Non-linear plane gravitational waves as space-time defects F. L. Carneiro, S. C. Ulhoa, J. W. Maluf and J. F. da Rocha-Neto Eur. Phys. J. C, 81 1 (2021) 67 Published online: 21 January 2021 DOI: 10.1140/epjc/s10052-021-08862-x