Pseudo-ductile Behavior Research Articles

Stability and robustness of a 300 m2 composite gridshell structure

In this paper, ductility aspects of a light-weight composite gridshell are developed. A gridshell is a very light structure that can support relative high loads. For many reasons, the materials used by the Navier laboratory are glass fibre reinforced polymers (GFRPs) that have elastic brittle behaviour. To ensure the safety of people these structures have to behave in a ductile way, that is to say they must not collapse without showing signs of weakness. This paper deals with the pseudo-ductile behaviour of the GFRP gridshells designed by the Architected Structures and Materials research unit of Navier laboratory.After a reminding context about gridshells, the buckling of the Solidays’ festival gridshell prototype (June 2011) is considered. It is demonstrated that buckling has to be avoided carefully because it fosters high stresses in the beams and may lead to a brutal collapse of the structure. Then it is shown that, under Ultimate Limit State (ULS), the gridshell prototype is far from buckling. Finally, a simulation of accident is performed: from the ULS, several elements are broken in order to understand the behaviour of the structure in case of accident. The pseudo-ductility of the structure is demonstrated thanks to the redundancy of the structural concept of the gridshell.

Interfacial control of uni-directional SiCf/SiC composites based on electrophoretic deposition and their mechanical properties

Abstract Interfacial control of uni-directional SiC f /SiC composites were performed by EPD, and their mechanical properties at room temperature were evaluated. The effect of the thickness of carbon interphase on SiC fibers by EPD on mechanical properties of uni-directional SiC f /SiC composites was also investigated. The average thickness of carbon coating on SiC fibers increased from 42 nm to 164 nm with an increase in the concentration of colloidal graphite suspension for EPD. Dense SiC f /SiC composites were achieved and their fiber volume fraction was 47–51%. The SiC f /SiC composites had a bending strength of 210–240 MPa. As the thickness of carbon coating was below 100 nm, the SiC f /SiC composites (SC01 and SC02) fractured in almost brittle manner. In contrast, the SiC f /SiC composites (SC03) showed a pseudo-ductile fracture behavior with a large number of fiber pullout as the thickness of carbon coating was above 100 nm. The fracture energy of SC03 was 3–4 times as high as those of SC01 and SC02 and the value was about 1.7 kJ/m 2 . In consideration of the results of mechanical properties, the thickness of carbon coating on SiC fibers should be at least 100 nm to obtain high-performance SiC f /SiC composites. The fabrication process based on EPD method is expected to be an effective way to control the interfaces of SiC f /SiC composites and to obtain high-performance SiC f /SiC composites.

Flexural Performance and Moment Connection of Concrete-Filled GFRP Tube–Encased Steel I-Sections

A hybrid system, concrete-filled fiber-reinforced polymer (FRP) tube (CFFT)-encased steel I-section, is introduced. The embedded steel section enhances flexural strength and stiffness, and provides a pseudoductile behavior. It also facilitates connection of the CFFT member to footings or other members. Phase I of the experimental program addresses the flexural behavior of the system through ten beam specimens, including steel and CFFT control specimens. The glass fiber-reinforced polymer (GFRP) tubes varied in thickness and laminate structure. The steel section enhanced performance considerably, especially pseudoductility, in tubes with cross-ply laminates, in which a significant sustained reserved strength remains stable over large deflections on fracture of the tube. CFFTs with angle-ply tubes showed a considerable inherent ductility on their own, in which case, adding the steel section enhanced strength and stiffness only. Phase II addresses a moment connection through five cantilever tests. The connections consist of steel base plates welded to the steel sections, which are embedded into the CFFT members at various length (Ls)-to-span-length (L) ratios ranging from 0.1 to 1.0. Three distinct failure modes are observed. At (Ls/L) ratios of up to 0.17, premature bond failure occurs. At ratios of 0.17 to 0.47, the CFFT member achieves flexural tension failure of the tube just beyond the free end of the steel section. Beyond a 0.47 ratio, the full plastic hinge capacity is developed at the fixed end. A simple design-oriented model that predicts strengths at the full range of (Ls/L) ratios is developed and validated.

Low-grade glued laminated timber reinforced with FRP plate

Fibre-reinforced polymers (FRPs) are an accepted material by structural engineers for the strengthening of structural elements. With increasing environmental concerns, further emphasis is being placed on the use of sustainable construction materials such as timber. This paper describes an experimental programme which examines the reinforcement in flexure of low-grade glued laminated timber (glulam) with a commercially attractive recyclable FRP. Unreinforced glulam beams, FRP plate reinforced glulam beams, unreinforced glulam beams incorporating an additional sacrificial lamination and FRP plate reinforced glulam beams, which had a sacrificial lamination bonded beneath the FRP plate, were fabricated and tested in flexure. The unreinforced glulam demonstrated linear elastic behaviour and exhibited brittle tensile-flexural failures in comparison to the pseudo-ductile behaviour of the reinforced beams. The addition of reasonable reinforcement percentages strategically located in the tension zone resulted in moderate enhancements in the stiffness while more significant improvements in the ultimate moment capacity were obtained. Strain profiles illustrated that increased utilisation of the compression strength of the timber is obtained when FRP reinforcement is included.

Steel Fiber-Reinforced Self-Compacting Concrete: Experimental Research and Numerical Simulation

Over the last few decades, the astonishing developments of superplasticizers technology allowed great achievements in the conception of concrete mixes exhibiting self-compacting ability. Since the 1980s, some methodologies have been proposed to achieve self-compacting requirements in fresh concrete mixes, based on the evaluation of the flowing properties of these mixes. There still persist, however, some doubts about the most appropriate strategy to define the optimum composition of a self-compacting concrete (SCC) mix, based on a required performance. The behavior of SCC as a structural material can be improved if adequate steel fiber reinforcement is added to SCC mix composition. In fact, the fiber-reinforcement mechanisms can convert the brittle behavior of this cement-based material into a pseudoductile behavior up to a crack width that is acceptable under the structural design point of view. Fiber addition, however, increases the complexity of the mix design process, due to the strong perturbation effect that steel fibers cause on fresh concrete flow. In the present work, a mix design method is proposed to develop cost effective and high performance steel fiber-reinforced self-compacting concrete (SFRSCC). The material properties of the developed SFRSCC are assessed as well as its potentiality as a structural material, carrying out punching and flexural tests on panel prototypes. A material nonlinear analysis is carried out, aiming to address the possibility of calibrating the constitutive model parameters by obtaining, with an inverse analysis, the fracture parameters using force-deflection relationships recorded in simpler laboratory tests, such as the three-point notched beam bending test. The contribution of steel fibers for punching resistance is also discussed.

유사연성 하이브리드 FRP 시트를 이용한 RC 보의 휨 보강

Use of both carbon fiber (CF) and glass fiber (GF) at the same time to strengthen existing flexural members was exploited. Using a proper volumetric GF / CF ratio, the CF can rupture first followed by subsequent rupture of GF at higher stress and strain showing a pseudo-ductile behavior. A theoretical study indicated that the ratio is 4.62 : 1 and higher where the pseudoductile effect can be shown. Flexural tests of plain concrete beams strengthened using fibers were first carried out. Hybrid FRP sheet using 8.8 : 1 ratio was then fabricated and the sheet was used to strengthen reinforced concrete beams. The RC beams strengthened using 1-ply and 2-ply hybrid sheets both revealed increased strength over a non-strengthened beam and ductile flexural behavior. A comparable beam strengthened using CF also showed increased strength but with limited ductility.

Tenacidade à fratura de blendas PA 6/ABS avaliada através do método EWF (Trabalho Essencial de Fratura) - parte a: avaliação do efeito do compatibilizante

A poliamida 6 (PA6) é um polímero termoplástico semicristalino que apresenta alta susceptibilidade ao entalhe quando solicitado mecanicamente. Apesar de apresentar um comportamento dúctil sob deformação e sob fratura quando solicitado sem entalhes, torna-se essencialmente frágil e quebradiço quando entalhado. Assim, o comportamento pseudo-dúctil dessas poliamidas requer cuidados especiais na confecção do entalhe e da pré-trinca para análise de seu comportamento mecânico. Devem ser observadas ainda as dimensões ideais dos corpos de prova para garantir a condição de estado de tensão plana no ensaio de tração uniaxial para aplicação do método EWF (Trabalho Essencial de Fratura). Neste trabalho, o método EWF foi utilizado para a avaliação da tenacidade à fratura de blendas poliméricas com uma matriz PA6 modificada com ABS e também a eficiência de um compatibilizante acrílico reativo (MMA-MA) sintetizado em laboratório, na tenacificação da blenda PA6/ABS. Através da aplicação do método observou-se que a simples incorporação do ABS à poliamida aumenta o termo relativo ao trabalho específico de fratura e reduz a sensibilidade ao entalhe da blenda embora com perda na capacidade de absorção de energia plástica. A incorporação do copolímero MMA-MA à blenda refletiu-se nos acréscimos do trabalho específico e recuperação da capacidade de absorção de energia plástica em relação à blenda não-compatibilizada. Os resultados ilustram a aplicabilidade do método EWF para determinação da tenacidade à fratura de blendas poliméricas em relação aos polímeros puros, bem como a avaliação do efeito do compatibilizante reativo na tenacidade da blenda PA6/ABS.

Mechanical properties and microstructure of carbon fibre reinforced carbon materials produced by chemical vapour infiltration

Abstract Correlations between mechanical properties, fibre architecture, matrix texture and fracture behaviour of carbon fibre reinforced carbon CFC-materials produced by chemical vapour infiltration are investigated using results from tensile and four-point-bending tests, polarized light microscopy and scanning electron microscopy. Mechanical testing was performed at temperatures from 20 up to 1600 °C. Samples were tested in the as-received state and after heat treatment at T = 2200 °C. The heat treatment leads to an increasing Young’s modulus and brittleness, which corresponds with an increasingly distinct layer structure of high textured matrix carbon. A rise of test temperature up to 1400 °C also leads to a larger Young’s modulus and increasing brittleness. However, at T = 1600 °C the material shows a pronounced pseudoductile behaviour. Scanning electron micrographs show in this case a lot of concentric cracks between the sublayers of the high textured matrix. The fracture behaviour is explained by a multiple delamination between these layers.

Strengthening of Concrete Beams Using Innovative Ductile Fiber-Reinforced Polymer Fabric

An innovative, uniaxial ductile fiber-reinforced polymer (FRP) fabric for strengthening structures is described. The fabric is a hybrid of 2 types of carbon fibers and 1 type of glass fiber, and has been designed to provide a pseudo-ductile behavior with a low yield-equivalent strain value in tension. The effectiveness and ductility of the developed fabric was studied by strengthening and testing 8 concrete beams under flexural load. Similar beams strengthened with currently available uniaxial carbon fiber sheets, fabrics, and plates were also tested to compare their behavior with those strengthened with the developed fabric. This fabric has been designed so that it has the potential to yield simultaneously with the steel reinforcement of strengthened beams and hence, a ductile plateau similar to that of nonstrengthened beams can be achieved. The beams strengthened with the newly developed fabric exhibited higher yield loads and achieved higher ductility indices than those strengthened with the currently available carbon fiber strengthening systems. The fabric shows a more effective contribution to the strengthening mechanism.

Ductile Hybrid Fiber Reinforced Plastic Reinforcing Bar for Concrete Structures: Design Methodology

This paper addresses the need for a ductile or pseudo-ductile fiber reinforced plastic (FRP) reinforcement for concrete structures. The criteria to be met by the FRP, which are based on the properties of the steel reinforcing bar it is to replace, are threefold: high initial modulus, a definite yield point, and a high level of ultimate strain. It is shown that the use of a fiber architecture-based design methodology facilitates the optimization of the performance of FRP through material and geometric hybrid. Consequently, the advantages of FRP, such as high strength, low weight, and chemical inertness or noncorrosiveness, can be fully exploited. Using the material hybrid and geometric hybrid, it is demonstrated that the pseudoductility characteristic can be generated in FRP reinforcing bar. Critical material and geometric parameters such as elastic modulus, fiber volume fraction, twisting, crimp, and helical effect in the specimen components were investigated and parametric studies are reported. Ductile hybrid FRP bars were successfully fabricated at 3-mm and 5-mm nominal diameters using an in-line braiding and pultrusion process. Tensile specimens from these bars were tested and found to have consistent pseudoductile behavior and good agreement with the analytical predictions.

Strain-rate-dependent mechanical properties of the equine hoof wall.

The mechanical properties of fully hydrated equine hoof wall were examined at various loading rates in compact tension (CT) fracture, tensile and three-point bending dynamic tests to determine possible effects of hoof wall viscoelasticity on fracture toughness and tensile parameters. Four cross-head rates were used in CT tests: 1.7 x 10(-5), 1.7 x 10 (-3), 1.7 x 10(-2) and 2.5ms-1; four strain rates were used in tensile tests: 1.6 x 10(-3), 3.2 x 10(-2), 0.33 and 70s(-1). Speeds for the highest test rates were achieved using a large, custom-built impact pendulum. Bending test frequencies ranged from 0.04 to 200 Hz. In CT tests, both the initial modulus Ei and the stress intensity factor K rose with increasing strain rate (from 0.38 to 0.76 GPa for Ei and from 0.71 to 1.4 MN m-3/2 for K), whereas the fracture toughness parameter J remained constant at 12kJm-2. All tensile parameters except ultimate strain were sensitive to strain rate. Ei, total energy to breakage and maximum stress rose with increasing strain rate from 0.28 to 0.85 GPa, from 5.4 to 9.7 MJm-3 and from 17 to 31 MPa, respectively. Data from low-amplitude dynamic tests agreed well with Ei trends from CT and tensile tests. Direction of crack growth differed through the thickness of the wall, the pattern of which resembled a trilaminar ply. Although scanning electron microscopic examination of fracture surfaces revealed a decreasing pseudo-ductile behaviour with increasing strain rate, and ultimate tensile parameters are positively affected, equine hoof wall viscoelasticity does not appear to compromise fracture toughness at high strain rates.

Design Criteria for Pseudoductile Fiber-Reinforced Composites

The toughness of brittle materials can be significantly improved by the incorporation of fibers. Moreover, both theoretical and experimental investigations have shown that properly designed fiber composites can exhibit pseudoductile tensile behavior with multiple cracking rather than brittle failure with the formation of one single crack. To attain pseudoductile behavior, two criteria have to be satisfied: (1) the steady-state cracking criterion; and (2) the further cracking criterion. In the literature, criteria for steady-state cracking are usually obtained numerically or derived approximately with assumed profiles of the bridged crack. In this paper, a general analytical approach for the determination of an exact condition for steady-state cracking is presented. The steady-state cracking criteria for several important cases are then derived and compared with available numerical or approximate solutions. Then, an approximate but conservative criterion for further cracking is also developed. Since the tw...

The mechanical behavior of an alumina carbon/epoxy laminate

An experimental study has been made of a laminate consisting of monolithic thin alumina plates alternating with unidirectional carbon/epoxy (C/E) prepreg tapes. The main advantages of this system over the traditional means of reinforcing ceramics, are the avoidance of large flaws due to processing, which occur in fiber reinforced brittle matrix composites, and the nearly isotropic behavior under biaxial loading. In addition, the multiple fracture mechanism occurring in the system gives rise to pseudo ductile behavior and enhanced strain energy dissipation. The mechanical behavior of the laminate is explored. The effects of the number of layers, volume fraction and transverse properties are also investigated. The loss of stiffness with increase of the applied strain is estimated using a simple shear lag theory, which includes the plastic behavior of the interface.

The failure of fresh-water granular ice under multiaxial compressive loading

The failure of fresh-water granular ice under multiaxial compressive loading was investigated at −10, −20 and −40°C at 10 −3 s −1, using a true multiaxial servo-hydraulic testing system. The tests were carried out on cubic samples proportionately loaded under different confinement ratios R = σ 3/σ 1 = σ 2/σ 1. Two regimes were revealed at all three temperatures. Under lower confinement the failure stress, σ 1f, increases sharply with R, whereas at higher confinement σ 1f is roughly constant. Both brittle and pseudo-ductile behavior were observed under lower confinement, depending upon the confining stress and the temperature, while the behavior was brittle under higher confinement. An analysis of the brittle to pseudo—ductile transition within the lower confinement regime was performed in terms of the frictional crack sliding/wing-crack mechanism. The high-confinement regime is characterized by intense intergranular damage. The final failure, however, is caused not by the linking of the uniformly distributed microcracking, but by damage localized at the ice/platen interface. The saturation in the failure stress with increasing confinement is attributed to a mechanism specific to the boundary conditions and not to the ice per se.

Uniaxial tensile and creep behaviour of an alumina fibre-reinforced ceramic matrix composite: I. Experimental study

Abstract The uniaxial tensile and creep behaviour of an alumina fibre-reinforced silicon carbide composite is studied. The damage mechanisms during tensile loading are identified on the basis of the elastic response and in-situ morphological analysis. Tensile tests show that the composite presents a pseudoductile behaviour due to matrix microcracking and fibre-matrix debonding. Temperature induces changes in the tensile behaviour because of variations in load transfer conditions and in the axial residual stress borne by the fibres and the matrix. The creep curves at 1100°C under vacuum present an extended tertiary part, especially at low creep stress. The unloading-reloading loops periodically performed during creep show a progressive decrease in longitudinal stiffness. Progressive interface debonding during creep is invoked to explain: (i) the strain rate increase during tertiary creep, (ii) the decrease of the elastic modulus and (iii) the large fibre pull-out observed on the creep fracture surface. The different creep rupture modes at low and high stresses are related to the capability of the remaining intact fibres to support the overload failure of the first fibres.