Arbitrary bi-dimensional finite strain crack propagation

In the past two decades numerous numerical procedures for crack propagation have been developed. Lately, enrichment methods (either local, such as SDA or global, such as XFEM) have been applied with success to simple problems, typically involving some intersections. For arbitrary finite strain propagation, numerous difficulties are encountered: modeling of intersection and coalescence, step size dependence and the presence of distorted finite elements. In order to overcome these difficulties, an approach fully capable of dealing with multiple advancing cracks and self-contact is presented (see Fig.1). This approach makes use of a coupled Arbitrary Lagrangian-Eulerian method (ALE) and local tip remeshing. This is substantially less costly than a full remeshing while retaining its full versatility. Compared to full remeshing, angle measures and crack paths are superior. A consistent continuationbased linear control is used to force the critical tip to be exactly critical, while moving around the candidate set. The critical crack front is identified and propagated when one of the following criteria reaches a material limiting value: (i) the stress intensity factor; or (ii) the element-ahead tip stress. These are the control equations. The ability to solve crack intersection and coalescence problems is shown. Additionally, the independence from crack tip and step size and the absence of blade and dagger-shaped finite elements is observed. Classic benchmarks are computed leading to excellent crack path and load-deflection results, where convergence rate is quadratic

Influence of rebar diameter in concrete cracking studied using a discrete crack approach

The ability to model fracture propagation is critical to predict the structural response of a concrete member and possible failure mechanisms. Even for structures under normal service loads, good estimations on the overall deflections and associated stiffness are highly dependent on the onset of fracture and resulting crack pattern, both directly related with the tension-stiffening effect. With the aim of developing robust models to capture this behaviour, a preliminary numerical study is herein presented on the influence of rebar diameter in the cracking pattern of concrete beams captured numerically. Focus is first given to the performance of the numerical model and its mesh objectivity. Experimental results from concrete beams tested under flexural loads are adopted for validation. Two very distinct rebar diameters are used to assess the ability of the model in predicting the average crack opening, maximum crack opening and average crack spacing for a wide range of loads.ARC DE150101703, ARC DP14010052

Shear strength of cross laminated timber-concrete connections reinforced with carbon fibre polymer composites

This paper presents an experimental study on the mechanical behaviour of four different timber concrete composite (TCC) connections. Experimental shear tests were conducted to assess the strength and stiffness of the composite connection system. A new type of connection based on carbon fibre reinforced polymer (CFRP) was also proposed to achieve enhanced performance at the cross laminated timber. The stiffness and strength for each type of connection were identified by assessing the load-slip behaviour and load capacity. In addition, the relationship between applied force and slip characteristic of each type of connecting system was also established. Results showed that the addition of CFRP to the connection can effectively improve the maximum shear capacity and stiffness of the connection.ARC DE150101703, ARC DP140100529, ARC LP14010059

Seismic vulnerability of multi-span continuous girder bridges with steel fibre reinforced concrete columns

The occurrence of earthquakes during the lifetime of bridges can result in the closure or even the failure of the structure. The vulnerability of multi-span continuous girder (MSCG) bridges strongly depends on the seismic performance of its columns. This paper investigates the seismic vulnerability of MSCG bridges with reinforced concrete (RC), steel fibre reinforced concrete (SFRC) and RC-SFRC columns. Quasi-static tests were conducted on eight different columns to obtain the limit-state capacities and validate numerical models. Numerical analyses were performed on MSCG reference bridges with RC, SFRC or RC-SFRC columns to derive probabilistic models of the demands upon critical components. Fragility curves were established as a function of peak ground acceleration by integrating the capacity distributions with correlated component demand distributions. Results indicate that: i) SFRC columns with 1.0% fibre ratio show a better performance-price ratio for improving the structural capacity, compared with those with 1.5%; ii) MSCG bridges with SFRC and RC-SFRC columns are less vulnerable to earthquakes when compared with those constructed using only RC, and these differences increasing with the earthquake intensity; and iii) the seismic vulnerability of MSCG bridges with SFRC placed only at the plastic hinges is similar to the one found SFRC on the whole column. On a broader perspective, the conclusions drawn could offer a new strategy for the seismic enhancement or retrofit of MSCG bridges in earthquake regions, with optimal use of SFRC.National Natural Science Foundation of China 51508276, Fundamental Research Funds for the Central Universities of China (30915011329), the China Postdoctoral Science Foundation (Grant No.2015M570399), the Australian Research Council DE150101703

Comparison of Reynolds averaging Navier-Stokes (RANS) turbulent models in predicting wind pressure on tall buildings

This paper presents a detailed comparison of using Reynolds Averaging Navier-Stokes (RANS) approach in predicting wind pressure on a super-tall 406 m slender tower with circular cross-section. The results obtained from wind tunnel tests using a rigid model approach in a boundary layer wind tunnel (BLWT) were compared to that of Computational Fluid Dynamics (CFD) numerical simulations. The main objective of this study is to critically investigate the possibility of using RANS turbulent model based CFD approach in tall building design. Three different RANS turbulence models were compared with the wind tunnel data in predicting flow characteristics. The detailed wind tunnel experimental procedure and numerical approach are discussed and presented. It was shown that the shear stress transport (SST) variant model,could predict pressure coefficients comparable to that of the wind tunnel experiments. The influence of flow separation point on flow characterisation and pressure prediction is highlighted. The improvement that can be made in the near-wall region in the finite volume mesh to achieve an accurate separation point is presented. The effects of Reynolds number produced in the wind tunnel and scaled-down numerical models were compared with the anticipated full-scale flow Reynolds number. Hence, it is shown that a correct modelling technique in CFD using RANS turbulence models can be used as an alternative design approach of super-tall structures to estimate wind-induced pressures.ARC DE150101703, CERDS USy

An embedded formulation with conforming

Use of strong discontinuities with satisfaction of compatibilit

Influence of fibres on the mechanical behaviour of fibre reinforced concrete matrixes

An experimental analysis focused on the mechanical behaviour of fibre reinforced concrete matrixes (FRCM) is presented using a total of three hundred and twelve specimens. A reference plain mixture was first defined and then three types of fibres were chosen to reinforce it (polypropylene, glass and steel fibres). Within each type of reinforcement, four volumetric proportions were adopted, ranging from 0.5% to 2% in 0.5% increments. The influence of each type of fibre and dosage on the properties of the FRCM, including compressive strength, bending behaviour, cracking and maximum loads and ductility was analysed. In summary, it was observed that the compressive strength generally grows with the reinforcement dosage, and that this growth is greatly affected by the properties of the fibre, namely by its tensile strength. The load-displacement curves are also highly affected by the type of reinforcement. Steel and polypropylene fibres provide the composite material a better capacity to withstand high deformations. Glass fibres have a reduced effect on this regard, due to their brittle behaviour. For each type of fibre, by increasing the fibres percentage, an increase in the load capacity is also observed, with a maximum of 160% for an addition of 2.0% of steel fibres. The cracking loads are consistently lower than that of the reference mixture, due to the loss of homogeneity and increased porosity caused by fibre addition, in spite of the favourable influence associated to the mechanical properties of the fibres. For polypropylene FRCM the cracking loads were approximately 35% lower than that of the reference mixture. For steel and polypropylene fibres the toughness indexes (I5, I10 and I20) were defined, being observed that for 1.5% volume fraction of steel fibres the I5 and I20 are respectively 6.80 and 35.08, whereas for the polypropylene fibres those indexes are respectively of 3.61 and 15.75 for the same fraction.FCT PTDC/ECM/119214/2010, FCT SFRH/BD/84355/2012, ARC DE15010170

Numerical modelling of concrete beams at serviceability conditions with a discrete crack approach and non-iterative solution-finding algorithms

This paper describes the development and validation of a comprehensive numerical model enabling the simulation of reinforced concrete beams at serviceability conditions using a discrete crack approach. The highly non-linear behaviour introduced by the different material models and the multiple cracks localising and propagating within the member pose a challenging task to classic iterative solvers, which often fail to converge. This limitation is solved with a non-iterative solution-finding algorithm, in which a total approach is used to overcome critical bifurcation points. The finite element model is validated using experimental data concerning lightweight aggregate concrete beams under flexural loading. The model is shown to properly simulate both overall and particular features of the structural response, including curvature, crack openings and crack patterns. The model is then applied to carry out a small parametric study on the role of the longitudinal reinforcement ratio and crack widths in reinforced concrete beams.ARC DE150101703, FCT UID/ ECI/04029/201

Numerical modelling of concrete beams at serviceability conditions with a discrete crack approach and noniterative solution-finding algorithms

This paper describes the development and validation of a comprehensive numerical model enabling the simulation of reinforced concrete beams at serviceability conditions using a discrete crack approach. The highly non-linear behaviour introduced by the different material models and the many cracks localising and propagating within the member pose a challenging task to classic iterative solvers, which often fail to converge. This limitation is solved with a non-iterative solution-finding algorithm, in which a total approach was used to overcome critical bifurcation points. The finite element model was validated using experimental data concerning lightweight aggregate concrete beams under flexural loading. The model was shown to properly simulate both overall and localised features of the structural response, including curvature, crack openings and crack patterns. The model was used to carry out a numerical study on the role of the longitudinal reinforcement ratio and crack widths in reinforced concrete beams. It was observed that the total crack openings along the member seem to remain nearly independent of the tensile reinforcement for ratios above 2.5% and the same level of strength.ARC DE15010170

Element-wise fracture algorithm based on rotation of edges

We propose an alternative, simpler algorithm for FEM-based computational fracture in brittle, quasi-brittle and ductile materials based on edge rotations. Rotation axes are the crack front edges (respectively nodes in surface discretizations) and each rotated edge affects the position of only one or two nodes. Modified positions of the entities minimize the difference between the predicted crack path (which depends on the specific propagation theory in use) and the edge or face orientation. The construction of all many-to-many relations between geometrical entities in a finite element code motivates operations on existing entities retaining most of the relations, in contrast with remeshing (even tip remeshing) and enrichment which alter the structure of the relations and introduce additional entities to the relation graph (in the case of XFEM, enriched elements which can be significantly different than classical FEM elements and still pose challenges for ductile fracture or large amplitude sliding). In this sense, the proposed solution has algorithmic and generality advantages. The propagation algorithm is simpler than the aforementioned alternatives and the approach is independent of the underlying element used for discretization. For history-dependent materials, there are still some transfer of relevant quantities between meshes. However, diffusion of results is more limited than with tip or full remeshing. To illustrate the advantages of our approach, two prototype models are used: tip energy dissipation (LEFM) and cohesive-zone approaches. The Sutton crack path criterion is employed. Traditional fracture benchmarks and newly proposed verification tests are solved. These were found to be very good in terms of crack path and load/deflection accuracy