A. Torres Marques

Comparative analysis of drills for composite laminates

The characteristics of carbon fiber-reinforced plastics allow a very broad range of uses. Drilling is often necessary to assemble different components, but this can lead to various forms of damage, such as delamination which is the most severe. However, a reduced thrust force can decrease the risk of delamination. In this work, two variables of the drilling process were compared: tool material and geometry, as well as the effect of feed rate and cutting speed. The parameters that were analyzed include: thrust force, delamination extension and mechanical strength through open-hole tensile test, bearing test, and flexural test on drilled plates. The present work shows that a proper combination of all the factors involved in drilling operations, like tool material, tool geometry and cutting parameters, such as feed rate or cutting speed, can lead to the reduction of delamination damage and, consequently, to the enhancement of the mechanical properties of laminated parts in complex structures, evaluated by open-hole, bearing, or flexural tests.

Optimization of laminated composite structures using a bilevel strategy

This paper aims to obtain the optimal design of laminated composite structures. The design variables are specified as the ply angles and the laminate thicknesses. The objective is to obtain a structure with minimum weight that is capable of carrying a given set of static external forces without failure. The solution strategy consists of addressing the problem sequentially at two levels, each having a different algorithm and a separate set of design variables. The optimization techniques use a combination of sensitivity analysis combined with optimality criteria and mathematical programming. The adequacy of the solution is assured by constraints limiting stresses and displacements.

Reliability based design with a degradation model of laminated composite structures

Uncertainties in deviations of physical properties lead to a probabilistic failure analysis of the composite materials. The proposed optimization model for laminate composites is based on reliability analysis considering the ultimate failure state. To avoid difficulties associated with the complete analysis of the failure modes, bounds are established for the failure probability of the structural system. These bounds are related with theintact and degraded configurations of the structure. Using thefirst ply failure and thelast ply failure theories and a degradation model for the mechanical properties with load sharing rules we obtain the failure probabilities corresponding to the two above configurations. The failure probability of each configuration is obtained using level 2 reliability analysis and the Lind-Hasofer method.The optimization algorithm is developed based on the problem decomposition into three subproblems having as objectives the maximization of the structural efficiency atintact and degraded configurations of the structure and weight minimization subjected to allowable values for the structural reliability. Additionally, the search for the initial design is performed introducing a weight minimization level. It is expected to explore the remaining load capacity of the structures afterfirst ply failure as a function of the anisotropic properties of the composites. The design variables are the ply angles and the thicknesses of the laminates. The structural analysis for the model developed is performed through the finite element method mainly using the isoparametric degenerated shell finite element. The sensitivities are obtained using the discrete approach through the adjoint variable method. In order to show the performance of the analysis two examples are presented.

Analysis of reinforced concrete with external composite strengthening

A finite element model for the analysis of reinforced concrete with external composite materials strengthening is presented in this paper. The model is based on a concrete material model, external unidirectional composites, the Ahmad Shell element and geometric and material non-linearities. The first-order shear-deformation theory is used, in order to describe the deformation of the shell under the total lagrangian formulation. An example of a RC plate without and with external FRP reinforcement is presented and discussed.

Comparison of Three Shear-Deformation Theories in the Non-Linear Analysis of Sandwich Shell Elements

Sandwich shell structures are tipically found in may structural applications. The correct modelling of its structural behaviour is of relevant interest. Shear-deformation effects are always present in sandwich shells due to the difference between the core and skin characteristics. In this work it is formulated and compared a 1st order, a 3rd order and a layerwise shear deformation theories in the geometric and material nonlinear range. In the first two theories both translational and rotational degrees of freedom are laminate dependent, while in the layerwise theory the rotational degrees of freedom are layer dependent. This last theory produces constant shear deformations in each layer, but different from one layer to another. In the 1 st order theory the shear deformations are constant throughout the laminate. In the 3rd order theory, parabolic deformations are directly achieved. The finite element discretisation is made through the degenerated shell element, known as the Ahmad-Irons-Zienkiewicz element [1]. This element has proved to be very good in the analysis of not only arbitrary isotropic shells [2–6], but also composite layered shells [7–14]. Some modifications in the element basic matrices were made in order to follow the new kinematics according to each theory. Some examples are presented in order to discuss the performance of such theories in the analysis of sandwich shells.