Residual Stress Superposition and the Design and Testing of Coupons for Residual Stress Driven Stress Corrosion Testing

Tests to determine fracture mechanics properties of materials, including susceptibility to stress corrosion cracking (SCC) and the corresponding crack growth rates, regularly employ externally applied loads to provide crack driving force. The design and testing of a test coupon for which SCC crack growth rates can be determined without applied load, where the crack is driven by (intentionally) imposed residual stress through local out of plane compression, is presented. Measurements of residual stress profiles across two perpendicular planes in aluminum compact tension coupons with supplemental stress analysis are first discussed. Residual stress was introduced into the coupons by laser shock peening, and measurements were performed using the slitting method. For each coupon, the measured stress profile on the first plane was used to compute stress released on the second plane through the use of FEA. By adding this released stress to measured stress for the second plane, we obtain a stress profile for the second plane in the original configuration. Results of a numerical model that predicts residual stress due to laser shock peening using eigenstrain are presented, and agreement between the model and experimental results gives confidence in the superposition method applied. The same eigenstrain approach is used in the development of a coupon for a residual stress driven SCC test. The goal is to generate a useful residual stress field such that a single coupon can provide SCC growth rates under both Krs-increasing and Krs-decreasing conditions, in a single test. After a preliminary 2D study using eigenstrain, 3D contact simulations are performed for further detailed design, followed by experimental measurements for verification. The proposed SCC testing will make use of direct current potential drop (DCPD), crack mouth opening displacement (CMOD), and back-face strain measurements (BFS) in a corrosive environment. The DCPD data gives crack length as a function of time. When this data is combined with the CMOD data to find crack mouth opening displacement as a function of crack length and then differentiated, the stress intensity factor can be determined as a function of crack length by using Schindler's influence function with a corresponding Z(a). Crack growth rate as a function of stress intensity factor can then be ultimately determined by differentiating the crack length versus time data and correlating with the stress intensity factor data.