Structural health monitoring of metal components: A new approach based on electrochemical measurements

摘要

Corrosion fatigue is defined as the sequential stages of metal damage that evolve with accumulated load cycling, in an aggressive environment, and resulting from the interaction of irreversible cyclic plastic deformation with localized chemical or electrochemical reactions. It appears then evident the need to know and understand all phenomena that are involved in pit formation and growth in the presence of both an aggressive environment and a cyclic load. Although each single effect of both fatigue and corrosion have been extensively documented for aluminium alloys, their synergic action is not thoroughly understood and it continues to be an area of considerable scientific and industrial interest. Fatigue experiments were conducted on bare AA 2024 T3 aluminium alloy specimens in the presence of an aggressive environment consisting of a water aerated solution with 3.5% of NaCl in weight. The specimen was stressed in a four-point bending test; the upper surface of the specimen, experiencing only tensile stress, was the monitored one. True strain on this surface was monitored by means of fibre optic Bragg grating. The strain and open circuit potential variations observed as a function of number of cycles are interpreted as a result, and taking into account the numerous and complex phenomena occurring as time goes on. The pits birth and their subsequent growth seem to play a significant role in crack initiation and subsequent propagation up to final catastrophic failure. This has been verified also by means of finite element method analysis.