Small-Strain Elastic Properties of EPS: Insights from Video Extensometry

Accurate determination of the elastic parameters of expanded polystyrene (EPS) in the small-strain range is essential for its structural applications. This study investigates the compression modulus of elasticity and Poisson’s ratio of EPS with a nominal density of 20 kg/m³ through unconfined compression tests on cubic specimens of 50, 100, 200 and 300 mm side length and rectangular 50 × 50 × 100 mm specimens. Three deformation measurement methods were compared: video extensometer (VE) and crosshead displacement measurements performed at two independent laboratories. Two moduli of elasticity, E_0–0.5 and E_0.5–1.0, were determined within vertical strain (ε_v) ranges of 0–0.5% and 0.5–1.0%, respectively. VE-based moduli were up to twice the values obtained from crosshead displacement measurements, demonstrating that global deformation measurements significantly underestimate material stiffness in the small-strain range due to bedding error. The stress–strain relationship within ε_v ≤ 1%, commonly regarded as the elastic range of EPS, was found to be clearly nonlinear and is well described by a bilinear model. No size effect on moduli was observed in VE measurements, while crosshead displacement measurements showed increasing moduli with specimen height, stabilising at 100 mm and above. VE-based Poisson’s ratio v_0–0.5 was 0.23 for cubic specimens and 0.09 for rectangular specimens, suggesting a significant effect of specimen geometry. The results highlight the importance of local strain measurement for accurate characterisation of EPS elastic behaviour, particularly in structural applications.