Pre-stretching Process Research Articles

Influence of pre-stretching and annealing on microstructure and mechanical properties evolution of twin-roll cast Mg-2Al-1Zn-1Ca (wt.%) plates

Pre-stretching and annealing treatments were conducted on twin roll cast Mg-2Al-1Zn-1Ca (AZX211, in wt.%) plates with a rare earth-like texture. Varying amounts of deformation were applied along the rolling direction (RD) and transverse direction (TD) of AZX211 alloy in order to modify its mechanical properties at room temperature. The results demonstrate that pre-stretching treatment effectively enhances the yield strength (YS), especially along the RD. The strengthening mechanism is attributed to the production of a large number of dislocations and sub-grain boundaries, but the work-hardening ability of the plate will be greatly weakened. Additionally, annealing treatment substantially improves the plasticity and in-plane anisotropy and restores the work-hardening ability. The notable distinction in the pre-stretching process between different directions lies in the underlying deformation mechanism. In case of RD, deformation is predominantly governed by the slip mechanism of {0002} 〈11−20〉 basal slip and {10−10} 〈11−20〉 prismatic slip, while along the TD, deformation is primarily controlled by {0002} 〈11−20〉 basal slip without significant twinning deformation. When a 6 % pre-stretching is conducted, the initial rare earth-like texture of the sample transforms into a symmetrically distributed double-peak basal texture, accompanied by grain refinement. This texture transformation is chiefly due to the dominance of {0002} 〈11−20〉 basal slip-driven deformation. Moreover, the annealed sample maintains a strong basal texture, owing to strain-induced recrystallization.

Effect of pre-stretching on enhancing yield strength of as-extruded Mg-Sm-Ce based alloy

In this work, a new Mg-1.6Sm-0.4Ce (wt%) alloy sheets was fabricated by extrusion process, which exhibits the high yield strength of 184 MPa, tensile strength of 222 MPa, and elongation of 29.9%. Subsequently, the novel pre-stretching process was conducted on the as-extruded Mg-Sm-Ce sample, and the yield strength is apparently increased by ∼ 84 MPa when the pre-stretching strain reaches ∼ 15%. Microstructural analysis shows that pre-stretching increases the strength of Mg sheet via dislocation strengthening and texture strengthening. As pre-stretching strain increasing from 5% to 15%, the Mg-Sm-Ce alloy sheets undergo grain rotation through non-basal/basal slipping. The typical basal texture of < 10–11 > //ND (normal direction) can be gradually formed, which would exert the obvious texture hardening effect. Simultaneously, the density of dislocations within the Mg-Sm-Ce alloy sheet are also increased which would induce the strong lattice distortion. Consequently, the movement of dislocations is inhibited and the yield strength can be largely enhanced. The relevant results can provide important guidance for developing novel Mg wrought alloy sheet with high strength.

Wrinkle Clamp Down on Structure Crack Strain Sensor Based on High Poisson's Ratio Material for Home Health Monitoring and Human-Machine Interaction.

Flexible wearable crack strain sensors are currently receiving significant attention because they can be used in a wide range of physiological signal monitoring and human-machine interaction applications. However, sensors with high sensitivity, great repeatability, and wide sensing range remain challenging. Herein, a tunable wrinkle clamp down structure (WCDS) crack strain sensor based on high Poisson's ratio material with high sensitivity, high stability, and wide strain range is proposed. Based on the high Poisson's ratio of the acrylic acid film, the WCDS was prepared by a prestretching process. The wrinkle structures can clamp down the crack to improve the cyclic stability of the crack strain sensor while maintaining its high sensitivity. Moreover, the tensile properties of the crack strain sensor are improved by introducing wrinkles in the bridge-like gold stripes connecting each separated gold flake. Owing to this structure, the sensitivity of the sensor can reach 3627, stable operation over 10 000 cycles is achieved, and the strain range can reach about 9%. In addition, the sensor exhibits low dynamic response and good frequency characteristics. Because of its demonstrated excellent performance, the strain sensor can be used in pulse wave and heart rate monitoring, as well as posture recognition and game control.

Nonprestretching double-network enabled by physical interaction-induced aggregation

The design of double-network (DN) improves the properties of matrix effectively, which has attracted wide attention. The recently reported construction of DN structure usually involves the prestretching process (swelling process). However, such complicated multistep swelling processes become more difficult in high molecular weight matrix, which are not suitable for constructing DN structure in the universal elastomers (for example, natural rubber and polyisoprene rubber). To expand the application fields of DN, this work designs the dense crosslinking domain and the sparse crosslinking domain to construct nonprestretching DN structure through physical interaction-induced aggregation. We find that physical interaction induced-aggregation form the dense crosslinking domain and the sparse crosslinking domain, which builds the two contrasting DN structures. Double-quantum nuclear magnetic resonance, dynamic mechanical analyzer, and mechanical property characterization demonstrate that the dense crosslinking domain as the brittle first network is preferentially ruptured upon deformation, leading to a large energy dissipation. This design concept represents a general approach to develop the nonprestretching DN structure, which expands the application fields of DN structure in the elastomer matrix.

Effect of the Inhomogeneity of Material Properties on the Quenching and Pre-Stretching Residual Stress in Aluminum Alloy

The effect of the inhomogeneity of the material properties on the quenching residual stress in the aluminum alloy was studied by using the finite element method simulation and the evolution of the quenching residual stress during the pre-stretching process was discussed in this paper. The results show that both the x-component and y-component of the quenching residual stresses in the non-uniform model are nearly overlapped with that in the uniform model. The effect of the inhomogeneity of the material properties on the quenching residual stress is negligible. After pre-stretching, the surface stress of the non-uniform model decreases from the middle line to the edge while the center stress increases. Contrastingly, both the surface and center stress of the uniform model are nearly stable along the same path, and the stress magnitudes are lower than that in the non-uniform model. The inhomogeneity of material properties mainly affects the pre-stretching residual stress distribution, while it has little effect on the quenching residual stress distribution.

The phase transition of polyamide 6 with pre-stretching process at different temperatures

The temperature and strain fields are two key factors in the regulation of fibrous aggregate structure. In this paper, plate specimens of polyamide 6 were prepared by compression molding method. The oriented polyamide 6 plates were systematically analyzed from aspects of mechanical properties, thermal properties, crystal structure and crystal morphology. The maximum tensile strength and elastic modulus of polyamide 6 plate appeared at 120 °C. The stretching process induced the formation of a more thermal stable α-form crystals. With the increase of stretching temperature, α-form crystals transformed into γ-form crystals gradually. The crystal chips transformed into fibrils during the stretching process. The structural evolution model of materials under the temperature and strain fields was established.

Effects of abnormal grain growth at longitudinal weld on the aging behavior and mechanical properties of 2196 Al[sbnd]Cu[sbnd]Li alloy profile

Abnormal grain growth (AGG) at longitudinal weld of 2196 AlCuLi profile was observed during solution treatment, and the effects of AGG on the aging behavior and mechanical properties of the profile were investigated. The results show that the AGG area almost contained no low angle grain boundaries (LAGBs) while the fine extrusion fibers in the matrix area contained numerous LAGBs. The AGG structure at weld seam leads to precipitated phases coarsening, PFZ widening and a sharp decrease of elongation in profiles. During aging process, the AGG area precipitated more coarse S′/S and T2 phases and exhibited a faster age hardening rate than the matrix area, while the numerous LAGBs in the matrix area promoted the precipitation of strengthening phases (T1, δ′/β′) and suppressed the formation of coarse phases. The phases formed along the grain boundaries are mainly coarse quasi-crystal T2 phases while abundant fine T1 phases formed along the LAGBs of the matrix area, and the PFZ of AGG area is far wider than that in matrix area. During the pre-stretching process, the fine grains and LAGBs of the matrix area conduced to form and store more dislocations, which provided more nucleation sites for T1 phases.

Research on Resistance to External Pressure of Automobile Cover

The forming process of automobile covers is relatively complicated, various defects often appear. Insufficient resistance to external pressure is one of them. To solve this problem, this paper proposes to increase a pre-stretching process before the stamping process. Through building a series of finite element models for an automobile cover forming process, the influence of different pre-stretching amounts on the thickness and resistance to external pressure are compared. Finally determine the appropriate pre-stretching amount.

Durable, Sensitive, and Wide‐Range Wearable Pressure Sensors Based on Wavy‐Structured Flexible Conductive Composite Film

AbstractFlexible pressure sensors have potential applications in human motion monitoring and electronic skins. To satisfy the practical applications, pressure sensors with a high sensitivity, a low detection limit, a broad response range, and an excellent stability are highly needed. Here, a piezoresistive pressure sensor based on wavy‐structured single‐walled carbon nanotube/graphite flake/thermoplastic polyurethane (SWCNT/GF/TPU) composite film is fabricated by a prestretching process. Due to the random wavy structure, high conductivity, and good flexibility, the prepared sensor displays a low detection limit of 2 Pa, a wide sensing range of 0–60 kPa, and a high sensitivity of 5.49 kPa−1 for 0–50 Pa. Furthermore, the sensor shows a remarkable repeatability of over 1.1 × 104, 9.0 × 103, and 2.0 × 103 pressure loading/unloading cycles at 50 Pa, 500 Pa, and 30 kPa, respectively, and a fast responsibility of 100–150 ms of loading response time and 400–600 ms of relaxation time. Therefore, the pressure sensor is successfully adopted to monitor both the large‐scale human activities (e.g., walk and jump) and the small‐scale signals (e.g., wrist pulse). Furthermore, a sensor array is assembled to map the weight and shape of an object, indicating its various potential applications including human–machine interactions, human health monitoring, and other wearable electronics.

Flexible Wire Based on Silver Nanowires and Elastomer

In order to solve the problem that the wire is difficult to have both elasticity and conductivity, a novel method for producing flexible wires based on silver nanowires (AgNWs) and elastomer has been proposed. This type of flexible wire is fabricated using a pre-stretching process and a glass sheet transfer process. Stretching test shows that after 50 times 0-30%-0 cycles of tensile test, the resistance growth rate relative to initial resistance is only 510%. The flexible wire can be widely used in flexible electronic devices.

Carbon nanotube film based flexible bi-directional strain sensor for large deformation

To overcome the limitation of conventional uniaxial strain sensors and satisfy the increasing demand for flexible electronic devices, a highly sensitive and flexible bi-directional strain sensor is fabricated with aligned carbon nanotube (CNT) film on silicon rubber substrate via a pre-stretching process. Highly anisotropic gauge factors have been realized for the A-direction (alignment direction of CNTs) and B-direction (transverse direction of CNTs alignment) strain sensors with large deformation. The alignment of CNTs in the axial direction as well as their entanglements in the transverse direction contribute, respectively, to the strain sensing capabilities of A-direction and B-direction strain sensors. The demonstration of finger knuckle motion detection confirmed the potential of this flexible bi-directional strain sensor in detecting complex large deformation.

Patterning sub-30 µm liquid metal wires on PDMS substrates via stencil lithography and pre-stretching

Presented is a simple method combining a pre-stretching process with stencil lithography for high-resolution patterning of liquid metal wires on polydimethylsiloxane (PDMS) substrates. The PDMS substrate is pre-stretched to several times its original length before casting liquid metals, it is released and restores back to its original length after casting liquid metals through a stencil mask; hence, the width and spacing of the patterned liquid metal wires decrease simultaneously with the PDMS substrate. The degree to which the width and spacing of liquid metal wires decrease is approximately equal to the times that the PDMS substrate is pre-stretched. An ultraviolet-ozone surface modification method was used to enhance the adhesion between liquid metals and PDMS substrates, and an alternative method based on applying an external electric field was proposed to remove excess liquid metals over the stencil mask. Sub-30 µm liquid metal wires were successfully patterned. As a demonstration, a liquid metal based flexible strain sensor was fabricated, the sensor exhibited excellent stretchability, stability, sensitivity and durability.

Modified Pre-stretching Assembly Method for Cable-Driven Systems

Soft cable-driven systems have been employed in many assembled mechanisms, such as industrial robots, parallel kinematic mechanism machines, medical devices, and humaniform hands. A pre-stretching process is necessary to guarantee the quality of cable-driven systems during the assembly process. However, the stress relaxation of cables becomes a critical concern during long-term operation. This study investigates the effects of non-uniform deformation and long-term stress relaxation of the driven cables owing to moving parts in the system. A simple closed-loop cable-driven system is built and an alternating load is applied to it to replicate the operation of transmission cables. Under different experimental conditions, the cable tension is recorded and the boundary data are selected to be curve-fitted. Based on the fitted results, a formula is presented to estimate the stress relaxation of cables to evaluate the assembly performance. Further experimental results show that the stress relaxation is mainly caused by cable creep and the assembly procedure. To remove the influence of the assembly procedure, a modified pre-stretching assembly method based on the stress relaxation theory is proposed and verification experiments are performed. Finally, the assembly performance is optimized using a cable-driven surgical robot as an example. This paper proposes a dual stretching method instead of the pre-stretching method to assemble the cable-driven system to improve its performance and prolong its service life.

Material Enhancement Model for Austenitic Stainless Steel Sheets Subjected to Pre-stretching

Austenitic stainless steel has considerable strain hardening property, which can be utilized in the design of cold-formed stainless steel structures to effectively reduce project costs. Pre-stretching is a simple and fundamental cold-forming process. Material enhancement model developed for pre-stretching lays a basis for characterizing material properties in more complicated cold-forming process(e.g. cold-rolling and press-braking). In this study, a series of material tensile tests were performed to develop material enhancement models for austenitic stainless steel sheets subjected to pre-stretching. Austenitic stainless steel sheets were tensioned in six different levels to introduce cold working into the sheets. Material properties of the sheets in two directions (i.e. along and perpendicular to the pre-stretching direction) were obtained through coupon tensile tests. To facilitate the development of the material enhancement models, a simplified three-stage material model with six independent parameters was proposed. Enhancement model for each material parameter was developed with the plastic strain experienced in the pre-stretching process as a key factor. Key material parameters and full-range stress–strain curves at different levels of pre-stretching were generated based on the proposed and the available predictive models in literatures. Comparisons of the generated material parameters and the stress–strain curves with the test ones show good agreement.

Investigation on Residual Stresses in Pre-Stretching Process of Aluminum Alloy Thick Plate Based on Material Non-Uniformity

A discretization method is used in pre-stretching simulation to separate an aluminum alloy thick plate into layers. When the equivalent plastic strain in each layer is defined according to the residual stresses during quenching, the yield strength model can be developed to estimate its yield strength. Again, the curve of elasticity modulus is fitted in the light of the experimental data, which can reveal how the elasticity modulus changes with plastic deformation. Finally, in consideration of the non-uniformity of yield strength and elasticity modulus, the pre-stretching of the quenched 7075 aluminum alloy thick plate is taken as an example to numerically simulate the value and distribution of residual stresses at the pre-stretching ratio of 1.8%, 2.2% and 2.5%, respectively. It is known from the simulated results that, by using the proposed method in this paper, the obtained results of the distribution and magnitude of the residual stresses are in good agreement with the experimental measurements.

A stretchable humidity sensor based on a wrinkled polyaniline nanostructure

A stretchable humidity sensor which has mechanical stability has remained a challenging issue in stretchable electronics. Here, we report a novel stretchable nanostructured polyaniline humidity sensor. The sensor was fabricated by simple fabrication methods such as a pre-stretching process and self-assembly texturing process. The periodically wrinkled structure supported by microstructured elastomeric substrates provides an excellent stretchability. The micro-textured substrate was introduced for solving the crack problem by the Poisson effect as a stretchable sensor. The sensor maintains its humidity sensitivity well at different elongations. To the best of our knowledge, this is the first report of a stretchable humidity sensor.

Analytical evaluation on uniaxial tensile deformation behavior of fiber metal laminate based on SRPP and its experimental confirmation

Uniaxial tensile tests have been carried out to accurately evaluate the in-plain mechanical properties of fiber metal laminates (FMLs). The FMLs in this paper comprised of a layer of self-reinforced polypropylene (SRPP) sandwiched between two layers of aluminum alloy 5052-H34. In this study, nonlinear tensile and fracture behavior of FMLs under in-plane loading conditions has been investigated with numerical simulations and theoretical analysis. The numerical simulation based on finite element modeling using the ABAQUS/Explicit and the theoretical constitutive model based on the volume fraction approach using the rule of mixture and the modified classical lamination theory, which incorporates the elastic–plastic behavior of the aluminum alloy and SRPP, are used to predict the in-plain mechanical properties such as stress–strain response and deformation behavior of the FMLs. In addition, the pre-stretching process is used to reduce the thermal residual stresses before the uniaxial tensile tests of the FMLs. Through comparing the numerical simulations and the theoretical analysis with the experimental results, it is concluded that the adopted numerical simulation model and the theoretical approach can describe with sufficient accuracy of the actual tensile stress–strain curve.

Influence of warm pre-stretching on microstructure and properties of AZ31 magnesium alloy

A pre-stretching process was carried out at 250°C to improve the microstructure of an AZ31 alloy to investigate its effects on microstructure and properties. This process produced two effects on the microstructure: slight increasing of the grain size and texture weakening. The microtexture and macrotexture data showed that the c-axis of some grains tended to rotate towards the transverse direction after warm pre-stretching. Compared with the as-received sample, the pre-stretched sample exhibited larger fracture elongations both in the rolling direction and transverse direction. Erichsen tests were conducted at room temperature to evaluate influence of warm pre-stretching on the press formability of AZ31 alloy sheets. Here, the Erichsen value (IE) of the pre-stretched sample was increased by ∼54%.

EFFECT OF THE CYTOSKELETON FIBERS AND SUBSTRATE RIGIDITY ON ADHERENT CELLS

A pre-stretching process was carried out at 250 °C to improve the microstructure of an AZ31 alloy to investigate its effects on microstructure and properties. This process produced two effects on the microstructure: slight increasing of the grain size and texture weakening. The microtexture and macrotexture data showed that the c-axis of some grains tended to rotate towards the transverse direction after warm pre-stretching. Compared with the as-received sample, the pre-stretched sample exhibited larger fracture elongations both in the rolling direction and transverse direction. Erichsen tests were conducted at room temperature to evaluate influence of warm pre-stretching on the press formability of AZ31 alloy sheets. Here, the Erichsen value (IE) of the pre-stretched sample was increased by ∼54%.

Stretch rate and deformation for pre-stretching aluminum alloy sheet

Numerical simulation combined with experimental test was carried out to analyze the pre-stretching process of the 7075 aluminum alloy sheet, from which the stress variation curves and residual stress of aluminum alloy sheet in different stretch rates were obtained. The results show that the residual stress in length direction is released after unloading the stretch force, while the residual stress in width direction is released during the stretching process. The study of residual stress elimination is beneficial for optimizing stretch rate on the basis of residual stress distribution law. By comparing the variation principle of residual stress in length direction, the size range of three deformation areas and elimination percentage of residual stress were obtained. The residual stresses of clamping area and transition area are not eliminated effectively, so sawing quantity should be the sum of both the areas. The elimination rate of residual stress in even deformation area could reach 90% after choosing a proper stretch rate, which is verified by both simulation and experiment.