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.

An induction heating procedure coupled with an advanced alternating-current potential drop (ACPD) system to study thermal fatigue crack initiation and growth is presented. This technique can be adapted for various specimen geometries and is well suited for studying isotropic as well as anisotropic alloys. Details of the experimental apparatus and ACPD system are given. To illustrate the procedure, the effect of three cyclic thermal histories on the number of cycles to crack initiation of double-edge wedge specimens of IN-100 and MA-6000 superalloys was studied in air. The thermoelasto plastic finite-element analyses (FEA) to determine the critical locations and their respective stress-strain histories are presented. For this particular geometry of specimen, the initiation of microcracks about 10 to 20 μm in length could be detected and crack growth rates lower than 1 μm/cycle were measured. The experimental results combined with the FEA showed that the peak compressive strains encountered on specimen heat-up are more critical to crack initiation and surface degradation than total stress or strain ranges.

Strain controlled thermal-mechanical fatigue crack growth (TMFCG) tests were conducted on two titanium alloys, namely Ti-6A1-4V and Ti-6Al-2Sn-4Zr-6Mo, to evaluate the effect of phase angle between strain and temperature on the TMFCG rates. Three fracture mechanics parameters were used to correlate the data: the ΔK, ΔKe and ΔKeff. A fractographic study of the specimens tested under TMF was carried-out to identify the mechanisms responsible for cracking in these two titanium alloys. Hence, specimens tested under in-phase (εmax at Tmax), out-of-phase (εmin at Tmax) and counter-clockwise diamond (90° out-of-phase) conditions were compared to specimens tested under isothermal conditions (Tmin and Tmax) for different ΔKeff levels. The dominant TMF cracking mechanisms were mechanical fatigue (crack tip plasticity) and oxygen-induced embrittlement. The ΔKeff was found to be the only parameter to properly correlate all the data obtained under various testing conditions. A model is developed to predict the TMFCG rates based solely on isothermal data. The model uses a linear summation of the contributions to crack growth of the two dominant mechanisms which are active at the minimum and maximum temperature of the cycle. A discussion on the applicability of the model to predict the fatigue lives of actual components is discussed.