Abstract
In this study, a non-linear viscoelastic–viscoplastic constitutive equation for polyamide 6 (PA6) is formulated and a new model is suggested for the viscoplastic part of the equation. The suggested model is empirical but can accurately predict the viscoplastic strain. In this study, creep and recovery tests are conducted to evaluate the viscoplastic strain. Using a non-linear dashpot, a viscoplastic strain is formulated and its parameters are identified for PA6. In addition, a stress relaxation test is conducted, and the relationship between the viscoelastic strain and stress is identified when considering the viscoplastic strain. In this study, the time–temperature superposition principle is thoroughly applied to include the effect of elevated temperature on the viscoelastic–viscoplastic behavior. All material constants in the non-linear viscoelastic–viscoplastic constitutive equation including the time–temperature superposition principle for PA6 are presented in this study.
Highlights
The application fields of polymeric composite materials are widely used in the aerospace, energy, and sporting equipment industries, and in the automobile industry and various types of infrastructure
A thermo-viscoelastic–viscoplastic constitutive equation for a polyamide 6 (PA6) material is presented, in which we suggest a new model for viscoplastic strain
A non-linear viscoelastic–viscoplastic constitutive equation for polyamide 6 (PA6) is formulated in which the new empirical model is suggested for the viscoplastic part in the equation
Summary
The application fields of polymeric composite materials are widely used in the aerospace, energy, and sporting equipment industries, and in the automobile industry and various types of infrastructure. For these uses, an accurate prediction of the lifetime and residual strength is becoming increasingly important [1, 2, 3]. The viscoelastic behavior is neglected and the numerical implementation into commercial software applied to a finite element analysis, such as ABAQUS, is relatively easy; this might be unsuitable for cases discussing long-term durability [13, 20, 21, 22, 23, 24, 25, 26]. Many studies have employed non-linear viscoelastic–viscoplastic constitutive equations to express the time-dependent stress–strain relationships of matrix resins [27, 28, 29, 30, 31, 32, 33, 34, 35, 36]. There have been studies on applying damage as a factor in a non-linear viscoelastic–viscoplastic constitutive equation [37, 38, 39, 40, 41]
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