A Low-Cost Energy-Storing Prosthetic Foot from Flat Spring Stock: Sizing a Leaf Spring by Gait Physics for Body Masses of 40 to 100 kg

Energy-storing prosthetic feet are effective but expensive, while the rigid SACH foot returns almost no energy. This paper designs a J-shaped leaf-spring foot cut from flat unidirectional glass-fibre/epoxy strip (width 60 mm, effective bending length 180 mm) and sizes its thickness from gait physics for body masses of 40, 60, 80 and 100 kg, using stated inputs of peak ankle moment 1.5 N m/kg, peak load factor 1.2 times body weight, and ankle work demand 0.30 J/kg per step. Thickness is chosen for a 200 MPa design allowable (fatigue limit 250 MPa, ultimate 600 MPa, modulus 22 GPa) and rounded up to the nearest 0.5 mm. For the 80 kg design: thickness 9.5 mm, stiffness 48,514 N/m, deflection 19.4 mm at 941.8 N, stored energy 9.14 J, returned energy 8.04 J per step at 88% return (33.5% of the 24.0 J ankle demand), peak stress 187.8 MPa, fatigue life 9,849,082 cycles (8.99 years at 3,000 cycles per day), and total parts cost $27.96. Across the range, returned energy is 5.03 J (40 kg), 7.58 J (60 kg), 8.04 J (80 kg) and 9.31 J (100 kg); parts costs are $26.52 to $28.53; estimated metabolic saving against a SACH foot from stated cost-of-transport inputs is 11.3% at 40 and 60 kg, 8.6% at 80 kg and 7.8% at 100 kg. The design serves walking up to load factor 1.2 for 40 to 100 kg, is not sized for running, and overload stress at 2.5 times body weight is 360.4 to 413.9 MPa. All results are simulated, not measured; nothing was built, worn, or tested on people, and all computations are reproducible from the deposited compute.py.