The Corrosion Damage And Its Effect On Life Of Aircraft Structure

Corrosion, in its various forms, was a serious issue to be considered in its effects on aircraft structural safety. The following dissertation described the experimental and theoretical study on the corrosion damage and fatigue life of aluminum alloy structure. Based on the experiments, morphological classification on corrosion damage of aluminum alloy was studied. The metric for evaluating the corrosion damage grade was proposed. The pitting corrosion, exfoliation corrosion and corrosion fatigue which were the most usually occurred on aircraft structures were studied systemically. The primary mathematical relationship of the three kinds of damages and the degradation of fatigue life were built. The methods of analyzing the corrosion damage of aircraft structure and the technique of evaluating the life were developed. The dissertation will provide some technical support to life assessment and life extension of aging aircraft structures.Two experiments were conducted in this dissertation. Firstly, LY12CZ aluminum alloy specimens were pre-corroded under the conditions of three different test temperatures and exposure durations. After corrosion exposure, fatigue tests were performed. Scanning electron microscopy and optical microscope analyses on corrosion damage were carried out. Experimental results showed that, with the lengthening in corrosion exposure duration and the corrosion temperature going up, the dimension of corrosion pits became larger and little corrosion pits were congregated together, and the corrosion damage expanded along the specimen surface and cross section directions. The near corrosion pits congregated together and the exfoliation corrosion were observed. It can be found that all of the cracks nucleated from corrosion pits. In general, there were multiple fatigue cracks for almost every specimen, but most fatigue failures were governed by a dominant pit. In the middle of the specimens, the stress concentration was the most serious, but the fatigue crack initiating seldom occurred at this place and fracture usually occurred at the borderline of corrosion and non-corrosion. The artificially produced corrosion damage considerably reduced the fatigue life of laboratory specimens because of shortening the crack nucleation stage and the corrosion damage decreased the fatigue lives by a factor of about 1.25 to 2.38.Secondly, The fatigue crack propagation behavior of LY12CZ aluminum alloy specimen with central hole of fastener has been investigated in air and 3.5%NaCl solution. Experimental results showed that, the corrosion fatigue crack growth rate decreased with the increasing of the loading frequencies, and in corrosive environment, the crack growth rate was slightly, larger than in air. In 3.5 %NaCl, the crack growth rate was slightly larger than in air and the reason maybe rest on the duration of corrosion fatigue test was too short, and this led to the role of corrosion process was not significant.Based on the experiments, some mathematical models were proposed as follows,(1) The image analyses indicated that the two key parameters (the depth of corrosion pit and the surface corrosion damage ratio) were the metrics for evaluating the corrosion damage grade. The effect of corrosion damage on fatigue lives was discussed. Using the metrics can fix on the critical place of the aircraft structure subjected to corrosion damage.(2) The AB (Area-Box) parameter was proposed to conveniently classify the configuration of corrosion pits. Based on the experimental observation of corrosion pits configuration, the proposed AB parameter can describe the evolution rhythm of corrosion pit. With the lengthening in corrosion exposure duration and rise of corrosion temperature, AB value tend to 0.7854 (the corrosion pit presented rotundity or ellipse) and the irregular corrosion pits gradually developed to lager circular or elliptical corrosion pits.(3) The mathematical model of corrosion damage evolution was proposed. Statistical analysis had been performed to draw the conclusion that the Logistic distribution was acceptable for the data sets of the corrosion damage ratio. Using the probability model of corrosion damage evolution, we can forecast corrosion damage in varied service environments. Combing the electrochemistry, probabilistic analysis and stochastic processes method, the mathematic model of corrosion damage evolution was built and the predicted results were in good agreement with experimental results.(4) The prediction method of residual strength and lives of corroded structure was presented. Based on the experimental observation, using the element life-death technique in ANSYS software, the 3D finite element model of corroded structure was built and the stress distribution was analyzed. A simple crack growth calculation method using AFGROW software successfully predicted the fatigue lives of the artificially corroded specimens using the depth and width of the corrosion pits as the initial crack size. The simulation results using AFGROW were in good agreement with the experimental data.(5) The virtual corrosion fatigue crack propagation tests on aluminum structure were developed. The virtual corrosion fatigue crack propagation of fastening hole structure were investigated. The stochastic processes method and AFGROW simulation method were presented. The probabilistic model of crack length for a given number of cycles was well approximated by a lognormal distribution. This was true for either normal or lognormal stochastic processes and in corrosive or non-corrosive environment.(6) The corrosion-life management method of aircraft structure served in inshore circumstance was proposed. Based on the practical corrosion damage information derived from the investigation on the served aircraft structures, the corrosion damage tolerance of aircraft structure was investigated. The pitting corrosion and exfoliation corrosion were studied particularly and the terse engineering schematics were provided. At last, the holistic life model of aging aircraft made of aluminum alloys served in corrosive environment was proposed schematically.