Paolo Bettini

Carbon Fiber Reinforced Smart Laminates with Embedded SMA Actuators—Part I: Embedding Techniques and Interface Analysis

Up to now one of the main limits for a large use of shape memory alloys (SMA)-based smart composite structures in the aerospace industry is the lack of useful numerical tools for design. Moreover, technological aspects still need a more detailed investigation. This paper shows how to overcome issues regarding embedding of NiTiNOL wires in carbon fibre/epoxy laminates. A crucial aspect of those structures is related to the load transfer capabilities between the SMA actuators and the host material during their activation. Embedding techniques developed for taking into account problems like thermal and electrical compatibility between actuators and host material and passive/active invasivity are reported in this paper. Simple smart laminates with several actuators were manufactured, tested, and deeply analyzed. In order to characterize the interface in the real operative conditions, pull-out tests were conducted on NiTiNOL wires embedded in composite fiber laminates. The results were compared to standard experiments on wires embedded in pure epoxy resin blocks.

Fused Deposition Technique for Continuous Fiber Reinforced Thermoplastic

A simple technique for the production of continuous fiber reinforced thermoplastic by fused deposition modeling, which involves a common 3D printer with quite limited modifications, is presented. An adequate setting of processing parameters and deposition path allows to obtain components with well-enhanced mechanical characteristics compared to conventional 3D printed items. The most relevant problems related to the simultaneous feeding of fibers and polymer are discussed. The properties of obtained aramid fiber reinforced polylactic acid (PLA) in terms of impregnation quality and of mechanical response are measured.

Design, manufacturing and characterization of aero-elastically scaled wind turbine blades for testing active and passive load alleviation techniques within a ABL wind tunnel

In the research described in this paper, a scaled wind turbine model featuring individual pitch control (IPC) capabilities, and equipped with aero-elastically scaled blades featuring passive load reduction capabilities (bend-twist coupling, BTC), was constructed to investigate, by means of wind tunnel testing, the load alleviation potential of BTC and its synergy with active load reduction techniques. The paper mainly focus on the design of the aero-elastic blades and their dynamic and static structural characterization. The experimental results highlight that manufactured blades show desired bend-twist coupling behavior and are a first milestone toward their testing in the wind tunnel.

An efficient approach for modeling interlaminar damage in composite laminates with explicit finite element codes

A computationally efficient technique for the analysis of delamination in composite structures with the explicit finite element method is proposed. In the approach, the in-plane and the out-of-plane stiffness of a composite laminate are represented by different material phases. The laminate is seen as a stack of sub-laminates modeled by 2D elements that carry the in-plane stress components. These sub-laminates are connected together with three-dimensional elements that carry the out-of-plane stress components and account for the transverse shear stiffness. Cohesive zone models are adapted to represent the interlaminar damage within the connection elements. The principal advantage of this hybrid modeling technique is that interlaminar damage can be modeled without introducing zero-thickness cohesive elements, which are characterized by means of very large penalty stiffnesses. Consequently, the material stiffness is specified according to physical considerations and, therefore, stable time steps in explicit analyses are not affected by non-physical stiffnesses. The theoretical aspects of the modeling technique and the assessment of a laminate’s model are introduced. Then, three different test cases are considered to confirm that the correct kinematic response is achieved and to demonstrate that sequences of complex delamination events can be predicted accurately and efficiently.

Development and Experimental Validation of a Numerical Tool for Structural Health and Usage Monitoring Systems Based on Chirped Grating Sensors

The interest of the aerospace industries in structural health and usage monitoring systems is continuously increasing. Among the techniques available in literature those based on Fibre Bragg Grating sensors are much promising thanks to their peculiarities. Different Chirped Bragg Grating sensor configurations have been investigated in this paper. Starting from a numerical model capable of simulating the spectral response of a grating subjected to a generic strain profile (direct problem), a new code has been developed, allowing strain reconstruction from the experimental validation of the program, carried out through different loading cases applied on a chirped grating. The wavelength of the reflection spectrum for a chirped FBG has a one-to-one correspondence to the position along the gauge section, thus allowing strain reconstruction over the entire sensor length. Tests conducted on chirped FBGs also evidenced their potential for SHM applications, if coupled with appropriate numerical strain reconstructions tools. Finally, a new class of sensors—Draw Tower Grating arrays—has been studied. These sensors are applicable to distributed sensing and load reconstruction over large structures, thanks to their greater length. Three configurations have been evaluated, having different spatial and spectral characteristics, in order to explore possible applications of such sensors to SHM systems.

Design and experimental characterization of a NiTi-based, high-frequency, centripetal peristaltic actuator

Development and experimental testing of a peristaltic device actuated by a single shape-memory NiTi wire are described. The actuator is designed to radially shrink a compliant silicone pipe, and must work on a sustained basis at an actuation frequency that is higher than those typical of NiTi actuators. Four rigid, aluminum-made circular sectors are sitting along the pipe circumference and provide the required NiTi wire housing. The aluminum assembly acts as geometrical amplifier of the wire contraction and as heat sink required to dissipate the thermal energy of the wire during the cooling phase. We present and discuss the full experimental investigation of the actuator performance, measured in terms of its ability to reduce the pipe diameter, at a sustained frequency of 1.5 Hz. Moreover, we investigate how the diameter contraction is affected by various design parameters as well as actuation frequencies up to 4 Hz. We manage to make the NiTi wire work at 3% in strain, cyclically providing the designed pipe wall displacement. The actuator performance is found to decay approximately linearly with actuation frequencies up to 4 Hz. Also, the interface between the wire and the aluminum parts is found to be essential in defining the functional performance of the actuator.

Failure and energy absorption of plastic and composite chiral honeycombs

This paper investigates the crushing response of honeycombs having a chiral geometry, a non centre-symmetric topology made of circular cylinders connected by ligaments. Buckling and post-buckling responses of flatwise compressed plastic chiral honeycombs are numerically investigated. Results indicate that plastic chiral honeycombs can carry increasing loads beyond local buckling of ligaments, though the post-buckling response does not lead to high energy absorption performances. The characteristics of chiral topology can be exploited by adopting composite materials. The composite units produced by using a specific technological process presented in the paper and endowed with a bevel trigger failed by progressive crushing and obtained promising specific energy absorption levels.

Carbon Fiber-Reinforced Smart Laminates with Embedded SMA Actuators—Part II: Numerical Models and Empirical Correlations

Up to now one of the main limitations for a large use of shape memory alloys (SMA)-based smart composite structures in the aerospace industry is the lack of useful numerical tools for design; in addition, some technological aspects still need a more detailed investigation. This article shows numerical modeling approaches adopted for the implementation of SMA constitutive laws in commercial codes such as ABAQUS. Two different approaches were selected. The first one is based on the thermomechanical model proposed by Turner and the other one follows the thermodynamic macromechanical constitutive law developed by Lagoudas. The implementation in ABAQUS code was followed by a procedure to evaluate model parameters and to experimentally validate the reliability of code predictions for specifically designed test situations. This article presents the test campaign carried out for the definition of these parameters and the numerical-experimental correlation for both the models.

Monito-Ring: An original fiber optic system for morphing application

An original Monito-Ring system based on chirped fiber optic and draw tower grating array is presented. The target of this research activity is the realization of a device able to measure deformations of morphing structures which may show large, global displacements due to nonstandard architectures and materials adopted. The occurring strain field results, in turns, much more than the standard sensors can handle. Modulations are then necessary to keep the measured strain low. The proposed solution was conceived to overcome this limitation assuring a suitable reduction of the revealed strain. The concept is made of a flexible ring pinned on a certain number of points to the structural component of interest. The fiber optic is integrated within the ring, and depending on the angular position of the sensor, the ratio between the diameter elongation (i.e. structural strain) and the measured deformation (strain) can be almost arbitrarily set in a large range of values. From each spectrum provided by draw tower grating array, the corresponding unknown strain field is retrieved by applying an inverse technique obtaining an accurate continuous strain map. This article deals with a proof of concept analytical study first and then numerical and experimental validation.

Ribbon Tapes, Shape Sensors, and Hardware

For fiber optic sensors to be integrated in aerospace structures and for these sensors to provide data that can be used for structural health monitoring (SHM), development work has been performed in the SARISTU project. This particular chapter describes the inclusion of fiber optic sensors in a tape format, called ribbon tape, for damage detection and load monitoring with a secondary bonding procedure to install these in an aerospace structure. The ribbon tapes can be installed on composite structures either by co-bonding or secondary bonding techniques. Both procedures have been applied to composite coupons that have been tested in different environmental conditions. Moreover, a damaged tape repair procedure is developed. This chapter also describes the development of a fiber optic sensor to be used as a shape sensor inside a morphing aerospace structure to provide feedback on the structures shape. The hardware required to analyze and format the sensor readings for use in SHM is also provided in this chapter.