2015 IR&D Annual Report

Effect of Cyclic Relative Humidity on Environmentally Assisted Cracking, 18-R8554

Principal Investigators
James Dante
Todd Mintz
Erica Macha
James Feiger

Inclusive Dates: 04/01/15 – Current

Figure 1: Environmental control system designed to control relative humidity and temperature around a fatigue sample.
Figure 1: Environmental control system designed to control relative humidity and temperature around a fatigue sample.

Background — The annual cost of corrosion for the U.S. Department of Defense aircraft systems is estimated to be over $10 billion. It is estimated that more than 80 percent of structural cracks that have been detected initiate at corrosion sites, raising concerns that corrosion has a potentially high impact on structural integrity. In an attempt to compensate for these environmental effects, a margin of safety during design considerations is employed. These margins of safety are based on crack growth rate (CGR) data that has been collected under immersion conditions. Recent work, however, has shown that CGR can be as much as 10 times higher than values acquired in lab air under certain environmental conditions. Measurements of CGR under aggressive atmospheric conditions are required not only to understand the specific effects of environmental spectra on CGR, but also to define how environmentally assisted cracking should be addressed in engineering design approaches for components and structures.

Approach — The overarching goal of this work is to develop an initial framework to address atmospheric environmentally assisted cracking. The approach includes the development of an atmospheric testing cell with controlled relative humidity (RH) and temperature constructed around a standard fatigue test system. Testing under controlled environmental conditions and dynamic loading conditions will be used to demonstrate how atmospheric parameters directly affect CGR. Using well-established multi-electrode array techniques developed at Southwest Research Institute, correlations between corrosion modes and changes in crack growth rate will be established. This will serve as the basis for understanding fundamental mechanisms of atmospheric cracking processes. Finally, existing environmental sensor data will be analyzed in a manner analogous with structural usage spectra. Environmental spectra developed from this analysis will be used to define conditions for measuring CGR under controlled mechanical loads.

Accomplishments — An environmental control system (Figure 1) was integrated around a CT fatigue sample. Within the test cell, RH values can be controlled between 15 percent and 95 percent (+/- 2 percent) while temperature is controllable from room temperature to 45°C (+/- 2 degrees). Initial fatigue testing has been performed using alloy Al 7075-T651 compact tension (C(T)) samples, which had 1 g/m2 of sodium chloride (NaCl) deposited on the surface. Fatigue crack growth rate (CGR) tests were run at a constant frequency of 1Hz with a load ratio (R) of 0.5. CGR and RH as a function of time (Figure 2a) reveal a factor of three increase in CGR at high RH. Further analysis reveals that CGR is always highest during intervals of decreasing RH (Figure 2b). A tool has been partially developed to compare environmental data acquired by corrosivity sensors. Data from the tool can also be used to identify the duration of conditions critical for accounting for environmental effects on fatigue. Eventually, it is believed that the approach developed within this project will be used for predicting crack growth rate for aircraft structural integrity.

Figure 2. (a) Crack growth rate of Al7075-T651 compact tension specimen, loaded with 1 g/m2 of sodium chloride, and relative humidity as a function of exposure time.  (b) Crack growth rate as a function of relative humidity.
Figure 2: (a) Crack growth rate of Al7075-T651 compact tension specimen, loaded with 1 g/m2 of sodium chloride, and relative humidity as a function of exposure time. (b) Crack growth rate as a function of relative humidity.
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Southwest Research Institute® (SwRI®), headquartered in San Antonio, Texas, is a multidisciplinary, independent, nonprofit, applied engineering and physical sciences research and development organization with 10 technical divisions.
04/15/14