It is 100 years since the well-know Mohr-Coulomb strength theory was established in 1900. A considerable amount of theoretical and experimental research on strength theory of materials under complex stress state was done in the 20th Century. This review article presents a survey of the advances in strength theory (yield criteria, failure criterion, etc) of materials (including matellic materials, rock, soil, concrete, ice, iron, polymers, energetic material, etc) under complex stress, discusses the relationship among various criteria, and gives a method of choosing a reasonable failure criterion for applications in research and engineering. Three series of strength theories, the unified yield criterion, the unified strength theory, and others are summarized. This review article contains 1163 references regarding the strength theories. This review also includes a biref discussion of the computational implementation of the strength theories and multi-axial fatigue.

Advances in strength theories for materials under complex stress state in the 20th Century

A system, method and apparatus for eliminating image tearing effects and other visual artifacts perceived when scanning moving subject matter with a scanned beam imaging device. The system, method and apparatus uses a motion detection means in conjunction with an image processor to alter the native image to one without image tearing or other visual artifacts. The image processor monitors the motion detection means and reduces the image resolution or translates portions of the imaged subject matter in response to the detected motion.

Sbi motion artifact removal apparatus and method

An apparatus and method for allowing multiple high and low resolution SBI and conventional FPA imaging devices to use a common high resolution monitor and archive device without increasing or significantly changing the footprint of existing devices. This system and method uses a frame grabber for digitizing video from the legacy FPA devices, a frame mapper for rendering or mapping the FPA video into the SBI digital format, a converter for rasterizing SBI data streams into pixel-oriented FPA video frames, an input selector for selecting which FPA or SBI imaging device to display on a high resolution monitor, an processor for storing and manipulating frames of video, a video output encoder for converting the SBI frames into a video signal appropriate for display on the high resolution monitor, and an output means for connecting to a storage device for archiving video and images.

Combined SBI and conventional image processor

The mechanical behavior of metals has been investigated by many researchers since long ago. A large number of experimental works on the plastic behavior of metals are found from earlier times. In the initial stage of those studies [1], uniaxial behavior of various metals is examined, as related to yield points and the Bauschinger effect. From the beginning of the twentieth century [10], systematic experiments are conducted on the mechanical behavior of metals under combined stress state. These studies have started by the determination of the initial yielding of metals and were followed by the verification of the validity of the flow theory and the deformation theory [15, 68] for the case of non-proportional loading. A detailed chronological table of research on experimental plasticity is given in the article [11].

Experimental Plasticity on the Anisotropy of Metals

Shape memory alloys (SMAs) are a class of smart materials characterized by shape memory effect and pseudo-elastic behavior. They have the capability to retain their original form when subjected to certain stimuli, such as heat or a magnetic field. These unique properties have attracted many researchers to seek their application in various fields including transportation, aerospace, and biomedical. The ease process adaption from semiconductor manufacturing technology provides many opportunities for designing micro-scale devices using this material. This paper gives an overview of the fabrication and manufacturing technique of thin-film and bulk micromachined SMAs. Key features such as material properties, transformation temperature, material composition, and actuation method are also presented. The application and micromechanism for both thin-film and bulk SMA are described. Finally, the microactuator devices emphasized for biomedical applications such as microgrippers and micropumps are highlighted. The presented review will provide information for researchers who are actively working on the development of SMA-based microscale biomedical devices.

https://www.mdpi.com/2072-666X/6/7/879/pdf

Micromachined Shape-Memory-Alloy Microactuators and Their Application in Biomedical Devices

The manufacturing methods according to the present teachings provide shape memory alloy products having both a uniform composition and a precise shape memory recovery temperature. In this manufacturing method, raw material powders (e.g., Ti and Ni powders) may be precisely mixed. Next, a compound may be synthesized from the raw material powder mixture using a combustion synthesis method. The combustion synthesized compound may be melted and cast into a desired shape (e.g., a shape of the final product or a shape close to that of the final product).

Methods for making shape memory alloy products

The hypothesis of local determinability proposed by V.S. Lensky was experimentally confirmed to hold with high accuracy on a curved part of strain path. That is, a rate dθ/ds (θ is a difference between directions of stress and strain increment vectors, and s is a length of strain path) depends only on the current values of θ(s) and curvature K(s) of strain path. Experiments were performed along some strain paths with two circular parts in a two-dimensional space of strain, by applying combined loads of axial force and torque to thin-walled tubular specimens of initially isotropic brass. A simple stress-strain relation which reflects the curvature effect was formulated by using the hypothesis for a practically important set of strain paths with intermediate curvature.

On the Hypothesis of Local Determinability and a Concise Stress-Strain Relation for Curved Strain Path

A plastic deformation whose strain trajectory has constant curvature is investigated on mild steel by applying the combined load of axial force and torsion to thin-walled tubular specimen. The experiment is performed by using a full-automatic complex loading testing machine. Experimental result shows that the relation between the magnitude of stress vector and the arc length of plastic strain trajectory depends mainly on the curveture, and the curve showing such a relation appears higher than the hardening curve for pure torsion. On the other hand, cyclic loading makes the curve lower than the hardening curve for pure torsion owing to the effect of cyclic softening. This may be an experimental evidence that the conventional flow theory is not satisfactory for complex loading. However, the flow theory may be found as a sufficient approximation to plastic deformation under complex loading with strain trajectory of constant radius of curvature larger than 2 percent.

Plastic deformation of mild steel under combined load of axial force and torsion with strain trajectories of constant curvature

As a fundamental example of experimental work to examine the plastic behaviour of metals for deformations along curved strain trajectories under complex loadings, an experiment was performed on the deformation of a thin-walled tubular specimen of brass along orthogonal bilinear trajectories with a rounded corner for various constant radii under the combined loadings of axial force and torque. As the result of experiment, the variation of deformation behaviour under complex loading was clarified in the decreasing stage of the curvature of the rounded corner by comparing the behaviour when there is a significant history effect at the point corner to the behaviour along the so-called trajectory with small curvature which is close to that of proportional deformation. The experimental result was expressed as a vector relation of integral type between stress- and strain-increment vectors. The influence functions in this relation were determined according to the features of the experimental data with the use of an intrinsic time-scale in referring to the endochronic theory proposed by V alanis (1971). The stress-strain relation thus obtained was found to approximate the experimental result with high accuracy.

Effect of curvature of the strain trajectory on the plastic behaviour of brass

Self-propagating high-temperature synthesis (SHS) is a new method of making an ingot of TiNi. This method shows little gravity segregation. The purpose of this study is to investigate the difference of the shape memory characteristics of the TiNi casting alloys made from a SHS ingot and from a conventional melt cast ingot. The samples used in this study were rods made by centrifugal casting. Differential scanning calorimetry (DSC), X-ray diffraction, and tensile test were used to examine the shape memory characteristics on the samples. The heat treatment conditions were 773 K–1.8 ks and 1073 K–3.6 ks, respectively. The DSC samples were both ends (the top and bottom area) of the rod samples. The results of the XRD measurements showed that TiNi phase was obtained in all the samples. In contrast, the result of the DSC test showed that more gravity segregation effect happened in the melt cast sample than in the SHS sample. As the conclusion of this study, gravity segregation had little effect in the SHS ingot sample.

Masataka Tokuda

Papers

Methods for making shape memory alloy products

On the Hypothesis of Local Determinability and a Concise Stress-Strain Relation for Curved Strain Path

Plastic deformation of mild steel under combined load of axial force and torsion with strain trajectories of constant curvature

Effect of curvature of the strain trajectory on the plastic behaviour of brass

Shape memory characteristics of TiNi casting alloys made by using self-propagating high-temperature synthesis