Stiffness Of Foams Research Articles

Incorporation of canola meal as a sustainable natural filler in PLA foams

The canola oil industry generates significant waste as canola meal (CM) which has limited scope and applications. This study demonstrates the possibility of valorization of CM as a sustainable natural filler in a biodegradable polymer composite of Poly(lactic acid) (PLA). Generally, interfacial bonding between natural fibers and the polymer matrix in the composite is weak and non-uniform. One possible solution is to derivatize natural fibre to introduce interfacial bond strength and compatibility with the PLA polymer matrix. Here, CM was succinylated in a reactive extrusion process using succinic anhydride at 30 wt% to get 14% derivatization with 0.02 g of -COOH density per g of CM. The CM or succinylated CM at 5 and 15 wt% was co-extruded with amorphous PLA to get composite fibers. CM-PLA and succinylated CM-PLA biocomposites were foamed using a mild and green microcellular foaming process, with CO2 as an impregnating agent without any addition of organic solvents. The properties of the foams were analyzed using differential scanning calorimetry (DSC), Dynamic mechanical thermal analysis (DMTA), shrinkage, and imaging. The addition of CM or succinylated CM as a natural filler did not significantly change the glass transition temperature, melting point, percent crystallization, stiffness, and thermal stability of PLA foams. This suggests succinylation (modification) of CM is not a mandatory step for improving interphase compatibility with the amorphous PLA. The new PLA-CM foams can be a good alternative in the packaging industry replacing the existing petroleum-based polymer foams.Graphical

Experimental approach for characterizing the nonlinear, time and temperature‐dependent constitutive response of open‐cell polyurethane foams

AbstractElastomeric foams are composite materials comprising of a polymeric elastomer and interconnected gas‐filled pores, endowing them with exceptional compliance and the ability to undergo large, reversible deformations along with substantial volume change. These foams find extensive utility in contexts demanding compliance and compressibility, such as impact protection and cushioning, spanning a diverse range of applications. Changing temperature can dramatically alter foam stiffness, strength and deformation characteristics specifically around the material's glassy‐rubbery transition temperature ( ). With the aim of informing new constitutive model developments for elastomeric foams, we conducted a comprehensive series of large deformation, homogeneous compression and tension tests across strain rates from 10−2 s−1 to 100 s−1 and ambient temperatures ranging from −10°C to 50°C covering an even range around the material's of 20°C. To achieve precise control of ambient temperatures during mechanical testing, we constructed a custom‐designed environmental chamber for controlling the ambient temperature from −10°C to 50°C with a variation of less than 1°C. The obtained digital image correlation based stress‐strain data shows significant tension‐compression asymmetry as well as significant dependence on strain rate and ambient temperature, especially above and below the glass transition temperature. We provide full access to these data sets for the future development of rate‐ and temperature‐dependent constitutive models.

Architected flexible syntactic foams: Additive manufacturing and reinforcing particle driven matrix segregation

Polymer syntactic foams are transforming materials that will shape the future of next-generation aerospace and marine structures. When manufactured using traditional processes, like compression molding, syntactic foams consist of a solid continuous polymer matrix reinforced with stiff hollow particles. However, polymer matrix segregation can be achieved during the selective laser sintering process with thermoplastic polyurethane (TPU). It is uncertain what role hollow particles play in forming this matrix segregation and its impact on the corresponding mechanical properties of syntactic foams. We show that the size of the hollow particles controls the internal microscale morphology of matrix segregation, leading to counter-intuitive macroscale mechanical responses. Particles with diameters greater than the gaps between the cell walls of the segregated matrix get lodged between and in the walls, bridging the gaps in the segregated matrix and increasing the stiffness of syntactic foams. In contrast, particles with smaller diameters with higher particle crushing strength get lodged only inside the cell walls of the segregated matrix, resulting in higher densification stresses (energy absorption). We show that stiffness and densification can be tuned while enabling lightweight syntactic foams. These novel discoveries will aid in facilitating functional and lightweight syntactic foams for cores in sandwich structures.

Exploring the design space of the effective thermal conductivity, permeability, and stiffness of high-porosity foams

With knowledge of only a few effective properties of a porous structure, the applicability of the structure for a given system can quickly be determined. This study numerically simulates the effective thermal conductivity, permeability, and stiffness of high porosity structures. Commonly used isotropic architected porous structures are compared with commercially available stochastic metal foams. The architected structures include body-centered cubic (BCC) lattice, shell-based triply periodic minimal surface (TPMS), and hybrid foam (HF) composed of beam and shell with multiple adjustable parameters. The simulated effective properties touch on the applicability in heat transfer, fluid flow, and mechanically stressful situations. The dimensionless effective properties of the structures are presented in graphical form to clearly illustrate structurally dependent properties. Compared to the stochastic metal foams, the architected structures (BCC, TPMS, and most HF) showed higher effective thermal conductivities and permeabilities. This indicates a potential to improve the efficiency of a thermal or fluid flow system by replacing the stochastic foam with architected foam. Additionally, the HF structure shows broad tunability of specific properties. All effective properties simulated were rendered dimensionless to only reflect the impact of topology, and plotted in charts to show trends. These charts can aid in the selection of porous structures in diverse applications.

Prediction of anisotropic foam stiffness properties by a Neural Network

In this work, an Artificial Neural Network is developed to predict the orthotropic stiffness tensor of anisotropic foam structures utilizing a tessellation-based foam RVE database. Based on a validated mesoscale simulation model, which is able to predict the mechanical properties of foams including the consideration of the cell structure, in a first step a simulative database is created. Variable parameters are: Foam density, Young’s modulus of foam base material, cell size, cell orientation, cell shape, etc. In total, more than 2000 simulations were performed leading to a novel RVE database. These simulations are subsequently used to train and compare different networks in a supervised manner. It can be seen that with suitable network settings a Mean Relative Error of max 2% compared to the RVE simulations occurs. As a result, the anisotropic mechanical stiffness values of complex foam structures can be determined via the Artificial Neural Network within seconds instead of performing time-consuming simulations (up to hours). In a final study, the effect of the database size on the prediction accuracy is examined. It can be observed that at least 500 training datapoints are required to obtain sufficient accuracy.

Ice-templating hydrogels with high concentrations of cellulose nanofibers to produce architected cellular materials for structural applications

Ice-templated cellular materials made of entangled networks of slender cellulose nanocrystals (CNCs) or cellulose nanofibrils (CNFs) exhibit excellent specific physical properties but their mechanical properties are still insufficient: these biosourced systems cannot be used as structural materials, e.g., as cores of composite sandwich structures. To overcome this limitation, we ice-templated hydrogels with high concentrations of CNFs and CNCs by using a specific setup that enabled their unidirectional solidification. The microstructure and the mechanical properties of the freeze-dried foams were investigated using SEM, X-Ray nanotomography and compression tests, respectively. Increasing the content of nanofibers yielded to drastic shifts of (i) the foam microstructures from highly anisotropic (columnar with CNFs, lamellar with CNCs) to more isotropic ones (ii) the mechanical properties towards isotropy together with a significant increase of the foam stiffness, yield stress and absorbed energy. In addition, due to the high aspect ratio of the CNFs, CNF foams with elevated relative densities exhibit noteworthy specific mechanical properties.

Robust polymer foams from 2-hydroxyethyl cellulose: Fabrication, stability, and chemical functionalization

We describe the fabrication of polymer foams from aqueous solutions of 2-hydroxyethyl cellulose (HEC), using freezing-induced phase-separation and solvent removal methods. Freezing aqueous solutions of HEC produced 3D polymer networks by a mechanism involving polymer exclusion from ice crystals and concentration at grain boundaries. This phase-separation is viewed as a kinetically controlled process, with properties of the resultant foams reliant on the processing temperatures and other conditions employed. Specifically, HEC foams were obtained by removal of the ice phase either by lyophilization or solvent exchange techniques. The morphology and pore characteristics of the resultant foams were visualized by scanning electron microscopy (SEM), which showed fibrillar and flake-like polymer material embedded in an open-cell porous structure. Foam density and stiffness increased as a function of the initial HEC concentration employed, while the mechanical integrity of the resultant foams was improved by crosslinking with poly(acrylic acid) (PAA) or maleic anhydride (MA). Such crosslinking successfully enabled liquid water sorption into the foams without loss of structural integrity, thus providing opportunities for further incorporation of chemical moieties. Dynamic vapor sorption (DVS) was employed to assess water uptake into the HEC foams, which was generally greater for foams produced by lyophilization, and in all cases exceeded that achieved by conventional, commercially available cellulose foams.

Joining High Thickness Materials by Sewing \u2013 first Modelling Steps of the Stitched Place

Many textile products are using materials with higher thickness like foams, spacer fabrics and nonwoven materials in order to provide comfortable softness of the contact surface to the body. Such materials are often joint to another external layer who have to provide optical and mechanical stability of the system, and are joined together into a 3-dimensional geometry. This paper analyses some steps of joining such materials by sewing. The forces in the sewing threads are analyzed analytically. A numerical model based on finite element method (FEM) is implemented in LS-Dyna and the process of compaction of the sewing line, based on the thread force is simulated. The thread force propagation is numerically analyzed in the time, at different friction coefficients and for different foam stiffness. All simulations demonstrate logical results and can be used as a fundament for more complex investigations and optimizations of such products.

Effective stiffness, strength, buckling and anisotropy of foams based on nine unique triple periodic minimal surfaces

Recent advancements in the field of additive manufacturing has led to an increased interest in the study of cellular architectures especially triply periodic minimal surfaces (TPMS) and their properties. In this study, 9 sheet-based TPMS architectures are studied by comparing their effective stiffness, strength, buckling and anisotropic properties. The TPMS lattices are generated using level set equations where the relative density of each lattice is varied to study the effective properties as a function of relative density. Finite element models of these lattices are built to study the elastic and plastic deformation behaviour under uniaxial, shear, hydrostatic and buckling loading conditions. Effective properties such as Young’s modulus, shear modulus, bulk modulus, Poisson’s ratio and strengths under uniaxial, shear, hydrostatic and buckling loading are computed against each other and other common lattice structures. In addition, anisotropic behaviour as well as the orientation dependence of the properties of each lattice are also studied. The geometric efficiency criterion is predominantly used to evaluate the different structures and the results obtained are consolidated in both quantitative and qualitative manner. SC lattice outperforms all the other lattices in terms of strength and stiffness efficiencies at lower and higher relative densities. On the other hand, I2 − Y** lattice shows the weakest stiffness and strength efficiency especially at higher relative densities. The current study focuses on the properties of some unique lattice architectures that have not been studied previously and shows promising efficiencies compared to the common architectures studied such as the Gyroid lattice. This study opens the door for employing these lattices in various applications and the results are very useful to multi-scale topology-optimization studies.

Effect of surface curing temperature on depth-specific physical properties of poly(urethane-isocyanurate) foam panels in relation with local chemical composition and morphology

Processing low density poly(urethane-isocyanurate) (PIR) rigid foam boards still remains challenging, especially for high thickness panels with the spontaneous formation of two-dimensional wrinkles. Increasing the double-belt conveyor temperature (mold temperature in this work) was regarded as a way to increase the rigidity of the foam base material and, ultimately to increase the stiffness of the PIR foam under the facing. For that purpose, PIR foam samples simulating real panels were prepared at different mold temperatures and were chemically analyzed using ATR-FTIR spectroscopy while a Shore durometer (type OO) was used to estimate the foam rigidity in various locations of the model panel. Obviously, the isocyanurate content of the foam under the facing was found to increase with the increase in mold temperature but surprisingly the Shore hardness demonstrated the reverse effect. This result was explained by the decrease in the density of the foam layer under the facing as the mold temperature increases, thereby counteracting the probable increase in stiffness of the foam base material itself. In addition, foam characterizations at different depths (rise direction) indicate that the chemical and mechanical gradients occurring in the first 20–30 mm tend to disappear as the mold temperature increases.

Understanding the effect of processing temperature and carbon nanotube addition on the viscoelastic response of polyurethane foams

AbstractWe experimentally demonstrate a strategy to design the microstructure and hence viscoelastic behavior of polyurethane (PU) foams. PU foams are fabricated at two temperatures (room and −5°C) and reinforced with varying concentrations of oxidized carbon nanotubes (CNTs). Hierarchical pores, thicker cell walls, higher density, and low porosity are recorded in PU foams processed at −5°C. The addition of CNTs improves the cell wall stiffness and thermal stability of PU foams. Master curves constructed from dynamic mechanical analysis show that processing at −5°C and addition of 1.6 wt.% oxidized CNTs enhance the storage modulus of PU foams over a wide temperature and frequency range. Prony series analysis reveals that PU foams fabricated at −5°C have significantly lower relaxation time. Moreover, the elastic modulus of PU foams processed using optimized conditions shows stronger rate dependency in comparison to other types of PU foams fabricated in this work.

How muscle stiffness affects human body model behavior

BackgroundActive human body models (AHBM) consider musculoskeletal movement and joint stiffness via active muscle truss elements in the finite element (FE) codes in dynamic application. In the latest models, such as THUMS™ Version 5, nearly all human muscle groups are modeled in form of one-dimensional truss elements connecting each joint. While a lot of work has been done to improve the active and passive behavior of this 1D muscle system in the past, the volumetric muscle system of THUMS was modeled in a much more simplified way based on Post Mortem Human Subject (PMHS) test data. The stiffness changing effect of isometric contraction was hardly considered for the volumetric muscle system of whole human body models so far. While previous works considered this aspect for single muscles, the effect of a change in stiffness due to isometric contraction of volumetric muscles on the AHBM behavior and computation time is yet unknown.MethodsIn this study, a simplified frontal impact using the THUMS Version 5 AM50 occupant model was simulated. Key parameters to regulate muscle tissue stiffness of solid elements in THUMS were identified for the material model MAT_SIMPLIFIED_FOAM and different stiffness states were predefined for the buttock and thigh.ResultsDuring frontal crash, changes in muscle stiffness had an effect on the overall AHBM behavior including expected injury outcome. Changes in muscle stiffness for the thigh and pelvis, as well as for the entire human body model and for strain-rate-dependent stiffness definitions based on literature data had no significant effect on the computation time.DiscussionKinematics, peak impact force and stiffness changes were in general compliance with the literature data. However, different experimental setups had to be considered for comparison, as this topic has not been fully investigated experimentally in automotive applications in the past. Therefore, this study has limitations regarding validation of the frontal impact results.ConclusionVariations of default THUMS material model parameters allow an efficient change in stiffness of volumetric muscles for whole AHBM applications. The computation time is unaffected by altering muscle stiffness using the method suggested in this work. Due to a lack of validation data, the results of this work can only be validated with certain limitations. In future works, the default material models of THUMS could be replaced with recently published models to achieve a possibly more biofidelic muscle behavior, which would even allow a functional dependency of the 1D and 3D muscle systems. However, the effect on calculation time and model stability of these models is yet unknown and should be considered in future studies for efficient AHBM applications.

The Use of Polyurethane Injection as a Geotechnical Seismic Isolation Method in Large-Scale Applications: A Numerical Study

This paper analyses the effect of polyurethane injections on the seismic surficial response of cohesionless soils. For this purpose, dynamic finite element numerical analyses were performed through GiD + OpenSees. Both the soil and the composite material, resulted after the expansion of the injected polyurethane, are modelled with a nonlinear hysteretic constitutive model. Based on the polyurethane percentage, a homogenisation of the characteristics was considered for the composite material: linear for density and damping, and exponential (experimentally calibrated) for the stiffness. An expansion coefficient quantifies how much the injected polyurethane expands: three expansion coefficients were considered, each of them related to a different polyurethane density. For the evaluation of the foam stiffness, a linear stiffness–density correlation was used, derived after impact tests. Results showed that polyurethane reduces the surficial accelerations proportionally to the ratio of its seismic impedance and volumetric percentage with respect to the soil seismic impedance and total volume. This is a preliminary indication for the design of polyurethane injections in cohesionless soils for seismic acceleration reduction.

Combat helmet liner design for blunt impact absorption using multi-output Gaussian process surrogates

A finite element based computational model simulating the standard drop tower test for military helmets was created and used in conjunction with a multi-output Gaussian process surrogate to seek different designs of helmets for improved blunt impact performance. Experimental drop test results were used for the validation of the model’s ability to simulate impact. The influence of foam stiffness, impact velocity, strap tension, as well as pad placement and size on parameters on the peak linear acceleration (PLA) of the headform was investigated for the first time through a surrogate model trained by strategically choosing simulation points. Impact velocity was found to have the greatest effect. The strap tension and foam pad stiffness ranges examined within this sampling plan were found to have less of an effect on the performance of the helmet than the pad size and shape parameters examined. The surrogate modeling approach was used to quantify the influence of design parameters and can lead to not only improved helmet designs but also new data-driven design metrics and testing standards to accelerate the development of TBI-mitigating helmets.

Cushioning capability analysis of seat foundations considering the sitter’s anthropometric dimensions

Sitting comfort is primarily determined by the cushioning capability of the seat foundation. Limited literature has been found related to the effects that different sized human buttocks had on the cushioning capability of the seat foundation. Moreover, there is no testing method specialized to measure the load-deformation behavior of foam cushions that imitates the sitting behavior between indenters with different sizes and seat support. This study investigated the effects of various indenter diameters (20 cm, 30 cm, 36 cm, 41 cm, 51 cm, and 58 cm), foam stiffness levels (high and low), and seat bases (spring versus solid flat panel) on the compressive load-deformation behavior of upholstered seat foundations. The load-deformation curves of all the tested foam-seat base combinations exhibited three typical regions, i.e., linear elasticity, plateau, and densification, when subjected to the loads applied through different indenter diameters. Statistical results indicated that the primary effects of the indenter diameter, foam stiffness level, and seat base had significant effects on the spring constants, which represented the slopes of lines in these three regions. In addition, a regression technique was proposed to derive power equations for the estimation of the spring constants of a seat foundation as a function of the indenter diameter, foam material stiffness, and seat base type.

Mechanical properties of 3D printed architected polymer foams under large deformation

We report a new type of three-dimensional architected polymer foams composed of perforated spherical shells and flat strut connectors, which can be precisely produced by 3D printing techniques. We investigate the effects of foam architectures, manufacturing process, and constitutive material on the deformation patterns and failure modes of the proposed architected foams. We demonstrate that flat strut connectors offer unprecedented design flexibility for controlling the mechanical performance. By tuning the geometric parameter of flat strut connectors, the stiffness of architected foams can increase about one order of magnitude while the relative density increases only by 5%. Furthermore, the failure modes can be engineered from a catastrophic one to a progressive one by using weak flat strut connectors. Our experiments elucidate the salient roles of the layer-by-layer manufacturing process and constitutive polymer on the mechanical behavior of the proposed architected foams.

Reinforcement Efficiency of Cellulose Microfibers for the Tensile Stiffness and Strength of Rigid Low-Density Polyurethane Foams.

Rigid low-density closed-cell polyurethane (PU) foams are widely used in both thermal insulation and structural applications. The sustainability of PU foam production can be increased by using bio-based components and fillers that ensure both enhanced mechanical properties and higher renewable material content. Such bio-based foams were produced using polyols derived from rapeseed oil and microcrystalline cellulose (MCC) fibers as filler. The effect of MCC fiber loading of up to 10 wt % on the morphology, tensile stiffness, and strength of foams has been evaluated. For estimation of the mechanical reinforcement efficiency of foams, a model allowing for the partial alignment of filler fibers in foam struts was developed and validated against test results. It is shown that although applying MCC fibers leads to modest gains in the mechanical properties of PU foams compared with cellulose nanocrystal reinforcement, it may provide a higher content of renewable material in the foams.

Bio-inspired self-healing polymer foams with bilayered capsule systems

Bio-inspired, self-healing polymer foams containing novel calcium-alginate capsule system was developed for load-bearing application. The capsules were created by a multi-stage encapsulating process that can encapsulate two mutually reactive healing agents within single capsules. The capsules had a bilayered structure with the epoxy resin encapsulated within the inner layer and the hardener within the outer layer. To evaluate the mechanical self-healing performance, the following tests were conducted, i.e. (1) cyclic quasi-static compression tests for foams; (2) quasi-static three-point bending tests for foam core sandwich beams; and (3) high-speed soft impact tests for foam core sandwich beams. Cyclic quasi-static compression tests demonstrated (1) bilayered capsule systems had better multiple self-healing effect compared to the dual capsule system without external interventions; and (2) bilayered capsules could enhance the stiffness and strength of foams. For foam core sandwich beams, the bilayered capsules did not have a noticeable effect on the effective stiffness of the beams; however, it could provide a noticeable self–healing effect when the damage occurred. The images obtained from scanning electron microscope (SEM) and X-ray microcomputed tomography (μCT) suggested that the micro-cracks induced by the static and impact loading were either fully or partially healed by the released healing agents without external interventions.

Simulation and experiment study on the deviation influence factors of dynamic collision test for child safety seat

To investigate the prime causes of test deviations for child restraint systems testing in different laboratories, this article selected four types of child safety seats. Simulation and experiment had been done to quantitatively evaluate the factors which influence test deviations based on China GB27887 standard. The results show that the difference of acceleration pulse in different laboratories, the foam stiffness of sled cushion, the friction coefficient of safety belt guide, the three-point seat belt initial tension, the five-point harness initial tension, the five-point harness initial position offset, the overall position offset of child safety seat, the initial installation angle of supporting leg and the image recognition method of head excursion have significant effects on the deviations of test data. This article put forward a modifier formula which reduced the deviation of image recognition method of head excursion effectively. In addition, the effective control methods and measures were recommended to reduce the test deviations and ensure the consistency of the test data.

Microstructural characterization and compression mechanical response of glass hollow spheres/Al syntactic foams with different Mg additions

Aluminum syntactic foams with different magnesium contents were fabricated by pressure infiltration of glass hollow spheres. Effect of Mg addition on microstructure, compression properties and corresponding deformation behavior as well as energy absorption under different strain rates loading was experimental investigated. Increasing Mg contents resulted in interface reactions between matrix and glass spheres forming MgAl2O4, Si and Mg2Si hard phases, leading to the improvement of compression strength, structural stiffness and energy absorption capacity of syntactic foams, however, reducing the strain rate sensitivity of the syntactic foams. The addition of Mg content decreased the stress relaxation and load transfer ability of the matrix and developed the syntactic foam into severe brittle fracture during quasi-static compression. Nevertheless, the influence of Mg addition on the macroscopic compression deformation behavior of syntactic foams was weakened at increasing high strain rate loading.