Damage Evolution During Fatigue in Structural Materials

AbstractEarly stages of damage evolution in cyclic loading are described and discussed. The importance of the role of cyclic plastic strain in damage evolution is emphasized and the relation between stress and strain in cyclic straining is clarified. The principal stages of damage evolution in fatigued crystalline structural material are identified. The basic characteristic and theories of fatigue crack initiation are sketched and confronted with experimental observations. Early fatigue crack growth is characterized and quantitatively described. The relation between the growth of short cracks and fatigue life in the form of Manson-Coffin law is established

Cyclic Plastic Response and Damage in Materials for High Temperature Applications

In this contribution, the methods allowing to study the sources of cyclic stress and early fatigue damage at room and elevated temperatures are presented and used to reveal details of the mechanisms of cyclic plastic straining and fatigue damage evolution in austenitic steels and nickel superalloys.Анализ кривых гистерезиса показывает, что при различной температуре имеют место характерные изменения зависимости между циклическими пластическими деформациями и напряжениями в нержавеющей стали и суперсплавах. Соответствующее изменение микрорельефа поверхности при мягком нагружении зафиксировано с помощью сканирующей и томографической электронной микроскопии с высоким разрешением. Проанализированы механизмы локализации циклической деформации, образования поверхностного микрорельефа, инициирования усталостных трещин и роста коротких трещин.Аналіз кривих гістерезису показує, що за різних температур відбуваються характерні зміни залежності між циклічними пластичними деформаціями і напруженнями в нержавіючій сталі і суперсплавах. Відповідні зміни мікрорельєфу поверхні за м’якого навантаження зафіксовано за допомогою сканувальної і томографічної електронної мікроскопії з високим розділенням. Проаналізовано механізми локалізації циклічної деформації, утворення поверхневого мікрорельєфу, ініціювання тріщин від утомленості і росту коротких тріщин

IN-SITU HIGH TEMPERATURE LOW CYCLE FATIGUE STUDY OF SURFACE TOPOGRAPHY EVOLUTION IN NICKEL SUPERALLOY

In-situ Low Cycle Fatigue test (LCF) at temperature 635 degrees C have been performed in Scanning Electron Microscope (SEM) equipped with Electron Backscatter Diffraction analysis (EBSD) on a small dog-bone-shaped specimen of cast Inconel 713LC superalloy. The aim of the work was to study early stage fatigue damage at high temperature by the observations of the characteristic surface relief evolution and crystallographic characterization changes by EBSD. The detail of slip bands shape was checked by FIB and AFM microscopes. The LCF test was conducted on GATAN stage with pre tilted position and constant stress amplitude of total cycle number of 20. The relief produced in the first cycle determines the other locations of the localized cyclic slip to the primary slip planes (111). The relief was modified in the next cycles but without forming additionally new slip traces in the primary system. Based on EBSD analysis before and after LCF, the orientation of two grains was changed which caused activation of second slip system. The damage mechanism evolution is closely connected with the cyclic strain localization to the persistent slip bands where the fatigue cracks were initiated

IN-SITU HIGH TEMPERATURE LOW CYCLE FATIGUE STUDY

In-situ Low Cycle Fatigue test (LCF) at temperature 635 °C have been performed in Scanning Electron Microscope (SEM) equipped with Electron Backscatter Diffraction analysis (EBSD) on a small dog-bone-shaped specimen of cast Inconel 713LC superalloy. The aim of the work was to study early stage fatigue damage at high temperature by the observations of the characteristic surface relief evolution and crystallographic characterization changes by EBSD. The detail of slip bands shape was checked by FIB and AFM microscopes. The LCF test was conducted on GATAN stage with pre tilted position and constant stress amplitude of total cycle number of 20. The relief produced in the first cycle determines the other locations of the localized cyclic slip to the primary slip planes (111). The relief was modified in the next cycles but without forming additionally new slip traces in the primary system. Based on EBSD analysis before and after LCF, the orientation of two grains was changed which caused activation of second slip system. The damage mechanism evolution is closely connected with the cyclic strain localization to the persistent slip bands where the fatigue cracks were initiated

NANOSTRUCTURE CHARACTERIZATION OF IN738LC SUPERALLOY FATIGUED AT HIGH TEMPERATURE

The nanostructure of Inconel 738LC Ni-superalloy strengthened by trimodal γ’ precipitates distribution was investigated after Low Cycle Fatigue (LCF) loading at temperature 700°C. Different microscopic techniques as Scanning Electron Microscope (SEM) equipped with STEM detector, transmission Kikuchi diffraction in the SEM, transmission electron microscope (TEM) in the bright field mode and high resolution transmission electron microscopes in STEM mode were used for the characterization of nanostructure. The characteristic morphology of γ’ precipitates was examined by ex-situ and in-situ Small Angle Neutron Scattering (SANS) at high temperatures. All microscopic techniques indicate that the morphology of γ’ precipitates distributed in the γ matrix as received state corresponds to two types, i.e. large cuboid-like precipitates with the size around 670 nm, and the spherical precipitates with the diameter 52 nm. After the LCF tests at temperature 700°C, the ex-situ SANS measurement yielded additional scattering intensities coming from another small γ’ precipitates with estimated size up to 10 nm

ELASTIC-PLASTIC BEHAVIOUR OF ADVANCED ADI STUDIED BY IN-SITU SEM TENSILE TEST

In-situ SEM tensile tests at room temperature have been performed on flat specimen of advanced austempered ductile cast iron (ADI) alloyed with nickel with the aim to study elastic-plastic behavior together with the nucleation and growth of cracks. During tensile loading the systematic observation of selected locations was studied. The in-situ observation was used to elucidate the tensile curves in agreement with the deformation mechanisms. In the early stage of loading the stress-strain response is elastic. After exceeding elastic limit decohesion of graphite nodules from the matrix was observed. It can be related to the departure of the tensile curve from the elastic behavior. The cracks initiated preferably from interface of graphite nodule and the ausferrite matrix. Elastic-plastic region is connected with the plastic deformation of the matrix and growth and linking of short cracks