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Plasticity in material is typically described as a function of strain, but recent observations from torsion and indentation experiments in metals suggested that plasticity is also dependent on strain gradient. The effects of strain gradient on plastic deformation in thermosetting epoxy and polycarbonate thermoplastic were experimentally investigated by nanoindentation and atomic force microscopy in this study. Both thermosetting and thermoplastic polymers exhibited hardening as a result of imposed strain gradients. Strain gradient plasticity theory developed on the basis of a molecular kinking mechanism has predicted strain gradient hardening in polymers. Comparisons made between indentation data and theoretical predictions correlated well. This suggests that strain gradient plasticity in glassy polymers is determined by molecular kinking mechanisms.

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The elastic modulus ( E ) , the critical strain energy release rate (GJ, and the flexural strength (u) have been determined for two partially dense alumina bodies produced from' the same powder but with different initial densities. The mechanical properties were measured for specimens fabricated at four different relative densities. The measured elastic modulus, critical strain energy release rate, and a calculated critical stress intensity factor ( K , ) were observed to be linearly related to (p -p,,)/(l -p,,), where p is the current relative density and po is the initial relative density of the powder compact. With the observed linear relations for E , G, (or K c ) , and the assumption that the crack length responsible for failure was present in the initial powder compact and shrunk in proportion to the relative density change, a Griffith equation was constructed to estimate the strength at any relative density. This relation was in good agreement with measurements.

Structural development of tellurite glasses with the addition of LizO and NazO has been studied using infrared, Raman, and X-ray photoelectron spectroscopies. The increase in intensity of the peak at 755 cm-' in the infrared spectra as compared to the peak at 620 cm-' suggests the transformation of Te04 building units to Te03 pyramids with the addition of alkali oxide. Proposed structural change is further supported by the strong compositional dependence of the 755-cm-' peak in the Raman spectra as well as by the formation of a shoulder in the 0 l s peak of X-ray photoelectron spectra. In contrast to alkali silicate glasses, formation of nonbridging oxygens with the addition of alkali oxide is not observed. [

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. Purpose:  Intraocular pressure (IOP) measured using Goldmann Applanation Tonometry (GAT) changes with individual’s corneal properties, but the method to measure the in vivo corneal material properties to account for individual variation in GAT IOP is not available. In this study, a new method to measure the IOP‐dependent corneal tangent modulus in vivo is developed to address this research gap. Methods:  Instrumented indentation and analysis were developed to measure the corneal tangent modulus. The validity of the method and procedure was verified using model silicone eye pressurized to different IOP. In addition, 15 porcine eyes and 3 rabbit eyes were tested using the corneal indentation at different set intraocular pressure and different indentation rates. Results:  The results from silicone eye showed that the measured tangent modulus is in good agreement with the standard silicone rubber modulus. The results on the porcine eyes and rabbit eyes showed that the method can be used to measure corneal tangent modulus in vivo in the human range of intraocular pressure from 10 to 40 mmHg. Conclusions:  An indentation method to measure the corneal tangent modulus in vivo was developed, and the IOP dependence of the corneal tangent modulus was characterized. The developed indentation method provides a new means to measure the in vivo corneal tangent modulus to account for individual and pressure variations in measurement of intraocular pressure.