Composite Materials in Aerospace Design

The solution of stability problems of laminated composite plates is based on the classical theory of thin homogeneous anisotropic plates.

Traditional structured analysis and design methods have been criticized because the methods lack formality to provide a design for rigorous development. Several approaches have been developed for verifying a design by integrating traditional structured analysis and design methods with formal specification languages. However, the integration of traditional structured analysis and design methods with formal specification languages may confuse the problem definition and understanding with the attempt to define system structures and algorithms in the design phase. In addition, formal specification languages may add complexity to the design due to the complexity of the language syntax and semantics. In this paper, we are presenting an approach to verification of a structured design in HOS without using formal specification languages. Rules for the partial and total correctness of the design are also discussed.

Static strength tests of mechanically fastened (bolted and riveted) joints of elements made of composites show that, depending on layup and type of stress state, their fracture can be both viscous and quasi-brittle. In the first case, net failure stress (referred to net cross-sectional area, equal to the gross one except for the area of holes) is equal to the connecting element’s ultimate strength; in the second case, it is lower than the ultimate strength. Viscous fracture in tension is peculiar to composites with [± 45°] sl layup. Quasi-brittle fracture is peculiar to the joints of elements with multidirectional layups of [(0°) i /(90°) j ] sl , [(0°) i / ± 45°] sl , [(90°) j / ± 45°] sl , [(0°) i / ± 45°/(90°) j ], etc., type, where i, j = 1, 2, 3 (s means that layups shown in square brackets are once symmetrically repeated, l = index reflecting the number of symmetrically repeated layup fragments). A change of stress type — for example, addition of shear to tension — can change viscous fracture of joint elements with [± 45°] layups to quasi-brittle fracture. Simultaneously, the direction of fracture is changed to 45°. Failure of the joints considered can also take place under bearing stress in conjunction with shear, or as a result of fracture of the fastening element. The possibility of such types of failures also requires appropriate analysis.

Stress concentration factors (SCF) derived from both FEA and measurements from laboratory experiments are associated with uncertainties. To understand these uncertainties, SCF values can be determined from a series of tests or possibly also several SCF-values estimated from converged finite element analysis. The challenge is to determine a recommended value for the SCF based on these tests or analysis, ensuring that the resulting SCF along with the SN-curve meets the code required 2.3% exceedance probability. In this paper, structural reliability analysis (SRA) methods are employed to model the combined uncertainty of the parameters representing the SN-curve and the measured SCFs from experimental test joints. Based on this analysis, the SCF that, when paired with the design SN-curve, provides the minimum fatigue life is established. From this analysis, a method to provide characteristic values of the SCF is suggested. Finally, FEA of the same detail, with and without modelling the weld, using both first and second-order elements are performed. From these analyses the SCF is estimated using linear extrapolation and direct extraction. Based on these findings and their comparison to the characteristic SCF, a suggested method to extract SCFs from FEA is provided.

In the design of structures made of composites, the material itself is also a subject for design, owing to the anisotropy of its properties. Multilayer composites are formed of a large number of monolayers (laminae) with different orientations, and this provides for the creation of materials with various sets of characteristics. Experimental investigations of the characteristics of laminated composites are very time-consuming because of the variety of their configurations, and therefore theoretical methods of analysis are of major importance. Numerical methods for determining composite stiffness, strength and fatigue performance and parametric investigations based upon these methods contribute to the development of the optimal structures that are best suited for the operational conditions of a specific article. Described below are numerical procedures for determining the stiffness and strength characteristics of laminated composites (laminates).

Analysis and design of tee joints with double support

The purposes of this paper are sixfold. First, any micromechanics model for predicting elastic property (stiffness) of a composite is applicable to a reasonable prediction of a composite strength, provided that the homogenized internal stresses in the matrix are converted into true values. The conversion for all of the stress components free of biaxially transverse loads is presented in the paper. Second, the predictability of 12 well-known micromechanics models for stiffness and strength of a unidirectional composite is assessed against the measured data of all of the nine unidirectional composites used in three world-wide failure exercises. An accuracy ranking is made accordingly. Third, it is demonstrated that the smallest fiber volume in a representative volume element for a finite element approach plays a more dominant role than other issues such as a random fiber array to achieve the highest simulation accuracy. This feature has been largely ignored in the current literature. Fourth, a consistency of a micromechanics model in calculating the internal stresses in the fiber and matrix of the composite is an issue that should be taken into account. Among the 12 models considered, only Bridging Model is consistent. A non-consistency implies that a full three-dimensional approach should be used to predict an effective property of the composite even though it is subjected to a uniaxial load. Fifth, an effective method to detect an interface debonding between the fiber and matrix subjected to an arbitrary load is presented in the paper. Only a transverse tensile strength of the composite, in addition to the original fiber and matrix properties, is needed. Whereas essentially any load can cause the interface to debond earlier before a composite failure, only a transverse tensile load carrying capacity of the composite is influenced significantly by the debonding. Specifically, an interface debonding has insignificant effect on a shear strength of the composite. Sixth, a fiber misalignment-induced kink band failure is analyzed by virtue of the original fiber and matrix properties, and a longitudinal compressive strength is predicted micromechanically.

Approximate upper and lower bounds for the strength of unidirectional composites

Experimental and numerical investigation of stress concentration and strength prediction of carbon/epoxy composites

A novel semi-analytical method for notch stress and fatigue strength analysis of tube-flange welded joints

Aluminum alloy matrix hybrid composites, in which alumina particles were dispersed among continuous alumina fibers, were fabricated by squeeze casting, and the influence of temperature and the effects of the particle-dispersion on the strength of the composites were then investigated. The particle-dispersion among the fibers minimized preform contraction and fiber-to-fiber contact due to the melt infiltration during the squeeze casting. The tensile strength, 0.2% proof stress and elastic modulus of the composites in the longitudinal direction increased with increasing the fiber volume fraction, retaining nearly the same values up to 623 K. These values of the hybrid composite were larger than those of the particle-free composite at every temperature. This is because the fibers were distributed uniformly owing to the particles that prevented the fiber-to-fiber contact, leading to the reduction of stress concentration at the points of direct fiber contact, and stress transmission between the fiber and the matrix becomes easy. At every temperature, the transverse tensile strength and proof stress of the particle-free composite was lower than that of the unreinforced alloy, because the fracture was initiated at the fiber-to-fiber contact point and the cracks propagated mainly along the fiber-matrix interface. In contrast, the strength of the hybrid composite was close to that of the unreinforced alloy even at high-temperature, because the cracks propagated mainly throughout the matrix owing to the uniform distribution of the fibers and the strong fiber-matrix bond.

20] Isolation and characterization of the link proteins

Reduction of metal consumption in engineering structures attainable by the energy-based method for strength analysis

Probabilistic analysis of stress concentration factors in tubular KT-joints reinforced with internal ring stiffeners under in-plane bending loads

수평하중이 지배적인 해상 풍력발전기 설계 시에는 지반-기초구조물 거동을 정확히 모사하여야 상부구조물에 대한 정확한 거동예측이 가능하며, 합리적 설계가 이루어질 수 있다. 현재 다양한 지반 모델링 기법이 존재하나, 모노파일 기초 설계 시, 각 해석 기법에 대한 충분한 검증 절차 없이 해석 결과를 그대로 사용할 경우 구조물을 과다 및 과소하게 설계할 우려가 있다. 이에 본 연구에서는 지반 모델링 기법 차에 따른 모노파일의 부재력 및 수평변위 차를 비교 분석하였다. 검토 결과 고정단 모델은 최대 수평변위를 과소평가 하여 사용성 검토 측면에서 적합하지 않은 것으로 나타났으며, 고정단 모델, 지반강성행렬 모델은 모노파일의 부재력을 과소평가하는 것으로 나타났다. 반면 가상고정점 모델은 모노파일의 부재력을 과대평가하여 경제성 측면에서 적합하지 않은 것으로 나타났다. 지반반력계수 모델과 p-y곡선 모델의 경우 3D 지반 모델링 해석 결과와 비교적 유사한 수평변위 및 부재력을 나타냈으며, 지반을 2D로 모델링한 경우 타 모델링 기법에 비해 과대한 수평변위와 부재력을 산정했다. To predict behaviors of offshore wind turbines which are highly laterally loaded structures and to design them rationally, evaluating the soil-foundation interaction is important. Nowadays, there are many soil modeling methods for structural analysis of general structures subjected to vertical loads, but using the methods without any consideration for design of a monopile foundation is eschewed because it might cause wrong structural design due to the deferent loading state. In this paper, we identify the differences of the member forces and displacements by design methods. The results show that fixed end method is barely suitable for monopile design in terms of checking the serviceability because it underestimate the lateral displacement. Fixed end method and stiffness matrix method underestimate the member forces, whereas virtual fixed end method overestimates them. The results of p-y curve method and coefficient of subgrade reaction method are similar to the results of 3D soil modeling method, and 2D soil modeling method overestimates the displacement and member forces as compared with other methods.