D. Amodio

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.

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.

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.

Performance Assessment of Inverse Methods in Large Strain Plasticity

The Virtual Fields Method (VFM) is an inverse method which allows to identify the constitutive parameters of materials from full-field measurements. The method relies on the principle of virtual work and, traditionally, is mainly used to characterize the material properties. Nonetheless the VFM can also be used as an inspection tool to compare constitutive models or experimental configurations. For instance, it can be adopted to compare different constitutive models in order to find out which one is more suitable to describe the mechanical behaviour observed during an experiment. In this paper this idea is applied in the case of large strain plasticity. A sensitivity study was conducted to evaluate how the specimen geometry and the texture orientation influence the identification of the constitutive parameters. Then the method was used to compare the performances of three constitutive models in reproducing the plastic behaviour of a given material. The studied models are isotropic von Mises, Hill48 for normal and planar anisotropy. The work was conducted on simulated data.

Identification of the plastic zone using digital image correlation

In this paper Digital Image Correlation (DIC) is used to study the evolution of the plastic zone close to a crack tip. A modified CT-specimen was used in order to fulfill the plane stress condition. The strain field around the crack tip was measured using two cameras and stereo DIC, so that out-of-plane movements are taken into account. Then, the Virtual Fields Method was used to identify the plastic zone, looking at the parts of the specimen which deviates from the linear elastic behavior. With such approach, it was possible to individuate the onset of plasticity close to the crack tip and follow its evolution. A comparison with FEM results is also provided.

Study of the local and global deformation process of an aluminium alloy using full-field measurements

The yielding process of some aluminium alloys is often characterized by an inhomogeneous diffusion of shear bands during deformation, which cause oscillations in both the force level and the strain history. This phenomenon can be observed during a tensile test, however, in order to describe the mechanical behaviour of components at the continuum level, the hardening behaviour is usually calibrated according to the averaged stress vs strain curve. In this paper an inverse method and full-field measurement were used to identify the hardening behaviour in different zones of the specimen. Different stress/strain curves were obtained at different zones of the test, showing that, at the mesoscopic level, the material can be described as a heterogenous material. The difference in the hardening curves is consistent with the occurrence of the strain bands during deformation.

Identification of Plastic Behaviour of Sheet Metals in High Strain Rate Tests

In this work, dynamic tension tests have been conducted by an SHB on sheet metals in order to characterize the plastic behaviour of the materials. First of all, the sample geometry and the clamping system were optimized by FEM simulations in order to: (i) reduce impedance disturbance due to the fasteners, (ii) maximize the specimen cross-section to increase the force measurement sensitivity, (iii) reduce the elongation measurement errors due to deformation of the clamping system. Pictures of the samples were acquired during the test by means of a fast camera. On the one hand, this permitted to validate the strain measurement by the classical SHB theory formulas; on the other hand, application of DIC method permitted to obtain the actual strain distribution maps. These strain maps have been used to extract the parameters of a strain hardening constitutive model.

Application of VFM for the Simultaneous Identification of Visco-pseudo-hyper Elastic Constants of Rubbers

In this work, the Virtual Fields Method is employed in order to identify the parameters of constitutive models describing the visco-pseudo-hyper elastic behavior of rubber-like materials; a cruciform specimen is subjected to several biaxial loading/unloading cycles at different strain amplitudes, while load and full-field strain histories are recorded. The material is considered from its virgin state and no conditioning is performed, so not only the hyperelastic and viscoelastic characteristics have to be considered, but also the damage or Mullins effect has to be accounted for.