Experimental Shape Sensing and Load Identification on a Stiffened Panel: A Comparative Study

The monitoring of loads and displacements during service life is proving to be crucial for developing a modern Structural Health Monitoring framework. The continuous monitoring of these physical quantities can provide fundamental information on the actual health status of the structure and can accurately guide pro-active condition-based maintenance operations, thus reducing the maintenance costs and extending the service life of the monitored structures. Pushed by these needs and by the simultaneous development in the field of strain sensing technologies, several displacement reconstruction and load identification methods have been developed that are based on discrete strain measurements. Among the different formulations, the inverse Finite Element Method (iFEM), the Modal Method (MM) and the 2-step method, the latter being the only one able to also compute the loads together with the displacements, have emerged as the most accurate and reliable ones. In this paper, the formulation of the three methods is summarized in order to set the numerical framework for a comparative study. The three methods are tested on the reconstruction of the external load and of the displacement field of a stiffened aluminium plate starting from experimentally measured strains. A fibre optic sensing system has been used to measure surface strains and an optimization procedure has been performed to provide the best fibre pattern, based on five lines running along the stiffeners’ direction and with a back-to-back measuring scheme. Additional sensors are used to measure the applied force and the plate’s deflection in some locations. The comparison of the results obtained by each method proves the extreme accuracy and reliability of the iFEM in the reconstruction of the deformed shape of the panel. On the other hand, the Modal Method leads to a good reconstruction of the displacements, but also exhibits a sensitivity to the choice of the modes considered for the specific application. Finally, the 2-step approach is able to correctly identify the loads and to reconstruct the displacements with an accuracy that depends on the modeling of the experimental setup

METRIC SURVEY AND BIM TECHNOLOGIES TO RECORD DECAY CONDITIONS

The paper proposes a method able to describe, classify and organize information assets concerned with Architectural Heritage, through the use of integrated survey procedures, mainly based on Terrestrial Laser Scanner (TLS). The point clouds are then imported into the Building Information Modeling (BIM) software to start with the modeling phase. With regard to this issue, in the last period Building Information Modeling is emerging as the most reliable method to manage architectural design and building processes. Literature supplies both theoretical approaches and several practical applications. However, very little researches are devoted to BIM applied to historical architecture, even if some initial results indicate the actual HBIM (Historic/Heritage BIM) as a possible instrument for the design of an intervention aimed at the conservation of the Cultural Heritage. The focus of the research is the creation of parametric objects representing the preservation status of materials and building components: 3D modeling of decays in the BIM platform ensures to enrich the related database with graphic, geometric and alphanumeric data that can be effectively used to design and manage future interventions. The added value consists in its capability to associate new parameters that describe both the state of conservation of the materials and the detailed description of interventions needed to restore the building. The analyzed case study belongs to Ferrovie dello Stato (the main Italian Railways company) and it is part of the maintenance area, which was originally constituted by a roundhouse containing 51 sheltered railroad tracks and two big sheds

FEM modeling of a composite sandwich laminate with LS-DYNA for Aerospace applications

This project investigates how best to correctly predict the energy absorption of composite sandwich laminate under impact

C0 triangular elements based on the Refined Zigzag Theory for multilayered composite and sandwich plates

The Refined Zigzag Theory (RZT) has been recently developed for the analysis of homogeneous, multilayer composite and sandwich plates. The theory has a number of practical and theoretical advantages over the widely used First-order Shear Deformation Theory (FSDT) and other types of higher-order and zigzag theories. Using FSDT as a baseline, RZT takes into account the stretching, bending, and transverse shear deformations. Unlike FSDT, this novel theory does not require shear correction factors to yield accurate results for a wide range of material systems including homogeneous, laminated composite, and sandwich laminates. The inplane zigzag kinematic assumptions, which compared to FSDT add two additional rotation-type kinematic variables, give rise to two types of transverse shear strain measures - the classical average shear strain (as in FSDT) and another one related to the cross-sectional distortions enabled by the zigzag kinematic terms. Consequently, with a fixed number of kinematic variables, the theory enables a highly accurate modeling of multilayer composite and sandwich plates even when the laminate stacking sequence exhibits a high degree of transverse heterogeneity. Unlike most zigzag formulations, this theory is not affected by such theoretical anomalies as the vanishing of transverse shear stresses and forces along clamped boundaries. In this paper, six- and three-node, C0-continuous, RZT-based triangular plate finite elements are developed; they provide the best compromise between computational efficiency and accuracy. The element shape functions are based on anisoparametric (aka interdependent) interpolations that ensure proper element behavior even when very thin plates are modeled. Continuous edge constraints are imposed on the transverse shear strain measures to derive coupled-field deflection shape functions, resulting in a simple and efficient three-node element. The elements are implemented in ABAQUS - a widely used commercial finite element code - by way of a user-element subroutine. The predictive capabilities of the new elements are assessed on several elasto-static problems, which include simply supported and cantilevered laminated composite and sandwich plates. The numerical results demonstrate that the new RZT-based elements provide superior predictions for modeling a wide range of laminates including highly heterogeneous sandwich laminations. They also offer substantial improvements over the existing plate elements based on FSDT as well as other higher-order and zigzag-type element

Shape sensing methods: Review and experimental comparison on a wing-shaped plate

Shape sensing, i.e., the reconstruction of the displacement field of a structure from some discrete surface strain measurements, is a fundamental capability for the structural health management of critical components. In this paper, a review of the shape sensing methodologies available in the open literature and of the different applications is provided. Then, for the first time, an experimental comparative study is presented among the main approaches in order to highlight their relative merits in presence of uncertainties affecting real applications. These approaches are, namely, the inverse Finite Element Method, the Modal Method and Ko’s Displacement Theory. A brief description of these methods is followed by the presentation of the experimental test results. A cantilevered, wing-shaped aluminum plate is let deform under its own weight, leading to bending and twisting. Using the experimental strain measurements as input data, the deflection field of the plate is reconstructed using the three aforementioned approaches and compared with the actual measured deflection. The inverse Finite Element Method is proven to be slightly more accurate and particularly attractive because it is versatile with respect to the boundary conditions and it does not require any information about material properties and loading conditions

Detection and localisation of structural damage based on the Polynomial Annihilation Edge Detection: an experimental verification

This paper describes an experimental validation of a new structural damage detection method based on the Polynomial Annihilation Edge Detection (PAED) technique, a numerical method for detecting discontinuities in smooth piecewise functions and their derivatives. Since it is well known that concentrated damage such as a crack, causes a discontinuity in the rotations and consequently in the first derivatives of the mode shapes and in general of the deflection shapes of beam-like structures, the PAED can be applied to the problem of damage detection and localisation in these kind of structures for which only post-damage vibration measurements are available. As described in this paper, in order to verify this approach experimentally, vibration tests on a cantilever steel beam with a saw-cut have been performed and Operational Deflection Shapes (ODSs) determined. Since the approach requires a reasonably high spatial resolution of the ODSs, a scanning laser vibrometer, capable of acquiring data rapidly at a very large number of observation points, was used

Dal rilievo metrico all'HBIM per l'analisi dello stato di conservazione della fabbrica

The paper proposes a method able to describe, classify and organize information assets concerned with Architectural Heritage, through the use of integrated survey procedures, mainly based on Terrestrial Laser Scanner (TLS). The point clouds have been imported into the Building Information Modelling (BIM) software to start with the modelling phase. The focus of the research is the creation of a parametric object representing the preservation status of materials and building components: 3D modelling of decays in the BIM platform ensures to enrich the related database with graphic, geometric and alphanumeric data that can be effectively used to design and manage future interventions

Bender elements: bad source - good receiver

Shear wave velocity measurement in the laboratory, by means of Bender Elements is becoming more and more popular for geotechnical characterization. The reasons of the increasing popularity of such method are well known and among the many the “apparent” simplicity of the testing method should be mentioned. Anyway, several researchers have pointed out the difficulties in interpreting test results. Difficulties are mainly due to the fact that real test conditions mismatch the given assumptions. The paper clearly shows, using laser measurements, that a source-bender, excited by a sine wave, oscillates non-symmetrically showing a waveform very different than the electrical input. This make questionable the possible use of cross-correlation (referring to the input waveform). On the other hand the source-bender does not bend as a cantilever when using high input frequencies. Under such circumstances, deformation mode of the BE becomes very complex and a very complex wave field is generated. Numerical simulations clearly show what above indicated. The authors conclude suggesting the use of BE only as receivers; generation of the wave motion should be accomplished with other means

Application of wave propagation to pyroshock analysis

Pyroshocks are high frequency transients due to pyrotechnic devices used in aerospace engineering in order to deploy solar arrays and antennae, separate subsystems from the spacecraft or separate the spacecraft itself from the base stage booster; their prediction is usually complex and very time consuming. The aim of this article is to investigate on the application of the analysis of transmission of waves inelastic media in order to predict the dynamic response to pyroshocks. The work is completed by numerical examples, related to components of common use in the aerospace engineering field, showing the comparison between results obtained by using both MSC-NASTRAN and this novel application of wave propagation analysis