Elsevier

Polymer

Volume 48, Issue 10, 4 May 2007, Pages 2958-2968
Polymer

Effect of temperature and strain rate on the tensile deformation of polyamide 6

https://doi.org/10.1016/j.polymer.2007.03.013Get rights and content

Abstract

The effects of the draw temperature and the strain rate on the tensile deformation of polyamide 6 (PA6) were investigated using three PA6 samples with different initial shapes and physical dimensions. It is observed that the special double yielding phenomenon is indeed present in PA6, provided that certain temperature and strain rate are given, as well as the appropriate initial structure. The results also show that the dependence of the first yield stress on temperature is nearly linear while on strain-rate is logarithmic. The temperature and strain-rate sensitivity change at the draw temperature in the vicinity of the glass transition temperature of PA6. The double yielding of PA6 is not only the combination of two thermally activated rate processes depending on temperature and strain rate, but also associated with the initial structure of samples. The yielding manner for PA6 seems to be determined by the synergetic effect of both the deformation of amorphous and crystalline phases. Thus some special structure involving the crystalline and amorphous phases should come into being in PA6 exhibiting double yielding. Especially the important role of inter- and intra-link should be taken into account. The theory of partial melting–recrystallization cannot account fully for the double yielding of PA6.

Introduction

The study of the tensile deformation behavior of polymeric materials has been the subject of numerous investigations in a number of previous publications and the extensive work has established that both the draw temperature and the strain rate are the crucial factors in determining the deformation characteristics of polymers [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13]. It is well known that the Eyring formalization [1] for thermally activated rate processes has been the most largely used model for clarifying the yield mechanism of glassy and semicrystalline polymers. A detailed investigation of the yielding behavior of poly(methyl methacrylate) (PMMA) and polycarbonate (PC) over a wide range of strain rates and temperatures by Roetling [3] and Bauwens [4] has shown that the yield stress increases rapidly with increasing strain rate and decreasing temperature at low temperatures and high strain rates than at high temperatures and low strain rates. Therefore, it has been proposed that the Eyring equation can be extended, under the assumption that there is more than one activated rate process, the stresses being additive. Vinogradov et al. [5], [6] have dealt with the tensile flow and stress–strain behavior of poly(butyl methacrylate) at different testing conditions and found that: (1) over broad ranges of stresses and strain rates such polymers behave as linear viscoelastic bodies and the peculiar deformation properties are due to transitions from the fluid to rubbery, leathery, and glassy states as stress- and strain-rate increase; (2) its dependence on the stress is exponential in the temperature range below Tg, whereas at temperature above Tg a power law fits the data. Most recently, Mahieux and Reifsnider [7] have put forward a statistical model to describe the stiffness variation of polymers over a wide range of temperatures. Also, starting from the consideration of strain-rate dependence of the stiffness, Richeton et al. [8] have proposed a robust physically based model for predicting the stiffness for a wide range of temperatures and strain rates. Additionally, much attention has been focussed on the effect of temperature as well as strain rate on the tensile behavior of thermoplastic matrix composites. The results from Mallick and associates [9] have revealed that short fiber reinforced polyamide 6 (PA6) composite is a strain rate and temperature dependent material. The strain rate and temperature sensitivity differ at a temperature between 25 °C and 50 °C as a result of the glass transition of the PA6 matrix.

In particular, Seguela and co-workers [10], [11] have made a comprehensive researches on the yielding behavior of medium density polyethylene (MDPE) and drawn the conclusion that the yielding process of MDPE involves two thermally activated rate processes operating cooperatively, but having different activation parameters relative to both temperature and strain rate. It has been pointed out that PEs which give rise to a sharp yield point at room temperature may display a double yield point at higher draw temperature. Subsequently, they have paid special attention to studying the yielding behavior of PE and related copolymers and found that either homogeneous or heterogeneous deformation under tensile testing should be in existence in PEs. The stronger thermal activation of homogeneous slip makes it become more favorable than heterogeneous slip as temperature increases and strain rate slows down. However, the heterogeneous slip may be activated as deformation proceeds owing to its lower strain hardening [12]. In the past two decades, various investigations have been carried out in order to gain a better insight into the double yielding of PEs and some valuable results and models can be available [10], [11], [12], [13], [14], [15], [16], [17], [18].

The occurrence of double yielding in PA6 films, carefully dried under vacuum, was first recognised by Hoashi et al. [19] without any comments. However, to the best of the authors' knowledge, no extra literatures were reported concerning this special phenomenon of PA6 after that, except our recent publications [20], [21], [22], [23]. A qualitative analysis about the effect of temperature and strain rate on tensile deformation including the double yielding of PA6 is provided in present paper, and the origin of double yielding of PA6 will be discussed in more detail in subsequent reports.

Section snippets

Material

The polyamide 6 (PA6) resin used here was a commercial product of Xinhui Meida-DSM Nylon Slice Company LTD., supplied in pellets, with the trade mark M2800, as described previously [20], [21], [22], [23]. The melt flow rate (MFR) of the resin is 4.09 g/10 min at 275 °C, exerting a force of 325 g. The resin was dried for 12 h under vacuum at 100 °C before injection molding to avoid its hydrolytic degradation.

Sample preparation and tensile measurement

The strictly dried PA6 resin was injection molded into dog-bone specimens using an injection

Temperature and strain-rate dependence

Fig. 2 illustrates the nominal stress–strain curves of plain PA6 for sample A as a function of draw temperature at different strain rates. The crosshead speeds of apparatus were set at 1 mm/min, 5 mm/min, 10 mm/min and 50 mm/min. According to the definition of nominal strain rate above, it can be calculated that the corresponding nominal strain rates were 0.33 × 10−3 s−1, 1.67 × 10−3 s−1, 3.33 × 10−3 s−1 and 1.67 × 10−2 s−1. First we consider the influence of the temperature. On the whole, the sample exhibits

Discussion

Based on the extensive set of data generated with diverse PA6 samples we can draw the conclusion that double yielding is indeed in existence in PA6, provided that an appropriate initial structure and testing condition can be achieved. Very distinct changes in the shape of stress–strain curves of PA6 samples can be observed as there dependent variables are altered. The experimental observation has given the direct information evident sharp necking was discerned in the vicinity of the second

Conclusions

The major objective of this work is to establish the relation of the dependence of the complex yielding process on the structural factors and testing conditions. The double yielding really exists in PA6, provided that the definite temperature and strain rate are given, as well as the appropriate initial structure. As the temperature reaches a certain level, the stress–strain curves display a rubber-like deformation and exhibit no evident maximum. It is also revealed that the temperature

Acknowledgements

The authors gratefully acknowledge the financial support of National Natural Science Foundation of China (Grant Nos. 50503014, 50533050), Doctoral Research Foundation granted by the National Ministry of Education, China (Grant No. 20060610029) and the Special Funds for Major Basic Research (Grant No. 2005CB623808).

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