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The computation of the stress intensity factor at a crack tip can be determined from the nodal displacements along the crack face. Amongst the existing techniques available are the Displacement Correlation Technique (DCT), the Quarter-Point Displacement Technique (QPDT) and the Displacement Extrapolation Technique (DET). As each of these techniques are popular in general LEFM analysis, an evaluation of their relative performances would seem appropriate. Previously, only limited comparisons have been made. In this paper the comparison is made on the basis of extensive numerical analysis. In addition two new variants to the DET are introduced and shown to be more efficient computationally.The results indicate that the QPDT is generally more accurate and consistent in performance than the DCT. The DET, however, exhibited some erratic characteristics. Detailed examinations revealed that the linear regression analysis employed in the DET for the extrapolation is highly sensitive to the nodal displacement distribution. Both the new variant DETs exhibited much more consistent behaviour. Nomenclatures a = crack length LN = size of normal element K *i --K .... t LQ = size of QPE element e.i _ K exact Ls = size of singular element e DcT= error in SIF computation associated Lr = size of transition element with DCT N = number of K *~ in the set evaluated eQPDT = e *i, error in SIF computation asso-r *~ = distance between ith node and the ciated with QPDT crack tip G = shear modulus u', v' = local displacement along and normal Knu m = numerically computed SIF to crack axis Kana~ = analytical solution for SIF p = linear regression coefficient K *~ = SIF computed from ith nodal pair v = Poisson's ratio along crack face K xT = SIF computed using X technique

SUMMARYIn conjunction with the quarter-point element, a wide range of stress intensity factor computation techniques may be employed. Of these, the displacement correlation technique (DCT) and the quarterpoint displacement technique (QPDT) have been in common use. It has been suggested by various investigators that the DCT is more rational in formulation and should therefore be more accurate than the QPDT. However, published numerical studies provide conflicting results on the relative performance of either technique. To resolve this anomaly, this study provides a detailed investigation on the DCT and QPDT performance. On the basis of considering the second term in Westergaard's expression and supported by numerical experiments, it is concluded that the QPDT would generally perform better than the DCT.