Fatigue life study of ITO/PET specimens in terms of electrical resistance and stress/strain via cyclic bending tests

Bending tester in reciprocating motion is adopted to study the fatigue resistance and fatigue life of indium tin oxide (ITO) films deposited on poly(ethylene terephthalate) (PET) substrate with various prestrain levels. A commercial micro-compression tester and ANSYS-Workbench software are used to determine the stress versus strain curves, and thus the strain energy of a fixed volume in the specimen. A bending tester without a cycle limitation is designed to carry out ITO-film fatigue life tests. Two points symmetric with respect to the specimen's central line are set to measure the film's electrical resistance variation with the number of cycles in reciprocating motion. The number of cycles in the bending tests corresponding to the fatigue life of ITO film is determined on the bases of both the strain at which 63% of the material fails and the electrical resistance which has 10% rise w.r.t. the initial one before bending test. The mean electrical resistance increases with increasing number of cycles and prestrain applied to the PET substrate. Of the four substrate prestrains tested (0%, 2%, 4%, and 6%), the change in electrical resistance (ΔR) between the beginning and end of the test was highest for the PET-6%/ITO specimen regardless of the maximum acceleration of the tester. With the same substrate's prestrain and at the same maximum acceleration, the number of cycles for fatigue life predicted by these two models are fairly close. Increasing either the substrate prestrain or the jig's acceleration decreases the alternating stress and increases the strain during reciprocating motion, lowering the fatigue life of the film. The strain energy defined for a fixed volume in the specimen is found always asymptotic to a constant value. The number of cycles corresponding to the beginning of this constant strain energy is found close to those of fatigue cycles predicted by the models mentioned above. The strain energy method is thus provided as an effective way to predict the fatigue life of the specimens without fatigue limit in the SN curve.