Técnicas que contribuem para melhorar a qualidade dos compósitos produzidos por laminação manual ... more Técnicas que contribuem para melhorar a qualidade dos compósitos produzidos por laminação manual e cura em autoclave e/ou visando à redução no custo de processamento têm sido intensamente pesquisadas. Uma das maneiras de se obter esse ganho de produtividade é eliminar operações de usinagem da borda por meio da técnica de fabricação de laminados com bordas moldadas. Neste trabalho é apresentada uma revisão da tecnologia de fabricação de peças estruturais de compósitos poliméricos avançados, com ênfase na produção de laminados com bordas moldadas. Exemplos deste tipo de acabamento são mostrados utilizando laminados cruzados simétricos [(0,90)/0]s e [(0,90)/90]s, fabricados com fita unidirecional contínua e tecido bidirecional de carbono, pré-impregnados com resina epóxi. O arranjo de fibras e resina na região da borda livre de todos os corpos-de-prova foi analisado usando um microscópio óptico. Foi observado que as fibras longitudinais (a 0°) podem se mover lateralmente na direção da ...
An experimental program to characterize the effect of voids on the strength of composite laminate... more An experimental program to characterize the effect of voids on the strength of composite laminates is presented, and the adequacy of a fracture criterion to represent the experimental data for the effect of voids on the compressive and interlaminar shear strength of composite laminates is assessed. The experimental program investigates the effect of the material system (epoxy matrix vs bismaleimide matrix), type of reinforcement (unidirectional tape vs woven fabric), and the type of loading (compression vs interlaminar shear) on the critical void content. Laminates produced with carbon fabric/epoxy, carbon tape/epoxy, and carbon fabric/bismaleimide were produced with an intentionally high void content. The ultrasonic absorption coefficient was measured for all specimens and shown to vary approximately linearly with the void content, with the exception of the carbon fabric/epoxy laminates that presented a bilinear relationship with void content, corroborating previous experimental results. The effects of these factors on the strength of composite laminates are discussed in terms of the fracture parameters involved in the fracture criterion. The critical void content is estimated for each case both in terms of void content and ultrasonic attenuation.
Typically, aircraft wing structural panels are designed against buckling for a very large number ... more Typically, aircraft wing structural panels are designed against buckling for a very large number of possible loadings that may occur during the operation of the aircraft. If the optimisation procedure accounts only for a limited number of design loads, the structure may be vulnerable to a specific type of loading that may cause the structure to fail. A novel approach for the optimisation of ribs or plates of arbitrary shapes under uncertain loads is proposed. The geometry of the rib is defined by a single closed spline or several connected splines. The loading distribution is not considered to be uniform but it is allowed to vary within an admissible set, conferring uncertainty to the applied loads. The admissible load space comprises distributed normal and shear loadings that can be represented through a collection of piecewise linear functions defined along the plate boundary. A special procedure is applied to handle the constraint that the loading must be self equilibrating. A mi...
International Journal of Solids and Structures, 2003
... A min–max strategy is used to handle the loading variability such that the resulting optimal ... more ... A min–max strategy is used to handle the loading variability such that the resulting optimal design is able to withstand an entire ... investigations have been conducted in the past addressing the topic (Chao et al., 1975; Hirano, 1979; Haftka and Walsh, 1992; Foldager, 1999; Miki ...
International Journal of Aerospace Engineering, 2009
A fully 3D failure model to predict damage in composite structures subjected to multiaxial loadin... more A fully 3D failure model to predict damage in composite structures subjected to multiaxial loading is presented in this paper. The formulation incorporates shear nonlinearities effects, irreversible strains, damage and strain rate effects by using a viscoplastic damageable constitutive law. The proposed formulation enables the prediction of failure initiation and failure propagation by combining stress-based, damage mechanics and fracture mechanics approaches within an unified energy based context. An objectivity algorithm has been embedded into the formulation to avoid problems associated with strain localization and mesh dependence. The proposed model has been implemented into ABAQUS/Explicit FE code within brick elements as a userdefined material model. Numerical predictions for standard uniaxial tests at element and coupon levels are presented and discussed.
ABSTRACT This work presents an experimental characterization of the curvature effects on the comp... more ABSTRACT This work presents an experimental characterization of the curvature effects on the compression-after-impact strength of laminated composite shells. Curved panels impacted on the outer (convex) face and with normal pressure on the inner (concave) face with three different curvatures at three different impact energy levels were tested. A compression-after-impact testing setup was designed and implemented to evaluate the impact-induced damage tolerance of the composite shells. An analytical modeling methodology for compression-after-impact strength predictions based on the Mar-Lin and Whitney-Nuismer failure criteria is also proposed. The approach proposed herein consists of replacing the damaged area of the impacted coupon by an equivalent hole. The analytical compression-after-impact predictions obtained using the Mar-Lin and Whitney-Nuismer failure criteria were compared with experimental results. A good agreement between analytical predictions and experimental results was found. The experimental results also indicate that the compressive residual strength of the composite shells is significantly affected by the shell curvature and internal pressure effects.
Técnicas que contribuem para melhorar a qualidade dos compósitos produzidos por laminação manual ... more Técnicas que contribuem para melhorar a qualidade dos compósitos produzidos por laminação manual e cura em autoclave e/ou visando à redução no custo de processamento têm sido intensamente pesquisadas. Uma das maneiras de se obter esse ganho de produtividade é eliminar operações de usinagem da borda por meio da técnica de fabricação de laminados com bordas moldadas. Neste trabalho é apresentada uma revisão da tecnologia de fabricação de peças estruturais de compósitos poliméricos avançados, com ênfase na produção de laminados com bordas moldadas. Exemplos deste tipo de acabamento são mostrados utilizando laminados cruzados simétricos [(0,90)/0]s e [(0,90)/90]s, fabricados com fita unidirecional contínua e tecido bidirecional de carbono, pré-impregnados com resina epóxi. O arranjo de fibras e resina na região da borda livre de todos os corpos-de-prova foi analisado usando um microscópio óptico. Foi observado que as fibras longitudinais (a 0°) podem se mover lateralmente na direção da ...
An experimental program to characterize the effect of voids on the strength of composite laminate... more An experimental program to characterize the effect of voids on the strength of composite laminates is presented, and the adequacy of a fracture criterion to represent the experimental data for the effect of voids on the compressive and interlaminar shear strength of composite laminates is assessed. The experimental program investigates the effect of the material system (epoxy matrix vs bismaleimide matrix), type of reinforcement (unidirectional tape vs woven fabric), and the type of loading (compression vs interlaminar shear) on the critical void content. Laminates produced with carbon fabric/epoxy, carbon tape/epoxy, and carbon fabric/bismaleimide were produced with an intentionally high void content. The ultrasonic absorption coefficient was measured for all specimens and shown to vary approximately linearly with the void content, with the exception of the carbon fabric/epoxy laminates that presented a bilinear relationship with void content, corroborating previous experimental results. The effects of these factors on the strength of composite laminates are discussed in terms of the fracture parameters involved in the fracture criterion. The critical void content is estimated for each case both in terms of void content and ultrasonic attenuation.
Typically, aircraft wing structural panels are designed against buckling for a very large number ... more Typically, aircraft wing structural panels are designed against buckling for a very large number of possible loadings that may occur during the operation of the aircraft. If the optimisation procedure accounts only for a limited number of design loads, the structure may be vulnerable to a specific type of loading that may cause the structure to fail. A novel approach for the optimisation of ribs or plates of arbitrary shapes under uncertain loads is proposed. The geometry of the rib is defined by a single closed spline or several connected splines. The loading distribution is not considered to be uniform but it is allowed to vary within an admissible set, conferring uncertainty to the applied loads. The admissible load space comprises distributed normal and shear loadings that can be represented through a collection of piecewise linear functions defined along the plate boundary. A special procedure is applied to handle the constraint that the loading must be self equilibrating. A mi...
International Journal of Solids and Structures, 2003
... A min–max strategy is used to handle the loading variability such that the resulting optimal ... more ... A min–max strategy is used to handle the loading variability such that the resulting optimal design is able to withstand an entire ... investigations have been conducted in the past addressing the topic (Chao et al., 1975; Hirano, 1979; Haftka and Walsh, 1992; Foldager, 1999; Miki ...
International Journal of Aerospace Engineering, 2009
A fully 3D failure model to predict damage in composite structures subjected to multiaxial loadin... more A fully 3D failure model to predict damage in composite structures subjected to multiaxial loading is presented in this paper. The formulation incorporates shear nonlinearities effects, irreversible strains, damage and strain rate effects by using a viscoplastic damageable constitutive law. The proposed formulation enables the prediction of failure initiation and failure propagation by combining stress-based, damage mechanics and fracture mechanics approaches within an unified energy based context. An objectivity algorithm has been embedded into the formulation to avoid problems associated with strain localization and mesh dependence. The proposed model has been implemented into ABAQUS/Explicit FE code within brick elements as a userdefined material model. Numerical predictions for standard uniaxial tests at element and coupon levels are presented and discussed.
ABSTRACT This work presents an experimental characterization of the curvature effects on the comp... more ABSTRACT This work presents an experimental characterization of the curvature effects on the compression-after-impact strength of laminated composite shells. Curved panels impacted on the outer (convex) face and with normal pressure on the inner (concave) face with three different curvatures at three different impact energy levels were tested. A compression-after-impact testing setup was designed and implemented to evaluate the impact-induced damage tolerance of the composite shells. An analytical modeling methodology for compression-after-impact strength predictions based on the Mar-Lin and Whitney-Nuismer failure criteria is also proposed. The approach proposed herein consists of replacing the damaged area of the impacted coupon by an equivalent hole. The analytical compression-after-impact predictions obtained using the Mar-Lin and Whitney-Nuismer failure criteria were compared with experimental results. A good agreement between analytical predictions and experimental results was found. The experimental results also indicate that the compressive residual strength of the composite shells is significantly affected by the shell curvature and internal pressure effects.
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Papers by Sergio Almeida