Characterization of strain bursts in high density polyethylene by means of a novel nano creep test. (May 2019)

• Record Type: Journal Article
• Authors: Wilhelm, H.; Spieckermann, F.; Fischer, C.; Polt, G.; Zehetbauer, M.
• Is Part Of: International journal of plasticity. Volume 116(2019:May)
• Page Start: 297
• Page End: 313
• Publication Date: 2019-05
• Subjects: Dislocations -- Crystal plasticity -- Polymeric material -- Mechanical testing -- Dislocation avalanches; Plasticity -- Periodicals; Plasticité -- Périodiques; Plasticity; Periodicals; 620.11233
• DOI: 10.1016/j.ijplas.2019.01.010
• Languages: English
• ISSNs: 0749-6419

Abstract

Abstract: Recent nanoindentation experiments have shown that in semi-crystalline polymers, strain bursts emerge during creep experiments. A shortcome of nanoindentation is that due to the inhomogeneous stress field, a critical stress for the onset of strain bursts cannot be determined, and the resolution of the method seems to be limited. Since the strain bursts in polymers are in the nm range, an extremely high resolution in deformation measurement is necessary. Such a resolution is provided by modern solid state rheometers having a resolution < 10 −6 rad. With such a facility, in this work single strain bursts in PE-HD have been investigated in creep mode at stresses as low as < 0.5 MPa. As the strain bursts only occur rarely, hundreds of creep experiments were carried out and evaluated statistically. Through variation of the temperature and the stress, the activation energy for the strain bursts was determined as 0.65 (±0.06) eV. In addition, negative strain bursts (contrary to the direction of deformation) with an activation energy of 0.49 (±0.08) eV and post-oscillations after strain bursts could be observed for the first time. In summary the new method represents a powerful tool for a quantitative characterization of strain bursts and gives a new insight to dislocation kinetics in semi-crystalline polymers. Graphical abstract: Image 1 Highlights: Exact determination of the activation energy for strain bursts. First observation of negative strain bursts and oscillations after a strain burst. Plastic deformation already at stresses <1% of the yield stress. Deformation at low stress (strain bursts) is governed by the mobility of dislocations. For marked plastic flow additional dislocations must be mechanically generated.