Damage occurring under tensile loading and the resulting increase of electrical resistance of indium-doped tin oxide and amorphous graphite coatings on various polymer substrates are investigated, using an electro-fragmentation method in situ in an optical microscope. The electrical resistance is modeled as a function of damage state assuming that the resistance of coating cracks is proportional to their opening, and that there exists a conducting path of constant thickness at the coating/substrate interface across the crack. The model reproduces the experimental data with good accuracy apart from the damage initiation and saturation stages where it underestimates the measured coating resistance. This is due to the presence of stable cracks of finite length in the initiation stage and delamination in the saturation stage. Impedance spectroscopy measurements confirm the purely resistive nature of the conducting path, whose resistivity is found to be 3 to 4 orders of magnitude higher than that of the un- cracked coating.