Modeling nanoindentation using the Material Point Method
- PDF / 611,253 Bytes
- 13 Pages / 584.957 x 782.986 pts Page_size
- 7 Downloads / 253 Views
A numerical nanoindentation model was developed using the Material Point Method (MPM), which was chosen because it can handle both large deformations and dynamic contact under the indenter. Because of the importance of contact, prior MPM contact methods were enhanced to improve their accuracy for contact detection. Axisymmetric and full 3D simulations investigated the effects of hardening, strain-rate dependent yield properties, and local structure under the indenter. Convergence of load–displacement curves required small cells under the indenter. To reduce computation time, we used an effective nonregular grid, called a tartan grid and describe its implementation. Tartan grids reduced simulation times by an order of magnitude. A series of simulated load–displacement curves were analyzed as “virtual experiments” by standard Oliver–Pharr methods to extract effective modulus and hardness of the indented material. We found that standard analysis methods give results that are affected by hardening parameters and strain-rate dependence of plasticity. Because these parameters are not known during experiments, extracted properties will always have limited accuracy. We describe an approach for extracting more properties and more accurate properties by combining MPM simulations with inverse methods to fit simulation results to entire load–displacement curves.
I. INTRODUCTION
Humans have been using indentation to test material properties since the first person poked a stick into soft ground to see if it was firm enough to walk on. More modern techniques are described by Oliver and Pharr1 who developed a method for analyzing microscale indentation experiments using the maximum indentation load, maximum indentation depth, and initial unloading stiffness. This method for analyzing nanoindentation, which is generally referred to as the “Oliver–Pharr method,” extracts material properties from nanoindentation, load–displacement curves. Since then, much work has been done in analyzing, numerically modeling, and developing new experimental techniques. Most numerical modeling has used finite element analysis.2–4 This paper describes a new simulation method for modeling nanoindentation using the particle-based Material Point Method (MPM). MPM has been used for modeling nanoindentation experiments5 and for modeling coupled with molecular dynamics.6 This paper describes new axisymmetric and 3D MPM simulations of nanoindentation that added three improvements to increase accuracy and efficiency of prior MPM simulations.5 First, accurate modeling of nanoindentation requires that contact between the indenter and the material is well modeled. We describe an a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2018.75
improvement to the standard MPM contact algorithms7–9 that more accurately detects contact based on displacements of the two material surfaces. Second, converged nanoindentation results require small cells under the indenter. We describe a mesh refinement scheme, called a “tartan” grid
Data Loading...
