Crack detection in elastic beams by static measurements

AbstractThis paper deals with the identification of a single crack in a beam based on the knowledge of the damage-induced variations in the static deflection of the beam. The crack is simulated by an equivalent linear spring connecting the two adjacent segments of the beam. Sufficient conditions on static measurements which allow for the unique identification of the crack are presented and discussed. The inverse analysis provides exact closed-form expressions of position and severity of the crack as functions of deflection measurements for different boundary conditions. The theoretical results are confirmed by a comparison with static measurements on steel beams with a crack. Extension of the presented analysis to multiple cracks is briefly discussed

A procedure for the identification of multiple cracks on beams and frames by static measurements

In this work, a model of the Euler-Bernoulli beam in presence of multiple-concentrated open cracks, based on the adoption of a localized flexibility model, is adopted. The closed-form solution in terms of transversal displacements due to static loads and general boundary condition is exploited to propose an inverse damage identification procedure. The proposed identification procedure does not require any solution algorithm, on the contrary is formulated by means of simple explicit sequential expressions for the crack positions and intensities including the identification of the integration constants. The number of possible detected cracks depends on the couples of adopted sensors. Undamaged beam zones can also be easily detected in relation to the sensor positions. The analytical character of the explicit expressions of the identification procedure makes the inverse formulation applicable to damaged beams included in more complex frame structures. The proposed procedure is applied for the identification of the number, position, and intensity of the cracks along simple straight beams and also to more complex frame structures with the aim of showing its simplicity for engineering applications. In addition, the robustness of the methodology here described is shown through an accurate analysis of the basic assumptions on which the theory relies and by means of a study of the effect of noise on the identification results

New Frontiers on Seismic Modeling of Masonry Structures

An accurate evaluation of the non-linear behavior of masonry structural elements in existing buildings still represents a complex issue that rigorously requires non-linear finite element strategies difficult to apply to real large structures. Nevertheless, for the static and seismic assessment of existing structures, involving the contribution of masonry materials, engineers need reliable and efficient numerical tools, whose complexity and computational demand should be suitable for practical purposes. For these reasons, the formulation and the validation of simplified numerical strategies represent a very important issue in masonry computational research. In this paper, an innovative macroelement approach, developed by the authors in the last decade, is presented. The proposed macroelement formulation is based on different, plane and spatial, macroelements for the simulation of both the in-plane and out-of-plane behavior of masonry structures also in presence of masonry elements with curved geometry. The mechanical response of the adopted macroelement is governed by non-linear zero-thickness interfaces, whose calibration follows a straightforward fiber discretization, and the non-linear internal shear deformability is ruled by equivalence with a corresponding geometrically consistent homogenized medium. The approach can be considered as "parsimonious" since the kinematics of the adopted elements is controlled by very few degrees of freedom, if compared to a corresponding discretization performed by using non-linear finite element method strategies. This innovative discrete element strategy has been implemented in two user-oriented software codes 3DMacro (Caliò et al., 2012b) and HiStrA (Historical Structures Analysis) (Caliò et al., 2015), which simplify the modeling of buildings and historical structures by means of several wizard generation tools and input/output facilities. The proposed approach, that represents a powerful tool for the structural assessment of structures in which the masonry plays a key role, is here validated against experimental results involving typical masonry monumental substructural elements and numerical results involving real-scale structures

Light Exposure Effects on the DC Kink of AlGaN/GaN HEMTs

This paper presents the effects of optical radiation on the behavior of two scaled-gate aluminum gallium nitride/gallium nitride (AlGaN/GaN) high electron mobility transistors (HEMTs). The tested devices, having a gate width of 100 and 200 µm and a gate length of 0.25 µm, were exposed to a laser beam with a wavelength of 404 nm (blue-ray) in order to investigate the main optical effects on the DC characteristics. Owing to the threshold shift and the charge generation, a marked increase of the gate and drain current was noticed. The occurrence of the kink effect in the absence of light exposure was identified, and a hypothesis about its origin is provided. The obtained results agree with the analysis previously carried out on gallium arsenide (GaAs)-based devices

A Review of Simplified Numerical Beam-like Models of Multi-Storey Framed Buildings

Modern computational techniques have greatly influenced the numerical analyses of structures, not only in terms of calculation speed, but also in terms of procedural approach. In particular, great importance has been given to structural modelling, that is, the process by which a structure and the actions to which it is subjected are reduced to a simplified scheme. The use of a simplified calculation scheme is necessary since the structures are, in general, considerably complex physical systems whose behaviour is influenced by a large number of variables. The definition of a structural scheme that is at the same time simple enough to be easily computable as well as sufficiently reliable in reproducing the main characteristics of the behaviour of the analysed structure is, therefore, a crucial task. In particular, with reference to multi-storey framed buildings, the extensive use of three-dimensional finite element models (FEM) has been made in recent decades by researchers and structural engineers. However, an interesting and alternative research field concerns the possibility of studying multi-storey buildings through the use of equivalent beam-like models in which the number of degrees of freedom and the required computational effort are reduced with respect to more demanding FEM models. Several researchers have proposed single or coupled continuous beams to simulate either the static or dynamic response of multi-storey buildings assuming elastic or inelastic behaviour of the constitutive material. In this paper, a review of several scientific papers proposing elastic or inelastic beam-like models for the structural analyses of framed multi-storey buildings is presented. Considerations about limits and potentialities of these models are also included

A Fibre Smart Displacement Based (FSDB) beam element for the nonlinear analysis of reinforced concrete members

Beam finite elements for non linear plastic analysis of beam-like structures are formulated according to Displacement Based (DB) or Force Based (FB) approaches. DB formulations rely on modelling the displacement field by means of displacement shape functions. Despite the greater simplicity of DB over FB approaches, the latter provide more accurate responses requiring a coarser mesh. To fill the existent gap between the two approaches the improvement of the DB formulation without the introduction of mesh refinement is needed. To this aim the authors recently provided a contribution to the improvement of the DB approach by proposing new enriched adaptive displacement shape functions leading to the Smart Displacement Based (SDB) beam element. In this paper the SDB element is extended to include the axial force-bending moment interaction, crucial for the analysis of r/c cross sections. The proposed extension requires the formulation of discontinuous axial displacement shape functions dependent on the diffusion of plastic deformations. The stiffness matrix of the extended smart element is provided explicitly and shown to be dependent on the displacement shape functions updating. The axial force-bending moment interaction is approached by means of a fibre discretisation of the r/c cross section. The extended element, addressed as Fibre Smart Displacement Based (FSDB) beam element, is shown to be accurate and furthermore accompanied by the proposal of an optional procedure to verify strong equilibrium of the axial force along the beam element, which is usually not accomplished by DB beam elements. Given a fixed mesh discretisation the performance of the FSDB beam element is compared with the DB approach to show the better accuracy of the proposed element.Comment: the paper is composed of 23 pages, 14 figures. It repports the results of an original research performed by the author

A Discrete Macro Element Method for Modelling Ductile Steel Frames around the Openings of URM Buildings as Low Impact Retrofitting Strategy

This paper adopts the use of steel frames around existing openings as a low-impact seismic retrofitting strategy for unreinforced masonry structures (URM). Although elastic steel frames have been commonly adopted for strengthening masonry walls in case of the realization of new openings, the use of elasto-plastic frames has been proposed only recently. This study adopts the application of low-resistance ductile steel frames on the openings of existing masonry buildings as a low-impact retrofitting strategy. The adopted low-invasive solution possesses the advantage of increasing the in-plane resistance of the masonry wall, improving the displacement capacity, introducing additional energy dissipation under dynamic loadings, and providing a confinement effect on the adjacent masonry piers. An original aspect of the present paper is related to the adopted numerical method for modelling the presence of the steel frames around the openings. Namely, a Discrete Macro-Element Method (DMEM), which allows an efficient and reliable simulation of the involved collapse mechanisms of the masonry walls interacting with the frames, has been adopted. After the validation of the numerical approach, through a comparison with experimental results already reported in the literature, the low-impact strategy has been applied on a benchmark known as the “via Martoglio building”. The obtained results suggest that this low-impact retrofitting strategy can be successfully proposed for URM buildings and can be efficiently modelled by means of the DMEM

Targeted steel frames by means of innovative moment resisting connections

The present paper proposes the use of stepped cross section devices on steel frames aiming at reproducing a pre-established target push-over curve. To this aim a Limited Resistance Plastic Device (LRPD) to be inserted along selected structural members is proposed. The following two main specific features for LRPD are required: any elastic flexural stiffness variation of the original selected member must be avoided; an ultimate plastic bending moment value equal to an assigned percentage of the original limit resistance value must be ensured. Steel frames equipped with LRPD are modeled by means of an extension of a recently proposed Fibre Smart Displacement Based (FSDB) beam element model, characterized by the adoption of updating shape functions. The novelty of this research, with respect to previous studies, regards: i) the formulation of a targeted plane steel frame by means of a new optimal design procedure for the LRPD with assigned stiffness and resistance features, independent of each other; ii) the development of a FSDB beam element approach suitably devoted to such a special application of discontinuous cross sections devices. The design procedure and the FSDB model for the analysis of the real behaviour of the proposed LRPD device in the field of distributed plasticity is validated against detailed 3D finite element models