In situ study on damage evolution of additive manufactured short-fibre-reinforced composite: Improved design of nozzle based on mechanic principle

•Damage evolution of 3D printed short fiber reinforced composite was in situ observed.•Shear stress and internal voids joint regulated damage mechanisms was put forward.•Optimal ideas of internal microstructure from the view of mechanical were proposed.•Improved design of nozzle contraction section with quadratic curve shape was proposed.