Investigation of Cyclic and Frequency Nanoindentation Effects in Polydimethylsiloxane

The nanoindentation response of polydimethylsiloxane (PDMS) is examined using single nanoindentation loading and small-scale fatigue. It is well known that viscoelastic material response is inherently related to the local loading and environmental conditions. First, quasistatic nanoindentation experiments were performed at various depths through the specimen to benchmark our nanoindentation results with literature data. The PDMS cyclic and frequency dependence to quasi-static and dynamic nanoindentation loading was studied and a ‘load/partial-unload’ technique was employed to investigate nanoindentation modulus variation through the thickness of the specimen. The frequencies of the small-scale fatigue tests were varied to study periodic response. The average indentation modulus for PDMS at 2mN load-controlled tests was 4.37 ± 0.1 MPa. The PDMS sample had an average indentation modulus value of 3.94 ± 0.06 MPa for 3mN load-controlled tests. The indentation moduli decreased as the maximum depth increased because the stiffness reduced when indentations were performed further from the surface. The single nanoindentation data was confirmed with literature values and validated the precision of nanoindentation testing. Small-scale fatigue tests were implemented at 50 cycles with frequencies of 1, 0.5, and 0.033 Hz. The lower frequencies displayed an increase in maximum depth at a given controlled load due to relaxation and creep effects. As with the single nanoindentations, the small-scale fatigue tests confirmed the decreasing trend of indentation moduli as the maximum depth increased. Overall, the two nanoindentation methods corroborated similar trends in changes of the PDMS mechanical response.

An Investigation on the Viscoelastic Behavior of Eggshell Membrane by Nanoindentation Technology

The mechanical properties of eggshell membrane (ESM) are comparable to those of human patellar tendon or skin. This paper, based on static and dynamic nanoindentation technique, studied the time-dependent behavior, i.e., creep and relaxation of the ESM. The effect of loading level, loading rate and holding time on viscoelastic behavior were discussed by using the static measurement method of nanoindentation. The storage modulus, loss modulus and loss factor were obtained as a function of frequency. It was found that the outer membrane (OM) exhibits more apparent relaxation properties and stronger capacity to resist creep deformation than that of the inner membrane (IM). In addition, the loss factor of the IM is larger than that of the OM which caused a larger viscous damping of IM. This work can contribute to the bioinspired applications of ESM.