Fatigue crack initiation and growth under mixed mode loading in aluminum alloys 2017-T3 and 7075-T6

• Published in: Engineering fracture mechanics Vol. 28; no. 5; pp. 721 - 732
• Main Authors: Otsuka, Akio, Tohgo, Keiichiro, Matsuyama, Hidenobu
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 01.01.1987
• ISSN: 0013-7944; 1873-7315
• DOI: 10.1016/0013-7944(87)90065-8

Fatigue crack initiation and growth characteristics under mixed mode loading have been investigated on aluminum alloys 2017-T3 and 7075-T6, using a newly developed apparatus for mixed mode loading tests. In 2017-T3, the fatigue crack initiation and growth characteristics from a precrack under mixed mode loading are divided into three regions—shear mode growth, tensile mode growth and no growth—on the ΔK I - ΔK II plane. The shear mode growth is observed in the region expressed approximately by ΔK II > 3MPa√m and ΔK II / ΔK I > 1.6. In 7075-T6, the condition of shear mode crack initiation is expressed by ΔK II > 8 MPa√m and ΔK II / ΔK I > 1.6, and continuous crack growth in shear mode is observed only in the case of ΔK I / ΔK II , ≅ 0. The threshold condition of fatigue crack growth in tensile mode is described by the maximum tensile stress criterion, which is given by Δσ θmax √2π r ≅ 1.6MPa√m, in both aluminum alloys. The direction of shear mode crack growth approaches the plane in which K I decreases and K II increases towards the maximum with crack growth. da/ dN- ΔK II relations of the curved cracks growing in shear mode under mixed mode loading agree well with the da/ dN- ΔK II relation of a straight crack under pure mode II loading.