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RESEARCH PAPERS

Nonlinear Viscoelasticity and Viscoplasticity of Flax/Polypropylene Composites

[+] Author and Article Information
Erik Marklund

Division of Polymer Engineering, LuleåUniversity of Technology, SE-97187, Luleå, Swedenerik.marklund@ltu.se

Janis Varna

Division of Polymer Engineering, LuleåUniversity of Technology, SE-97187, Luleå, Swedenjanis.varna@ltu.se

Lennart Wallström

Division of Polymer Engineering, LuleåUniversity of Technology, SE-97187, Luleå, Swedenlennart.wallstrom@ltu.se

J. Eng. Mater. Technol 128(4), 527-536 (Jun 30, 2006) (10 pages) doi:10.1115/1.2345444 History: Received January 12, 2006; Revised June 30, 2006

In tensile tests the flax/polypropylene composites clearly show nonlinear behavior in loading and hysteresis loops in unloading. In creep tests performed at different load levels the response was nonlinear viscoelastic, and after recovery, viscoplastic strains were detected. No degradation in stiffness could be seen and thus nonlinear viscoelasticity and viscoplasticity were assumed to be the main cause for the observed behavior. The fracture surface of a specimen that experienced creep rupture at 24 MPa was investigated using a scanning electron microscope. The viscoplastic response was studied experimentally and described by a power law with respect to time and stress level in the creep test. The nonlinear viscoelasticity was described using Schapery’s model. The application of Prony series and a power law to approximate the viscoelastic compliance was investigated. Both descriptions have accuracy sufficient for practical applications. However, at high stresses the attempts to describe the viscoelastic compliance by a power law with a stress-independent exponent failed and therefore stress dependence of this exponent was included in the data analysis. The accuracy within the considered stress range is good, but the thermodynamic consistency of this procedure has to be proven.

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Copyright © 2006 by American Society of Mechanical Engineers
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References

Figures

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Figure 1

(a) Creep test followed by recovery period, (b) strain response to creep and recovery

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Figure 2

(a) SEM micrograph showing the creep fracture surface (cross section) of a flax/PP composite. The specimen ruptured after 11∕2h of creep at 24MPa, (b) SEM micrograph of an artificially introduced fracture surface along the longitudinal axis of the same specimen.

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Figure 3

Single flax fibers in a flax/PP composite. Detail of the fracture surface.

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Figure 4

(a) Dependence of viscoplastic strains at 15MPa on time in log scale, (b) development of plastic strain simulated using Eq. 18 dots are experimental values

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Figure 5

Nonlinear creep compliance (including the nonlinear elastic part) using Prony series. Model prediction (dots) and experimental data (solid line).

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Figure 6

Comparison of model prediction (dots) and experimental data (solid line) of strain recovery for (a)8MPa and (b)20MPa using Prony series creep compliance

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Figure 7

Determination of the exponent n in the power law and determination of the stress-dependent functions using data fitting to the defined master curve (solid line)

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Figure 8

The time-dependent nonlinear viscoelastic compliance (εc−ε0−εpl)∕σ of the composite in logarithmic axes. Time is in hours. The value of n=0.164 is used. ε0 is the fitting parameter.

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Figure 9

The predicted (solid line) and experimental (dots) strain curves in creep tests at (a)8MPa, (b)20MPa using the power law creep compliance

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Figure 10

The strain recovery after creep tests at (a)8MPa, (b)20MPa. Experimental data (dots) and model predictions (solid line) using the power law creep compliance.

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Figure 11

Creep compliance at 24MPa. Time is in seconds.

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Figure 12

Comparison of model prediction (dots) and experimental data for the viscoelastic response using the modified power law creep compliance

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Figure 13

Comparison of model prediction (dots) and experimental data for viscoelastic strain recovery in time interval t1<t<2t1 using the modified power law creep compliance

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Figure 14

Creep strain at 24MPa (including both viscoplastic and viscoelastic part) and model predictions

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