A study of the relationship between indentation creep and uniaxial creep

by 1978- LaManna, James Anthony

The purpose of this work is to further the scientific understanding of the relationship between indentation creep and uniaxial creep. The data for this study was obtained by conducting both indentation and uniaxial creep experiments on amorphous selenium. Experiments were designed to collect data over a wide range of creep rates. Specific temperatures were chosen to collect creep data above and below the glass transition temperature of 31°C. The indentation and uniaxial compression data was used to accomplish several objectives. The first objective was to test the ability of the Oliver-Pharr stiffness equation to predict contact area. The current contact area calculation technique relies on elastic recovery to account for sink-in and can not account for pile-up. It is shown that if the modulus of the material is known as a function of temperature, the Oliver-Pharr stiffness equation can generally predict the contact area within 10 percent of areas measured from optical and interference photomicrographs. This method accurately predicts areas for predominately plastic indents that display almost no elastic recovery where photomicrographs show that the indent has experienced considerable sink-in. The second objective was to analyze Bower et. al’s prediction of the sink-in/pileup parameter, c, as a function of the creep exponent, n. The parameter c is defined as the ratio of indent contact depth to indent total depth. The calculations of the parameter c from indent profiles are within 10 percent of Bower’s model. Bower over predicts between 15 and 30 percent when c is calculated from measured areas. The third and primary objective of this study was to calculate the relationship between the uniaxial creep parameter, A, and the indentation creep parameter, B. The A/B ratio was calculated for both nominal mean pressure, that does not account for sinkin and pile up, and actual mean pressure. The A/B ratio calculated by the nominal mean pressure method, when computed with 2% plastic strain compression stresses, is consistent with Bower et. al’s prediction. The A/B ratio calculated with the actual mean iv pressure method, when computed with 2% plastic strain compression stresses, is approximately 100 percent higher then the prediction by Bower. v