ACTUAL LIGHT DEFLECTIONS IN REGIONS OF CRACK TIPS AND THEIR INFLUENCEON MEASUREMENTS IN PHOTOMECHANICS

Crack-tip photomechanics procedures are based on certain simplifying a ssumptions that are seldom discussed. In a recent paper the theoretica l bases of the shadow optical methods of caustics have been analysed a nd tested using the results obtained by three analytical-experimental procedures, namely classical strain gage techniques, isodynes, and str ain-gradient index method. It has been concluded that the strain-gradi ent index method appears to be a suitable tool for analysis of stress states near crack tips and notches and, in particular, for testing the predictive power of the pertinent singular solutions of the linear el astic fracture mechanics and the ranges of their applicability. In the present paper, a more derailed analysis of all results obtained in li ght deflection experiments allows to quantify the contribution of both involved effects and to determine the distortion of the faces of the investigated plates along their crack planes. The ability of the strai n-gradient light bending method to analyse some features of the three- dimensional stress-stare is reported. Finally, the presented experimen tal evidence allows to draw conclusions related to limits of applicabi lity of certain photomechanical measurements near crack tips.