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Herein,we fabricated a heterogeneous nanostructured Cu with deformation nanotwin bundles(NTBs)embedded in a matrix of nanograins by means of dynamic plastic deformation at liquid nitrogen temperature.We conducted fracture mechanical mea-surements to investigate the effect of longitudinal length of the NTBs on their strengthening and toughening.Results suggest that an increase in the NTB length had a marginal influence on the tensile strength;however,it remarkably promoted both fracture initiation toughness and crack growth toughness.Longer NTBs are more effective not only in intrinsically enhancing crack tip plasticity by suppressing the strain localization and void nucleation but also in serving as crack bridges behind the crack front to extrinsically resist the fracture by shielding the crack tip.