Giovanni B. Broggiato

Bio: Giovanni B. Broggiato is an academic researcher from Sapienza University of Rome. The author has contributed to research in topics: Digital image processing & Stamping. The author has an hindex of 9, co-authored 27 publications receiving 371 citations.

Digital speckle correlation for strain measurement by image analysis

Abstract: This paper is concerned with small strain measurement utilizing the numerical processing of digital images. The proposed method has its theoretical basis in digital signal analysis and, from a methodological point of view, it can be considered as an extension to digital images of the wellknown white light speckle photography technique. That conventional method is based on the analysis of photographic plates that are exposed twice (before and after the specimen deformation) with the image of a random speckle pattern that has been previously printed on the test piece surface. The digital speckle correlation advantages consist of requiring a very simple specimen preparation and, mainly, of allowing the strain field computation just by numerical elaboration of the acquired images. In this paper, the theoretical basis of the technique and some valuable improvements to the known analogous methodologies are presented. Finally, test results for an application of digital speckle correlation are shown and advantages and disadvantages of the technique are elaborated. In addition, further developments in this area are discussed.

Development of Micro-Grippers for Tissue and Cell Manipulation with Direct Morphological Comparison

Abstract: Although tissue and cell manipulation nowadays is a common task in biomedical analysis, there are still many different ways to accomplish it, most of which are still not sufficiently general, inexpensive, accurate, efficient or effective. Several problems arise both for in vivo or in vitro analysis, such as the maximum overall size of the device and the gripper jaws (like in minimally-invasive open biopsy) or very limited manipulating capability, degrees of freedom or dexterity (like in tissues or cell-handling operations). This paper presents a new approach to tissue and cell manipulation, which employs a conceptually new conjugate surfaces flexure hinge (CSFH) silicon MEMS-based technology micro-gripper that solves most of the above-mentioned problems. The article describes all of the phases of the development, including topology conception, structural design, simulation, construction, actuation testing and in vitro observation. The latter phase deals with the assessment of the function capability, which consists of taking a series of in vitro images by optical microscopy. They offer a direct morphological comparison between the gripper and a variety of tissues.

The Chaboche nonlinear kinematic hardening model: calibration methodology and validation

Abstract: This work studies how a nonlinear kinematic model aimed for cyclic plasticity could be put into effect and used within a FEM code. A correct modeling of cyclic elasto-plastic behavior can be exploited in low-cycle fatigue life investigation as well as in manufacturing problems related to springback prediction. The chosen formulation has been proposed by Chaboche, and it is implemented in most of the commercial codes used for nonlinear FEM simulations. At first, a procedure for the proper identification of unknown material model parameters has been put forward. This calibration, based on the data collected from experimental low-cycle fatigue tests, has been performed by means of an inverse method. Laboratory tests differ according to the type of material under investigation. A classification can be operated distinguishing between specimens obtained from bulk materials or from sheet metals. For the former, standard tension-compression tests have been performed, while for the latter, a dedicated testing equipment for three-point bend cyclic tests has been devised. Then, further experimental tests have been run to check model transferability: different strain per cycle amplitudes, asymmetric strain cycling and different stress triaxiality levels have been investigated. For each of these tests, experimental vs. FEM results have been analyzed to show the level of agreement that has been reached.

Application of digital image correlation to the study of planar anisotropy of sheet metals at large strains

Abstract: The aim of this work is to measure and model the planar anisotropy of thin steel sheets. The experimental data have been collected using the digital image correlation technique. This is a powerful tool to measure the strain field on differently shaped specimens subjected to large plastic deformations. In this manner, it is possible to observe the material behaviour under different stress-strain states, from small to large deformation conditions, on the entire specimen surface. The experimental results on smooth and notched samples have been used to characterize the flow stress curve after necking and a nonassociated plastic flow rule is proposed to describe the anisotropic behaviour of the material. To compare the experimental data with the predictions of the adopted constitutive model, a novel method, based on the image correlation results, has been implemented.

A J2–J3 approach in plastic and damage description of ductile materials:

Abstract: This paper illustrates a methodology to improve the description of the plastic behavior and the fracture prediction for ductile materials under complex loading conditions. To this purpose, a plasticity model and a damage estimation model are proposed. The former, differently from the classic J2 plasticity theory, takes into account the effect of the third deviatoric invariant on the plastic flow. The latter assumes that damage accumulation is governed by both stress triaxiality and deviatoric parameters, and takes advantage of the new plasticity formulation. The two models rely on the same theoretical foundation, where a specific function is invoked to describe the subsequent yield surfaces and the damage accumulation up to fracture. Both have been implemented into a commercial finite element code via user subroutines.Three steel alloys have been tested under very different stress states: tensile tests on smooth and round notched bars, plane strain tests, torsion tests, and combined tension–torsion tests ...

A numerical model of severe shot peening (SSP) to predict the generation of a nanostructured surface layer of material

Abstract: Generation of a surface layer of material characterized by grains with dimensions up to 100 nm by means of severe plastic deformation is one of the most interesting methods to improve the mechanical behaviour of materials and structural elements. Among the ways to obtain a surface layer with this characteristic, shot peening is one of the most promising processes, since it is applicable to very general geometries and to all metals and metal alloys without high-tech equipments. Notwithstanding the fact that the ability of shot peening to obtain nanostructured surfaces by using particular process parameters (mainly high impact energy and long exposure time) is proved, deep knowledge of the correct choice of quantitative values of process parameters and their relation to the grain size and the thickness and uniformity of the nanostructured layer is still lacking. In this paper a finite element model of severe shot peening (SSP) is developed with the aim of predicting the treatment conditions that lead to surface nanocrystallization. After having assessed the accuracy of the model as regards mesh parameters and constitutive law of the material, the results are discussed and interpreted in terms of induced residual stresses and surface work hardening. A method to assess the formation of nanostructured layer of materials based on the value of the equivalent plastic strain is developed. The comparison with experimental results allow to affirm that the model is a useful tool to predict the generation of a nanostructured surface layer by shot peening and to relate the peening parameters with the treated surface layer in terms of residual stresses, work hardening, and depth of the nanostructured layer.

Investigation on dissimilar laser welding of advanced high strength steel sheets for the automotive industry

Abstract: To support the use of advanced high strength steels in car body design and fabrication, an investigation was carried out on dissimilar butt laser welding between TWinning Induced Plasticity (TWIP) steels, Dual Phase (DP) steels, hot stamping boron (22MnB5) steels, and TRansformation Induced Plasticity (TRIP) steels. The base materials and the weldments were fully characterized by means of metallography, microhardness, and tensile tests. Digital image analysis was also used to provide additional information on the local strain field in the joint during the tensile tests. Fractographic examination was finally performed on the fracture surfaces of the tensile samples. The dissimilar joints between the DP, 22MnB5, and TRIP steels exhibit good resistance properties. On the contrary, the dissimilar joints encompassing the TWIP steel exhibit poor mechanical strength and fail along the weld seam by intergranular fracture, probably due to presence of Mn segregations. Therefore, the laser welding of TWIP steel with other advanced high strength steels is not recommended without the use of proper metal fillers. Dissimilar laser welding of DP, TRIP and 22MnB5 combinations, on the contrary, can be a solution to assemble car body parts made of these steel grades.