A B S T R A C T The relation is studied between crack initiation from micro-notches in a fully lamellar intermetallic -TiAl alloy and the local strain field. These micro-notches were introduced using femtosecond-laser ablation and had dimensions below the average colony size. The specimen under investigation was then subjected to fatigue loading. Continuous monitoring using a travelling optical microscope allowed detecting microcracks at an early stage. Prior to fatigue loading, a sustained load was applied and the local strain field was determined using digital image correlation. This was supplemented by a Finite Element analysis of the notches and their neighbourhood. It was found that a crack was initiated from a notch causing high normal strains in lamella direction, whereas no crack was initiated from notches with high shear strains. a = Notch depth [μm] b = Notch width [μm] c = Notch length [μm] da/dN = Crack growth rate [mm/cycle] DIC = Digital image correlation E i = values of elastic modulus [MPa] FE = Finite element method FSD = Forward scatter diode G ij = Values of shear modulus [MPa] R p0.2 = Yield strength [MPa] R p0.01 = Elastic limit [MPa] ΔK = Range of stress intensity factor [MPa√m] ε, ε ij = Strain tensor V ij = Values of Poisson's ratio σ = Stress [MPa]
In this work, we demonstrate two-color inline refractivity compensation in a heterodyne synthetic wavelength interferometer for a measurement of absolute distances over several hundred meters with sub-millimeter accuracy. Two frequency-doubled Nd:YAG lasers with a coherence length of more than 1 km are used as light sources. Direct SI traceability is achieved by controlling the lasers' frequency difference in the radio frequency regime. The resulting synthetic wavelengths at 532 nm and 1064 nm are used for the absolute distance measurement and dispersion-based inline refractive index compensation. A standard deviation of 50 μm is achieved for distances up to 864 m. This performance corresponds to a standard deviation of the observable, the difference of the four optical wavelengths, on a sub-nanometer level. Comparison against white light interferometry confirms sub-millimeter accuracy over this distance. Temporally resolved data over 864 m provide quantitative insights into the influence of chromatic beam paths.
Well‐defined starter cracks at preselected sites in the microstructures are needed for systematic investigation of the characteristic features of microstructure controlled growth of small cracks. A kinked ellipsoidal micronotch with very high notch factor at the trailing kink is proposed, which triggers controlled crack initiation along the notch contour. These micronotches can be machined by femtosecond laser ablation with virtually no heat‐affected zone at the edges. Crack growth results obtained for an intermetallic γ‐TiAl alloy are presented as an illustrative example.
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