this work, high strain rate split Hopkinson bar torsion experiments on Ti6Al4V are carried out to study the constitutive material behaviour at large plastic strain and strain rate. In torsion experiments, due to the absence of cross sectional area reduction, higher strains than in tensile tests can be obtained. In addition, a modified torsional split Hopkinson bar setup is developed to perform dynamic Bauschinger tests. A shear reversed-shear load is applied instead of the classical tension-compression load cycle. The test results are analysed to find out if the technique can be used for characterisation of the kinematic material behaviour. Digital image correlation and finite element simulations are used to improve the interpretation of the experimental results.

Non-homogeneous and multi-axial stress distribution in concrete specimens during split Hopkinson tensile tests

Abstract A split Hopkinson bar set-up is often used for the dynamic testing of materials. Test execution is relatively simple, and interpretation of test results is considered to be straightforward. Classical treatment of Hopkinson bar test results is based on the assumption of a homogeneous, uniaxial stress state in the specimen. However, non-axial stress components inevitably accompany the axial stress. These stresses have a large influence on the mechanical behaviour of concrete. To investigate the dynamic tensile properties of concrete, a split Hopkinson bar set-up was built. To obtain the full understanding, numerical simulations of the behaviour of the test set-up have been performed. The results are presented in this paper.