Strain energy deployable composite structures offer spacecraft designs reduced payload and compact volume. One of the greatest advantages presented by deployable composite structures arises from their ability to maintain high-strain configurations for extended periods of stowage. Because of the viscoelastic nature of the polymer matrix, the stowed composite structure undergoes stress relaxation that results in a decrease of the energy available for deployment. This paper focuses on a three-layered (±45 deg plain weave/0 deg unidirectional/±45 deg plain weave) carbon-fiber-reinforced polymer composite deployable structure, known as a tape spring. Stress relaxation testing was used to define the viscoelastic behavior of the epoxy matrix. Experimental long-term stowage and deployment testing was performed on the (±45/0/±45 deg) tape spring specimens. Finite element simulations considering viscoelastic, orthotropic stress relaxation were developed to predict the effects of stress relaxation on the deployment of a (±45/0/±45 deg) tape spring. [ABSTRACT FROM AUTHOR]