Fatigue crack propagation behavior of TiNi50.6 shape memory alloy

The fatigue crack propagation behavior of TiNi50.6 shape memory alloy was s tudied. The experiment results showed that the crack propagation properties of this alloy display difference and similarity in comparison with common metallic materials. Because of the stress concentration there was stress in duced martensite transformation (SIMT) near the crack tip though the nomina l stress was lower than the threshold stress of SIMT. The position and the amount of SIMT was in situ observed by a quester remote measurement system (QRMS). The observation results showed that the position of SIMT was beside the crack tip and was not in the plastic, zone of common metallic material s (in front of the crack tip). The SIMT zone at an angle of about 45 degree s to the direction of the crack propagation, like a butterfly, appeared in the loading process, disappeared in the unloading process and grew larger w ith the increase of DeltaK. The crack propagation rate(da/dN) followed the linear law in Ig-Ig plot. Observation of the crack surface showed fatigue s triation clearly. The relationship between the site and the size of the pla stic zone and the SIMT zone is discussed and a model is given to explain bo th the similarity and the difference of the crack propagation property of T iNi50.6 to common metal materials.