© 2022, The Author(s), under exclusive licence to Springer Science+Business Media, LLC, part of Springer Nature.Adhesively bonded external composite patch repairs are one of the most common types of repairs. The shape and form of the damage in the repaired composite and the limits of this damage are interesting topics for researchers. In this direction, the detection of the damage in the patch and damaged main material can be determined by various methods. This study aims to investigate the impact delamination behavior of composite laminates repaired by external patches by using 3D finite element analysis and ultrasonic testing (UT). The non-destructive testing method is preferred for the actual delamination damage caused and the accuracy of the proposed numerical model. The proposed numerical model estimated delamination damage by degrading to material constants according to the delamination damage criterion. UT consisting of pulse-echo and through-transmission (TT) technique was evaluated under different ambient temperatures. These ambient temperatures were chosen due to field and laboratory conditions. Numerical analysis of patched-composites was carried out by using ABAQUS-PYTHON scripting language with VUMAT and 3D Hashin shear and delamination damage model. The effect of the material and thickness of the composite patch affecting the delamination damage under impact was also investigated in order to precisely determine the accuracy of the ultrasonic methods and the proposed numerical model. The damage areas and regions at the top (patch) and bottom (damaged composite plate) faces of the patched composite specimen were in good agreement. Numerical analysis results correctly predicted the delamination areas. The use of patches increased the peak contact force (glass fiber and carbon fiber, respectively) in the unpatched damaged composite by 11% and 18%. The patch material change (from glass fiber to carbon fiber) increased the peak contact force by 6%. With the increase in patch thickness (glass fiber and carbon fiber, respectively), the peak contact force was increased by 21 and 15%. Ultrasonic scanning results show that the through-transmission technique is more successful in detecting damage to the back of the plates (volumetric damages), and the pulse-echo technique is more successful in detecting damage to the impact surface of the plate.