A comprehensive review is given of methods to determine the stress intensity factor at crack tips using thermoelastic stress analysis. In order to obtain accurate results a number of areas of experimental procedure need to be considered and these are discussed in detail. The paper concludes with a discussion on the future potential of the use oj thermoelasticity for the analysis of cracks.

A B S T R A C T Four optical techniques for evaluating stress intensity factors in opaque specimens are described in outline, and compared for both an artificial crack and a fatigue crack. The results are compared to a standard solution for the geometry considered. All the techniques gave acceptable results over a range of stress levels and crack lengths. The methods of caustics and strain gauges were less good, whilst photoelasticity gave consistent results over a wide range of stress levels. Comments on the ease of application and the resource implications are also made in order to assist practitioners.= diameter of caustic (as measured on screen) E = modulus of elasticity f σ = material stress fringe constant F 0 , F 1 , G 0 = unknown coefficients in strain gauge solution g i = error function (photoelastic MPODM) h i = error function (thermoelastic MPODM) j = √ −1 = complex number k = (1 − υ)/(1 + υ) K I , K II = mode I and II stress intensity factors, respectively K AP = normalized apparent stress intensity factor (K AP = τ max √ 8πr) l = length of light path through photoelastic specimen L = length of strain gauge N (N m ) = photoelastic fringe order (value at (r m , θ m )) N x = moiré fringe order at (r, π/ 2) p = pitch of moiré reference grating P = applied load q = focal length of lens in caustic apparatus r = polar coordinate with origin at crack tip r c = distance from crack tip to geometric centre of strain gauge r m = distance from crack tip to photoelastic fringe apogee s(σ 0 i,j ) = a set of constants S = thermoelastic signal t = specimen thickness u = distance from specimen to lens in caustic apparatusCorrespondence: E. A. Patterson.