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A Cu bicrystal with a twin boundary (TB) inclined at 45° to the loading direction and its component monocrystal were specially designed to reveal their fatigue cracking mechanisms. Experimental results show that slip bands (SBs) parallel to the TB plane operated easily in both component grains and concentrated at the TB, while the SBs distributed more homogeneously in the monocrystal. It was revealed by atomistic modeling simulations and anisotropic elasticity theory that the screw dislocations transmitted to the TB could dissociate into two far-separated partial dislocations. Therefore, these dislocations would be confined in the TB with a high difficulty of cross-slip, leading to strain localization near the TB. With cyclic deformation, the fatigue crack nucleated along the TB preferentially in the bicrystal with fewer cycles than that for the SB crack in the monocrystal. Besides, unlike the impingement fatigue crack along …