The current development of digital image correlation, whose displacement uncertainty is well below the pixel value, enables one to better characterise the behaviour of materials and the response of structures to external loads. A general presentation of the extraction of displacement fields from the knowledge of pictures taken at different instants of an experiment is given. Different strategies can be followed to achieve a sub-pixel uncertainty. From these measurements, new identification procedures are devised making use of full-field measures. A priori or a posteriori routes can be followed. They are illustrated on the analysis of a Brazilian test.

Digital Image Correlation: From Displacement Measurement to Identification of Elastic Properties - A Review

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Digital Image Correlation: from Displacement Measurement to Identification of Elastic Properties – a Review

The methods of preparation and formation of supermolecular structures in quiescent and sheared melts and the properties of the β-modification of isotactic polypropylene (β-iPP) are reviewed. The introduction of selective β-nucleants is the most reliable method for preparation of samples rich in β-modification or of pure β-iPP. The advantages and drawbacks of the known β-nucleating agents are summarized. It is emphasized that pure β-iPP can be prepared under laboratory and processing conditions in the presence of highly active and selective β-nucleants. Nevertheless, there are no literature data—apart from that of the author's groups—which evidenced unambiguously the formation of pure β-iPP. It hints at the insufficient selectivity of β-nucleants used or at the inappropriate crystallization or melting conditions applied by other scientists. The structure formation during the high-temperature hedritic crystallization is discussed comprehensively and illustrated by polarized light microscopy and scanning ele...

β-MODIFICATION OF ISOTACTIC POLYPROPYLENE: PREPARATION, STRUCTURE, PROCESSING, PROPERTIES, AND APPLICATION

Strength and toughness of crystalline polymer systems

When polyethylene (PE) is deformed to large strains, the stress originates from both the viscous forces associated with the plastic deformation of the crystallites by slip and fragmentation processes and the entropic elastic forces arising from the stretching of the entangled amorphous regions. Relative weights of the different processes change with the crystallinity. The dependencies were analyzed in a comprehensive study on a series of samples encompassing a large range of crystallinities:  PE, low-density PE (LDPE), and ethylene−vinylacetate copolymers. The comparison was based on measured true stress−strain curves for constant strain rates. For the samples with higher crystallinity, which show a necking, this was achieved by employing a video-controlled tensile testing machine. Recovery properties of the sample were studied in step-cycle runs, where the load was applied stepwise and interrupted after each step by an unloading−reloading loop. Simultaneous with the mechanical testing, the related textur...

Network Stretching, Slip Processes, and Fragmentation of Crystallites during Uniaxial Drawing of Polyethylene and Related Copolymers. A Comparative Study

A novel technique has been developed to record the intrinsic plastic behaviour of ductile materials by monitoring the effective strain and the effective stress in the mid-plane of hour-glass-shaped tensile specimens. The method utilizes a computer-aided video extensometer which analyses the sample profile in real time. The effective strain is computed automatically from the minimum diameter, and the effective stress is deduced from the applied load by taking into account the stress triaxiality corresponding to the local radius of curvature of the sample profile. Furthermore, a digital closed-loop system controls the ram speed of the hydraulic tensile testing machine in such a way that the local effective strain rate is maintained at a constant value. It is shown that most polymeric and metallic materials are entitled to be investigated by this method, which gives access in real time to the constitutive plastic equation, up to strains far beyond the necking point. The capabilities of the technique are illustrated and discussed critically, with more details for two polymers of different structures: polyethylene and polycarbonate.

Video-controlled tensile testing of polymers and metals beyond the necking point

Experimental characterization of deformation damage in solid polymers under tension, and its interrelation with necking

In situ observation of the plastic deformation of polypropylene spherulites under uniaxial tension and simple shear in the scanning electron microscope

Characterization of volume strain at large deformation under uniaxial tension in high-density polyethylene

The tensile behaviour of a commercial grade of isotactic polypropylene was tested in a temperature range between 20 and 150 °C with a video-controlled testing system which is capable of imposing a constant true strain-rate within the neck automatically. The results are displayed in the form of effective stress-strain curves and modelled by a constitutive equation in a multiplicative form. It is thus shown that, for each temperature, the plastic response can be described up to very large strains (ɛ ≃ 2.0) by a set of four parameters. The assumptions introduced in this modelling are critically discussed in order to check the validity of the simplifying hypotheses (strain homogeneity, isochoric deformation, etc.). The constitutive equation thus obtained was utilized in a finite difference code in order to predict the development of stretching instabilities of polypropylene. The simulation gives access to the engineering stress-strain response of the stretched test piece and to the detailed kinetics of the incipient neck. It is found that the severity of the instabilities is less at room temperature than near the melting point because of the decrease of the strain-hardening and of the strain-rate sensitivity with temperature.

Abdesselam Dahoun

Papers

Video-controlled tensile testing of polymers and metals beyond the necking point

Experimental characterization of deformation damage in solid polymers under tension, and its interrelation with necking

In situ observation of the plastic deformation of polypropylene spherulites under uniaxial tension and simple shear in the scanning electron microscope

Characterization of volume strain at large deformation under uniaxial tension in high-density polyethylene

Rheology of polypropylene in the solid state