An investigation of flow patterns and hardness distributions using different anvil alignments in high-pressure torsion

A super duplex stainless steel was selected as a model material to evaluate the origins of unusual double-swirl flow patterns that have been reported on the upper surfaces of discs processed by high-pressure torsion (HPT). The experiments were conducted by making changes in the anvil alignment prior to HPT processing. Experiments were conducted under two different conditions: using essentially a perfect anvil alignment and with an initial anvil misalignment of 100 μm. The experimental results show that no double swirls are visible on the surfaces of discs processed under conditions of perfect anvil alignment but double swirls become visible when processing with a misalignment of 100 μm. The presence of double swirls also affects the measured hardness distributions, and for a misalignment of 100 μm the hardness distribution along a diameter may be non-symmetric with respect to the centre of the disc.     

An evaluation of the shearing patterns introduced by different anvil alignments in high-pressure torsion

A two-phase duplex stainless steel was used to investigate the influence of different amounts of anvil misalignment on the shearing patterns produced during high-pressure torsion (HPT) and subsequently on the hardness distributions across the disc upper surfaces. The results show that for a perfect alignment of the anvils, to within <25 μm, the discs exhibit regular flow patterns as anticipated from conventional analysis whereas for anvil misalignments of 100 or 200 μm the discs develop double-swirl patterns on the upper surfaces. These double swirls increase in size with increasing anvil misalignments and decrease in size with increasing numbers of torsional turns. The origin of these double swirls is investigated by examining a model based on a buckling theory for loaded beams and the development of instabilities within the discs during HPT processing. It is demonstrated that this model provides a satisfactory explanation for the reports of swirls and vortices in samples processed by HPT.     

Effect of anvil roughness on the flow patterns and hardness development in high-pressure torsion

Two sets of anvils having different surface roughness were used to systematically investigate the flow patterns developed on the top and bottom surfaces of stainless steel discs with an anvil misalignment of 100 µm during high-pressure torsion. It is shown that the flow patterns on the disc surfaces have different variation tendencies depending on whether the anvils have rough or smooth surfaces. Double-swirl flow patterns were observed on the top and bottom surfaces of discs after 1 and 5 turns when using an anvil with a smooth surface. In contrast, when using an anvil with a rough surface the double-swirl flow patterns appeared only on the top surface after 1 turn and a single swirl appeared on both surfaces after 5 turns. Hardness measurements on the top surfaces showed that discs processed using an anvil with a rough surface have greater hardness than discs processed using an anvil with a smooth surface. There was no obvious hardness difference on the bottom surfaces for discs processed using anvils with rough or smooth surfaces.     

Using finite element modeling to examine the flow processes in quasi-constrained high-pressure torsion

Finite element modeling was used to examine the flow processes in high-pressure torsion (HPT) when using quasi-constrained conditions where disks are contained within depressions on the inner surfaces of the upper and lower anvils. Separate simulations were performed using applied pressures from 0.5 to 2.0 GPa, rotations up to 1.5 turns and friction coefficients from 0 to 1.0 outside of the depressions. The simulations demonstrate the distribution of effective strain within the depressions is comparable to the prediction by ideal torsion, and the applied pressure and the friction coefficient outside the depressions play only a minor role in the distribution of effective strain. The mean stresses during processing vary linearly with the distance from the center of the disk such that there are higher compressive stresses in the disk centers and lower stresses at the edges. The torque required for rotation of the anvil is strongly dependent upon the friction coefficient between the sample and the anvil outside the depressions.     

Inhomogeneous softening during annealing of ultrafine-grained silver processed by HPT

The softening in ultrafine-grained silver processed by high-pressure torsion (HPT) was studied during annealing using differential scanning calorimetry (DSC). Two separate exothermic peaks were observed in the DSC thermogram of the HPT-processed sample. It is shown that the first and the second peaks are related to the recrystallization of the middle volume and the surface regions of the HPT-processed disk, respectively. Therefore, a very inhomogeneous sandwich-like microstructure develops during annealing with a soft interior and hard surface layers. The lower thermal stability of the middle region appears to be related to the stronger twinning activity since the twinned volumes can act as nuclei for recrystallized grains. The higher twin-fault probability in the interior is attributed to the larger strain due to the outflow of material between the anvils of the HPT facility during quasi-constrained processing.     

Thin-film thickness and density determination from x-ray reflectivity data using a conventional power diffractometer

The determination of thin-film thickness and density from X-ray specular and off-specular reflectivity data obtained using a conventional powder diffractometer has been investigated. An analysis of specular reflectivity data for a 565.9 Å Pt film showed that the results were determined precisely and agreed with those obtained previously from a high-resolution reflectometer. A systematic study of the effect of film-surface misalignment revealed that the values of thickness were insensitive to the alignment. A precision of about 1% or less was obtained from off-specular reflectivity data with a surface misalignment of 0.20° or less. The insensitivity makes conventional powder diffractometers attractive for film thickness determination and opens this technique to many laboratories. The values of density were found to be relatively more sensitive to surface alignment. Nevertheless, densities with a precision of 3% and better were obtained when the film surfaces were aligned to within the effective divergence of the incident X-ray beam.     

Different models of hardness evolution in ultrafine-grained materials processed by high-pressure torsion

High-pressure torsion (HPT) is an attractive processing method in severe plastic deformation techniques involving the application of high compressive pressure with concurrent torsional straining. Excellent grain refinement is anticipated when using this technique to average grain sizes of the submicrometer or even nanometer ranges. Because of the significant microstructural changes during processing, there are numerous reports showing evolution in local hardness toward homogeneity throughout a disk diameter with increasing numbers of revolutions. The achieved hardness after HPT is mostly much higher than that in the as-received condition because of exceptional grain refinement although there are a limited number of metals and alloys showing softening or weakening after HPT processing. This paper was initiated to review recent discoveries in the experimental results on hardness evolution toward homogeneity during HPT processing and discuss the different models of hardness developments with respect to imposed equivalent strain by HPT processing for a range of metals and alloys. Moreover, recent results of hardness homogeneity and heterogeneity through thicknesses of the processed disks are discussed toward a complete understanding of hardness evolution in the UFG metals processed by HPT.     

Noise-equivalent change in radiance for misalignment noise in a double-sided interferogram

Engineers designing optical alignment servo systems for Michelson interferometers and Fourier-transform infrared spectrometers need to predict the amount of noise expected from the small and randomly varying amounts of misalignment that occur as the servo attempts to maintain alignment while taking data. A formula is derived for the noise-equivalent change in radiance due to this effect and the formula's accuracy is demonstrated by comparison of its predictions to the errors found in simulated interferometer measurements contaminated by misalignment noise.     

Unusual macroscopic shearing patterns observed in metals processed by high-pressure torsion

A duplex stainless steel was processed by high-pressure torsion (HPT) and then examined by optical microscopy. The results reveal unusual flow patterns including double-swirl strains, shear vortices, and the presence of significant local turbulence. Similar flow behavior was also visible in disks of high-purity aluminum and a Zn–22%Al eutectoid alloy. These complex flow patterns and the presence of double-swirls are consistent with the presence of a Kelvin–Helmholtz instability during HPT processing where this may arise if there are local shear velocity gradients between adjacent positions within the HPT disks.     

Photodeformable Azobenzene‐Containing Liquid Crystal Polymers and Soft Actuators

Photodeformable liquid crystal polymers (LCPs) that adapt their shapes in response to light have aroused a dramatic growth of interest in the past decades, since light as a stimulus enables the remote control and diverse deformations of materials. This review focuses on the growing research on photodeformable LCPs, including their basic actuation mechanisms, the various deformation modes, the newly designed molecular structures, and the improvement of processing techniques. Special attention is devoted to the novel molecular structures of LCPs, which allow for easy processing and alignment. The soft actuators with various deformation modes such as bending, twisting, and rolling in response to light are also covered with the emphasis on their photo‐induced bionic functions. Potential applications in energy harvesting, self‐cleaning surfaces, sensors, and photo‐controlled microfluidics are further illustrated. The existing challenges and future directions are discussed at the end of this review.     

Ge2Sb2Te5‐Based Tunable Perfect Absorber Cavity with Phase Singularity at Visible Frequencies

The metal‐dielectric stacks‐based asymmetric Fabry–Perot (F–P) cavity systems have recently attracted much interest from the scientific community for realizing perfect absorption over the spectral bands from visible to infrared since they possess a lithography‐free design that is cost‐effective and scalable. This study experimentally demonstrates an asymmetric F–P cavity system for achieving tunable wide angle perfect absorption and phase singularity. The proposed system shows tunable multiband perfect absorption in the visible spectral region by incorporating an ultrathin layer of phase change material such as Ge₂Sb₂Te₅ (GST) in the stack. The system shows multi‐narrowband perfect absorption with a maximum of 99.8% at a specific incident angle and polarization state when the GST is in amorphous phase; however, the absorption bands blueshift and broaden after switching to the crystalline phase. More importantly, the proposed scheme shows tunable phase singularity at the reflection‐less point. The obtained tunable perfect absorption and abrupt phase change are solely due to the presence of a highly absorbing ultrathin layer of GST in the stack. Experimental results are validated using an analytical simulation model based on a transfer matrix method. The proposed scheme could find potential applications in active photonic devices such as phase‐sensitive biosensors and absorption filters.     

CO2激光器虚共焦腔凸面镜失调的实验研究

高功率激光器工作时，由于谐振腔反射镜上的光强分布不均匀，会导致镜面温度不均匀，从而造成镜面的角向偏转，致使激光器光轴发生偏转，严重影响输出光束质量，甚至造成不出光。文中主要从实验研究了CO2激光器虚共焦腔凸面镜失调对激光器输出光束质量的影响，实验结果表明，当凸面镜失调角接近于由理论计算而得的失调临界角时，输出光斑约为半个圆环，与理论分析非常符合，当凸面镜失调角达到约为失调临界角的4．5倍时，完全停止振荡。     

Investigation of the 2D assumption in the image‐based inertial impact test

The image‐based inertial impact (IBII) test has shown promise for measuring properties of composites at strain rates where existing test methods become unreliable due to inertial effects (> 10² s^?1 ). Typically, the IBII tests are performed with a single camera, and therefore, to use surface measurements for material property identification, it is necessary to assume that the test is two‐dimensional. In this work, synchronised ultra‐high‐speed cameras are used to quantify the relevance of this assumption when nonuniform, through‐the‐thickness loading is applied to interlaminar samples. Initial experiments revealed that an angular misalignment of approximately 1° between the impact faces of the waveguide and projectile created a bending wave that propagated along the sample behind the axial pulse. Even under these conditions, consistent measurements of stiffness were made by assuming a linear distribution of the behaviour through‐the‐thickness. When the misalignment was reduced to 0.2°, the effects on single‐sided measurements were significantly reduced. The two alignment cases were compared to show that three‐dimensional loading had a small effect on stiffness identification (approximately 5% bias) relative to failure stress (approximately 30% bias). This study highlights the importance of impact alignment for reliable characterisation of the interlaminar failure stress and was used to establish guidelines for diagnosing loading issues from single‐sided measurements.     

气囊隔振装置对中扰动控制研究*

将气囊隔振装置应用于船舶主机能大幅度衰减振动向船体的传递,但其主要难点之一是解决主机与轴系的适配性问题。主机运行过程中的各种对中扰动会增大主机的不对中量,对其进行控制是适配性研究的重要组成部分。根据气囊隔振装置的特点,提出通过气囊的充放气调整来抵消对中扰动的影响。在试验数据的基础上,分析了对中扰动控制的可行性并给出了控制方法。分析结果表明对中扰动控制能取得良好的效果。     

Using high-pressure torsion for metal processing: Fundamentals and applications

High-pressure torsion (HPT) refers to the processing of metals whereby samples are subjected to a compressive force and concurrent torsional straining. Although the fundamental principles of this procedure were first proposed more than 60 years ago, processing by HPT became of major importance only within the last 20 years when it was recognized that this metal forming process provides an opportunity for achieving exceptional grain refinement, often to the nanometer level, and exceptionally high strength. This review summarizes the background and basic principles of processing by HPT and then outlines the most significant recent developments reported for materials processed by HPT. It is demonstrated that HPT processing leads to an excellent value for the strength of the material, reasonable microstructural homogeneity if the processing is continued through a sufficient number of torsional revolutions and there is a potential for achieving a capability for various attractive features including superplastic forming and hydrogen storage. The review also describes very recent developments including the application of HPT processing to bulk and ring samples and the use of HPT for the consolidation of powders.     

Structural and hardness inhomogeneities in Mg–Al–Zn alloys processed by high-pressure torsion

Experiments were conducted to evaluate the evolution of structure and hardness in processing by high-pressure torsion (HPT) of the magnesium AZ91 and AZ31 alloys. Both alloys were processed by HPT at room temperature for 1/4, 1, and 5 turns using a rotation speed of 1 rpm. Structure observations and microhardness measurements were undertaken on vertical cross-sectional planes cut through the HPT disks. The results demonstrate that the deformation is heterogeneous across the vertical cross sections but with a gradual evolution toward homogeneity with increasing numbers of revolutions.     

Performance degradation of a Michelson interferometer when its misalignment angle is a rapidly varying, random time series

When subjected to random background vibrations, a standard, circular-aperture Michelson interferometer with a dynamic alignment servo system has a misalignment angle that is a random function of time. Here we derive formulas for the loss in performance when the misalignment angle is modeled as a stationary stochastic time series and show how these formulas are simplified when the power spectrum is band-limited white noise.     

Interpretation of hardness evolution in metals processed by high-pressure torsion

The processing of metals through the application of high-pressure torsion (HPT) provides the potential for achieving exceptional grain refinement in bulk disks. Numerous reports are now available describing the application of HPT to a range of pure metals and simple alloys. Excellent grain refinement was achieved using this processing technique with the average grain size often reduced to the nanoscale range. By contrast, the development of microstructure and local hardness is different depending upon the material properties. In order to make HPT processing more practical, it is indispensable to investigate the nature of the sample characteristics immediately after conventional HPT processing. Accordingly, this report demonstrates the different models of hardness evolution using representative materials of AZ31 magnesium alloy, high-purity aluminum, and Zn–22 % Al eutectoid alloy processed by HPT. Separate models are described for the evolution of hardness with equivalent strain, and the correlation between these models is suggested by the homologous temperature of HPT processing. A special emphasis is placed on examining the numerical expression of the level of strain hardening or softening of these metals with increasing equivalent strain.     

A frontal Delaunay quad mesh generator using the L ∞ norm

In a recent work, a new indirect method to generate all‐quad meshes has been developed. It takes advantage of a well‐known algorithm of the graph theory, namely the Blossom algorithm, which computes in polynomial time the minimum cost perfect matching in a graph. In this paper, we describe a method that allows to build triangular meshes that are better suited for recombination into quadrangles. This is performed by using the infinity norm to compute distances in the meshing process. The alignment of the elements in the frontal Delaunay procedure is controlled by a cross field defined on the domain. Meshes constructed this way have their points aligned with the cross‐field directions, and their triangles are almost right everywhere. Then, recombination with the Blossom‐based approach yields quadrilateral meshes of excellent quality. Copyright © 2013 John Wiley & Sons, Ltd.     

Effect of ply angle misalignment on out-of-plane deformation of symmetrical cross-ply CFRP laminates: Accuracy of the ply angle alignment

This paper discusses the accuracy of ply angle alignment and how it relates to out-of-plane deformation in carbon fiber reinforced plastics (CFRP) laminates. We investigated the deformation of symmetrical cross-ply laminates under hot and humid conditions. In spite of the symmetrically stacked laminates, unpredictable out-of-plane deformation occurred over time due to ply angle misalignment. The deformation was unstable and disproportionate to the absorbed moisture. A Monte Carlo simulation based on laminate theory was performed to quantify the deformation induced by the ply angle misalignment. Symmetrical cross-ply laminates were found to twist as they absorbed water when they underwent ply angle misalignments. By comparing the analytical results with experimental results, we concluded that a standard deviation of approximately 0.4° exists as ply angle misalignment in the laminates used in this study and that this slight ply angle misalignment can be a significant factor in out-of-plane deformation of cross-ply laminates.