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Mechanical size effects are a well known phenomenon when the sample volume is reduced or the characteristic length of the microstructure is changed. While size effects in micropillar compression (smaller is stronger) or due to grain refinement (Hall-Petch) are well understood, this is less so in fracture mechanics. Given this lack of knowledge, the main question addressed in this work is: What happens to the fatigue crack growth properties when extrinsic size effects play a role? To answer this question, nanocrystalline nickel cantilevers, ranging in width from 5 to ▪, were subjected to fatigue crack growth. The crack growth rates and stress intensity factors were calculated and the Paris exponent in the stable crack growth regime was determined. It was found that the results scatter more strongly for the smaller cantilevers compared to the larger cantilevers. Results are interpreted in terms of plastic zone size and ligament size which are found to be critical for small cantilevers. • Scatter of Paris Exponents increases with decreasing microcantilever dimensions. • Differences in the extent of grain growth are associated with the different stages of fatigue crack initiation and growth. • Fatigue pre-crack ensures consistent crack growth experiments in ductile samples. [ABSTRACT FROM AUTHOR]

[Display omitted] • Fracture behaviour of BaTiO3 thin film attached to different substrates were studied using microcantilever facture experiments. • These experiments revealed a significant effect of the substrate elastic modulus on the fracture toughness of the films. • Increase in fracture toughness of the bi-layered system was found as the crack tip approach the film-substrate interface. • Presence of ductile interface shielded the crack tip and changed the crack trajectory. • These measurements provide a template for design of thin films with increased fracture resistance. Damage tolerance of a thin film attached to a substrate is dependent on several parameters such as film thickness, film orientation, residual stresses, nature of interfaces, microstructure and defects present. Here we study the fracture resistance and crack growth trajectory in BaTiO 3 thin films attached to elasticially stiff substrates using micromechanical experiments and finite element modeling. Microcantilever fracture tests are carried out on bi-layered systems of BaTiO 3 film on Pt-Si and SrTiO 3 substrates to study the effect of interfaces, texture and elastic mismatch on fracture toughness. The substrates in the bi-layers are chosen to force the crack to experience a shielding effect in terms of a decrease in the crack driving force. Experiments revealed an unexpectedly large increase in fracture toughness when the crack tip is closer to a stiffer substrate, qualitatively matching the predictions from the numerical model. The bi-layered films attached to the substrate showed 125% (on Pt-Si) and 160% (on SrTiO 3) increase in fracture toughness compared to the free-standing films, for the first time, revealing the significant effect of elastic modulus of the substrate on improving the fracture resistance at such micrometer length scales. [ABSTRACT FROM AUTHOR]

Fatigue and fracture mechanics testing is strictly regulated by standards that specify a minimum specimen size. However, when component sizes or material volume decrease below this limit, test methods need to be adapted for small scales. In fatigue crack growth studies, experiments become complex and challenging. Thus, there is lack of systematic scale-bridging studies from meso- to micro-scales on ductile materials. This study outlines a method to bridge this scale gap by performing meso-scale studies. Cantilver bending specimens with a cross section of 50x50 µm2 and a lever arm length of 200 µm of nanocrystalline nickel, as a model material, are milled by Xe plasma focused ion beam technique and tested following ASTM E647 standard. The gathered d a / d N -curves are highly reproducible. The threshold stress intensity factor Δ K th is obtained by step-wise load reduction and discussed in context of crack closure. So, not only the gap between macro and micro-scaled testing was bridged, but also a pre-cracking step was shown to be essential for obtaining reliable data, and a new method to determine the intrinsic threshold stress intensity factor was introduced. [Display omitted] [ABSTRACT FROM AUTHOR]

The evolution and distribution of stresses and geometrically necessary dislocation densities during microcantilever testing were studied in-situ in a scanning electron microscope using high-angular resolution electron back scatter diffraction. Focused ion beam milling was used to prepare the beams with dimensions of ca. 5–7 µm in height and width and 20 µm in length in a tungsten single crystal along a 〈100〉 axis. Un-notched cantilevers were tested to gain insight into dislocation mechanisms leading to significant strengthening during loading. Locally increased yield and flow stresses were linked to pronounced stress gradients on specific slip planes and dislocation pile-ups at regions of zero stress. The limited plastic deformation behaviour of tungsten at room temperature allowed also detailed observations of the interplay between crack initiation and the formation of a plastic zone at the crack tip. Due to the lack of dislocation sources in the crack tip vicinity, dislocations were initially emitted on {110} planes from the crack tip, which led to finite crack tip blunting. The initiated crack was stopped by compressive stresses and a significant plastic zone developed, which kept the crack from growing with ongoing loading. The combined experimental and analytical observations allow a mechanistic understanding, which helps in explaining the recently reported size-dependent plastic deformation and fracture behaviour of semi-brittle materials. [ABSTRACT FROM AUTHOR]