Optimized placement of parasitic vibration energy harvesters for autonomous structural health monitoring

Energy harvesting, based on sources including vibration and thermal gradients, has been exploited in recent years to power telemetry, small devices, or to charge batteries or capacitors. Generating the higher levels of power which have thus far been required to run sensor systems such as those needed for structural health monitoring has been more challenging. In addition, harvesters such as those required to capture vibration often require additional elements (e.g. cantilevers) to be added to the structure and harvest over a relatively narrow band of frequencies. In aerospace applications, where weight is at a premium and vibrations occur over a broader range of frequencies, this is non-ideal. With the advent of new, lower power monitoring systems, the potential for energy harvesting to be utilized is significantly increased. This article optimizes the placement of a set of parasitic piezoelectric patches to harvest over the broad band of frequencies found in an aircraft wing and validates the results experimentally. Results are compared with the requirements of a low-power structural health monitoring system, with a closing of the gap between the energy generated and that required being demonstrated

Optimization of postbuckled stiffened panels with multiple stiffener sizes

The panel analysis and optimization code VICONOPT, based on exact strip theory, is utilized to investigate the optimum design of stiffened panels with multiple stiffener sizes or substiffeners. The optimization ensures that the buckling stability of the panel includes an allowance for postbuckling reserve of strength. The adoption of this approach necessarily results in the local buckling stress being lower than the overall buckling stress and with the introduction of substiffeners introduces extra buckling modes. This complicates the post buckling behavior of the panel which is investigated by examining the case when the smaller stiffeners lose stiffness, i.e. there is a change from a local to a torsional mode. The panels are loaded in axial compression with a sinusoidal imperfection. It is found that small mass savings are achieved by using stiffeners of more than one size and there is an increase in the spacing of the major stiffeners and transverse supports. The optimum panel designs obtained by VICONOPT are evaluated by comparison with the optimum designs produced with one size of stiffener

Improved acoustic emission source location during fatigue and impact events in metallic and composite structures

In order to overcome the difficulties in applying traditional Time-Of Arrival (TOA) techniques for locating Acoustic Emission (AE) events in complex structures and materials, a technique termed “delta-t mapping” was developed. This paper presents a significant improvement on this, in which the difficulties in identifying the precise arrival time of an AE signal are addressed by incorporating the Akaike Information Criteria (AIC). The performance of the TOA, the delta-t mapping and the AIC delta-t mapping techniques is assessed by locating artificial AE sources, fatigue damage and impact events in aluminium and composite materials respectively. For all investigations conducted the improved AIC delta-t technique shows a reduction in average Euclidean source location error irrespective of material or source type. For locating H-N sources on a complex aluminium specimen the average source location error (Euclidean) is 32.6, (TOA), 5.8 (delta-t) and 3mm (AIC delta-t). For locating fatigue damage on the same specimen the average error is 20.2, (TOA), 4.2 (delta-t) and 3.4mm (AIC delta-t). For locating H-N sources on a composite panel the average error is 19.3, (TOA), 18.9 (delta-t) and 4.2mm (AIC delta-t). Finally the AIC delta-t mapping technique had the lowest average error (3.3mm) when locating impact events when compared with the delta-t (18.9mm) and TOA (124.7mm) techniques. Overall the AIC delta-t mapping technique is the only technique which demonstrates consistently the lowest average source location error (greatest average error 4.2mm) when compared with the delta-t (greatest average error 18.9mm) and TOA (greatest average error 124.7mm) techniques. These results demonstrate that the AIC delta-t mapping technique is a viable option for AE source location, increasing the accuracy and likelihood of damage detection, irrespective of material, geometry and source type

Buckling optimization of blended composite structures using lamination parameters

In this paper, a new lamination parameter based method is proposed for the layup optimization of built-up composite laminates with ply drop-offs. The optimization process is divided into two stages. In the first stage, the multilevel optimization feature of the exact strip software VICONOPT MLO is extended to use the lamination parameters and laminate thicknesses of each component panel as design variables to minimize the weight of the whole structure subject to buckling and lamination parameter constraints. For the second stage, instead of using the common heuristic optimization methods, a novel dummy layerwise branch and bound (DLBB) method is proposed to search the manufacturable stacking sequences to find those needed to achieve a blended structure based on the use of 0°, 90°, +45° and −45° plies and having lamination parameters equivalent to those determined in the first stage. The DLBB method carries out a logical search to circumvent the stochastic search feature of heuristic methods for the determination of stacking sequences. This two-stage method is an extension of a previous highly efficient two-stage method for a single laminate (Liu et al., 2019) [1]. The effectiveness of the presented method is demonstrated through the optimization of a benchmark wing box

Buckling and vibration of stiffened panels or single plates with clamped ends

An efficient method for the buckling and vibration analysis of plates or stiffened panels with clamped ends is presented. The method uses Lagrangian multipliers to couple sinusoidal modes with appropriate half-wavelengths of response, thereby enforcing the end conditions at discrete point supports. Clamped ends can usually be modelled accurately using only a few point supports, while arguments from symmetry often enable some of the required end conditions to be satisfied without explicitly applying constraints. In such cases few half-wavelengths are needed to obtain excellent accuracy. Solutions obtained for the simple limiting case of single plates are exact or within 1% of the classical or other reported solutions. Solutions obtained for stiffened panels are in close agreement with those obtained using finite element analysis

A novel parallel method for layup optimization of composite structures with ply drop-offs

This paper presents a novel parallel optimization method for obtaining blended layups of composite laminates which closely match target lamination parameters. Firstly, a guide-based adaptive genetic algorithm (GAGA) which stochastically searches the layups is developed. Then the parallel optimization method DLBB-GAGA is developed by combining GAGA and a dummy layerwise branch and bound method (DLBB) which performs logic-based search in a parallel computation, during which optimization information is shared between the two methods. The combination of these two different methods gives the parallel DLBB-GAGA method the advantages of both, enhancing the searching ability for the blended layup optimization. The superiority of the parallel DLBB-GAGA method is demonstrated through comparisons between the three methods, and it is concluded that the method is particularly effective for practical design where more layup design constraints are considered