Anàlisi del comportament a compressió de perfils prims oberts d'acer conformats en fred

Cold-formed thin-walled steel sections are frequently used in pallet rack structures. This type of cross-sections are normally used as a columns and contains perforations uniformly distributed enabling the connection between other structural elements. One of the most typical reasons of failure for pallet-racking systems is the buckling of the uprights. As a consequence, the calculation of ultimate strength under compression is a key factor for a safe structural design. The influence of distortional and local buckling in cold formed thin-walled sections is really important, whereas the influence in hot-rolled sections (IPN, IPE, HEB, etc) is insignificant. The analysis of distortional and local buckling in the upright behaviour is difficult because the classical beam theories cannot handle sectional deformations. As a result, different new methodologies have been developed and finite element analysis has taken an important role. Several recent research activities are focused to study the buckling behaviour of rack sections under pure compression in order to improve the current design codes. Specifically, most of the present research is dealing with the following factors: interaction effects involving the different modes of buckling, initial geometric imperfections in nonlinear finite element analysis, cold-work effects, residual stresses and its influence in the upright ultimate load and the modelling of the perforations. The present doctoral thesis aims to provide new knowledge in some of the previously cited factors. It has been studied the influence of the cold-work effect, residual stresses and the load carrying capacity of cold-formed steel rack columns via nonlinear finite element analysis. Moreover, several experimental tests have been done in order to validate the numerical results. Furthermore, a new software has been developed to calculate the lineal elastic buckling load for any mode of deformation. These values are used for the current design codes and for a new numerical methodology developed. Keywords: Thin-walled section, rack, geometric imperfection, distortional buckling, GBT, residual stresses, cold-work effects, finite element analysisEls perfils prims oberts d'acer conformats en fred són elements estructurals molt utilitzats en les prestatgeries per a emmagatzematge de palets. Aquests perfils són habitualment emprats com a columnes i contenen perforacions uniformement distribuïdes per garantir la connexió entre els diferents elements estructurals. Un dels motius més freqüents de fallida d'aquestes prestatgeries és el col·lapse a compressió dels puntals, i per tant, la determinació de la seva capacitat resistent és un factor clau per assegurar el correcte dimensionament de l'estructura. Els perfils prims conformats en fred, a diferència dels elements estructurals conformats en calent (IPN, IPE, HEB, etc), presenten una sensibilitat molt més elevada als modes de vinclament local i distorsional a causa de les seves característiques geomètriques. La inclusió d'aquests modes complica considerablement el procés de càlcul resistent, ja que les teories clàssiques de barres no tenen en compte les deformacions seccionals. Com a conseqüència, el desenvolupament de nous mètodes i les anàlisis mitjançant elements finits han cobrat una gran importància. Nombroses investigacions actuals tenen com a objectiu l'adquisició de nous coneixements sobre el comportament a compressió dels perfils prims que permetin millorar les normatives de càlcul vigents. Alguns dels aspectes on més s'està aprofundint són: la interacció dels diferents modes de fallida, el tipus d'imperfecció geomètrica a utilitzar en les anàlisis per elements finits, l'efecte de l'enduriment per deformació, l'obtenció de les tensions residuals i la importància d'aquestes en la capacitat resistent del puntal i la caracterització de les perforacions en els diferents mètodes de càlcul. La present tesi doctoral pretén aportar nous coneixements en alguns dels factors prèviament mencionats. S'ha estudiat l'efecte de l'enduriment per deformació, les tensions residuals i la modelització del problema d'estudi mitjançant el mètode dels elements finits verificant els valors obtinguts amb resultats experimentals sempre que hagi estat possible. També s'ha desenvolupat un programa per calcular càrregues elàstiques de vinclament per a qualsevol tipus de mode de fallida, ja que aquesta informació és necessària pels càlculs utilitzant la normativa i també per alguna de les metodologies de càlcul proposades mitjançant la simulació numèrica. Paraules clau: Perfil prim obert, rack, imperfecció geomètrica, vinclament distorsional, GBT, tensions residuals, enduriment per deformació, anàlisi per elements finits

Influence of exposure time on mechanical properties and photocuring conversion ratios for photosensitive materials used in additive manufacturing

The influence of UV post - curing process on mechanical properties as well as photocuring conversion ratios is presented. An analytical model to determinate the conversion ratio for frontal polymerization is used t o define 3D printing parameters in order to obtain a conversion profile as homogeno us as possible. The mechanical properties of 3D p rinted coupons with and without UV post - curing process were obtained through experimental tensile and bending tests. Furthermore, the experimental conversion ratios of printed samples were obtained by means of FTIR spectrometry analysis. It was observed that conversion ratios and some mechanical properties increase because of UV post - curing treatment, enabling the chance to optimize the whole manufacturing process in function of the 3D printed part requiremen ts.Postprint (author's final draft

Epoxy Doped, Nano-scale Phase-separated Poly-Acrylates with Potential in 3D Printing

An efficient method to improve the mechanical performance of a commercially available photocure resin is described wherein the resin is modified with a mixture of a cycloaliphatic epoxy and an anhydride curing agent. Photocured samples are thermally treated in a subsequent step to cure the epoxy to obtain an interpenetrated polymer network (IPN) and also complete reaction of the acrylate monomers remaining from the photocure. The latter is accomplished by a thermal radical initiator added earlier into the formulation together with the epoxy-anhydride. The thermal properties and microstructure of the resulting IPN are analyzed. Uniform and quantitative conversions are obtained, with glass transition temperatures comparable to conventional epoxies. The liquid, uncured samples containing different amounts of epoxy are stable at 30 °C for several weeks. In the fully cured epoxy-rich materials, nano-scale phase separation is observed by atomic force microscopy. This is corroborated by the existence of multiple relaxations determined by dynamic mechanical analysis analysis. Specimens from a formulation containing 50% by weight of epoxy-anhydride are 3D printed in a customized masked image processing stereolithography, thermally treated, and are subjected to compression tests. Results show that Young's modulus increases by 900% over the neat resinPeer ReviewedPostprint (published version

Analysis of printing parameters for metal additive manufactured parts through Direct Ink Writing process

Direct Ink Writing is an Additive Manufacturing process in which a metal ink is continuously extruded to built-up a green metal part. Therefore, a debinding and sintering process is required to obtain the final metal part. This thermal process produces a shrinkage of the green printed part according to several material and printing parameters. In this paper, the influence of printing process planning on the width of printed rods for a copper ink is analyzed by means of a Design of Experiments procedure to optimize the printing and equipment parameters and characterize the shrinkage after the sintering processPeer ReviewedPostprint (published version

Influence of the cold work effects in perforated rack columns under pure compression load

This paper analyses the influence of residual stresses and strain hardening due to the cold rollforming process on the load carrying capacity of perforated rack columns. First of all, a residual strain distribution of the manufacturing process is obtained via a finite element analysis and then it is introduced in the model as initial state to carry out a nonlinear buckling analysis. Two different methodologies to introduce the residual strain pattern are presented in order to reproduce the local stress effects because of perforations. Moreover, the changes of the mechanical properties of the material by strain hardening are evaluated through experimental tests and its influence on the rack column behaviour is investigated. The results obtained agree well with experimental tests and show a method to predict the load carrying capacity of perforated rack column taking into account the effects of the cold roll-forming process.Postprint (author’s final draft

Distortional influence of pallet rack uprights subject to combined compression and bending

A pallet rack is a structure in which the beams and uprights are cold-formed steel sections. The beam-upright connections are normally clipped in order to select the beam levels required and for ease of assembly. For this reason, the uprights contain holes and perforations distributed along their length. The complexity of these components, with local, distortional and torsional/flexural buckling behaviour, represents a challenge in structural design. The loads acting on the structure are induced by the weight of pallets to be stored, and transmitted to the uprights by the semi-rigid connection system. Therefore, the uprights are thin-walled singly symmetrical perforated open crosssections bearing axial load and bending moment simultaneously. The main objective of this paper is to analyse the influence of the bending moment on the load bearing capacity of rack uprights subject to axial load together with bending moment. Bending is induced by means of axial forces eccentrically applied. The influence of bending on the strength capacity of rack uprights is analysed through finite element analysis, where residual stresses and strength enhancement induced during cold forming of sections are also considered. Experimental tests have been performed reproducing the load conditions in order to validate the FEA results. The ultimate load has also been calculated according to European standards and compared to experimental resultsPostprint (author's final draft

Simulation of the Roll-Forming Process for the Determination of the Relevant Residual Stresses of Cold-Formed Steel Members

The uprights for pallet rack systems are usually thin-walled open cold-formed steel sections. Their behaviour under compression can be analysed experimentally, analytically or by finite element simulation. In the finite element simulation usually material non linearity, large displacements and initial geometric imperfections are introduced. But until now, the effect of the residual stresses due to the forming process, is usually non considered. With the aim of introducing these stresses in the finite element simulation, our group has started to simulate the roll-forming of open steel sections. In this paper we present our first experiences in the simulation of roll-forming of U-, C- and trapezoidal sections, some comparisons with experimental data available in the literature and the first results of the forming of a medium load upright section.Postprint (published version

Experimental study of the compression behavior of mask image projection based on stereolithography manufactured parts

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State-of-the-art review on adjustable pallet racks testing for seismic design

This paper presents a state-of-the-art review on seismic tests of adjustable pallet rack systems: on particular components of racks, subassemblies, full racks and stored goods. The tested particular components are the most critical connections: beams-to-uprights, braces-to-uprights, and floor-to-uprights; subassemblies also include beam-to-upright connections, among other components. Tests on full racks can be static (pushover), pseudo-dynamic, or dynamic (pullout and shaking table). Finally, tests on stored goods are sliding (aimed to identify the friction coefficient with the supporting members) and tilting (to check their confinement). The examination of the discussed experiments provides relevant conclusions and allows identifying research needs.Peer ReviewedPostprint (published version

Investigation on the down-aisle ductility of multiple bay pallet racks by means of pushover analyses

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