Mechanisms of Fatigue Crack Growth in Boron/Aluminum Laminates

Laminated boron/aluminum sheets of 0, ± 60, 0, ± 60, and 0-deg layer orientation were notched and subjected to tension-tension (T-T) fatigue loading. Fatigue cracks were monitored visually and the fatigue cycling terminated at various amounts of fatigue crack growth.

Crack growth rates were evaluated in terms of a fatigue crack growth rate law based on fracture mechanics.

Some specimens were then loaded to fracture and the fracture surfaces examined by scanning electron fractography. Other similar specimens were examined by progressively removing the aluminum matrix by a selective etching technique. Each layer of fibers was thus exposed and could be examined in the scanning electron microscope (SEM).

The results obtained show that the extent of fracture of the 0-deg layers of fibers is the same as the extent of the visible fatigue crack on the surface of the specimen. For the 60-deg layers, the extent of fiber fracture may be significantly less than the extent of the visible crack.

The extent of fatigue crack growth in the matrix material in the interior of the specimen exceeds the length of the surface crack by several fiber diameters.