High strain rate characterization of polymers

Journal de Physique IV (Proceedings), 2006

The stress-strain behaviours of polycarbonate (PC) and polyvinylidene difluoride (PVDF) have been measured over a range of strain rates at room temperature and a range of temperatures at high strain rate. Both materials show an approximately bilinear dependence of yield stress on strain rate over the rates examined. The experiments at different temperatures allow the high strain rate glass and transitions to be identified in PC, and the melting point and glass transition to be identified in PVDF. These can be confirmed by comparison to Dynamic Mechanical Analysis (DMA) measurements on the materials. Applying a timetemperature superposition to the data shows that these transitions are the cause of the bilinearity in the strain rate dependence of the materials. The behaviour of nominal (or engineering) stress in PC is also examined.

The European Physical Journal Conferences

In the field of high rate testing of polymers the measured properties are highly dependent on the applied methodology. Hence, the test setup as whole but in particular also the geometrical type of specimen plays a decisive role. The widely used standard for the determination of tensile properties of polymers (ISO527-2) was extended by a novel standard (ISO18872:2007), which is targeted on the determination of tensile properties at high strain rates. In this standard also a novel specimen shape is proposed. Hand in hand with the introduction of new specimen geometry the question of comparability arises. To point out the differences in stress-strain response of the ISO18872 specimen and the ISO527-2 multipurpose specimen tensile tests over a wide loading rate range were conducted in this paper. A digital image correlation system in combination with a high speed camera was used to characterize the local material behaviour. Different parameters like nominal stress, true stress, nominal strain, true strain as well as volumetric strain were determined and used to compare the two specimen geometries.

Le Journal de Physique IV, 1991

-La réponse mécanique de plusieurs polymères ductiles d'utilisation courante a été étudiée en compression à des vitesses de déformation de 2,5 x 10 3 s-1 , à température ambiante et dans l'azote liquide (100 K). Une barre d'Hopkinson à impact direct (BHID) est utilisée pour déterminer les courbes contrainte-déformation dans ces conditions d'expérimentation. La cinématographie ultra-rapide (7 u.s entre images) est l'outil majeur pour déterminer la déformation à rupture, en particulier pour les polymères ayant une grande déformation à rupture, pour lesquels celles-ci n'apparaît pas dans la fenêtre de temps imposée par la BHID (285 us).

Journal of Theoretical and Applied Mechanics

The increasing use of polymeric materials in transport �elds requires knowledge of their mechanical behaviour at high strain rate to optimize the structures. The particular behaviour of polymers, compared to metals, is characterized by low Young modulus, weak density and vis- coplastic behaviour, which make conventional experimental test inopera- tive. The objective of this work is the development of a dynamic tensile test reaching from 100 to 500 s􀀀1 (strain rate) based on the Charpy test- ing machine. The proposed test device is composed of an instrumented bar and a sensing block for wave strain measurement to determine the stress and strain on the tested material. The main objective consists in studying mechanical behaviour of polymer material at high strain rate. Key words: high strain rate, dynamic behaviour, shock, Hopkinson bar, sensing block.

Journal of The Mechanics and Physics of Solids, 2008

This paper details a methodology to test the mechanical response of soft, pressure sensitive materials, over a wide range of strain rates. A hybrid experimental-numerical procedure is used to assess the constitutive parameters. The experimental phase involves axial compression of a cylindrical specimen which is confined by a tightly-fit sleeve that is allowed to yield plastically, thus applying a constant confining pressure. The usually neglected frictional effects between the specimen and the sleeve are fully accounted for and characterized in detail. With commercial polycarbonate as a typical example, it is shown that pressure-sensitivity and rate-sensitivity are not coupled, thus reducing the number of tests needed to characterize a material. The results of numerical simulations indicate that the pressure sensitivity index (angle β in the Drucker-Prager material model) has little influence on the hydrostatic and confining pressures, whereas the equivalent stress sustained by the specimen increases with β, which for commercial polycarbonate is found to be 0 15 β = .

Journal of Physics: Conference Series, 2012

Advanced polymer materials are finding an increasing range of industrial and defence applications. Ultra-high molecular weight polymers (UHMWPE) are already used in lightweight body armour because of their good impact resistance with light weight. However, a broader use of such materials is limited by the complexity of the manufacturing processes and the lack of experimental data on their behaviour and failure evolution under high-strain rate loading conditions. The current study deals with an investigation of the internal heat generation during tensile of UHMWPE. A 3D finite element (FE) model of the tensile test is developed and validated the with experimental work. An elastic-plastic material model is used with adiabatic heat generation. The temperature and stresses obtained with FE analysis are found to be in a good agreement with the experimental results. The model can be used as a simple and cost effective tool to predict the thermo-mechanical behaviour of UHMWPE part under various loading conditions.

Le Journal de Physique Colloques, 1985

Resume-Une b a r r e de Kolsky B choc d i r e c t a 6% u t i l i s e e pour obtenir des courbes contrainte-deformation 3 v i t e s s e de dgformation de 2 x 10' s-' dans un e s s a i de compression s u r polymSres. De plus, l a deformation de disques de polymbres imposee par l a chute dlun poids a Bt6 6tudiee par photographie u l t r a r a p i d e ; l ' i n t e r v a l l e e n t r e chaque image d t a i t de 7 u s e t l a v i t e s s e de deformation de 5 x l o 3 s-'. Enfin, une nouvelle technique bas6e s u r l t u t i l i s a t i o n de f i l m s e n s i b l e 3 l a chaleur a Bt6 dkveloppde a f i n d'kvaluer l'accroissement de temperature associe B une deformation rapide.

International Journal of Impact Engineering, 2010

The need to model fracture in crashworthiness by means of finite element codes is a real challenge for research. Before implementing fracture criteria, an excellent knowledge of the stress and strain states in the material just before the crack appearance is the first condition necessary to ensure the model development. At present, most of the material behaviour laws, for example for steel, are only defined until the maximum force when necking occurs. For polymers, the early occurrence of the diffuse necking leads to an experimental technique in which the speed loading is controlled in real time to maintain a constant strain rate during the test. This technique is not however used, due to technical limitations, for high strain rate behaviour laws. In this paper, the authors propose to use the heterogeneity of the displacement field on the surface of the tensile specimen as an initial condition to identify behaviour laws. The method developed uses the information in all the surface zone of the specimen by using digital image correlation. Stresses, strains and strain rates are then obtained to build a surface behaviour called the SEĖ surface. By cutting it, the experimental behaviour laws for a range of large strains and strain rates are then defined for model identification.

2012

Polymers are increasingly used in impact and complex high rate loading applications. Generally, the mechanical response of glassy polymers under high strain rates has been determined in compression. Some research programs have studied the combined effects of temperature and strain rate, still primarily in compression, providing better understanding of the physics behind the observed response and enhancing the models for these materials. However, limited data are available in tension, and even more limited are data describing both the compressive and tensile response of the same glassy polymer. This paper investigates the compressive and tensile response of glassy polymers across a range of stain rates from quasi-static to dynamic. Experimental results from dynamic mechanical analysis, quasi-static compression and tension, and split Hopkinson tension/pressure bars on several representative glassy polymers will be presented. The pressure dependant yield in these materials will be discussed through comparison of the tensile and compressive yield stresses.

Experimental Mechanics, 2019

The simple shear response of soft polymers under large deformation (>50%) and strain rates spanning 10 −3-10 3 s −1 is characterized by developing quasi-static and split-Hopkinson pressure bar based single-pulse dynamic simple shear experiments rooted in continuum mechanics fundamentals. Cross-linked polydimethylsiloxane (PDMS) is chosen as a model material. By examining the evolution of stress, strain and strain rate, the latter two parameters measured using two-dimensional digital image correlation (DIC), it is demonstrated that dynamic simple shear deformation consists of four distinct stages: momentum diffusion, inertia effect, steady-state material response, and strain rate decay. By isolating the unsteady and steady-state deformation stages, inertia-free material response is captured under a uniform strain rate. It is shown that the shear response of PDMS is nearly linear with a weakly rate-sensitive shear modulus in the investigated strain rate range. Further, by analyzing the DIC strain-field and comparing the kinematic experimental results with those predicted by classical continuum mechanics, it is demonstrated that the proposed experiments not only achieve a nearly theoretical simple shear state that is uniform across the specimen, but also allow for post-test validation of individual experiments based on these criteria.