Initial Compression Stage Research Articles

The effect of twinning on dynamic recrystallization behavior of Mg-Gd-Y alloy during hot compression

The effects of twinning on the dynamic recrystallization (DRX) of the Mg-6.58Gd-5.7Y-0.55Zr alloy were investigated through uniaxial compression tests at 400 °C and 450 °C with an initial strain rate of 0.001–0.1 s−1 and were characterized through optical microscopy (OM), scanning electron microscopy (SEM), electron back-scattering diffraction (EBSD) and transmission electron microscopy (TEM). At a relatively high strain rate, extension twins can mainly be activated during the initial stage of hot compression, and the subsequent DRX is closely related to the twin-twin and twin-dislocation interactions. The intersected twin boundaries of the extension twins from multiple twin variants may provide effective DRX sites and promote microstructure refinement. However, extension twins from a single twin variant pair may quickly grow, soon swallowing the matrix and decreasing the DRX efficiency.

Characteristics of self-potential of coal samples under uniaxial compression

Deformation and fracture of a coal body under load may cause potential changes. Using coal samples under uniaxial compression, the coupling relationships between coal mass potential (surface potential) and stress during a complete stress–strain process were determined. Based on the potential difference of coal samples during loading rupture, the time–space evolution signatures of the electrical potential and stress of bituminous and anthracitic coal samples under load during a complete stress–strain process were dynamically depicted with high precision. Experimental results demonstrated that the parameters of self-potential and stress of these coals showed consistent variation with change of uniaxial loading pressure. During the initial stage of uniaxial compression (crack closure and elastic deformation), the self-potential difference of bituminous and anthracitic coals showed a linear increase or decrease with increasing uniaxial pressure, with average rates of change of 0.3565 mV/MPa and 1.1128 mV/MPa, respectively. With a gradual increase of uniaxial loading pressure (into the softening stage), the catastrophic points of uniaxial loading pressure corresponded to the inflection points of the self-potential change. The magnitude of the changes in the self-potentials of both bituminous and anthracitic coals was 10–100 mV before and after the last inflection points. The surface potential distribution of coal samples during uniaxial loading was well described by the self-potential profile, which indicated the degree of damage to the sample. The self-potential of a coal sample caused by deformation and fracture can therefore be used to evaluate the stress state of the coal mass. This work provides a theoretical foundation for application to a method for early prediction of coal and rock dynamic disasters.

Characterisation of rain erosion at ex-service turbofan blade leading edges

Rain erosion of turbofan blades creates problems in the aeronautics community, since the changes of profiles on leading edges affect aerodynamic performance that subsequently leads to significant efficiency drop for the aircraft engine. Water-hammer pressure induced by droplet impingements is primarily responsible for the damaged induced during the initial compression stage. As the surface roughened, the damage mechanism is then governed by lateral flow jetting assisted with hydraulic penetration. Studies on WDE are restricted to laboratory testing with simplified conditions and samples. Hence, this will be the first study to demonstrate real-life WDE damage on real turbofan blades during their ‘complicated’ in-service conditions.A set of Ti-6Al-4V ex-service turbofan blades are sectioned to examine their in-service degradation. Different parts of the blade, with impact velocities ranging from 144 m s−1 to 396 m s−1, were selected to study the effect of impact velocity on rain erosion damage. Erosion morphology and the damage mechanisms are characterised with Alicona profilometer and SEM. The results reveal that rain erosion occurs exclusively at the leading edges. Increase of rain erosion severity with increasing impact velocity is identified. The normalised volume loss is proportional to Vn, where V is the impact velocity, ranging from 286 m s−1 to 348 m s−1, and the exponent (n) is estimated to be around 8 for Ti-6Al-4V ex-service turbofan blade, which agrees with the literature from laboratory tests. The damage features during early incubation period cannot be detected due to the roughening of the leading edges. However, intergranular fracture is detected at the tip of the leading edge with less severe damage. In the steady state, material removal appears to be due to coalescence of microcracks to form erosion craters. The cracks continue to propagate from the side wall and bottom of the erosion craters, attributable to the joint effect of lateral outflow jetting and hydraulic penetration induced by repetitive droplet impingements. Thus, the present research is an invaluable opportunity to validate the results obtained from laboratory tests by comparing with the real-life WDE, which then helps to further elucidate the damage mechanisms behind WDE.

Cutting-induced end surface effect on compressive behaviour of aluminium foams

Three cutting methods, i.e. electrical discharge machining (EDM), band saw (BS) and water jet (WJ), were used to prepare cuboid samples of closed-cell aluminium Alporas foam. On the end surfaces of the prepared samples, local roughness of mesoscopic structural components (i.e. cell wall, node and facet) and global roughness for the entire cut surface were measured using confocal microscopy. Furthermore, quasi-static uniaxial compression tests were performed with intermittent unloading-reloading. In the initial stage of compression, the measured loading stiffness and unloading elastic modulus are highest, intermediate and lowest for the EDM-cut, BS-cut and WJ-cut samples, respectively. This difference in measured compressive properties has been correlated with the difference in end-surface roughness associated with different cutting methods. However, the peak and plateau of compressive stress and the unloading elastic modulus in the plateau stage are insensitive to cutting method. An analytical model has been developed to elucidate the observed compressive behaviour and shed light on the responsible mechanisms.

‘Optical-acoustic-stress’ responses in failure progress of cemented gangue-fly ash backfill material under uniaxial compression

ABSTRACTCemented gangue-fly ash backfill material (CGFBM) is made of coal gangue, fly ash, cementation materials and additives. It has been widely used in the field of backfill mining to control surface subsidence and to protect the environment. Partial backfill mining technology attracts much attention for its lower cost, and a large number of unconfined backfill structures (UBS) are formed based on it. Aimed at the stability monitoring of UBS in partial backfill, the failure progress of the CGFBM were obtained by uniaxial compression tests. The ‘optical-acoustic-stress’ responses of the specimens were monitored by Digital Image Correlation (DIC) technique, Acoustic Emission (AE) technique, Ultrasonic Testing (UT) technique and embedded pressure sensors. The advantages of each method in the stability monitoring of UBS was discussed. It is found that different monitoring methods have different sensitivity to different failure characteristics. The AE ringing count signal response is more sensitive to the pore compression and micro-crack generation in initial compaction stage and elastic compression stage. The central stress can be used to acquire the bearing capacity of the core bearing area after the top stress. The DIC response and UT response can be used to quantitatively evaluate the damage degree of the specimen. The stability of the UBS can be monitored by the jointly methods of ‘optical-acoustic-stress’ in partial backfill mining.

Atomistic simulation of effects of surface notches and loading mode on deformation and mechanics of ZrNi metallic glass

The effects of surface notches and loading mode on the mechanical deformation and mechanics of ZrNi metallic glass (MG) are studied using molecular dynamics simulations based on the many-body embedded-atom potential. The effects are investigated in terms of atomic trajectories, shear strain distributions, and stress-strain curves. The simulation results show that for ZrNi MG, resistance to shear deformation (shear strain > 200%) before breaking is much greater than that to tensile and compressive deformation. For ZrNi MG under tension, a pre-existing notch leads to earlier necking and breaking. Significant stress concentration occurs around the notch root when the notch length (L) is 3 nm or above, and dominates plastic deformation. For ZrNi MG under compression, a pre-existing notch is completely filled by neighboring atoms at the initial stage of compression. A pre-existing notch leads to single-edge barreling and weakens a sample’s ultimate strength when the L value is 3 nm or above. For ZrNi MG under shear loading, a pre-existing notch does not influence the shear modulus of samples; however, their ultimate strength decreases with increasing L value.

Experimental analysis of dynamic and static mechanical properties of deep thick anhydrite cap rocks under high-stress conditions

Across the globe, the formation of large oil and gas fields is always associated with a wide range of anhydrite cap rocks. Thus, an accurate analysis of the dynamic and static mechanical properties of anhydrite rocks is required. A deep thick anhydrite cap rock is developed in the Paleogene formation of the southwestern Tarim Basin, NW China. In this paper, we designed mechanical and acoustic synchronization tests and acquired the corresponding parameters. The effects of stress and mineral composition on the dynamic and static mechanical properties of the anhydrite rock were analyzed systematically. The results indicate that under uniaxial conditions, the anhydrite rock showed obvious brittle rupture characteristics, while under triaxial conditions, the shear rupture characteristics were obvious. The stress–strain curves of the anhydrite rocks under triaxial conditions can be divided into five stages: initial compression stage, linear elasticity stage, nonlinear steady extension stage, nonlinear unsteady extension and failure stage and post-peak stress stage. An increase in the anhydrite content can delay the weakening time of the anhydrite rocks, greatly improving the rock strength. Under high-stress conditions, the anhydrite mineral particles can accumulate large amounts of energy, exhibit a long period of creep behavior and produce significant additional stresses. The anhydrite composition can improve the strength and rigidity of the anhydrite rocks. However, anhydrite rock with a high anhydrite content will also have strong plasticity. The greater the stress environment that the anhydrite rock bears, the stronger its plasticity will be. The anhydrite component can improve the ability of the anhydrite rocks to resist shear rupture. Compared to the anhydrite mudrocks (AMR), argillaceous anhydrite rocks (AR) have higher VP, VS and VP/VS values. There is a very good positive correlation between VP/VS and the anhydrite content. A regression model of the longitudinal and shear wave time differences of the anhydrite rocks is obtained, which can be used for the prediction of the shear wave time difference. The dynamic Young’s modulus, Poisson’s ratio and shear modulus of the AR are higher than those of the AMR. The regression models of dynamic and static mechanical parameters of the anhydrite rocks are also obtained. These models will be valuable in the evaluation of mechanical characteristics for similar types of anhydrite rocks worldwide.

Permeability stress-sensitivity in 4D flow-geomechanical coupling of Shouyang CBM reservoir, Qinshui Basin, China

Permeability stress-sensitivity is the critical characteristic in reservoir development, especially for coalbed methane (CBM) reservoir. To investigate the permeability evolution during production in Shouyang CBM reservoir, Qinshui Basin, China, a new stress-sensitive permeability analytical model is proposed, while a time-lapse three-dimensional (4D) multi-physical coupling model is constructed. Heterogeneity and anisotropy of the reservoir flow and geomechanical properties as well as the permeability stress-sensitivity are adequately considered in the model. A coupling method is proposed to bridge data between finite difference (FD) grids and finite element (FE) grids with an interface code complied by Python. The reservoir flow data and geomechanical data are connected with each other by the interface code. Permeability stress-sensitivity experiments are carried out and compared with the numerical simulation results. According to the dynamic simulation results of 230 producing days, the stress increases with pore pressure decline, and the permeability stress-sensitivity is very strong in Shouyang 15# coalbed, especially in the initial stage of compression. The dynamic evolutions of stress and permeability are significantly affected by the anisotropy and heterogeneity of coalbed during depletion. The proposed stress-dependent permeability model is more suitable for characterizing the permeability evolution of the Shouyang CBM reservoir.

First-principles molecular dynamics simulations of anorthite (CaAl2Si2O8) glass at high pressure

We report first-principles molecular dynamics study of the equation of state, structural, and elastic properties of CaAl2Si2O8 glass at 300 K as a function of pressure up to 155 GPa. Our results for the ambient pressure glass show that: (1) as with other silicates, Si atoms remain mostly (> 95%) under tetrahedral oxygen surroundings; (2) unlike anorthite crystal, presence of high-coordination (> 4) Al atoms with ~ 30% abundance; (3) and significant presence of both non-bridging (8%) and triply (17%) coordinated oxygen. To achieve the glass configurations at various pressures, we use two different simulation schedules: cold and hot compression. Cold compression refers to sequential compression at 300 K. Compression at 3000 K and subsequent isochoric quenching to 300 K is considered as hot compression. At the initial stages of compression (0–10 GPa), smooth increase in bond distance and coordination occurs in the hot-compressed glass. Whereas in cold compression, Si (also Al to some extent) displays mainly topological changes (without significantly affecting the average bond distance or coordination) in this pressure interval. Further increase in pressure results in gradual increases in mean coordination, with Si–O (Al–O) coordination eventually reaching and remaining 6 (6.5) at the highest compression. Similarly, the ambient pressure Ca–O coordination of 5.9 increases to 9.5 at 155 GPa. The continuous pressure-induced increase in the proportion of oxygen triclusters along with the appearance and increasing abundance of tetrahedral oxygens results in mean O–T (T = Si and Al) coordination of > 3 from a value of 2.1 at ambient pressure. Due to the absence of kinetic barrier, the hot-compressed glasses consistently produce greater densities and higher coordination numbers than the cold compression cases. Decompressed glasses show irreversible compaction along with retention of high-coordination species when decompressed from pressure ≥ 10 GPa. The different density retention amounts (12, 17, and 20% when decompressed from 12, 40, and 155 GPa, respectively) signifies that the degree of irreversibility depends on the peak pressure of decompression. The calculated compressional and shear wave velocities (5 and 3 km/s at 0 GPa) for the cold-compressed case display sluggish pressure response in the 0–10 GPa interval as opposed to smooth increase in the hot-compressed one. Shear velocity saturates rather rapidly with a value of ~ 5 km/s, whereas compressional wave velocity displays continuous increase, reaching/exceeding 12.5 km/s at 155 GPa. These structural details suggest that the pressure response of the cold-compressed glasses is not only inherently different at the 0–10 GPa interval, the density, coordination, and wave velocity data are consistently lower than the hot-compressed glasses. Hot-compressed glasses may, therefore, be the better analog in the study of high-pressure silicate melts.

Investigation of Energy Mechanism and Acoustic Emission Characteristics of Mudstone with Different Moisture Contents

Characteristics of energy accumulation, evolution, and dissipation in conventional triaxial compression of mudstones with different moisture contents were explored. Stress‐strain relations and acoustic emission (AE) characteristics of the deformation and failure of rock specimens were analyzed. The densities and rates of stored energy, elastic energy, and dissipated energy under different confining pressures were confirmed. The results demonstrated that the growth rate of absorbed total energy decreases with the increase of moisture content, indicating that the higher the moisture content is, the less the total energy mudstone samples absorb. The dissipated energy of the soaking sample, by contrast, has the first increase speed, and natural sample comes second at the beginning. When entering the crack stable development stage, the dry sample has the fastest growing rate of dissipated energy, meaning that dissipated energy used for crack propagation gradually decreases with the increase of moisture content. The AE signals significantly enhance at the initial compression stage and plastic deformation stage with the moisture content decreasing. The AE location events at the failure moment decrease as the moisture content increasing. The time that the maximum AE even rate appears is slightly lagged behind the macroscopic failure time, and the AE even rates increase with the decrease of confining pressure. The above results indicate that the water erosion process on rock reduces the cohesive energy and cohesive force, destroys the micromechanical structure, and minimizes the energy states of rock.

Study on quasi static and dynamic mechanical properties of chemical foaming concrete

Chemical foaming concrete is a kind of new building material. In this paper, the quasi-static conditions of foam concrete and dynamic mechanical properties under impact dynamic conditions are studied. This paper also analyzes in detail the damage mechanism, deformation characteristics and energy absorption characteristics of the foam concrete under compression and stretch conditions. The results indicate that, the lower the density is, the less the cells on the surface and the inside of the concrete material is, and the larger the diameter of the pores is. When the density increases, the number of cells increases as well, and the diameter of the pores decrease gradually. The whole process of uniaxial compression of foam concrete is divided into three stages, that is, the material's elasticity and plasticity stage, platform stage and material densification stage. In the initial stage of stretch and compression, the stress-strain curve of foam concrete increases linearly basically. In platform stage, the foam concrete is mainly absorbing the energy, and there are both strain- softening and strain-hardening behaviors in this stage at the same time. The greater the density of the concrete, the earlier the densification stage occurs. The research on the impact dynamic properties of foam concrete shows that the dynamic strain rate is obviously positively correlated with the impact velocity, i.e., the greater the impact velocity, the greater the strain rate. The dynamic stress-strain curve of foam concrete is almost the same as that of static curve. It is also divided into three stages: linear elastic stage, material yield stage and material failure stage.

Majoritic garnet grains within shock-induced melt veins in amphibolites from the Ries impact crater suggest ultrahigh crystallization pressures between 18 and 9 GPa

Shock-induced melt veins in amphibolites from the Nordlinger Ries often have chemical compositions that are similar to bulk rock (i.e., basaltic), but there are other veins that are confined to chlorite-rich cracks that formed before the impact and these are poor in Ca and Na. Majoritic garnets within the shock veins show a broad chemical variation between three endmembers: (1) $${}^{\text{VIII}}{{\text{M}^{2+}}_3} {}^{\text{VI}}{\text{Al}}_{2} ({}^{\text{IV}}{\text{SiO}}_{4} )_{3}$$ (normal garnet, Grt), (2) $${}^{\text{VIII}}{{\text{M}^{2+}}_3} {}^{\text{VI}}[{\text{M}}^{2 + } ({\text{Si,Ti}})]({}^{\text{IV}}{\text{SiO}}_{4} )_{3}$$ (majorite, Maj), and (3) $${}^{\text{VIII}}({{\text {Na} {\text M}^{2+}}_2}) {}^{\text{VI}}[ ({\text{Si,Ti}}){\text {Al}}]({}^{\text{IV}}{\text{SiO}}_{4} )_{3}$$ (Na-majorite50Grt50), whereby M2+ = Mg2+, Fe2+, Mn2+, Ca2+. In particular, we observed a broad variation in VI(Si,Ti) which ranges from 0.12 to 0.58 cations per formula unit (cpfu). All these majoritic garnets crystallized during shock pressure release at different ultrahigh pressures. Those with high VI(Si,Ti) (0.36–0.58 cpfu) formed at high pressures and temperatures from amphibole-rich melts, while majoritic garnets with lower VI(Si,Ti) of 0.12–0.27 cpfu formed at lower pressures and temperatures from chlorite-rich melts. Furthermore, majoritic garnets with intermediate values of VI(Si,Ti) (0.24–0.39) crystallized from melts with intermediate contents of Ca and Na. To the best of our knowledge the ‘MORB-type’ Ca–Na-rich majoritic garnets with maximum contents of 2.99 wt% Na2O and calculated crystallisation pressures of 16–18 GPa are the most extreme representatives ever found in terrestrial shocked materials. At the Ries, the duration of the initial contact and compression stage at the central location of impact is estimated to only ~ 0.1 s. We used a ~ 200-µm-thick shock-induced vein in a moderately shocked amphibolite to model its pressure–temperature–time (P–T–t) path. The graphic model manifests a peak temperature of ~ 2600 °C for the vein, continuum pressure lasting for ~ 0.02 s, a quench duration of ~ 0.02 s and a shock pulse of ~ 0.038 s. The small difference between the continuum pressure and the pressure of majoritic garnet crystallization underlines the usefulness of applying crystallisation pressures of majoritic garnets from metabasites for calculation of dynamic shock pressures of host rocks. Majoritic garnets of chlorite provenance, however, are not suitable for the determination of continuum pressure since they crystallized relatively late during shock release. An extraordinary glass- and majorite-bearing amphibole fragment in a shock-vein of one amphibolite documents the whole unloading path.

Heat transfer losses in reciprocating compressors with valve actuation for energy storage applications

Understanding the exergy losses stemming from heat transfer in compressors and expanders is important for many energy storage applications such as compressed air and pumped thermal storage. In order to obtain a better understanding of these losses, CFD simulations were performed for simple gas springs, for a gas spring with an internal grid to mimic valve flow, and for a reciprocating compressor with functioning inlet and outlet valves. The wall heat exchanges for these three cases were examined and compared. The model adopted has previously been validated for a simple gas spring using experimental data from literature. For the gas spring with an internal grid it was found that increased mixing leads to higher heat-transfer-induced hysteresis losses and (at high piston speeds) to a significant pressure loss. These two types of loss can be distinguished by undertaking adiabatic-wall calculations. For a compressor (i.e., with valve flows) heat transfer over the cycle depends very much on valve timing. For example, at 1500rpm, when the delivery valve is opened at 7bar the heat transfer coefficient for the initial stages of compression is similar to that for a simple gas spring, whereas for the same speed at 6bar it is more than doubled.

Strength degradation and anchoring behavior of rock mass in the fault fracture zone

Rock mass in the fault fracture zone has some characteristics such as low strength and poor self-stability, so the control mechanism of stability has been a difficulty in the research of underground engineering. A set of laboratory simulation method of fault fractured rock mass is developed to reflect the natural forming process of fault fracture zone. Compared with intact rock mass, the fault fractured rock mass has an obvious degradation in strength and deformation parameters, and the degradation index is between 22.79 and 84.06%. The bolt has a certain supporting effect on the fault fractured rock mass, and in the situation of end anchoring, the greater the pretightening force is, the better the enforcement effect will be. The stress field produced by high pretightening force can relieve the stress concentration around the bolt hole and make the initial cracks of rock mass away from the bolt plate. The evolution curve of bolt axial force in the process of uniaxial compression of large-scale specimen shows four stages, which are the initial compression stage, pre-peak joint load-bearing stage, post-peak joint load-bearing stage and the residual stage. Research results could provide some theory reference for the stability control of rock mass in the fault fracture zone.

Estimates of crystalline LiF thermal conductivity at high temperature and pressure by a Green-Kubo method

Given the unique optical properties of LiF, it is often used as an observation window in high-temperature and pressure experiments; and, hence, estimates of its transmission properties are necessary to interpret observations. Since direct measurements of the thermal conductivity of LiF at the appropriate conditions are difficult, we resort to molecular simulation methods. Using the Belonoshko et al. (2000) empirical potential validated against ab initio phonon density of states, we estimate the thermal conductivity of LiF at high temperatures (1000-4000K) and pressures (100-400 GPa) with the Green-Kubo method. We also compare these estimates to those derived directly from ab initio data. To ascertain the correct phase of LiF at these extreme conditions we calculate the (relative) phase stability of the B1 and B2 structures using a quasiharmonic ab initio model of the free energy. We also estimate the thermal conductivity of LiF in an uniaxial loading state that emulates initial stages of compression in high-stress ramp loading experiments and show the degree of anisotropy induced in the conductivity due to deformation.

New insight on the recent tectonic evolution and uplift of the southern Ecuadorian Andes from gravity and structural analysis of the Neogene-Quaternary intramontane basins

The sedimentary basins of Loja, Malacatos-Vilcabamba and Catamayo belong to the Neogene-Quaternary synorogenic intramontane basins of South Ecuador. They were formed during uplift of the Andes since Middle-Late Miocene as a result of the Nazca plate subduction beneath the South American continental margin. This E-W compressional tectonic event allowed for the development of NNE-SSW oriented folds and faults, determining the pattern and thickness of sedimentary infill. New gravity measurements in the sedimentary basins indicate negative Bouguer anomalies reaching up to −292 mGal related to thick continental crust and sedimentary infill. 2D gravity models along profiles orthogonal to N-S elongated basins determine their deep structure. Loja Basin is asymmetrical, with a thickness of sedimentary infill reaching more than 1200 m in the eastern part, which coincides with a zone of most intense compressive deformation. The tectonic structures include N-S, NW-SE and NE-SW oriented folds and associated east-facing reverse faults. The presence of liquefaction structures strongly suggests the occurrence of large earthquakes just after the sedimentation. The basin of Malacatos-Vilcabamba has some folds with N-S orientation. However, both Catamayo and Malacatos-Vilcabamba basins are essentially dominated by N-S to NW-SE normal faults, producing a strong asymmetry in the Catamayo Basin area. The initial stages of compression developed folds, reverse faults and the relief uplift determining the high altitude of the Loja Basin. As a consequence of the crustal thickening and in association with the dismantling of the top of the Andes Cordillera, extensional events favored the development of normal faults that mainly affect the basins of Catamayo and Malacatos-Vilcabamba. Gravity research helps to constrain the geometry of the Neogene-Quaternary sedimentary infill, shedding some light on its relationship with tectonic events and geodynamic processes during intramontane basin development.

Anisotropic deformation behavior under various strain paths in commercially pure titanium Grade 1 and Grade 2 sheets

In this study, the anisotropic deformation behavior in commercially pure titanium Grade 1 and Grade 2 sheets under various strain paths was examined. A small but sharp stress peak arose during tension following compression in the Grade 1 sheet when loaded in the rolling direction, and the occurrence of a stress peak was retarded when the sheet was subjected to cyclic loading; however, such behavior did not occur in the Grade 2 sheet. Similarly, the work-hardening rate was different between the initial and latter stages of compression following tension. These behaviors did not arise when the sheets were loaded in other directions. The type of active twin mode was different depending on the loading path. When loaded in the rolling direction in both Grade 1 and Grade 2 sheets, <101̅2> twinning and detwinning were active, respectively, during compression and tension following compression, whereas <112̅2> twinning and detwinning were active during tension and compression following tension. The twinning activity was much more pronounced in the Grade 1 sheet than in the Grade 2 sheet. The abovementioned stress–strain responses were presumed to result from twinning and detwinning activities as for magnesium alloy sheets. However, we concluded that the effect of twinning activity on the stress–strain curves was much smaller in the titanium sheet than in magnesium alloy sheets.

Implementation of computation-reduced DCT using a novel method

The discrete cosine transform (DCT) performs a very important role in the application of lossy compression for representing the pixel values of an image using lesser number of coefficients. Recently, many algorithms have been devised to compute DCT. In the initial stage of image compression, the image is generally subdivided into smaller subblocks, and these subblocks are converted into DCT coefficients. In this paper, we present a novel DCT architecture that reduces the power consumption by decreasing the computational complexity based on the correlation between two successive rows. The unwanted forward DCT computations in each 8 × 8 sub-image are eliminated, thereby making a significant reduction of forward DCT computation for the whole image. This algorithm is verified with various high- and less-correlated images, and the result shows that image quality is not much affected when only the most significant 4 bits per pixel are considered for row comparison. The proposed architecture is synthesized using Cadence SoC Encounter® with TSMC 180 nm standard cell library. This architecture consumes 1.257 mW power instead of 8.027 mW when the pixels of two rows have very less difference. The experimental result shows that the proposed DCT architecture reduces the average power consumption by 50.02 % and the total processing time by 61.4 % for high-correlated images. For less-correlated images, the reduction in power consumption and the total processing time is 23.63 and 35 %, respectively.

Twinning-Detwinning Behavior during Cyclic Deformation of Magnesium Alloy

In situ neutron diffraction has been used to examine the deformation mechanisms of a precipitation-hardened and extruded Mg-8.5wt.%Al alloy subjected to (i) compression followed by reverse tension (texture T1) and (ii) tension followed by reverse compression (texture T2). Two starting textures are used: (1) as-extruded texture, T1, in which the basal pole of most grains is normal to the extrusion axis and a small portion of grains are oriented with the basal pole parallel to the extrusion axis; (2) a reoriented texture, T2, in which the basal pole of most grains is parallel to the extrusion axis. For texture T1, the onset of extension twinning corresponds well with the macroscopic elastic-plastic transition during the initial compression stage. The non-linear macroscopic stress/strain behavior during unloading after compression is more significant than during unloading after tension. For texture T2, little detwinning occurs after the initial tension stage, but almost all of the twinned volumes are detwinned during loading in reverse compression.

Dynamics of polymer response to nanosecond shock compression

The high strain rate mechanical dynamics of a polymer under shock compression, poly-methyl methacrylate (PMMA, MW = 996 000), were probed in experiments that combined two measurement techniques. The km s−1 impacts with laser-launched flyer plates were characterized by a fast interferometer, and the polymer dynamics were characterized by the nanosecond emission redshifting of Rhodamine 6G dye in PMMA. The thicknesses and velocities of the flyer plates were varied to create shock durations in the 5–22 ns range and pressures in the 2.6–9.2 GPa range. The strain rates were on the order of 107 s−1. With shock durations greater than 10 ns, a two-part rise of the redshift transients was observed, which was interpreted with reference to the well-known viscoelastic shock compression paradigm. However, the initial compression stage, lasting ∼10 ns, was found to be only partially elastic. With an elastic compression, after a shock the density should promptly return to its initial value. But even with the shortest-duration 5 ns shocks, the density relaxation after the shock ended was quite small. The critical shear stress where PMMA loses strength and the viscous compression process begins, appeared to be considerably larger with nanosecond shocks than with microsecond shocks. The loss of strength always took ∼10 ns, and could not be accelerated by increasing the shock pressure, suggesting the rate of strength loss was limited by the need for collective motions of the 30–40 nm polymer chains. The rates and amplitudes of the viscous density relaxation process increased with shock duration and shock pressure.