Experimental Investigations on the Induced Anisotropy of Mechanical Properties in Polycarbonate Films

Wibbeke, Andrea; Schöeppner, Volker; Mahnken, Rolf

doi:10.1155/2013/649043

Cited by 6 publications

(27 citation statements)

References 10 publications

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“…. We provide an explanation, that anisotropy is independent on the pre-strain of initial loading, as observed e. g. in Wibbeke et al (2013). .…”

Section: Introductionsupporting

confidence: 65%

“…It is extruded by the institute Kunststofftechnik Paderborn to get af ilm with a thickness of t initial ¼ 0 : 35 mm. From the experimental results of Wibbeke et al (2013) we recall that initially isotropic material is verified by ac omparison of stress-strain curves for various initially loaded PC-films. Specimens with ag eometry illustrated in Fig.…”

Section: Materials and Specimen Preparation For Initial Loadingmentioning

confidence: 88%

“…Following the approach in Wibbeke et al (2013), the experimental investigations are structured in two loading phasesin order to investigate the induced anisotropic material behavior: 1. an initial loading phase and 2. ar e-loading phase. The fol-lowing subsections give abrief description of the material and the specimen preparation for both loading phases.…”

Section: S Pecimens and Testing Equipmentmentioning

confidence: 99%

“…To this end, plastic incompressibility is assumed in some approaches, which gives an estimate for the stretch in the lateral direction and therefore an estimate for the reduced area in the current configuration, see e. g. Ames et al (2009) or Wibbeke et al (2013). Parsons et al (2004) use am ore complex technique to determine the true stress.…”

Section: Introductionmentioning

confidence: 99%

“…The initially isotropic material behavior becomes anisotropic To the authors knowledge, only af ew publications are concerned with experimental investigations on induced anisotropy. Arruda et al (1993) investigate this effect resulting from compression of polycarbonate, whereas Wibbeke et al (2013) provide an ansatz for tensile tests of polycarbonate films. In both approaches, an initial loading phase is applied to induce anisotropy, followed by as econd phase for re-loading, where also the material direction is varied.…”

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Experimental Investigation of PC-Films Using Optical Measurements

Dammann

Caylak

Mahnken

2014

International Polymer Processing

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The alignment of polymer chains is aw ell known microstructural evolution effect due to straining of polymers. This has a drastic influence on the macroscopic properties of the initially isotropic material. In this work, cold forming is performed at room temperature on at ensile testing machine. Polycarbonate films are examined in two loading phases. In the first phase, the specimen is loaded to induce anisotropy, and in the second, it is re-loaded, while the material direction is varied. The investigations are supported by an optical measurement system to gain knowledge about the inhomogeneous material behavior in the initial loading phase and about the anisotropic homogeneous behavior during the re-loading phase. Two dimensional strain contours are obtained from the test data. Additionally, we propose am ethod for approximation of the macroscopic true stress and compare the results with ac ommon approach based on volume consistency. In the future, the test data will set ab asis for parameter identification of constitutive equations taking into account ac ombination of inhomogenous and homogenous material behavior,exhibiting strain induced anisotropy. 1I ntroductionGlassy polymers consist of an umber of polymer chains that form anetwork at amicroscopic level. As illustrated schematically in Fig. 1, when exposed to external loading, these chains orient in acertain manner. The consequences of this alignment on the mechanical properties are twofold: . There is ap ronounced inhomogeneous behavior of strains and stresses within the specimen . The initially isotropic material behavior becomes anisotropic To the authors knowledge, only af ew publications are concerned with experimental investigations on induced anisotropy. Arruda et al. (1993) investigate this effect resulting from compression of polycarbonate, whereas Wibbeke et al. (2013) provide an ansatz for tensile tests of polycarbonate films. In both approaches, an initial loading phase is applied to induce anisotropy, followed by as econd phase for re-loading, where also the material direction is varied.The need for exact knowledge of inhomogeneous and anisotropic stress and strain states is important in several fields in material science. Aprominent example is parameter identification of constitutive equations. As shown from finite element calculations for example by Miehe et al. (2009) the stress and strain states are highly non-uniform during coldforming. Consequently, material parameters identified on basis of homogeneous data, as obtained for example in Mahnken and Damman (2014) or Damman and Mahnken (2013), can only be regarded as estimates. Therefore, parameter identification with the finite element method, see e. g. Mahnken and Stein (1997) or Widany and Mahnken (2012),based on the inhomogeneous experimental data is required and will be pursued in future work.The determination of the macroscopic true stress-strain behavior requires knowledge of the reduced area in the current configuration to calculate the true stress. To this end, plastic incompressibi...

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“…. We provide an explanation, that anisotropy is independent on the pre-strain of initial loading, as observed e. g. in Wibbeke et al (2013). .…”

Section: Introductionsupporting

confidence: 65%

Section: Materials and Specimen Preparation For Initial Loadingmentioning

confidence: 88%

Section: S Pecimens and Testing Equipmentmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

mentioning

confidence: 99%

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Experimental Investigation of PC-Films Using Optical Measurements

Dammann

Caylak

Mahnken

2014

International Polymer Processing

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Sequential biaxial stretching of polycarbonate‐films for characterization of strain‐induced anisotropy

2018

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Amorphous polymers consist of polymer‐chains whose alignment is a well‐known microstructural evolution effect due to stretching of polymers. This so‐called morphology‐development is investigated in the literature, where for thin films simultaneous biaxial stretching is distinguished from sequential biaxial stretching. Both procedures have a strong influence on the macroscopic properties of an initially isotropic material, resulting in induced anisotropy. This work concentrates on sequential biaxial stretching of cold forming realized by tensile testing. To this end, thin polycarbonate films are examined in 2 loading phases: In the first preloading phase anisotropy is induced and in the consecutive second reloading phase, the material direction is varied. Optical measurements support the analysis of highly inhomogenous strain fields. As a main result two‐dimensional strain contours are obtained from test data which will set a basis for parameter identification of constitutive equations for strain‐induced anisotropy in the future.

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A macroscopic consitutive model on induced anisotropy for polymers with weighting functions

Dammann

Mahnken

2014

Proc Appl Math and Mech

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The alignment of polymer chains is a well known microstructural evolution effect due to straining of polymers. This has a drastic influence on the macroscopic properties of the initially isotropic material, such as a pronounced strength in the loading direction of stretched films. Experiments on strain induced anisotropy at room temperature are analyzed by optical measurements. For modeling the effect of strain induced anisotropy a macroscopic constitutive model is presented. As a key idea, weighting functions are introduced to represent a strain-softening/hardening-effect to account for induced anisotropy. These functions represent the ratio between the total strain rate and a structural tensor. In this way, material parameters are used as a sum of weighted direction related quantities. In the finite element examples we simulate the cold-forming of amorphous thermoplastic films below the glass transition temperature subjected to different re-loading directions. Experiments on induced anisotropyThis section summarizes some results on experimental investigations on strain-induced anisotropy for polycarbonate films through cold-forming. For evaluation, we use the true strain ε , the 1. Piola Kirchhoff stress and the major strain ε M . More details are given in [1].Two loading phases are carried out: The initial loading phase, where the maximal pre-strain is ε c = 0.41 and the re-loading phase in which the material direction α ∈ [0 • , 90 • ] is varied as illustrated in Fig. 1c. The stress strain profiles from both loading phases are shown in Fig. 1a. Contourplots of the major strain are illustrated in Fig. 1b for initial loading and in Fig. 1d for re-loading. EXP INITIAL α = 90 o α = 0 o α = 90 o α = 45 o ε M ε M 0 10 20 30 40 50 60 70 80 90 100 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 1.PK. stress [MPa] True strain [-] EXP INITIAL EXP 0°E XP 45°E XP 90°a ) F F d) c) b) Fig. 1: Experiments on induced anisotropy: a) 1.PK. stress vs. true strain for initial loading with ε c = 0.41 as well as for re-loading in 0 • , 45 • and 90 • direction, b) contourplot of the major strain for initial loading, c) illustration of variation of material direction α, d) contourplot for re-loading in α = 90 • direction.The results on initial loading render the following consequences: A highly non-uniform behaviour is revealed by showing the stabilization and propagation of the neck. This results in an almost homogenous distribution of strain in the region of the lateral contraction. As shown in Fig. 1b, by use of optical measurements we obtain a local saturation strain of ε Msat ≈ 80%. This explains the experimental result of [2], that anisotropy for re-loading is almost independent on the pre-strain ε c . For the re-loading phase we observe a decreasing initial yielding for increasing α ∈ [0 • , 90 • ], varying hardening behaviour dependent on α and almost uniform strain states, as shown in Fig. 1d. Constitutive Modelling of induced anisotropyAn alternative approach to model induced anisotropy on the basis of weighting function is used in [3], mo...

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Experimental Investigations on the Induced Anisotropy of Mechanical Properties in Polycarbonate Films

Cited by 6 publications

References 10 publications

Experimental Investigation of PC-Films Using Optical Measurements

Experimental Investigation of PC-Films Using Optical Measurements

Sequential biaxial stretching of polycarbonate‐films for characterization of strain‐induced anisotropy

A macroscopic consitutive model on induced anisotropy for polymers with weighting functions

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