This work aims at presenting techniques for the damage identification in single lap joints (SLJs). The two proposed experimental approaches, exploiting particular interactions of the structure with vibrational waves produced by piezoelectric sensors, allow to perform a Structural Health Monitoring (SHM) without a baseline. The first technique involves the excitation of the structure by means of stationary sinusoidal waves: the presence of a subharmonic in the frequency response spectrum at a receiver point indicates the presence of damage in the joint. In addition, through a simplified analytical model it is possible to relate the frequency of this subharmonic to the size of the damage. The second technique is based on the use of a tone burst: the exciting sensor sends this transient signal that travels through the bonded area and is subsequently read by the receiving sensor; the information received is the result of an interaction between the sent wave and the reflection of the boundaries, sensitive to possible damages. The attenuation of the burst, studied through the wave equations, gives indications on the size of the damage. Both experimental campaigns were carried out on aluminum SLJs bonded with acrylic adhesive, using piezoelectric sensors (one exciting and one receiving). Simplified analytical models were used to validate the experimental results. The good analytical-experimental correlation confirms the validity of the proposed approaches.
This work is concerned with the mechanical characterization of bistable composite plates in order to investigate their nonlinear behavior dependence on mechanical factors, e.g. strain, stress trends and potential energy. The bistable laminates have two stable shapes that are actuated by a variety of mechanisms (piezoelectric ceramic based actuators, shape memory alloys or thermal actuation) to induce “snap-through” between states. These composite structures are receiving interest in several aeronautic applications such as shape changing applications without the need of servoactivated control systems. Scope of the work is to describe the “0” strain-stress status of the asymmetric bistable laminates, immediately after the manufacturing process. An experimental testing is carried out with the purpose of collecting enough data for the numerical and analytical analyses. Numerical simulations based on Finite Element Models (FEM) are used to study strain and stress fields of the laminates and successively to validate semi-analytical results. By the Classic Plate Lamination Theory (CLPT), an analytical model is developed to provide an interpretation of the bistability phenomenon. The experimental results, with FE and CLPT models, help to understand the relation between the mechanical features of the composite laminate and the bistability phenomenon. This paper reports on detailed nonlinear characterization of bistable plates using numerical, analytical and experimental data in order to provide a starting point for future works characterizing bistable strain-stress evolution over the time.
This study is concerned with the activation energy threshold of bistable composite plates in order to tailor a bistable
system for specific aeronautical applications. The aim is to explore potential configurations of the bistable plates and
their dynamic behavior for designing novel morphing structure suitable for aerodynamic surfaces and, as a possible
further application, for power harvesters. Bistable laminates have two stable mechanical shapes that can withstand
aerodynamic loads without additional constraint forces or locking mechanisms. This kind of structures, when properly
loaded, snap-through from one stable configuration to another, causing large strains that can also be used for power
harvesting scopes. The transition between the stable states of the composite laminate can be triggered, in principle,
simply by aerodynamic loads (pilot, disturbance or passive inputs) without the need of servo-activated control systems.
Both numerical simulations based on Finite Element models and experimental testing based on different activating
forcing spectra are used to validate this concept. The results show that dynamic activation of bistable plates depend on
different parameters that need to be carefully managed for their use as aircraft passive wing flaps.
This paper is aimed at developing a theoretical model able to predict the generation of nonlinear elastic effects associated to the interaction of ultrasonic waves with the steady-state nonlinear response of local defect resonance (LDR). The LDR effect is used in nonlinear elastic wave spectroscopy to enhance the excitation of the material damage at its local resonance, thus to dramatically increase the vibrational amplitude of material nonlinear phenomena. The main result of this work is to prove both analytically and experimentally the generation of novel nonlinear elastic wave effects, here named as nonlinear damage resonance intermodulation, which correspond to a nonlinear intermodulation between the driving frequency and the LDR one. Beside this intermodulation effect, other nonlinear elastic wave phenomena such as higher harmonics of the input frequency and superharmonics of LDR frequency were found. The analytical model relies on solving the nonlinear equation of motion governing bending displacement under the assumption of both quadratic and cubic nonlinear defect approximation. Experimental tests on a damaged composite laminate confirmed and validated these predictions and showed that using continuous periodic excitation, the nonlinear structural phenomena associated to LDR could also be featured at locations different from the damage resonance. These findings will provide new opportunities for material damage detection using nonlinear ultrasounds.
In this study the amplitude and the phase of the structural response of samples of Single Lap Joint (SLJ) subjected to ultrasonic harmonic excitation was evaluated to identify and characterize the defects within the bonded region. Different parameters such as frequency, shape, and amplitude of the response signal coming back from the adhesive joint are key criteria for understanding the quality of the adhesion. Different metallic samples with the same geometry were experimentally tested: the defects were artificially introduced bonding partially two plates and changing the extension of the debonded region: two piezoelectric sensors (one exciting, one receiver) were attached on each of the two bonded plates. In this way, different experimental tests were carried out in order to study the influence of debonded regions on SLJ structural behavior. The structural dynamic response of the debonded samples was investigated and compared with the predictions of numerical models, for each SLJ, introducing viscoelastic properties for the adhesive layer, and applying the harmonic excitation. Moreover the numerical modal analysis was used to understand the experimental results by a proper description of viscoelastic tape behavior. The numerical simulations were used to find correlation between the content of the acquired signals and the defects of adhesion.
The coupling between structural support and protection makes biological systems an important source of inspiration for the development of advanced smart composite structures. In particular, some particular material configurations can be implemented into traditional composites in order to improve their impact resistance and the out-of-plane properties, which represents one of the major weakness of commercial carbon fibres reinforced polymers (CFRP) structures. Based on this premise, a three-dimensional twisted arrangement shown in a vast multitude of biological systems (such as the armoured cuticles of Scarabei, the scales of Arapaima Gigas and the smashing club of Odontodactylus Scyllarus) has been replicated to develop an improved structural material characterised by a high level of in-plane isotropy and a higher interfacial strength generated by the smooth stiffness transition between each layer of fibrils. Indeed, due to their intrinsic layered nature, interlaminar stresses are one of the major causes of failure of traditional CFRP and are generated by the mismatch of the elastic properties between plies in a traditional laminate. Since the energy required to open a crack or a delamination between two adjacent plies is due to the difference between their orientations, the gradual angle variation obtained by mimicking the Bouligand Structures could improve energy absorption and the residual properties of carbon laminates when they are subjected to low velocity impact event. Two different bioinspired laminates were manufactured following a double helicoidal approach and a rotational one and were subjected to a complete test campaign including low velocity impact loading and compared to a traditional quasi-isotropic panel. Fractography analysis via X-Ray tomography was used to understand the mechanical behaviour of the different laminates and the residual properties were evaluated via Compression After Impact (CAI) tests. Results confirmed that the biological twisted structures can be replicated into traditional layered composites and are able to enhance the out-of-plane properties without a dangerous degradation of the in-plane properties.
SMArt Thermography exploits the electrothermal properties of multifunctional smart structures, which are created by embedding shape memory alloy (SMA) wires in traditional carbon fibre reinforced composite laminates (known as SMArt composites), in order to detect the structural flaws using an embedded source. Such a system enables a built-in, fast, cost-effective and in-depth assessment of the structural damage as it overcomes the limitations of standard thermography techniques. However, a theoretical background of the thermal wave propagation behaviour, especially in the presence of internal structural defects, is needed to better interpret the observations/data acquired during the experiments and to optimise those critical parameters such as the mechanical and thermal properties of the composite laminate, the depth of the SMA wires and the intensity of the excitation energy. This information is essential to enhance the sensitivity of the system, thus to evaluate the integrity of the medium with different types of damage. For this purpose, this paper aims at developing an analytical model for SMArt composites, which is able to predict the temperature contrast on the surface of the laminate in the presence of in-plane internal damage (delamination-like) using pulsed thermography. Such a model, based on the Green’s function formalism for one-dimensional heat equation, takes into account the thermal lateral diffusion around the defect and it can be used to compute the defect depth within the laminate. The results showed good agreement between the analytical model and the measured thermal waves using an infrared (IR) camera. Particularly, the contrast temperature curves were found to change significantly depending on the defect opening.
Nonlinear ultrasonic non-destructive evaluation (NDE) methods can be used for the identification of defects within adhesive bonds as they rely on the detection of nonlinear elastic features for the evaluation of the bond strength. In this paper the nonlinear content of the structural response of a single lap joint subjected to ultrasonic harmonic excitation is both numerically and experimentally evaluated to identify and characterize the defects within the bonded region. Different metallic samples with the same geometry were experimentally tested in order to characterize the debonding between two plates by using two surface bonded piezoelectric transducers in pitch-catch mode. The dynamic response of the damaged samples acquired by the single receiver sensor showed the presence of higher harmonics (2nd and 3rd) and subharmonics of the fundamental frequencies. These nonlinear elastic phenomena are clearly due to nonlinear effects induced by the poor adhesion between the two plates. A new constitutive model aimed at representing the nonlinear material response generated by the interaction of the ultrasonic waves with the adhesive joint is also presented. Such a model is implemented in an explicit FE software and uses a nonlinear user defined traction-displacement relationship implemented by means of a cohesive material user model interface. The developed model is verified for the different geometrical and material configurations. Good agreement between the experimental and numerical nonlinear response showed that this model can be used as a simple and useful tool for understanding the quality of the adhesive joint.
Literature offers a quantitative number of diagnostic methods that can continuously provide detailed information of the
material defects and damages in aerospace and civil engineering applications. Indeed, low velocity impact damages can
considerably degrade the integrity of structural components and, if not detected, they can result in catastrophic failure
conditions. This paper presents a nonlinear Structural Health Monitoring (SHM) method, based on ultrasonic guided
waves (GW), for the detection of the nonlinear signature in a damaged composite structure. The proposed technique,
based on a bispectral analysis of ultrasonic input waveforms, allows for the evaluation of the nonlinear response due to
the presence of cracks and delaminations. Indeed, such a methodology was used to characterize the nonlinear behaviour
of the structure, by exploiting the frequency mixing of the original waveform acquired from a sparse array of sensors.
The robustness of bispectral analysis was experimentally demonstrated on a damaged carbon fibre reinforce plastic
(CFRP) composite panel, and the nonlinear source was retrieved with a high level of accuracy. Unlike other linear and
nonlinear ultrasonic methods for damage detection, this methodology does not require any baseline with the undamaged
structure for the evaluation of the nonlinear source, nor a priori knowledge of the mechanical properties of the specimen.
Moreover, bispectral analysis can be considered as a nonlinear elastic wave spectroscopy (NEWS) technique for
materials showing either classical or non-classical nonlinear behaviour.