Measurement Of Power-law Creep Parameters By Instrumented Indentation Methods

New experimental methods have been developed to measure the uniaxial power-law creep parameters α [alpha] and n in the relation έ[epsilon dot]=α[alpha]σⁿ[sigma] (έ [epsilon dot] is the creep strain rate and σ [sigma] is the creep stress) from the load, time, displacement and stiffness data recorded during an instrumented indentation experiment performed with a conical or pyramidal indenter. The methods are based on an analysis of Bower et al., which relates the indentation creep rate to the uniaxial creep parameters based on simple assumptions about the constitutive behavior (Bower et al., 1993). Using finite element simulations to establish the influences of finite deformation and transients caused by elasticity, the proposed methods are explored experimentally using amorphous selenium as a model material. Cylindrical specimens compressed at high temperatures and low strain rates deformed stably into barrel-like shapes, while tests at low temperatures and high strain rates caused the sample to catastrophically shatter. These observations are consistent with the stress exponent and kinetic activation parameters extracted from the nanoindentation creep tests. With a few notable exceptions, the values of both α [alpha] and n derived from the indentation data at 35°C [Celsius] are generally in good agreement with those measured in uniaxial compression, thus demonstrating the validity of the approach. Another technique, based on the measured elastic contact stiffness and assumed values of Young’s modulus and Poisson’s ratio, is proposed as an alternative method to extract α [alpha] and n when the affect of thermal drift is such that the measured displacement in the indentation test is no longer reliable. Experimental verification of the stiffness method is provided through constant load and hold indentation creep experiments performed with a Berkovich indenter on amorphous selenium at 35°C [Celsius], where the affect of drift is extremely small, and high purity polycrystalline aluminum at 250°C [Celsius], where the affect of drift is so strong that the measured displacement is completely useless. Results show that the stiffness method can accurately predict the projected contact area and α [alpha] and n for both materials, thus demonstrating the validity of the proposed stiffness method.