Original Article J Strain Analysis 2015, Vol. 50(1) 4–14 Ó IMechE 2014 Reprints and permissions: sagepub.co.uk/journalsPermissions.nav DOI: 10.1177/0309324714554965 sdj.sagepub.com Modified creep constitutive equation for an epoxy-based adhesive with nonlinear viscoelastic behavior Mohammad Zehsaz, Farid Vakili-Tahami and Mohammad-Ali Saeimi-Sadigh Abstract This article presents the procedure to obtain constitutive equations to express the creep behavior of Araldite 2015 epoxy adhesive using the results of the uniaxial creep tests. The experimental data show that the examined adhesive has a nonlinear viscoelastic behavior. Three types of constitutive equations have been used: (a) Bailey–Norton equation, (b) generalized time-hardening model and (c) rheological model which is a series combination of springs and dampers known as Maxwell and Zener combination. The first two models have vast application in commercial finite element– based software. It is shown that the generalized time-hardening model can simulate the creep behavior of the adhesive better than the Bailey–Norton model. However, this model is less accurate at the elevated temperatures. Therefore, an empirical equation based on Maxwell and Zener’s was proposed which demonstrates a very good consistency with the results of the experimental data over the assumed range of stress and temperatures. Keywords Adhesive, creep, nonlinear viscoelastic material Date received: 8 May 2014; accepted: 2 September 2014 Introduction Adhesive joints are being used widely in engineering industries. This is because of increasing demand to design lightweight structures such as aircrafts and vehi- cle body frames. In addition, the ability of adhesives to join dissimilar materials makes them popular in joining different components. To design and for proper use of adhesively bonding, knowledge of the stress and strain distribution in the adhesive joint is required. Accurate predictions are obtained by means of models that pre- cisely describe the deformation behavior of adhesives. The behavior of adhesives, like most polymers, is time- dependent. They exhibit viscoelastic and viscoplastic behavior when subjected to relatively high temperature and high stress levels. Thus, close attention needs to be paid to the creep behavior of the adhesive joints in structural applications to increase durability and relia- bility of them. 1 Experiments reveal that deformation behavior of epoxies is approximately linear below moderate stress levels; however, with increasing time under load, the stiffness of material decreases and it exhibits nonlinear behavior. 2 Thus, extensive studies were carried out to model the nonlinear behavior of adhesives. Many vis- coelastic and viscoplastic models have been defined to perform stress analysis of adhesively bonded structures. The simplest viscoelastic models are the classical models of Voigt and Maxwell. 3 Other improved viscoelastic and viscoplastic models are available in the literature. 4,5 Feng et al. 6 established a method to study the long-term creep behavior of epoxy adhesives based on series of short-term accelerated tests. They investigated the tem- perature and moisture effects by means of mechanical response and they used a coupling model to analyze creep behavior in epoxies. An important attribute of this model is its ability to physically describe the charac- teristics of the molecular mobility by means of coupling parameters. Dean 7 modeled the uniaxial tension creep behavior of polypropylene using stretched exponential Department of Mechanical Engineering, The University of Tabriz, Tabriz, Iran Corresponding author: Mohammad-Ali Saeimi-Sadigh, Department of Mechanical Engineering, The University of Tabriz, 29 Bahman Str., No2, 5166-16471 Tabriz, Iran. Email: saeimi.sadigh@tabrizu.ac.ir by guest on December 4, 2016 sdj.sagepub.com Downloaded from