On the bulk modulus of open cell foams.(Report)

ABSTRACT

Bulk properties of open cell polyurethane foam are studied in a hydrostatic compression experiment under strain control. A linear region of behaviour is observed in the stress-strain curve, followed by a non-monotonic region corresponding to a negative incremental bulk modulus. The bulk modulus in the linear region is in reasonable agreement with the value calculated from compressional Young's modulus and Poisson's ratio. The linear region of behaviour in hydrostatic compression corresponds to less than half the axial strain range observed in axial compression.

1. INTRODUCTION

Most experimental studies of foam properties have been conducted in tension/compression (1). In compression of a flexible polymer foam, a linear region in the load-deformation curve is observed up to about 5 % strain. This region is associated with bending (2) of the cell ribs, though early models advocated rib extension (3). Rib bending gives rise to a modulus which increases as the square of the relative density in the linear region (4). If the foam does not deviate too much from isotropy, the Poisson's ratio is close to 0.3. At higher compressive strain, there is a plateau region of reduced slope in the stress-strain curve due to buckling of the cell ribs, followed by densification in which the curve's slope increases due to contact of the cell ribs. In the plateau region, localization of strain has been observed (5) and interpreted in the context of stability (6,7). As for tension, a linear region in the load-deformation curve is observed, followed by a region of increasing slope due to cell rib orientation.

Hydrostatic compression of foam is of interest in a variety of contexts including under-sea applications. Syntactic foam, for instance, consists of hollow glass inclusions in an epoxy matrix (8,9). Such foam is intended to have a bulk modulus comparable to that of water (about 2 GPa), and sufficient compressive strength. This foam is not permeable to water since the cells do not have communicating paths. A syntactic foam was found to have (10) a Young's modulus of E = 2.76 GPa and a shear modulus of G = 1.03 GPa, The Poisson's ratio v based on the isotropic relation

E = 2G(1 + v) (1)

was v = 0.33, typical of a normal solid.

Indentation of foam, in contrast to homogeneous material, depends upon the compressibility (11,12). Strength measurement in triaxial compression, including hydrostatic compression is relevant to understanding how foam responds to impacts (13), particularly in the application of closed cell foam in packaging. The bulk modulus of these foams is less than the prediction of theoretical models, as a result of influence by wrinkles in the cell walls (14). Strength measurements (15) in hydrostatic compression have also been done on closed cell polymeric structural foam (Rohacell) in the evaluation of its crushing behavior. Related studies of strength have been conducted of multiaxial yield of aluminum alloy foam (16).

In contrast to syntactic foam and other closed cell foams, open cell foams are permeable to fluids; but …