Material behaviour at low temperatures for calibrating cryogenic machining numerical simulations

Abstract Recently, cryogenic machining of difficult-to-cut alloys has been adopted to increase tool life and improve the machined components surface integrity. Numerical models of cryogenic machining are being developed to evaluate the influence of the different process parameters. Up to now, their calibration in terms of material flow stress is fulfilled using data at conventional temperatures, whereas the material sensitivity to temperatures lower than the environment one should be taken into account. To this regard, the paper objective is to present material data, obtained through a newly developed Split Hopkinson Tension Bar, at cryogenic temperatures and high strain rates to properly calibrate cryogenic machining numerical models

Non-direct tensile loading of sheet specimens on a classical split Hopkinson bars apparatus

Impulsive and impact loading are applied on reduced specimens to determine viscoplastic behaviour laws and to fix their strain rate sensitivity on large domains. Conventional tensile testing on sheet specimens raise the problem of the specimen holding mode which generally induces impedance mismatches and perturbs the elastic pulses on Hopkinson bars testing. This paper presents a non-direct tensile testing configuration using a classical Hopkinson bars apparatus. The interest of this configuration is to reduce section arrangements and to keep compression loading capabilities. A test programme is carried out on a low carbon steel alloy at plastic strain rates between 200 and 440 s$^{-1}$. Full FE simulations of the testing device have been conducted on the basis of the same testing conditions so as to evaluate the quality of the elastic pulses and to observe local phenomena. Numerical results revealed deviations in stress and strain rate values regarding comparable experimental databases. Hypotheses are made and investigated by the authors to identify the origin of the problem. Further investigations will be found out for a better understanding of experimental set-u

Enabling OpenVX support in mW-scale parallel accelerators

MW-scale parallel accelerators are a promising target for application domains such as the Internet of Thing (IoT), which require a strong compliance with a limited power budget combined with high performance capabilities. An important use case is given by smart sensing devices featuring increasingly sophisticated vision capabilities, at the cost of an increasing amount of near-sensor computation power. OpenVX is an emerging standard for the embedded vision, and provides a C-based application programming interface and a runtime environment. OpenVX is designed to maximize functional and performance portability across diverse hardware platforms. However, state-of-the-art implementations rely on memory-hungry data structures, which cannot be supported in constrained devices. In this paper we propose an alternative and novel approach to provide OpenVX support in mW-scale parallel accelerators. Our main contributions are: (i) an extension to the original OpenVX model to support static management of application graphs in the form of binary files; (ii) the definition of a companion runtime environment providing a lightweight support to execute binary graphs in a resource-constrained environment. Our approach achieves 68% memory footprint reduction and 3 7 execution speed-up compared to a baseline implementation. At the same time, data memory bandwidth is reduced by 10% and energy efficiency is improved by 2 7

Hardware-Accelerated Energy-Efficient Synchronization and Communication for Ultra-Low-Power Tightly Coupled Clusters

Parallel ultra low power computing is emerging as an enabler to meet the growing performance and energy efficiency demands in deeply embedded systems such as the end-nodes of the internet-of-things (IoT). The parallel nature of these systems however adds a significant degree of complexity as processing elements (PEs) need to communicate in various ways to organize and synchronize execution. Naive implementations of these central and non-trivial mechanisms can quickly jeopardize overall system performance and limit the achievable speedup and energy efficiency. To avoid this bottleneck, we present an event-based solution centered around a technology-independent, light-weight and scalable (up to 16 cores) synchronization and communication unit (SCU) and its integration into a shared-memory multicore cluster. Careful design and tight coupling of the SCU to the data interfaces of the cores allows to execute common synchronization procedures with a single instruction. Furthermore, we present hardware support for the common barrier and lock synchronization primitives with a barrier latency of only eleven cycles, independent of the number of involved cores. We demonstrate the efficiency of the solution based on experiments with a post-layout implementation of the multicore cluster in a 22 nm CMOS process where the SCU constitutes less than 2 % of area overhead. Our solution supports parallel sections as small as 100 or 72 cycles with a synchronization overhead of just 10 %, an improvement of up to 14× or 30× with respect to cycle count or energy, respectively, compared to a test-and-set based implementation

Combined effects of the in-plane orientation angle and the loading angle on the dynamic enhancement of honeycombs under mixed shear-compression loading

The combined effect of the loading angle (ψ) and the in-plane orientation angle (β) on the dynamic enhancement of aluminium alloy honeycombs is investigated. Experimental results are analysed on the crushing surfaces (initial peak and average crushing forces). A significant effect of the loading angle is reported. The dynamic enhancement rate depends on the loading angle until a critical loading angle (ψcritical). Beyond, a negative dynamic enhancement rate is observed. Concerning the in-plane orientation angle β effect, it depends on the loading angle ψ under quasi-static conditions. Under dynamic conditions, a significant effect is reported independently of the loading angle ψ. Therefore, the dynamic enhancement rate depends on the combined effects of ψ and β angles. A global analysis of the buckling mechanisms allowed us to explain the combined effect of ψ and β angles on the initial peak force. The collapse mechanisms analysis explain the negative dynamic enhancement rate for large loading angles

Characterisation and modelling of structural bonding at high strain rate

These paper deals with the development of new bonded joint modelling for crash application. A new testing device has been set up on Split Hopkinson bars in order to identify adhesive’s properties in assemblies for high strain rate and for different loading angles. These tests led to the development of a new cohesive element model used for solving nonlinear dynamic problems with an explicit integration time scheme. An example illustrates and justifies the development of such a cohesive element under dynamic loading by a good efficiency and a significant saving in calculation time

Quasi-static and dynamic behaviour of the bone structures with fine geometric and materials modelling aspects

The principle aim of this study is to highlight the influence of velocity on mechanical responses of cortical bones. Quasi-static tests are performed on cubic samples from bovine femurs in order to highlight the anisotropic effect of cortical structure. Thanks to the Hopkinson bars technique, a set of curves will be obtained and analysed to define precisely mechanical behaviour of porosity and loading directions. Therefore, this technique combined with a precise geometrical measurement based on μCT technique is expected to provide a more accurate representation of the mechanical behaviour of biological tissues. This protocol will be applied on human tissues after validation of geometrical and material correlation in order to increase the biofidelity of human body models

Thermal effect of the welding process on the dynamic behavior of the HSS constitutive materials of a fillet welded joint

Welded joints, due to their manufacturing process, are commonly weakened areas. This study provides a pragmatic methodology to analyze the dynamic behavior of the Base Metal (BM) and the Heat-Affected Zone (HAZ) of an HSS (High Strength Steel) fillet welded joint. Firstly, a specific approach has been developed to generate the HAZ material using a thermal treatment. Hardness and grain size are used to validate the replicated HAZ. This approach appears efficient and repeatable. Secondly, the true stress-strain quasi-static and dynamic behaviors up to failure of the BM and the HAZ have been determined. This characterization was performed thanks to a video tracking procedure and Bridgman-LeRoy correction. The comparison between these two materials shows that the thermal field of the welding process increases the HAZ yield stress and hardening while decreasing the strain at failure. It appears that the base metal is not rate sensitive from quasi-static up to 1350s-1. On the contrary, the heat affected material appears to be rate sensitive, but by softening. This unexpected dynamic material softening requires further analyses. A first Finite Element (FE) numerical analysis on a welded substructure submitted to impact loading shows a strong influence of the HAZ on the initiation of failure mechanisms

Modélisation et caractérisation des joints collés à hautes vitesses de déformation Modeling and characterization of bonded joints at high strain rates

Ce papier traite de la modélisation de joints collés pour les structures soumises à des sollicitations de type crash. Cette nouvelle modélisation basée sur un élément cohésif tient compte du comportement viscoplastique, de l'endommagement ainsi que de la rupture de l'adhésive. Sensible à la vitesse de déformation l'identification du critère de rupture nécessite une base expérimentale allant jusqu'à de très hautes vitesses de déformations. Un nouveau dispositif d'essais a donc été mis en place sur les barres de Hopkinson afin de solliciter des assemblages à haute vitesse et sous différents angles de chargement. <br> This paper deals with the modeling of bonded joints for structures subjected to dynamic crash loading. This new model based on a cohesive element takes into account the viscoelastic behavior, the damage and the failure of the adhesive. Due to the strain rate sensitivity, the identification of failure criterion requires experimental tests, up to very high strain rates. A new testing device has then been set up on the Hopkinson bar in order to load the assemblies with high strain rates and with different angles

Characterisation and modelling of structural bonding at high strain rate

These paper deals with the development of new bonded joint modelling for crash application. A new testing device has been set up on Split Hopkinson bars in order to identify adhesive’s properties in assemblies for high strain rate and for different loading angles. These tests led to the development of a new cohesive element model used for solving nonlinear dynamic problems with an explicit integration time scheme. An example illustrates and justifies the development of such a cohesive element under dynamic loading by a good efficiency and a significant saving in calculation time