Variable Material Length Scale Associated with Nanoindentation Experiments

Material length scale that can be used in nonlocal gradient theories is obtained in this work based on experimental observations for two metals using nanoindentation experiments. The materials are cold rolled 1018 steel and oxygen free high conductivity copper. A fixed value of the material length scale is not always realistic and different problems under various conditions could require different values. Therefore, two models are proposed for a dynamic length scale that depends on strain rates and temperature. First the model is physically based, with parameters related to dislocation densities. This model introduces strain rate and temperature dependency in a coupled form. The second model is a phenomenological one that is based on hardness tests. Both models show that length scale decreases with increasing equivalent strain rate. The temperature effects are not studied in this work.