Experimentelle und numerische Analyse von Risswachstum bei Polypropylen

Observations were presented on crack growth behavior of polypropylene (PP) on virgin and annealed compact tension (CT) specimens at various loading amplitude. The fracture behavior was evaluated using J integral method. J integral fracture resistance decrease with annealing process. Higher loading levels on fracture behavior of PP is more influential than lower loading levels, and fluctuating loading conditions does not influence overall fracture behavior of PP. Additionally, the fracture behavi- or of virgin and annealed PP specimens have been investigated at cross head speed of between 0.1 and 100 mm/ min. Fracture behavior of PP changes from ductile to brittle transition with increase in cross head speed

Stress relaxation behavior of double network hydrogels under compressive loading

Akrilamid (AAM) zayıf mekanik özellikleri dolayı, biyodoku iskeleleri ve yumuşak doku aktüatörleri gibi biyomedikal uygulamalarda istenilen özellikleri sergileyememektedir. Bu nedenle Aljinat (ALG) kullanı-larak çift ağa sahip hidrojel şeklinde kullanılmaktadır. Bu çalışmada AAM-ALG hidrojelindeki kovalent çapraz bağlayıcı (BIS) ve iyonik çapraz bağlayıcı (CaCl2) miktarının mekanik özellikler etkisini araştırmak üzere beş farklı hidrojel üretilmiştir. Silindirik basma numuneleri %20 gerinim seviyesine kadar yüklenmiş, daha sonra 300 saniye süre ile gevşeme testi yapılmıştır. Sonuçlar iyonik çarpaz bağlayıcı miktarının me-kanik özelliklere etkisinin çok güçlü olduğunu göstermiştir. Kovalent çapraz bağlayıcı miktarının ise belli bir kritik seviyenin üzerinde etkili olduğu gözlemlenmiştir.Because of moderate stretchable properties of acrylamide (AAM), it is used as double network hydrogel with alginates (AAL). We prepared AAM-ALG hydrogels containing various amount of crosslinker (BIS) and concentration of calcium chloride (CaCl2) solution. A series of uniaxial compression loading tests were performed on five different AAM-ALG specimens. The specimens are loaded up to 20% strain and hold at the same strain level for 300 seconds. Concentration of CaCl2 solution dependency is clear; increasing molarity of CaCl2 yields an increase in the stress level. On the other hand, effect of amount of BIS is not dominant on mechanical properties

Modeling finite deformation behavior of semicrystalline polymers under uniaxial loading-unloading

WOS: 000282243000003The aim of this work is to investigate the finite deformation behavior of polymeric materials under monotonic loading-unloading. The strain rate sensitivity behaviors of polymeric materials were modeled using viscoplasticity theory based on an overstress (VBO) model. The modeling capability of the VBO model was improved to describe the nonlinear stress-strain behavior of the fully inelastic flow region in loading at small strain level. In this model, the tangent modulus (E(t)) is taken nonlinearly to simulate this polymeric material behavior. The numerical results were compared to the experimental data in the literature. These results were in good agreement with the experimental data

The unusual creep and relaxation behaviour of polypropylene

This paper focuses on experimental investigation and modelling of the unusual creep and relaxation behaviour of polypropylene. A polypropylene specimen was loaded up to three different levels of maximum strain (ɛmax), unloaded to various minimum stresses (σmin), creep and relaxation tests were performed at seven different strain levels. In addition to investigate the effect of loading history on unloading paths, multiple creep experiments were conducted at various stress levels. It was observed that unusual creep and relaxation behaviour of polypropylene depended on a strain increment (Δɛ) which is the difference between the minimum (ɛmin) and maximum strain level (ɛmax). Increasing the strain increment affects the creep and relaxation behaviour on loading path. As a result, the creep and relaxation behaviours of polypropylene show non-monotonic characteristics. The viscoplasticity theory based on overstress for polymer (VBOP) model was used to model the unusual creep and relaxation behaviour of polypropylene. From the comparison between the VBOP model predictions and the experimental results, it can be seen that the VBOP model can model both the unusual creep and relaxation behaviours of a semicrystalline polymer

An observation of the evolution of equilibrium stress on poly(lactic acid) and poly(lactic acid)/hydroxyapatite nanocomposites

Relaxation behavior provides specific information that indicates stress development with elapsed time for the determination of material characterization and constituting of material modeling. In addition, anomalous relaxation behavior of polymeric materials enables experimental analysis of some theoretical variables in constitutive equation. Equilibrium stress is one of the most prevailing variables in material modeling and it is generally considered as unmeasurable. In this study, evolution of equilibrium stress was examined with relaxation tests of two semi crystalline polymer materials, one of which was found to reach precise equilibrium stress levels. In accordance with this purpose, extensive relaxation tests were conducted on poly(lactic acid) and poly(lactic acid)/hydroxyapatite nanocomposites specimens at a wide range of temperatures from 23 ℃ and 55 ℃, and stress levels from 1 MPa to 50 MPa for poly(lactic acid) and 51 MPa for poly(lactic acid)/hydroxyapatite nanocomposites. All the specimens were subjected tensile loading–unloading and partial retraction process. The starting points of the relaxation test were chosen on unloading segment of stress–strain curves. Evolution of stress may decay, increase or decay then increase depending on the test point on unloading curves. Relaxation behavior of poly(lactic acid) and poly(lactic acid)/hydroxyapatite nanocomposites was simulated using viscoplasticity theory based on overstress for polymeric materials. Experimental results of poly(lactic acid) and poly(lactic acid)/hydroxyapatite nanocomposites were matched with numerical results of viscoplasticity theory based on overstress for polymeric materials, and viscoplasticity theory based on overstress for polymeric material model was found to have an aptitude for predicting anomalous relaxation behavior of poly(lactic acid) and poly(lactic acid)/hydroxyapatite nanocomposites. Additionally, the effect of temperature on relaxation time was investigated with using Kohlraush–Williams–Watts time-decay function. Modeling capability of Kohlrausch–Williams–Watts model was reasonable to predict short-term simple relaxation of poly(lactic acid) and poly(lactic acid)/hydroxyapatite nanocomposites. Responses of the model showed that interaction between relaxation time and environmental temperature was related to transition from glass state to rubbery state

Universal mechanical response of polypropylene under cyclic deformation

WOS: 000313415700005Experimental data are reported on isotactic polypropylene in uniaxial tensile (i) cyclic tests (oscillations between maximum strain 0.15 and the zero minimum stress) with various cross-head speeds and (ii) relaxation tests at temperatures ranging from room temperature to 100 degrees C. Observations in cyclic tests show that maximum stresses decrease, while minimum strains grow with the number of cycles. A universal character is revealed of the dependence of minimum strain on the number of cycles: this function is independent of temperature and weakly affected by strain rate. To rationalize this observation, constitutive equations are proposed in cyclic viscoelasticity and viscoplasticity of semicrystalline polymers and a scenario is suggested for damage accumulation. Two types of damage are distinguished which are induced by (i) lamellar fragmentation in the crystalline phase and (ii) nucleation and growth of micro-voids in the amorphous matrix. The stress-strain relations involve five adjustable parameters that are found by fitting observations (50 cycles of loading-retraction) at room temperature and alpha-transition point. Numerical simulation demonstrates that the model correctly describes experimental data and can predict observations in tests with a large number of cycles.European Commission [Nanotough-213436]; TUBITAKADD acknowledges financial support by the European Commission through project Nanotough-213436. ND acknowledges fellowship support by TUBITAK

The effects of loading history and manufacturing methods on the mechanical behavior of high-density polyethylene

WOS: 000294480500003This article describes a series of experiments conducted to determine the effects of loading history and manufacturing techniques on mechanical behavior of high-density polyethylene (HDPE). The main reason for undertaking the research was to investigate multiple creep, multiple relaxation, and cyclic loading on uniaxial tension. The samples used for tensile tests were obtained from extruded pipe and compression-molded sheets. The stress-strain responses of both samples under uniaxial tensile were found to be independent of the loading history. It was observed that the compression-molded specimens exhibit greater deformation ratio than the extruded specimen. Understanding the deformation behavior under different loading can offer the designer of high-density polyethylene products reliable data relevant to practical applications

Mechanical properties of polymer-matrix cellulose-based composite materials

This study focuses on composite cardboard panels, determining their mechanical properties. The panels were manufactured of a minced waste-paper-based composite material. First, four types of mixture were prepared using minced-material composite cardboards with different ratios of polyethylene (PE). Second, the mixtures were compressed and heated in a mold, and then a composite cardboard panel (CCP), 7-mm thick, was produced. In the third stage, tensile, three-point-bending, nail-pulling and water-absorption tests were applied to specimens of different CCPs. Mechanical tests were conducted using a tensile testing machine in accordance with ASTM 1037. In this study, the mechanical behaviors of the CCPs associated with the added PE were investigated. Observations showed that the presence of PE enhanced the mechanical strength of the CCPs and resulted in an increase in the nail-pulling-load and water-absorption resistance. The investigation was extended to conventional wooden panels (oriented strand board and fiberboard), which were exposed to the same tests. The experimental data demonstrated that the strength of the new product was higher than that of the conventional wooden panels. The results suggest that CCPs can be used as an alternative to wooden panels

Cyclic deformations of polypropylene with a strain-controlled program

WOS: 000310030700008Experimental data are reported on isotactic polypropylene in uniaxial tensile cyclic tests with a strain-controlled program (oscillations between fixed minimum and maximum strains). The following characteristic features of stressstrain diagrams are observed: (i) logarithmic decay in maximum and minimum stresses with number of cycles (cyclic softening), (ii) more pronounced reduction in minimum stress than in maximum stress (cyclic strengthening), (iii) independence of rates of decrease in maximum and minimum stresses of strain rate, (iv) decrease in hysteresis energy with number of cycles. To rationalize these observations, a constitutive model is derived in cyclic viscoelasticity and viscoplasticity of semicrystalline polymers. Numerical simulation demonstrates that the model correctly describes experimental stressstrain curves and quantitatively predicts evolution of maximum and minimum stresses with number of cycles. POLYM. ENG. SCI., 2012.Danish Agency for Science, Technology and Innovation [RK 2010-2012 B4]; TUBITAKContract grant sponsor: Danish Agency for Science, Technology and Innovation; contract grant number: RK 2010-2012 B4; contract grant sponsor: TUBITAK

Unusual mechanical response of carbon black-filled thermoplastic elastomers

WOS: 000331352000009Observations are reported on carbon black-filled thermoplastic elastomer in multi-step uniaxial tensile cyclic tests with various strain rates at room temperature. Experimental data reveal several unusual features of stress-strain diagrams: (i) fading memory of deformation history in cyclic tests with increasing maximum elongation ratios, (ii) transitions from simple to mixed to inverse relaxation and creep in specimens subjected to stretching and subsequent retraction down to various minimum stresses, and (iii) strain rate-induced acceleration of inverse relaxation and creep. A constitutive model is developed for the time- and rate-dependent response of polymer composites under an arbitrary three-dimensional deformation with finite strains. Adjustable parameters in the stress-strain relations are found by fitting the experimental data. Ability of the constitutive equations to describe the mechanical behavior of thermoplastic-elastomer composites under cyclic deformation and to predict the observed phenomena is confirmed by numerical simulation.EU Commission [314744]ADD acknowledges financial support by the EU Commission through project Evolution-314744