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Residual stresses play a crucial role in determining material properties and behaviour, in terms of structural integrity under monotonic and cyclic loading, and for functional performance, in terms of capacitance, conductivity, band gap, and other characteristics. The methods for experimental residual stress analysis at the macro-and micro-scales are well established, but residual stress evaluation at the nanoscale faces major challenges, e.g. the need for sample sectioning to prepare thin lamellae, by its very nature introducing major modifications to the quantity being evaluated.Residual stress analysis by micro-ring core Focused Ion Beam milling directly at sample surface offers lateral resolution better than 1µm, and encodes information about residual stress depth variation. We report a new method for residual stress depth profiling at the resolution better than 50nm by the application of a mathematically straightforward and robust approach based on the concept of eigenstrain. The results are validated by direct comparison with measurements by nano-focus synchrotron X-ray diffraction.

The growing demand for reliable, durable electrical energy systems to power electric and hybrid vehicles motivates worldwide efforts aimed at developing high-energy, high-power density batteries. One of the obstacles to widespread industry adoption is the lack of profound understanding and the ability to monitor and control the long-term degradation and capacity fading observed in these systems. Focused Ion Beam-Scanning Electron Microscopy (FIB-SEM) serial sectioning is used to reconstruct the evolution of the three-dimensional structure of Li-ion battery electrodes during extended cycling. High resolution imaging reveals microstructural information at the level of the composite framework consisting of the spheroidal micro-particles of the active material held together by the polymer matrix. The evolution of damage within the micro-particles of the active material can be seen in the form of voiding, cracking and ultimate fragmentation. In particular, when spherical micro-particles of Li-rich layered oxides are used as the cathode, it is found that the extent of fragmentation varied in the direction of Li + diffusion current from the particle surface inwards. We use a simple model of the strain and strain gradient effects of Li + transient diffusion within the electrode to identify the driving force for particle fragmentation, and discuss the implication of these results. † Electronic supplementary information (ESI) available: 3D reconstructions of electrodes aer the 1st charge and the 1st cycle in addition to XRD and SEM observations of Li-rich particles. See

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The strain-induced softening of thermoplastic polyurethane elastomers (TPUs), known as the Mullins effect, arises from their multi-phase structure. We used the combination of small- and wide- angle X-ray scattering (SAXS/WAXS) during in situ repeated tensile loading to elucidate the relationship between molecular architecture, nano-strain, and macro-scale mechanical properties. Insights obtained from our analysis highlight the importance of the ‘fuzzy interface’ between the hard and soft regions that governs the structure evolution at nanometre length scales and leads to macroscopic stiffness reduction. We propose a hierarchical Eshelby inclusion model of phase interaction mediated by the ‘fuzzy interface’ that accommodates the nano-strain gradient between hard and soft regions and undergoes tension-induced softening, causing the Mullins effect that becomes apparent in TPUs even at moderate tensile strains.

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Quantification of residual stress gradients can provide great improvements in understanding the complex interactions between microstructure, mechanical state, mode(s) of failure and structural integrity. Highly focused local probe nondestructive techniques such as X-ray diffraction, electron diffraction or Raman spectroscopy have an established track record in determining spatial variations in the relative changes in residual stress with respect to a reference state for many structural materials. However, the interpretation of these measurements in terms of absolute stress values requires a strain-free sample often difficult to obtain due to the influence of chemistry, microstructure or processing route. With the increasing availability of focused ion beam instruments, a new approach has been developed which is known as the micro-scale ring-core focused ion beam-digital image correlation technique. This technique is becoming the principal tool for quantifying absolute in-plane residual stresses. It can be applied to a broad range of materials: crystalline and amorphous metallic alloys and ceramics, polymers, composites and biomaterials. The precise nano-scale positioning and welldefined gauge volume of this experimental technique make it eminently suitable for spatially resolved analysis, that is, residual stress profiling and mapping. Following a summary of micro-stress evaluation approaches, we focus our attention on focused ion beam-digital image correlation methods and assess the application of micro-scale ring-core methods for spatially resolved residual stress profiling. The sequential ring-core milling focused ion beam-digital image correlation method allows micro-to macro-scale mapping at the step of 10-1000 mm, while the parallel focused ion beam-digital image correlation approach exploits simultaneous milling operation to quantify stress profiles at the micron scale (1-10 mm). Cross-validation against X-ray diffraction results confirms that these approaches represent accurate, reliable and effective residual stress mapping methods.