Fracture Energy Measurements at the Micro-Scale

This study aims to develop a methodology for quantifying uncertainties in Small Cantilever Beam (SCB) testing, enhancing the accuracy and reliability of micro-scale fracture toughness evaluations.

A significant source of uncertainty arises during the microcantilever fabrication process. Techniques such as focused ion beam (FIB) milling using Gallium (Ga⁺) ions can result in ion implantation that adversely affects mechanical properties. Literature suggests employing low beam currents to mitigate damage. Femtosecond laser processing, while precise, can induce localised heating that alters microstructure and fracture toughness. In contrast, plasma FIB, which uses xenon ions, generally results in less ion damage but may create surface roughness that impacts test outcomes.

Accurate measurement of pre-crack and crack lengths is essential; discrepancies can lead to considerable errors in stress intensity factor calculations. While scanning electron microscopy (SEM) offers high-resolution imaging, it is susceptible to operator error and resolution limitations.

Symmetrical loading is vital for credible SCB test results, with deviations beyond 14% from the centre regarded as unacceptable. Asymmetrical loading can introduce additional bending moments and shear forces that distort fracture property measurements. Additionally, precise load and displacement measurements are critical, requiring regular calibration of load cells and high-resolution sensors to detect small-scale deformations.

The geometry and size of the microcantilever significantly influence results; even minor dimensional differences can alter stress distribution and crack propagation. Localised deformation near the crack tip, including plasticity, further complicates measurement accuracy. Sharp indenters on quasi-brittle materials may penetrate the specimen, affecting load and displacement readings.

Strain rate during testing is also crucial; quasi-static rates between 10⁻³ to 10⁻⁵ s⁻¹ are preferred to minimise dynamic effects. Lastly, challenges arise in interpreting fracture data, particularly concerning the transition between plane stress and plane strain. Accurate modelling of the stress state is necessary, along with careful assumptions when interpreting load-displacement data. Validating calculated fracture toughness against known values for similar materials is advisable.

By systematically evaluating these factors, the project seeks to improve the precision and consistency of SCB testing, enabling more accurate micro-mechanical characterisation in both research and industry contexts.