M. Sasso

Design of an Innovative System for Wave Generation in Direct Tension–Compression Split Hopkinson Bar

The split Hopkinson bar is a well-known apparatus for performing tests at high strain-rate on engineering materials. The specimen is placed between two long bars and quickly loaded by a pressure wave, which travels at sound speed along the input bar, passes through the specimen, then is partly reflected and partly transmitted to the output bar. In the classical version, the input wave is generated by the impact of a striker bar against the input bar at a given velocity. In the direct version of the Hopkinson bar, the wave is generated by pre-loading and releasing a portion of the input bar. However, with this approach the rise time of the pressure wave is usually longer respect to the classical version. In this paper, an innovative wave generation system is presented, which exploits the abrupt shear fracture of a thin brittle disc. In this way, the rise time of the incident waves can be close to that obtained by the classical impact-based Hopkison bars (80xa0µs instead of 50xa0µs). Moreover, both tension and compression tests can be carried out with the same experimental system.

Prediction of ductile failure in materials for onshore and offshore pipeline applications

This paper shows the procedures needed to calibrate a numerical model intended for ductile damage estimation of bulk materials. For this purpose, an extensive experimental campaign has been carried out on three steels used for offshore/onshore pipe applications. Tests have been performed providing very different stress states: tensile and compressive uniaxial tests, multiaxial tensile tests on round notched bars, 3-point bend tests, again on notched geometries, and plane strain tensile tests on large grooved specimens. Based on the gathered results, a standard plasticity model has been tuned and then the damage model parameters have been identified for each investigated material. The chosen theoretical formulation can take into account all of the experimental evidence: hence, the numerical model represents a useful tool for finite element simulation of engineering problems where information concerning the materials ultimate resistance capability is needed. Moreover, the proposed calibration technique has general validity and can be used to tune other similar damage models.

Superimposed fringe projection for three-dimensional shape acquisition by image analysis

The aim in this work is the development of an image analysis technique for 3D shape acquisition, based on luminous fringe projections. In more detail, the method is based on the simultaneous use of several projectors, which is desirable whenever the surface under inspection has a complex geometry, with undercuts or shadow areas. In these cases, the usual fringe projection technique needs to perform several acquisitions, each time moving the projector or using several projectors alternately. Besides the procedure of fringe projection and phase calculation, an unwrap algorithm has been developed in order to obtain continuous phase maps needed in following calculations for shape extraction. With the technique of simultaneous projections, oriented in such a way to cover all of the surface, it is possible to increase the speed of the acquisition process and avoid the postprocessing problems related to the matching of different point clouds.

Surface Defect Generation and Recovery in Cold Rolling of Stainless Steel Strips

Previously researchers investigated the mechanism of surface defect evolution in rolling. It was highlighted how the lubricant plays an essential role for the final strip surface quality. In some cases the lubricant can be entrapped in pits or in other defects where hydrostatic pressure tends to prevent its elimination; however, when some favorable conditions are satisfied, the lubricant can be drawn out by hydrodynamic actions and defects can be recovered. This mechanism has been described as microplastohydrodynamic lubrication (MPHL) and recent studies report a suitable parameter (the ratio of the oil drawn out from the pit to the initial pit volume) as MPHL characterization coefficient. The present paper deals with the recovery of isolated surface defects in the Sendzimir rolling process of AISI 304 stainless steel; the analyses have been conducted on two rolling conditions, which although quite similar, regularly showed opposite capability of defect recovery, moreover, with a trend that is in contrast with the predictions made by standard MPHL. Two effects, which are usually ignored in literature modeling, have been considered in this work: The former is the back-tension, which has relevant outcome on the contact pressure and the latter is the position of the neutral point, which cannot be assumed to lie at the end of the roll bite. The analytical treatment was supported by FEM simulations, which permitted to put realistic data into the MPHL equations, thus, to explain the experimental behavior. The analysis was then validated with two further rolling schedules that seem to confirm the proposed approach.

Characterization of aluminum alloys using a 3D full field measurement

In the 2009 SEM Conference in Albuquerque, a measurement technique that combines the digital image correlation and the fringe projection was proposed by the authors to evaluate the 3D displacement of a specimen during a tensile test: the DIC is used to measure the in-plane displacement on both faces of the specimen while the fringe projection and phase shifting is used to reconstruct the shape with a high spatial resolution. In this work, the mentioned experimental technique was employed to characterize the plastic behavior of an aluminum alloy. First a 3D mesh was utilized to regularize the measured data and to get the strain field inside the specimen, in such way the full strain history of the test was reconstructed until failure. Then the Virtual Fields Method was adopted to identify the parameters of an anisotropic plasticity model. A comparison with FEM was also made to assess the correctness of the identified parameters.

High Strain Rate Behaviour of AA7075 Aluminum Alloy at Different Initial Temper States

The aim of this work is to study the mechanical properties of alloy AA7075 in both T6 and O temper states, in terms of visco-plastic and fracture behavior. Tension and compression tests were carried out starting from the quasi-static loading condition 10-3 up to strain rates as high as 2 x 103 s-1. The high strain rate tests were performed using a Split Hopkinson Tension-Compression Bar (SHTCB) apparatus. The tensile specimens were also subjected to micro-fractography analysis by Scanning Electronic Microscope (SEM) to evaluate the characteristics of the fracture. The results show a different behavior for the two temper states: AA7075-O showed a significant sensitivity to strain rate, with a ductile behavior and a fracture morphology characterized by coalescence of microvoids, whilst AA7075-T6 is generally characterized by a less ductile behaviour, both as elongation at break and as fracture morphology. Brittle cleavage is accentuated with increasing strain rate. The Johnson-Cook viscoplastic model wad also used to fit the experimental data with an optimum matching.

Evaluation of friction at high strain rate using the Split Hopkinson Bar

The present work aims at studying the influence of strain rate on the frictional behaviour of AA7075 aluminium alloy in the O-annealed temper state. To this purpose, ring compression tests were performed both under quasi-static and dynamic loading conditions. The high strain rate tests were carried out by means of the Split Hopkinson Tension-Compression Bar in the direct version. In both cases, hollow cylindrical samples, characterised by an initial outer diameter to inner diameter to height ratio of 6:3:2, were tested under dry condition and by lubricating with molybdenum disulphide grease. The different frictional behaviour exhibited by AA7075-O under quasi-static and dynamic loading conditions can be attributed to the strain rate effect both on the plastic flow behaviour of the deformed material, and on the thickness of the lubricant film.

Advanced Test Simulator to Reproduce Experiments at Small and Large Deformations

Full field measurements and inverse methods can be conveniently used to identify the constitutive properties of materials. Several methods are available in the literature which can be applied to many different types of materials and constitutive models (linear elasticity, elasto-plasticity, hyper-elasticity, etc.). The effectiveness of the identification procedure is related to the specimen geometry and the quality of the optical measurement technique. A method to improve and optimize the identification procedure is to numerically simulate the whole process. In such a way it is possible to compare different configurations and chose the one that shows the lowest identification error.

Experimental and Numerical Analysis of Pressure Waves Propagation in a Viscoelastic Hopkinson Bar

In this paper, the viscoelastic behaviour of PET is assessed in order to study the wave propagation in long SHPB made of polymeric materials.

Strain Assessment in Cracked Sheet Metals by Optical Grid Method

The present work is an extension of the optical grid method for the experimental investigation of deformation in stamped sheet metals. The classical method presents difficulty or is inapplicable where the deformation has resulted in the tearing of the sheet. In these cases, the measurement result is not available right where the most interesting data are expected.