Yield stress distribution in injection-mouldedglassy polymers

A methodology for structural analysis simulations is presented that incorporates the distribution of mechanical propertiesalong the geometrical dimensions of injection-moulded amorphous polymer products. It is based on a previously developedmodelling approach, where the thermomechanical history experienced during processing was used to determine the yield stressat the end of an injection-moulding cycle. Comparison between experimental data and simulation results showed an excellentquantitative agreement, both for short-term tensile tests as well as long-term creep experiments over a range of strain rates,applied stresses, and testing temperatures. Changes in mould temperature and component wall thickness, which directly affectthe cooling profiles and, hence, the mechanical properties, were well captured by the methodology presented. Furthermore, itturns out that the distribution of the yield stress along a tensile bar is one of the triggers for the onset of the (strong) localizationgenerally observed in experiments.Spanish Government (Ministry of Sci-ence and Innovation, Ministry of Economy and Competitiveness)through grant numbers RYC-2010-07171 and DPI2011-25470This is the peer reviewed version of the following article: Verbeeten, W. M., Kanters, M. J., Engels, T. A. and Govaert, L. E. (2015), Yield stress distribution in injection-moulded glassy polymers. Polym. Int., 64(11): 1527–1536, which has been published in final form at http://dx.doi.org/10.1002/pi.4898. This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Self-Archivin

Sticky Multicolor Mechanochromic Labels

Sticky-colored labels are an efficient way to communicate visual information. However, most labels are static. Here, we propose a new category of dynamic sticky labels that change structural colors when stretched. The sticky mechanochromic labels can be pasted on flexible surfaces such as fabric and rubber or even on brittle materials. To enhance their applicability, we demonstrate a simple method for imprinting structural color patterns that are either always visible or reversibly revealed or concealed upon mechanical deformation. The mechanochromic patterns are imprinted with a photomask during the ultraviolet (UV) cross-linking of acrylate-terminated cholesteric liquid crystal oligomers in a single step at room temperature. The photomask locally controls the cross-linking degree and volumetric response of the cholesteric liquid crystal elastomers (CLCEs). A nonuniform thickness change induced by the Poisson’s ratio contrast between the pattern and the surrounding background might lead to a color-separation effect. Our sticky multicolor mechanochromic labels may be utilized in stress-strain sensing, building environments, smart clothing, security labels, and decoration.</p

A macroscopic viscoelastic viscoplastic constitutive model for porous polymers under multiaxial loading conditions

A macroscopic constitutive model, the Porous Eindhoven Glass Polymer (Porous EGP) model, is presented to describe the deformation behavior of cavitated rubber toughened polymers under multiaxial loading conditions. It is shown that the proposed macroscopic constitutive model is able to describe the non-linear pre-yield regime, strain rate dependence, post-yield behavior (strain softening and hardening) and void evolution for loading conditions ranging from shear to equi-triaxial (pure triaxial) tension and compression. The Porous EGP model is a combination of a well established non-linear viscoelastic viscoplastic model, the Eindhoven Glassy Polymer (EGP) model, and the modified Gurson model. The Gurson model is adopted to determine the equivalent stress and plastic rate of deformation tensor making it depending on the void volume fraction, deviatoric and hydrostatic stress. The macroscopic constitutive model is developed based on the response of realistic 3D representative volume elements (RVEs) containing randomly positioned mono-disperse inclusions. The constitutive behavior of the matrix phase in this full-field model is described by the EGP model, and the cavitated inclusions are idealized as voids. Their response is studied for a range of void volume fractions, multiaxial loading conditions, strain rates and thermodynamic states. The yield behavior of the heterogeneous material depends non-linearly on the macroscopic hydrostatic stress. This response is well captured with the proposed macroscopic constitutive model.</p

Melt-Extruded Thermoplastic Liquid Crystal Elastomer Rotating Fiber Actuators

Untethered soft fiber actuators are advancing toward next-generation artificial muscles, with rotating polymer fibers allowing controlled rotational deformations and contractions accompanied by torque and longitudinal forces. Current approaches, however, are based either on non-recyclable and non-reprogrammable thermosets, exhibit rotational deformations and torques with inadequate actuation performance, or involve intricate multistep processing and photopolymerization impeding scalable fabrication and manufacturing of millimeter-thick fibers. Here, the melt-extrusion and drawing of a 50 m long thermoplastic liquid crystal elastomer fiber with a ≈1.3 mm diameter on a large scale is reported. With the responsive thermoplastic material, rotating actuators are fabricated via easily exploited programming freedom resulting in large, reversible rotational deformations and torques. The actuation performance of the twisted fibers may be controlled by the programmed twisting density without complicated preparation steps or photocuring being required. The thermoplastic behavior enables fabrication of plied fibers, demonstrated as a triple helical twisted rope constructed from individual rotating fibers delivering up to three times as great rotational and longitudinal forces capable of reversibly opening and lifting a screw cap vial. Besides the programmability, the thermoplastic material employed lends itself to be completely reprocessed into other configurations with self-healing properties in contrast to thermosets.</p

Processing and Properties of Melt Processable UHMW-PE Based Fibers Using Low Molecular Weight Linear Polyethylene's

The rheology, solid state drawability, morphology, and mechanical properties of polyethylene blends containing ultrahigh molecular weight polyethylene (UHMW-PE) and linear-low molecular weight polyethylene waxes (PEwax) are explored. Addition of PEwax enables melt processing of UHMW-PE and improves solid state drawability, providing opportunities in recycling of UHMW-PE waste. Small angle X-ray scattering results show that both PEwax and UHMW-PE align fully in the drawing direction, irrespective whether the PEwax has an Mn below or above the critical molecular weight at which entanglements can form (Mc). Tensile moduli of drawn specimen are in accordance to the Irvine–Smith model confirming that both UHMW-PE and PEwax align in the drawing direction and no chain slip occurs toward zero strain and both UHMW-PE and PEwax fractions, irrespective of their molecular weight, contribute fully to the modulus. Tensile strength of the blends scales according to the rule of mixtures where PEwax below Mc scale toward zero and those above Mc do contribute to the tensile strength. Modulus hence can be regarded as insensitive to the molecular weight of the PEwax used, whereas strength does show to be sensitive to the molecular weight of the PEwax

Time-dependent failure in load-bearing polymers. A potential hazard in structural applications of polylactides

Polylactides are commonly praised for their excellent mechanical properties (e.g. a high modulus and yield strength). In combination with their bioresorbability and biocompatibility, they are considered prime candidates for application in load-bearing biomedical implants. Unfortunately, however, their long-term performance under static load is far from impressive. In a previous in vivo study on degradable polylactide spinal cages in a goat model it was observed that, although short-term mechanical and real-time degradation experiments predicted otherwise, the implants failed prematurely under the specified loads. In this chapter we demonstrate that this premature failure is attributed to the time-dependent character of the material used. The phenomenon is common to all polymers, and finds its origin in stress-activated segmental molecular mobility leading to a steady rate of plastic flow. The main conclusion is that knowledge of the instantaneous strength of a polymeric material is insufficient to predict its long-term performance

Influence of fiber orientation, temperature and relative humidity on the long-term performance of short glass fiber reinforced polyamide 6

As a result of processing of short fiber reinforced thermoplastics, the fiber orientation varies throughout a product giving rise to a pronounced anisotropic mechanical response. Different flow conditions in a product result in spatial variation in both short- and long-term mechanical properties. In this study, a modeling approach is presented to evaluate the lifetime of short fiber reinforced polyamide 6, both in plasticity- and crack growth controlled regions of failure. In the plasticity-controlled region, a viscoplastic model based on separation of the load angle (by means of Hill's equivalent stress formulation) and time dependence of the yield stress is used in the form of an associative flow rule. The influence of temperature and relative humidity on the magnitude of the plastic flow rate is described by using an apparent temperature approach combined with a Ree-Eyring formulation. The depression of the glass transition temperature in the polyamide 6 matrix with increasing amount of absorbed moisture was used to predict the anisotropic deformation kinetics in a humid environment. Similar to the plasticity controlled failure, in slow crack growth controlled failure region the effect of temperature, relative humidity, and load angle on the lifetime under a fatigue load is investigated. The apparent temperature approach could also be successfully applied to predict the slow crack growth failure, while the load angle dependence is shown to scale similar to the plasticity-controlled failure with the Hill's equivalent stress

Physical background of the endurance limit in poly(ether ether ketone)

In this study, it is demonstrated that the apparent endurance (fatigue) limit for plasticity‐controlled failure in poly(ether ether ketone) is related to an evolution of the yield stress. The increase of the yield stress has two separate causes: (a) stress‐ and temperature‐accelerated physical aging of the amorphous phase and (b) strain hardening as a result of texture development. Yield stress evolution is monitored using thermomechanical treatments during which the material is exposed to temperature and load. The combined contributions of both temperature and applied stress to yield stress evolution (below T g ) can be effectively modeled using an effective time approach employing an Arrhenius temperature‐activation as well as Eyring stress activation. Combination of the yield stress evolution with a previously developed model for plasticity‐controlled failure allows prediction of time‐to‐failure under both static and cyclic load, quantitatively capturing the observed apparent endurance limit

Predicting long-term crack growth dominated static fatigue based on short-term cyclic testing

In the present work, the time to failure of a glass fibre reinforced glassy polymer is studied in cyclic fatigue at various frequencies and stress ratios with the goal to predict long-term crack-growth dominated static fatigue. It is demonstrated that the crack propagation rate can be regarded to consist of two components: a time-dependent creep component, and a frequency dependent cyclic component. In static loading, the time-dependent component prevails, while for cyclic loads with large load amplitudes the frequency dependent, cyclic component dominates. For intermediate load amplitudes, the total propagation rate is shown to be a combination of both. Consequently, the contribution of the cyclic component diminishes with decreasing frequency or load amplitude, revealing the contribution of the static component. As such, extrapolation of the stress ratio dependence of the fatigue life to R=1 allows estimation of the long-term static performance. A phenomenological model is provided that captures all relevant aspects and provides an accurate description of the stress ratio and frequency dependence of the lifetime in fatigue loading and allows prediction of the long-term failure, based on short-term cyclic fatigue experiments only