Inelastic behavior of thermoplastic composites

Ilsup Chung, Purdue University

Abstract

The mechanical behaviors of AS4 reinforced PEKK-based composites were studied. These new thermoplastic composite system developed by Du Pont exhibit pronounced nonlinear and time dependent behaviors. To characterize the inelastic behavior, classical plasticity and creep theory have been adopted and modified for the orthotropic case by using a one-parameter potential function. The elastic-plastic behavior of LDF$\sp{\rm TM}$ AS-4/PEKK system employing aligned, long discontinuous fiber reinforced composites has been studied in comparison with the continuous fiber reinforced counterpart. A dual mode plastic potential action was employed to model the plastic behaviors of these composites at room and elevated temperatures up to 177$\sp\circ$C. A one-parameter failure criterion was also applied to predict the failure strength. Off-axis coupon specimens were used to measure the failure strength and generate stress-strain curves from which the elastic-plastic properties were extracted. It was observed that the elastic-plastic performance and strength of LDF$\sp{\rm TM}$ are as good as those of the continuous fiber system. A creep model for orthotropic material has been developed based on the same one-parameter potential function. The material constants were determined by using uniaxial creep tests of off-axis coupon specimens. In terms of effective stress and effective creep strain, the anisotropic creep behavior was described by a stress dependent master creep curve irrespective of loading direction relative to fiber orientation. This model was implemented into classical laminate theory to predict the creep behavior of composite laminate, and compared with experimental data of ($\pm$45) $\sb{\rm 4s}$ laminate specimens. The influence of thermal stress was included. The micromechanical approach also was tried to model the inelastic behavior of continuous fiber reinforced composites. The fiber was assumed to be orthotropically elastic, and the matrix to conform to the classical elastic-plasticity and creep theories. For the so called mechanics of material type of approach, a representative volume element assuming simplified distribution of stress and strain was employed, and numerical prediction was compared with other existing models or experiments.

Degree

Ph.D.

Advisors

Sun, Purdue University.

Subject Area

Mechanics|Aerospace materials|Mechanical engineering

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