The computationally efficient analysis and optimum design of the buckling of stiffened, thin-walled shear panels in aircraft structures is discussed. Namely, the postbuckling behavior of these panels is assessed using the iterative procedure developed by Grisham. This procedure requires only linear finite element analyses, whereas convergence is typically achieved in as few as five iterations. An algorithm developed by (A. F. Grisham, "A Method for Including Post-Buckling of Plate Elements in the Internal Loads Analysis of Any Complex Structure Idealized Using Finite Element Analysis Methods," AIAA Paper 78-515, April 1978) using connect format, is compared with empirical methods of analysis frequently used in aircraft structures and also with a refined, nonlinear quasi-static finite element analysis. It is shown that the procedure proposed by Grisham overcomes some of the conservatism inherent in conventional methods of analysis. In addition, the method is notably less expensive than a complete nonlinear finite element analysis, which makes it attractive for use during initial design iterations, even though global collapse of a structure cannot be predicted. As an illustration of the optimal design of buckled, stiffened thin-walled structures, the Grisham algorithm is combined with a microgenetic algorithm. Important reductions in weight are obtained within relatively few function evaluations.
Nomenclature
A= area E = Young's modulus G = shear modulus k = diagonal tension factor L x = length of web along the x axis L y = length of web along the y axis N = internal loads of structure t = thickness of web α = diagonal tension angle γ xy = web shear distortion γ xyc = web shear distortion component due to compressive buckling γ xy DT = Postbuckled shear distortion component of web x , y = web normal strain xc , yc = web compressive buckling strain x DT , y DT = web diagonal tension strain µ = poisson's ratio σ x , σ y = web normal stress σ xc , σ yc = web compressive buckling stress σ xcr , σ ycr = web modified critical normal buckling stress σ x DT , σ y DT = web diagonal tension stress τ cr = web critical buckling shear stress τ xy = web shear stress τ xycr = web modified critical buckling shear stress
In total, 136 taxa of larval fishes representing at least 49 families were
recorded in the Mhlathuze Estuary over one year. Community structure was very
similar to that found in a similar study at the adjacent Richards Bay Harbour,
with dominating families being the Engraulidae, Gobiidae, Sciaenidae and
Clupeidae. Marine-spawned larvae dominated the catch, with the most dominant
being euryhaline species that usually breed at sea and whose juveniles show
varying degrees of dependence on estuaries. High recruitment (238 per 100
m3 and 308 per 100 m3) occurred
during November 1998 and July 1999 respectively, these being the months of
lowest river discharge. Results indicated that a physical variable, in this
case river flow, is probably the forcing factor regulating spawning and
recruitment.
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