101,917 research outputs found
Least-thickness symmetric circular masonry arch of maximum horizontal thrust
No full textThis analytical note shall provide a contribution to the understanding of general principles in the Mechanics of (symmetric circular) masonry arches. Within a mainstream of previous research work by the authors (and competent framing in the dedicated literature), devoted to investigate the classical structural optimization problem leading to the least-thickness condition under self-weight (“Couplet-Heyman problem”), and the relevant characteristics of the purely rotational five-hinge collapse mode, new and complementary information is here analytically derived. Peculiar extremal conditions are explicitly inspected, as those leading
to the maximum intrinsic non-dimensional horizontal thrust and to the foremost wide angular inner-hinge position from the crown, both occurring for specific instances of over-complete (horseshoe) arches. The whole is obtained, and confronted, for three typical solution cases, i.e., Heyman, “CCR” andMilankovitch instances, all together, by full closed-form explicit representations, and elucidated by relevant illustrations
Selective mass scaling for distorted solid-shell elements in explicit dynamics: optimal scaling factor and stable time step estimate
No full textThe use of solid-shell elements in explicit dynamics has been so far limited by the small critical time step resulting from the small thickness of these elements in comparison with the in-plane dimensions. To reduce the element highest eigenfrequency in inertia dominated problems, the selective mass scaling approach previously proposed in [G. Cocchetti, M. Pagani and U. Perego, Comp. \& Struct. 2013; 127:39-52.] for parallelepiped elements is here reformulated for distorted solid-shell elements. The two following objectives are achieved: the critical time step is governed by the smallest element in-plane dimension and not anymore by the thickness; the mass matrix remains diagonal after the selective mass scaling. The proposed approach makes reference to one Gauss point, trilinear brick element, for which the maximum eigenfrequency can be computed analytically. For this element, it is shown that the proposed mass scaling can be interpreted as a geometric thickness scaling, obtaining in this way a simple criterion for the definition of the optimal mass scaling factor. A strategy for the effective computation of the element maximum eigenfrequency is also proposed. The considered mass scaling preserves the element translational inertia, while it modifies the rotational one, leading to errors in the kinetic energy when the motion rotational component is dominant. The error has been rigorously assessed for an individual element, and a simple formula for its estimate has been derived. Numerical tests, both in small and large displacements and rotations, using a state-of-the-art solid-shell element taken from the literature, confirm the effectiveness and accuracy of the proposed approach. Copyright {\copyright} 2014 John Wiley \& Sons, Ltd
Computational elastoplastic structural analysis of carbon nanotubes
No full textCarbon nanotubes represent, in modern application trends, an innovative material with excellent physical characteristics, specifically from a mechanical standpoint. Their growing usage fosters a challenging quest of computational methods suitable for reliable and effective structural analyses [1]. To this aim, modelling approaches capable to deal with nanoscale and atomistic aspects, through classic continuum and structural mechanics methods, turn out to exhibit a particular interest [2,3]. In the present contribution, the carbon-carbon bonds of carbon nanotubes are set to be modelled by beam finite elements, focusing, in particular, on lattice structures for Single-Walled Carbon NanoTubes. Consistently, a first investigation step is devoted to a validation of proposing a constitutive elastoplastic behaviour modelling, with interpretation reference also to experimental data available in the literature. Furthermore, two subsequent structural stages are considered: (a) an evolutive elastoplastic step-by-step analysis, developed according to an algorithm proposed in [4], and (b) a direct, kinematic Limit
Analysis collapse study, consistently devised with a numerical implementation as in [5].
The employment of such devoted, self-implemented, computational modelling approaches shall display advantageous features both from a numerical viewpoint and from a practical engineering standpoint. In particular, as discussed by the results of the present contribution, Limit Analysis methodologies are newly proven to constitute effective and robust modelling strategies, toward the assessment of structural and collapse behaviours, also within the context of carbon nanotube structures.
References
[1] Silvestre, N. (Ed), 2016, Advanced Computational Nanomechanics, Wiley, Chichester.
[2] Rafiee, R., Moghadam, R. M., 2014, On the modeling of carbon nanotubes: A critical review,
Compos Part B-Eng, 56: 435-449.
[3] Li, C., Chou, T.-W., 2003, A structural mechanics approach for the analysis of carbon
nanotubes, Int J Solids Struct, 40: 2487-2499.
[4] Ferrari, R., Cocchetti, G., Rizzi, E., 2016, Limit Analysis of a historical iron arch bridge.
Formulation and computational implementation, Comput Struct, 175: 184-196.
[5] Ferrari, R., Cocchetti, G., Rizzi, E., 2018, Effective iterative algorithm for the Limit
Analysis of truss-frame structures by a kinematic approach, Comput Struct, 197: 28-41
Finite-Friction Effects in Self-standing Symmetric Circular Masonry Arches
No full textThis note concerns a general issue, in the mechanics of masonry arches, with reference to symmetric circular geometries, with variable opening, and possible stereotomy with radial joints (to be potentially formed, at failure, within the ideal continuous arch), in a least-thickness condition, under self-weight, namely the role that a finite inherent friction, among the theoretical joints, may play in ruling out the selfstanding conditions and the mechanical features at incipient collapse, setting a change from purely-rotational modes to mechanisms that may include sliding. The issue is systematically investigated, by a full analytical derivation, and validated through an original Complementarity Problem/Mathematical Programming formulation, and numerical implementation, reconstructing the complete underlying map of thicknessto-radius ratio versus friction coefficient of all arch states, and corresponding collapse mechanisms. This investigation shall clear the issue, of the theoretical influence of finite friction, in the above-stated setting, and contribute to provide a full understanding of basic aspects in the methodological description, and physical interpretation, of the mechanics
of masonry arches, with implications that may come up to appear also in practical terms, once dealing with this traditional and remarkable structures, in real cases, possibly endowed of historical character and architectural value, to be preserved and renewed
Estimation of residual stresses by inverse analysis based on experimental data from sample removal for “small punch” tests
No full text“Small Punch” (SP) tests, at present frequently employed for mechanical characterizations of structural metals, particularly for diagnosis of plant components, are here considered in view of employment also for assessments of stresses. In the procedure proposed herein the standardised sample removal from an in-service component for SP tests is exploited as external action altering the residual stress state possibly present near to the surface in the location considered. Full-field measurements of consequent displacements in the surrounding surface are employed as input for inverse analysis based on the following features: computer simulations of the sample removal as for its consequences due to relaxation of the pre-existing stress state; “non-uniformity” of residual-stress dependence on depth described as layer-dependent with uniformity in each layer of a pre-defined set of layers; “discrepancy function” minimization with employment of the elasticity parameters provided by the subsequent SP test. The advantages of the novel method consist of no-more need of traditional usual “Hole Drilling” (HD) tests or other tests for residual-stress estimation. The SP experimental procedure proposed herein for estimations of both stress state and elastic-plastic material properties would imply reductions of damages, costs and times in structural diagnoses
Assessment of residual stresses and mechanical characterization of materials by "hole drilling" and indentation tests combined and by inverse analysis
No full textHole Drilling (HD) tests are frequently employed as "quasi-non-destructive" experiments, for assessments of residual stresses in metallic components of power plants and of other industrial structures. With respect to the present broadly standardized HD method, the following methodological developments are proposed and computationally validated in this paper: assessments of elastic and plastic parameters by indentation exploiting the hole generated by HD tests; employment of "Digital Image Correlation" (DIC) for full-field displacement measurements, instead of the strain measurements by gauge "rosettes" usually adopted so far; transitions from experimental data to sought parameters by inverse analyses based on computer simulations of both tests and on minimizations of a "discrepancy function". Interactions between the two experiments are here investigated, besides the elastic parameters transition from indentation (IND) to HD test interpretation. The main advantage achievable by the procedure proposed herein is reduction of additional "damage" and cost due to usual experimental procedures for diagnosis of structural components (e.g. frequently adopted "small punch" experiments or laboratory tension tests)
Materials mechanical characterizations and structural diagnoses by inverse analyses
No full textMechanical damages in structures, in structural components of plants and in industrial products usually imply changes of parameters which have central roles in computational modelling apt to assess safety margins with respect to service loading. Such parameters may depend also on the production processes in industrial environments. In this chapter, the parameter identification methodology by inverse analyses is dealt with under the following limitations: experiments at macroscale level, deterministic approaches, statical external actions and time independence in material behaviours. Semiempirical approaches frequently adopted in codes of practice are not dealt with here. The inverse analysis methods outlined here are centered on computational simulations of tests (namely, direct analyses), sensitivity analyses for the optimal design of experiments, model reduction procedures and other provisions apt to make fast and economical the parameter estimation in engineering practice. The applications summarized here as examples concern structural diagnoses based on indentation tests, in situ diagnostic experiments on concrete dams and laboratory mechanical characterization of membranes and laminates
Evolutive and Kinematic Limit Analysis Algorithms for Large-Scale 3D Truss-Frame Structures: Comparison Application to Historic Iron Bridge Arch
No full textTwo new computational algorithms for the Limit Analysis (LA) of large-scale 3D truss-frame structures recently proposed by the authors are reconsidered and adapted for a comparison prediction of the elastoplastic response of a strategic beautiful historic infrastructure, namely the Paderno d'Adda bridge (or San Michele bridge), a riveted wrought iron railway viaduct that was built in northern Italy in 1889. The first LA algorithm traces a fully exact evolutive piece-wise linear elastoplastic response of the structure, up to plastic collapse, by reconstructing the true sequence of activation of made-Available plastic joints (as a generalization of plastic hinges), in the true spirit of LA. The second LA algorithm develops an independent kinematic iterative approach apt to directly determine the plastic collapse state, in terms of collapse load multiplier and plastic mechanism, based on the upper-bound theorem of LA. Specifically, the marvelous doubly built-in parabolic arch of the bridge is analyzed, under a static loading configuration at try-out stage, and its elastoplastic response is investigated, in terms of evolutive load-displacement curve, collapse load multiplier and plastic collapse mechanism. The two LA algorithms are found to much effectively run and perform, despite the rather large size of the computational model, with a number of dofs in the order of four thousands, by achieving good corresponding matches in terms of the estimate of the load-bearing capacity and of the collapse characteristics of the arch substructure, showing this to constitute a well-set structural element. Moreover, the direct kinematic method displays a rather dramatic performance, in truly precipitating from above onto the collapse load multiplier and rapidly adjusting to the collapse mode, in very few iterations, by a considerable saving of computational time, with respect to the complete evolutive elastoplastic analysis. This shall open up the way for further adoption of such advanced LA tools, with LA regaining a new momentum within the modern optimization analysis of structural design and form-finding problems
Reference Structural Investigation on a 19th-Century Arch Iron Bridge Loyal to Design-Stage Conditions
No full textThis work gathers the outcomes of a comprehensive case study concerning a reference structural investigation on the bridge at Paderno d’Adda, a monumental arch iron viaduct completed in 1889, situated in Lombardy (northern Italy), still keeping today a crucial position within the local railway and road transportation networks. The bridge is part of precious architectural heritage, together with a few rather similar bridges in Europe, which may be in the predicate to become part of the UNESCO list. A comprehensive structural analysis loyal to design-stage conditions was developed. After the constitution of a detailed linear elastic numerical model of the structure, various static analyses were first performed; comparisons with available recorded data at the try-out stage proved the model’s consistency. Then, further insight into the linear range, concerning the modal dynamic behaviour, was investigated, with a reasonable matching with available experimental modal characteristics. Finally, a complete non-linear inelastic analysis was developed, up to detect plastic-collapse features, to assess the safety margin at increasing live loads. This case study shall contribute in setting up a reference scene about the present bridge structural capacity, within the current debate on the destinies of the structure and the screening of possible intervention scenarios
Inverse Structural Analyses on Small Punch Tests, with Model Reduction and Stochastic Approach
No full textIntegrity and durability of metal structures at present can be economically assessed by quasi-non-destructive experiments such as Small Punch Tests already envisaged by international industrial codes. In this communication the following innovative provisions are presented and proposed in view of their contributions to accuracy and economy of diagnostic analyses of structural components: the test is simulated by a finite element computer code; parameters in a popular elastic-plastic material model are computed by inverse analysis; diverse parameters identification procedures are comparatively employed; Proper Orthogonal Decomposition is employed for preliminary model reduction in order to make parameters identification more economical for multiple engineering applications; Kalman filter approach is adopted for stochastic back-analysis in structural diagnosis
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