In situ mechanics across length scales: From elastic isotropy to high strain rate mechanics (03/06/26)
Speaker and Affliation:
Dr. Lalith Kumar Bhaskar
Max Planck Institute for Sustainable Materials, Department of Structure and Micro-/Nano- Mechanics of Materials, Düsseldorf, Germany
When?
03rd June, 2026 (Wednesday), 3.00 PM (India Standard Time)
Where
KPA Auditorium, Dept. of Materials Engineering, IISc, Bangalore
Abstract:
The global mechanical response of materials is governed by the interplay of phases, grains, defects, and interfaces across multiple length scales. Understanding the contribution of these individual features requires experimental methodologies specifically tailored to the relevant length scale. In this talk, three studies will be presented at milli-, micro-, and nanometer length scales, respectively, that explore size-specific open research questions in material mechanics. Specifically, a unique combination of custom instrumentation, protocols, and methodologies was developed to understand underlying deformation physics.
Starting at the millimetre scale, one fundamental question is how elastic anisotropy or isotropy can be experimentally determined from polycrystalline materials. In this context, the entropy-stabilized oxide (MgNiCoCuZn)O is particularly interesting, as Density functional theory (DFT) studies reported in the literature predicted near-perfect elastic isotropy, yet no experimental validation exists. To address this, a robust micromechanical method combining in situ X-ray diffraction during mechanical loading, along with an optimization sub-routine, was developed to extract single-crystal elastic constants directly from polycrystalline samples. The experimentally determined single-crystal elastic constants were further validated against orientation-dependent constants from nanoindentation, thereby providing the first experimental confirmation of elastic isotropy in this entropy-stabilized oxide of composition (MgNiCoCuZn)O.
Moving to the microstructural length scale of microns, one of the long-standing open questions was whether the rate-dependent material physics at this scale would be similar to or different from the dynamic macroscale studies. Specifically focusing on the strain rate range of ~1000-10000/s, where a sudden increase or upturn in strength is typically observed in macroscale explorations. To achieve such high strain rates at the microscale, a piezo-based in situ micromechanical tester was heavily custom-modified, and necessary protocols and post-processing methodologies were developed. This allowed, for the first time, nanoindentation-based hardness measurements spanning six orders of magnitude from 10/s to 100000/s across a variety of material systems. In this talk, the case study on single-crystal molybdenum will be presented. Strength upturn was indeed present even at this microscale at strain rates between 1000 to 3000/s. Using TEM explorations, the underlying deformation mechanism responsible for the strength upturn was identified as increase in dislocation density.
Finally, at the nanoscale, exploring material mechanics is intrinsically challenging owing to the need for electron-transparent, thin samples in the ~100-200 nm range. Usually, lithography-based MEMS (Micro-Electro-Mechanical Systems) testing platforms are used, but they offer less flexibility for quantitative and controlled loading. To address this bottleneck, a new additive metal-micromanufacturing methodology called localized electrodeposition in liquid was used to develop 3D-printed copper microdevices. To these devices, any micro-/nano-sized sample of interest can be attached and strained in both a scanning electron microscope (SEM) and a transmission electron microscope (TEM). As part of this ongoing work, proof-of-concept results for 3D-printed metal microdevices and a case study on single-crystal nickel samples will be presented.
Short Bio:
Dr. Lalith Kumar Bhaskar received his PhD from the Indian Institute of Technology (IIT) Madras and is currently a postdoctoral researcher in the Nanomechanical Instrumentation and Extreme Nanomechanics group at the Max Planck Institute for Sustainable Materials, Germany. His research focuses on high-strain-rate mechanics, structure-property correlation in ceramics, and 3D metal micromanufacturing. His work focuses on developing custom experimental instrumentation that can be integrated in situ with advanced characterization techniques to probe and understand the deformation physics of ceramics and metals under extreme and complex loading conditions. He is also the co-founder of InsituMicron Pvt. Ltd., focused on advanced in situ mechanical testing technologies.