Abstract:
Polymeric foam cores are used by composite engineers striving to increase the impact resistance of high-performance sandwich structures in multiple disciplines. There is a growing demand to better
understand how dynamic loadings, particularly in marine applications, affect foam-cored sandwich
structures' response. The transverse shear characteristics of the polymeric foam cores typically drive
design choices but are still poorly understood in dynamic loading scenarios due to current testing
methods producing difficult-to-analyse data.
This work has developed existing dynamic beam testing methods to produce easier-to-analyse results.
A typical 4-point flexure rig has had two load cells added to the support points, allowing for more
direct force measurement on the specimen from the IMATEK IM10 Drop Weight Impact Testing
system. A series of strain gauges on the facesheets were applied and found very few signs of higher-order oscillatory behaviour that could have interfered with the results. Quasi-static 4-point flexure
tests were carried out, forming a baseline for comparisons. Far less noisy data was produced by the
addition of the support load cells than historically experienced, allowing for points of interest other
than ultimate failure to be analysed.
Digital Image Correlation was used to take a more detailed look at the strain fields within the core
throughout the test duration. Contour plots and transverse sections of the strains helped to elevate
the understanding of when and where plasticity begins within the foam cores. It was found that the
typically assumed yield point has a significantly non-uniform strain field. This indicates that material
properties derived using the ASTM C393 standard should be treated cautiously.
Energy and failure prediction methods were assessed and found to be accurate within the linear-elastic region when actual material properties were used, as opposed to datasheet properties.
Understanding individual constituent capacities have increased the knowledge of how a sandwich
structure supports a load. A simple expression was derived that allows the structural response of the
material and geometric choices to be evaluated efficiently. Higher-density foam cores were found to
carry less energy per kilonewton squared than a lower density core. However, they are able to carry
more energy before yield. Strain energy density prediction methods were highlighted to have the
potential to capture true material properties better. Overall, the assessment concluded that
predictions would not vary within the linear-elastic region given any starting point, even when
considering simple dynamic impact methodology.
Four beam constructions were assessed using the new dynamic methodology. It was shown that
meaningful conclusions could be extracted from the test data. The ability to generate meaningful data
will enable better informed future structural designs when using polymeric foam core sandwich
constructions subject to dynamic loading scenarios.