Materials Science Forum Vol. 813

Abstract: This paper proposes an efficient analytical failure analysis approach for multilayered composite risers used in the offshore oil industry. This approach is based on a layer-by-layer progressive damage model and a homogenization stress analysis method from which the discontinuous stresses through layers can be efficiently calculated. Different failure theories can be easily integrated into the approach to determine failure initiation in layers with different materials. Progressive failure analysis is based on layer-by-layer material degradation schemes, taking into consideration different failure modes such as yielding, fracture, matrix cracking, fiber broken, etc., in layers with different materials. In this approach, progressive failure information involving failed layers and their failure sequences as well as failure modes can be efficiently predicted for multilayered composite risers under given loading conditions. Failure envelopes of composite risers are generated for either initial failure or ultimate failure in different load spaces, and strengths of composite risers can be predicted under given load ratios. This analytical approach is efficient for failure analysis or strength prediction of composite risers with many layers because stress redistributions in all layers during failure progression can be easily and quickly calculated. A user-friendly interface based on Excel sheets is used to carry out this analytical failure analysis approach. Failure analyses of a 22-layer composite riser under several typical loading conditions are presented to demonstrate the application of the proposed approach. Initial and ultimate failure envelopes of the composite riser are shown in different force spaces. This failure analysis approach provides an efficient way for design of composite risers in the offshore oil and gas industry.

Abstract: An aerodynamic and structural integrated design optimization method of composite wind turbine blade based on multidisciplinary design optimization (MDO) is presented. The optimization aims to reduce the mass of blade under some constraints, including the power and deflection at the rated wind speed, and the strength and deflection under ultimate case. The design variables include parameters both in aerodynamic and structural disciplines. In order to keep the shape of blade smooth,the chord and twist distributions are controlled by the Bezier function in the optimization process. 3D parameterization of blade was carried out in Finite Element Analysis (FEA) software. Considering tip-loss and hub-loss, aerodynamic analysis was performed by using Blade Element Momentum (BEM) theory. Finite Element Method (FEM) was used in structural analysis. Multi-island Genetic Algorithm (MIGA) which has excellent exploration abilities was used to optimize wind turbine blade. RBF meta-model was construct to approximate the accurate structural analysis model by Optimal Latin Hypercube DOE sample points. An example was given to verify the method in this paper. The result shows that the optimization method has good optimization efficiency and the RBF meta-model could reduce the computational cost a lot.

Abstract: There are challenges of using composite laminates in the marine engineering, i.e., composites are frequently suffering from the effects of impaction including wave impaction, ship or other objects hitting, missiles or bullets hitting and other especially conditions. It is significant to understand the impact behaviors of laminates, in this research, the impact responses of typical laminates are investigated numerically. The delamination responses among the plies and fibre and/or matrix damage responses within the plies are simulated to understand the impaction behaviours of laminates under impaction conditions. The impact damage of composite laminates in the form of intra-and/or inter-laminar cracking is modelled by using stress-based criteria for damage initiation, and fracture mechanics technique is used to capture its evolution. Interface cohesive elements are inserted between plies with appropriate mixed-mode damage laws to predict the delamination. A group of graphite fibre/epoxy laminates with impact energies of 5, 10, 15 and 20 J, respectively, are simulated with a full scale FE model and a simplified FE model respectively. Through comparing the simulation results with each other, we find out that the impact behaviors obtained in the simplified FE model is comparable to experiments with a short computing time, but the simplified model cannot represent the properties of laminate after impact.

Abstract: This paper discussed the residual thermal stress on single lap joint of various materials. Finite element method (FEM) was adopted to simulate the experimental phenomena. The adherend and adhesive were the main research objects. Mismatch of adherend, adhesive thickness, temperature variation which were the three key factors on the residual thermal stress were analyzed. While, four kinds of materials that include C/SiC, SiC, high temperature nickel based alloy GH1035 (GH1035) and high temperature boron fiber reinforced epoxy composite (composite) formed seven approaches for numerical analysis. The results showed that the adhesive with the C/SiC has the best performance and the best thickness of every approach was determined. Moreover, the ladder temperature is better than other temperature styles.

Abstract: Scarf repair parameters were optimized in this paper, to find the best repair parameter, and get better strength restoration effect. Composite laminates material was IM6/3501-6. Considering symmetry of structure, 1/4 repaired structure was taken as research object, to be built up finite element model and optimized for repair parameter. Element SOLID46 in software ANSYS was used to simulate main board and patch, element SOLID45 was used to simulate adhesive layer, to build up composite scarf repair finite element model. Because of small scarf repair angle and small ratio of adhesive layer thickness to composite laminate thickness, repair structure geometry was complex, to get exact calculation result, mesh was refined at place where stress is concentrated. There were 4488 element in the built up finite element model, with nodes number as 6263. Tsai-Wu rule and maximum shear stress was taken as failure criteria for composites laminate and adhesive layer respectively. With consideration on scarf ratio and adhesive layer thickness on repaired specimen strength, three dimension finite element models for scarf repair on composite laminates was constructed. Compared with experiment, it was found that finite element model agreed with experiment, thus validity of scarf repair model was proved. By Changing the value of each factor, finite element calculation of the repaired structure failure load showed that: the optimal value of scarf ratio is 1:20; influence of adhesive layer thickness of failure load of scarf repair structure is not obvious, generally 0.1mm meets the requirements; failure load increases when adhesive layer elastic modulus increases, but the rate of increase slows down gradually. Then the earliest failure site was analyzed according to finite element stress calculation result, failure process of adhesive layer was simulated, adhesive layer failure process was predicted, from 0 degrees to 45 degrees, and the adhesive layer top part first destroys, then spreads to 45 degrees to 90 degrees, and spreads to the adhesive layer lower end.

Abstract: In the past few years, owing to the vital role in maritime safety monitoring and marine hazard warning, stratospheric airship has become a research hotspot around the world. The structure of stratospheric airship is a typical aerated flexible membrane one. It is geometrical nonlinear in essence and it bears a distinct mechanical nature of large deformation. Therefore it makes optimizing the structure more difficult. The current studies of air-supported flexible membrane structural optimization ignore the uncertainty influence which is resulted from the loading environments, material properties and structural parameters. According to the Lagrangian Coordinate method, the deformed configuration is used as a reference in order to build accurate strain/displacement geometry. The Equilibrium equation is created by using the principle of virtual work. Meanwhile, the accuracy and stability of the solution is ensured by using the Updated Lagrangian Formulation. The uncertain parameters which are introduced during the analysis and optimization of the aerated flexible membrane structure are portrayed by interval mathematical theory. With respect to engineering application, the factors of uncertainty are considered. Maximizing the structural fundamental frequency (or first-order natural frequency) is taken as the objective, while structural integrity is taken as the constraint. Then the dynamic optimization is carried out under interval uncertainty. In this way the reliability of the membrane structure in complex conditions is improved and the probability of failure is reduced.

Abstract: Aerodynamic heating will be caused when a hypersonic vehicle is in flight, so it should be taken into account when a structural dynamic analysis is carried out. A method for structural dynamic analysis under thermal environment is presented in this paper. Firstly, Computational Fluid Dynamic (CFD) method which is based on solving the three-dimensional compressible Navier-Stokes (N-S) equations is employed to perform aerodynamic heating analysis. Secondly, Finite Element Method (FEM) is used to assess structural heat conduction and get temperature field. Lastly, based on the quasi-linear structural model, a structural dynamic analysis is analyzed under thermal environment. A hypersonic composite wing is investigated as a numerical example. Compared with the case without considering aerodynamic heating, major modes and frequencies of the wing change a lot because of the decrease of the elastic modulus and the alteration of structural stiffness distribution. The results indicate that the aerodynamic heating has a high influence on the vibration characteristics. Therefore, aerodynamic heating should be considered in hypersonic structural dynamic analysis.

Abstract: During the last decades the application of composite materials in various structures has become increasingly popular. Advanced hybrid composites are useful in marine and aerospace engineering. In this contribution, impact response and damage process by a drop-weight instrument on glass reinforced (GLARE) 5 fibre-metal laminates (FMLs) with different impacted energy were presented. After comparing and analyzing the histories of contact force, central displacement, absorbed energy and force-deflection for GLARE 5 (2/1) with impacted energy of 8J, 10J and 15J, respectively. The fact that aluminium layers play an important role in absorbing energy was proved. Moreover, A numerical methodology including user material subroutine VUMAT, Johnson–Cook flow stress model is employed to simulate the response of the contact force, deflection, absorbed energy and corresponding failure modes in low-velocity impact of FML. After comparing the five simulation results with different mesh density, the influence of mesh density on simulation results was investigated and presented. The optimalizing mesh size which considered both computational efficiency and accuracy was found.

Abstract: Composites are getting more and more attention in the usage of major structures on ships owing to many advantages such as high specific strength and rigidity. However, composites are sensitive to impact, which indicates that the load-carrying capacity and stability of composite structures will decrease dramatically even for an invisible impact damage. Hence, a real-time impact monitoring system which has already been applied in aviation industry is important for reliability of ships, which inevitably encounter impacts during their lifetime due to different sources. This paper deals with the realization of an operational processing scheme to monitor the impact on composite structures, and estimate the impact location and load history by power distribution approach and system identification method. A PZT (piezoelectric lead zirconate titanate) sensor network was designed and employed to collect sensing signals for monitoring the impact. In the monitoring process, stress wave signals caused by impact were captured, and used to determine the impacted location and load history. Results demonstrate that the proposed method can be successfully applied in identifying the location and load history of impact on composite structures. The impact identification approach is of great significance for marine applications.