Identification of Crack Tip Location in DCB Specimen Using Optical Methods

Determination of crack tip location in full scale and laboratory specimens is an important issue in fracture mechanics and especially in standard fracture tests. It is required for further studies on different fields. For example in CTOA measurement of ductile fracture, knowing the exact location of crack tip increases the accuracy of the results. Another application of crack tip identification is measurement of deformation and strain fields at the crack tip. In this paper, a Modified Double Cantilever Beam (MDCB) is used to investigate crack tip location ahead of a growing crack. The test material is API X65 steel, that it is widely used in Iran's natural gas transmission pipelines. Experimental data is obtained from test specimens taken from actual pipes manufactured in industrial scale (with 1219 mm diameter, 14.3 mm wall thickness). A squared reference grid is scribed on both sides of the sample surface using laser etching. The specimen is subjected then to loading in opening mode-I. Digital images from growing crack are captured and stored on a computer for further analysis and processing. Three methods are then used to identify crack tip location, and the results are discussed and compared with each other.