A Practical Procedure for Measuring the Stiffness of Foam like Materials

Abstract

Polymer foams are used extensively in everyday life, from disposable packaging and soft furnishings through to engineering applications such as core structural materials in the marine industry or bone analogue materials for orthopaedic device testing. In the engineering field it is important that the mechanical behaviour of these materials is characterised correctly, as computationally based predictions of structural performance rely heavily on accurate input data. Mechanical property data from standard physical tests such as uniaxial compression are subject to artefacts including non-uniformity of applied loading, test fixture-sample contact conditions, and test machine compliance. These are well-documented problems, which techniques such as extensometry and point tracking of marker pairs attempt to resolve. In particular, in addition to being non-contact, the use of individual marker pairs can reveal non-linear behaviours because of alignment issues. In the current work, a practical, accurate experimental methodology is introduced to investigate this issue. Uniaxial compression tests were conducted on cellular polyurethane foam blocks. Both faces of the foam specimens were monitored using point tracking on multiple marker pairs to account for misalignment. Sample deformation was simultaneously measured by test-machine crosshead displacement. The Young’s modulus and Poisson’s ratio were calculated in both cases. To verify the measurements, digital volume correlation (DVC) was applied. DVC is a full-field non-contact strain measurement method that interrogates the interior structure of the foam to determine the physical response. Results demonstrated that misalignment effects could easily be followed during testing, which averaged out on both front and back surfaces to produce a single modulus value. Considerable differences were evident between crosshead displacement calculated modulus and point tracking, indicating that artefacts can lead to substantial errors, as evidenced in the published literature.