Micro-scale residual stress relaxation measurements using focused ion beam slotting and digital image correlation

The research presented in this summary was carried out by the author as part of his fourth-year group industrial project [1], in pursuit of the degree Master of Engineering in Mechanical Engineering at the University of Bristol. The project, Measurement of residual stress in heterogeneous materials, was conducted with three other students and consisted of experimental work at the microand macro-scale and the development of an analytical solution. The crack compliance technique is well established on the macro-scale, determining the residual stress state from relaxation measurements taken while slotting a sample. In the last decade, due to technological advances, this technique has been applied on the microscale using a focused ion beam (FIB) instrument and digital image correlation (DIC) [2]. This technique is particularly interesting to researchers, improving understanding of residual stresses on the micro-scale and has many potential applications, for example, in NEMS and MEMS devices and the additive layer manufacturing method. The FIB instrument is similar to a scanning electron microscope (SEM); however, it uses a gallium ion beam rather than an electron beam. These heavy gallium ions interact with the surface atoms of the specimen and cause sputtering; atoms are dislodged, etching the surface. This gives the FIB both imaging and micro-machining capabilities. However, imaging is inherently destructive due to the sputtering effect. In contrast to previous research, which exploits the non-destructive imaging benefits of a dual beam FIB (that is one with both an FIB and SEM for imaging), a single beam FIB was used. Surface degradation, caused by sputtering and re-deposition, adversely affects DIC correlation accuracy. Furthermore, most previous research has been in thin-film/amorphous materials, rather than traditional engineering materials. DIC software can be used to analyse successive images, mapping pixel displacement between the images. In this case, preand post-slot images are taken with the FIB instrument. A slot (20× 1.5× 6 μm) is milled in the sample using a high beam current. DIC can measure sub-pixel displacements, to the order of hundredths of pixels [3], at a scale of 5 k× corresponding to 0.594 nm. There are various commercial DIC software packages available, in this case Dantec Dynamics Istra 4D was used. Previous work with low carbon mild steel [4] encountered problems due to surface redeposition of the sputtered material while slotting. This led to the so-called “halo” effect.