We have studied the pinning force Fp(B,T) in YBa2Cu3O7 crystals as a function of fluence of 3-MeV protons. For a given ion-damage level, we could reduce all Fp data to a single function of reduced field b=B/B*, where the scaling field B*(T)Hc2(T) is related to the irreversibility line. This result rules out the possibility of matching effects between the vortex lattice and the distribution of pinning sites. We emphasize the influence of thermally activated relaxation in the determination of Fp and compare the data with simple pinning models. © 1991 The American Physical Society.