An Experimental Investigation of Creep and Viscoelastic Properties Using Depth-Sensing Indentation Techniques
Date of Award
Doctor of Philosophy
Materials Science and Engineering
Carl J. McHargue
Warren C. Oliver, Ben F. Oliver, Thomas G. Carley
Broad band, quasi-static and frequency specific dynamic techniques adapted to depth-sensing indentation testing have been utilized to measure both time dependent plasticity (creep) at both room and elevated temperatures as well as time dependent elasticity (viscoelasticity) at room temperature.
Using a variety of depth-sensing indentation techniques at both room and elevated temperatures, the dependency of the indentation hardness on the variables of indentation strain rate (stress exponent for creep, n) and temperature (apparent activation energy for creep, Q), and the existence of a steady state behavior in an indentation creep test with a Berkovich indenter were investigated. The indentation creep response of five materials, Pb-65 at% In (at RT), high purity indium (from RT to 75°C), high purity aluminum (from RT to 250°C), a vapor deposited amorphous alumina film (at RT), and single crystal alumina (sapphire) (at RT), was measured. It was shown for the first time that the indentation strain rate, defined as ˙h/h,could be held constant during an experiment using a Berkovich indenter by controlling the loading rate such that the loading rate divided by the load, ˙P/P, remained constant. This technique yields the most unambiguous determination of the stress exponent for creep and seems to most closely approximate the steady state results from uniaxial testing. The results from the constant ˙P/P experiments were compared to the results from conventional indentation creep experiments where the load is ramped on at a high rate and then held constant for a period of time. It was shown that the transition from the loading segment to hold segment shows a transient period with an apparent higher stress exponent for creep which has previously been mistaken for power law breakdown type behavior. The apparent activation energy for indentation creep in indium was found to be approximately 78 kJ/mol, in excellent agreement with the activation energy for self diffusion in the material. The temperature dependence of the indentation creep process in aluminum was found to be best described by an effective diffusion coefficient as described in the literature for bulk aluminum at intermediate temperatures. By performing ˙P/P change experiments it was shown that a steady state path independent hardness could be reached in an indentation test with a geometrically similar indenter. The arrival at a new steady state value of hardness seems to depend on the accumulation of strain rather than a relaxation time. The measurements on amorphous alumina and sapphire demonstrate the technique's ability to measure differences in the time dependent response of materials that can not be tested with other techniques.
Using a frequency specific dynamic indentation technique, a general method to measure the linear viscoelastic properties of polymers was determined. The polymer tested was an amorphous unvulcanized natural rubber, poly-cis 1,4-isoprene. By imposing a small harmonic force excitation on the specimen during the indentation process and measuring the displacement response at the same frequency, the complex elastic modulus, G*=G'+ iG" , of the polymer was determined. The portion of the displacement signal which is "in phase" with the excitation represents the elastic response of the contact and is related to the stiffness (S) of the contact and to the elastic modulus, or storage modulus (G') , of the material. The "out of phase" portion of the displacement signal represents the energy being absorbed by the material, i.e. the damping (Cω where ω=2 π f) of the contact, and thus the loss modulus (G") of the material. It was shown that the storage, S, and loss, Cω, components of the displacement response scale as the respective component of the complex modulus multiplied by the square root of the contact area.
Lucas, Barry Neal, "An Experimental Investigation of Creep and Viscoelastic Properties Using Depth-Sensing Indentation Techniques. " PhD diss., University of Tennessee, 1997.