This project aims at contributing to the advancement in the field of composite materials modelling, from the theoretical and experimental point of view, using multiscale approaches to increase the capability to describe the behavior these materials exhibit at the macroscale. Characterization of the elastic properties of composites will be also dealt with. An increased ability to predict the material behaviour is crucial in view of the conception of materials with extreme and unusual properties and for condition monitoring of in-service composite structures to detect possible degradations. The research project entails computational and experimental activities are arranged in three workpackages:
WP1:ANALYSIS AND COMPUTATIONAL METHODS-MULTISCALE METHODS implementation of models able to capture the macroscale behavior of composite materials retaining memory of the response at the micro-scale of their components (inclusions, fibers, grains), like Second Gradient, Cosserat. WP2:DESIGN AND OPTIMIZATION application of models derived in WP1 to the definition of internal arrangements of material components which enable to gain unusual and ultraperforming properties; resorting to 3D printing, high performance damping materials and solids with structural hierarchy and unusual properties will be investigated.
WP3:NODESTRUCTIVE EVALUATION AND MATERIAL CHARACTERIZATION, EXPERIMENTS AND APPLICATIONS OF LSV The computational approach developed in WP1 is applied to acoustoelastic problems, to investigate the velocities of bulk and guided acoustic waves propagating along nonclassical continua. This will allow to investigate the inverse problem of material characterization.LSV experiments will be performed with laser scanning of waves in fabricated composite beams, plates and solids, in view of the identification of constitutive parameters. The research team reached significant experience in the field, and will also benefit from consolidated national and international collaborations.
An increased ability to theoretically predict the material behaviour of multiscale hierarchic composite materials by equivalent continuum models is crucial in view of the conception of materials with extreme and unusual properties. In composite materials, different mechanisms occur at each scale, while the inter- and intrahierarchal interactions are of competing or reinforcing character. The multiscale modeling is a particularly useful approach to gain insight into these complex problems. In order to allow the best resolution at any length and time scale, computational methods can be integrated seamlessly, enabling one to bridge scales from nano to macro. Special attention is paid to advanced numerical methods able to rapidly converge to the problem solution with a high accuracy. Among the various computational methods, the viability of the recently-proposed and innovative Virtual Element Method (VEM) will be evaluated. This method has various interesting characteristics and provides a very good description in terms of modeling and of numerical solution for composite materials when applied to non-local models. The VEM appears promising for multi-scale analysis of composite materials with a random microstructure. Homogenization techniques applied to composite materials with internal random structure give rise to a considerable computational burden due to statistical-based homogenization procedures, which is an issue yet to be addressed.
We believe advancements in the field of composite materials modeling could be produced by an experimental validation of theoretical models, in particular by looking for ways to experimentally identify constitutive constants and stress state of multifield/microstructured continua. Ultrasonic guided-wave propagation is a technique with potential for stress identification and property characterization. The inversion scheme can be based on matching phase velocity dispersion curves of relevant guided wave modes to model curves depending on constitutive constants. Proof-of principle numerical studies are needed to demonstrate the potential of each selected wave mode as well as detailed investigations of the related inverse problem. Research on ultrasonic stress identification and material characterization of non-local, continuous micromorphic and multi-field models, including micropolar and second-gradient continua is extremely limited, not to say absent.
This activity could benefit from the hopefully soon availability of the 3D Scanner Laser Doppler Vibrometer, acquired by our department with the recently-funded research project Wide-Range Laser Scanning Station for 3D Shape Reconstruction and Dynamic Measurements, 2018 (Sapienza Grandi Attrezzature). This is an extremely advanced and performing noncontact acquisition system, which enables to dynamically acquire the kinematic 3D evolution of a structure at high frequencies. By virtue of the noncontact working principle, vibrations can be measured without any disturbance caused by the sensor itself, which is very appropriate to for the measurement of ultrasonic wave propagation.
The outcome of the deeper vision and better understanding of the theory of multi-field continua and improved reliability of instruments for the analysis of problems at the coarse scale dominated by the microstructure and in particular by scale effects will enable to design and synthesize materials with extreme and unusual physical properties, which will benefit from the availability of a 3D printing system (Fab Lab, school of Architecture). It will make possible to investigate the freedom of natural and synthesized materials to behave in ways not anticipated in elementary continuum representations, in view to ameliorate stress concentrations, and to attain physical properties of much higher magnitude than anticipated from standard theories.
Investigations will concern materials beyond the state-of-the-art such as:
- materials with heterogeneous structure, including natural viscoelastic composites such as bone, ligament, tendon and wood, as well as synthetic composites, biomaterials, and cellular solids with structural hierarchy. Viscoelastic materials are of particular interest as high performance damping materials, with performance depending on internal friction and strain rate.
- materials with reversed properties, including 3D materials with a negative Poisson's ratio (auxetic), materials with arbitrarily large magnitudes of positive or negative thermal expansion or even with zero thermal expansion, materials with negative stiffness inclusions in composites, composite materials with very high stiffness and designed Cosserat solids which are all doable with 3D printing. These materials exhibit reduced stress concentrations compared with classical elastic materials.