Fatigue Crack Growth Measurements in TMF Testing of Titanium Alloys Using an ACPD Technique

Abstract

A thermal-mechanical fatigue (TMF) testing system has been developed which is capable of studying the fatigue behaviors of gas turbine materials under simultaneous changes of temperatures and strains (or stresses). Furthermore, an advanced alternating current potential difference (ACPD) measurement technique has been developed successfully to perform on-line monitoring of fatigue crack initiation and growth in specimens tested under isothermal and TMF conditions. In this paper, the basic principles of the ACPD technique as well as all the relevant experimental procedures for performing ACPD measurements, including probe setup, choice of alternating currents (AC) and frequencies, noise rejection, data acquisition, and signal processing, are described. The linear relationship between ACPD signals and crack lengths, as well as the effects of thermal cycling on the ACPD signal, are presented and discussed. The capabilities of the TMF and ACPD systems are well illustrated by fatigue crack initiation and growth test results under isothermal and TMF conditions. These tests were performed on two titanium forgings, Ti-6Al-4V (Ti64) and Ti-6Al-2Sn-6Mo (Ti6246), respectively. Alloy Ti64 was TMF cycled between 150 and 400°C, while Ti6246 was cycled between 200 and 482°C. The resolution for detecting crack initiation at the root of notches was found to be 50 μm with 95% confidence while the resolution for crack growth was 2 μm per mV change of ACPD. An environmental assisted cracking model applied to TMF crack growth is proposed for rationalizing the data.

Electromagnetic methods for measuring material properties of cylindrical rods and array probes for rapid flaw inspection

“…The depth of penetration of the AC fields into the metal is determined by the electromagnetic "skin depth", which is a function of the AC frequency, magnetic permeability and conductivity of the material. A lot of research work has been carried out to investigate the ACPD method in crack measurements [17][18][19][20][21][22][23][24][25].…”

Potential difference methods for measuring crack growth: A review

“…Barnett and Troiano [60] first used this technique for laboratory research to measure crack growth kinetics in a study of hydrogen embrittlement in notched tensile specimens. Electrical PD methods, which were also named as electrical resistance methods, have been used with success of detection crack closure in fatigue crack studies [61][62][63][64][65][66][67][68][69][70], crack initiation in crackopening displacement and fatigue tests [23,45,46,[71][72][73][74][75][76][77], measurement of crack growth under various of loading conditions (including fatigue [9,21,55,[78][79][80][81], creep [19,21,37,42,[81][82][83][84][85], stress corrosion [21,[86][87][88], sustained loading [89][90][91], fretting fatigue [12], and hydrogen embrittlement [21,…”

“…However, an opposite argument has been reported that a linear slope of calibration established in WOL specimens was independent of operating frequency [134]. Error signals in calibration measurement may be introduced by plastic deformation [11,47,56,60,72,79,80,134], crack closure [34,48,100,135,136], and temperature fluctuation (which is related to temperature dependent resistivity) [2,76]. With respect to the last point, constant current supply and controlled temperature conditions, such as an immersion in an ice bath [21], is desirable [39].…”

“…However, in DCPD tests the whole specimen is tested in terms of the current path, and so the specimen geometry itself (rather than just some flaw geometry) influences the calibration [13]. Moreover, a relatively high current level is required in order to achieve measurable potential outputs [41,76,137,138]. According to [134], a DC of 30-50A is necessary for a steel specimen in normal size.…”

Alternating current potential drop and eddy current methods for nondestructive evaluation of case depth

“…The alternating current potential drop method was used extensively to detect and char acterize surface cracks [15][16][17][18][19][20][21][22][23][24][25]. Suppose an infinite plate contains an infinitely long surface crack of uniform depth.…”