Mechanical and Civil Engineering Seminar

Great progress has been made over the past decade in making mechanical property measurements at small scales by load- and depth-sensing indentation methods, also known as nanoindentation. Making such measurements with sharp pyramidal indenters allows for high point-to-point spatial mapping of properties as well as the mechanical characterization of very thin films, thin surface layers, and even small particles or individual phases in complex multiphase microstructures. Although most nanoindentation testing has been done at room temperature, recent advances in nanoindentation testing methods have expanded the horizons to very high temperatures, thus paving the way for the small-scale measurement of parameters characteristic of time-dependent creep deformation such as the stress exponent, n, and the activation energy, Qc. However, in doing so, serious experimental difficulties are often encountered, and how one converts the data obtained in nanoindentation tests to the parameters normally used to characterize uniaxial creep is not at all straightforward because of the complex, non-uniform stress state produced during indentation contact.

In this presentation, we report on progress in making meaningful measurements of power law creep parameters by nanoindentation based on recent experience with a new high temperature nanoindentation system capable of testing at temperatures up to 1100°C. Special attention is given to the models and data analysis procedures needed to convert the nanoindentation load-displacement-time data into creep parameters normally measured in uniaxial tension or compression testing. The models and procedures are evaluated by comparison to several sets of creep data in which the material behavior has been probed independently by both nanoindentation and uniaxial testing methods.

* Work sponsored in part by the National Science Foundation through grant number DMR-174343.