Nonlinear Creep

If a creep experiment is carried out at a constant stress that is sufficiently large that the sample is strained into the nonlinear regime before achieving a steady strain rate, the compliance function will be a function of stress as well as time. Data from this type of experiment have been reported by Agarwal and Plazek [ 53 ], but creep has not been widely used for the study of nonlinear viscoelasticity. 

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The material behaviors considered will include linear elasticity plus linear or nonlinear creep behavior. The nonlinear case will be restricted to power-law rheologies. In some cases the elasticity will be idealized as rigid. In ceramics, it is commonly the case that creep occurs by mass transport on the grain boundaries.1 This usually leads to a linear rheology. In the models considered,... 

Kelly and Street21 analyzed this model but their deductions were not consistent with the mechanics. McMeeking23 has remedied this deficiency for nonlinear materials. His results for n = 2 are relevant to composite materials with nonlinearly creeping ceramic matrices which tend to have low creep indices. In that case, the steady-state creep rate is given by Eqn. (63) with n = 2 and... 

Crack Tip Fields Under Nonlinear Creep Conditions... 

R Mohan and D F Adams, Nonlinear creep-recovery response of a polymer matrix and its composites , Exper Mech 1985 25 262-271. 

In the general case, the nonlinear creep behavior could be described by a multiple integral representation using several kernels. The practical application of such equations is limited by the absence of a clearly defined strategy of creep kernel determination as well as relations between the kernel and resolvent. Various simplified versions of this approach have also been proposed. Usually it is assumed that the creep kernel and relaxation time are independent of the stress value. At the same time, it is known that a good approximation to nonlinear creep may be obtained by using the following equation ... 

Nonlinear Creep Power Law It has been empirically observed that the creep of metals and other materials can be approximated using a creep power law of the form ... 

Using the nonlinear creep and creep recovery data given below, find the seven material parameters (Dg, Dj, n, go, gi, g2 aj needed for representation by the Schapery equations. 

Starkova, O., Yang, J.L., and Zhang, Z. (2007) Application of time-stress superstition to nonlinear creep of polyamide 66 filled with nanoparticles of various sizes. Compos. Sci. Technol., 67 (13), 2691-2698. 

Stress relaxation data, revealed that time temperature superposition broke down at longer times with small stresses. This observation indicated that complex dissociation occurred only at longer times under small strain. Nonlinear creep compliance was independent of stress below 1.0 mPa but above this it deviated from linearity and showed a sigmoidal shape. 

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