Development of the marks tracking technique for strain field and volume variation measurements

The mark tracking method is a simple and efficient technique for a local strain measurement. We show the possibilities of the mark tracking for field measurement. We present a procedure which allows us to experimentally characterise this technique depending on the sensor and the marks' dimensions. The method presented is insensitive to illumination variation and to in-plane rotation of the specimen. The strain values are obtained by analysis of four marks. In order to obtain a displacement field, we analyse a series of small groups of spots. With the help of an average of the parameters (using a series of initial images), we suppress the systematic error that we can observe when we use a method comparing an initial state to a deformed one. So the strain uncertainty becomes less that 0.0002 for a local study and 0.002 for a whole field study (measurement base 20×20 pixel2).