Effect of thermal history on the fracture and fatigue behaviors of semi-crystalline polymers prepared via material extrusion additive manufacturing

• Fatigue life for material extrusion additive manufacturing (MEX-AM) was explored. • Lower nozzle temperatures in MEX-AM lead to rapid, brittle crack growth. • Increased interfacial temperatures in MEX-AM lead to greater fatigue life. • Increased temperature in MEX-AM reduced void coalescence ahead of the crack tip. • Increasing temperature increased plasticity at the crack tip, enhancing fracture energy dissipation. Material extrusion (MEX) additive manufacturing (AM) is transforming the design and production of complex structures, providing reliable on-demand components. However, the effect of thermal history on the resultant microstructure and damage tolerance of MEX-AM materials is not fully understood. This research investigates the critical role of interfacial thermal history, which is dependent on processing conditions, in determining the fracture and fatigue behaviors of semi-crystalline polymers, as exemplified by polyamide-6 (PA-6). Utilizing infrared thermography, the thermal history, and its dependence on nozzle temperature of extruded PA-6, was investigated. Quasi-static and cyclic tests of compact tension specimens were used to evaluate fracture and fatigue performance. The K IC in samples produced at a nozzle temperature of 260 °C were 201% and 18% higher than those fabricated at 240 °C and 280 °C, respectively. X-ray computed tomography showed thermal history significantly influences interfacial diffusion and void content, directly affecting performance. Optical microscopy and digital image correlation identified damage mechanisms and examined strain evolution around crack tips, revealing that interfacial thermal history governed crack tip plasticity, impacting the energy release rate. This study establishes a crucial process-structure–property-performance relationship and highlights the damage tolerance of MEX-AM polymers, showcasing their potential for advanced structural applications.