Title

Several non-destructive inspection methods applied to quantify fretting fatigue damage in simulated Ti-6Al-4V turbine engine dovetail components

Date of Award

2012

Degree Name

Ph.D. in Materials Engineering

Department

Department of Chemical and Materials Engineering

Advisor/Chair

Advisor: Daniel Eylon

Abstract

The objective of this research is to determine the ability of several Non-Destructive Inspection (NDI) methods to detect various levels of High Cycle Fatigue fretting fatigue damage induced in simulated Ti-6Al-4V dovetail engine components. To generate various levels of fretting fatigue damage; a specially designed dovetail specimen is utilized which more accuracy simulates the cyclic loading interaction between a compressor blade and disk of an aircraft turbine engine. All fretting fatigue tests were conducted with un-coated Ti-6Al-4V alloy at ambient temperature, at a load ratio of 0.1, and two 30 Hz cyclic load levels (10% and 30% of expected life). In addition, two microstructures ([alpha]+[beta], [beta]-annealed) are utilized to determine their effect on fretting fatigue as well as the NDI signal response. To quantify the extent of fretting fatigue damage; mini-C specimens are extracted from the fretted dovetail specimens and step-tested" to quantify the debit in fatigue strength. Specimens are heat-tinted after fretting fatigue to help qualify the extent of fretting fatigue damage and aid in crack initiation site identification using both optical microscope and the Scanning Electron Microscope (SEM). Three NDI techniques are used to qualify the extent of fretting fatigue damage in Ti-6Al-4V and relate this damage to the NDI signal response and the debit in fatigue strength. The NDI techniques utilized in this research include: White Light Interference Microscopy (WLIM), Wyle Lab Eddy Current Inspection System (ECIS) and the JENTEK Meandering Winding Magnetometer (MWM) Array. Note: these NDI techniques are used "as is" and were not modified for fretting fatigue detection. However, in the case of the WLIM, a fretting fatigue damage parameter methodology is utilized to specifically quantify the extent of fretting damage. In addition, the Scanning Electron Microscope (SEM), Auger Electron Spectroscopy (AES), and Knoop micro-hardness tester were utilized to investigate the compacted fretting fatigue layer beneath the fretting fatigue scar and determine the existence of either a tribologolically transformed structure (TTS) or hard alpha case (HAC). In general, all three NDI techniques were able to detect various degrees of fretting fatigue damage (i.e., fretting fatigue cracks). However, as fretting fatigue cycles increased, the white light surface measurement technique's ability to discern higher levels of crack damage is suppressed by the modification of the contact fretting surface features (i.e., particle compaction with reduced asperity heights with nano sized contact debris) to include debris filled pits and cracks that help promote the loss of surface fidelity. The WLIM damage parameter and elements of the JENTEK MWM signal did correlate with the Mode I Newman-Raju stress intensity factor. However, the detection of fretting fatigue damage beyond fretting fatigue cracking was not attempted and would require special calibration specimens of a specific damage type (i.e., TTS, HAC, multiple co-linear cracking or perpendicular, slanted or zig-zag cracking, or corrosion effects) to correlate the NDI signal response to that damage."

Keywords

Alloys Fatigue Testing, Turbines Deterioration Testing, Nondestructive testing

Rights Statement

Copyright 2012, author

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