unisa ITA  unisa ENG

PRIN 2017: Multiscale Innovative Materials and Structures - MIMS19

Work Packages

Inspired by nature, where multiscale materials and structures are ubiquitous, MIMS is aimed at deepening the fundamental understanding of the mechanical response of highly nonlinear lattice materials and nanomaterials and exploiting such knowledge for the creation of new engineering materials and devices. By building from well-studied local phenomena (e.g., the nonlinear response of tensegrity building blocks or nanostructures), the project will create complex global systems (the metamaterials) in 1-D, 2-D, and 3-D arrangements, at different scales. These systems will exhibit unprecedented mechanical properties that are tunable by local and global prestress. Additionally, carbon-derived nanomaterials, such as fullerenes and CNTs, will be employed as nano-enhancers in new-generation nanocomposites with advanced mechanical properties. The above research goals will be accomplished through an integrated design-modeling-build-testing procedure in the following Work Packages (WPs):

i) WP1 - "D&M": Design and modeling of innovative materials and structures at different scales;
ii) WP2 - "ENG": Development of novel engineering applications of multiscale materials and structures;
iii) WP3 - "CTR": Formulation and implementation of structural control and health monitoring techniques for next-generation engineering materials and structures.

The main goals and methods of the different WPs are as follows.

i) WP1 - "D&M": Design and modeling of innovative materials and structures at different scales.
A challenging way to manufacture unconventional materials, which are able to fill holes in the current material property charts, is to control the architecture of the material, so as to optimally combine material and space, forming ground-breaking materials with unmatched properties [11]-[15]. The focus of the D&M WP is on novel lattice materials and nanostructures that show arbitrarily nonlinear mechanical response at different scales. These materials will open up new horizons in the multidisciplinary area of multiscale mechanics, due to the following innovative features: the employment of optimized  architectures at different scales with nonlinear mechanical response that are tunable by local (or internal) and global (or external) prestress; the achievement of extreme properties over more than several orders of magnitude in density; and the possibility to match arbitrary, user-defined nonlinear constitutive laws. The nonlinear response of the material will descend from geometric and/or mechanical nonlinearities, such as large displacements, multistable response, fracture, inelastic/plastic deformation, and viscoelasticity.

Recent exciting numerical and experimental results obtained by MIMS researchers regarding macro- and small-scale lattice materials (see, e.g., [13]-[20], [23]-[25], [30]-[35]), will be generalized to two- and three-dimensions (cf. Fig. 4). The D&M phase will use fractal shapes to design tensegrity structures inspired by nature's geometries [16]-[20] through a finite or infinite number of self-similar subdivisions of basic modules. The examined lattice structures will be either freestanding or combined with a support matrix that will be typically (but not exclusively) made of soft media, e.g., plastics and rubber-like materials either in their virgin state or recycled. 

Additionally, novel nanocomposites will be designed by using carbon-derived nanomaterials such as nanotubes, fullerenes, and graphene, and/or additively manufactured particles, fibers or fabrics as fillers. The D&M WP of such materials will take advantage of the tensegrity modeling of fullerenes [10], and the advanced modeling of nanostructures developed by MIMS researchers in recent years [30]-[35]. The modeling of polymeric and cementitious multiscale composites reinforced with micro- and/or nano-scale reinforcing elements (MNREL) will be performed through multiscale approaches. A basic step will consist in modeling the mechanical behavior of the reinforcing
elements, through generalization and refinement of stress-driven, nonlocal models developed by MIMS researchers in recent years [69]-[71]. Next, the D&M WP will focus on the design and modeling of multiscale composites, which show polymeric, geopolymeric or cementitious matrices reinforced with MNRELs [70]-[75]. 

The D&M phase will also employ quasicontinuum models of periodic systems with different arrangements of soft and hard units [3]. More complex nonlinear behaviors in 1D, and 2D, and 3D composite materials will be designed and analyzed by computation through adaptive finite element approaches, nonlinear homogenization techniques (NHOM), molecular dynamics simulations, and/or mixed discrete-continuum methods developed by members of the MIMS consortium, with the aim of achieving enhanced performances in terms of cost and weight reduction, and multi-functional behavior. The adopted numerical approaches will be able to capture microstructure evolution (due, e.g., to debonding, cracking, instability, contact and plasticity [44],[47]-[50],[55]-[57], and damage growth taking place at multiple scales (delamination, damage, fracture) (MULTIDAM)

ii) WP2 - "ENG": Development of novel engineering applications of multiscale materials and structures. 

The research area of linear and weakly nonlinear wave dynamics has devoted much attention to socalled "phononic band gap" theory [1], which extends the previously investigated theory of photonic band gaps. It has been shown that composite materials with periodic variations in density and/or wave velocity can display band gaps where the propagation of mechanical waves is forbidden. A first goal of the ENG WP aims to reveal band gaps in tensegrity-based metamaterials and to exploit the possibility of their tuning for the design and test of novel waveguides, sound proof layers and vibration protection devices. Building on established results for granular crystals, such a line of the ENG WP will initially focus on the optimal design of 1D, 2D, and 3D bandgap metamaterials formed by tensegrity units and lumped masses, which will be tunable by varying the unit's parameters for both the initial static precompression of the constituent units (internal selfstress) and the whole structure (external prestress). Additionally, locally resonant mechanisms generated by the use of composite, soft-hard materials for the lattice members and the junctions will be introduced, with the aim of achieving wave propagation control at low frequencies (~100 Hz) and large wavelengths, while using small-scale lattices [6].

An additional line of investigation of the ENG WP is concerned with the development of innovative vibration control devices, exploiting their use in conjunction with the aforementioned artificial materials as a cutting-edge technology for structural base-isolation (EVIB task). This research will be based upon recent studies [27]- [28] showing the beneficial effect of novel passive control devices, such as tuned liquid column dampers (TLCDs) and multi-mass dampers [36], for vibration mitigation of flexible and base-isolated systems. The high tunability and easy installation of TLCD devices will be exploited, and novel multi-mass dampers will be developed for vibration control, through
generalization of classical single-mass dampers. Enhanced vibration-attenuation systems will also be obtained via inclusion of fractional viscoelastic connections between the masses forming the device [37]-[43]. The tuning of multiple resonances will permit the development of metamaterials featuring wave-filtering properties over a wide frequency range [67][68].

A further goal of the ENG WP is concerned with the use of carbon nanotubes, lattices and/or particles with hierarchical structure to form fillers, particles, fibers and fabrics for the reinforcement of novel, groundbreaking composites. Fullerenes, nanotubes and carbon nanostructures will be used for the reinforcement of polymeric-, geopolymeric- or cement-matrix composites (Fig. 5a), as well as for the coating of multiscale glass or carbon fibers [72]-[73]. In addition, particles and fibers with structural hierarchy originating from their geometric design will be directly manufactured from computer-aided design data, employing additive manufacturing techniques based on polymeric and/or metallic materials, at different scales [4]-[5] (cf. Fig. 5b,c). The use of such reinforcements will lead to obtain advanced composites with enhanced interlaminar shear strength (ILSS), 
and improved overall strength and fracture toughness [72]-[73]. 

Novel additive manufacturing (AM) techniques will be employed in the manufacturing (MANU) phase to build prototypes of the engineered materials. Macro- and small-scale models may be fabricated, for example, via multi-jet technologies using materials with different coefficients of thermal expansion for struts and cables, in order to create internal self-stress. Microlattices will be manufactured through a projection micro-stereolithography (PμSL) system [29], that is built in-house and which employs swelling materials for the tensile members (Fig. 2a). Fibers and particles covered with microscopic lattices will be fabricated through PμSL, while metallic reinforcing elements with 100-500 um resolution will be manufactured using the EOSINT M 270 system for the additive manufacturing of metal parts that is available for use at UNISA. Multiscale plasma irradiation techniques for manufacturing hierarchical modifications of the surface of the reinforcing elements will also be applied [77]. Carbon nanotubes will be synthesized and  functionalized at the Nanomates Research Center of the University of Salerno.
The TEST phase of small-scale systems will employ an experimental setup composed of three parts: a structural support for a sample, a wave field generator, and sensors to measure the input and output signals. A high-resolution CCD camera will be employed to track the motions of the lattice elements. Tensile tests, short-beam shear tests, and bending tests will be employed to characterize the mechanical properties of the composite materials delivered by the project, using scanning electron microscopy to investigate on the morphologies of the reinforcing elements and the fracture surfaces, before and after test [72][73][78]. Large-scale tests will employ an in-house built testing system equipped with a 500 kN dynamic actuator (+/-500mm stroke).

ii) WP3 - "CTR": Formulation and implementation of structural control and health monitoring techniques for next-generation engineering materials and structures.

The present WP deals with the formulation and implementation of novel tools and methods for the control and structural health monitoring (SHM) of innovative materials and structures at different scales. By employing cutting-edge techniques based on the Operational Modal Analysis (OMA) [58]-[60], [65], the CTR WP will develop innovative procedures for the fast SHM of the materials and structures developed within the ENG WP. Quasi-real-time SHM procedures will be formulated through the fast processing of the vibration data collected when the tested material or structure is under operation. Furthermore, novel and efficient control systems will be studied and tested with focus on the materials and structures delivered by the project.

OMA will be employed to identify the modal properties of materials and structures based on vibration data collected when these systems are under their operating conditions; that is, no initial conditions or known artificial external excitations will be required. The CTR WP aims at developing an innovative OMA method exploiting the properties of the Hilbert transform, in order to obtain the analytical representation of the system response in terms of the so-called correlation function. Notably, this function is particularly useful for a correct representation of the system response in a probabilistic framework, which naturally arises when dealing with unknown external excitations that can be assumed as stochastic processes. This procedure will be therefore applied with particular focus on the innovative materials and structures at different scales developed in the ENG WP, thus leading to an accurate and reliable tool for a fast non-destructive identification of investigated materials properties, as well as for the early damage detection in these novel systems.

A first implementation of the control strategies developed by the current WP regards the optimal tuning of the next-generation vibration attenuation devices delivered by the ENG WP. It is worth mentioning that the trend towards the use of materials with enhanced mechanical properties, together with the application of modern computer methods for the design of engineering systems, have led to the construction of increasingly flexible and vibration prone structures. The CTR WP will investigate the optimal tuning of local/global prestress variables and multiple resonances effects in vibration-isolation devices coupling lattice metamaterials with passive control devices [27]-[28]. Specifically, the potential use of cutting-edge technologies in vibration control, such as the so-called inerters [66], will be exploited for achieving higher control performances for TLCD and related control systems. The inerter is a two-terminal flywheel device developing resisting forces proportional to the relative acceleration of its terminals, thus developing a beneficial "mass amplification effect". This research will be founded upon the preliminary analytic and/or numerical studies [66] that have shown how inerter devices are particularly promising tools for enhancing the performance of vibration control systems. The vibration attenuation devices developed by the ENG WP will be implemented and experimentally tested to reduce the oscillations induced by wind-wave loads in floating offshore wind turbines, with the aim of making the floating concept an economically viable alternative to classical onshore installation [61],[62]. To this end, a prototype of a multi-mass damper will be developed and applied on a 1:30 scale floating-spar support for offshore wind turbines, installed directly in sea water at the NOEL Laboratory, University of Reggio Calabria [63],[64]. In addition, the CTR WP will develop diffuse SHM techniques for civil structures and historical building heritage, through application of CNT enriched cement composites on the surface of such structures. Such a research line will exploit the strain-sensing ability of the embedded CNTs, which is due to the variation of their electrical properties under mechanical loading. The mechanism of strain control through electric fields in electro-active polymer (EAP) elements will be also considered [79]. Finally, novel optimization techniques will be developed for the control of biomedical devices; bio-inspired damage-tolerant materials for structural strengthening and retrofitting; and multifunctional nanostructured materials (such as self-healing [55], self-cleaning and self-assembling components, sensors and actuators, vibration damping and energy harvesting devices).