Journal of Aerospace Engineering

Gradient‐based mathematical‐optimization algorithms usually seek a solution in the neighborhood of the starting point. If more than one local optimum exists, the solution will depend on the choice of the starting point, and the global optimum cannot be found. This paper presents the optimization of space structures by integrating a genetic algorithm with the penalty‐function method. Genetic algorithms are inspired by the basic mechanism of natural evolution, and are efficient for global‐searches. The technique employs the Darwinian survival‐of‐the‐fittest theory to yield the best or better characters among the old population, and performs a random information exchange to create superior offspring. Different types of crossover operations are used in this paper, and their relative merit is investigated. The integrated genetic algorithm has been implemented in C language and is applied to optimization of three space truss structures. In each case, an optimum solution was obtained after a limited number of iterations.

An existing computer code to solve a supersonic potential equation around three‐dimensional configurations was modified. Using a generalized boundary‐conforming coordinate system, a full potential equation in conservation form was upwind differenced in supersonic direction. Grids were generated in each two‐dimensional cross‐flow plane using the algebraic method. A new topology for grid generation produced appropriate grids around thin wings and wing bodies. Usage of orthogonal grid generation in cross‐flow plane improved results and extended the various cases that can be handled by the code. A zonal flow field solution was obtained using a new method for the implementation of boundary conditions at wing locations on boundaries of each zone. Approximate factorizations were used to solve the resulting equations implicitly. Using these capabilities thin‐wing and wing‐body results are presented that compare well with other results. The capability of running this code on microcomputers makes this program a potentially valuable design tool in primary and middesign stages.

This paper describes the development of methodology that provides for quantification of uncertainty in the lifetime material strength degradation of structural components of aerospace propulsion systems subjected to a number of diverse random effects. The methodology is embodied in the two computer programs PROMISS and PROMISC. These programs form a material‐resistance model used in an aerospace structural‐reliability program NESSUS. A probabilistic material‐degradation model, in the form of a postulated randomized multifactor interaction equation, is used to quantify lifetime material strength. Each multiplicative term in the model has the property that if the current value of an effect equals the ultimate value, then the lifetime strength will be zero. Also, if the current value of an effect equals the reference value, the term equals one and lifetime strength is not affected by that particular effect. Presently, the model includes three effects that typically reduce lifetime strength: high temperature, mechanical fatigue, and creep. The paper also includes the statistical analysis of experimental data for INCONEL 718 obtained from the open literature. This statistical analysis of data provided regression parameters for use as the empirical material constants of the model, thus calibrating the model specifically for INCONEL 718. Model calibration was carried out for three variables, namely, high temperature, mechanical fatigue, and creep. Finally, using the PROMISS computer program, a sensitivity study was performed with the calibrated random model illustrating the effect of each variable upon random lifetime strength.

The present paper describes an approach for damage detection in composite structures that has its basis in methods of system identification. Response of a damaged structure differs from predictions obtained from a mathematical model of the original structure, where such a model is typically a finite‐element representation of the structure. In the present work dealing with composite materials, two distinct analytical models, one using two‐dimensional (2D) elements in conjunction with the classical lamination theory and another using three‐dimensional (3D) elements were considered. The output error approach of system identification was employed to determine changes in the analytical model necessary to minimize differences between the measured and predicted response. The proposed method is an extension of the stiffness‐reduction approach for damage detection to realistic structures. Numerical simulation of measurements of static deflections, strains, and vibration modes were used in the identification procedure. The methodology was implemented for representative composite structures. Principal shortcomings in the proposed approach and possible methods to circumvent these problems are discussed in the paper.

An analytical and experimental investigation was conducted to determine the effect of localized bending at through‐flaws in pressurized composite cylinders. A finite‐difference solution was formulated to determine the stress, strain, and displacement fields in the vicinity of a slit. Tests were conducted on 305‐mm‐diameter cylinders made from graphite/epoxy fabric in a $(0\text{,}45{)}_s$ configuration, with axial slits. Surface‐strain‐field measurements made in the vicinity of the slit showed a significant bending, which verified the finite‐difference solution. This significant bending near the slit tip results in a large magnification of the stresses there. An average stress criterion was employed to predict the failure response of these cylinders based on data obtained from coupon specimens. The finite‐difference solution provided correction factors to account for the localized bending. The prediction utilizing this methodology was excellent in all cases. A generalized methodology to assess damage tolerance of structural configurations with notches is proposed.

Optimal ratio of the thickness of the inner layers made of a low‐stiffness fiber‐reinforced material to the total laminate thickness is determined for sandwich hybrid laminates, with the objective of maximizing the buckling load for a given laminate thickness. The sandwich plate is constructed as a symmetric, cross‐ply laminate with outer layers made of a high‐stiffness fiber composite material and inner layers made of a low‐stiffness fiber‐composite material. The fiber orientations of high‐ and low‐stiffness materials (0° or 90°) are determined optimally, noting that the orientations of outer and inner layers affect the maximum buckling load in the case of hybrid laminates. The relative stiffness of outer layers to inner layers is shown to affect the optimal level of hybridization. The optimal laminate configurations are shown to be sensitive to the relative magnitudes of biaxial buckling loads as well as to the changes in the aspect ratio.

A parabolic shear‐deformation beam theory assuming a higher‐order variation for axial displacement has been recently presented. In this theory, the axial displacement variation can be selected so that it results in a suitable admissible transverse shear‐strain variation across the depth of the beam. This paper examines several transverse shear‐strain variations that can go with the aforementioned higher‐order theory. Apart from the usual simple parabolic variation, six other shear‐strain variations are considered: the sinusoidal variation, cubic, quartic, quintic, and sixth‐order polynomials. All these variations for transverse shear‐strain satisfy the requirement that the shear strain be zero at the extreme fibers $(z=\pm h/2)$ and nonzero elsewhere along the depth of the beam. Comparison of the results from this paper with results from others show that the simple parabolic distribution for transverse shear strain gives most accurate results. Also, Timoshenko's theory (with a shear factor of five‐sixths) and the current formulation which uses the parabolic shear‐strain distribution, give identical values for deflections.