Fatigue Failure Characteristics of Monolithic Carbon TPU Insoles and Their Potential Use as a Mechanical Indicator of Load Distribution During Walking

Monolithic carbon-reinforced thermoplastic polyurethane (TPU) insoles with a thickness of 1.0 mm and Shore hardness of 80A were fabricated and evaluated under real-world walking conditions using a 70 kg subject. Initial crack formation was observed in the forefoot region at approximately 10–12 km of cumulative walking distance. Crack propagation progressed gradually with continued use, ultimately resulting in complete structural failure at approximately 65 km. Crack initiation and propagation occurred consistently in the forefoot region, with no structural failure observed in the heel or arch regions. This reproducible failure pattern suggests a stable stress concentration mechanism associated with forefoot loading during walking. In contrast, switching to a conventional EVA insole resulted in a pronounced sensation of arch support pressure, suggesting stiffness-dependent differences in load distribution. These findings indicate that monolithic carbon TPU insoles may function not only as structural components but also as passive mechanical indicators of gait-related load distribution, with potential applications in biomechanical assessment, gait training, and rehabilitation monitoring.