Structural testing of a juvenile prosthetic foot pylon

Clinicians at the Shriner's Hospitals for Children have noticed unexpected catastrophic failures in the 2024-T3 aluminum prosthetic pylons of their juvenile patients. They postulated that small "dings", caused by sharp impacts with playground equipment or other obstacles, might be causing stress concentrations in the pylons. To investigate this, students at the United States Air Force Academy performed a series of tests to determine potential failure modes for the pylon. Initially, a technique for introducing small reproducible "dings" was established. These damaged specimens were then subjected to compressive axial tests to failure, axial fatigue tests (5-100 lbs at 5 Hz), and finally four point bending fatigue tests (370 in-lbs at 5 Hz). The maximum compressive load of the damaged specimen was virtually identical to that of the control specimen (14 kips), and the axial fatigue test was halted with no damage after one million cycles. Two bending fatigue tests resulted in cracks and eventual failure at the load application points of the fixture on the pylon. This has caused us to speculate that failure of the pylons is not due to the dings, but may be due to over-tightening of the pylon attachment points to the foot and the deficient limb. Further testing will be conducted to examine this possibility.