Anastasia Muliana

Effect of Prestress on the Mechanical Performance of Composites

AbstractBodies are prestressed with the intention to enhance the load-carrying capacity of the body. The primary objective of this study is to understand the effect of prestressing the constituents in composite bodies on the overall mechanical performance of the composites. This paper considers composites having linearized viscoelastic constituents that can exhibit fluidlike and solidlike behavior. It also examines the effect of stress relaxation in a component of the prestressed composite on the overall load-carrying capacity of the composite. The properties of the composite, whether a brittle inclusion embedded in ductile matrix or a ductile inclusion in brittle matrix, are greatly influenced by the ratio of the induced prestress with respect to the external load and thereby influence the load-carrying capacity of the composite.

A Multi-scale Formulation for Smart Composites with Field Coupling Effects

This study presents a two-scale homogenization scheme for determining effective thermal, mechanical, electrical, and piezoelectric properties of smart laminated composites. The studied smart composite is composed of a unidirectional fiber reinforced laminated system as a host structure and an active unidirectional fiber reinforced piezocomposite. The effective response of the composites, in a representative unit-cell model, is formulated based on a volume average of the field quantities of the constituents. Each unit-cell is divided into a number of subcells. A unit-cell model, consisting of four fiber and matrix subcells, is generated to homogenize mechanical and non-mechanical responses of a lamina. Material parameters in the constitutive models of the constituents, i.e., fiber and matrix, are allowed to vary with temperature and time. The macro-scale consists of a sublaminate model that homogenizes responses of representative layers in the laminated systems. Perfect bonds are assumed at the fiber–matrix interphases and at the interphases between laminae. The effective properties obtained from the present micromechanical model are comparable to the ones generated using an asymptotic homogenization scheme. Available experimental data in the literature are used to verify the multi-scale model formulation. The effects of temperature dependent material constants on the overall coupled electro-mechanical properties of composites during transient heat conduction are also examined.

Nonlinear Hysteretic Response of Piezoelectric Ceramics

Ferroelectric materials, such as lead zirconate titanate (PZT), have been widely used in sensor, actuator, and energy conversion devices. In this paper, we are primarily interested in the electro-mechanical response of polarized ferroelectric ceramics subject to cyclic electric fields at various magnitudes and frequencies. There have been experimental studies on understanding the effect of electric fields and loading rates on the overall electro-mechanical response of PZT (see for examples Crawley and Anderson 1990, Zhou and Kamlah 2006). The electrical and mechanical responses of PZT are also shown to be time-dependent, especially under high electric field (Fett and Thun 1998; Cao and Evans 1993; Schaufele and Hardtl, 1996). Ben Atitallah et al. (2010) studied the hysteretic response of PZT5A and active PZT fiber composite at several frequencies and isothermal temperatures. They show the nonlinear and time-dependent piezoelectric constants of the PZTs and PZT fiber composites. In a review of nonlinear response of piezoelectric ceramics, Hall (2001) discussed experimental studies that show strong time-dependent and nonlinear behavior in the electro-mechanical response of ferroelectric ceramics. The time-dependent effect becomes more prominent at electric fields close to the coercive electric field of the ferroelectric ceramics and under high magnitude of electric fields a ferroelectric ceramics exhibits nonlinear electro-mechanical response. Furthermore, high mechanical stresses could result in nonlinear mechanical, electrical, and electro-mechanical responses of the ferroelectric materials. Within a context of a purely mechanical loading in viscoelastic materials, timedependent response is shown by a stress relaxation (or a creep strain). This results in stressstrain hysteretic response when a viscoelastic material is subjected to a cyclic mechanical loading. There are different types of viscoelastic materials, such as polymers, biological tissues, asphalts, and geological materials. It is understood that these materials possess different microstructural characteristics at several length scales; however the macroscopic (overall)1 mechanical response of these materials, i.e. stress relaxation and hysteretic response, especially for a linear response, follows similar trends. Experimental studies have shown that there are similarities with regards to the macroscopic time-dependent (or