English: Crack closure effect (example of R=0). Stress intensity factor, K, is calculated from the external applied force P, and the crack tip opening displacement, CTOD, varies with time during a sinusoidal cycle of K. Nominal stress intensity factor range, delta_K, is calculated from K_max and K_min. However, crack closure occurs when K < K_cl even though under positive K,allowing us to define an effective stress intensity range, delta_K_eff from K_max and K_cl, which is less than the nominal applied delta_K.