Elastic-plastic behavior of thermoplastic composites under cyclic loadings
Abstract
The experimental results reveal that AS4/PEEK thermoplastic composite exhibits cyclic softening and loading range effect under symmetric cycles of strain or stress, and shows ratchetting effect under unsymmetric cycles of stress and mean stress relaxation under unsymmetric cycles of strain. The cyclic softening, loading range effect, ratchetting and mean stress relaxation are anisotropic. Based on the essential features of the material behavior, an elastic-plastic constitutive model is proposed to describe the material response under cyclic loadings. A one-parameter yield surface is used. A one-parameter memory surface is introduced to characterize the loading range effect and to distinguish the deformation process during subsequent loading from the deformation process during initial loading and over-loading. Cyclic softening is achieved by the evolution of generalized plastic modulus. The ratchetting effect is modeled in the frame of time-independent cyclic plasticity by the change of the generalized plastic modulus. The viscous effect of the material, which is secondary order shown by the experimental data within certain ranges of stress and loading rate, is not considered here. Good agreement between the model simulations and the experimental data is obtained. The proposed model is applied to form the constitutive relations of laminates with and without thermal stress by using classical lamination theory. A numerical iteration scheme is presented for the nonlinear finite element analysis of structural laminate. The model predictions and the experimental data for a ($\pm45\sp\circ$) $\sb{\rm 3s}$ laminate and a ($\pm45\sp\circ$) $\sb{\rm 3s}$ plate with a center hole under cyclic loadings are presented and discussed.
Degree
Ph.D.
Advisors
Sun, Purdue University.
Subject Area
Aerospace materials|Mechanical engineering
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