Abstract:
The strong relationship between localised stresses in sandwich composites and their overall structural performance demands a robust understanding of the core materials' constitutive properties in order for informed design decisions to be made. However, in the standardised tests used to determine the core's properties the material is assumed to be in a state of spatially invariant deformation for the duration of the experiment. This assumption is often violated when testing cores which exhibit considerable post-yield deformation. In order to better understand the mechanical performance of such materials, more comprehensive experimental data characterising their response to applied loads must be obtained; full eld deformation measurement using digital image correlation (DIC) presents a potential means of achieving this. The overall aim of this work is to determine if DIC based full eld strain measurement techniques can be used to achieve more accurate constitutive characterisation of ductile polymeric foam cores in order to enable more accurate failure predictions for the structures in which they are utilised. To this end a signi cant amount of work is carried out re ning the Centre for Advanced Composite Materials' in-house DIC system, determining the tracking implementation which is most appropriate for the analysis of cellular foams, and attempting to minimise errors inherent in the DIC method in general. This is followed by a critical comparison of the full eld deformation exhibited during standardised mechanical characterisation tests with the constitutive data obtained from them. The use of colour images is found to be problematic for DIC due to the interpolation carried out during the demosaicing process; novel methods are proposed for dealing with the systematic measurement errors often encountered with the technique; and the use of a subset shape function, often recommended to improve tracking accuracy, is found to detrimentally a ect tracking robustness when analysing polymeric foams. The global material responses during characterisation tests are found to be a poor representation of the local constitutive properties of the materials, particularly post-yield. While full eld deformation measurement shows promise for obtaining more accurate data, there is a need for test methods for which the post-yield stress states are well understood.