An Inclusion Size Based Fatigue Prediction Model For Nitinol

As recently demonstrated [1], maximum inclusion size significantly affects Nitinol fatigue life: fracture surfaces of superelastic wire samples fatigue tested  by means of Rotation Beam  Testing (RBT) showed either oxide or carbide inclusions at the crack origin; moreover, larger inclusions where correlated with worse fatigue performance.

Inclusions can be assimilated to small cracks that, if a threshold stress intensity factor is exceeded, grow until fracture. Thus fatigue can be in principle be described by a suitable crack propagation model, provided its validity in the “small crack” regime.

We propose the NASGRO [2] propagation equation coupled with the El-Haddad small crack correction.

The NASGRO propagation equation parameters are calibrated by means of literature crack propagation data for Nitinol [3], shifted to a stress ration R=-1 considering the Shjivie method.

The El-Haddad correction is calibrated through the identification of one fatigue limit corresponding to a certain defect size by analyzing RBT fatigue tested samples.

The model, once calibrated, allows for the life prediction given a strain level and an initial defect size. Thus, fatigue predicted data can be compared with experimental data showing a good agreement.

[1] M. Urbano et. Al., The Effect of Inclusions on Fatigue Properties for Nitinol, in press.

[2] Fatigue crack growth computer propram NASGRO version 3.0—Reference Manual.

Rapp. tecn. JSC-22267. NASA, 2001.

[3] S.W. Robertson e R.O. Ritchie. “A Fracture-Mechanics-Based Approach to

Fracture Control in Biomedical Devices Manufactured from Superelastic Nitinol

Tube”. In. J. Biomedical Materials Research - Part B Applied Biomaterials

84 (2007), 26–33