Publisher Summary This chapter focuses on micromechanics of crystals and polycrystals. In Section II of the chapter, a brief outline of only some of the important features of the micromechanics of crystalline plasticity is given. The discussion is confined to plastic flow caused by dislocation slip, and face-centered-cubic crystals are used in the examples of dislocation mechanisms. Particular attention is paid to kinematics and to the phenomenology of strain hardening, because these are shown to play dominant roles in macroscopic response. In Section III, constitutive laws for elastic-plastic crystals are developed. The framework draws heavily on Hill's analysis of the mechanics of elasticplastic crystals, but the theory is extended by incorporating the possibility of deviations from the Schmid rule of a critical resolved shear stress for slip. Deviations from the Schmid rule are motivated by micromechanical models for dislocation motion and are shown to lead to deviations from the “normality flow rule” of continuum plasticity. The implications of these “non-Schmid effects” regarding the stability of plastic flow are brought out via some examples of models for kinks bands and shear bands in Section IV. In Section IV some examples of analyses of elastic-plastic deformation in crystals are discussed. The chapter concludes with some suggestions for fruitful research. These involve extensions of the theory to finite-strain rate-dependent polycrystalline models.

Micromechanics of Crystals and Polycrystals

Within the last several years, the field of terahertz science and technology has changed dramatically. Many new advances in the technology for generation, manipulation, and detection of terahertz radiation have revolutionized the field. Much of this interest has been inspired by the promise of valuable new applications for terahertz imaging and sensing. Among a long list of proposed uses, one finds compelling needs such as security screening and quality control, as well as whimsical notions such as counting the almonds in a bar of chocolate. This list has grown in parallel with the development of new technologies and new paradigms for imaging and sensing. Many of these proposed applications exploit the unique capabilities of terahertz radiation to penetrate common packaging materials and provide spectroscopic information about the materials within. Several of the techniques used for terahertz imaging have been borrowed from other, more well established fields such as x-ray computed tomography and synthetic aperture radar. Others have been developed exclusively for the terahertz field, and have no analogies in other portions of the spectrum. This review provides a comprehensive description of the various techniques which have been employed for terahertz image formation, as well as discussing numerous examples which illustrate the many exciting potential uses for these emerging technologies.

https://iopscience.iop.org/article/10.1088/0034-4885/70/8/R02/pdf

Imaging with terahertz radiation

A review of the various optical methods to detect ultrasound (bulk and surface waves) at the surface of opaque solids is presented. The most useful techniques are thoroughly analyzed. Their performance when nonideal conditions are encountered, such as vibrations, air turbulence, and rough light scattering surfaces is evaluated. This review includes a description of knife-edge techniques, optical heterodyning, differential interferometry, and velocity (time-delay) interferometry methods, plus a mention of various less-important tech-

Optical Detection of Ultrasound

The nondestructive assessment of the damage that occurs in components during service plays a key role for condition monitoring and residual life estimation of in-service components/structures. Ultrasound has been widely utilized for this; however most of these conventional methods using ultrasonic characteristics in the linear elastic region are only sensitive to gross defects but much less sensitive to micro-damage. Recently, the nonlinear ultrasonic technique, which uses nonlinear ultrasonic behavior such as higher-harmonic generation, subharmonic generation, nonlinear resonance, or mixed frequency response, has been studied as a positive method for overcoming this limitation. In this paper, overall progress in this technique is reviewed with the brief introduction of basic principle in the application of each nonlinear ultrasonic phenomenon.

Nonlinear ultrasonic techniques for nondestructive assessment of micro damage in material: A review

Visualization of stress distribution has been realized by a nondestructive mechanoluminescence (ML) from SrAl2O4:Eu, which can emit three magnitudes higher visible light than that of well-known ML substance of quartz. A simulation result confirms that such a ML image successfully reflects the stress distribution. A kinetic model for ML of SrAl2O4:Eu is proposed.

Direct view of stress distribution in solid by mechanoluminescence

Non-contact ultrasonic techniques.

Thermographic investigation of high-power ultrasonic heating in materials

There is currently widespread use of high-strength steel tendons for prestressing and post-tensioning of concrete structures and as suspension cables for long-span bridges. Tendons normally consist of one or more seven-wire, helically wound steel strands or solid rods. There are more than 130,000 prestressed bridges in the United States that contain these tendons, and approximately 3,000 new bridges are constructed each year. The prestressing tendons are critical structural elements because the forces in the tendons counteract tensile stresses in the concrete that result from loads acting on a bridge. The tendons are frequently inaccessible for visual inspections and there is currently no accepted nondestructive evaluation technique to assess the condition of these tendons. The goal of this research is to examine ultrasonic stress measurement techniques for the condition assessment of prestressing tendons. This information could be used to compare the actual force in the tendon with its design va...

Velocity Constants for Ultrasonic Stress Measurement in Prestressing Tendons

A computer-controlled narrowband ultrasonic pulser/receiver system was used to determine changes in the coefficient of attenuation and the acoustic velocity, with high precision, in pearlitic steel specimens during cyclic tensile fatigue. The specimens were interrogated through the fatigue-damaged zone in a direction normal to the loading and parallel to the crack formation planes, using shear wave bursts at 5 MHz. Attenuation and velocity are found to be sensitive to stress accumulation and dislocation pileup, but appear to be unaffected by the onset and growth of microcracks. Using the Granato and Lucke dislocation damping theory, the change in the coefficient of attenuation and the acoustic velocity was used to derive the relative change in dislocation density and loop length. The change these four parameters experience is more pronounced during the early stages of fatigue, when dislocations are mobile, than during the more advanced stages of fatigue. As fatigue damage accumulates, dislocations begin to multiply, tangle up and their mobility diminishes progressively. Consequently, the rate at which these four parameters change decreases with the diminishing mobility of dislocations.

Ultrasonic monitoring of dislocations during fatigue of pearlitic rail steel

Laser based and air coupled ultrasound test methods are used for testing for internal and surface breaking cracks in railroad tracks. Signals were generated with an infrared pulse laser and detected with a confocal Fabry-Perot interferometer (CFPI) or a micromachined capacitive air coupled transducer. The experiments demonstrate the flexibility and capability of this hybrid technique to detect cracks that are undetectable with current testing techniques. Cracks of different orientations and locations on the rail head, web and base were interrogated, in addition to other common discontinuities in the rail track. The noncontact and remote nature of these techniques renders them suitable for inservice applications.

Robert E. Green

Papers

Non-contact ultrasonic techniques.

Thermographic investigation of high-power ultrasonic heating in materials

Velocity Constants for Ultrasonic Stress Measurement in Prestressing Tendons

Ultrasonic monitoring of dislocations during fatigue of pearlitic rail steel

Laser Based and Air Coupled Ultrasound as Noncontact and Remote Techniques for Testing of Railroad Tracks