*Editorial* **Dislocation Mechanics Pile-Up and Thermal Activation Roles in Metal Plasticity and Fracturing**

## **Ronald W. Armstrong**

Center for Engineering Concepts Development, Department of Mechanical Engineering, University of Maryland, College Park, MD 20742, USA; rona@umd.edu; Tel.: +1-410-723-4616

Received: 14 January 2019; Accepted: 27 January 2019; Published: 31 January 2019

**Abstract:** Dislocation pile-up and thermal activation influences on the deformation and fracturing behaviors of polycrystalline metals are briefly reviewed, as examples of dislocation mechanics applications to understanding mechanical properties. To start, a reciprocal square root of grain size dependence was demonstrated for historical hardness measurements reported for cartridge brass, in line with a similar Hall-Petch grain size characterization of stress-strain measurements made on conventional grain size and nano-polycrystalline copper, nickel, and aluminum materials. Additional influences of loading rate (and temperature) were shown to be included in a dislocation model thermal activation basis, for calculated deformation shapes of impacted solid cylinders of copper and Armco iron materials. Connection was established for such grain size, temperature, and strain rate influences on the brittle fracturing transition exhibited by steel and other related metals. Lastly, for AISI 1040 steel material, a fracture mechanics based failure stress dependence on the inverse square root of crack size was shown to approach the yield stress at a very small crack size, also in line with a Hall-Petch dependence of the stress intensity on polycrystal grain size.

**Keywords:** dislocation mechanics; yield strength; grain size; thermal activation; strain rate; impact tests; brittleness transition; fracturing; crack size; fracture mechanics
