This study is part of a continuing project toward the development of a low cost modular prosthesis system. Specifically, durability tests were developed for a new foot component, the Niagara foot. The Niagara foot is a single piece injection molded device capable of plantarflexion and dorsiflexion with limited energy return capabilities. It is currently fabricated using Delrin® 100P and 100ST.

A four station cyclic testing device was designed, fabricated and validated. In each station, two pneumatic cylinders are used to apply alternating forces to the heel and the toe under computer control. The shape of the force application waveform was validated using a strain gage instrumented pylon and compared to the tibial axial forces determined during normal gait in volunteer subjects. The magnitude of the peak forces was validated using a piezoelectric load cell directly between the cylinder and the foot component.

Cyclic testing was performed at a frequency of 1 Hz with a force range of 50-970 N at both the heel and the toe. Failures were observed consistent with those expected on the basis of the static analysis. One sample of Delrin® 100P failed at 1,700,000 cycles with fracture surface morphology consistent with low-cycle fatigue failure. A second sample, of a slightly different design, failed at 310,000 cycles, consistent with early failures in field trials.

The NPO Fatigue Tester conforms to ISO 10328, incorporates a force waveform similar to the ground reaction forces on the tibia during normal gait and is capable of predicting failure in the Niagara foot.