Mechanical Performance of Composite Laminates and Sandwich Panels Upcycled from Carbon and Glass Fiber Prepreg Scraps

Composite manufacturing generates large quantities of uncured prepreg scraps that retain continuous fibers and uncured resin, providing an opportunity for high-value reuse. Unlike previous studies that have largely focused on coupon-level samples with relatively simple scrap layouts, the present work comparatively evaluates three patch-assembly configurations in both glass and carbon fiber epoxy systems, extends the approach to foam-core sandwich structures, and assesses its structure-level feasibility through a head–neck board demonstrator. Tensile and flexural properties were evaluated against virgin reference laminates, followed by three-point bending tests on sandwich beams. Failure mechanisms were analyzed using stress-based calculations and optical microscopy. Results showed scrap assembly reduced tensile strength because of ply-joining interfaces, whereas flexural performance was less affected, particularly for rectangular patch assemblies. Among the investigated geometries, rectangular patches consistently achieved the highest mechanical performance retention for both fiber systems. In sandwich structures, glass fiber specimens failed predominantly by foam-core shear, while carbon fiber specimens exhibited local skin wrinkling caused by elastic instability. A head–neck board demonstrator manufactured from rectangular scrap-assembled carbon fiber sandwich panels retained 77% of the bending capacity of a commercial reference while exceeding the load-bearing requirement for clinical use. These results demonstrate that direct reuse of uncured prepreg scraps is a technically viable route for producing structural composite components and advancing circular composite manufacturing.