Evolution of the microstructure and mechanical properties of polyamide 6 during impact

Low-energy impact tests were performed on a polyamide 6 using a dart test technique with a 20-mm-diameter hemispherical striker. Partial penetrations at controlled and increasing energies allowed to visualize the different steps of the deformation. The evolution of the deformation is described, taking into account the shape and the thickness of the samples, the crystalline orientation, the microstructure and the resulting mechanical properties of the polymer. The deformation results from several steps, which are revealed by the evolution of the force measured on the striker. The deformation induces important and inhomogeneous changes in the microstructure and the orientation of the polymer. Variations may be abrupt. The resulting mechanical behavior is strongly dependent on this evolution and, as a consequence, depends on the location in the sample. These observations confirm that it is impossible to predict such a complex behavior using data from simple tensile tests.     

Analysis of multiaxial impact behavior of polymers

Impact performance is a primary concern in many applications of polymers. In this paper, finite element analysis (FEA) and ABAQUS/Explicit are used to simulate the deformation and failure of polymers in the standard ASTM D3763 multiaxial impact test. The specimen geometry and loading mode in this multiaxial impact test provides a close correlation with practical impact conditions. A previously developed constitutive model (“DSGZ” model) for polymers under monotonic compressive loading is generalized and extended for any loading mode and takes into account the different behavior of polymers in uniaxial tensile and compression tests. The phenomenon of thermomechanical coupling during plastic deformation is also included in the analysis. This generalized DSGZ model, along with thermomechanical coupling and a failure criterion based on maximum plastic strain, is incorporated in the FEA model as a coupled‐field user material subroutine to produce a unique tool for the prediction of the impact behavior of polymeric materials. Load‐displacement curves from FEA simulations are compared with experimental data for two glassy polymers, ABS‐1 and ABS‐2. The simulations and experimental data are in excellent agreement up to the maximum impact load. It is shown that not accounting for the different behavior of the polymer in uniaxial tensile and compression tests and thermomechanical coupling effects leads to an overestimation of the load and impact energy, especially at large displacements and plastic deformations. Friction also plays an important role in the impact behavior. If one neglects the friction between the striker and polymer disk, the predicted impact loads are lower as compared with experimental data at large displacements.     

Anisotropic mechanical behavior of semi‐crystalline polymers: Characterization and modeling of non‐monotonic loading including damage

In this work, the mechanical response of high density polyethylene (HDPE) to complex uniaxial tensile loadings is firstly characterized experimentally, taking into account the damage occurring in large deformation and the initial anisotropy induced by the forming process. Anisotropic effects are characterized through tensile tests using several complex loading paths involving large deformation, and for different orientation with respect to the extrusion direction. A mechanical model is then developed, based on a non‐equilibrium thermodynamic approach of irreversible processes, resulting in a new thermodynamic potential describing both the elasto‐viscoelastic–viscoplastic behavior and the volume variation due to damage. Results show that transverse strains and volume strain of HDPE highly depend on specimen orientation, whereas the apparent Young's modulus is not affected by this orientation. The developed model is validated for HDPE, and satisfyingly predicts the complex response of HDPE to complex loadings paths. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 44468.     

Toughness of SMI‐modified ABS alloys and the associated deformation behavior

The mechanical toughness of modified ABS (acrylonitrile–butadiene–styrene) alloys was evaluated using Izod impact, tensile, and compact tension tests. The modified ABS alloys contain 20 wt % of styrene–N‐phenylmaleimide (SMI) that is added to enhance the thermal resistance of the ABS. In this study, the effects of matrix composition, rubber/matrix adhesion, and rubber particle structure on the alloy toughness were investigated. Results from the tensile test and Izod impact test ranked the alloys in an order that is different from that given by K_Ii (stress intensity factor for crack initiation), measured from compact tension specimens. This is due to the difference in energy‐absorption characteristics for crack initiation and crack growth. The conclusion is supported by optical micrographs on the deformation zone size. The microdeformation behavior of the alloys was examined using transmission electron microscopy (TEM), which revealed different rubber‐toughening mechanisms between Izod and tensile specimens. The former contains numerous extensive crazes, while the latter, only a very few short crazes, except in regions within a few micrometers from the fracture surface. The dominant matrix deformation mechanism for the tensile specimens is believed to be shear deformation. Another interesting observation from the study is rubber particle cavitation, commonly observed in tensile specimens and Izod specimens with solid rubber particles; it did not occur in the Izod specimens containing salami‐type rubber particles. This is attributed to the salami structure that increased the straining rate for the rubber phase, leading to ductile–brittle transition of the rubber. The transition to brittle deformation of the rubber phase prevented rubber particle cavitation. The microscopic examination indicated that toughening mechanisms by the rubber particles can be very different among the mechanical tests, which should be taken into account for the rubber toughening of polymers. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 71: 1543–1553, 1999     

Studying deformation behavior of a single spherulite with in-situ infrared microspectroscopic imaging

The deformation behavior of a single spherulite embedded in quenched isotactic polypropylene (iPP) film is studied with in-situ infrared microspectroscopic imaging (FTIRI) with Focal Plane Array (FPA) detector during uniaxial tensile test. Imaging an area of 250 × 250 μm² with 4096 Fourier transform infrared (FTIR) spectra, the absorption distributions of crystalline (998 cm^?1 band) and amorphous (1153 cm^?1 band) phases with radiation polarized parallel and perpendicular to tensile direction are obtained, which are employed to construct the orientation distribution images at different strains. The daughter lamellae in equatorial region are slightly rotated first toward tensile direction, which may postpone the sliding deformation of parent lamellae. The orientation evolution of crystal during tensile deformation suggests that a single spherulite can be divided into three different mechanical zones, corresponding with the crystallinity distribution at different regions of spherulite as estimated through the ratio of 998 cm^?1 to the summation of 998 cm^?1 and 1153 cm^?1. The results may provide a more realistically mechanical model for computer simulation and demonstrate the advantages of FTIRI on the study of structure-property of polymers.     

Effects of β-α transformation on the static and dynamic tensile behavior of isotactic polypropylene

It was demonstrated that the mechanical stress-induced βα-transformation in isotactic polypropylene (iPP) is associated with considerable toughness enhancement. This toughness improvement depends on the test conditions (loading frequency). The toughness of β-iPP was superior to the α-iPP by 13% under static (characterized by a load frequency of ca. 5 × 10⁻⁴ Hz) and 70% under dynamic (tensile impact with a loading frequency in the range of ca. 3 × 10²…10³ Hz) conditions, respectively. By applying the essential work of fracture (EWF) concept to single-edge notched tensile (SEN-T) specimens it was shown that for the toughness upgrading observed, energy dissipation in the enlarged plastic zone is responsible. The occurrence of the βα-transformation was evidenced by differential scanning calorimetry (DSC). Based on DSC measurements it was found that the degree of βα-trans-formation depends on the local strain. At high strain values the βα-conversion is complete (at elogation at break in uniaxial static tensile test), while this transformation is only partial at lower strains (at tensile impact). In addition, in the plastic (or deformation) zone the βα-conversion changed locally, and can be used for mapping of this region. © 1996 John Wiley & Sons, Inc.     

Dilatometric studies of polymers undergoing high and low rate tensile deformation

Specific volume change and stress-strain data were obtained simultaneously during tensile deformation on several plastics known to be resistant to impact loading. Tensile deformation rates of 20 percent/minute and 10⁶ percent/minute and temperatures of ?190° to 55°C were employed. A common sequence of deformation modes was observed in all materials studied (rubber modified acrylics and styrene, ABS materials, polycarbonate, impact grade polypropylenes, and high density polyethylene). In all cases the major mode of deformation to failure at low rates and/or higher temperatures is volume conserving and primarily a shear flow process. At higher rates of deformation or lower temperatures, a transition occurs and the specific volume of the material increases in direct proportionality to the tensile strain above the apparent yield point. Volume increases of 17 to 50% were observed and these were equal to 85 percent or more of the observed tensile strain at failure. These observations indicate that microcavitation may be the major process available for the absorption of mechanical energy at impact rates in plastic materials.     

The properties of dry-spun carbon nanotube fibers and their interfacial shear strength in an epoxy composite

Continuous fibers composed of carbon nanotubes have been adopted as reinforcements for polymeric composites. This paper presents several fundamental studies relevant to the mechanical behavior of CNT fibers, including fiber tensile behavior; in situ SEM observation of fiber deformation mechanisms; and fiber modulus, ultimate strength and fracture strain measurements. A modified Weibull strength distribution model that takes into account the flaw density variation with fiber diameter has been adopted for the statistical strength analysis. The interfacial shear strength between the carbon nanotube fiber and the epoxy matrix has been measured using fragmentation tests of single-fiber composites.     

Post-curing process and visco-elasto-plastic behavior of two structural adhesives

The aim of this paper is to reveal original visco-elasto-plastic phenomena for two commercial epoxy adhesives (D609 and E20HP) subjected to uniaxial tension and compression. First, a post-curing heat treatment is proposed by means of thermal analyses in order to ensure stable mechanical properties. Bulk adhesive specimens are prepared to analyze the mechanical response of both materials. Monotonic tensile and compressive tests are carried out at different strain rates. Both adhesives exhibit first a linear elastic behavior but once a yield stress is reached, a visco-elasto-plastic behavior appears. Creep tensile tests are also carried out and confirm that strain rate phenomena take place and that non-negligible negative volumetric inelastic strains appear. Cyclic tests are also performed and reveal ratcheting effects. The applicability of the results to thin bondlines is discussed. The experimental observations must be taken into account in any model which aims at predicting accurately the behavior of the adhesives considered in this paper.     

Deformational behavior of polyolefins at high temperature and strain rate: Experimental analysis and constitutive laws

Large deformation behavior of two polyolefins has been investigated under high temperatures and high strain rate conditions. During uniaxial tensile tests at constant true strain rate, volume, stress, and deformation ratio were measured. At the temperatures considered, the mechanical behavior tends to be elastic (rate‐independent) as strain rate increases. Thus elastic models can be used to describe the deformational behavior of these resins under such conditions. Models proposed by Ogden, by Coleman as modified by Radi, and by Sweeney‐Ward have been considered to fit experimental data and their adequacy has been discussed. Sweeney‐Ward's model has been found to provide the best fit of the examined polymer's behavior at high temperatures and high strain rates.     

Effect of fiber properties and matrix composition on the tensile behavior of strain-hardening cement-based composites (SHCCs) subject to impact loading

The article at hand describes the behavior of high-strength and normal-strength strain-hardening cement-based composites (SHCCs) made of fine-grained matrix and high-density polyethylene fibers under quasi-static and impact tensile loading. The dynamic tension testing of unnotched and notched cylinders was performed using the Hopkinson bar at strain rates of around 150 s^− 1. The responses of the materials under dynamic and quasi-static tensile loading were compared to the corresponding results for normal-strength SHCC made of polyvinyl-alcohol fibers as obtained in previous investigations. To explain the pronounced differences in rate effects on the material performance of various SHCC compositions, cracking pattern and fracture surface conditions were studied. Additionally, strain rate dependent changes in the mechanical behavior of individual fibers and in the fiber–matrix interfacial properties were deduced from single-fiber tension tests and fiber pullout tests, respectively. Altogether, the results obtained provide clear indications as to the decisive parameters for a purposeful material design of impact resistant types of SHCC for use in structural elements or protective overlays.     

Assessment of weld line performance of PP/Talc moldings produced in hot runner injection molds

Weld lines are weak regions in thermoplastic injection moldings caused by low molecular entanglement and unfavorable orientation. Their occurrence may lead to a significantly reduced mechanical performance of the products. Therefore, when weld lines are likely to occur in molded products, they must be taken into account during the mechanical and technological design processes. The weld lines become more critical when particulate fillers are compounded with the polymer. The performance of weld lines in talc‐filled polypropylene box moldings produced with a double‐gated hot runner mold is assessed in this work. The processing conditions were varied in order to cause morphology and tensile‐impact resistance changes. The weld performance at room temperature was assessed in terms of the energy absorbed in the impact tests. It was found that the performance depends on the injection temperature, the injection rate, and the orientation of the talc particles in the weld‐line plane. J. VINYL ADDIT. TECHNOL., 13:159–165, 2007. © 2007 Society of Plastics Engineers     

Fracture toughness of modified polypropylene/poly(styrene‐ran‐butadiene) blends

Fracture toughness of polypropylene (PP)/poly(styrene‐ran‐butadiene) rubber (SBR) blends as a function of concentration of maleic anhydride (MA) in the maleated polypropylene (MAPP) compatibilizer was investigated under uniaxial static and impact loading conditions. The addition of MAPP to the unmodified PP/rubber blend enhanced the tensile modulus and yield stress as well as the Charpy impact strength. The maximum values were recorded at 1.0 wt% grafted MA in the compatibilizer. V‐shaped blunt‐notched specimens exhibited typical ductile behavior and no breakage of the specimens occurred during the impact fracture tests. Sharp‐notched specimens of uncompatibilized and low‐content MA blends broke in a semibrittle manner, supported by a rapid crack propagation process. Increasing MA content in the blends led to semibrittle‐to‐ductile transition characterized by stable crack propagation. Fracture mechanics experiments, supplemented by scanning electron microscopy (SEM), were also employed to obtain a better understanding of the fracture and deformation behavior. Copyright © 2005 Society of Chemical Industry     

Effect of the resin/hardener ratio on yield,post‐yield,and fracture behavior of nanofilled epoxies

In this work, the effect of the resin/hardener ratio on the small deformation, yield, post‐yield, and fracture behavior of a series of DGEBA‐Jeffamine epoxy‐clay nanocomposites with a fixed organo‐clay content (6 phr), and of the corresponding unfilled resins, was investigated. The mechanical behavior at small deformation was studied by means of uniaxial tensile tests, whereas compression tests were employed to investigate the large (yield and post‐yield) deformation levels. The fracture behavior was studied by the application of fracture mechanics testing methods. The results pointed out that small variations in the resin/hardener ratio used for the preparation of the resin can give rise to remarkable differences in the mechanical behavior at large deformation levels and at fracture. These effects were related to the parameters characteristic of the macromolecular architecture of the resins (chain segments flexibility and crosslink density). The results obtained on nanofilled systems showed that the effect of the resin/hardener ratio on the mechanical behavior of the resins is reduced in presence of organoclay particles. POLYM. COMPOS., 2011. © 2011 Society of Plastics Engineers     

Unimpaired highly extensible tough chemically crosslinked hydrogel after experiencing freeze/thaw and boiling processes

A highly extensible, tough, chemically crosslinked double-network (DN) hydrogel was synthesized from acrylamide. Three samples of this hydrogel were swelled using different solutions. One swelled in water, one in an aqueous glycerol solution, and one in an aqueous sodium chloride solution. The freezing points of the hydrogel samples were determined using differential scanning calorimetry (DSC). Then, the samples underwent controlled freeze–thaw cycles, and their mechanical behavior under loading-unloading-reloading large-strain tensile deformation were analyzed. The results of these mechanical tests indicated that all the data points for deformation cycles coincided, despite the large water content of the samples. This means that no mechanical damage occurred during the deformation process. The results of hydrogel samples boiled in these solutions also showed no damage. Thus, it can be concluded that the tough chemically crosslinked DN hydrogel does not damage under large-strain tensile deformations even after experiencing harsh environmental conditions, such as freeze/thaw or boiling processes, which makes it a great candidate for applications that involve large temperature variations. The resistance of the DN hydrogel to damage is attributed to the specific molecular architecture of this hydrogel, in that the building block of this material is a loosely crosslinked polymeric network.     

Deformation behavior of an epoxy resin subject to multiaxial loadings. Part I: Experimental investigations

The mechanical behavior of an epoxy resin (Epon 826) was studied by performing a series of tests on thin‐walled tubular specimens. These tests deal with different aspects of the mechanical behavior of this epoxy resin. The deformation behavior, such as viscoelastic behavior, hydrostatic stress effect, multiaxial behavior and loading path effect, was investigated. It was found that the Epon 826 epoxy resin is a highly nonlinear viscoelastic material. The effect of hydrostatic pressure on the deformation behavior of this epoxy is not significant. However, it shows different tensile and compressive deformation behavior. The loading path was found to have an observable effect on the deformation response of this epoxy, especially in the high stress/strain range.     

Investigation of the thermoformability of various D-Lactide content poly(lactic acid) films by ball burst test

In this article, we investigated the thermoformability of poly(lactic acid) (PLA) films with various D -Lactide contents and therefore different crystallization properties, performing tensile and ball burst tests at various temperatures and testing rates. We found that the behavior of the PLA films tested above the glass transition temperature significantly differs due to the difference in D -Lactide content, and thus crystallinity. During tensile testing, elevated temperatures and mechanical stress caused the crystallization temperature to decrease and thus highly induced crystallization. At the same time, as testing speed was increased, the ability of the polymer to crystallize decreased. In ball burst tests, the PLA films crystallized more than during tensile testing. We described the differences found between tensile testing and ball burst testing, which latter better represents the conditions of thermoforming through inducing biaxial deformation.     

Morphology and properties of the biosourced poly(lactic acid)/poly(ethylene oxide‐b‐amide‐12) blends

Biosourced poly(lactic acid) (PLA) blends with different content of poly(ethylene oxide‐b‐amide‐12) (PEBA) were prepared by melt compounding. The miscibility, phase structure, crystallization behavior, mechanical properties, and toughening mechanism were investigated. The blend was an immiscible system with the PEBA domains evenly dispersed in the PLA matrix. The PEBA component suppressed the nonisothermal melt crystallization of PLA. With the addition of PEBA, marked improvement in toughness of PLA was achieved. The maximum for elongation at break and impact strength of the blend reached the level of 346% and 60.5 kJ/m², respectively. The phase morphology evolution in the PLA/PEBA blends after tensile and impact tests was investigated, and the corresponding toughening mechanism was discussed. It was found that the PLA matrix demonstrates obvious shear yielding in the blend during the tensile and impact tests, which induced energy dissipation and therefore lead to improvement in toughness of the PLA/PEBA blends. POLYM. COMPOS., 2013. © 2012 Society of Plastics Engineers     

Mechanical Characterization of Si–C(O) Fiber/SiC (CVI) Matrix Composites witrTa BN–Interphase

The mechanical behavior of three CVT-processed 2D woven SiC/BN/SiC composite materials with different initial BN interphase thicknesses has been investigated by means of tensile and impact tests. The results have established the efficiency of a BN interphase in promoting a nonlinear/non–catastrophic tensile behavior and high impact resistance. The effect of the initial BN interphase thickness on the resulting mechanical behavior has also been demonstrated. Characterization of the fiber/matrix interfacial zones by AES and TEM has revealed the presence of a SiO₂/C double layer at the BN/fiber interface, which might result from a decomposition undergone by the Si–C(O) Nicalon fiber during processing. It has been suggested that the influence of the initial BN interphase thickness on the mechanical properties of the composites results from both changes occurring in the composition and morphology of the interfacial zones and modifications of the interfacial forces due to accommodation of the radial residual clamping stress.     

Effect of thermal pretreatment on the fracture behavior of polycarbonate in medium strain rate tensile tests

The fracture properties of as-received and annealed (48 h at 403 K) commericial polycarbonates (PC) were examined in tensile tests with an average strain rate of ? = 2 s^?1. Both materials were subjected to tensile tests at temperatures ranging from 223 to 423 K. The results were processed by a computer interfaced to the testing machine. The tensile strength of the annealed (pretreated) PC was superior to that of the as-received (untreated) material. The elongation at break and the fracture energy, then, decreased due to annealing at all temperatures, yet followed impact strength curves. Fracture analysis and fracture morphology revealed clear differences in the behavior of the materials. Fracture nucleation occurred commonly at several points in the pretreated specimens, whereas only one nucleation point was found in the untreated specimens. Shear yielding morphology, which indicated plastic deformation, started to appear at a lower temperature in the pretreated specimens than in the untreated ones.