Based on a penalty Hellinger-Reissner principle, two special hybrid stress
elements ensuring traction-free conditions along arbitrary boundaries are i
ntroduced to improve the penalty-equilibrium hybrid stress element method (
PEHS). In the element formulation, the stresses are approximated by complet
e polynomials of global co-ordinates, The path independence of J(1)- and I-
1*-integrals is examined. These integrals are then used for the determinati
on of the stress intensity factor (SIF) at the tip of a crack in mode I or
II in a linear isotropic or orthotropic material. Studies on the validity o
f the dual analysis concept for both materials are also carried out. Confir
mative numerical results for typical cracked specimens are presented and di
scussed. (C) 1999 Elsevier Science Ltd. All rights reserved.