EXPERIMENTAL STUDY AND NUMERICAL MODELLING FOR FLEXURAL CAPACITY OF FRC STRUCTURAL ELEMENTS

Concrete reinforced by short steel fibres is typical brittle matrix composite, in which fibres are impeding cracks growth, such way increasing material’s tensile strength. The use of steel fibre reinforced concrete (SFRC) in structures with high physical and mechanical characteristics makes possible to reduce their weight and cost, to simplify their production technology, to reduce or eliminate reinforcement labour, at the same time increasing reliability and durability. Randomly distributed discontinuous fibres are bridging the crack’s flanks providing material’s “ductility”- like non-linear behaviour at cracking stage. The current study is focused on one formulation of a specific type of concrete matrix with added fibres and without fibres. Concrete cubes and prisms without fibres and having in every situation the same content of 60 mm long fibres were fabricated. Cubes (100×100×100 mm) were tested in compression and beams (100×100×400 mm prisms) were tested under four-point bending (4PBT). Fracture process (crack growth) in the material was modelled, based on experimental results (part of experimental data was used). Finite element method (FEM) using the ANSYS program analysis were realized modelling stress distributions in the broken beams with the goal to predict fracture process. Model’s prediction was validated.

EFFECT OF SHORT FIBERS ORIENTATION ON MECHANICAL PROPERTIES OF COMPOSITE MATERIAL – FIBER REINFORCED CONCRETE

Traditional fiberconcrete structures have fibres in the mix oriented in all spatial directions, distributed in the struc­tural element volume homogenously, what not easy to obtain in practice. In many situations, structurally more effective is the insertion of fibres into the concrete structural element body by forming layers, with a predetermined fibre concentration and orientation in every layer. In the present investigation, layered fibre concrete is under investigation. Short steel fibres were at­tached to flexible warps with the necessary fibres concentration and orientation. Warps were placed into the prismatic mould separating them by concrete layers without fibres. Prisms were matured and tested under four-point bending. The bending-affected mechanical behaviour of cracked fibre concrete was simulated numerically by using a developed struc­tural model. Comparing the simulation results with experimental data, material micromechanical fracture mechanisms were analysed and evaluated.

CRACKING AND STRAIN ANALYSIS OF BEAMS REINFORCED WITH COMPOSITE BARS / KOMPOZITINIAIS STRYPAIS ARMUOTŲ SIJŲ PLEIŠĖJIMO IR DEFORMACIJŲ ANALIZĖ

The paper discusses the results of experimental and numerical modelling using two beams reinforced with GFRP bars. One beam was made of plain concrete while the other contained short steel fibres. The influence of steel fibres on deflection and cracking behaviour was studied. A comparative analysis of experimental results has shown that steel fibres significantly reduce deflections and average crack width of the beam. Moreover, an addition of steel fibres to the concrete mix led to a more ductile failure mode of the beam. Numerical analysis employing nonlinear finite element software ATENA has revealed that a good agreement between calculated and experimental results regarding an ordinary concrete GFRP reinforced beam can be obtained. Santrauka Kompozitinės armatūros taikymas betono konstrukcijoms dažnai sukelia šias problemas: neleistinai didelius įlinkius ir atsiveriančius plyšio pločius, trapų konstrukcijų irimą. Straipsnyje pateikiami dviejų vienodos geometrijos kompozitiniais stiklo pluošto strypais armuotų sijų eksperimentiniai ir skaitiniai tyrimai. Vienoje iš sijų į betono mišinį papildomai įdėta plieno plaušo fibrų. Bandymo metu matuoti vidutiniai sijų kreiviai, įlinkiai ir atsiveriantys plyšio pločiai. Atlikus lyginamąją analizę pastebėta, kad plieno plaušo naudojimas padidina stiklo pluoštu armuotų sijų standumą, atsparumą pleišėjimui, sijų suirimo pobūdis tampa plastiškesnis. Sijas sumodeliavus netiesine baigtinių elementų programa ATENA gautas geras eksperimentinių ir skaitinių rezultatų sutapimas įprasto betono stiklo pluošto armatūra armuotai sijai.

Characterization of Epoxy/Steel Fibres Composites for Hybrid Injection Moulds

Metallic fibres in polymeric matrix are used for mouldings blocks of hybrid injection moulds improving the mechanical and thermal properties. This paper reports on the characterization of epoxy resin/short steel fibres (SSF) composites. The effect of the concentration of 2,4,6-tris (dimethylamino-methyl) phenol as accelerator in the epoxy system was evaluated by viscosity and dynamical mechanical analyses. The composites were characterized by compression and microhardness tests. The fibres dispersion into the epoxy matrix was analysed by optical microscopy. It was found that the best accelerator concentration for this type of composite was 5,0 parts by weight

Assessment of Injection Moulded Parts of PP/Nanoclay Produced with Hybrid Moulds

The concept of hybrid mould combines the conventional techniques of mould manufacturing and Rapid Prototyping and Rapid Tooling, resorting to non-conventional materials for producing moulding blocks, e. g., epoxy resin composites. Composites based on an epoxy system with 15% weight fraction of short steel fibres (SSF) were considered adequate for improving the performance of moulding blocks. The epoxy/short steel fibre composite moulding blocks were produced by vacuum casting in silicone moulds. Polypropylene (PP) was mixed with a commercial PP masterbatch with 50% of nanoclay and injected in a hybrid mould under various processing conditions. These were chosen from a central composite design with 15 experiments. The moulding microstructure was assessed by polarized light microscopy and differential scanning calorimetry. The skin-core morphology was observed and suggested that the low thermal conductivity of the epoxy composite produces a thinner skin when compared to all-steel moulds. The nanoclay concentration was the variable with the most significant effect on skin thickness and crystallinity. The addition of 1 wt% nanoclay under certain processing conditions favours the formation of β-form spherulites and the increase of crystallinity.

Mechanical Properties of Epoxy Composites Filled with Short Steel Fibres for Hybrid Injection Moulds

Epoxide filled composites are being increasingly used for mouldings blocks of hybrid injection moulds. The filling is sought for improving both mechanical and thermal properties that are relevant for the mould performance. In spite of several works investigating the particulate filling of resins, there are only a few reports on fibre reinforcement. Composites based on an epoxy system with varying volume fractions of short steel fibres (SSF) were investigated. The mechanical properties were determined for each composite, and the topography of the fracture surfaces was analyzed by SEM. The mechanical properties of the epoxy filled composites were also compared to commercial particulate composites that are used for producing casting moulds. In spite of the SSF being more difficult to mix that the usual metal particulate fillers, it was found that the resulting composites show some improvement in the mechanical properties.

Friction Properties of Steel Fibre Reinforced Epoxy Composites Used in Moulding Blocks of Hybrid Moulds

Rapid tools for injection moulding are often produced by casting of epoxide filled composites. The moulding blocks obtained in this way are likely to undergo wear especially at the surfaces of the moulding cores during the ejection phase. In this study, tribological properties of epoxide composites containing different volume fractions of short steel fibres used in moulding blocks were assessed. Friction tests with the composites and moulded polypropylene were carried out with a prototype equipment that reproduces the actual ejection phase of injection moulding. The friction data were interpreted in terms of the roughness and compared with the microscopic features of the epoxy composite surface.