Large Residual Strain Research Articles

Effect of cold work level on the crack propagation behaviour of 316LN stainless steel in high-temperature pressurized water

We investigated the microstructure, mechanical properties, and stress corrosion cracking (SCC) propagation behaviour of 316LN austenitic stainless steel with various cold work (CW) levels. CW increased the crack growth rate of 316LN. The stress corrosion crack growth rate (SCCGR) for 316LN with 5% CW was approximately 6.52% higher than that of the solution-annealed state; 10%, 20%, and 30% CW resulted in 1.20, 3.18, and 6.10 times higher SCCGRs, respectively. A larger residual strain and proliferation of slip bands via cold deformation augmented the SCCGR. The SCC propagation mechanism of 316LN changed with increasing CW levels owing to slip bands.

Preparation of Gradient Polyurethane and Its Performance for Flexible Sensors.

Flexible sensors are prone to the problems of slow recovery rate and large residual strain in practical use. In this paper, a polyurethane functional composite with a gradient change in elastic modulus is proposed as a flexible sensor to meet the recovery rate and residual strain without affecting the motion. Different hard and soft segment ratios are used to synthesize a gradient polyurethane structure. The conductive percolation threshold was obtained between 45 wt% and 50 wt% of flake silver powder. Both gradient polyurethane and gradient polyurethane composites demonstrated that gradient materials can increase the recovery rate and reduce residual strain. The gradient polyurethane composites had a tensile strength of 3.26 MPa, an elastic modulus of 2.58 MPa, an elongation at break of 245%, a sensitivity coefficient of 1.20 at 0-25% deformation, a sensitivity coefficient of 11.38 at 25-75% deformation, a rate of recovery of 1.95 s at a time, and a resistance to fatigue (over 1000 cycles at a fixed strain of 20% showed a stable electrical response). The sensing performance under different cyclic strain frequencies was also investigated. The process has practical applications in the field of wearable skin motion and health monitoring.

Strain engineering and strain measurement by spring tethers on suspended epitaxial GaN-on-Si photonic crystal devices

Suspended epitaxial gallium nitride (GaN) on silicon (Si) photonic crystal devices suffer from large residual tensile strain, especially for long waveguides, because fine structures tend to crack due to large stress. By introducing spring-like tethers, designed by the combination of a spring network model and finite element method simulations, the stress at critical locations was mitigated and the cracking issue was solved. Meanwhile, the tethered-beam structure was found to be potentially a powerful method for high-precision strain measurement in tensile thin films, and in this case, a strain of 2.27(±0.01)×10−3 was measured in 350 nm epitaxial GaN-on-Si.

Ab initio stability prediction of [formula omitted] titanium and [formula omitted] and [formula omitted] precipitates in [formula omitted] titanium matrix for titanium alloys using density functional theory and micromechanics

β titanium alloys have a wide range of applications, including in aerospace and transport. The β phase is stable only at high temperatures, and the phase stability can be improved by adding stabilizers. However, the β-phase-stabilization effects of β stabilizers have not yet been clearly elucidated. Here, I report the ab initio prediction of the energetic phase stability of β titanium alloys with Mo, V, W, Nb, and Ta as additive β stabilizer elements. The stability is predicted using a combination of atomistic simulation via density functional theory and continuum micromechanics (Eshelby’s ellipsoidal inclusion analysis). In particular, I consider the heterogeneity of the secondary ω and α phases (precipitation) in the β matrix. All β stabilizer species led to a significant energetic and elastic stabilization of the β phase. Mo and W additives stabilized the β phase relatively stronger and secondary ω and α precipitates may hardly nucleate in β phase in high concentrate condition. Although V, Nb, and Ta additives stabilized the β phase significantly, the β phase remained metastable. Regarding the morphology of these secondary phases, V helped α precipitation and Ta helped ω precipitation. The possibility of a coexistence of ω and α in the β matrix was suggested for Nb addition. The strain fields around the precipitates were also investigated and the results suggested that α precipitates cause a large residual strain around it though ω precipitates do not.

Investigation of real-time strain responses of the insulating system of superconducting magnets by using fiber Bragg grating sensors

The normal operation of various superconducting magnets is greatly influenced by the thermal stress of the insulating system, commonly made of epoxy resins with vacuum pressure impregnation (VPI) technology, throughout the curing and cooling processes. In this work, we developed a Fiber Bragg Grating (FBG) strain measurement method to monitor the real-time strain responses of an epoxy resin IR-3 and its glass fiber-reinforced composite during both curing and cooling processes. Then, we also monitored the same process of a Nb–Ti superconducting magnet coil prepared by VPI technology. With the help of FBG strain sensors, the residual strains of the coil at various positions and directions were investigated. The results show that the large residual strain occurred near the end of the coil in the axial direction. In addition, the interlaminar shear stress properties were measured at both room temperature and liquid nitrogen temperature. The strain characteristics of the epoxy resin IR-3 and the insulating system can provide useful guidance for the design and construction of high-field Nb–Ti superconducting magnets.

Experimental Study on Mechanical Characteristics of Li2TiO3 Pebble Beds Under Cyclic Loading

The lithium titanate (Li2TiO3) ceramic pebble bed is one of the main tritium breeder candidates in the solid blankets of fusion reactors. Under the extreme operating conditions of fusion blankets, such as neutron irradiation, high temperatures, structural material extrusion, and stress concentration, the mechanical characteristics of tritium breeding pebble beds not only affect the mechanical performance of the blanket but also affect tritium production and extraction. Therefore, an experimental apparatus was built to characterize the mechanical behavior of 0.47 and 0.99 mm Li2TiO3 pebble beds. A uniaxial compression test was performed under the cyclic mechanical loads of 4, 6, and 8 MPa, respectively. It was shown that large irreversible residual strain appeared in the Li2TiO3 pebble bed with the increase of loading cycles and that the mechanical characteristics of the pebble beds were greatly affected by different mechanical loads and particle sizes. The current results provide relevant experimental data that can support the design of fusion blankets.

Effect of sodium addition on structural and magnetic properties of solid state processed SrFe12−xAlxO19 (x ≤ 4)

Aluminum-substituted M-type strontium hexaferrite (SrM-Al, SrFe12−xAlxO19, 0 ≤ x ≤ 4) with uniform microstructure and excellent magnetic properties was synthesized by conventional ceramic processing based on solid state reaction. Coercive field was highly enhanced by heavy doping with Al3+ although magnetization was significantly reduced. However, formation of coarse microstructure and poor development of Hc with low level doping of Al3+ disturbed practical application. To promote phase formation and to suppress growth of coarse grains which were induced by incorporation of Al3+ ions, SrM-Al ceramics was processed with NaOH. Such Na-assisted processing of SrM-Al reduced formation of non-magnetic phases and assisted development of uniform fine grains. In addition, the large residual strain by Al3+ doping was partially relieved by the same approach. As a result, saturation magnetization of SrFe12−xAlxO19 was remarkably enhanced and the BHmax of randomly oriented hard magnetic ceramics was improved to 1.23MGOe by concomitant addition of Na and Al ions.

Experimental study on hysteretic behavior of Q1100 ultra-high strength steel

In order to study the hysteretic behavior of the ultra-high strength steel (UHSS), cyclic tests on Q1100 UHSS with nominal yield strength equal to 1100 MPa are conducted in the present study. The strength softening, preserved strength, and energy dissipation capacity of Q1100 UHSS are analyzed. The influences of the loading scheme and initial residual strain on the hysteretic behavior of UHSS are investigated. It is found that Q1100 UHSS shows a significant softening behavior under cyclic loading. The strength decrease is relatively rapid at the beginning of the cyclic loading and then tends to stabilize. The decrease of the strength of UHSS with larger residual strain tends to be more slowly. The energy dissipation coefficient of Q1100 UHSS increases with the increase of strain amplitude. The sampling direction has a negligible effect on the hysteretic behavior of Q1100 UHSS. A nonlinear combined hardening constitutive model is calibrated for various cyclic loading schemes by using an automatic calibration method and the cyclic behavior of Q1100 UHSS is predicted well.

Collagen pre-strain discontinuity at the bone-Cartilage interface.

The bone-cartilage unit (BCU) is a universal feature in diarthrodial joints, which is mechanically-graded and subjected to shear and compressive strains. Changes in the BCU have been linked to osteoarthritis (OA) progression. Here we report existence of a physiological internal strain gradient (pre-strain) across the BCU at the ultrastructural scale of the extracellular matrix (ECM) constituents, specifically the collagen fibril. We use X-ray scattering that probes changes in the axial periodicity of fibril-level D-stagger of tropocollagen molecules in the matrix fibrils, as a measure of microscopic pre-strain. We find that mineralized collagen nanofibrils in the calcified plate are in tensile pre-strain relative to the underlying trabecular bone. This behaviour contrasts with the previously accepted notion that fibrillar pre-strain (or D-stagger) in collagenous tissues always reduces with mineralization, via reduced hydration and associated swelling pressure. Within the calcified part of the BCU, a finer-scale gradient in pre-strain (0.6% increase over ~50μm) is observed. The increased fibrillar pre-strain is linked to prior research reporting large tissue-level residual strains under compression. The findings may have biomechanical adaptative significance: higher in-built molecular level resilience/damage resistance to physiological compression, and disruption of the molecular-level pre-strains during remodelling of the bone-cartilage interface may be potential factors in osteoarthritis-based degeneration.

Effects of reconsolidation degree on reliquefaction resistance under mainshock–aftershock sequences: A DEM investigation

AbstractAftershock events may immediately follow mainshock events. In liquefiable deposits, the excess pore pressure caused by mainshock events may not dissipate completely in a short time interval between the mainshock and the aftershock. Sandy soil with different reconsolidation degrees (Ur) may present different reliquefaction resistances during the aftershock, which was not fully considered into the previous studies of reliquefaction. In this study, discrete element method (DEM) was employed to simulate a series of cyclic triaxial tests to evaluate the effects of Ur on reliquefaction resistance under mainshock–aftershock seismic sequence. The initially liquefied specimen was reconsolidated from two states with small and large residual shear strain respectively. Reconsolidated specimens reliquefied under different cyclic stress ratios (CSRs). Simulation results show that reliquefaction resistance increased gradually with increasing Ur, which was closely related to the previous residual shear strain during the first liquefaction event. Specimens that having large residual shear strain reliquefied easily, and the reliquefaction resistance of those specimens increased slightly with increasing Ur. On the contrary, reliquefaction resistance of the specimens that having small residual shear stain increased obviously with increasing Ur. The ratio of initial mechanical average coordination number to initial mesoscopic fabric anisotropy of reconsolidated specimens had a good correlation to the contraction potential and increased gradually with increasing Ur.

Small-angle neutron scattering study on phase separation in a super duplex stainless steel at 300 °C – Comparing hot-rolled and TIG welded material

The evolution of nanoscale phase separation in the ferrite phase of super duplex stainless steel 25Cr 7Ni (wt%) (SDSS 2507) and two SDSS TIG (tungsten inert gas) weldments have been quantitatively investigated by small-angle neutron scattering (SANS). The results show that the phase separation is more pronounced in the SDSS weldments in comparison to the base metal SDSS 2507, especially after aging for 35,000 h at 300 ° C. These results correlate with the higher ferrite micro-hardness in the aged TIG weldments than in the SDSS 2507. The enhanced phase separation is partly due to the higher contents of Cr and Ni in the ferrite of TIG weldments compared to SDSS 2507 base metal, revealed by energy-dispersive X-ray spectroscopy (EDS). Additionally, the residual strain measurements through focused ion beam and digital image correlation (FIB-DIC), indicate larger residual strains in the ferrite of weldments than in the base metal SDSS 2507. This is also believed to contribute to the enhanced phase separation. • TIG weldments show a faster rate of phase separation at 300 ° C than the base metal of SDSS 2507. • Higher contents of Cr and Ni in the ferrite are partly responsible for the accelerated kinetics of phase separation in the weldments. • Higher heat input during TIG welding leads to higher residual stresses which enhance phase separation

Study on Stress Corrosion Cracking Behavior of Incoloy825/X65 Bimetallic Composite Pipe Welded Joint in Wet Hydrogen Sulfide Environment

The stress corrosion cracking behavior of an Incoloy825/X65 bimetallic composite pipe welded joint in wet hydrogen sulfide (H2S) environment was investigated by means of the creviced bent beam (CBB) test in this study. The microstructure, element distribution and crack propagation behavior of the welded joint were analyzed by optical microscope (OM), scanning electron microscope (SEM), electron dispersive spectroscopy (EDS) and electron backscatter diffraction (EBSD). The results showed that two types of cracks were observed in the Incoloy825/X65 bimetallic composite pipe welded joint in wet H2S environment, they initiated from the notch and the intersection of the three zones (cladding Incoloy825, base X65 and weld), respectively, and propagated along the fusion boundary(FB) and the Type-II-like grain boundary. The mechanisms of the two types of cracks are due to the combination of anodic dissolution, stress and hydrogen. Near the FB, there are high angle grain boundaries, Type-I, Type-II and the Type-II-like grain boundaries, which have high SCC sensitivity. The element distribution in the intersection of the three zones and the crack tip is complex, with element diffusion, Cr loss and large residual strain. All these provide the conditions for cracks initiation and propagation.

Experimental investigation of the pseudoelastic behavior on zirconium modified Cu–Al–Be shape memory alloys for seismic applications

This paper examines the effect of 0.05–0.3 wt.% of zirconium-doping on microstructure, transformation temperatures, tensile properties, and pseudoelastic behavior of the parent -phase Cu87.93–Al11.5–Be0.57 shape memory alloys (SMAs). Results reveal that alloying zirconium in the evaluated SMA samples exhibits an excellent grain refinement up to 0.15 wt.%. Further, higher additions of Zr ⩾ 0.2 wt.% lowers the grain refinement efficiency due to precipitates agglomeration. Larger the size and volume fraction of Al3Zr precipitates led to higher transformation temperatures. Tensile properties were improved with Zr-doping, resulting enhancements in the maximum tensile strength and ductility with the addition of 0.15 wt.% Zr. The alloy with 0.05 wt.% of Zr-dope showed a good pseudoelastic strain recovery of deformation strain and then lowered by retaining large residual strain, indicating deterioration in the pseudoelasticity of SMAs.

Recyclable ZnO/Fe3O4 nanocomposite with piezotronic effect for high performance photocatalysis

The unrecyclability and low photocatalytic performance are two important problems that significantly limit the usages of ZnO in photocatalysis. In this article, ZnO/Fe3O4 nanocomposites with residual expansion strain have been synthesized by solvothermal method and sintering-cooling processes. On one hand, the addition of Fe3O4 nanoparticles provides a possible way for recycling the composites by magnetic attraction. On the other hand, the existence of residual expansion strain in ZnO is favorable for the introduction of piezoelectric potential, which provides an internal driving force for decreasing the recombination rate of photoinduced electron-hole pairs, and improves photocatalytic ability of the ZnO/Fe3O4 nanocomposites. Compared with the composites with little residual expansion strain, the photocatalytic activity of the composites with large residual expansion strain has been increased by 66%. This research shows the possible applications of piezotronic effect in photocatalysis and related areas, providing a new strategy to prepare recyclable photocatalysts with improved photocatalytic performance.

Shape memory effect in Na0.5Bi0.5TiO3-based ferroelectric ceramics

Shape memory materials (SMMs) are a class of smart materials that generate recoverable residual deformation. Among the family of SMMs, shape memory ceramics (SMC) are more applicable to extreme situations with high temperature, high stress, or corrosive environments. Most of the well-known SMCs are zirconia-based ceramics. Herein, we demonstrate the shape memory behavior of ferroelectric ceramics through a case study on Na0.5Bi0.5TiO3-BaTiO3 (NBT-BT) ceramics. The results show that after heat treatment under stress, there was a large residual strain left in the NBT-BT ceramics. After they were annealed at a temperature above the phase-transition temperature, the residual strain recovered. The maximum recoverable residual strain observed in the NBT ceramic plate was approximately 0.369%. Because most ferroelectric ceramics have a smaller critical stress regarding the stress-induced structural orientation or transformation compared with that of the zirconia-based SMCs, they should be more suitable for SMC applications at a medium stress level.

Effects of Strain History and Induced Anisotropy on Reliquefaction Resistance of Toyoura Sand

Recent earthquake events have indicated that liquefaction histories can greatly affect the reliquefaction resistance of saturated sandy ground. This paper reports a fundamental experimental study of medium-dense/dense Toyoura sand with a relative density of 70% using a hollow cylinder torsional shear apparatus (HCTSA) to provide a better understanding of the integrated effects of strain history and soil fabric on reliquefaction resistance of Toyoura sand. A series of postliquefaction monotonic tests was conducted to investigate the degrees of induced fabric anisotropy at different states of liquefaction and subsequent reconsolidation. A series of reliquefaction tests was performed on sandy specimens that had experienced medium to large residual shear strains (γres) and that had reconsolidated in different states, under various cyclic stress ratios (CSRs). It was shown that reliquefaction resistance of Toyoura sand is influenced highly by the residual shear strain during liquefaction and fabric anisotropy after reconsolidation. The experimental results were used to predict excess pore-water pressure (EPWP) buildup during liquefaction and reliquefaction as a function of shear strain or dissipated energy. The liquefaction/reliquefaction resistance of Toyoura sand was quantified using an energy-based criterion considering the integrated effects of strain history and soil fabric.

Measurement of residual strain in tantalum-clad tungsten after hot isostatic pressing

Tantalum-clad tungsten targets are a popular choice for spallation neutron production, due to the combination of high neutron yield and corrosion resistance. Such targets typically use the Hot Isostatic Press (HIP) process to bond the cladding to the core; this produces a strong bond but also introduces large residual stresses in the target and cladding. This is of particular interest at the ISIS neutron source, because cladding breaches are currently believed to limit the lifetime of ISIS TS2 targets. Two different and complementary methods were used to measure the residual strain in a tantalum-clad tungsten strip manufactured using the same HIP process as ISIS targets. The strip was produced with deliberately asymmetric cladding, causing it to deflect in proportion to the residual stress. FEA simulations were used to back-calculate the stress from the measured deflection. The strip was then placed on the ISIS instrument ENGIN-X, which allowed detailed through-thickness strain profiles to be measured via neutron diffraction. The results of both methods confirm the presence of large residual strains, and agree reasonably well with FEA simulations of the cladding process.

Room temperature magnetic shape-memory effect in strontium-doped lanthanum cobaltite single crystals

In this study, we investigated the temperature dependence of magnetostriction in the twinned rhombohedral perovskite structure-based lanthanum cobaltite La0.8Sr0.2CoO3. The magnetostriction measured in a magnetic field applied along the pseudocubic [111]c axis showed large hysteresis and residual strains. The residual strains are caused by the twin deformation, which we observed using x-ray diffraction. We found that the critical magnetic field for the twin deformation of La0.8Sr0.2CoO3 increases with temperature. The twin deformation occurs in a sufficiently strong magnetic field (∼6 T), even in the paramagnetic region at room temperature. We also confirmed that the temperature dependence of the critical magnetic field for twin deformation is related to the temperature dependence of the magnetization.

Load–Displacement Behavior of Helical Shape Memory Alloy Spring Actuators with Small Spring Diameter to Wire Diameter Ratios

A design methodology for fabricating and predicting the load–displacement behavior of shape memory alloy (SMA) spring actuators has been previously created and validated (Nicholson et al., Smart Mater Struct 23(12):13–23, 2014). However, it was found in this work that for springs with a spring index (ratio of spring diameter to wire diameter) below five, the load–displacement response of shape memory spring actuators no longer fits the previously established model. This paper explores the use of a correction factor in the governing equations to account for the plastic deformation that occurs when fabricating springs with very small spring indices. The plastic deformation induces an effective reduction in the volume of the transforming material, thus reducing the load-bearing and actuation capabilities of the spring. A semi-analytical solution to this problem is found which can be used to predict the load–displacement behavior. This is accomplished through a reduction in the effective wire diameter based on the approximate shear strain during loading. This approach is consistent with observations during thermomechanical cycling in SMAs, where the phase transformation remained mostly unhindered despite large residual inelastic and/or plastic strains, and has direct application in the design of actuators with small spring indices by quantifying the drop in actuation force and stroke.

Wear of ZhS6U Nickel Superalloy Tool in Friction Stir Processing on Commercially Pure Titanium

The use of electric arc or gas welding in the manufacture of titanium components often results in low quality welded joints due to large residual stresses and strains. A successful solution to this problem can be found in the application of friction stir welding. However, friction stir welding (FSW) of titanium alloys is complicated by rapid tool wear under high loads and temperatures achieved in the process. This paper studies the durability of a tool made of ZhS6U Ni-based superalloy used for friction stir processing of commercially pure titanium and the effect of the tool wear on the weld quality. The total length of the titanium weld formed by the tool without failure comprised 2755 mm. The highest wear of the tool is observed at the base of the pin, which brings about the formation of macrodefects in the processed material. The tool overheating causes an increase in the dendrite element size of ZhS6U alloy. The transfer layer contains chemical elements of this alloy, indicating that the tool wear occurs by diffusion and adhesion. As a result of processing, the tensile strength of commercially pure titanium increased by 25%.