Mechanical analysis of composite leaf springs made by 4D printing method

The paper explains why composite curved beams made by 4D Printing of Composites (4DPC) can endure many fatigue cycles without obvious degradation. The beam is built from an unsymmetric laminate with 0° and 90° fiber/epoxy layers laid flat on a flat mold. After curing and cooling to room temperature, the flat stack curls into a curved beam. The authors attribute this shape change to anisotropy: different thermal contraction coefficients between the 0° and 90° layers create interactions that generate curvature. Their mechanical analysis focuses on residual strains and residual stresses formed at the surface of the 90° layers (described as the concave surface). They report that these residual stresses/strains are large enough to reduce the tensile strains and stresses produced during mechanical loading, so the combined values stay below the material’s surface strain/stress limits, which they use to explain the improved fatigue resistance. The abstract also notes practical implications: the method uses only a flat mold and may be attractive for custom-made springs, and the same structure could be used as actuators where thermal variation changes curvature.