Effect of Water Vapor Pressure on the Fatigue Crack Propagation of Aerospace Aluminum Alloys 7075-T651 and 2199-T86
Abstract
Fatigue testing over a wide range of delta K-values, extending into the threshold regime, at various water vapor pressures (PH2o) applicable to airframe operation yielded a novel set of fatigue crack growth rates for two aerospace aluminum alloys (7075-T651 and 2199-T86). Data were analyzed with respect to prominent environmental fatigue theories and used to inform a next generation prognosis aimed at incorporating the effects of loading environment. AA 2199-T86 exhibited enhanced fatigue crack growth rate resistance compared with AA 7075-T651 over a wide range of environmental exposures and delta K. The da/dN-delta K data for both alloys shows the expected decrease in crack growth rate with decreasing exposure parameter (PH2df). Plotting the growth rate data versus PH2o/f demonstrates consistency with prominent environmental theories where the crack growth is limited by either molecular flow or H-diffusion in the crack tip process zone. In both alloys, delta K-shed data at intermediate-low exposures shows an apparent threshold followed by an increase in growth rates with falling delta K. Speculatively, this is due to development of crack wake roughness that initially impedes molecular flow, increasing roughness then causes convective mixing which enhances flow and increases growth rates. A protocol to select environment appropriate crack growth rates for linear elastic fracture mechanics (LEFM) modeling is presented.
Document Details
- Document Type
- Technical Report
- Publication Date
- Sep 16, 2013
- Accession Number
- AD1035407
Entities
People
- J. T. Burns
Organizations
- University of Virginia