Optical analysis of crack tip stress fields: a comparative study

Abstract

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.

Three‐dimensional stress fields due to notches in plates under linear elastic and elastic–plastic conditions

“…The caustics have been successfully applied to determine the stress intensity factor of cracks under mode I loading as well as the triaxial state of stresses near the crack tip. That method has been also used to study the dynamic fracture behaviours of homogenous and isotropic material …”

Observation of Stress Field Around an Oscillating Crack Tip in a Quenched Thin Glass Plate

“…Photoelasticity is one of the most reliable and widely accepted techniques for fracture studies, and it has long been used for the investigation of stress field around a propagating crack tip [19,20]. However, it is found that isochromatic fringes cannot be observed for a thin glass specimen because glass exhibits low birefringence.…”

A computational tool for estimating stress fields along a surface crack front

“…There are no modern experimental methods that allow directly determining the stress distribution as well as the SIFs and T ‐stresses along the curved crack front from the experimental data in certain areas of the object. Thus, the second approach can be connected with experimental methods for determination of the SIF and the T ‐stress at narrow range of the problem, namely experimental measurement of the SIF and the T xx ‐stress in the vicinity of the through‐thickness crack.…”