Hybrid mechanical characterization of materials by optical and numerical techniques

One of the most important phases of the mechanical design of any engineering structure is the choice of the most suitable material for a particular application. Engineer’s capabilities and pride turn out in a reasonable choice that balances different material properties such as strength, ductility, machinability, availability, cost and mutual interactions between these and other factors. The objectives for implementing a valid project are realized in terms of security, high mechanical performances and low production costs. Mechanical design becomes more complex in all the cases in which new materials are used. Actually, these materials have the advantage of being designed for specific applications, offering higher performances than the traditional ones; moreover, due to their specificity, a precise analysis of mechanical, physical and chemical properties is required. Thus, defining and realizing new materials (i.e. three-dimensional composites, alloys of aluminum foam or polyethylene, sintered, biodegradable, etc..) represents a growing research activity due to the interesting perspectives it can offer. The possibility to use innovative materials, or conventional materials with greater mechanical properties could be a huge advantage for companies over competitors. Using innovative materials requires a deepening of traditional techniques of mechanical design, combined with the use of innovative procedures typical of the mechanical experimentation. Mechanical characterization proposes to evaluate materials’ properties, such as elastic constants, using standard testing or ad hoc procedures. However, it is not unusual that existing regulations and methodologies are not adequate for these kind of materials (i.e. foam, titanium alloys). The purpose of this thesis is to develop a new hybrid procedure (numerical-experimental) for the characterization of new materials, not recognized in regulations and in technical literature. This hybrid procedure is tested and validated on traditional materials with known mechanical performances. For my purpose, simple numerical models and experimental techniques are used. This approach is widely applied in literature but not specifically used for materials characterization. Ph.D. research activities are inspired by actual needs of industrial partners, involved in both automotive and aerospace field. This study contributes to the development of valid non-destructive instruments for the characterization of any type of material (traditional and innovative), with known and unknown mechanical properties, and provides reliable results in short time. This thesis is divided into two major themes dealing with two different aspects of new materials characterization. The first is related to the static mechanical characterization in terms of elastic and plastic properties of materials, in order to fully determine the Sigma-Epsilon curve; the second aspect is related to the effects of residual stresses, that could more or less compromise components’ life. A numerical-experimental approach has been defined to carry out research activities. The steps are described as follows: literature search on the state of art, set-up laboratory planning based on ad hoc procedures, definition of a finite element model for the mechanical properties optimization and/or validation of the model itself. Two different approaches are followed in the mechanical characterization of different types of materials. The first deals with traditional tests on an innovative composite material and on a validation of a finite element model by experimental tests. It provides advance information on materials’ mechanical response. A long-fiber stitched composite material is studied, as part of a collaborative project with Alenia Aeronautica. The composite consists of ply containing parallel fibers, held together through the thickness using staples and then impregnated with hot resin under pressure (RFI). All experimental tests are performed according to standard regulations ASTM and under different conditions of temperature, in order to simulate the real working conditions in flight and close to the engines. Numerical validation is realized only referring to tensile and compressive tests. The second most innovative approach of mechanical characterization defines a precise hybrid procedure, based on a combination of interferometric speckle pattern technique (PS-ESPI) and the finite elements modeling. This procedure aims to solve an inverse problem for the identification of unknown materials’ elastic constants, minimizing the difference between numerical FEM displacements and the same measured experimentally. The second part of the research is dedicated to the development of a new methodology for analysis and relaxation of residual stresses. It provides an alternatives to the most widespread traditional techniques. This section consists of two different aspects. The development and application of a full field measurement methodology by using the hole drilling method associated with the Electronic Speckle Pattern Interferometry. It can provide accurate results of the stress field surface in short time and with a non-contact technique. The optimization of working parameters of a high power diode laser, focused on a ductile material, for the stresses relaxation and subsequent evaluation using X-ray diffraction.