ANALES DE MECÂNICA DE LA FRACTURA 357 USE OF OPTICAL TECNIQUES IN THE ASSESSEMENT OF THE DISPLACEMENT FIELD NEAR THE CRACK TIP J. Ribeiro *, M. Vaz **, H. Lopes *, F. Q. de Melo***, J. Monteiro **** * ESTG - Instituto Politécnico de Bragança Campus de Sta Apolónia, Apt 1134, 5301-857 Bragança ** DEMec - Faculdade de Engenharia da Universidade do Porto Rua Dr. Roberto Frias, 4200-465 Porto *** UA – Universidade de Aveiro Campo Universitário de Santiago, 3810-193 Aveiro **** Laboratory of Optics and Experimental Mechanics Instituto de Engenharia Mecânica e Gestão Industrial Rua do Barroco, 174, 4465-591 Abstract. A crack tip represents a highly singular stress field existing in a structural component. The evaluation of the associate strain gradient is difficult using experimental discrete methods. An efficient alternative relies on optical methods which are non contact and give continuous information about displacements fields and its derivatives for strain evaluation. This paper describes some experimental methods to fully characterize the displacement field near a crack tip existing in flat plates. Three optical field techniques based on image analysis were used in the present work; respectively ESPI (Electronic speckle pattern interferometry), MI (Moiré Interferometry), and DIC (digital image correlation). These methods present different resolutions which can be adjusted according to the expected strain gradient. While the first method depends on the laser wavelength, the second depends on the grid pitch and the last on the surface texture. 1. Optical Methods 1.1 – ESPI All these techniques use light to codify the surface information before and after loading to enhance displacements or strains. ESPI and MI need coherent illumination to generate the fringe patterns which characterize the object behaviour. Both set-ups have capability for in-plane displacement measurement; however, the ESPI set-up can be adapted to simultaneously measure in-plane and out-of-plane displacements. The applications of this method to fracture mechanics has been carried out in previous work were the stress intensity factor in a part through crack was measured [1]. To achieve this step the experimental setup was designed to cancel rigid body motion and coupled with accurate image processing tools, obtain the above mention displacements and rotations of the plate along the crack line. Once calculated these parameters it is followed an hybrid methodology were the compliance factors of each line spring element placed along the crack plane allow the evaluation of the consequent stress field and further assessment of the stress intensity factor. Figure 1. Fringe pattern and phase map obtained for in-plane and out-of-plane displacement measured with ESPI [1]