A study of fatigue crack closure in polycarbonate CT specimens

Plane strain and plane stress finite element (FE) models were built to simulate fatigue crack closure behaviour in polycarbonate. Effective driving forces for crack growth were determined in terms of crack closure, where the opening load was taken either as corresponding to full opening of the two crack faces (Definition 1), or identified by the instant when the principal stress perpendicular to the crack-tip stress becomes tensile (Definition 2). Experimental work was conducted to determine crack growth rates and to characterise crack closure levels, using the compliance technique, in compact tension specimens either 2 mm (plane stress) or 10 mm (plane strain) thick. The FE predictions of crack closure using Definition 1 were found to agree reasonably with the experimental results. It was also found that higher crack closure levels were predicted from Definition 2. A two-parameter function of Delta K-eff and K-max was utilised to model the fatigue crack growth rate data. The equation derived from Definition 1 theoretical results described the growth rate data better than that obtained from Definition 2. (C) 2000 Elsevier Science Ltd. All rights reserved.