Computational study of nanoindentation on an elastoplastic solid with an interface parallel to the indentation direction

The dense, anhydrous zeolitic imidazolate frameworks (ZIFs), Zn(Im)(2) (1) and LiB(Im)(4) (2), adopt the same zni topology and differ only in terms of the inorganic species present in their structures. Their mechanical properties (specifically the Young's and bulk moduli, along with the hardness) have been elucidated by using high pressure, synchrotron X-ray diffraction, density functional calculations and nanoindentation studies. Under hydrostatic pressure, framework 2 undergoes a phase transition at 1.69 GPa, which is somewhat higher than the transition previously reported in 1. The Young's modulus (E) and hardness (H) of 1 (E≈8.5, H≈1 GPa) is substantially higher than that of 2 (E≈3, H≈0.1 GPa), whilst its bulk modulus is relatively lower (≈14 GPa cf. ≈16.6 GPa). The heavier, zinc-containing material was also found to be significantly harder than its light analogue. The differential behaviour of the two materials is discussed in terms of the smaller pore volume of 2 and the greater flexibility of the LiN(4) tetrathedron compared with the ZnN(4) and BN(4) units.

Computational study on the effect of anisotropy in the transversely isotropic elasticity of elastoplastic solids on depth-sensing indentation with a point-sharp indenter

Developing angular trapping methods, which enable optical tweezers to rotate a micronsized bead, is of great importance for studies of biomacromolecules in a wide range of torque‐generation processes. Here a novel controlled angular trapping method based on model composite Janus particles is reported, which consist of two hemispheres made of polystyrene and poly(methyl methacrylate). Through computational and experimental studies, the feasibility to control the rotation of a Janus particle in a linearly polarized laser trap is demonstrated. The results show that the Janus particle aligned its two hemispheres interface parallel to the laser propagation direction and polarization direction. The rotational state of the particle can be directly visualized by using a camera. The rotation of the Janus particle in the laser trap can be fully controlled in real time by controlling the laser polarization direction. The newly developed angular trapping technique has the great advantage of easy implementation and real‐time controllability. Considering the easy chemical preparation of Janus particles and implementation of the angular trapping, this novel method has the potential of becoming a general angular trapping method. It is anticipated that this new method will significantly broaden the availability of angular trapping in the biophysics community.

A finite-strain phase-field model is developed for the analysis of multivariant martensitic transformation during nano-indentation. Variational formulation of the complete evolution problem is developed within the incremental energy minimization framework. Computer implementation is performed based on the finite-element method which allows a natural treatment of the finite-strain formulation and of the contact interactions. A detailed computational study of nano-indentation reveals several interesting effects including the pop-in effect associated with nucleation of martensite and the energy-lowering breakdown of the symmetry of microstructure. The effect of the indenter radius is also examined revealing significant size effects governed by the interfacial energy.

Large calcifications often develop in advanced atherosclerotic plaques. Prior computational studies showed these macrocalcifications to stabilize arterial wall stress with calcification moduli (E calc) of 2.5 MPa. However, recent nanoindentation studies measured E calc as 10–25 GPa, suggesting underestimation by up to 104 in previous models. This study investigated the effects of E calc and calcification geometry on stress in models of atherosclerotic plaque, with the modifying factor of fibrous component constitutive relation. Stress was calculated in idealized plane-strain finite element models of pressurized coronary arteries containing calcified lesions. Lesions were modeled as arc-shaped, circular, and elliptical regions with varying lumen separation, length, and thickness. E calc varied from 1.0 MPa to 10 GPa. Various orthotropic and hyperelastic constitutive relations for arterial wall and fibrous plaque were assigned, representing a range of literature values. In all models, stress concentration at the calcification-fibrous plaque interface increased with increasing E calc, with highest stresses in orthotropic models for higher Poisson’s ratios and lower radial and circumferential moduli. This effect was more pronounced in arc-shaped calcifications and was highly sensitive to geometry, with peak stress dependent on calcification distance from the lumen and increasing dramatically with increased length and decreased thickness. This study indicates the importance of using accurate material properties and geometries in models of atherosclerotic arteries. Results suggest calcification geometry, rather than calcification area, is a better predictor of high stresses in the arterial cross section and that some macrocalcifications, instead of providing a stabilizing influence, may predispose a plaque to rupture at the calcified interface.