The addition of Kaowool fibers substantially increases the fatigue strength
of 339 aluminum at 300 degrees C. However, the fatigue life of these compo
sites is limited by the presence of Kaowool "shot" particles, which act as
crack initiation sites. An elastic analysis of the stress concentrations as
sociated with this type of defect shows that a crucial parameter is the dis
tance of the particle from the free surface. As the distance from the surfa
ce decreases, the stress concentration factor increases substantially and a
ttains maximum value when a small portion of the particle has been removed
by machining. The relationship between the distance from the surface and th
e stress concentration factor is in excellent agreement with the particle g
eometries observed at fatigue crack initiation sites.