Dept. of Mech. Eng., Univ. of Patras, Patras, Greece
The replacement of human heart valves by mechanical ones was pioneered in the 1960s. Shiley, Inc., of Irvine, California, was one of the early manufacturers of such valves. The Bjoerk--Shiley 60 convexo-concave valve was introduced in 1967 and about 85 000 people received this valve until 1986, when it was withdrawn from the market. Analysis of sound emitted from the strut of the Bjoerk--Shiley 60o CC valve due to impact was used to monitor the propagation of the fatigue crack before it would lead to the failure of one or both legs of the outlet strut. Analytical and experimental results established that the range of the fundamental natural frequency is 2500 to 8000 Hz. It was found that previous experimental investigations were performed within a window of lower frequencies and they could not capture the transition of the natural frequency during the propagation of the fatigue crack, which would take several months. It was found that for crack depths above 50% of the diameter, the change in the natural frequency, thus the dominant component of the signal, was substantial and easily measurable. This may lead not only to diagnosis of the fracture at one end of the outlet strut but also to detection of the propagating crack long before it would lead to the partial or complete strut failure.