Auxetic compliant flexible PU foams: static and dynamic properties

Abstract

The paper describes the manufacturing and tensile testing of auxetic (negative Poisson's ratio) thermoplastic polyurethane foams, both under constant strain rate and sinusoidal excitation. The foams are produced from conventional flexible polyurethane basis following a manufacturing route developed in previous works. The Poisson's ratio behaviour over tensile strain has been analyzed using an Image Data processing technique based on Edge Detection from digital images recorded during quasi-static tensile test. The samples have been subjected to tensile and compressive tests at quasi-static and constant strain-rate values (up to 12 s -1 ). Analogous tests have been performed over iso-volumetric foams samples, i.e., foams subjected to the same volumetric compression of the auxetic ones, exhibiting a near zero Poisson's ratio behaviour. The auxetic and non-auxetic foams have been also tested under sinusoidal cycling load up to 10 Hz, with maximum pre-strain applied of 12%. The hysteresis of the cycling loading curve has been measured to determine the damping hysteretic loss factor for the various foams. The measurements indicate that auxetic foams have increased damping loss factor of 20% compared to the conventional foams. The energy dissipation is particularly relevant in the tensile segment of the curve, with effects given by the pre-strain level imposed on the samples.

The manufacture and characterisation of a novel, low modulus, negative Poisson’s ratio composite

“…It has been known to be theoretically possible since Love [7]. Auxetic behaviour has been observed in both naturally occurring and synthetic materials [8][9][10][11][12][13] and may provide many benefits over conventional positive Poisson's ratio behaviour, such as increased shear stiffness, synclastic curvature, increased fracture toughness and enhanced indentation resistance [5 and Scarpa [15]. Interestingly there are signs that auxetic materials may have advantages over conventional equivalents in damping applications [15].…”

“…Auxetic behaviour has been observed in both naturally occurring and synthetic materials [8][9][10][11][12][13] and may provide many benefits over conventional positive Poisson's ratio behaviour, such as increased shear stiffness, synclastic curvature, increased fracture toughness and enhanced indentation resistance [5 and Scarpa [15]. Interestingly there are signs that auxetic materials may have advantages over conventional equivalents in damping applications [15]. It has been the goal to produce an auxetic composite for some time and some examples exist which rely upon specific stacking sequences of otherwise conventional lamina and fibres [16,17], rather than fibres which are auxetic in their own right.…”

“…It is therefore conceivable that composites of this nature could be manufactured commercially without the need for developing new manufacturing techniques. Auxetics are known to have many realised and potential benefits over positive Poisson's ratio equivalents [6][7][8][9][10][11][12] but these low modulus examples may be especially useful for damping applications [15], particularly in aerospace or automotive roles.…”

Auxetic Foam for Snow-Sport Safety Devices

“…For a given foam, the energy absorbed in the compression cycle increased with pad Figure 8a shows example compressive stress-strain curves for samples from the large foam conversion. The curves all show an extended quasi-linear region followed by progressive stiffening at approximately 30% compression, characteristic of auxetic foam [32,35]. The corner sample was slightly stiffer above approximately 20% compression, with maximum stress ranging from 13 to 16 kPa in comparison to 11 to 13 kPa for the edge and centre samples.…”

On the Damped Beams with Hysteresis

“…The nonlocal damping force is modeled as a weighted average of the velocity field over the spatial domain, and it is determined by a kernel function based on distance measures [17]. The damping capacity of the auxetic foam, in particular the conventional grey open-cells polyurethane foam [18], is tested by adding it under the form of the external elements to conventional materials [19][20][21].…”