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Content archived on 2024-04-15

METHODS FO)R PREDICTING THE EFFECT OF SURFACE DEGRADATION OF FATIGUE AND FRACTURE BEHAVIOUR

Objective


The research aimed to quantitatively describe the effect of corrosion pitting on the high cycle fatigue behaviour of engineering components, using a novel fracture mechanics approach. Crack initiation and growth parameters and the interactive effects of corrosion pit nucleation and growth kinetics have been incorporated into a mathematical model. Two materials were evaluated; a 13% chromium (X2013Cr) turbine blade steel and a 3.5% nickel rotor steel. The evaluation took the form of mechanical testing in both air and corrosive environments. The data required for the modelling necessitated the generation of mechanical data on crack initiation, crack growth, fatigue threshold, S-N data and corrosion pit initiation and growth kinetics.

Two theoretical approaches have been put forward both based on linear elastic fracture mechanics. The first provides a method of predicting in fatigue strength of components caused by the presence of corrosion pits of various sizes operating at various R-ratios. The second predicts the fatigue crack growth from corrosion pits for lifetime prediction. Both the models were combined on one computer program with the specific constants of the equation used in the model being defined from the experimental programmes. Good agreement was found with experimental results.

An examination of surface deposits formed during the initial stages of corrosion of 2 steels led to important information of the species primarily responsible for the formation of corrosion pits. The effect of laser treatment on the fatigue strength of the X20Cr13 steel with a view to its possible use as a repair method on certain components was also studied. The results have shown that any benefits are only obtained in the absence of corrosive environment.

Two theoretical models were established to describe quantitatively the influence of corrosion pits on fatigue behaviour of chromium steel and nickel steel and to simulate fatigue crack growth starting from a corrosion pit. The first model predicted the dependence of fatigue strength on pit size and R-value and the second simulated the fatigue crack growth of pitted specimens. Both theoretical approaches have been combined into one computer program for life time calculation. Both models are based on a linear elastic fracture mechanics (LEFM) defect assessment. Investigations on pitted specimens have shown that a corrosion pit can be treated as a semielliptical crack with the same depth and surface length as a pit.

Topographic and statistical scanning electron microscope (SEM) investigations of fracture surfaces have shown that stress fields generated by the pits on the surface of the specimen overlap during the fatigue test. Due to this fact the interaction of neighbouring pits was included in the model. There are various applications of the program. With the first model the fatigue strength of pitted specimens can be predicted. Predicted fatigue strength reduction of S-N push pull specimens for different pit sizes showed excellent agreement with the model, whereas the predicted results for 3.5% nickel steel were conservative. The model was further used to simulate complete S-N curves for different load geometries in air and under pitting conditions. With respect to the scatter of the measured values and the difficulties in defining exactly the pit depth, the obtained results were in good agreement with the experiments.

Round specimens (push pull and bending) of chromium steel (X20Cr13) for high cycle fatigue tests (S-N tests) and crack initiation (CI) specimens were laser hardened with the aim of enhancing fatigue strength. The experiments were carried out with a continuous wave carbon dioxide laser of 10.6 um wavelength and 3.5 kW maximum output power connected to a 3 axes workstationwith computer numerical control. The treatment conditions (laser power, traverse speed and spot shape and size) were adjusted in order to avoid surface melting of the specimen.

Microhardness (Vickers hardness under 100 g load) profiles, performed after the treatment showed that the surface microhardness may reach 640 to 750 HV (depending on the kind of specimen), to be compared with an initial value of 250 to 275 HV. In S-N specimens, the zones of overlapping laser paths exhibit a hardness decrease resulting from local annealing in previously treated zones (helical treatment). There was an increase in fatigue strength in chromium steel.

An experimental methodology which would afford the production of well defined corrosion pits on 13% chromium steel (X20Cr13) and 3.5% nickel steel surfaces was established. The methodology developed was utilised for the production of prepitted specimens for use in mechanical lifetime tests. The fundamental aspects of the kinetics of pit initiation and propagation were also examined. X20Cr13 specimens use pitted in a dilute aqueous chloride solution (0.5 molar sodium chloride) using a potentiostatic technique. Pitting was readily accomplished upon application of a pitting potential of 25 mV. The method can be applied both to disc specimens, mechanical testpieces and component samples.

An experimental methodology for the generation of spatially defined single pits was also developed. The technique involved masking off the specimen surface and subsequent use of a high frequency, high voltage discharge to break down the protective coating at the chosen location on the specimen surface. This treatment resulted in the erosion of a small pit on the surface of the metal. The latter was subsequently enlarged by a process of chemical etching using dilute (0.5 molar) nitric acid solution.
CORROSIVE ENVIRONMENTS ARE KNOWN TO HAVE DETRIMENTAL EFFECTS ON FATIGUE AND FRACTURE PROPERTIES OF METAL COMPONENTS; PITTING CORROSION CAN DECREASE THE FATIGUE LIMIT SERIOUSLY.
HENCE THE OBJECTIVE OF THIS PROJECT IS TO QUANTITATIVELY DESCRIBE THE INFLUENCE OF CORROSION PITS ON FATIGUE BEHAVIOUR OF COMPONENTS, SUCH AS TURBINE BLADES AND ROTORS. TWO ALLOYS OF DIFFERENT MECHANICAL STRENGTHS WILL BE INVESTIGATED AND EVALUATED WITH RESPECT TO A QUANTITATIVE DESCRIPTION OF THE INFLUENCE ON FATIGUE BEHAVIOUR EXERTED BY DIMENSIONS AND AREAS DENSITY OF PITS AND, FURTHERMORE, BY THE GROWTH KINETICS OF PITS DURING DYNAMIC LOADING IN CHLORIDE CONTAINING ENVIRONMENTS. COMPUTER PROGRAMS WILL BE DEVELOPED ENABLING PREDICTIONS OF LIFETIME FOR PITTED COMPONENTS IN AIR AND IN CORROSIVE MEDIA TO BE MADE.IN ADDITION THE INFLUENCE OF BASE MATERIALS, I.E. THEIR MECHANICAL STRENGTH, CHEMICAL COMPOSITION, SURFACE TREATMENT, RESIDUAL STRESSES AND ROUGHNESS WILL BE CONSIDERED.

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Coordinator

Allgemeine Elektrizitäts Gesellschaft AG (AEG)
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Goldsteinstraße 235
60528 Frankfurt am Main
Germany

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