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This paper describes an equi-biaxial tension fatigue test system which utilises the bubble inflation method to subject elastomers to equi-biaxial fatigue loading between user-defined limits of pressure, volume, stretch ratio or stress. The test system integrates a hydraulic inflation system, a high speed vision system and a control system. The high-speed vision system allows the stretch ratio and stress acting on the test specimen to be evaluated in real-time during testing. This in turn allows either stretch ratio or stress to be used as a direct control limit. In this research, constant maximum engineering stress control tests have been carried out to evaluate the suitability of the developed dynamic bubble inflation system for equi-biaxial fatigue testing of elastomers. The resulting test data was used to produce Wöhler (S/N) curves and stress/stretch ratio plots for ethylene-propylene rubber (EPM). [ABSTRACT FROM AUTHOR]

The composite-structure protective systems in head-on collision with objects are largely subjected to dynamic compression load along the thickness of composite structure. A typical plain weave aramid fabric reinforced polyamide (PA) composite, which is defined as one of single polymer composites (SPCs), is addressed in this paper. Firstly, in the process of sample preparation, processing characteristics of the single polymer composites are skillfully achieved and discussed using differential scanning calorimetry (DSC) and capillary rheometer. Secondly, the out-of-plane compression properties of the composite are studied on Split Hopkinson Pressure Bar (SHPB) apparatus in the strain rate range of 400–1200s −1 . Effects of fiber content and strain rate on dynamic off-plane compression properties are investigated and quasi-static properties are obtained on a universal testing machine as a comparison. Results provide a basis for selecting composite composition and lay-up for designing armor with improved impact resistance. Additionally, penetration of the resin through the fabric is observed by the digital microscope and the internal damage of the laminates is qualitatively predicted by the microstructure of the internal fabric yarns. [ABSTRACT FROM AUTHOR]

Magnetorheological Elastomers (MREs) are “smart” materials whose physical properties are altered by the application of magnetic fields. In previous studies the properties of MREs have been evaluated under a variety of conditions, however little attention has been paid to the recording and reporting of the magnetic fields used in these tests [1]. Currently there is no standard accepted method for specifying the magnetic field applied during MRE testing. This study presents a detailed map of a magnetic field applied during MRE tests as well as providing the first comparative results for uniaxial and biaxial testing under high strain fatigue test conditions. Both uniaxial tension tests and equi-biaxial bubble inflation tests were performed on isotropic natural rubber MREs using the same magnetic fields having magnetic flux densities up to 206 mT. The samples were cycled between pre-set strain limits. The magnetic field was switched on for a number of consecutive cycles and off for the same number of following cycles. The resultant change in stress due to the application and removal of the magnetic field was recorded and results are presented. [ABSTRACT FROM AUTHOR]

The presented literature review of cruciform shapes used for biaxial characterization of materials indicates that the majority of shapes can be divided into two large groups when the following selection criteria are taken into consideration: (i) the shape of the outer boundaries and (ii) the load capacity needed to achieve failure in the biaxial region. Manipulation of the outer shape boundaries appears to be essential to bundle the applied loads to the central zone where failure is intended to be built up. For each group, one particular cruciform design is reported whereby the outer boundaries are based on a single curved shape. Although the use of discontinuous double radii edges should be avoided according to earlier reports [1,2], it is shown here through the construction of an optimization algorithm, that the use of a single curve for the outer boundaries leads to strains in the arms that are strongly dependent on these single curved edges. Numerical simulations based on the finite element method as well as experiments performed on polymeric test pieces in combination with DIC measurements, show good agreement on this matter and demonstrate this sensitivity very clearly. [ABSTRACT FROM AUTHOR]

Abstract: Investigating the fatigue behaviour of a material often requires a lot of experiments in order to have statistically valid results. However, the number of successful fatigue experiments, where tab failure did not occur, can be significantly smaller than the total number of experiments. This manuscript studies the fatigue behaviour of a carbon fabric reinforced PPS using the specimen geometry proposed by ASTM D3479/D3479M. All specimens failed in the tabbed section, meaning that fatigue life may be significantly underestimated and the proposed geometry needs improvement. Therefore, the main emphasis of this manuscript is on stiffness reduction and permanent deformation of the material, rather than on the number of cycles to failure. It may be concluded that for the [(0°,90°)]4s stacking sequence the material does not show significant stiffness reduction and that only limited permanent deformation is present. Furthermore, the material shows very brittle failure behaviour and the stress-span between infinite fatigue life and failure in a few dozen cycles is very narrow. [Copyright &y& Elsevier]

Abstract: This paper presents the measurement of the nonlinear mechanical properties of polydimethylsiloxane (PDMS) elastomer based on the mixing ratio of base polymer to curing agent. Strip-type PDMS samples with different mixing ratios were prepared using a simple coating, curing, and cutting process. A cyclic uniaxial tension test with a fixed magnitude of applied strain and a single-pull-to-failure tension test were performed with a micro-tensile tester at room temperature. Our new finding is that when the PDMS is mixed with excessive curing agent, stress softening occurs and residual strain exists in cyclic tension tests when the magnitude of the applied strain increases. For the PDMS-05 samples, in which the mixing ratio of base polymer to curing agent was 5 to 1, there were large differences in the stresses for the same strain level under loading and unloading during the first cycle with a 100% fixed strain amplitude, but the softening effect of the stress in the PDMS dropped rapidly starting from the second cycle. Nonlinear mechanical Neo-Hookean, third-order Mooney, and second-order Ogden models of three different PDMS films were computed from the stress–strain data. The results showed that all models were preferable for the small strain region of PDMS compared with other models. In the nonlinear, large strain region, only the second-order Ogden model properly described the mechanical behavior of the PDMS, while the Neo-Hookean and third-order Mooney-Rivlin models were too stiff or flexible in the measurement range. The bulk modulus of PDMS increased with the amount of curing agent in it. Therefore, the second-order Ogden model is preferable for analyzing the PDMS structure over the entire measurement range. This could provide reasonable mechanical models of PDMS for rapid computational prototyping and for designing active and passive components from PDMS. [Copyright &y& Elsevier]

Abstract: The mechanical behaviour of a copolymer polypropylene material (PPC7712) was studied under high strain rate tension loading conditions. For this purpose, a servo-hydraulic machine was equipped with a novel experimental device involving an original grip system. Stress–strain measurements were performed with strain gauges. The aim of the study was to determine the effects of the strain rate on the stress–strain behaviour, the tensile strength and the onset of the fracture process. Various strain rates ranging from 0.8 to 200 s−1 were tested. The tensile strength was found to increase with the strain rate. The strain rate dependent behaviour of the material was described using the phenomenological Hyperelasto-Visco-Hysteresis model (HVH model) previously used with the results of quasi-static tensile tests on this material . The effects of the strain rate were given in this model by the changes in the viscous parameters. [Copyright &y& Elsevier]

Abstract: Observations are reported on isotactic polypropylene in tensile tests with various strain rates, relaxation tests at various strains, and creep tests with various stresses at room temperature. Constitutive equations are derived for the viscoelastic and viscoplastic responses of semicrystalline polymers at three-dimensional deformations with small strains. The stress–strain relations involve eight material constants that are found by fitting the experimental data. The model is applied to the numerical analysis of creep failure of polypropylene under various deformation modes (uniaxial tension, equi-biaxial tension, shear, multiple-step creep tests). [Copyright &y& Elsevier]

Failure of polymer liquids under flow has been the subject of a relatively strong computational, theoretical, and experimental effort over the years, despite which a clear picture of the phenomenon is still nonexistent. For example, there are still arguments in the literature whether the maximum in engineering stress, also known as the Considère limit for failure, is a point of true yielding of the polymer network or simply a purely elastic mechanical instability and if this onset of yielding also corresponds to the true onset of non-homogeneous deformation, as expressed for example by necking. The main objective of the present work is to contribute to the ongoing discussion on this matter by studying a linear polyisobutylene melt in terms of its failure and rupture behavior in both the viscoelastic and purely elastic deformation regimes of deformation showing that homogeneous flow still occurs after the maximum in engineering stress and quantifying this deviation. This is ultimately connected with the observation of a limiting stress at rupture in the rubbery (elastic) regime. From the results, it is apparent that, for this particular polydisperse melt and in the viscoelastic regime, the Considère criterion underestimates by approximately 60% the point of onset of non-homogeneous deformation, i.e., necking. The calculated true onset of failure, on the other hand, agrees almost exactly with the visually determined onset. However, it is clear that a general working criterion for failure of entangled polymer melts, especially in the all-important viscoelastic deformation regime, is still lacking. [ABSTRACT FROM AUTHOR]

Abstract: In this work, measurements of in situ solidification and post-fabrication strains in two commercial stereolithography photopolymers are presented. The photopolymers examined in the present investigation are a multifunctional acrylate-based resin and an organic–inorganic polymer known as ORMOCER®, which have been used in single and two-photon stereolithography, respectively. The resulted solidification strain build up is monitored via the use of an embedded Fibre Bragg grating (FBG) sensor during photopolymers stepwise exposure to UV-light irradiation. Analysis of the recorded spectra demonstrate the development of high consolidation strains in both UV cured photopolymers as well as a significant increase of their magnitude after post-fabrication thermal treatment at 75°C. Based on the strain data from the utilized optical sensor, information on the degree of cure for one of the two photopolymers is also provided. [Copyright &y& Elsevier]

Abstract: The determination of the energy of fracture of tough polymeric materials in plane strain conditions by means of the essential work of fracture procedure has nowadays two open questions. The first is related to the ligament length that has to be used when the impact specimens break partially upon testing, and the second is related to the way to assure that the crack propagation occurs in an already yielded zone. The use of i) a “true ligament length” associated with only the broken area and ii) impact specimens yielded in the notch zone in a previous tensile test, are evaluated and proposed. [Copyright &y& Elsevier]

Abstract: Dynamic tensile tests were performed on polycarbonate using a split Hopkinson tension bar (SHTB) system. A prefixed short metal bar was used to generate the incident stress pulse. The shape of the incident pulse was controlled to meet the requirement of the one-dimensional experimental principle of SHTB. The dynamic tensile stress–strain responses of polycarbonate at high strain rates up to a rate of 1750s−1 were obtained. Experimental results indicate that the tensile behavior of polycarbonate is dependent on the strain rate. Its yield stress and unstable strain all increase with the increased strain rate. The yield behavior was modeled for a wide range of strain rates based on the thermally activated theory. The correlation between the experimental data and the model is good. [Copyright &y& Elsevier]

Abstract: Dynamic tests at high strain rates involving a weight dropped from different heights have been carried out on acrylic stick cylindrical specimens, in order to determine their mechanical behavior. The experimental device consists of a mass M dropped from an initial height , impacting the plate of a compressive apparatus lying on the upper surface of the specimen. The stress and the strain are derived from measurements of the compressive force and relative displacement of the upper plate. Regarding theoretical aspects, a novel strategy for a constitutive model, based on a combination of measurements and the use of Lie groups analysis, is proposed. The methodology consists in condensing experimental data obtained for different initial drop heights into master curves, which are further interpreted as Lie symmetries of the postulated constitutive equation for the stick sample. The constitutive model involves two functions of the strain rate, obtained from the Lie symmetry condition; this condition expresses the postulated invariance of the material behavior when the impact conditions vary. The material parameters are obtained from the adjustment of the constitutive model with experimental data. The model is found to be similar to the one used in [Naik, N.K., Perla, Y., 2008. Mechanical behavior of acrylic under high strain rate tensile loading. Polymer Testing 27, 504–512] for acrylic. Predictions of the model at constant strain rates show a viscoelastic behavior typical of such polymers. [Copyright &y& Elsevier]

Abstract: Low-velocity impact tests on cast acrylic (PMMA) plates were conducted using an instrumented drop-weight impact machine under various temperatures. Radial cracks were induced and the impact force histories were recorded. Results indicated that there was a glass transition temperature of cast acrylic between 185 and 200°F. Ultrasonic assessments on the impacted acrylic plates were then performed using different transducers. Focused transducers provided more accurate results than flat ones. In addition, the higher the ultrasound frequency, the better the resolution. Finally, the drop-weight impact responses were simulated using LS-DYNA. Two node-splitting techniques, sudden releasing and stepwise releasing, based on maximum tensile stress failure criterion, were implemented for this study. The maximum tensile stress failure criterion was adopted to capture the onset of crack initiation and crack propagation. The node release methods enabled us to calculate the strain energy release rates of cast acrylic. Numerical simulations were compared with experimental results and good agreements were achieved. [Copyright &y& Elsevier]

Abstract: Following the ASTM and ISO test standards, a series of scratch tests were carried out on four categories of polymers: I) ductile and strong, II) ductile and weak, III) brittle and weak, and IV) brittle and strong. The scratch damage features were characterized by using a desktop scanner for scratch visibility assessment, and optical and electron microscopes for detailed damage mechanisms investigation. Various scratch damage mechanisms were identified for the different categories of polymers. The effect of testing rate on possible alteration of scratch damage mechanisms was also studied. The stress fields experienced by the polymer during scratch were determined using three-dimensional finite element methods modeling. It is found that both the material characteristics and the complex stress state exerted on the scratched surface are responsible for the various scratch damage mechanisms observed. A generalized scratch damage mechanism map for polymers is presented. The usefulness of the above understanding for designing scratch-resistant polymers is also discussed. [Copyright &y& Elsevier]

Morphology and deformation mechanisms and tensile properties of tetrafunctional multigraft (MG) polystrene-g-polyisoprene (PS-g-PI) copolymers were investigated dependent on PS volume fraction and number of branch points. The combination of various methods such as TEM, real time synchrotron SAXS, rheo-optical FTIR, and tensile tests provides comprehensive information at different dimension levels. TEM and SAXS studies revealed that the number of branch points has no obvious influence on the microphase-separated morphology of tetrafunction MG copolymers with 16 wt % PS. But for tetrafunctional MG copolymers with 25 wt % PS, the size and integrity of PS microdomains decrease with increasing number of branch point. The deformation mechanisms of MG copolymers are highly related to the morphology. Dependent on the microphase-separated morphology and integrity of the PS phase, the strain-induced orientation of the PS phase is at different size scales. Polarized FT-IR spectra analysis reveals that, for all investigated MG copolymers, the PI phase shows strain-induced orientation along SD at molecular scale. The proportion of the PI block effectively bridging PS domains controls the tensile properties of the MG copolymers at high strain, while the stress−strain behavior in the low-mediate strain region is controlled by the continuity of PS microdomains. The special molecular architecture, which leads to the higher effective functionality of PS domains and the higher possibility for an individual PI backbone being tethered with a large number of PS domains, is proposed to be the origin of the superelasticity for MG copolymers. [ABSTRACT FROM AUTHOR]

Abstract: Polarisation sensitive optical coherence tomography (PS-OCT) is an interferometric technique to determine and map the internal birefringence properties of translucent and turbid samples. Because of its non-destructive character, PS-OCT is an ideal method for material characterisation and testing. When birefringence patterns induced by mechanical strain are analysed, information about the stress within a material can be obtained. In this work, we investigate the stress in polymer samples using PS-OCT. From strain experiments under well defined loading conditions, we determine the stress optical coefficients for different materials by evaluation of the birefringence patterns. Applied to strained samples fabricated from these materials, the stress calibration allows a correlation of the birefringence with quantitative stress values within the materials. [Copyright &y& Elsevier]

Optical Coherence Tomography (OCT) is an emerging technology for high-resolution non-contact imaging of semi-transparent structures. Originally developed for medical diagnostics, we extend the OCT-technique to problems posed in material testing and characterisation. Layer thickness and refractive indices as well as internal structures of polymer parts have been determined within this study. An extension of OCT, namely polarisation-sensitive OCT (PS-OCT) has been used to identify regions of increased anisotropy within injection-moulded plastic parts and to determine quantitatively internal strain within translucent materials in a non-destructive way. [ABSTRACT FROM AUTHOR]