Release Of Internal Stress Research Articles

Effects of Hydrothermal Environment on the Deformation of the Thin Bamboo Bundle Veneer Laminated Composites

To overcome warping in thin bamboo bundle veneer laminated composites (TBLC), their hydrothermal deformation characteristics were systematically investigated in this study. It was found that TBLCs accelerated the release of internal stress in the thickness direction in a hydrothermal environment, which increased their warpage. TBLCs showed increased warpage in the width and diagonal directions upon increasing the temperature. The warpage of Type E increased by 155.88% and 66.67% in the width and diagonal directions, respectively, when the temperature increased from 25 C to 100 C. The symmetrical TBLC with cross-lay-up and odd layers displayed better hydrothermal stability. We revealed that the deformation of the TBLCs could be regulated under the synergistic effect of water and temperature. These results provide a scientific basis for improving the uniformity of bamboo bundle composite materials and for developing thin bamboo bundle fiber composite materials with designable structures and controllable performance.

Structure and Thermal Expansion of Cu−90 vol. % Graphite Composites

Copper–graphite composites are promising functional materials exhibiting application potential in electrical equipment and heat exchangers, due to their lower expansion coefficient and high electrical and thermal conductivities. Here, copper–graphite composites with 10–90 vol. % graphite were prepared by hot isostatic pressing, and their microstructure and coefficient of thermal expansion (CTE) were experimentally examined. The CTE decreased with increasing graphite volume fraction, from 17.8 × 10−6 K−1 for HIPed pure copper to 4.9 × 10−6 K−1 for 90 vol. % graphite. In the HIPed pure copper, the presence of cuprous oxide was detected by SEM-EDS. In contrast, Cu–graphite composites contained only a very small amount of oxygen (OHN analysis). There was only one exception, the composite with 90 vol. % graphite contained around 1.8 wt. % water absorbed inside the structure. The internal stresses in the composites were released during the first heating cycle of the CTE measurement. The permanent prolongation and shape of CTE curves were strongly affected by composition. After the release of internal stresses, the CTE curves of composites did not change any further. Finally, the modified Schapery model, including anisotropy and the clustering of graphite, was used to model the dependence of CTE on graphite volume fraction. Modeling suggested that the clustering of graphite via van der Waals bonds (out of hexagonal plane) is the most critical parameter and significantly affects the microstructure and CTE of the Cu–graphite composites when more than 30 vol. % graphite is present.

Experimental investigation on the thermo-mechanical deformation of thermo-induced shape memory polyurethane

The influences of loading conditions on the shape memory effect (SME) of thermo-induced shape memory polyurethane (TSMPU) are investigated by series of thermo-mechanical deformation experiments. The variables related to mechanical responses and the strain contours are extracted to analyze the SME. It is shown that the recovery ratio gradually decreases and the residual strain gradually accumulates with the increase of number of cycles. Then, the internal stress is obtained from the experimental results under different deformation histories. It is unexpectedly found that there is a linear relationship between the internal stress and recovery ratio under the same peak strain with different strain rates and number of cycles. Meanwhile, the formation and release of internal stress as the deformation mechanism can reasonably reflect the influence of deformation history on the SME. The experimental results are useful to deepen the understanding of SME and establish a constitutive model considering the thermo-mechanical deformation history of TSMPU.

Phase composition, microstructure, and microwave dielectric properties of non-stoichiometric yttrium aluminum garnet ceramics

A series of Y3+xAl5O12 (−0.12 ≤ x ≤ 0.12) ceramics were prepared via a solid-state reaction under a vacuum sintering environment to investigate the impact of variations in different cation composition on the phase composition, microstructure, and microwave dielectric properties of yttrium aluminum garnet (Y3Al5O12; YAG) ceramics. X-ray diffraction analyses revealed that Al2O3 precipitates in the Al-excess samples. In contrast, the YAlO3 secondary phase appeared in the Y-excess samples, and variations in the content of both contributed to an increase in lattice parameters. The Rietveld refinement results showed that the lattice parameters and unit cell volume increased as the |x| value increased. The secondary phases of granular Al2O3 and YAlO3 crystallized at the extended grain boundaries in samples with excess Al and Y were visible in the scanning electron microscopy images. The electron diffraction patterns confirmed that a small amount of Y2O3 promoted continuous phase boundaries and facilitated the release of internal stresses. Ultimately, after sintering at 1750 °C for 12 h and at x = 0.03, the microwave dielectric properties of the non-stoichiometric YAG ceramic sample were εr = 11.18, Q×f = 236936 GHz, and τf = −35.9 ppm/℃.

Synergistic effects of fluid flow and hydrostatic pressure on the degradation of epoxy coating in the simulated deep-sea environment

The anticorrosion performance of epoxy varnish coating/steel systems under a simulated deep-sea fluid-pressure (FP) environment was studied using physical performance tests, electrochemical measurements and finite element modeling. The results show that fluid flow provides an initial kinetic energy to water, and the hydrostatic pressure difference reduces the energy barrier for water absorption, leading to premature corrosion of the coated steel under FP condition. Detachment of polymer pieces from the coating surface and large areas of blistering are found to occur under the synergistic effects of fluid and pressure, which can be attributed to the release of internal stress at coating defects.

A comprehensive investigation of acrylates photopolymerization shrinkage stress from micro and macro perspectives by real time MIR-photo-rheology

The acrylate photocuring system with the high curing rate are widely used in various industries. However, the rapid polymerization kinetics can cause the surge of resin viscosity and the gel point can be reached in advance. This can results in the restriction of molecular chain movement and internal stress release is thus hindered during the photopolymerization. Finally, it usually leads to the poor performance of UV-cured materials with lower double bond conversion and large volume shrinkage and shrinkage stress. In order to understand the relationship between properties and polymerization process, this paper employed the combined technique of the mid-infrared spectrometer and photo-rheometer to monitor the acrylate free radical photopolymerization process. The effects of curing conditions (curing temperature, radiant intensity and sample thickness) and system compositions (diluent functionality, diluent segment length) on the rheological properties and chemical conversion of materials were fully investigated. Finally, a standard protocol is established to evaluate the reactivity, mechanical properties and volume shrinkage of the photocuring system. Simultaneously, the double bond conversion of the reaction system can be achieved through calculation of peak area of IR absorption. A complex nonlinear relationship between the storage modulus, shrinkage stress and double bond conversion is established.

Enzymatically Triggered Jack-in-the-Box-like Hydrogels.

Enzymes can support the synthesis or degradation of biomacromolecules in natural processes. Here, we demonstrate that enzymes can induce a macroscopic-directed movement of microstructured hydrogels following a mechanism that we call a "Jack-in-the-box" effect. The material's design is based on the formation of internal stresses induced by a deformation load on an architectured microscale, which are kinetically frozen by the generation of polyester locking domains, similar to a Jack-in-the-box toy (i.e., a compressed spring stabilized by a closed box lid). To induce the controlled macroscopic movement, the locking domains are equipped with enzyme-specific cleavable bonds (i.e., a box with a lock and key system). As a result of enzymatic reaction, a transformed shape is achieved by the release of internal stresses. There is an increase in entropy in combination with a swelling-supported stretching of polymer chains within the microarchitectured hydrogel (i.e., the encased clown pops-up with a pre-stressed movement when the box is unlocked). This utilization of an enzyme as a physiological stimulus may offer new approaches to create interactive and enzyme-specific materials for different applications such as an optical indicator of the enzyme's presence or actuators and sensors in biotechnology and in fermentation processes.

Significant reduction of phase-transition hysteresis for magnetocaloric (La1-xCex)2Fe11Si2Hy alloys by microstructural manipulation

First-order magnetostructural phase transitions are inevitably accompanied by large hysteresis, which evokes non-ignorable energy losses a main challenge for the utilization of giant magnetocaloric effect (MCE) materials in the emerging magnetic cooling technology. In this work, we present a novel approach to reduce the hysteresis and simultaneously to remain the giant entropy change in La-Fe-Si-based MCE alloys by microstructural manipulation. The microstructure evolution is comprehensively studied by high angle annular dark field-scanning transmission electron microscope, three-dimensional atom probe and geometric phase analysis. For the LaFe13-xSix system via co-doping of Ce and H atoms, we have observed the appearance of nanograins in size range of 5 – 50 nm that is totally different from the widely reported compositions. Such refinement can be ascribed to the release of internal stress caused by the inhomogeneous distribution of hydrogen atoms. With the formation of the nanocrystals in (La1-xCex)2Fe11Si2Hy alloys, the value of hysteresis loss can be monotonously reduced from 48.3 to 0.6 J kg−1. More importantly, the magnetostructural transition keeps an obvious first-order type, which leads to a large adiabatic temperature change of 2.03 K in 1.3 T upon 105 magnetic cycles, as well as a high reversible refrigeration capacity of 89.4 J kg−1 for (La0.6Ce0.4)2Fe11Si2Hy.

Shape recovery analysis of the additive manufactured 3D smart surfaces through reverse engineering

Novel methodology to estimate the shape memory behavior of smart structure fabricated by additive manufacturing is presented by merging 3D laser scanning and reverse engineering (RE). The specimens made from a temperature-stimulated smart material-PLA, is printed by fused deposition modelling (FDM), which adds the functionality of shape memory with additive manufacturing. Initially, to verify the estimation of shape memory behavior, standard rectangular samples are designed, printed, scanned and regenerated by RE. The results of the shape memory analyses are in close agreement with the previous works, which validate the methodology to be adopted from more complex geometries. Subsequently, the procedure is employed to the three-dimensional paraboloid surface for the development of surfaces during the different phases of the cycle. Successful rapid restoration of all the surfaces has been achieved with the accurate measurements which could be a time-consuming process using the conventional measurement techniques or involve high sophistication methodology. The shape recovery and shape fixity of the paraboloid surfaces are in congruence with the values calculated for the standard samples. RE models are also used to estimate the effect of stresses applied during the programming stage and subsequent release of internal stresses in the recovery stage above the glass transition temperature of PLA.

Structural Plasticity in the Frank‐Kasper Realm: Chemical Pressure Roles of the μ‐ and χ‐Phase Units in the Mo−Fe−Cr System

AbstractIntermetallic phases have been known to exhibit a wide diversity since Pauling's seminal investigations into NaCd2 in the 1920s that, along with Cd3Cu4 and Mg2Al3, was shown by Samson to crystallize with a giant cubic cell containing >1000 atoms. The concept of structural plasticity – the notion that complex structures emerge from the release of internal stresses that would arise in simpler structures – has recently been used to account for one family of intermetallics, tracing the structures of Ca2Ag7, Ca14Cd51, CaPd5, and CaCd6 to chemical pressure (CP) issues in the CaCu5 type. Here, we extend the ideal of structural plasticity closer to the giant cells elucidated by Pauling and Samson through its application to a series of Mo−Fe−Cr Frank‐Kasper phases. We begin with a DFT‐CP analysis of the MgZn2‐type phase MoFe2, which serves as a parent structure to μ‐Mo6Fe7 and χ‐Mo5Cr6Fe18. The analysis reveals negative CPs around the Mo atoms arising from collisions between the Fe atoms. Tighter Mo coordination is provided in the μ‐ or χ‐phases by substituting some of the Friauf polyhedra of MoFe2 with either μ‐ and χ‐phase units, resulting in layers or blocks of Laves‐like connectivity. Sites preferences in the μ‐phase and the role of Cr substitution in the χ‐phase are explained through the dual lenses of CP and electronegativity. Parallels to the features of NaCd2 hint that such giant‐unit‐celled intermetallics can represent striking manifestations of structural plasticity.

Effect of Nonbonded Hydrogen on Structure and Properties of DLC Films

a-C: H films were prepared on the surface of 316 L stainless steel, aluminum foil and single crystal silicon (100) waferby PE-CVD. The effect of nonbonded hydrogen on the thermal stability and properties of the films was discussed. The analysis results show that the H elements in this a-C: H film mainly exists as free states of H molecules or H atoms. The graphitization stage of the a-C: H films prepared in this experiment are obviously divided into two stages by the precipitation of non-bonded H 564°C. At the same time, the escape of non-bonded H significantly resulted in the release of internal stress and significantly promoted the second stage graphitization. Under the action of hydrogen evolution and graphitization, the microstructure of carbon film is rearranged and agglomerated in a certain direction.

The effect of strain rate on mechanical properties and microstructure of a metastable FeMnCoCr high entropy alloy

The relationship between mechanical properties and microstructures of a metastable dual-phase high entropy alloy Fe50Mn30Co10Cr10 under uniaxial tensile testing at different strain rates (10-3 s-1~103 s-1) has been studied systematically. As the strain rate increases, yield strength, ultimate tensile strength and uniform elongation decrease first and then increase. Namely, when the strain rate is 10-3 s-1, yield strength and ultimate tensile strength are 280 MPa and 720 MPa, respectively, with the uniform elongation of 64.2%. When the strain rate is increased to 1 s-1, yield strength and ultimate tensile strength are 300 MPa and 672 MPa, respectively, and uniform elongation decreases to 49.2%. When the strain rate reaches 103 s-1, yield strength and ultimate tensile strength increase to 380 MPa and 810 MPa, respectively, while uniform elongation is elevated to 67%. As dynamic deformation is affected by the adiabatic heating, the stacking fault energy of the alloy is increased by ~13 mJ m-2 at a strain rate of 103 s-1 compared with that in the quasi-static condition. Under quasi-static loading, martensitic transformation is the dominant deformation mechanism. Under dynamic loading, when the strain is low the deformation induced phase transformation dominates, whereas as the loading proceeds mechanical twinning becomes the dominant deformation mode. At the same time, the adiabatic temperature rise under dynamic tests also causes a reverse transformation from ε-martensite to austenite. Accordingly, the release of internal stress and the formation of soft and ductile austenite jointly contribute to the elevated uniform elongation of the material. Both mechanical twinning and martensitic reverse transformation promote the microstructure to be dynamically refined, so that the alloy shows the good plasticity while maintaining the high ultimate tensile strength at dynamic strain rates.

Significant Reduction of Phase-Transition Hysteresis for Magnetocaloric (La 1-xCe x) 2Fe 11Si 2H y Alloys by Microstructural Manipulation

First-order magnetostructural phase transitions are inevitably accompanied by large hysteresis, which evokes non-ignorable energy losses a main challenge for the utilization of giant magnetocaloric effect (MCE) materials in the emerging magnetic cooling technology. In this work, we present a novel approach to reduce the hysteresis and simultaneously to remain the giant entropy change for La-Fe-Si-based MCE alloys by microstructural manipulation. The microstructure evolution is comprehensively studied by high angle annular dark field-scanning transmission electron microscope (HAADF-STEM), three-dimensional atom probe (3DAP) and geometric phase analysis (GAP). For the LaFe13-xSix system via co-doping of Ce and H atoms, we have observed the appearance of nano-sized grains (5 ~ 50 nm) that is totally different from the conventional compositions. Such refinement can be ascribed to the release of internal stresses caused by the inhomogeneous distribution of hydrogen atoms. With the increase of Ce content for (La1-xCex)2Fe11Si2Hy alloys, the value of hysteresis losses can be significantly reduced from 48.3 to 0.6 J kg-1. More importantly, the magnetostructural transition keeps the obvious first-order type, which leads to a large adiabatic temperature change of 2.2 K in 1.3 T upon 105 magnetic cycles, as well as a high refrigeration capacity of 89.4 J kg-1.

Study on Dynamic Strain Distribution in Rock Broken by Abrasive Slurry Jet

To reveal the stress and strain field in rock under abrasive slurry jet (ASJ) impacting, this paper carried out experiments to measure the dynamic rock strain using a novel strain measurement approach, which is the non-contact strain measurement system. Moreover, it was compared with the theoretical analysis of stress propagation in rock. The results showed that dynamic strain will propagate within rock in the form of spherical waves, whose maximum value is proportional to the jet pressure and inversely proportional to the square of the propagation distance. In the process of ASJ impacting rock, strain will increase from zero to a maximum value in milliseconds. Dynamic strain in rock is fluctuates due to the release of internal stress caused by the rock crushing, which can reflect that rock will be broken by ASJ in the form of stepped-failure. The paper reveals the dynamic strain distribution in rock broken by ASJ and provides a new idea and method for studying the rock failure mechanism under ASJ impacting.

Internal stress and deformation analysis of ultra-thin plate-shaped optical parts in thinning process.

During the thinning process of plate-shaped optical parts (PSOP), the release of internal stress would cause the deformation of ultra-thin PSOP, which deteriorates the processed surface figure. The stress-release-induced deformation is hard to be predicted and controlled due to the difficulty in measuring the tiny internal stress in ultra-thin PSOP. In this paper, an analytical model is established to depict the variation of internal stress and deformation during the thinning process. It can be used to calculate the initial internal stress distribution along the thickness according to the deformation and the residual thickness of the sample. Meanwhile, the model can predict the residual stress distribution and deformation in the whole thinning process. The prediction results obtained from the proposed model agree well with the experimental results, and the prediction error is less than 13%. The presented model has great significance for the analysis of the tiny internal stress and then guide the process of making ultra-thin PSOP.

Controllable Shape Changing and Tristability of Bilayer Composite.

The programmable shape transition of a two-dimensional sheet to a three-dimensional (3D) structure in response to a variety of external stimuli has recently attracted increasing attention. Among the various shape changing materials, shape memory polymers (SMPs) can fix their temporary shape and/or their length and recover under proper thermal treatment. In this work, we create a bilayer composite by bonding one layer of elastomer with one layer of stretched SMPs, which can undergo a series of shape transitions via the storage and release of internal stresses. The programed shapes are achieved by adjusting the orientation and elongation of the SMPs. Meanwhile, the 3D structures exhibit tristability and can transit between hemihelical, left-handed helical, and right-handed helical shapes. Both theoretical analysis and finite element simulations were conducted to understand the mechanism of shape transformation and used to predict the deformed configuration by adjusting preprogramming parameters. Our work provides a new strategy and design space for fabricating smart reconfigurable structures and paves way for the design and development of bioinspired four-dimensional active matter for a broad range of applications in intelligent materials.

Influence of heat treatment on microstructures and magnetic properties of Fe-based soft magnetic composites prepared by co-precipitation method

Abstract Fe/(NiZn)Fe2O4 soft magnetic composites (SMCs) were prepared by the co-precipitation method using pure atomised iron powder and chloride as raw materials. The morphology and element content of coated powders were analysed using scanning electron microscopy and energy dispersive spectroscopy. Moreover, we used electron probe micro-analyser, X-ray diffraction and Raman spectrum to systematically investigate the effect of heat treatment temperature in argon on the microstructure of SMCs, including O element distribution, phase formation and internal stress. The results show that by increasing heated temperature, the oxygen content at the boundary of the SMCs increased and the internal stress release of SMCs was more complete. Moreover, non-magnetic FeO would be generated in SMCs when the heat treatment temperature reached 550 °C. We also discussed the relationship of heat treatment temperature and atmosphere with AC magnetic properties of the samples, including permeability, core loss and coercivity. The results indicate that the SMCs annealed at 450 °C in argon had the optimum magnetic performance (µ = 55.8, P cv = 12.2 kw/m3, H c = 22.6 A/m). At the heat treatment temperature of 500 °C, the argon-annealed samples presented better magnetic properties and higher frequency stability than air-annealed samples.

Temperature-Time Superposition Effect on Compressive Properties of AZ31B Magnesium Composite Foams

Magnesium composite foams with 10 vol. % of hollow ceramic microspheres (CMs) were prepared by modified melt foaming method. Specimens with homogeneous pore structures were subjected to various heating temperature (150, 250, 320, 400, and 500 °C, respectively) and enduring times (1, 2, 4, 6, and 24 h, respectively). Evolution of microstructure and mechanical properties of the samples, before and after the heating processes were examined by applying X-ray diffraction technique (XRD), differential scanning calorimetry (DSC), scanning electron microscopy (SEM), and quasi-static uniaxial compression test. The results showed that as heating temperature and enduring time increasing, β-Mg17Al12 phases gradually dissolved, resulting in a solid-solution strengthening effect. Meanwhile, internal stress relaxation in the matrix leads to the decrease of yield strength and micro hardness of the specimens. When compared with the unheated foams, the treated specimens possessed lower micro-hardness, yield strength, and energy absorption capacity due to the dissolution of β-Mg17Al12 phases and the release of internal stress. However, higher strain hardening exponents for almost all of the treated composite foams were observed and the reasons were discussed. It is proposed that more factors should be taken into account when using heated composite foams in practical applications.

First-order martensitic transformation in Heusler-type glass-coated microwires

Properly annealed Ni-Mn-Ga glass-covered microwires exhibit a hysteretic anomaly on the temperature dependence of magnetization attributed to the first order martensitic transformation. The temperatures of the structural and magnetic transitions are drastically affected by annealing conditions. Annealed glass-coated Ni-Mn-Ga microwires show a Curie temperature shift close to room temperature. The temperature and magnetic field dependences of magnetization are discussed in terms of atomic disorder, the release of internal stresses, and recrystallization after annealing.

Long term ageing of polyamide 6 and polyamide 6 reinforced with 30% of glass fibers: physicochemical, mechanical and morphological characterization

Hygrothermal ageing of polyamide 6 (PA6) and polyamide 6 reinforced with 30 wt% of glass fibers (PA6GF30) was undertaken. Immersion was conducted in distilled water at 90 degrees C and 100% relative humidity (RH) for up to 80 days (1920 h). Results revealed a noteworthy decrease either in glass transition temperature T-g or in tensile properties, at early stage of ageing, for both studied materials. This decline was mainly caused by the plasticization effect of water and the weakness of the interfacial interactions leading as a consequence to a loss of adhesion between fiber and matrix. Afterwards, physical and mechanical properties decrease monotonically testifying the occurrence of exhaustive damages and chemical reaction phenomena. Such phenomena were yellowing and crazing formation which were observed for both materials after 1920 h of conditioning. The former is caused by the thermo-oxidation whereas the latter results from the release of internal stresses induced by water sorption. These chemical reactions were monitored by infrared spectroscopy. Thus, an increase of the free N-H stretch and the carbonyl groups (imides) was noted. Accordingly, it seems that long term immersion in distilled water at high temperature induces chemical reactions which indicate the severity of the damage.