Discrete Stiffness Tailoring: Optimised design and testing of minimum mass stiffened panels

Discrete Stiffness Tailoring (DST) is a novel manufacturing concept where stiffness tailoring is achieved using discrete changes in ply angle to favourably redistribute stresses. Resulting performance increases can be exploited to potentially achieve lightweight rapidly manufacturable structures, uninhibited by the minimum tow-turning radii which limit continuous fibre steering approaches. An efficient two-stage optimisation routine is implemented to design a DST minimum-mass stiffened aircraft wing panel subject to buckling and manufacturing feasibility constraints. The panel is manufactured and compression tested to failure, extending the DST design concept to component level for the first time. A weight reduction of 14.4% is achieved compared to a constant stiffness optimum, through redistribution of load to the stiffener region. The optimum design removes material from the skin, between stiffeners. Experimentally, the optimised tailored panel achieved a buckling load, without failure, within 5% of that predicted, validating both the methodology and modelling

Minimum-mass optimisation for high-rate manufacture of damage tolerant and unbuckled composite components

A significant increase in the rate of composite manufacture is needed to meet demand for short-range commercial aircraft. The enabling automated manufacturing processes can, however, induce undesirable process features such as wrinkles. Additionally, the potential for Barely Visible Impact Damage has resulted in widespread use of overly-conservative strain allowables which has led to overweight aircraft structures. Two new constraints are presented which enable formability and damage tolerance to be incorporated into a two-stage minimum-mass optimisation framework for performance and manufacturability. An efficient, approximate method is presented for determining a conservative lower bound on the strain required to propagate a single, circular delamination, given a general through-thickness position and an upper bound on delamination size. A Compatibility Index is used to predict the propensity for wrinkles to occur during a forming manufacturing process. Optimised stacking sequences for two benchmark design problems; a flat plate and blade-stiffened panel, are obtained subject to minimum formability, damage tolerance and buckling constraints alongside common industry design rules. The damage tolerance and formability constraints are met for a diverse set of design requirements, without increasing mass or reducing buckling load, thereby demonstrating that components may be optimised for manufacture using high-rate processes without detriment to performance.</p