Bulk fatigue damage evolution in polyamide-6 and in a polyamide-6 nanocomposite

Materials Science and Engineering: A, 2010

Humidity affects the mechanical properties of polymers and their composites. Understanding the influence of humidity on the strength, stiffness and fatigue characteristics will aid in better product design. The effect of relative humidity (RH) on indentation hardness and flexural fatigue behavior of polyamide 6 nanocomposites is reported. Indentation hardness and indentation modulus of the material reduces up to ∼50% in the samples conditioned in water due to the plasticization and associated increased polymer chain mobility. Cantilever bending fatigue tests conducted at different relative humidity levels at constant displacement amplitude revealed increased fatigue life for polyamide 6 nanocomposites at high humidity. Hysteresis heating and molecular reorientation lowers the modulus during fatigue process and causes a reduction in the force amplitude at high humidity levels. The failure mechanisms at different humidity levels are discussed.

Polymer Testing, 2020

Tensile, fatigue and thermomechanical properties of poly(ethylene-co-vinyl acetate) nanocomposites incorporating low and high loadings of pre-swelled organically modified montmorillonite, Polymer Testing (2020), doi:

International Journal of Fatigue, 2019

Understanding fatigue damage mechanisms in short fiber reinforced thermoplastics is a key issue in order to optimize material processing and propose physically based multiscale fatigue damage models. The present work aims at further understanding observations of fatigue damage in the polyamide 6.6 matrix with respect to its semi-crystalline structure. In this paper the polymer and associated composite are tested in their ductile regime i.e. above the glass transition temperature. Tomographic and SEM observations are used in order to establish a damage scenario at the spherulitic scale. These observations prove that fatigue damage progresses by intra-spherulitic failure in their equatorial plane. Observations of the spherulite nuclei also evidence the oriented structure of the semi-crystalline polymer induced by the injection-molding manufacturing process.

Composites Science and Technology, 2007

The long-term tensile creep of polyamide 66 and its nanocomposites filled with 1 vol.% TiO 2 nanoparticles 21 and 300 nm in diameter is studied. It is assumed that the dominant mechanisms of creep deformation are of viscoelastic nature, while the contribution of plastic strains is not essential in the stress (< 0.6 of the ultimate stress) and time (about 100 h) ranges considered. The creep isochrones obtained show that the materials exhibit a nonlinear viscoelastic behaviour and the degree of nonlinearity is reduced significantly by incorporation of the nanoparticles. The evolution of viscoelastic strains is less pronounced for the nanocomposite filled with smaller nanoparticles. Smooth master curves are constructed by applying the Time-Stress Superposition (TSS). The Boltzmann-Volterra hereditary theory is used for the creep modeling. The nonlinearity of viscoelastic behaviour is taken into account by using the TSS principles and introducing a stress reduction function into an exponential creep kernel. The master curves are employed to predict the creep for time periods more than 60 times exceeding the test time. A comparison of relaxation spectra of the polymers shows that the incorporation of nanoparticles restricts the mobility of polymer chains.

Journal of Applied Polymer Science, 2012

The relative amounts of amorphous and crystalline cand a-phases in polyamide-6 nanocomposites, estimated from the deconvolution of X-ray diffraction peaks using Gaussian functions, correlates with their mechanical, thermomechanical, and barrier properties. The incorporation of organoclay platelets (Cloisite 15A and 30B) induced the crystallization of the polymer in the c form at expense of the amorphous phase, such that 1-2 wt % of Cloisite is enough to enhance the mechanical and the thermomechanical properties. However, higher nanofiller loads were necessary to achieve good barrier effects, because this property is mainly dependent on the tortuous path permeation mechanism of the gas molecules through the nanocomposite films. V

Compared to metal-to-metal tribology, polymer tribology presents further complexity as it is more prone to be influenced by operating conditions. Over the past two decades, progress in the field of wear of polymers has led to the establishment of more refined wear mechanisms. The current paper establishes the link between different load parameters and the wear rate of polymers, based on experimental investigations. A pin-on-plate reciprocating tribometer was used to examine the wear behaviour of polyamide sliding against a steel counterface, under constant and fluctuating loads, in dry conditions. In addition, the influence of controlled imperfections in the polymer surface upon its wear rate were examined, under cyclic and steady loading, in order to better understand surface fatigue wear of polymers. The imposed imperfections consisted of vertical artificial deep crack (slit) perpendicular or parallel to the direction of sliding. The study concludes with the followings findings; in general, wear of polymers shows a significant tendency to the type of applied load. Under cyclic loads, polymers show an increase in wear rate compared to those tested under static loads. Such increase was found to increase with the increase in cyclic load frequency. It is also demonstrated that surface cracks results in higher wear rates, particularly under cyclic loads.

Current Opinion in Solid State and Materials Science, 2014

This report was cleared for public release by the USAF 88th Air Base Wing (88 ABW) Public Affairs Office (PAO) and is available to the general public, including foreign nationals. Copies may be obtained from the Defense Technical Information Center (DTIC) ().

Macromolecules, 2008

A study on the mechanisms for embrittlement of polyamide-6 (PA-6) by nanometer-sized clay in nanocomposites is reported in this paper. Tensile modulus and yield strength in these composites were found to increase with clay concentration, while the strain at rupture decreased. Similar to the strain at rupture, fracture toughness also decreased dramatically. Investigation by transmission electron microscopy alone indicates that multiple crazing appears to be the only significant toughening mechanism for the nanocomposite with 2.5 wt % clay which does not explain why composites with more clay are significantly more brittle. The deformation behavior was therefore further studied using wide (WAXS) and small (SAXS) angle X-ray scattering, scanning electron microscopy (SEM), and optical microscopy. WAXS detected the existence of the γ-phase of PA-6, probably due to the presence of clay. However, the occurrence of this crystalline phase cannot explain the observed changes in mechanical behavior. SAXS study indicated that crazing occurred in the PA-6 matrix in tension and that the craze concentration depended on clay loading. The highest craze concentration was observed at 2.5 wt % clay. The existence of crazes in nanoclay composites was also confirmed by optical microscopy and transmission electron microscopy. In addition, SAXS showed that microcracking also occurred in these nanocomposites along with crazing, with the highest concentration of microcracks observed at 2.5 wt % clay loading. TEM showed that microcracks occurred near the interface of clay and matrix. The existence of a high concentration of crazes and microcracks in the nanocomposite with 2.5 wt % of clay is the likely cause for its high toughness relative to those composites containing higher clay loading. At higher clay loadings the high incidence of microcracks probably prevented the crazing mechanism from operating to its fullest possible extent, thus resulting in low toughness.

Macromolecules, 2017

Micromechanisms of deformation and cavitation within the spherulitic microstructure of a polyamide 6 (PA6) material have been observed with a true spatial resolution of 50 nm by magnified synchrotron radiation holotomography. Smooth round bars in PA6 were submitted to interrupted monotonic tensile tests. Before the engineering peak stress, only pre-existing nanometric voids were identified. At the peak stress, void growth and coalescence have been observed in the neck and 2 mm apart from the neck along the revolution axis. Two kinds of cavity arrangements have also been identified, studied quantitatively, and attributed to specific regions of the deformed spherulites. Voiding appeared in the equatorial plane of the spherulites as diffuse annular voided zones, and polar fans were characteristically penny-shaped voids stacked in columns parallel to the loading axis in the spherulite poles. A cylindrical volume located at the center of these spherulites remained intact (no voids).

Polymer, 2005

Polyamide 6/clay nanocomposites (PAn, where n is the mass fraction of clay) with various clay loading were prepared by melt compounding in a twin screw extruder. Exfoliation of clay in a PA matrix was confirmed by X-ray diffraction. Strain amplitude response of PAn in both melt and solution states has been investigated. In the melt state, critical strain amplitude of PAn is sensitive to strain amplitude response and decrease logarithmically with increasing clay loading. The elastic moduli (G 0 ) of PAn are reversible under frequency loop sweeps. Comparisons of strain amplitude response in both melt and solution states have been conducted. Two different responses have been observed: strain thinning in the melt state and weak strain overshoot in the solution state. FTIR studies show that amide II band of PAn shifts toward high wavenumbers, but amide I band and N-H stretching vibration are independent of clay loading. We suggest that two types of strain amplitude response of PAn can be explained: strain thinning which is dominant in PAn caused by physical adsorption and entanglement of PA chains on nanoclays and weak strain overshoot caused by weak bonds between PA chains and nanoclays. q Polymer 46 (2005) 6429-6436 www.elsevier.com/locate/polymer 0032-3861/$ -see front matter q

Journal of Materials Science, 2013

In this paper, a new model based on the micromechanical and normalized stiffness degradation approaches is established. It has been assumed that during the fatigue condition, only material properties of composites (fiber and matrix) were degraded and nanofillers remain intact under different states of stress. A normalized stiffness degradation model was proposed for laminated fibrous composites reinforced with nanoparticles to derive a novel model to predict the stiffness reduction. The developed model is capable of predicting the fatigue life of nanoparticle-filled fibrous composites based on the experimental data of fibrous composites without nanofillers. The new fatigue model is verified by applying it to different experimental data provided by different researchers. The obtained results by the new fatigue model are in very good agreement with the experimental data of nano-silica glass/ epoxy composites under constant cyclic stress amplitude fatigue and also for silica/epoxy nanocomposites in various states of stress with negligible error.

2014

The objective of this work is to identify and to analyze the main micro-mechanisms which govern the fatigue behavior of a short glass fiber reinforced polyamide 66 composite through a multi-scale experimental analysis. Tension-tension fatigue tests have been performed at different applied maximum stress and have been analyzed at both microscopic and macroscopic scale. Together with the progressive stiffness reduction, the temperature rise due to self-heating during cyclic loading has been measured using an infrared camera. Moreover, SEM fractography observations have been performed to assess the chronology of deformation mechanisms. Two principal mechanisms have been identified: matrix deformation due to selfheating and fiber-matrix interface damage. In addition, localized deformation zones have been observed around the fibers. The evolution of the size of these micro-ductile areas have been statistically related to the maximum applied stress. Finally, a competition between thermal ...

2013

This work focus on the mechanical properties of three-phase nanocomposites using multiscale reinforcements. The influence of the nano-fillers content, as well as the temperature were studied. Polyamide-6 reinforced with short glass fibre 30 wt.% and with an addition of nanoclay (montmorillonite) and/or nanosilica (SiO2) were tested in order to characterise their tensile properties at room temperature and at 65C just above the polyamide 6 glass transition temperature. SEM analysis were conducted on the fracture surface of the tensile bars. SEM investigations showed the importance of the interaction matrix/filler for the material behaviour. Our study also shows that the increase of OMMT percentage in polyamide-6/glass fibre composite made the material more brittle and had a negative effect on the tensile properties. Further, for the silica-based nanocomposites, an optimum was found for a nanofillers content of 1wt.%.

Journal of Applied Polymer Science, 2008

The rheological properties of a polyamide 6/clay nanocomposite with a low loading of clay (1 wt %) were studied. Linear viscoelastic measurements in oscillatory and steady shear with small strain amplitudes were carried out. The nanocomposite exhibited a higher elastic modulus, viscous modulus, and complex viscosity than neat polyamide 6 during dynamic and steady shear tests. Moreover, the addition of clay resulted in a reduction of the critical strain amplitude, an increase of the loss angle, and a reduction of the frequency at the intersection of the elastic and viscous moduli.

Journal of Applied Polymer Science, 2011

Polyamide 6/montmorillonite (MMT) nanocomposites were prepared by melt compounding method comprising 1-7.5 wt % of Nanomer I.24 TL or 5 and 10 wt % of Cloisite 15A organically modified nanoclays. The composite samples were characterized by synchrotron X-ray, thermal and FT-IR spectroscopy methods looking for changes in the micro-and nanostructure of both PA6 matrix and MMT reinforcement as a function of the clay content and type. These data were discussed in conjunction with the mechanical properties of the respective nanocomposites. Generally, the Young's modulus was found to increase proportionally to the clay content being the highest in samples with strong aggregation of MMT at micron length scale.