Abstract:

Objective In order to analyze and predict the tensile stress-stain relationship of core/sheath composite yarn with double filaments, a viscoelastic tensile model needs to be developed, which would enable numerical prediction of tensile strength of this composite yarn.

Method The yarn was regarded as a viscoelastic body and its tensile property was shown in the form of the stress-strain corresponding to time. The models commonly used for the research of the tensile fracture mechanism of yarn are varied, such as linear spring, nonlinear spring, nonlinear dashpot, and Maxwell and Kelvin models. Some models above are selected in series or parallel configuration to establish suitable tensile model for core/sheath composite yarn with double filament. Additionally, experimental tests are conducted to verify the correctness of the viscoelastic model.

Results In this study, a nonlinear viscoelastic model was established to predict the tensile stress-strain relationship of core/sheath composite yarn with double filaments. The tensile properties of the yarn were tested, and different stages of the tensile curve were analyzed. The tensile properties of the composite yarns were simulated and calculated by employing the developed model, and the stress-strain relationship was fitted by applying a polynomial on the foundation of the proposed model. According to the results of tensile tests, the strength of the composite yarn was the highest in comparison with filament and staple yarn. Nevertheless, Young's modulus of composite yarn was the lowest among three types of yarns. In terms of elongation, the fracture elongation of staple yarn was much lower than that of the filament and the composite yarn (Fig. 2). The experimental results revealed that the tensile fracture curve of the composite yarn comprised three stages (Fig. 3). In the first stage, the stress and strain of the yarn showed a linear relationship when only staple yarn understood the load. In the second stage, the stress increased rapidly, and the sheath filament began to be stressed. In the final stage, some monofilaments started to fracture and the strength fluctuated, but the overall strength increased slowly. In accordance with the tensile fracture characteristics, a five-element nonlinear viscoelastic model composed of Kelvin element, Maxwell element, and linear springs were established (Fig.4). Thus, the equation of stress-strain relationship on the core/sheath composite yarn with double filaments was attained based on the model. Then, a scipy.optimize.root function in Python was employed to make a solution to the equation and several parameters were calculated. The viscoelastic tensile model can decently explain the three-stage stress-strain characteristics of the composite yarn tensile curve. The theoretical results were consistent with the experimental results, and the pearson correlation coefficient was greater than 0.999 (Fig. 5).

Conclusion This paper presented a five-element nonlinear viscoelastic mechanical model to forecast the tensile properties of core/sheath composite yarn with double filaments, according to an experimental and analytical study of the composite yarn. A polynomial function corresponding to the stress-strain relationship of the composite yarn was constructed to analyze and predict the practical tensile curve. The theoretical predictions showed good consistency with the experimental results. The yield stress derived from the model accurately reflected the different stages of the experimental tensile curve. The proposed five-element viscoelastic tensile model can provide theoretical and experimental reference values for the tensile fracture mechanism of core/sheath composite yarn with double filament. Moreover, a novel concept for research on the yarn tensile fracture mechanism is proposed.

Key words: composite yarn, tensile property, fracture characteristic, viscoelastic mechanical model, strength prediction, composite core/sheath yarn