Variable stiffness and active damping technique for turbomachinery using shape memory alloys

High cycle fatigue (HCF) is a major concern for both military and commercial aircraft, as it is a leading cause of component or engine failure. Of the numerous techniques for HCF mitigation, over-design and constrainment layers are common; all resulting in added weight, increased operational costs, and lower performance. The use of the shape memory effect of shape memory alloys (SMA) (e.g. Nitinol) to allow variable stiffness of engine components is a novel approach to HCF mitigation. To quantify the effectiveness of Nitinol as a HCF mitigation technique, a composite beam consisting of an SMA actuator adhered to an Aluminum alloy substrate was designed. Analysis of preliminary designs with the use of finite element analysis software led to the selection of two final configurations: the first spanning the full beam (full sample) and the second spanning half the length of the beam (half sample). Complete modal analyses were taken over a selected frequency range using both single and scanning laser vibrometers. Experimental results showed that the actuation of the SMA patches led to a shift in modal frequency. Repeated tests on the half sample resulted in an mean increase in modal frequency, ranging from 3.77Hz (1.84%) at second bending to 36.8Hz (1.90%) at fifth bending during heating. Repeated tests on the full sample resulted in an mean increase in modal frequency, ranging from 9.43Hz (4.57%) at second bending to 74.5Hz (3.98%) at fifth bending during heating. Analysis of the third bending mode node location during the thermal cycle demonstrated a shift in the half sample, illustrating the material's capability to change the mode shape vector. Damping tests on both samples exhibited quality values, Q, around 100 at the highest temperatures, but no correlation with phase transformation was realized. Maps of temperature vs. modal frequency vs. beam tip amplitude were recorded for both the full and half sample for second bending mode. Complementary computational results were consistent with experimental investigations, illustrating the effectiveness of SMA has a HCF mitigator.