Torque tubes are a critical component of aircraft braking systems. Designed to absorb the torsional and thermal loads produced by the brake system, a torque tube is a heavily stressed component whose material properties must meet the required specification. Titanium’s mechanical and thermal properties make it an excellent material for aircraft brake torque tubes, but it is not impervious to heat damage from extreme braking events. Degradation of material properties and in-service failure may result if heat-damaged torque tubes are not detected and removed from service. Historically, the evaluation of surface color changes caused by oxidation has been used to identify titanium components with thermal damage. However, color evaluation has been found to be only marginally reliable. Color evaluation is subjective, qualitative, obscured by surface residue, and there is not a simple linear relationship between color and thermal history. The observed color change of the material is influenced by the temperature the part was exposed to, the distribution of heat on the object, by the amount of time held at a specific temperature, and the thickness of any previous oxide layer due to its thermal history. In some cases, variation in oxide layer thickness can cause the same colors to be produced on the surface of the part for different heat treatments. An objective, quantitative inspection that reliably and accurately detects thermal damage in the midst of confounding factors is needed. Published studies of heat effects on material properties found that heat treatment and quenching of Ti-6Al-4V (Ti-64) between 600 °C and 1200 °C produced as much as 10% variation in Young’s modulus, which was attributed to the change in composition and volume fraction of the constituent material phases \[1\]. Process Compensated Resonance Testing (PCRT) is sensitive to the microstructural changes in metallic components caused by thermal damage \[2\]. Because PCRT is sensitive to changes in a parts Young’s modulus, PCRT has the potential to replace qualitative, unreliable visual inspections with a quantitative measure of damage, while also estimating the part temperature exposure and tensile strength. This paper presents the results of an evaluation of PCRT for detection of heat damage in titanium material used for aircraft brake torque tubes. The study produced strong correlations between temperature exposure, ultimate tensile strength, and resonance frequency shift for Ti-64 coupons after various heat treatments ranging from 200°C-850°C. Additional nondestructive inspection (NDI) methods were also employed in this work to assess their capabilities in measuring temperature exposure. These included Ultrasonic testing (UT), Eddy Current testing (ET), color evaluation, and hardness testing. The relative effectiveness of each method was assessed by examining the relationships between the inspection results, thermal history, and mechanical testing results. Future work will expand evaluations to include titanium torque tubes with thermal damage, and larger populations of sample coupons with more complex thermal damage scenarios such as localized heating and varying exposure time.

Acknowledgment:

This research was supported through a Small Business Innovation Research (SBIR) Phase I Contract, FA8222-19-P-0002 sponsored by the Air Force Sustainment Center (AFSC/PZIEA). Any opinions, findings and conclusions, or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the PZIEA.

References:

\[1\] Peng, Wenwen et all. “Oxidation behavior and effect of oxidation on mircostructre and tensile properties of Ti-5Al-5Mo-5V-1Cr-1Fe alloy” Jouranl of Alloys and Compounds, 577, 633-542, (2013)

\[2\] Piotrowski and Weaver, “Enhancing Reliability with Process Compensated Resonance Testing (PCRT) at Delta TechOps”, ATA NDT Forum 2016 (September 28, 2016) https://www.airlines.org/wp-content/uploads/2016/10/9_28_1315.pdf


Detecting and Characterizing Overtemperature Exposure in Titanium via Process Compensated Resonance Testing (Pcrt)

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