Formation Of Internal Stresses Research Articles

Research on residual stress control in polycarbonate molding based on orthogonal analysis

Abstract Residual internal stress is a primary factor affecting the performance and quality of transparent food containers formed by polycarbonate injection molding. Adjusting the injection molding process parameters stands out as the most effective means to control residual internal stress. This study employs numerical simulation to analyze the influence of injection molding process parameters on material internal stress during the polymer injection molding process, providing a reference for optimizing molding process parameters. Initially, through an analysis of the mechanism of internal stress formation, it is determined that the internal stress of beverage containers is mainly composed of cooling internal stress. Consequently, a mathematical model based on thermo-viscoelastic constitutive relations is selected to numerically simulate and calculate the formation of cooling internal stress during the injection molding process of PC materials. Numerical simulation results are obtained accordingly. Building upon this, orthogonal experiments are conducted based on the analysis data from numerical simulation, and through experimental comparison, the consistency between numerical simulation results and experimental results is validated. Finally, combining the results of experiments with numerical simulation, it is concluded that melt temperature is the primary factor influencing molding internal stress, followed by mold temperature and holding pressure, with injection speed being the least influential factor on residual stress.

The Initial Stage of Climatic Aging of Basalt-Reinforced and Glass-Reinforced Plastics in Extremely Cold Climates: Regularities.

Detailed analyses of the reasons for changes in the mechanical parameters of fiberglass exposed to different climatic zones have been made available in the literature; however, such detailed studies of basalt plastic do not yet exist. It is possible to make reasonable conclusions on the climatic resistance of reinforced plastics by monitoring the deformation-strength characteristics in combination with fractographic and DMA analyses of the solar- and shadow-exposed parts of the plastics; additionally, one can conduct analyses of the IR spectrum and the moisture sorbtion kinetics. As a starting point for the climatic aging of polymer composite materials, it is necessary to accept the time of exposure in which the maximum values of the elastic strength properties of polymeric materials are achieved. Based on the results of the DMA analysis, it was found that, unlike basalt-reinforced plastics (where the material is post-cured exclusively at the initial stage of the exposure), in glass-reinforced plastic, a process of destruction occurs. The formation of internal stresses in the material and their growth were determined through observing the duration of climatic exposure. The formation of closed porosity, depending on the duration of exposure, can be assessed using the values of the increase in the average moisture content. A set of experimental studies has established that glass-reinforced plastics are subject to greater destruction under the influence of a very cold climate than the basalt-reinforced plastic.

Internal Stress Formation and Changes in Oxide Films on a Lead Alloy Anode Surface

Internal Stress Formation and Changes in Oxide Films on a Lead Alloy Anode Surface

Determination of the formulation and curing conditions of thermosetting epoxy resins for optimizing their properties and future use in gelcasting process

AbstractThis work investigates the suitability of epoxy resins for possible use in an industrial gelcasting process. Two different formulations are studied using complementary experimental techniques. Both reactive formulations are based on a difunctional epoxy prepolymer (aliphatic BDGE or aromatic RDGE) mixed with a polyamine hardener (TEPA). The combination of analytical chemistry (NMR and FTIR spectroscopy) and calorimetric measurements (DSC) are used to determine the optimal epoxy/amine ratio for each reactive formulation. The gelation and vitrification times are investigated by dynamic rheometry through kinetic analyses over a large range of fixed temperature. It is found that the RDGE‐TEPA mixture is more reactive than BDGE‐TEPA with shorter curing starting times. The thermomechanical analyses performed on both optimized formulations show that the RDGE‐TEPA material presents better performances after curing (Tg close to 80°C) than the BDGE‐TEPA system (Tg lower than ambient temperature). Then, the effects of the cooling cycle on the thermomechanical profile of each polymer are investigated. The RDGE‐TEPA formulation is found to be much more subject to the formation of internal stresses created during the cross‐linking and cooling step than the BDGE‐TEPA material. Considering all these criteria, the BDGE‐TEPA system is ultimately more suitable for the gelcasting process than RDGE‐TEPA.

Self-adhering implantable device of titanium: Enhanced soft-tissue adhesion by sandblast pretreatment

Self-adhering implantable devices, which can be immobilized inside the bodies without suturing nor organic glues, made of metallic biomaterials would be optimal devices for preventing device-related complications such as device migration after implantation. We reported previously that acid-treated commercially-pure titanium (CpTi) adhered directly and immediately on hydrous non-keratinized soft tissues. Herein, we investigated the influence of sandblasting as pretreatment for acid-treated CpTi to increase its soft tissue adhesiveness. First, the effects of sandblasting conditions (i.e., pressure, distance and time) were investigated in terms of the sandblasted surface area and the degree of deformation (i.e., internal stress formation) of CpTi films. The effect of the sandblasting on the immediate soft tissue adhesion of acid-treated CpTi was investigated using an ex vivo shear adhesion test with mouse dermal tissues. The optimal sandblasting pretreatment remarkably improved the soft tissue adhesion strength of acid-treated CpTi (102 ± 19 kPa) compared with the non-sandblasted counterparts (41 ± 2 kPa). Finally, the CpTi adhesive was applied for immobilizing a near field communication (NFC) device in vivo, and was shown to have strong immediate adhesion to muscle fascia.

Formation of residual stresses during quenching of Ti17 and Ti–6Al–4V alloys: Influence of phase transformations

The formation of internal stresses during quenching of titanium alloys from the β phase field are investigated both experimentally and by simulation, in order to show the effects of phase transformations. Two titanium alloys are considered: the β-metastable Ti17 alloy and the α+β Ti–6Al–4V alloy. During the quench into water of laboratory scale samples (40 mm diameter cylinders), no phase transformations occurred in the Ti17 alloy because of its β-metastable character and the fast cooling. However, β→α+β and β→α′ phase transformations occurred in the Ti–6Al–4V sample. Both alloys are compared in order to highlight the effects of the phase transformations. Residual stresses were determined by neutron diffraction, by the contour method and at the surface by the hole drilling method. A model for the coupled thermal, mechanical and metallurgical evolutions is established in order to simulate the quenching operations. The material model for the Ti17 alloy was established in a previous study. Regarding the Ti–6Al–4V alloy, modeling approaches and experimental data from literature are utilized to build the material model. From both experiment and simulation, it is found that the internal stress evolutions are governed by the thermal gradients. The phase transformations have a weak impact because of the small deformation strains induced by the phase change. Nevertheless, good prediction of the phase transformation kinetics is necessary for accurate simulations, because the α and α’ phases strengthen the alloy, thereby limiting the plastic straining at the origin of the residual stresses. As most plastic strains are cumulated at high temperature, the thermomechanical model should be established accurately over the temperature ranges in which there is a significant proportion of β phase.

Controlled composite processing based on off‐stoichiometric thiol‐epoxy dual‐curing systems with sequential heat release (SHR)

AbstractControl of curing rate and exothermicity during processing of thermosetting composite materials is essential in order to minimize the formation of internal stresses leading to mechanical and dimensional defects in the samples, especially in thick composite samples. It was recently proposed that sequential heat release, an approach based on the kinetic control of the curing sequence of dual‐curing thermosets, would enable a step‐wise release of the reaction heat and therefore a better control of conversion and temperature profiles during the crosslinking stage. In this article, it is shown experimental proof of this concept obtained by means of an instrumented mold that can be used for the processing of small samples with and without carbon fiber reinforcement. Safe processing scenarios have been defined by numerical simulation using a simplified two‐dimensional heat transfer model and validated experimentally.

Improvements of depolarization temperature, piezoelectric and energy harvesting properties of BNT-based ceramics by doping an interstitial dopant

An important problem of many BNT-based perovskite piezoelectric materials for real applications is to increase and/or to disappear the depolarization temperature (Td) as well as to achieve high piezoelectric properties. Many previous works, through various techniques, have tried to shift up the Td and also enhance piezoelectric properties. In the present work, we report an alternative way to increase the Td by using a boron oxide (B2O3) additive, where the boron ion in B2O3 is an interstitial dopant. The increase of Td is proposed to relate with an internal stress formation in the ceramic, due to the doping effect. Furthermore, improvements of remanent polarization (Pr), piezoelectric properties, loss tangent value and energy harvesting performance of the ceramics are also enhanced for the doped samples.

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.

Theoretical study of the prestressing strand's stress by computer modeling methods

The modern construction industry widely uses reinforced concrete structures, where high-strength prestressing strands are used. Key parameters determining strength and relaxation resistance are a steel microstructure and internal stresses. The aim of the work was a computer research of a stage-by-stage formation of internal stresses during production of prestressing strands of structure 1х7(1+6), 12.5 mm diameter, 1770 MPa strength grade, made of pearlitic steel, as well as study of various modes of mechanical and thermal treatment (MTT) influence on their distribution. To study the effect of every strand manufacturing operation on internal stresses of its wires, the authors developed three models: stranding and reducing a 7-wire strand; straightening of a laid strand, stranding and MTT of a 7-wire strand. It was shown that absolute values of residual stresses and their distribution in a wire used for strands of a specified structure significantly influence performance properties of strands. The use of MTT makes it possible to control in a wide range a redistribution of residual stresses in steel resulting from drawing and strand laying processes. It was established that during drawing of up to 80% degree, compressive stresses of 1100-1200 MPa degree are generated in the central layers of wire. The residual stresses on the wire surface accounted for 450-500 MPa and were tension in nature. The tension within a range of 70 kN to 82 kN combined with a temperature range of 360-380°С contributes to a two-fold decrease in residual stresses both in the central and surface layers of wire. When increasing temperature up to 400°С and maintaining the tension, it is possible to achieve maximum balance of residual stresses. Stranding stresses, whose high values entail failure of lay length and geometry of the studied strand may be fully eliminated only at tension of 82 kN and temperature of 400°С. Otherwise, stranding stresses result in opening of strands.

Investigation on microstructure and properties of laser solid formed low expansion Invar 36 alloy

This study elucidated the microstructure, defect formation, mechanical properties and thermal expansion properties of the laser solid formed (LSF) Invar 36. Its macrostructure is composed of irregular columnar dendritic grains grown epitaxially from the bottom up, and the average dendrite spacing gradually decreased as the scanning speed increased. The overlapping curved and horizontal bright white bands (planar growth region) alternated in the neighboring deposited layers. At a scanning speed of 500mmmin−1, cracks were absent, but when the scanning speed was significantly increased or decreased, cracks begin to form. Crack formation at extreme scanning speeds can be attributed to the segregation of impurity elements at the grain boundary and the formation of internal stresses during LSF. When the scanning speed increased from 400mmmin−1 to 800mmmin−1, the tensile strength increased by 22.1% and the elongation increased by 52.1%, and the fracture mechanism was also altered, from dimple plus cleavage fracture to dimple fracture. The coefficient of thermal expansion (CTE) of Invar 36 prepared by LSF is on par with those prepared via conventional processing method, which can be attributed to the presence of Ni in the alloy affecting the overall CTE of the alloy.

Finite Element Simulation of Heat Transfer in Laser Additive Manufacturing of AlSi10Mg Powders

Abstract In the digital world, direct metal laser sintering (DMLS) is known as a laser-based additive manufacturing process, which grabs the attention of manufacturing industries to make the components directly from metal powder. It is a high-temperature process in which the metal particles are fused with laser power to form a full dense homogenous structure. The quality of fabricated parts depends on the heat transfer mechanism, temperature distribution, and molten pool formation in the powder bed, which directly influence by the process parameters such as scan speed, laser power, hatch spacing, powder layer thickness, and laser spot diameter. So it is necessary and important to understand the characteristics and mechanism of these thermal processes. The heat transfer mechanism involves conduction, convection, and radiation, which affects the formation of internal stresses that leads to deformation. In response to this fact, it was indispensable to analyze the thermal behavior in the powder bed instantly during the DMLS process and subsequently monitor the process parameters in order to obtain acceptable manufacture outputs intended for further use. The present research work focused to develop a finite element model for the prediction of temperature distribution and molten pool formation during sintering process of AlSi10Mg composite powders using ANSYS 17.0. The temperature distribution, thermal history, molten pool dimension and sintering depth in direct metal laser sintering process were investigatedat different laser spot diameter and powder layer thickness. From the simulation result, it was observed that as the laser spot diameter increases from 2mm to 6mm, the surface temperature of the molten pool decreases 4626°C to 541°C. So that molten pool length, width and sintering depth of powder bed decrease 3mm to 0 mm. Similarly, as powder bed layer thickness increases from 1mm to 3mm, the field temperature of the molten pool decreases 1200°C to 1075°C and the sintering depth decreases 0.243mm to 0.112mm. This model will be an important tool for the development of optimized process controls and cooling strategies

Mechanism of internal stress formation in titanium alloy samples, obtained by additive manufacturing

The paper presents the results of residual stress formation study in Ti-6Al-4V alloy samples, obtained by additive manufacturing. An analogy of residual stress formation mechanism observed in samples during 3D-printing and laser beam welding technology is shown. The effect of annealing on residual stress level is investigated.

Research of the contact and deformation properties of lubricated surfaces

The article considers the issues of contact interaction of steel surfaces under various contact conditions. To assess the depth of ball penetration, an experiment was conducted based on the indentation procedure on the Brinell press. The hypothesis of the formation of internal stresses under conditions of non-lubricated and contact in the presence of a lubricant is formulated. The effect of lubricant on the occurrence of resistance forces to the movement of the indenter in a special laboratory setup is investigated.

Control of magneto-static and -dynamic properties by stress tuning in Fe-Si-B amorphous microwires with fixed dimensions

Abstract The factors, which influence the internal stresses in glass coated amorphous microwires were discussed in this work in relation to the magnetization behaviour and domain wall propagation. The approach taking into account the technological parameters of manufacturing by Ulitovsky-Taylor method is presented. The magnetization process and domain wall dynamics in Fe77.5Si7.5B15 microwires produced under different conditions but with identical geometric parameters were investigated experimentally and analysed on the basis of internal stress formation in different microwire states: as prepared, after glass removal and after annealing. It is demonstrated that the residual stresses in wires of the same composition are not defined exclusively by their dimensions but depend on the technological regime. The stress modification due to relaxing treatments also proceeds differently in wires produced under different conditions. These conclusions are evidenced by comparing the domain wall mobilities which strongly depend on the internal stress.

Structure and Properties of Al – ZrW2O8 Pseudo Alloys

Al – ZrW2O8 powder mixture was sintered to obtain alloys were Al – ZrW2O8 pseudo alloys. The structure formation process of the pseudo alloys obtained was investigated. During sintering process zirconium tungstate was found to decompose into constituent oxides with consequent formation of WAl12 and ZrAl3 intermetallic compounds. Increasing the sintering time up to 5 hours leads to re-synthesis of the zirconium tungstate represented with elongated particles - microfibers. The formation of internal stresses in the material was shown using XRD and Williamson-Hall analyses. The value of internal compression stresses was estimated to be 90 MPa. Vickers hardness of the pseudo alloys obtained increases by two times with increasing sintering time from 1h to 5h.

Joining of material compounds of aluminium matrix composites (AMC) by arc brazing using Al-Ag-Cu system filler alloy

Aluminium matrix composites (AMC) are in the focus of recent research. In the field of lightweight design, dissimilar joints with AMC are of current interest. To ensure the positive properties of AMC—like high specific strength, increased wear resistance, and low coefficient of thermal expansion (CTE)—a suitable joining technique is necessary. This work shows the state of the art of arc brazing using a filler material of the alloying system Al-Ag-Cu. Joints of AMC/stainless steel and AMC/aluminium alloy are investigated. The formation of brittle Al-Fe intermetallic phases in AMC/stainless steel joints must be reduced to obtain a sufficient joint strength. Therefore, an adapted alloying concept is used for the filler material. The filler (40 wt% Al, 40 wt% Ag, and 20 wt% Cu) is alloyed with various contents of Si (1.2, 1.3, and 1.4 wt%). Primarily solidified Si can be seen above 1.3 wt% Si in the microstructure. The melting temperature could be reduced about 10 K by additional Si. The microstructure analyzed by SEM, EDXS, and XRD, as well as the hardness profile of the joining zone, are characterized and discussed. In case of the interface braze metal/aluminium material, the formation of a thin (approx. 5 μm) phase layer, consisting of solid solution of Al, can be seen. Compared with that insights, the phase layer at the interface braze metal/stainless steel consists of intermetallics of the system Fe-Al. The results of the hardness measurements at the interface and XRD patterns prove the presence of Fe3Al, FeAl, and FeAl3. Cracks can occur between the brittle phases FeAl and FeAl3, due to their high hardness of approx. 600 HV0.005 (rep. FeAl) and 1020 HV0.005 (rep. FeAl3) and in combination of internal stress formation during cooling.

FEA simulation of thermal processes during the direct metal laser sintering of Ti64 titanium powder

Abstract With regard to the fact that laser sintering belongs to the high-temperature processes in which metal particles are sintered by a high-power laser, forming a homogenous structure, it is necessary and important to know the characteristics and the mechanism of these thermal processes. A high-power laser system produces three forms of heat that include convection, conduction, and radiation. These thermal processes affect the formation of internal stresses and tension that lead to deformations and rapidly influence the resulting quality, dimensions, density, micro-structure, and mechanical properties of fabricated parts. In response to this fact, it was important to analyse these heat transfer methods instantly during the direct metal laser sintering (DMLS) process simulation and subsequently monitor the parameters and settings of the sintering equipment in order to obtain acceptable manufacture outputs intended for further use. This work is focused on the creation of a FEA simulation model and the simulation of thermal processes across an object during and after the sintering process in the cooling stage, when it is important to consider a laser beam trajectory, temperatures of individual elements affected by the laser beam, and current laser energy in time. A 3D FEA simulation model was created in order to represent actual behaviour of a part during the sintering process. The simulation model consisted of two sub-models, particularly the building platform model with the dimensions of 250 mm × 250 mm × 22 mm, with stainless steel as the selected material, and the model of individual layers of sintered titanium powder with the dimensions of 10 mm × 10 mm × 0.03 mm. The total number of used layers was 12, which represents the total thickness of 0.36 mm. Applied power was P = 170 W. The simulation as such was carried out using the FEA software, Simulia Abaqus supported on the Windows x86-64 platform, which uses an integrated solver to make thermal and mechanic calculations. The calculations included also the impact of the protective argon atmosphere located in the process chamber. Mutual impact between individual layers was also considered. The simulation results were confronted with the results of already performed experimental studies of other scientific works, with the compliance and confirmation of assumptions being on a very good level.

Variation of internal stresses as a function of deformation and distance from sources in Cu–Al polycrystalline alloys

Internal stress fields in deformed Cu–Al polycrystalline alloys are studied via TEM. The sources of such stresses are determined. Internal stress fields are measured as a function of distance from different sources. The effect grain size has on the formation of internal stresses is determined.

EXPERIENCE OF APPLICATION DATA DIFFERENTIAL THERMAL ANALYSIS TO REDUCE INTERNAL STRESSES DURING POLYMERIZATION OF ADHESIVES, SUITABLE FOR THE ASSEMBLY OF ELECTRO-MECHANICAL DEVICES

The article presents the results of the study of polymerization processes of industrial adhesives, suitable for the assembly of electro-mechanical devices. According to [1, 2] were chosen for the study 14 samples of adhesives: Epotecny E505, Epotecny P102, Epotecny E207, EPO-TEK H74UNF, EPO-TEK H77S, EPO-TEK 353ND, Resbond 940LE, Resbond 989F, Cerambond 618N, ВК-21, Аnaterm 106, К400, ОС-52, ОС-92. For all samples of adhesives by differential thermal analysis is used to study parameters such as temperature mass loss, the number of stages and the temperature of polymerization, the temperature of the beginning of destruction in a predetermined tem-perature range. Presents the results of comparing the values of temperature of the beginning of destruction of adhesives, as claimed by the manufacturer and obtained experimentally. The paper identified the most important features of the process of polymerization of the adhesive samples. Investigated the rea-sons for the formation of internal stresses during polymerization of the adhesive joints in electro-mechanical devices and develop appropriate measures to reduce these stresses. On the basis of the results of differential thermal analysis of polymerization adhesives proposed effective modes of drying ad-hesives to improve the adhesive joints. An experimental study of the adhesive joints obtained by the proposed mode of drying and recommended by the manufacturer.