High strain rate testing of aluminium alloy & high speed photography

In order to gain in safety, it is necessary to study material responses in severe conditions, for instance under dynamic loadings, especially in the case of light alloys used in car, nautical and aeronautical industries. The initiated study concerns fatigue-impact coupling effects on a commercial aluminium alloy (AA5454) used in car industry. Through a part of this investigation, by modificating the pre-existent flywheel, we try to characterize its dynamic strain and failure properties. The first modification is the changing of the specimen holding device, using a new measurement technique, inspired by the Split Hopkinson Pressure Bar. This allows us to obtain data directly exploitable for the derivation of the intrinsie material behaviour, avoiding then a numerical post treatment. Furthermore, we present the multi-CCD camera system that we set up in addition to the flywheel apparatus. This photographie device is associated to this equipment in order to visualize localisation of plastic strain and subsequent failure phenomenon

Fatigue acceptable damage threshold and fatigue failure threshold according to a residual impact behaviour of an aluminium alloy

In a competitive economical context that nevertheless aims at gaining in safety, it is necessary to take into account the resistance of structural components in relation to their thermomechanical histories. Particularly, fatigue preloading influence with regard to residual dynamic strength is crucial, especially in the case of light alloys used in transport and aeronautic industries. The study has been carried out on a 2017A T3 aluminium alloy. To fatigue predamage, tensile-compression loadings are performed and the residual dynamic behaviour is characterized with tensile loading at the speed of 5 ms-l. Since a direct correlation between predamage configuration and residual response is not easy because of the high scattering effects with regard to fatigue phenomenon, an hybrid modal analysis method - sound emission / numerical - is used to detect the predamage level. Then, according to the classification obtained by this way, the evolution of the mechanical properties of pre-fatigued material under impact loading allows to define the best mechanical parameter for boundary thresholds determination between no-damage influence and weakened states. Hence, an acceptable damage threshold -damaged critical volume for which impact toughness is not affected by fatigue preloading- and a failure threshold -damaged critical volume for which we observe a fall of the mechanical properties- are established. Therefore, it is possible to estimate the fracture energy under impact loading for prefatigued specimens and therefore to predict a safe or a dangerous behaviour

Adaptation of a flywhell to aluminium alloy for studying the influence of a fatigue predamage on its dynamic plasticity

Through this study, we try to characterize fatigue loading influence on dynamic response of an aluminium alloy, a 2017A T3. In fact, if many works consist to describe the fatigue behaviour or the impact behaviour of materials, few ones concern effects of combined loadings. Tensile-compression loadings are performed for fatigue with an Amsler vibrophore whereas the impact is realized through tensile loading with a flywheel. To fatigue predamage, stress levels are chosen in the high cycle fatigue area and several configurations (i.e : stress level / fracture probability) have been performed. To carry out impact tests, first it is necessary to adapt the impact testing machine in order to test metallic materials and to obtain data directly exploitable for the derivation of the intrinsic material behaviour. Actually, dynamical perturbations induced by the high velocity loading have to be eliminated to observe material behaviour. This is made by the way of mechanical modifications of the specimen holding device and by a numerical processing of the load sensor signal ; the system is validated to 5 ms-1 loading speed. Then, particular behaviours for combined loadings appear for residual macroscopic constitutive equations : weakened behaviour, no predamage influence and strenghtened behaviour

Neutrino fast flavor oscillations with moments: Linear stability analysis and application to neutron star mergers

Providing an accurate modeling of neutrino physics in dense astrophysical environments such as binary neutron star mergers presents a challenge for hydrodynamic simulations. Nevertheless, understanding how flavor transformation can occur and affect the dynamics, the mass ejection, and the nucleosynthesis will need to be achieved in the future. Computationally expensive, large-scale simulations frequently evolve the first classical angular moments of the neutrino distributions. By promoting these quantities to matrices in flavor space, we develop a linear stability analysis of fast flavor oscillations using only the first two "quantum"moments, which notably requires generalizing the classical closure relations that appropriately truncate the hierarchy of moment equations in order to treat quantum flavor coherence. After showing the efficiency of this method on a well-understood test situation, we perform a systematic search of the occurrence of fast flavor instabilities in a neutron star merger simulation. We discuss the successes and shortcomings of moment linear stability analysis, as this framework provides a time-efficient way to design and study better closure prescriptions in the future

Preparation and mechanical characterisation of laminate composites made of glass fibre/epoxy resin filled with tri bloc copolymers

International audienceLaminate composites made of glass fibre/epoxy resin filled with acrylic tri-bloc copolymers (Nanostrength) have been successfully manufactured. Microstructure, thermomechanical properties and impact resistance have been investigated and compared with those of a glass fibre/epoxy resin system. Dynamic mechanical analysis (DMA) tests have been conducted to determine the effect of Nanostrength on storage, loss modulus and glass transition temperature. A drop weight tower was used to perform low-velocity impact tests on laminate composites. Addition of Nanostrength to the epoxy matrix led to an increase in both strength and impact resistance of the composite. Moreover, a moderate decrease of storage modulus and glass transition temperature was observed

Two-moment Neutrino Flavor Transformation with Applications to the Fast Flavor Instability in Neutron Star Mergers

Multi-messenger astrophysics has produced a wealth of data with much more to come in the future. This enormous data set will reveal new insights into the physics of core-collapse supernovae, neutron star mergers, and many other objects where it is actually possible, if not probable, that new physics is in operation. To tease out different possibilities, we will need to analyze signals from photons, neutrinos, gravitational waves, and chemical elements. This task is made all the more difficult when it is necessary to evolve the neutrino component of the radiation field and associated quantum-mechanical property of flavor in order to model the astrophysical system of interest—a numerical challenge that has not been addressed to this day. In this work, we take a step in this direction by adopting the technique of angular-integrated moments with a truncated tower of dynamical equations and a closure, convolving the flavor-transformation with spatial transport to evolve the neutrino radiation quantum field. We show that moments capture the dynamical features of fast flavor instabilities in a variety of systems, although our technique is by no means a universal blueprint for solving fast flavor transformation. To evaluate the effectiveness of our moment results, we compare to a more precise particle-in-cell method. Based on our results, we propose areas for improvement and application to complementary techniques in the future

Two-moment Neutrino Flavor Transformation with Applications to the Fast Flavor Instability in Neutron Star Mergers

Multi-messenger astrophysics has produced a wealth of data with much more to come in the future. This enormous data set will reveal new insights into the physics of core-collapse supernovae, neutron star mergers, and many other objects where it is actually possible, if not probable, that new physics is in operation. To tease out different possibilities, we will need to analyze signals from photons, neutrinos, gravitational waves, and chemical elements. This task is made all the more difficult when it is necessary to evolve the neutrino component of the radiation field and associated quantum-mechanical property of flavor in order to model the astrophysical system of interest—a numerical challenge that has not been addressed to this day. In this work, we take a step in this direction by adopting the technique of angular-integrated moments with a truncated tower of dynamical equations and a closure, convolving the flavor-transformation with spatial transport to evolve the neutrino radiation quantum field. We show that moments capture the dynamical features of fast flavor instabilities in a variety of systems, although our technique is by no means a universal blueprint for solving fast flavor transformation. To evaluate the effectiveness of our moment results, we compare to a more precise particle-in-cell method. Based on our results, we propose areas for improvement and application to complementary techniques in the future