Search

This work describes a series of investigations carried out on sets of pristine and auxetic (negative Poisson's ratio (NPR)) open‐cell polyurethane foams subjected to relative humidity (RH) conditioning ranging from 9% to 92% at room temperature. The foams have been produced using a uniaxially thermoforming process. Pristine and auxetic foams have then been subjected to quasi‐static compressive cyclic loading (with maximum strains of 10% and 80%, respectively), as well as vibration transmissibility tests with base accelerations up to 2.29 g. Increasing levels of RH do not seem to statistically affect the moduli and PRs of foams subjected to lower maximum strains, however, especially the auxetic foams at higher compressive deformations show a decrease in the stiffness with the increase of the RH. The vibration tests show an increasing trend of the dynamic modulus of the foams with the increase of the RH. The results indicate the complexity of the interaction between foam architecture and absorption/desorption mechanisms occurring inside these porous auxetic materials. [ABSTRACT FROM AUTHOR]

We study the shear behavior of pristine and uniaxially thermoformed auxetic foams using three different shear testing techniques and finite element simulations. A novel simple shear test rig with sliding lateral supports was developed and compared to fixed simple shear and diagonal pure shear rigs. The three rigs yield similar results at shear strains under 8 %, but the fixed simple shear rig shows a significant rise in shear modulus and stiffness at higher strains. In contrast, the sliding simple shear and diagonal pure shear rigs exhibit similar linear results up to 20 % shear strain. The thermoformed auxetic foam displays strong orthotropy, with a shear modulus of 35 kPa along the thermoforming direction and 0.1 MPa in the transverse directions. The pristine foam shows good isotropy under shear, with a similar 50 kPa shear modulus in all directions. The auxetic foam expands slightly along the thermoforming direction at high strains, while the pristine foam and auxetic foam in other directions show noticeable shrinkage. Finite element models based on 3D μ-CT scans, using a new boundary coupling element to apply periodic boundary conditions, closely matched the experimental results. The simulations revealed the deformation mechanisms within the foam cell structures that contributed to the observed orthotropic shear behavior. [ABSTRACT FROM AUTHOR]

Rigid polyurethane foams (RPUFs) are among the most used polymeric materials due to their tailorable properties. Greener and biobased polyol sources for synthesising RPUFs have been recently prototyped because of the ever-growing demand for more sustainable materials solutions. Castor Oil has also rapidly become one of the most successful alternatives to replace polyurethane fossil-based components. This study extensively investigates three types of RPUFs developed from commercially available Castor Oil-based resins, correlating their properties across different length scales. In particular, X-ray diffraction identified a characteristic peak whose intensity correlated with the apparent foam density, morphology (obtained from computed tomography scans) and mechanical properties. Furthermore, quasi-static compression and vibration transmissibility tests revealed distinctive anisotropic mechanical responses among the foams, with one type of RPUF consistently outperforming the others due to its lower porosity, reaching 13.3 MPa of compressive modulus. These findings demonstrate the possibility of tuning the mechanical properties of foams via compositional control. • Extensive characterisation of Castor Oil-based rigid polyurethane foams. • Foam morphology and mechanical properties can be tuned via composition control. • Correlation of the foam's XRD pattern with porosity and density. [ABSTRACT FROM AUTHOR]

[Display omitted] • Design of polyurethane foam composites with naturally abundant sepiolite nanoclay and biopolymer chitosan via simple and facile layer-by-layer coatings. • The coatings reinforce the foam skeleton, resulting in a 200% improvement of the Young's and dynamic moduli compared to untreated foams. • Transmitted impact forces are reduced by 50% with six bilayer coatings, demonstrating an improved impact energy dissipation. • Detailed discussions are provided about the deformation mechanisms and enhancement of the quasi-static and dynamic mechanical performance. Flexible polyurethane foam composites with enhanced stiffness and energy dissipation have been prepared via a facile layer-by-layer assembly approach. The composite foams consisted of naturally abundant nanoclay/chitosan multilayers (up to six) deposited onto the foam struts via dip-coating. The nanoclay/chitosan polyurethane foams were characterised using infrared spectroscopy, scanning electron microscopy, elemental mapping and μ-CT scanning. Quasi-static mechanical compression of the foams with 6 bilayers showed a 202% increase in the stiffness and a 33% enhancement in the damping loss factor compared to the uncoated pristine foam. Vibration transmissibility tests showed that the dynamic modulus of the 6-bilayer coated foams was 3 times that of the pristine foam. Remarkably, impact tests registered a 50% decrease in the transmitted impact force of these sepiolite/chitosan layer-by-layer coated open cell polyurethane foams, demonstrating their improved energy dissipation capability compared to other nanocoated foams in open literature. [ABSTRACT FROM AUTHOR]

This work proposes a new dynamic poroelastic model to describe the vibration transmissibility of auxetic polyurethane (PU) foams. The model includes the pneumatic damping effect provided by the porous structure of the foam, and the viscoelasticity of the PU material. The auxetic foam is manufactured using a simplified and relatively low-cost uniaxially thermoforming compression technique, which allows the production of a foam with transverse isotropic properties. Stress relaxation, quasi-static cyclic compressive-tensile and seismic base vibration tests in the small strains and linear elastic regimes have been performed in this work. The permeability and the viscoelasticity of the auxetic foams with different thermoforming compression ratios along the thermoforming and transverse directions are identified by performing a nonlinear regression from the mechanical and vibration-derived data. The auxetic foams possess lower stiffness along the thermoforming direction and higher modulus along the transverse one. The moduli of different auxetic foams obtained from quasi-static tests are 40%–70% lower than the corresponding dynamic values estimated from the seismic base excitation, while the static loss factors are all around 20%–100% lower than the corresponding dynamic ones. The enhancements of the dynamic stiffness and damping are mainly caused by the pneumatic damping effect and the viscoelasticity of the PU material. The permeability of the auxetic foam loaded along the thermoforming direction is ∼ 2 times larger than the one of the pristine foam, but 50% smaller when the foam is dynamically loaded along the transverse direction, due to the transverse isotropic microstructure of this porous material. [ABSTRACT FROM AUTHOR]

Auxetic (negative Poisson's ratio) open cell polymeric foams have been traditionally produced using thermoforming techniques coupled with the volumetric compression of conventional (pristine) off-the-shelf foams. The anisotropy of the pristine foam plays however a strong role on the performance of thermoformed auxetic foams, and this is the subject studied in this work. Micro computed tomography (μ-CT) scans indeed show that the cell structures of the pristine foam exhibit an elongated convex polyhedron shape, with more ribs oriented along the foam rising direction d 1. The modulus along d 1 can be up to 70% larger than the one along the transverse directions, and the Poisson's ratios in different planes are also noticeable different, ranging from 0.2 to 0.85. Three types of auxetic foams uniaxially thermoformed along the three orthogonal directions of the conventional porous material are here developed, and all show different mechanical performances due to the anisotropy of the conventional baseline foam. The auxetic foams show significant auxetic behavior along the thermoforming direction only when thermoformed along the transverse directions of the pristine foam (d 2 or d 3) and loaded along the foam rising direction (d 1); in that case, the negative Poisson's ratio reach values as low as −1. In other cases, the auxetic performance is not evident, and the Poisson's ratio is close to 0. Finite Element models of the pristine and auxetic foams have been also here developed based on the 3D models from μ-CT, and the stiffness-strain curved derived from those coincide well with the experimental results. The simulated deformation mechanisms of the cell structures inside different foams help to explain the anisotropic performance of the foams and the differences between compressive and tensile properties. [ABSTRACT FROM AUTHOR]

• New class of uniaxially thermoformed auxetic foam under quasi-static and impact tests. • Custom drop tower design described. • Strain fields, poisson's ratios and energy absorption metrics in different planes measured. • Parametric energy absorption metrics determined for different manufacturing parameters. • Nagy's model to relate quasi-static stiffness to the dynamic one of these foams developed. This work describes large strain quasi-static and impact tests of a new class of low-cost, uniaxially thermoformed, transverse isotropic auxetic foams with compression ratio ranging from 20 to 80%. A custom drop tower rig with high-speed cameras suitable for these soft porous foam materials is designed and used to perform impact tests with energy ranging from 0.38 to 1.90 J, corresponding to strain rates from 64/s to 143/s. The thermoformed foams only show auxeticity when loaded along the transverse direction to the uniaxial thermoforming compression, with Poisson's ratio V 21 reaching as low as −1.5 at 20% strain and then close to 0 at large strains. The samples deform with large shear band deformations, also due to the auxeticity. The negative Poisson's ratio foams along that transverse direction also show enhanced impact energy absorption performance, with normalized peak force reduction as high as ∼40% against the pristine foam; the reduction is however 20% along the thermoforming direction. A constitutive model based on Nagy's approach is applied to describe the enhancement of dynamic stress during the impact tests compared with the quasi-static one, due to strain rate effects. [ABSTRACT FROM AUTHOR]

[Display omitted] • Auxetic foam produced via uniaxial thermoforming in thick blocks and variable volumetric compressions. • Stiffness, Poisson's ratios and energy dissipated in quasi-static compression and tension tests. • Topological and quantitative characterization of the foams via μ-CT scanning. • High-fidelity and benchmarked Finite Element models of the foam microstructures. We present here a simplified procedure to manufacture auxetic PU foam via a single direction thermoforming compression process applied to conventional (pristine) open cell foam samples. The auxetic foams produced here have a Poisson's ratio ν 21 ranging from −1 to 0 in the 1–2 plane and a tangent modulus E t2 ranging from 0.2 MPa to 2 MPa. X-ray μ-CT and 3D skeletonization enabled the extraction of the topological parameters of the pristine and the auxetic foams. The auxetic foam is transverse isotropic with cell structures exhibiting a re-entrant shape along the thermoforming compression direction 1. More ribs are also oriented within the transverse plane 2–3, in which the stiffness is larger. Tensile and compression quasi-static tests have been carried out on samples cut along different directions and having various thermoforming compression ratios r c. Auxeticity is present in the in 1–2 plane only for compression ratios between 40% and 80%. Finite Element models for the pristine and auxetic foams have also been built from the μ-CT scanned 3D models. The numerical results show a good agreement with the experimental data and help explaining the deformation mechanisms of these auxetic foams. [ABSTRACT FROM AUTHOR]