Fracture toughness and crack growth rates in friction stir welded joints of AL 7075-T651

Friction stir welding is an aluminum plate joining process that has been found to be more superior to that of fusion welding. The need for information on this process regarding its response to fracture is vital in understanding the working life of high strength aluminum alloy plate structures in the aerospace industry. The objective of this study is 1) to perform plane strain fracture toughness tests to understand how the friction stir welded material will resist crack growth both in the longitudinal (T-L) and transverse (L-T) planes, 2) to perform plane strain fracture toughness tests through the weak areas of the weld, and 3) to perform tests on fatigue crack growth rates through the weak areas of the welds. Testing was performed on the MTS 810 Material Testing Machine. The fracture tests were conducted on compact specimens (CT) where the welded material will be fractured in the longitudinal and transverse planes, and through the Thermal Mechanical Affected Zone (TMAZ) and the Heat Affected Zone (HAZ) areas in the longitudinal direction. This study also examined the crack growth rates of each of the above areas of the weld. The crack growth rates of each of the three types of specimens was developed using an Acoustic Emission analyzer and the unloading compliance method. It is shown that the transverse plane of the weld will not perform as well as the longitudinal plane in its resistance to fracture. Also, this study’s fracture analysis shows that fracture toughness varies in the longitudinal plane of the TMAZ/HAZ areas --Abstract, page iii