1,721,497 research outputs found
Effective GA approach for a direct evaluation of reaction kinetic within EPDM accelerated sulphur crosslinking
No full textA direct genetic algorithm (GA) approach with kinetic base, to provide effective numerical estimates of vulcanization level for EPDMcross-linkedwith accelerated sulphur is presented. The model requires a preliminary characterization of rubber through standard rheometer tests. A recently presented kinetic exponential model is used as starting point to develop the algorithm proposed. In such a model, three kinetic constants have to be determined by means of a non-linear least-squares curve fitting. The approach proposed circumvents a sometimes inefficient and not convergent non-linear data fitting, disregarding at a first attempt reversion and finding the local minimum of a suitable two-variable error function, to have an estimate of the first two kinetic constants. A comparison between present GA approach and traditional gradient based algorithms is discussed. The last constant, representing reversion is again evaluated through a minimization performed on a single variable error function. The applicability of the approach is immediate and makes the model extremely appealing when fast and reliable estimates of crosslinking density of cured EPDM are required.
To show the capabilities of the approach proposed, a comprehensive comparison with both available experimental data and results obtained numerically with a least square exponential model for a real compound at different temperatures is provided
Homogenized limit analysis of FRP-reinforced masonry walls out-of-plane loaded
No full textA three-dimensional (3D) homogenized limit analysis model for the determination of collapse loads of out-of-plane loaded FRP reinforced masonry walls is presented. Homogenization is performed on unreinforced masonry, whereas strips are applied at a structural level on the already homogenized material. Unreinforced masonry strength domain is obtained by means of a compatible approach in which bricks are supposed infinitely resistant and joints are reduced to interfaces with frictional-cohesive behavior and associated flow rule. A sub-class of elementary deformation modes is a-priori chosen in the representative volume element (RVE), mimicking typical failures due to joints cracking and crushing. Masonry strength domains are obtained equating power dissipated in the heterogeneous model with power dissipated in a fictitious homogeneous macroscopic plate. Afterwards, an upper bound FE limit analysis code is implemented to study entire unreinforced and FRP reinforced walls out-of-plane l..
Upper bound sequential linear programming mesh adaptation scheme for collapse analysis of masonry vaults
Full text linkThe analysis of masonry double curvature structures by means of the kinematic theorem of limit analysis is traditionally the most diffused and straightforward method for an estimate of the load carrying capacity. However, the evaluation of the actual failure mechanism is not always trivial, especially for complex geometries and load conditions. Usually, the failure mechanism is simply hypothesized basing on previous experience, or - due to the complexity of the problem - FE rigid elements with interfaces are used. Both strategies may result in a wrong evaluation of the failure mechanism and hence, in the framework of the kinematic theorem of limit analysis, in an overestimation of the collapse load. In this paper, a simple discontinuous upper bound limit analysis approach with sequential linear programming mesh adaptation to analyze masonry double curvature structures is presented. The discretization of the vault is performed with infinitely resistant triangular elements (curved elements basing on a quadratic interpolation), with plastic dissipation allowed only at the interfaces for possible in- and out-of-plane jumps of velocities. Masonry is substituted with a fictitious material exhibiting an orthotropic behavior, by means of consolidated homogenization strategies. To progressively favor that the position of the interfaces coincide with the actual failure mechanism, an iterative mesh adaptation scheme based on sequential linear programming is proposed. Non-linear geometrical constraints on nodes positions are linearized with a first order Taylor expansion scheme, thus allowing to treat the NLP problem with consolidated LP routines. The choice of inequalities constraints on elements nodes coordinates turns out to be crucial on the algorithm convergence. The model performs poorly for coarse and unstructured meshes (i.e. at the initial iteration), but converges to the actual solution after few iterations. Several examples are treated, namely a straight circular and a skew parabolic arch, a cross vault and a dome. The results obtained at the final iteration fit well, for all the cases analyzed, previously presented numerical approaches
A genetic algorithm with zooming for the determination of the optimal open pit mines layout
No full textA Genetic Algorithm (GA) with nested zooming strategy is proposed for the determination of the optimal open pit mine design. Different genetic procedures are applied to increase robustness, namely two typologies of admissible mutations for the elite subpopulation subjected to zooming and mutation and reproduction for the remaining individuals. In order to further improve convergence rate, a user-defined population percentage, depending on individuals fitness, is replaced with new phenotypes, enforcing chromosomic renewal. Several comparisons with (traditionally used) dynamic programming approaches are provided both for 2D and 3D open pit mines. Both small and large scale mines are analyzed, to benchmark the code in presence of several variables. Results show that the procedure proposed requires a very limited computational effort, both for challenging problems with several variables and when a micro-GA (populations with few individuals) is adopted for small scale problems
Comparison between Upper and Lower Bound strategies to determine the homogenized strength domain of running bond masonry in-plane loaded
No full textFour models for the determination of the homogenized strength domain of running bond masonry in-plane loaded are compared. The first is a lower bound approach, where the elementary cell is subdivided into a few rectangular sub-domains, where the micro-stress field is expanded using polynomial expressions. The second is again a lower bound, where joints are reduced to interfaces and bricks are subdivided into a few constant stress triangular elements (CST). The third procedure is a compatible identification, which belongs to the upper bound family, where joints are reduced to interfaces and bricks are assumed infinitely resistant. The last model is again a kinematic (upper bound) procedure based on the so called Method of Cells (MoC), where the elementary cell is subdivided into six rectangular cells with pre-assigned polynomial fields of periodic-velocity. The first and latter models have the advantage that the reduction of joints to interfaces is not required. The second approach, albeit reduces joints to interfaces with frictional behavior, still allows to consider failure inside bricks. The third approach is the most straightforward, but is reliable only in case of thin joints and strong blocks. A critical comparison of pros and cons of all models is discussed, with reference to a real case study
Simple homogenization models for the limit and non-linear analysis of masonry structures in- and out-of-plane loaded
No full textThe paper addresses the capabilities of kinematic and static models of masonry homogenization in the prediction of both the non-linear behaviour and the homogenized strength domains for in- and out-of-plane loads. The first approach is based on an equilibrated polynomial expansion of the micro-stress field into rectangular sub-domains within the elementary cell. The second is again a model based on equilibrium, and relies into a rough FE discretization of the unit cell through triangular elements with constant stress field (CST), where mortar joints are reduced to interfaces with frictional behavior and limited strength in tension and compression. The extension to out-of-plane loads is handled by means of a standard integration of the micro-stress field along the thickness. The generalization to the non-linear range is also very straightforward. The third procedure is kinematic identification strategy, where joints are reduced to interfaces and bricks are assumed infinitely resistant. The last model is again a kinematic procedure based on the so called Method of Cells (MoC), where the Representative Element of Volume (REV) subdivided into six rectangular cells with pre-assigned polynomial fields of periodic-velocity. The first and latter models have the advantage that the reduction of joints to interfaces is not required. The second approach, albeit reduces joints to interfaces, still allows to consider failure inside bricks. The third approach is the most straightforward, but is reliable only in case of thin joints and strong blocks. At a cell level, a critical comparison of pros and cons of all models is discussed, with reference to real cases
FE homogenized limit analysis model for masonry strengthened by near surface bed joint FRP bars
No full textA homogenized limit analysis model for the prediction of collapse loads and failure mechanisms of
masonry walls reinforced with near surface bed joint GFRP bars is presented. Reinforced masonry homogenized
failure surfaces are obtained by means of a compatible identification procedure, where each brick
is supposed interacting with its six neighbors by means of finite thickness mortar joints, filler epoxy resin
and FRP rods.
In the framework of the kinematic theorem of limit analysis, a simple constrained minimization problem
is obtained on the unit cell, suitable to estimate – with a very limited computational effort – reinforced
masonry homogenized failure surfaces.
A FE strategy is adopted to solve the homogenization problem at a cell level, modeling joints, bricks,
filler and FRP rods by means of eight-noded infinitely resistant parallelepiped elements. A possible jump
of velocities is assumed at the interfaces between contiguous elements, where plastic dissipation occurs.
For mortar and bricks interfaces, a frictional behavior with possible limited tensile and compressive
strength is assumed, whereas for epoxy resin and FRP bars some formulas available in the literature
are adopted in order to take into account in an approximate but effective way, the delamination of the
bar from the epoxy and the failure of the filler at the interface with the joint.
In order to validate the model proposed, two meaningful examples are critically analyzed. The first
relies on a reinforced masonry beam in four-point bending, whereas the second is a full scale wall constrained
at three edges and loaded until failure with a distributed out-of-plane pressure. While the first
example is useful to test the model at a cell level, since only horizontal ultimate bending moment is
involved in the failure mechanism, the second provides a full assessment of the procedure proposed at
a structural level. In both cases, very good agreement is found with literature data, meaning that the
model proposed may provide useful information for all practitioners interested in the design of masonry
walls reinforced with bed joint FRP bars
Four approaches to determine masonry strength domain
Full text linkFour models to determine the homogenised strength domain of running bond masonry in-plane loaded are compared. The first is a lower bound, where the elementary cell is subdivided into a few rectangular sub-domains and the micro-stress field is expanded using polynomial expressions. The second is again a lower bound, where joints are reduced to interfaces and bricks are subdivided into constant stress triangular elements. The third procedure is a compatible identification (kinematic approach), where joints are reduced to interfaces and bricks are assumed infinitely resistant. The last model is again a kinematic procedure based on the so-called method of cells. The representative element of volume is subdivided into six rectangular sub-cells with pre-assigned polynomial fields of periodic velocity. The first and latter models have the advantage that the reduction of joints to interfaces is not required. The second approach, albeit reduces joints to interfaces, still allows considering failure inside bricks. The third approach is the most straightforward, but is reliable only for thin joints and strong blocks. Some illustrative examples regarding the determination of masonry homogenised strength domains are discussed, focusing on pros and cons of the models, role played by joint thickness, constituent materials failure surfaces and numerical efficiency
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