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Kinetics of fatigue crack growth under notches in duplex steel
Kotecký, Ondřej ; Degallaix, S. ; Polák, Jaroslav
While under elastic fatigue loading of structural components with a stress concentration the material response is quite well known and the predictions are accurate, under elastic-plastic fatigue loading the predictions are poor and result in over-dimensioning the structure. The present work studies the effect of the stress gradient and the microstructure in an austenitic-ferritic (or duplex) 2507 stainless steel. Fatigue tests were performed on double edge notched specimens. Direct optical observations and potential drop technique calibrated via the finite element method are used to measure the surface crack size and its depth and to determine the in-depth and surface crack growth. The effect of stress and strain gradients on the crack propagation kinetics is evaluated and discussed.
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Comparison of low cycle fatigue characteristics of cast nickel base superalloys Inconel 713 LC a Inconel 792-5A
Petrenec, Martin ; Obrtlík, Karel ; Polák, Jaroslav
Cylindrical specimens of cast polycrystalline nickel base superalloys Inconel 713 LC and Inconel 792-5A were cyclically strained under total strain control at room temperature and at 800 °C. Structural characteristics of both materials are documented. Metallographic observations revealed coarse grains with dendrites, shrinkage pores and carbide inclusions. TEM observation shows differences in the morphology of ordered γ´ precipitates in both materials. Cyclic hardening/softening curves, cyclic stress-strain curves, and fatigue life curves were obtained for both temperatures. The cyclic hardening/softening curves depend both on temperature and plastic strain amplitude. The cyclic stress-strain curves can be approximated by power law. Experimental points of fatigue life curves satisfy the Manson-Coffin and Basquin law. Stress-strain response and fatigue life characteristics are compared at both temperatures and discussed in relation to structural parameters of materials studied.
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Kalibrace potenciálové metody pro měření růstu únavové trhliny
Kotecký, Ondřej ; Degallaix, S. ; Polák, Jaroslav
The DC potential drop method widely used to follow long crack growth, was adapted to measure the growth and to estimate the shape of mechanically short fatigue cracks initiated on a small defect in an austenitic-ferritic stainless steel. The paper deals with problems of calibration of the method by the means of finite element analysis. A comparison between measured and actual crack length and crack front in notched specimens is presented.
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Initial stages of fatigue failure of polycrystalline materials
Man, Jiří ; Petrenec, Martin ; Obrtlík, Karel ; Polák, Jaroslav
Geometry of persistent slip markings (PSMs) formed during cyclic loading of polycrystalline stainless steels and superalloy was studied using atomic force microscopy (AFM) and high-resolution scanning electron microscopy (SEM-FEG). Internal dislocation structure was investigated using transmission electron microscopy (TEM). Persistent slip bands with dislocation structure corresponding to the cyclic strain localization were identified and correlated with PSMs observed on the specimen surface.
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CYCLIC PLASTICITY AND DISLOCATION SUBSTRUCTURE OF ferritic STAINLESS STEEL
Petrenec, Martin ; Polák, Jaroslav
The cyclic plastic response of polycrystalline ferritic stainless steel with b.c.c. structure is analysed in terms of the general statistical theory of the hysteresis loop and cycled in the magnetic field. Simultaneously, the internal dislocation substructure developed in the material during cyclic straining is studied using transmission electron microscopy and Kikuchi lines. The inhomogeneous distribution of dislocations into the persistent slip bands can be correlated with the localization of the cyclic plastic strain. The analysis of the loop shape by plotting the first and the second derivative of the half-loop allows determining the effective stress and the probability density function of the critical internal stresses. Extra peak of the second derivative of the half-loop was explained by the inverse magneto-elastic effect.
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Quantitative assessment of the surface relief in fatigue using AFM
Man, Jiří ; Obrtlík, Karel ; Polák, Jaroslav
Atomic force microscopy (AFM) was applied to study the early stages of fatigue damage in polycrystalline copper and stainless 316L steel. The analysis of the image formation using an AFM tip reveals the lateral distortion of the extrusions and intrusions in case of high extrusions and deep intrusions. The true extrusion height can be obtained in direct observation and the depth of intrusions using replica technique. The evolution of the shape and the height of extrusions allowed to judge the activity of the persistent slip bands (PSBs) during fatigue life.
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