Abstract
In parametric design studies, the strength of a structure is often considered as the primary design criteria, and consequently the optimal (best) structural design is often chosen as the one that minimises the maximum stress generated. However, for structures whereby failure is governed by fracture or fatigue, the residual strength, as distinct from stress, needs to be considered as the explicit design objective. In this study, the design space for the distribution of residual strength for different structural configurations is evaluated to demonstrate the utility of design space exploration for damage tolerance design optimisation. This was illustrated using the problem of the optimum design of a cutout shape under biaxial load. The geometry of the cutout was represented parametri- cally, and numerous flaws/cracks were assumed to be located along the structural bounda- ries. The maximum stress intensity factors associated with the flaws along the boundary were evaluated for each cutout geometry. Finite element modelling was used to calculate the stress field, and a semi-analytical method was used for computation of the stress intensity factors. The design surface for residual strength was found to resemble a 'ship hull'. This shape con- firms that a design based on residual strength indeed poses a well-behaved optimisation problem, i.e. a well-defined minimum/maximum region exists. The flatness of the design space for residual strength was demonstrated. The optimum values of the stress intensity factor ob- tained from the design space agreed well with those determined using various optimisation methods in the literature. It is shown that the residual strength optimised shape can be quite different from the corresponding stress optimised solution. This emphasises the need to ex- plicitly consider residual strength as the design objective. It is shown that a design space ex- ploration can provide a systematic way to reduce the weight of a structure by adopting a 'fea- sible non-optimal' solution that meets the design criteria, rather than aiming for the 'optimal' (best) solution.
Highlights
The use of light weight structures is of prime consideration in the aerospace industry
Design space exploration and optimisation are closely related in that in structural optimisation we move through the design space using an algorithm in order to improve the current design, whereas in a design space study we attempt to obtain an overall view of the variation in the design objective function with structural geometry/shape
A design space exploration study has been performed to gain an insight into the variation of the damage tolerance based objective function with structural geometry and to illustrate the utility of design space exploration in the context of design optimisation
Summary
The use of light weight structures is of prime consideration in the aerospace industry This often results in high operational stress levels and increases the likelihood of crack initiation and propagation. We previously developed a residual strength optimisation technique based on a heuristic evolutionary algorithm (Fracture ESO in [1]), and applied it for maximising the residual strength of a stringer cutout in a bulkhead, located in pressurised fuselages of transport aircrafts [2] We subsequently extended another heuristic method (Biological algorithm) for residual strength [3] and fatigue life [4, 5] optimisation of three-dimensional structures. Design optimisation including damage tolerance parameters is an inherently iterative process It usually involves analysis of several trial systems in order to identify an acceptable design. The optimum solutions obtained via the design space study are compared with those predicted by different structural optimisation methods
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