Untitled

Abstract. Force oscillation occurring during the tensile testing of poly(ethylene terephthalate) (PET), resulting in periodical cavitation of the test specimens, has been studied. A mathematical model has been developed to describe the phenomenon, wherein special fibre bundles are assigned to the amorphous molecular chains. In order to model the local periodical transformations and the rate dependent viscoelastic behaviour the coupled fibre bundle cells were supplemented with a twoelement Maxwell model. Using the parameters determined from the measurements the model was compared to the measured force-elongation diagrams and it has been concluded that the simple model can be well used to describe the phenomenon.

Section: Introductionmentioning

confidence: 99%

Modelling tensile force oscillation during the tensile test of PET specimens

Vas¹,

Ronkay²,

Czigány³

2009

“…In any case, the average value of normalised linear stiffness obtained when using paddocked and scutched fibres is comparable with values measured already on other laminates, although reinforced with a much larger volume of fibres, as reported in Table 5. Damping ratio has been defined, along the lines of what done already in [13][14][15], as the ratio between the non-elastic energy (A2+A3) and the hysteresis energy before damping (or rebound) (A1+A2): an example of this measurement is reported for example in Figure 7 for paddocked and scutched fibres laminates. Paddocking and scutching phormium fibres results in the production of laminates which are able to dissipate much more energy (actually, more that twice as much) in the damping phase of the impact event.…”

Section: Resultsmentioning

confidence: 99%

Mechanical and falling weight impact properties of unidirectional phormium fibre/epoxy laminates

Santulli

Jeronimidis²,

Rosa³

et al. 2009

Abstract. The falling weight impact properties of composites obtained by introducing 16 wt% of phormium tenax fibres extracted with two different methods i.e., either manually or by paddocking and scutching and in both cases chemically treated with a 1% solution of sodium hydroxide, have been investigated. The effect of the two extraction methods on fibre characteristics is compared by the tensile properties of the fibres and the flexural properties, fracture modes and hysteresis cycles parameters (impact energy partition, linear stiffness and normalised penetration energy) of final laminates. Laminates obtained using paddocked and scutched fibres are clearly superior to those manufactured using the manually extracted fibres: this is due to the more effective fibre impregnation in the former case than in the latter, which results in an improved dissipation of energy during the damping phase of the impact event. It is noteworthy, however, that the low volume of fibres introduced in the laminate with the manufacturing method adopted does not allow obtaining properties comparable with other semi-structural plant fibre composites, such as e.g., hemp fibre reinforced laminates.

“…Prior to testing all specimens were isothermally annealed at 80°C for 20 min to ensure high crystallinity which favors stress oscillation in sPP [15]. Specimens of both pure sPP and its nanocomposites with montmorillonite in the form of rectangular stripes of 80×7.5×0.34 mm 3 , were cut from pressmolded sheet for tensile testing. Tensile tests were performed on a Hounsfield HK20-W (Tinius Olsen, Surrey, England) desktop machine.…”

Section: Methodsmentioning

confidence: 99%

“…This stress fluctuation is accompanied by the appearance of alternating transparent/opaque zones in the necking area of the specimen [2]. The stress oscillation effect can appear either during semi-static [2] or tensile impact loading [3][4][5], spontaneously [2][3][4] or provoked [6], by an abrupt change in the specimen loading speed. It can be observed for both amorphous and semi-crystalline polymers too.…”

Section: Introductionmentioning

confidence: 99%

“…Temperature at the necking region tends to fluctuate, because of the polymer poor heat conductivity and this causes a reduction in the local elastic modulus of the material, triggering thus stress oscillation. One of the most plausible explanations was proposed by Karger-Kocsis et al [3]. He proposed that a combined effect of crystallization due to cold drawing of the polymer in the necking region accompanied by heat dissipation due to plastic flow leads to a 'stick/slip' mechanism of plastic deformation which results in the appearance of the alternating zones.…”

Section: Introductionmentioning

confidence: 99%

See 1 more Smart Citation

Study of the stress oscillation phenomenon in syndiotactic polypropylene/montmorillonite nanocomposites

Mouzakis¹

2010

Abstract. The phenomenon of Stress Oscillation (SO) was studied in syndiotactic polypropylene and syndiotactic polypropylene nanocomposites with montmorillonite. The effect was provoked by varying the crosshead speed during tensile testing of thin stripes. The internal morphology of the stress oscillated specimens was studied by scanning electron microscopy on chemically etched samples revealing that cavitation prevails inside the opaque stripes of the yielded areas. Differential scanning calorimetry proved that there exists virtually no crystallinity differentiation between the characteristic alternating opaque/transparent stripes that mark the stress oscillation during necking. Finally a simple finite element model of the necking area of the specimens revealed non-uniform internal stress distributions of yield-point magnitudes.