Enhanced Mechanical Properties of Smart Polymer-Matrix Hybrid Composite by Shape Memory Effect
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ABSTRACT New smart hybrid composite with TiNi shape memory alloy (SMA) fiber as reinforcement is designed to be composed of TiNi fiber and carbon/epoxy (CFRP) composite. One-dimensional composite modeling is utilized to evaluate residual stresses of the smart composite generated by shape memory effects. The modeling indicated that the tensile strength of TiNi/CFRP composite is improved due to the shape memory effect of TiNi fiber for both parallel and perpendicular configurations of TiNi fiber with respect to the direction of carbon fiber. Modeling also shows the tensile strength is increased with increasing both volume fraction of TiNi fiber and amount of prestrain. INTRODUCTION Two types of smart composite with TiNi SMA fiber as reinforcement have recently been designed. The composite designed includes metal-matrix and polymer-matrix composite, such as TiNi fiber/6061A] matrix(TiNi/Al) and TiNi fiber/epoxy matrix (TiNi/epoxy) composites [1,2,3]. Embedding SMA fiber in these matrix materials produces composites with improved mechanical properties such as stiffhess, tensile yield stress, fracture toughness, and fatigue resistance. This improvement is attributed to residual stresses formed due to shape memory effects. We are currently developing carbon fiber reinforced polymer (CFRP) composite containing continuous fiber of TiNi SMA both experimentally and theoretically. Here CFRP used is a Toray product and made of Carbon Toray T-300 fiber and bis-phenol epoxy. The objective of the present study is to evaluate the effects of prestrain and volume fraction of SMA fiber on tensile strength of the composite. From the results obtained, fracture properties can be predicted. ANALYTICAL MODEL One-Dimensional Composite Model Assuming that the absolute strain of the composite is so small that no sliding occurs between TiNi fiber and CFRP composite, the compatibility equations for the smart hybrid TiNi/CFRP composite can be expressed as sc = • f =
-
(1)
where E s, E f and E . are the total strain of the smart hybrid composite, TiNi fiber, and CFRP composite, respectively. In this case, the applied stress of the smart composite, a .. , can be expressed by the force equilibrium equation given by the stress of TiNi fiber, a r, and that of the composite, au. The relationship is given in form (2), 425 Mat. Res. Soc. Symp. Proc. Vol. 459 ©1997 Materials Research Society
Vf X
a
V±f) X oU,
f+(-
(2)
where Vf is the volume fraction of TiNi fiber. The total strain of TiNi fiber, Er, is given in form
(3), el CTE El= Er'+Ef + E•
trans
(3)
where 'el', 'CTE', and 'trans' stand for elastic strain, thermal strain, and transformation strain, respectively. In the case of CFRP composite, its total strain E , is given in form (4),
(4)
CTE
The constitutive equations of TiNi fiber and CFRP composite are given in normal forms, f = Ef X
(5)
E fel
o =E~X E el
(6)
where Ef and E, are the elastic moduli of TiNi fiber and CFRP composite, respectively. The thermal strain of TiNi fiber and CFRP composite for a temperature change of dT
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