**Preface to "Dislocation Mechanics of Metal Plasticity and Fracturing"**

We begin with a historical description beginning at the start of the 20th century, with a new focus on the effect of surface steps, notches, internal holes, and cracks in reducing the strength of engineering structures. In that first decade, Inglis reported pioneering mechanics calculations of the strength reduction produced by sharp notches and cracks; and, in the second decade, Griffith produced an inverse square root of crack size prediction for the fracture stress of a pre-cracked material. In the third decade, new model analyses were reported of smaller, atomic-scale "dislocation" defects determining the (permanent) plastic deformation behaviors of crystalline materials. That such dislocation defects in localized slip band "pile-ups" behave similarly to Griffith cracks on a continuum mechanics level was described both theoretically and experimentally at the beginning of the fifth decade.

In a complementary manner, the use of optical reflection microscopy to study the crystal microstructures of sectioned metal surfaces was developed by Sorby just before the beginning of the 20th century, and the follow-on discovery of crystal X-ray diffraction in the first decade of the new century led, by the middle of the 20th century, to the use of transmission electron microscopy for observing dislocations in deformed metal foils and, subsequently, to the multiple electron microscope methods that are employed today in modern research investigations probing beneath the surfaces of all types of crystals, almost all of which are full of dislocations. Such dislocations played a counterpart-biological "nematode" role in underlying the new 20th century subject of "Materials Science and Engineering".

Building onto such historical descriptions, the aim of the present *Metals* Special Issue is to provide a valuable sampling of updated research reports focusing on the strength and/or fracturing properties of a variety of modern engineering metals and their alloys. In the introductory article is given a description, based on dislocation mechanics, of the influences of polycrystalline grain size on the hardness, yield stress, and fracture stress of metals and alloys, and which influences are related to an analogously associated crack size dependence. The subsequent all-important research articles begin with a report on the serrated plastic stress–strain behavior exhibited in an aluminum–zinc–magnesium–copper alloy and analyzed in terms of mobile dislocation and atomic solute interactions. Then comes a report on crystallographic grain textures associated with elastic anisotropy measurements in steel materials, followed by an article on the evaluation of Charpy impact test measurements employed to evaluate steel loading rate and notch sensitivity dependencies. Tandem reports are given on dislocation-based assessments of severely deformed copper–zirconium alloy material strength dependencies on applied loading rate. Next, a computational model simulation is described at microscale dimensions of deformation twinning and detwinning in nanograin-sized gold–copper alloy crystals. Two reports follow: first, on the statistical aspects of dislocations tracked in small (fcc) crystal micropillars and, then, on fractal characterizations of dislocations relating to (bcc) iron micropillar test specimens. At the opposite dimensional scale, Weibull characterization of the strength of steel wires as employed in transportation-based bridge cables is reported. This subject connects with the next report on a fracture mechanics description of crack tip plasticity. Lastly, a holistic description is given of the dynamics of dislocation pile-ups in iron and steel materials as related to plastic yielding behavior, creep, and fatigue crack growth rate results. This Special Issue project has been an informative and appreciative effort for me as Guest Editor. Sincere thanks are expressed to the authors, reviewers, and especially to Ms. Maggie Guo for their super efforts in producing the present Special Issue.

**Ronald W. Armstrong**

*Editor*
