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Deformation Mechanisms in Semicrysalline Polymers

The aim of this work is to study the nano-scale deformation mechanisms within the spherulitic structure of semi-crystalline polymers. The deformation mechanisms are imaged by atomic force microscopy. The originality of this work is the use of a home-made tensile drawing stage under the AFM head in order to perform in situ tensile tests (Figure 1).

Figure 1 in situ tensile testing machine

The observations performed on several semi-crystalline polymers such as polyamide 6 (PA6), polybutene (PB) and polycaprolactone (PCL). Theses observations revealed fragmentation of crystalline lamellae (Figure 2) and micro-crazing (Figuree 3). These mechanisms appear from the end of elastic stage, and induce permanent deformation and degradation of mechanical properties.

Figure 2 AFM height image of fragmentation in PB

Figure 3 AFM height image of micro-crazing in PB

These experimental observations enable assuming that there is a coupling between plasticity and damage. The viscous contribution is first identified, then subtracted from the data of cyclic tensile tests in order to assess the damage and the plastic deformation. The damage evolution follows the same law for the various materials in spite of their large structural differences (lamellar thickness, spherulite diameter, glass transition temperature, ...) (Figure 5).

Figure 4 Law coupling damage and cumulated plastic strain

Figure 5 Rheological model

The mechanical behavior modeling based on the concept that the semi-crystalline polymers consist of a macromolecular network flanged by the crystalline structure. The behavior law has been developed with this concept including the damage law previously identified (Figure 5). This law has been implemented in a finite element program, ZéBuLon. Good fits of the experimental monotonic and cyclic tensile tests have been obtained together with fairly good predictions of the relaxation behavior of three studied materials (Figure 6).

Figure 6 Model versus experiment (Polybutène case)