193 8: Summary and Conclusions
This report has presented a numerical model for the prediction of the compression-after-impact strength of composite sandwich panels, and an accompanying experimental study which provided data for model validation. The experimental study revealed that the use of a thinner distal (undamaged) skin could improve the strength of mildly damaged sandwich panels over undamaged sandwich panels using the same asymmetric configuration. It is believed that this effect is due to the movement of the neutral plane of the sandwich panel caused by the reduction in the stability of the damaged skin through stiffness reduction and geometric imperfections. This removes the eccentricity of the compressive loading that exists in the undamaged asymmetric panels, which has mismatched axial stiffness between the indented skin and the thinner distal skin, and thus a noticeably lower ultimate strength than the undamaged symmetric panels.
The numerical model generally agreed well with the ultimate stress found in the experiments for these different configurations, but is quite poor at estimating the magnitude of the damage induced by the indentation. When used to model the experimental study, the model gave generally good, conservative estimates for the residual compressive strength of both the symmetric and asymmetric panels. The tendency of the asymmetric panels to become stronger with mild damage was not captured by the model per se, with the numerical results instead showing an insensitivity to damage in the asymmetric panels, which was not shared by the symmetric panels. However, the numerical model did exhibit erroneous strain-stress responses for both panel configurations, particularly for the undamaged and mildly damaged cases. Investigations revealed that this erroneous behaviour was caused by inconsistency in the material data, which had been collected partially via experimentation and partly from literature sources.
Finally, the sandwich panel model is utilised in a simulated design study looking at the effect of combined longitudinal and transverse compressive loads and out-of-plane pressure loads on panels of varying size and with and without impact damage. This final exercise made extensive use of mass-scaling to significantly improve computational efficiency, as well as further use of panel symmetry to produce a quarter-model. Thanks to these improvements, the complete study of 36 models was completed with little over one week of computational effort. This supplementary investigation demonstrated the utility of the sandwich model developed by this project in studying different candidate designs for a given application, though there was a reduction in accuracy due to the considerable mass-scaling used. Of particular interest is the apparent recovery of compressive strength seen in damaged panels with a pressure loading, with the highly-pressurised damaged panels considered in this study invariably showing an improvement in strength over their damaged, but unpressurised counterparts. On closer inspection, it appears that the bending induced by the pressure load (applied to the distal face) causes the dent in the opposing face to get pulled out by the tensile forces induced on this side of the panel, mitigating the instability induced by this geometric imperfection. This finding certainly warrants further study.
194 This project achieved its primary goal of producing a numerical model for damaged sandwich panels that can reliably produce good estimates of the ultimate compressive strength for a number of different structural and loading configurations, while taking into account all of the major damage mechanisms for composite sandwich panels. However, there remains plenty of scope for improvement. It has already been stated that consistency of material data is crucial to give trustworthy numerical results – this must be gathered via dedicated and comprehensive experimental work for the material system/s in question. The experimental study itself could have been improved via more extensive use of strain gauging to give better estimates of the overall panel stiffness, and to confirm the hypothesis that mild damage in the asymmetric panel is providing a stabilising influence under compressive loading. The addition of an LVDT or similar instrument would allow experimental verification of the global displacement observed in the numerical model, and a future study would ideally use some form of non-destructive testing, such as ultrasonic measurement, to establish the size of the delamination induced by the indentation, thus establishing the quality of the cohesive surface response used in the numerical model. Another useful addition to the experimental effort would be to choose composite lay-ups such that both symmetric and asymmetric panels have the same overall cross-sectional thickness, so that any change in strength would be purely due to changes in the neutral axis rather than simple differences in thickness. As already mentioned, the effect of pressurisation on the compression-after-impact behaviour of damaged sandwich panels would also be a worthwhile investigation, if potentially difficult to perform in practice.
Particular weaknesses identified in the sandwich model are the skin damage model and the core model. The deficiency of the skin damage model, particularly the inability to capture extensive fibre breakage, has already been discussed. This may be solved by careful adjustment of the solution and meshing parameters, for example via element deletion (which carries its own difficulties in terms of the robustness of the numerical solution), but there are also a myriad of different composite damage theories that have been developed over the past decade which may be applied to this problem. These would require a great deal of care and expertise to implement these developments correctly, as they all require self-programmed subroutines within Abaqus. The use of the cohesive zone method may also be used for this purpose, via the addition of a width-wise cohesive layer through the centre of the damaged region. The core material and crushing response used in this project was very basic, and could be readily improved. A common approach is to explicitly create the geometry of a honeycomb, and apply basic isotropic material properties to the resultant structure. Other approaches capturing the response of the core in various directions are obviously more involved, again requiring extensive experimentation to gather the needed material data and implemented via the use of user-defined subroutines. The use of a quarter-model could have been attempted earlier in the project, and the benefits this approach offered in reducing the computational effort in solving these problems make it a very valuable technique, especially if using more complex material responses, more refined meshes, or explicit honeycomb geometries (though obviously, this approach has some limitations – for example, it would not be suitable for the investigation of shear loadings, due to the non-symmetrical shape this would produce).
195 Finally, there are other problems that this model could potentially be applied to. The combined loading study in Chapter 7 considered more realistic multiple loading cases, and another study could perhaps consider the behaviour of singly- and doubly-curve sandwich panels, representative of realistic aerospace structural geometries. A different study may involve the effect of shear loading on sandwich panels, or a further look at different materials (such as form cores common to the marine industry). A useful contribution might be a numerical investigation into the effect of through-the-thickness reinforcement of the skins on the compression-after-impact performance of a sandwich, which could be perhaps be implemented simply by altering the homogenous properties of the skin and the bond. The model might also be used for the investigation of high-energy impacts, provided that strain-rate dependency of the material was included. Preparing these continuation studies would be fairly straightforward as far as the numerical model is concerned (presuming the deficiencies mentioned previously were adequately addressed), but as with the other investigations presented here, experimental verification of this would be required, and potentially difficult and costly to implement. Ultimately, the entire purpose of numerical modelling is minimise, if not eliminate, the need for experimental work. Despite the promising advancements made by this project, there remain sufficient shortcomings in the models as they currently stand that experimental validation remains necessary, particularly for these more involved loading cases and more complex geometries.
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