Stiffness based technique to probe cyclic damage accumulation in micro-structurally graded bond coats via micro-beam bending tests

ABSTRACT

This paper highlights a novel technique to delineate the fatigue response of different regions within thin microstructurally graded platinum nickel aluminide bond coats. Notched clamped beam structures fabricated from distinct microstructural zones of these coatings are subjected to programmed cyclic bending using the nano-indentation system. A methodical approach is established herein to quantify the cyclic damage preceding crack pop-in by using the cyclic stiffness of the beam as an indicator to mark failure. Preliminary results from these tests show that there is a characteristic change in the stiffness of the beam before a crack pop-in event occurs and different regions within the coating show different stiffening characteristics. Factors affecting the measured stiffness such as offsets in the loading position and blunting of the notch tip have been estimated using the finite element method. A graded flow stress model has been proposed and implemented in FEM to quantify the local flow stress changes accompanying the measured rise in stiffness. Electron transparent foils lifted off from the notched region of the beam post-testing suggests that the cyclic stiffening of the beams occurs due to dislocation hardening in the plastically deformed region close to the notch tip. Toughening mechanisms active in the crack wake have thus been investigated and correlated to the measured cyclic stiffness.

KEYWORDS:

• Micro-mechanics
• nano-indentation
• fatigue
• platinum nickel aluminide bond coats
• notched clamped beam geometry
• in situ micro-scale fatigue testing

Acknowledgements

We would like to acknowledge Hysitron India Inc. (Mr. Pratyank Rastogi and Mr. Syed Asif) for providing us the in situ pico-indentation system (PI-85), DMRL, Hyderabad for providing the Pt-aluminide coatings used in this study.

Disclosure statement

No potential conflict of interest was reported by the authors.

ORCID

Kaustubh Venkatraman http://orcid.org/0000-0003-3916-4507

Additional information

Funding

The funding was provided by: Aeronautical Research and Development Board (AR&DB) - DRDO, India through the Gas Turbine Materials Research program.