P.R. Cunningham

A temperature correction methodology for quantitative thermoelastic stress analysis and damage assessment

In thermoelastic stress analysis, an infrared detector is used to obtain the small temperature change resulting from the thermoelastic effect. The output from the detector, known as the thermoelastic signal, is dependent on both the surface stresses and the surface temperature of the component under investigation. For quantitative thermoelastic stress analysis, it is important that the response resulting from changes in the surface temperature is decoupled from the response resulting from the stress changes. In this paper, a means of decoupling the response is presented that involves making corrections for increases in surface temperature so that the thermoelastic signal is dependent only on the stresses. The underlying theory is presented and a correction factor is developed using an experimental approach. A methodology for applying the correction factor to full-field data is provided. The methodology is validated through a number of case studies and applied to a composite component subject to fatigue damage initiated at a central hole.

A new measurement technique for the estimation of core shear strain in closed sandwich structures

Sandwich structures have been widely used for many years in applications such as aircraft panels, marine-craft hulls, racing car bodies and spacecraft solar arrays. Most sandwich panel designs include a lightweight core such as paper honeycomb or closed cell foam encased between two face plates, and in the case of aircraft panels constructed from carbon fibre reinforced plastics, the core is bevelled and edge pan plies are included to totally enclose the core. This type of design restricts access to the core making it almost impossible for the engineer to measure the shear strain developed in the core during in-service static or dynamic loading. This paper introduces a new measurement technique whereby the shear strain in the core can be estimated from face plate measurements using a linear finite difference approximation. The estimation method is presented and supported by calculations on a statically loaded sandwich beam. Static and dynamic experiments were conducted in order to validate the technique using a honeycomb sandwich beam instrumented with strain gauges on the core and face plates. The results showed excellent agreement between measured and estimated core shear strain for sandwich configurations with thin face plates, such as those encountered in aircraft and marine-craft constructions.

Dynamic response of doubly curved honeycomb sandwich panels to random acoustic excitation. Part 2: Theoretical study

In this paper a single-degree-of-freedom model is developed to predict the dynamic response of an acoustially excited doubly curved sandwich panel. Three variants of the model are investigated, based on differing assumptions regarding the spatial distribution of the applied loading. When the loading is assumed to be uniform then the model reduces to the Miles approach, and when the loading is assumed to conform to the structural mode shape then the method is very similar to the Blevins approach. The third variant involves a more detailed consideration of the travelling wave characteristics of the applied loading, and this is found to give much improved agreement with experimental results obtained in a progressive wave tube facility. In addition, an approach using the finite element method is presented in which the response to grazing incidence excitation is computed, and this is also found to yield good agreement with the experimental results.

Parameter optimization of the dynamic behavior of inhomogeneous multifunctional power structures

For next generation microsatellites and nanosatellites, new design approaches will be required to significantly increase their payload to mass fraction. One proposed technology is the multifunctional design concept that incorporates spacecraft subsystems into the load carrying structure. The focus of the research is the multifunctional power structure which replaces conventional battery systems in a spacecraft. An analytical and finite element analysis of ten multifunctional sandwich structures is presented. The out-of-plane material properties are discussed and a parameter optimization of the ten sandwich panels is carried out to optimize their frequency to density ratio. The best configuration for an optimized multifunctional power structure is then identified from the analytical and finite element investigation. The optimized design provides a similar predicted dynamic response as a conventional honeycomb sandwich panel, and can be considered a serious alternative for future spacecraft. Copyright

Experimental Determination of the Dynamic Behavior of a Multifunctional Power Structure

A main aim of spacecraft design is cost reduction that can either be achieved by reducing the cost of the spacecraft or by lowering the cost to launch. One proposed technology to reduce the mass and therefore lower the launch costs is the multifunctional power structure concept that incorporates the secondary spacecraft power supply into the load carrying structure. Here a short introduction of the dynamic analysis and optimization of such a structure is presented. The manufacture and testing of a multifunctional power panel is discussed in detail and its dynamic response is compared to a conventional honeycomb panel. The multifunctional design successfully combined the structural and power storage functions. It provided a similar dynamic response to conventional spacecraft structures and improved the energy density.

A review of analytical methods for aircraft structures subjected to high-intensity random acoustic loads

Abstract A review of the acoustic fatigue design process for aircraft structures is presented in this paper, together with the current design guides, which are used to predict the stresses that an acousticallly loaded aircraft structure may experience in service. These methods are based on linear theory and use the single-degree-of-freedom approximation method. A recent programme of research which uses this method together with the finite element method to predict the root mean square strains experienced by acoustically excited, doubly curved sandwich panels is briefly discussed. Recent developments in prediction methods based on the non-linear dynamic response of thermoacoustic loaded structures are reviewed, and suggestions are made as to possible future directions in the area of acoustic fatigue research

Development of a Temperature Calibration Device for Thermoelastic Stress Analysis

A means of calibrating the effect of temperature on the thermoelastic signal obtained from the Deltatherm system is described. A design of a suitable calibration device is covered and sample results presented and discussed.

Experimental analysis of damping across joints in metal plates

In the current world of engineering, structural vibration problems continue impact the design and construction of a wide range of products. Amid the parameters that determine the dynamic behaviour of a structure the one that takes into account the dissipation of energy resulting in the decay of the vibration is the least understood and the most difficult to quantify [1]. The estimation of damping factors is of interest in most branches of engineering sciences. In the field of aircraft structures the damping directly affects the fatigue life, a parameter which is applied conservatively due to the inherent complexity in modelling the damping of built up structures and the potentially catastrophic consequences of a fatigue failure. One of the most important problems is the limited knowledge of how joints affect the damping of the complete structure. This work therefore addresses this issue and focuses on the damping of joints in metal plates as part of a larger project to investigate the damping of built up structures. Various plate configurations are experimentally investigated using two different approaches. The results from the configurations are compared and discussed along with the advantages and disadvantages of each experimental approach. This enables a link to be identified between the damping magnitudes and the mode shapes and joint stiffnesses.

Verification of various modelling techniques for simply-supported piezoelectric actuated thin panels

Abstract In this work, three techniques for the mathematical modelling of a piezoelectric actuated thin panel, namely the finite element method, a Lagrange Rayleigh—Ritz method, and a mechanical impedance-based method, are briefly presented. An accurate experimental implementation of a piezoelectric actuated simply-supported panel, whose dynamics have been simulated using the mathematical models, is described in detail. Since the differences between the results produced by the various mathematical models are very small, the accuracy of the experimental set-up is crucial. The results obtained via the numerical simulations are then compared with test results in order to assess the accuracy of the various modelling techniques.

Experimental and numerical investigation of the effect of asymmetry on the residual strength of a composite sandwich panel

Asymmetric sandwich panels with skins of differing thickness are subjected to various degrees of damage via quasi-static indentation before compressive loading to failure. These are compared with panels with skins of equal thickness. The experiments show that the asymmetric panels experience an improvement in strength with small amounts of indentation compared with undamaged asymmetric panels, and for more severe damage, show greater residual strength than the symmetric panels. The two configurations are numerically modelled using Abaqus, including inter- and intra-laminar damage, and core crushing. The strength predictions from the models agree well with the experiments.