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Mechanical fatigue under strain controlled tension/tension (T/T) of rectangular un-notched and notched specimens of high density polyethylene and polyamide 6 was performed. Under large amplitude oscillatory elongation, the stress response is nonlinear asymmetric due to a different stress response during loading and unloading and was analyzed via discrete Fourier transform. Dynamic strain ([[epsilon].sub.0]) sweep tests revealed odd ([I.sub.3/1] [alpha] [[epsilon].sub.0.sup.2]) and even ([I.sub.2/1] [alpha] [[epsilon].sub.0.sup.1]) harmonics in the stress, well described by the Neo-Hooke's law. Before crack onset for the Nc specimens, the storage and loss moduli and [I.sub.2/1] are constant, but [I.sub.3/1] increases with fatigue. The first derivative of [I.sub.3/1] (([I.sub.3/1]/dN) and the cumulative nonlinearity [Q.sub.f] parameter (integral of [I.sub.3/1] /[[epsilon].sub.0.sup.2]) were found to be strong criteria predicting fatigue. To break the unnotched specimens in the low and high cycle fatigue regime, large strain amplitudes were applied, resulting for both materials in initial plastic deformation followed by buckling when a critical compression force is exceeded. This results in a different time evolution of the linear and nonlinear parameters, especially as a minimum in the [I.sub.2/1] curve. Finally, a model is proposed to describe the nonlinear viscoelastic specimen response under T/T and the effect of buckling. KEYWORDS buckling, mechanical fatigue, nonlinear stress response, plastic deformation, 1 | INTRODUCTION Failure of a part under a cyclic load or deformation is called mechanical fatigue break-up. It is typically characterized via the average applied stress ([[sigma].sub.0]) or the [...]

Herein, a novel 3D printable ink designed for the fabrication of interpenetrating polymer network (IPN) hydrogels is introduced, ingeniously integrating a polyelectrolyte complex (PEC) of hyaluronic acid (HA) and chitosan (CS) with a photo‐crosslinkable P(OEGMA‐co‐EGDEMA) polymer. Initially, the carboxyl group of HA is modified with a photo‐labile ortho‐nitrobenzyl group, preventing premature PEC formation during 3D ink formulation. Subsequent UV illumination via digital light processing (DLP) simultaneously triggers the photo‐deprotection of the carboxyl group of HA and the photopolymerization of OEGMA crosslinked hydrogel, together creating a PEC‐integrated IPN hydrogel. Comprehensive characterizations, including NMR, IR, UV/Vis, TGA, DSC, SEM, and mechanical tests are conducted to evaluate the structural, morphological, and rheological properties of these hydrogels. Furthermore, adjustments to the composition of the 3D ink enable the production of hydrogels with a spectrum of mechanical strength and elastic modulus ranging from 1 to 10 kPa. The resultant PEC‐IPN hydrogels display excellent flexibility, compressive strength, high strain tolerance, pH responsiveness, and thermal stability. In essence, the approach seamlessly merges natural polyelectrolytes with technological innovation to refine 3D ink production. This method could mark a pivotal advancement in the realm of 3D materials, unlocking numerous prospective applications. [ABSTRACT FROM AUTHOR]

Strain hardening of long-chain branched polymers in elongational flow occurs due to the stretch of the backbone chain between branch points. With an increasing number of side arms, the length of the backbone chain segment between two branch points of a comb decreases. Of particular interest is the case when the number Nb of arms per entanglement length of the polymer is larger than one. This leads not only to larger strain hardening but also to hyperstretching, i.e., the elongational stress growth shows an enhanced increase with strain. We consider elongational data reported by Abbasi et al. [Macromolecules 50(15), 5964–5977 (2017)] and Faust et al. [Macromol. Chem. Phys. 224(1), 2200214 (2023)] on a series of comb and branch-on-branch polystyrene (PS) melts with the average number Nb of branches per entanglement segment of the backbone ranging from Nb = 0.2 to Nb = 9.5. In addition, we present measurements of the elongational viscosity of two PS combs with Nb = 4.7 as well as of blends consisting of 5 to 50 wt. % of a PS comb and a monodisperse linear PS. Analysis by the hierarchical multimode molecular stress function model shows that while backbone chains of loosely grafted combs with Nb < 1 are stretched affinely in elongational flow, backbone chains of more densely grafted combs with Nb > 1 show increasing hyperstretching with increasing Nb. The elongational data of the comb/linear blends confirm that hyperstretching is an intrinsic property of the comb macromolecule with Nb > 1, independent of its concentration in the blend. While this is of considerable interest from a modeling point of view, hyperstretching causing an enhanced increase of the elongational stress growth can also have a significant impact on the processability of polymers, and quantification of this effect is, therefore, important. [ABSTRACT FROM AUTHOR]

Polyamides serve as matrix material for fiber reinforced composites and are widely applied in many different engineering applications. In this context, they are exposed to various environmental influences ranging from temperature to humidity. Thus, the influence of these environmental conditions on the mechanical behavior and the associated implications on the performance of the material is of utmost importance. In this work, the thermoviscoelastic behavior of polyamide 6 (PA 6) for two equilibrium moisture contents is investigated. To this end, dynamic mechanical analysis tests with and without humidity control of the environmental chamber were performed. In terms of relaxation tests, the experimental results reveal drying effects and increased diffusion activities when the sample's equilibrium moisture content differs from the ambient humidity level within the testing chamber. Temperature-frequency tests quantify the humidity-induced shift of the glass transition temperature. The linear generalized Maxwell model (GMM) and time-temperature superposition are used to analyze the hydrothermal effects on the linear viscoelastic material properties and the onset of mechanical nonlinearity. Based on these investigations and findings, insight is gained on the humidity influence on the material properties and the limitations of linear thermoviscoelastic modeling. Furthermore, the computational construction of master curves and the parameter identification for a generalized Maxwell model are described in detail. [ABSTRACT FROM AUTHOR]

Polymeric foam applications have rapidly increased over the years and most of them are submitted to cyclic loading conditions. Therefore, their fatigue behavior is highly important to ensure safe operation for which limited information is available compared to compact materials like asphalt, concrete, and metals. Starting with an understanding of the main parameters controlling the mechanical properties of polymer foams, this literature review further presents a broad overview of the information available to understand the relations between loading conditions and fatigue lifetime, including damage level development. The review is composed into three main classes of polymers: thermoplastics, thermosets, and elastomers. The effect of loading conditions, including stress/strain amplitude, loading ratio (R, r), frequency (f), moisture content, type of deformation and temperature (T) is presented and discussed. Finally, the effect of foam morphology in terms of cell size (d) and density (ρ) on the fatigue performance is included to make a link between Paris' law and crack propagation. A conclusion is also provided with openings for future development. [ABSTRACT FROM AUTHOR]

The mechanical properties of virgin/recycled high-density polyethylene (HDPE) blends over the complete concentration range was thoroughly investigated in this work. In particular, a focus was made on the long-term properties via mechanical fatigue. Two different mixing methods, namely powder mixing (dry blending) and extrusion mixing (melt blending), were used to determine the effect of processing conditions on the tensile and fatigue behavior of the blends after compression molding. It was found that both tensile (modulus, ultimate strength) and fatigue performances were improved with increasing vHDPE content. Based on the obtained data, a correlation between the blends composition and mechanical properties is reported. Moreover, it was observed that increasing the vHDPE content led to slower crack propagation rate, probably due to less defects (contamination) in the blends. Finally, a negligible difference in mechanical properties (fatigue resistance) between both mixing approaches was observed, but samples produced via powder mixing showed less viscous dissipation (heat generation) as the vHDPE content increased, leading to lower surface temperature rise which can be an advantage for specific applications. [ABSTRACT FROM AUTHOR]

The high consumption rates of polymers generate large amounts of wastes imposing long‐term adverse effects on the environment combined with a significant carbon footprint. So the appeal for a circular economy is becoming loud enough to take actions. Despite increasing interests for polymer recycling, some reserves about their mechanical performances, especially long‐term properties such as fatigue resistance, are barriers to introduce more recycled polymers back into production lines. In this study, a comparison between the fatigue resistance of virgin and recycled high‐/low‐density polyethylene (H/LDPE) is made to provide more quantitative information to address these concerns. Although some recycled polymers (HDPE) show similar tensile properties compared to virgin ones, significant differences can be observed in their fatigue lifetime. So tensile testing alone is not sufficient to provide a complete information about the overall properties of recycled polymers. Our results show that recycling polymers does not necessarily result in reduced fatigue resistance. [ABSTRACT FROM AUTHOR]

The influence of topology on the strain hardening in uniaxial elongation is investigated using monodisperse comb and dendrigraft model polystyrenes (PS) synthesized via living anionic polymerization. A backbone with a molecular weight of Mw,bb = 310 kg mol−1 is used for all materials, while a number of 100 short (SCB, Mw,scb = 15 kg mol−1) or long chain branches (LCB, Mw,lcb = 40 kg mol−1) are grafted onto the backbone. The synthesized LCB comb serves as precursor for the dendrigraft‐type branch‐on‐branch (bob) structures to add a second generation of branches (SCB, Mw,scb ≈ 14 kg mol−1) that is varied in number from 120 to 460. The SCB and LCB combs achieve remarkable strain hardening factors (SHF) of around 200 at strain rates greater than 0.1 s−1. In contrast, the bob PS reach exceptionally high SHF of 1750 at very low strain rates of 0.005 s−1 using a tilted sample placement to extend the maximum Hencky strain from 4 to 6. To the best of the authors' knowledge, SHF this high have never been reported for polymer melts. Furthermore, the batch foaming with CO2 is investigated and the volume expansions of the resulting polymer foams are correlated to the uniaxial elongational properties. [ABSTRACT FROM AUTHOR]

The nonlinear mechanical behavior of viscoelastic materials is modeled based on the Schapery integral model containing internal variables. In this context, a new approach for the strain‐dependent material properties is introduced considering a one‐dimensional formulation with strain‐dependent nonlinear functions for an oscillatory load case. In addition to the viscoelastic storage and loss modulus, the higher order harmonic oscillations in the stress response are computed and compared to experimental data from Fourier transform rheology of Polyamide 6 (PA6). The comparison reveals a good agreement between the predictions of the nonlinear model and the experimental data for the higher harmonic intensity I2/1. [ABSTRACT FROM AUTHOR]