Optimisation of local in-plane constraining forces in double diaphragm forming
Abstracts:Rigid blocks (risers) were introduced in the double diaphragm forming (DDF) process to change the local in-plane strain distribution in the diaphragms, aimed at reducing wrinkling defects in the production of fabric preforms. A two-step optimisation method was developed to determine the position and dimension of each riser. In Step I, optimisation of the riser position was conducted using a simplified finite element (FE) model coupled with a genetic algorithm (GA). The height of each riser was optimised in Step II using a detailed FE model with the optimised riser positions from Step I. For demonstration, a hemisphere preform was manufactured by DDF using the optimum riser arrangement established by the optimisation routine. Results indicate that the optimum riser pattern (shape and position relative to the component boundary) can dramatically improve the preform quality through reduction of out-of-plane wrinkles, validating the feasibility of the two-step routine.
An experimental study on the effect of adding multi-walled carbon nanotubes on high-velocity impact behavior of fiber metal laminates
Abstracts:In this paper, the effect of adding multi-walled carbon nanotubes (MWCNTs) on high-velocity impact behavior of fiber metal laminates (FMLs) was investigated. The unreinforced and reinforced FMLs with different MWCNT weight percentages of 0.25, 0.5 and 1 were manufactured and tested under high-velocity impact loading using a gas gun and a spherical projectile. Moreover, tensile tests were performed on the unreinforced and reinforced composite laminates of FMLs. Incorporating 0.5 wt% of MWCNTs into the composite laminate of FML resulted the maximum reduction of 29.8% in projectile residual velocity and the maximum increase of 18.9% in the absorbed energy during projectile perforation compared to the unreinforced FMLs. This was consistent with the tensile test results in which maximum improvements in the strength, stiffness and toughness were obtained for the 0.5 wt% MWCNT-nanocomposite. The detailed visual inspections and SEM images showed that adding MWCNTs improved the resin-fiber adhesion consequently reduced the composite delamination and matrix cracking. Conversely, MWCNTs weakened bonding between the aluminum and composite layers and allowed the aluminum layer to experience larger plastic deformation.
A water-soluble magnesium sulfate bonded sand core material for manufacturing hollow composite castings
Abstracts:A high-strength and water-soluble magnesium sulfate bonded sand core (WSMBS) material cured by twice microwave heating was successfully fabricated using magnesium sulfate aqueous solution as a binder, which is suitable for manufacturing hollow composite structures castings. The effects of various factors on the properties of WSMBS were investigated. The results shows that WSMBS possesses some advantages of good water solubility, high curing speed and strength. The tensile strength of WSMBS is more than 0.6 MPa under optimal parameters, and when the temperature of WSMBS ranges from 105 °C to 116 °C, the tensile strength is superior and MgSO4·7H2O is dehydrated to MgSO4·4H2O or MgSO4·3H2O. The moisture absorbability of WSMBS is quite high, and it rises with increasing the stored time and the magnesium sulfate binder content. The scanning electron microscope analysis shows that there are some micro-cracks or holes in the bonding bridge that decreases the strength of WSMBS after being put in humidistat for several hours. The energy dispersive X-ray analysis shows that the bonding bridge mainly comprises magnesium sulfate and so the use of WSMBS in casting does not release toxic gases. The water-solubility rate of WSMBS is 57.9 kg·min−1·m−2, which can be dissolved in water quickly after casting and overcome the poor leachability of the common bonded sand core, and therefore the used WSMBS can be easily reclaimed by water scrubbing method and the use of WSMBS can improve the production efficiency of the complex hollow composite castings with many interior channels or passages, undercuts. What is more, the aqueous solution of magnesium sulfate can be made from waste water which is used for dissolving the sand core after casting, and therefore it can realize green casting with no toxic gas during casting and recycling magnesium sulfate binder.
A novel interface-treated micromechanics approach for accurate and efficient modeling of CNT/polymer composites
Abstracts:In this paper, a two-step incremental micromechanics formulation in conjunction with FEM and weakened interface model is utilized to characterize elasto-plastic behavior of CNT/polymer nanocomposites. For the validation purpose, results corresponding to the perfect bonding assumption are compared with the experimental data. The micromechanics approach considering the weakened interface is extended to represent the nanotube, its surrounding polymer and the interfacial interactions via an equivalent fiber. The most important factor in the developed method is the sliding parameter determined through comparing the results of the model with that of a molecular structural mechanics-finite element multiscale approach at various loading conditions. Subsequently, employing several case studies, various aspects of the effects of interfacial strength on the elastoplastic behavior of these nanocomposites are systematically examined. The results show that the interfacial bonding characteristics plays a crucial role in enhancing the mechanical behavior of the host polymer and thus, should be thoroughly studied.
Interfacial behavior and debonding failures of full-scale CFRP-strengthened H-section steel beams
Abstracts:Strengthening existing steel beams with externally bonded carbon fiber-reinforced polymer (CFRP) plate has attracted many interests in the research community. Debonding of the CFRP plate is the dominant failure mode in a flexurally strengthened steel beam, and the debonding failure is controlled by the interfacial responses between the CFRP plate and the substrate steel beam. Although some experimental investigations have been conducted on CFRP-strengthened steel beams, limited test results on the interfacial stress and strain responses especially in full-scale steel beams, are available to verify the numerical modeling. This paper presents an experimental study on the flexural behavior of full-scale CFRP-strengthened H-section steel beams. The effects of different bond lengths of CFRP plates and the presence of steel stiffeners are investigated. The test results in terms of the failure modes, load-deflection responses, CFRP strains, interfacial shear stress distributions are reported in detail. A three-dimensional finite element model is proposed to predict the flexural performance of full-scale CFRP-strengthened steel beams, and it is then validated extensively by the test results.
Experimental and numerical studies on seismic performance of traditional style steel-concrete composite frame
Abstracts:In order to study the seismic performance of a Chinese traditional style steel-concrete composite frame, a half-scale prototype was designed for a region with intensity seven in the Chinese seismic design code. In the experiment, the El Centro, Taft, Lanzhou and Wenchuan ground motions with different peak accelerations (0.035 g, 0.1 g, 0.22 g, 0.4 g) were used to simulate earthquakes of different characteristics and intensities. The displacement response, restoring force and acceleration response of the measured structure were analyzed, and the hysteresis characteristics, failure mechanism, deformability and energy dissipation capacity of the model were quantified. The results show that the damage caused is concentrated in the beams and at the transition zone between the upper and lower column as evidenced by noticeable pinching of the hysteresis curves. The ultimate story drift reached is about 1/157–1/153 (0.65%), satisfying the elastic-plastic limits in the code. At a peak acceleration of 0.4 g, the stiffness of the specimen is 66% lower than that at a peak acceleration of 0.035 g. The most obvious structural response is under the excitation of El Centro wave, while the smallest is under the action of Wenchuan ground motion. The finite element software SAP2000 was used to analyze the failure of the specimen, the displacement time history curve and sequence of the plastic hinges. The calculated values of the finite element are in reasonable agreement with the experimental values, which can supply some reference to the practical engineering application.
An experimental study on cracking and deformations of tensile concrete elements reinforced with multiple GFRP bars
Abstracts:Although different setups have been developed for analysis of the serviceability properties (cracking and deformations) of reinforced concrete elements, tensile tests are remaining the most often used testing layouts. Recent studies, however, have revealed noticeable limitations of the traditional tests of concrete prisms reinforced with a centre bar. The essential aspect responsible for adequate interpretation of the test outcomes could be addressed to inter-correlation of the basic cross-section parameters, i.e. bar diameter, reinforcement ratio, and cover depth. Furthermore, the test equipment has a limited possibility comparing outcomes of the tensile prisms reinforced with bars made of steel and fibre reinforced polymer materials. To solve these problems, a special equipment for the anchorage of multiple bars has been developed. This manuscript presents and discusses the tests results of 16 prismatic specimens reinforced with steel and glass fibre reinforced polymer (GFRP) bars provided by different producers. At the same deformation range of reinforcement, almost identical crack distances are characteristic of the prisms reinforced with steel and GFRP bars with similar axial stiffness. This result enables formulating a hypothesis that crack spacing in tensile elements of equivalent axial stiffness is predominantly dependent on geometry of the concrete and, particularly, on the cover depth.
Double lap shear test on steel fabric reinforced cementitious matrix (SFRCM)
Abstracts:The present work deals with the experimental characterization of the mechanical bond behaviour of a galvanized steel fabric reinforced cementitious matrix (SFRCM) laminated on concrete support. The specimens, made of two low strength concrete blocks connected with a galvanized steel fabric embedded in a geo-polymeric mortar layer, have been tested according to double lap test (DLT) set-up. Six different groups of specimens have been tested varying both the lamination length and the steel fabric density. In order to reproduce the load-slip or bond-slip curves, a tri-linear bond slip model together with its parameters identification has been proposed.
Optimal form and size characterization of planar isotropic petal-shaped auxetics with tunable effective properties using IGA
Abstracts:Focusing on planar isotropic petal-shaped auxetics, an isogeometric design framework is presented to achieve tunable effective properties. Specifically, the design framework includes (i) a NURBS-based parametric modelling scheme that characterizes petal-shaped auxetics with a small number of design variables; (ii) a systematic consideration of petal form, component widths and base material properties; (iii) a semi-analytical sensitivity analysis method based on material derivatives; and (iv) constraints for effective stiffness and target Poisson ratio. Three cases are considered: Case A with the same component width, Case B with different component widths, and Case C for composite designs with multiple base materials. For each case, a design limit curve is obtained for the effective Poisson ratio over a range of effective stiffness constraints, to give a quick overview on the properties attainable for each design setting. The optimization framework is next demonstrated for designing composite petal-shaped auxetics with target effective properties.
Dynamic fracture analysis of functionally graded materials under thermal shock loading by using the radial integration boundary element method
Abstracts:A coupled thermoelastic radial integration boundary element method is applied to analyze the dynamic fracture mechanics of functionally graded materials (FGMs) subjected to the thermal shock loadings. The dynamic stress intensity factor (DSIF) of the crack tip is defined by the crack open displacement (COD) near the crack tip. The effects of material gradating direction versus crack direction and the coupling effects on the stress intensity factor are studied for two- and three-dimensional crack structures. The results demonstrate that the present method is very accuracy and efficiency to analyze the dynamic fracture mechanics for the cracked FGMs. The research also provides a theoretical basis for engineering design, and can extend the application range of the boundary element method.