The crack tip strain field of AISI 4340 - Part I - Measurement technique

This is the first paper in a study on the influence of the environment on t he crack tip strain field for AISI 4340. A stressing stage for the environm ental scanning electron microscope (ESEM) was constructed which was capable of applying loads up to 60 kN to fracture-mechanics samples. The measureme nt of the crack tip strain field required preparation (by electron lithogra phy or chemical etching) of a system of reference points spaced at similar to 5 mu m intervals on the sample surface, loading the sample inside an ele ctron microscope, image processing procedures to measure the displacement a t each reference point and calculation of the strain field. Two algorithms to calculate strain were evaluated. Possible sources of errors were calcula tion errors due to the algorithm, errors inherent in the image processing p rocedure and errors due to the limited precision of the displacement measur ements. Estimation of the contribution of each source of error was performe d. The technique allows measurement of the crack tip strain field over an a rea of 50 x 40 mu m with a strain precision better than +/- 0.02 at distanc es larger than 5 mu m from the crack tip. (C) 1999 Kluwer Academic Publishe rs.