The method of fracture toughness measurement of brittle materials by means of high speed camera and DIC

The aim of the investigation was to determine the stress intensity factor values of WC–40 wt.% Co composite obtained by means of the impact sintering method. Mini-samples having a rectangle cross section of 4 × 2 mm with a mechanically machined notch of about 0.07 mm radius were subjected to 3-point bending tests.Obtaining fatigue pre-cracks in the brittle material to be tested by any typically used method was difficult because of the low stability of the crack. The samples’ dimensions were also too small to enable electromechanical extensometers to be used for measuring the displacement of the opening crack.To overcome these barriers a procedure for cyclical loading and the optical measurement of displacement fields on the sample’s surface was applied. The displacement maps were determined by a Digital Image Correlation (DIC) method. The images necessary for DIC method were recorded with a high speed camera in the short period immediately before fracture of the sample. Displacement fields arrays of the area in the vicinity of the crack tip were used as the input data for the inverse method for identifying the stress intensity factors. The procedures proposed in the paper can be used for the preparation of pre-cracks and the measurement of the stress intensity factor in testing of brittle materials which are available only in small volumes. The results obtained by the proposed method were compared with tests performed using monotonic loading on samples without pre-crack.

► Mini-samples made of WC–Co composite were tested in 3-point bending mode.
► Digital Image Correlation measurements and inverse method was utilized for stress intensity factors calculations.
► Checking if crack notch radius is sharp enough.
► Good agreement of results from proposed method and compliance method.