Strain Recovery Research Articles

Seeding the Infant Gut in Early Life-Effects of Maternal and Infant Seeding with Probiotics on Strain Transfer, Microbiota, and Gastrointestinal Symptoms in Healthy Breastfed Infants.

We investigated the effects of two dosing regimens of two multi-strain probiotic products on the gut microbiota of breastfed infants, including the transfer of the dosed strains and clinical outcomes. In forty-seven dyads, infants were either exposed through maternal intake (MS) of Lactobacillus acidophilus LA-5, Bifidobacterium animalis subsp. lactis BB-12, Lacticaseibacillus rhamnosus LGG, and Bifidobacterium longum subsp. infantis Bifin02 from gestational week thirty-three until four weeks after birth (n = 24) or dosed directly (IS) with the same strains except for LA-5 starting within 24 h after birth until day 28 (n = 23). Infant stool samples were collected on day 0, 14, 28, and 42 after birth. Gastrointestinal symptoms were assessed by parents using an electronic diary. Microbiota composition was determined using 16S rRNA sequencing, and strain recovery was analyzed by qPCR. Notably, 100% of the IS infants were colonized with Bifin02 after 14 days as opposed to only 25% of the MS infants. Mean stool frequency was significantly lower in IS infants compared to MS infants and IS infants had softer stools on day 14, 28, and 42. A significantly steeper slope of progression of inconsolable crying and fussing was observed in MS infants compared to IS infants. In conclusion, direct infant seeding induced a faster increase in fecal bifidobacteria abundancy and Bifin02 recovery compared to dosed through the maternal intake.

Phase Field Simulations of Microstructures in Porous Ferromagnetic Shape Memory Alloy Ni2MnGa

The magnetic domain structures and martensite microstructures of porous Ni2MnGa Heusler alloys with various circle-shaped and ellipse-shaped pores were systematically studied by the phase field method. The magnetization curves and magnetic field-induced strains (MFIS) at the external field were determined. A mesoscopic mechanism was proposed for simulation to reveal the influence of the pores on the microstructures and the MFIS of porous magnetic shape memory alloy. The stress concentration effect and the recovery strain of the porous alloy are studied. The results indicate the MFIS value increases when ellipse-shaped pores elongate along the twin boundary. The effects of porosity and pore size on MFIS for porous Ni-Mn-Ga alloys with randomly distributed pores were also explored. The present study is of guiding significance for understanding the role played by pores on the MFIS and may provide a possible way to adjust the functional properties of ferromagnetic shape memory alloys.

Study the correlation of B19' phase transformation and grain orientation evolution on shape recovery of NiTi alloys fabricated by powder bed fusion

In this study, NiTi shape memory alloys (SMAs) were fabricated by powder bed fusion (PBF), and the effect of energy densities on the microstructure, phase transformation behavior and shape memory effect (SME) was studied. The results indicated that the NiTi SMAs exhibit a microstructure with coarse and long grains surrounded by fine grains, while the increase in energy density from 44.5 J/mm3 to 133.3 J/mm3 makes the grain size larger, and different energy densities induce various grain orientations. Specifically, the NiTi alloy produces more martensite B19’ at two energy densities of 44.5 J/mm3 and 111.1 J/mm3 after aging at 773 K for 2 h. The phase transition behavior of B19’ is related to the high angle grain boundaries (HAGBs) fraction and the grain orientation spread (GOS) distribution. SME tests showed that the NiTi alloy prepared by PBF had unstable recovery strain compared to the specimens after aging, and the SME of NiTi alloy after aging was more consistent with the evolution of the residual B19’ content and grain orientation of the B2 phase. The sample prepared at an energy density of 111.1 J/mm3 exhibited the best recovery strain of 4.5%, depending on the greater residual B19’ content and grain orientations of the B2 phase with higher Schmid factors. These results reveal the relationship between the recovery strain and microstructural evolution of NiTi SMAs prepared by PBF, and provide new insights into the fabrication of NiTi alloy with better SME by appropriate PBF process.

Stress estimation in a 3 km-deep geothermal borehole: A snapshot of stress state in Southern Cathaysia block, China

The present-day stress state at depth in Southern Cathaysia block remains poorly understood. We retrieved twelve high-quality granite core samples in a 3 km deep borehole of the Huangshadong geothermal field in South China and conducted core-based stress measurements using the anelastic strain recovery (ASR) method to estimate the full stress tensors. The measured stress data of hydraulic fracturing in an adjacent, 600 m-deep geothermal borehole was also collected. The results show that the maximum principal stress σ1 is sub-horizontal from 1566 m to 3008 m in depth. The maximum and minimum horizontal principal stresses, SH and Sh, and the vertical principal stress, Sv, generally follow SH > Sv ≈ Sh in their magnitudes, thus indicating a thrust/strike-slip faulting transitional stress regime in the study area. Our stress orientations by ASR method based on paleomagnetic analysis and the impression test of hydraulic fracturing further suggest that SH is approximately oriented N72° ± 5°W, and this is consistent with both the regional stress field and the observed motion pattern of the Tonghu strike-slip fault in the study area. Interestingly, we found that the difference between the magnitudes of SH and Sh is small, indicating a low level of shear stress in the shallow crust. Also, because of the presence of the lithological changes and fracture zones, there are some snapshots of localized heterogeneous stress states in the granite geothermal reservoir. The present stress state has an impact on the occurrence and exposure of hot springs in the Huangshadong geothermal field. We indicated that the fracture zones with weak stress accumulation are potential transport channels of hot water. The stress field of the Southern Cathaysia block is mainly constrained by the southeast push of the Tibetan Plateau and the northwest subduction of the Philippine Sea plate.

The microstructural features and strain recovery characteristics of Ti–V–Al shape memory alloy with minor Sc addition

In the present work, minor Sc addition was employed to control the microstructural features, further optimizing the mechanical/functional performances of Ti–V–Al based shape memory alloy. The results revealed that phase constituents of Ti–V–Al based shape memory alloys were dominated by single αˊˊ martensite phase, regardless of Sc content. Nevertheless, Sc addition caused the significant lattice distortion of Ti–V–Al based shape memory alloy. Meanwhile, the grain size of Ti–V–Al based shape memory alloys were reduced due to Sc addition. Besides, the martensite morphologies evolved from self-accommodation configuration to optimal orientation configuration, but the types of twins kept unchanged. All Ti–V–Al based shape memory alloys showed the reversible αˊˊ→β martensitic transformation in heating curves, irrespective of Sc content. With Sc content increasing, the martensitic transformation temperatures increased linearly due to the chemical composition effect and mechanical effect. Moderate Sc addition was favor to the improvement of thermal cycling stability of Ti–V–Al based shape memory alloys, which can enhance its functional stability. In proportion, the mechanical and functional properties of Ti–V–Al based shape memory alloys firstly increased and then decreased. Ti–V–Al based shape memory alloy with moderate Sc content of 0.025 at.% possess the higher fracture strength of 748.4 MPa, superior ductility of 18.7% and the larger shape memory effect strain of 4.31% under the 8% pre-strain condition. The higher performances in Ti–V–Al shape memory alloy with 0.025 at.% Sc can be attributed to the compressive effect of solution strengthening and grain refinement.

Effects of Building Directions on Microstructure, Impurity Elements and Mechanical Properties of NiTi Alloys Fabricated by Laser Powder Bed Fusion.

For NiTi alloys prepared by the Laser Powder Bed Fusion (LPBF), changes in the building directions will directly change the preferred orientation and thus directly affect the smart properties, such as superelasticity, as well as change the distribution state of defects and impurity elements to affect the phase transformation behaviour, which in turn affects the smart properties at different temperatures. In this study, the relationship between impurity elements, the building directions, and functional properties; the effects of building directions on the crystallographic anisotropy; phase composition; superelastic properties; microhardness; geometrically necessary dislocation (GND) density; and impurity element content of NiTi SMAs fabricated by LPBF were systematically studied. Three building directions measured from the substrate, namely, 0°, 45° and 90°, were selected, and three sets of cylindrical samples were fabricated with the same process parameters. Along the building direction, a strong <100>//vertical direction (VD) texture was formed for all the samples. Because of the difference in transformation temperature, when tested at 15 °C, the sample with the 45° orientation possessed the highest strain recovery of 3.2%. When tested at the austenite phase transformation finish temperature (Af)+10 °C, the 90° sample had the highest strain recovery of 5.83% and a strain recovery rate of 83.3%. The sample with the 90° orientation presented the highest microhardness, which was attributed to its high dislocation density. Meanwhile, different building directions had an effect on the contents of O, C, and N impurity elements, which affected the transformation temperature by changing the Ni/Ti ratio. This study innovatively studied the impurity element content and GND densities of compressive samples with three building directions, providing theoretical guidance for LPBFed NiTi SMA structural parts.

Microstructural design for achieving high performances in Ti-V-Al lightweight shape memory alloys by optimizing Zr content

In the present study, the quaternary Zr element was introduced to replace Ti in Ti-V-Al based shape memory alloys. The addition of Zr element resulted in the microstructural features of Ti-V-Al based shape memory alloys, such as the phase evolution of αˊˊ → β, the precipitation of C14-type Laves phase, reduction of grain size etc. Moreover, the two-stage martensitic transformation corresponding to ω → β and αˊˊ → β transformation gradually evolved into single ω → β martensitic transformation during heating process in Ti-V-Al based shape memory alloys, as Zr content was increased from 0.5 at.% to 5.0 at.%, which was firstly reported in Ti-V-Al based shape memory alloys. Meanwhile, the martensitic transformation temperatures of Ti-V-Al based shape memory alloys were decreased with Zr increasing. By optimizing Zr content, the highest yield strength and largest microhardness can be gained in Ti-V-Al based shape memory alloy with controlling 5.0 at.% Zr, as a result of solution strengthening, grain refinement and precipitation strengthening. Besides, the moderate Zr content caused the precipitation of C14-type Laves phase and formed the quasi-continuous network structure, which contributed to the superior elongation. And the perfect fracture ductility with an elongation of 40% in the present Ti-V-Al based shape memory alloys with 3.0 at.% Zr was significantly larger than that the other reported Ti-V-Al based shape memory alloys. Moreover, the superior strain recovery characteristics with a fully recoverable strain of 4% can be achieved in Ti-V-Al based shape memory alloys through controlling 3.0 at.% Zr.

Body temperature NiTi alloys: effect of the heat treatment on the functional thermo-mechanical properties

The design process of devices based on shape memory alloys (SMA) is particularly demanding. It is fundamental the investigation of parameters describing the thermo-mechanical behavior of these materials, such as the phase transformation temperatures and the Clausius-Clapeyron coefficient (CCC). The aim of this study is to determine how heat treatment (HT) affects the above-mentioned parameters on two commercially available body-temperature NiTi alloys. This analysis is accomplished through differential scanning calorimetry (DSC) and strain recovery (SR) tests. Finally, the two NiTi alloys have been compared to determine whether exists a significant difference between different casting batches of the same commercial alloy conformed to the ASTM F2063. It results that both alloys present a similar overall trend of Af as a function of HT temperature and time, even if the values significantly differ for treatment at temperatures higher than 773 K for all HT times. Furthermore, it is noted that the HT time presents a minor effect on the value of Af. After the SR analysis, it has been possible to highlight the effect of the HT on the values of CCC for As, Ms and Rs. The overall trend is similar between the two NiTi alloys. These results lead to the conclusion that HT highly affect not only the transformation temperatures but also the transformation path of the alloy, thus its mechanical properties.

Highly lubricious SPMK-g-PEEK implant surfaces to facilitate rehydration of articular cartilage

To enable long lasting osteochondral defect repairs which preserve the native function of synovial joint counter-face, it is essential to develop surfaces which are optimised to support healthy cartilage function by providing a hydrated, low friction and compliant sliding interface. PEEK surfaces were modified using a biocompatible 3-sulfopropyl methacrylate potassium salt (SPMK) through UV photo-polymerisation, resulting in a ∼350 nm thick hydrophilic coating rich in hydrophilic anionic sulfonic acid groups. Characterisation was done through Fourier Transformed Infrared Spectroscopy, Focused Ion Beam Scanning Electron Microscopy, and Water Contact Angle measurements.Using a Bruker UMT TriboLab, bovine cartilage sliding tests were conducted with real-time strain and shear force measurements, comparing untreated PEEK, SPMK functionalised PEEK (SPMK-g-PEEK), and Cobalt Chrome Molybdenum alloy. Tribological tests over 2.5 h at physiological loads (0.75 MPa) revealed that SPMK-g-PEEK maintains low friction (μ< 0.024) and minimises equilibrium strain, significantly reducing forces on the cartilage interface. Post-test analysis showed no notable damage to the cartilage interfacing against the SPMK functionalised surfaces.The application of a constitutive biphasic cartilage model to the experimental strain data reveals that SPMK surfaces increase the interfacial permeability of cartilage in sliding, facilitating fluid and strain recovery. Unlike previous demonstrations of sliding-induced tribological rehydration requiring specific hydrodynamic conditions, the SPMK-g-PEEK introduces a novel mode of tribological rehydration operating at low speeds and in a stationary contact area. SPMK-g-PEEK surfaces provide an enhanced cartilage counter-surface, which provides a highly hydrated and lubricious boundary layer along with supporting biphasic lubrication.Soft polymer surface functionalisation of orthopaedic implant surfaces are a promising approach for minimally invasive synovial joint repair with an enhanced bioinspired polyelectrolyte interface for sliding against cartilage. These hydrophilic surface coatings offer an enabling technology for the next generation of focal cartilage repair and hemiarthroplasty implant surfaces.

Approach for accurately characterizing nonlinear viscoelastic and nonrecoverable properties of asphalt binders based on MSCR test

The multiple stress creep and recovery (MSCR) test has been widely accepted as a more practical method for accessing the rutting performance of asphalt binders than the conventional Superpave rutting factor test. Nevertheless, there still exists a major concern that 9-s recovery time in each cycle may not guarantee a sufficient recovery of the recoverable viscoelastic strain, especially for modified asphalts, and this could result in a potential inaccuracy in characterizing rutting resistance of binders. To effectively separate the recoverable nonlinear viscoelastic (NLVE) and nonrecoverable responses in the MSCR test, this paper presents a new approach that adopts an additional one-cycle low-stress creep and recovery (LSCR) test and the Schapery’s NLVE model in analyzing the MSCR test data. The LSCR test, covering the entire duration of the MSCR test (200 s), can well determine the linear viscoelastic (LVE) recoverable properties that the Schapery’s model requires for NLVE analysis. The results show that with the recoverable and nonrecoverable responses accurately isolated, the proposed approach rigorously redefines the indicators of the percent recovery R and nonrecoverable creep compliance Jnr in the MSCR test, and therefore effectively eliminates the inaccuracy of the original indicators caused by incomplete recovery of the recoverable strain component in each cycle. Also, the developed method exactly characterizes stress dependence and permanent deformation resistance of asphalt binder, thus offering an effective and accurate approach for high-temperature performance evaluation of asphalt binders.

Giant elastocaloric effect covering a wide temperature region in a directionally solidified Ni50Mn30Ti20 alloy

High-performance elastocaloric materials coupled with giant elastocaloric effect and broad working temperature range are highly demanded for solid-state refrigeration applications. Here, we demonstrate giant elastocaloric response covering a wide temperature region in a directionally solidified Ni50Mn30Ti20 alloy with the columnar-grained microstructure and <001>A preferred orientation. Owing to the effective reduction in the transformation temperatures (∼200 K) through composition tuning, large superelastic response with the recovery strain of 9% is obtained from 213 K to 333 K, giving rise to the maximum adiabatic temperature change as high as −31.3 K upon unloading. Moreover, giant |ΔTad| values higher than 20 K have been well achieved within a wide temperature window of 90 K ranging from 223 K to 313 K.

All-Nanochitin-Derived, Super-Compressible, Elastic, and Robust Carbon Honeycombs and Their Pressure-Sensing Properties over an Ultrawide Temperature Range.

Elastic carbon aerogels show great potential for various applications but are often hindered by structure-derived fatigue failure, weak elasticity with low compressibility, and low stress and height retention. Herein, we demonstrate a super-elastic and fatigue-resistant nanochitin-derived carbon honeycomb with honeycomb-like anisotropic microstructures and carbon-based molecular structures, which was tailored by optimizing the nanochitin concentrations and carbonization temperatures. The carbon honeycomb fabricated at a nanochitin concentration of 1.0 wt % and a carbonization temperature of 900 °C demonstrated anisotropic honeycomb channels, nanofibrous network channel walls with few cracks, and weak interactions between the carbonized nanochitin, which afforded high compressibility with up to 90% strain and complete recovery. In particular, the carbon honeycomb provided good fatigue resistance with high stress and height retentions of 87 and 94%, respectively, after more than 10,000 compression cycles at 90% strain. Moreover, the tailored anisotropic honeycomb channels and molecular structures endowed the carbon honeycomb with elasticity even under severe conditions, such as exposure to flame (approximately 1000 °C) and liquid nitrogen (approximately -196 °C). Owing to these properties, the nanochitin-derived carbon honeycomb could act as a high-sensitivity pressure sensor for a wide working pressure range of 0-185.5 kPa and ultrawide temperature range of -196-600 °C. This study can provide a promising route to develop all-biomass-derived, super-elastic, and fatigue-resistant carbon materials for pressure sensing under harsh conditions and for versatile electronic applications.

Oscillatory and rotational rheological characterization of jackfruit peel cellulose suspension: Effect of concentration, pH and ionic strength

Jackfruit peel (JP) is considered as an agro-industrial lignocellulosic waste which can be utilized for cellulose isolation. In this work, a yield of 25.6% cellulose was obtained from JP powder through alkali (NaOH) and bleaching (NaClO2) treatment along with the elimination of lignin, hemicellulose and other non-cellulosic constituents. The IR spectroscopy confirmed the existence of cellulose content in the final extracted product. The SEM images and EDX spectra demonstrated the microstructure of cellulose fibrils and their major elemental components as carbon and oxygen, respectively. The XRD data validated the crystalline (69.42%) nature of the cellulose. The various rheological attributes of cellulose suspension like the flow properties and dynamic rheological characteristics were examined at different pH, ionic strength and cellulose concentration. The cellulose suspensions exhibited shear thinning nature for all the pH, ionic concentration and cellulose concentrations. The change in viscoelastic behaviour of the cellulose suspension was noticed with the rise in storage modulus (G′) value with ionic concentration. The pH of the cellulose suspension had a significant impact on its viscoelastic properties due to the modifications in interfibrillar interactions of cellulose fibrils. The temperature ramp test showed that the G’ is independent of the heating of cellulose suspension from 20 °C to 80 °C. The creep and thixotropic recovery test revealed that upon removal of stress, there is the recovery of strain and viscosity up to a certain extent, which is a characteristic property of viscoelastic material.

Shape memory effect and thermal conductivity of PLA/h‐BN composites

AbstractThe use of polylactic acid‐based composites as self‐fixing materials is a promising technology for various applications due to the shape memory effect of polylactide. In this study, hexagonal boron nitride (h‐BN) was mixed with polylactic acid (PLA) with concentrations of 0, 0.5, 1, 2, 5, and 10 wt% for subsequent film casting. The morphology, mechanical and thermal characteristics of the composite were investigated. With a high percentage of filler content, a decrease in thermal conductivity was observed. The recovery strain decreased with increasing the filler content. The amount of h‐BN had a mixed effect on the recovery stress of the composite. Thus, PLA/h‐BN composites represent a new research idea to study the influence of a heat‐conducting filler on a shape memory effect in polylactic acid.Highlights Correlation between crystallinity, thermal and shape memory properties was described Decrease in thermal conductivity was observed with the large percentage of filling The composite with 2 wt% of h‐BN exhibits optimal characteristics of SME Recovery strain decreased with the rise of filler content Amount of h‐BN had a non‐trivial effect on the recovery stress of composite

A Simple Strain Recovery based A-posteriori Error Estimator for Laminated Composite Plates

A globally and locally reliable strain recovery based a-posteriori error estimator for laminated composite plates is proposed in this study. An extensive check of the local and global quality of the proposed error estimator is carried out for the bending problem. Effect of material orientation, ply stacking sequence, boundary condition and loading profile is carried out for a square plate. It is found that this estimator is reliable for almost all the cases, and even predicts errors for the preasymptotic range reliably.

Laser powder bed fusion of full martensite Cu-Al-Mn-Ti alloy with good superelasticity and shape memory effect

A copper-based shape memory alloy Cu-11.85Al-3.2Mn-0.1Ti with desirable superelasticity and shape memory effect was printed by laser powder bed fusion (LPBF), which demonstrated the feasibility of engineering applications through the deformation and recovery of complex corrugated structural components. The microstructure of LPBF-printed Cu-11.85Al-3.2Mn-0.1Ti was composed of full β1' martensite with a high density of stacking faults and twin crystals. The abnormal phase constitution of full martensite in the printed Cu-11.85Al-3.2Mn-0.1Ti is correlated to substrate preheating and thermal cycling during LPBF. The start and finish transformation temperatures of martensite (Ms and Mf) and austenite (As and Af) were 172.5 °C, 144.4 °C, 180.2 °C and 209.2 °C, respectively. The printed samples exhibited compressive strength of 1319 MPa with a compressive fracture strain of 16.1%. The printed alloy displayed remarkable superelasticity and shape memory effect. The superelastic recovery rate reached a maximum value of 87% at 6% pre-strain, corresponding to a superelastic recovery strain of 5.22%. The mechanism of superelasticity is attributed to the rubber-like deformation behavior. Moreover, the pre-strained samples demonstrated a desirable shape memory effect under the temperature stimulation, where the shape memory recovery rate of up to 100% at a pre-strain of 6%, corresponding to a shape memory recovery strain of 0.78%. The reasons for the favorable shape memory effect were: (i) the thermoelastic inversion of martensitic transformation and (ii) the precipitation of γ2(Cu9Al4) phase along the grain boundaries. As a result, these findings suggest that the LPBF technique can produce Cu-Al-Mn-Ti parts with superelasticity and good shape memory properties, which provided valuable references for printed copper-based shape memory alloys with full martensitic structures.

Highly elastic and degradable vitrimeric elastomers using polycondensation

Introducing covalently crosslinked network to polymer matrix can merge the advantages in reprocessing and durability of polymers. In this contribution, a series of high-performance vitrimeric elastomers were achieved via polycondensation. The topological structures of polymers were tuned by varying the feeding ratios of bisacetoacetate, hex‑substituted bisacetoacetate, bisamine and tris(2-aminoethyl)amine. With these structural manipulations, the vitrimeric elastomers presented great elastic recovery properties (strain recovery value up to 80%) benefiting from the introduction of long chain branch. Furthermore, the elastomers exhibited excellent reprocessing property, water vapor/oxygen barrier and adhesive properties. Specially, the elastomers could be degraded into monomer under acid conditions which enabled the elastomer synthesis again in closed loop recycling system. The ease of the polycondensation in this work to prepare highly elastic and recyclable vitrimeric elastomers demonstrated exciting opportunities for the synthesis of sustainable polymers.

Wrap-like transfer printing for three-dimensional curvy electronics.

Three-dimensional (3D) curvy electronics has wide-ranging application in biomedical health care, soft machine, and high-density curved imager. Limited by material properties, complex procedures, and coverage ability of existing fabrication techniques, the development of high-performance 3D curvy electronics remains challenging. Here, we propose an automated wrap-like transfer printing prototype for fabricating 3D curvy electronics. Assisted by a gentle and uniform pressure field, the prefabricated planar circuits on the petal-like stamp are integrated onto the target surface intactly with full coverage. The driving pressure for the wrapping is provided by the strain recovery of a prestrained elastic film triggered by the air pressure control. The wrapping configuration and strain distribution of the stamp are simulated by finite element analysis, and the pattern and thickness of the stamps are optimized. Demonstration of this strategy including spherical meander antenna, spherical light-emitting diode array, and spherical solar cell array illustrates its feasibility in the development of complex 3D curvy electronics.

Relationship between programming stress and residual strain in FDM 4D printing

4D printing with fused deposition modeling (FDM) enables the production of smart structures using smart materials that can change their shape over time. During the printing process, stresses are introduced into the structure that are relieved when exposed to an external stimulus, in this case, when raising the temperature above the glass-transition temperature TTrans\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${T}_{Trans}$$\\end{document}. This article investigates the relationship between stress and strain during 4D printing. We mounted the print platform on load cells to measure the forces in x-direction during printing. Flat hinges made of PLA are printed and are activated by immersion in hot water, which causes bending of the hinge areas. We varied nozzle temperature, print speed, and melt zone length to investigate their influence on programming force and post-activation curvature. Programming force and curvature translate into stress and strain, when specimen geometry is taken into account. The results are approximated by a linear relationship between programming stress and recovery strain. However, these gradients are different for each printing parameter. Lower nozzle temperature, shorter melt zone and higher print speed all result in higher forces and higher curvatures. However, increasing the forces by raising print speed results in a smaller increase in curvature than when the same increase in force is applied by lowering nozzle temperature. This is partly due to the heating and cooling process of the structure, which in turn depends on the printing parameters.

Nonlinear unloading-reloading behavior and macro/micro models during multi-pass cyclic hot compression of aluminum alloys

Unloading-reloading behavior is of crucial importance in multi-pass forming of metal materials, the nonlinear of which creates challenges for the analysis of springback and residual stress of the precision manufacture components. Especially for the hot bulk-formed material at elevated temperature, the thermal-mechanical coupling effect makes the unloading-reloading process more complicated and thus more difficult to model and control, which remains to be solved. In this paper, the unloading-reloading behavior dependent on strain rate and deformation temperature and its model of hot compressive deformed aluminum alloys at elevated temperature were studied from the macroscopic and microscopic perspectives. The effect of deformation temperature, strain rate and strain on the unloading-reloading behavior of hot compressive deformed aluminum alloys were revealed, and corresponding micro-mechanism was elucidated and then discussed. Macroscopically, a strain-, strain rate- and temperature-dependent nonlinear macroscopic model describing unloading–reloading process was established, which can be used to describe the unloading stress relaxation and predict springback during multi-pass hot bulk forming. Microscopically, it is suggested that the relaxations of back stress and forest dislocation hardening stress collectively affect the unloading stress relaxation, due to the decrease of mobile and immobile (forest) dislocation density in unloading, while the dislocation-related behavior before yield, i.e. the dislocations’ bowing-out under pinning, affects the reloading behavior. Based on the micro-mechanism, a physically-based micro-model for describing unloading–reloading was constructed which further is used to clarify the physical mechanism of internal stress relaxation and inelastic strain recovery under multi-pass hot compression deformation. The comparison of the calculation results of the models with those of experiments indicated that the established unloading–reloading macroscopic and microscopic models are able to predict the stress–strain response upon the unloading–reloading loop of multi-pass cyclic hot compression deformation.