Effect of pre-stretching on microstructure of aged 2524 aluminium alloy

The effects of pre-stretching following solution treatment on the hardness and microstructures of aged 2524 aluminium alloy at 170 ℃ were studied.Ageing hardness values under different pre-stretching conditions were measured,and the corresponding microstructures were characterized by transmission electron microscopy（TEM）.The results show that compared with unstretched samples,the peak hardness is increased and the time to reach the peak hardness is reduced with the increase of pre-strain;the number density of S（Al2CuMg） phases is increased and the length is shortened in pre-stretched alloy.Additionally,the number density of GPB zones is decreased with the increase of pre-strain in peak-aged samples.When the pre-strain is up to 5%,S phases play the predominant contribution to the peak hardness.Fine and uniformly distributed S phases lead to a higher hardness than GPB zones together with S phases existing in conventionally aged 2524 alloy.

Influence of pre-stretching on quench sensitive effect of high-strength Al-Zn-Mg-Cu-Zr alloy sheet

The influence of pre-stretching on quench sensitive effect of high strength Al-Zn-Mg-Cu-Zr alloy AA 7085 sheet was investigated by tensile testing at room temperature,transmission electron microscopy(TEM)and differential scanning calorimetry(DSC).The water-cooled and aged alloy exhibits higher strength than the air-cooled and aged alloy;2.5%pre-stretching of tensile deformation exerts little effect on strength of water-cooled and aged alloy but increases that of air-cooled and aged one,and therefore the yield strength reduction rate due to slow quenching decreases from about 3.8%to about 1.0%,reducing quench sensitive effect.For the air-cooled alloy,pre-stretching increases the sizes ofη'strengthening precipitates but also increases their quantity and the ratio of diameter to thickness,resulting in enhanced strengthening and higher strength after aging.The reason has been discussed based on microstructure examination by TEM and DSC.

Modifed 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.

Ageing behavior of an Al-Zn-Mg-Cu alloy pre-stretched thick plate

The ageing behavior of a pre-stretched thick plate of Al-Zn-Mg-Cu alloy was systemically studied including one-step ageing, two-step ageing, and retrogression and reageing treatment （RRA）. One-step ageing of the alloy resulted in peak ultimate tensile strengths of 595 and 575 MPa after 22 and 6 h at 120 and 135°C, respectively. The strengthening phase in peak aged （T6 temper） alloy contained GP zones and the η′ phase predominantly. After two-step ageing, the electrical conductivity was increased markedly, but the pre-stretched thick plate sacrificed a great loss of strength. RRA treatment provided a method for maintaining the strength close to that obtained by T6 temper and for obtaining the high electrical conductivity close to that obtained by T7 temper; the ultimate tensile strength and electrical conductivity were 583 MPa and 21.0 MS/m, respectively. TEM analysis of T7 and RRA specimens revealed two types of precipitates that contributed to age strengthening i.e. the η′ and η phases.

MODIFIED LAYER REMOVAL METHOD FOR MEASUREMENT OF RESIDUAL STRESS DISTRIBUTION IN THICK PRE-STRETCHED ALUMINUM PLATE

The integrated structure parts are widely used in aircraft. The distortion caused by residual stresses in thick pre-stretched aluminum plates during machining integrated parts is a common and serious problem. To predict and control the machining distortion, the residual stress distribution in the thick plate must be measured firstly. The modified removal method for measuring residual stress in thick pre-stretched aluminum plates is proposed and the stress-strain relation matrix is deduced by elasticity theory. The residual stress distribution in specimen of 7050T7451 plate is measured by using the method, and measurement results are analyzed and compared with data obtained by other methods. The method is effective to measure the residual stress.

Modified Layer-Removal Method for Measurement of Residual Stress in Pre-stretched Aluminium Alloy Plate

Residual stress is one of the factors affecting the machining deformation of monolithic structure parts in the aviation industry. Thus, the studies on machining deformation rules induced by residual stresses largely depend on correctly and efficiently measuring the residual stresses of workpieccs. A modified layer-removal method is proposed to measure residual stress by analysing the characteristics of a traditional, layer-removal method. The coefficients of strain release are then deduced according to the simulation results using the finite element method （FEM）. Moreover, the residual stress in a 7075T651 aluminium alloy plate is measured using the proposed method, and the results are then analyzed and compared with the data obtained by the traditional methods. The analysis indicates that the modified layer-removal method is effective and practical for measuring the residual stress distribution in pre-stretched aluminium alloy plates.

Compressive properties and energy absorption characteristics of open-cell nickel foams

Open-cell nickel foams with different relative densities and pre-stretching degrees were subjected to room temperature quasi-static compressive tests to explore their compressive properties. The compressive properties of the nickel foams including yield strength, elastic modulus, energy absorption density and energy absorption efficiency were calculated accurately. The results show that the compressive properties of yield strength, elastic modulus and energy absorption density increase with the increase of relative density of nickel foams. The compressive properties are sensitive to the pre-stretching degree, and the values of yield strength, elastic modulus and energy absorption density decrease with the increase of pre-stretching degree. However, the energy absorption efficiency at the densification strain state exhibits the independence of relative density and pre-stretching degree. The value of energy absorption efficiency reaches its peak when the strain is at the end of the collapse plateau region.

Effect of thermo-mechanical treatment process on microstructure and mechanical properties of 2A97 Al-Li alloy

2A97 Al-Li alloy was processed by thermo-mechanical treatment at different pre-stretch deformations of 0, 3% and 6%. The microstrucatre observation results reveal that some δ＇ and T1 precipitates are found in a（Al） matrix of 2A97 alloy processed by the heat treatment with no pre-stretch deformation. When the pre-stretch deformation is 3% and 6%, respectively, amounts of tiny T1 and a few of S＇ precipitates precipitates are observed in the microstructures of 2A97 alloy. The tensile test results show that the tensile properties of 2A97 alloys are improved via thermo-mechanical treatment. When the pre-stretch deformation is from 0, 3% to 6%, the ultimate tensile strength values of the 2A97 alloys increase gradually from 447.7, 516.5 to 534.3 MPa, and the elongations decrease from 17.6%, 12.8% to 10.2%, respectively. Moreover, with increasing pre-stretch deformation amount from 0 to 6%, the in-plane anisotropy value of 2A97 alloys becomes more obvious.

Indentation of a compressible soft electroactive half-space: Some theoretical aspects

We theoretically study the indentation response of a compressible soft electroactive material by a rigid punch. The half-space material is assumed to be initially subjected to a finite deformation and an electric biasing field. By adopting the linearized theory for incremental fields, which is established on the basis of a general nonlinear theory for electroelasticity, the appropriate equations governing the perturbed infinitesimal elastic and electric fields are derived particularly when the material is subjected to a uniform equibiaxial stretch and a uniform electric displacement. A general solution to the governing equations is presented, which is concisely expressed in terms of four quasi-harmonic functions. By adopting the potential theory method, exact contact solutions for three common perfectly conducting rigid indenters of fiat-ended circular, conical and spherical geometries can be derived, and some explicit relations that are of practical importance are outlined.

Experimental investigation on electromechanical deformation of dielectric elastomers under different temperatures

Under an applied voltage, dielectric elastomers （DEs） produce an actuation strain that is nonlinear, partly because of the material properties. In this study, an experimental characterization is conducted to evaluate how the ambient temperature and pre-stretch affected the actuation performance. For DEs with a pre-stretch of 2 × 2, an increase of temperature from -10° to 80° results in a variation in the actuation strain of more than 1700%. Low pre-stretched DEs are more susceptible to temperature change; while highly pre-stretched DEs are relatively insensitive to temperature, because in this case the energy conversion was dominated by mechanical stretching, rather than thermal conduction, during the actuation.

WAVES IN PRE-STRETCHED INCOMPRESSIBLE SOFT ELECTROACTIVE CYLINDERS:EXACT SOLUTION

This paper studies wave propagation in a soft electroactive cylinder with an under- lying finite deformation in the presence of an electric biasing field. Based on a recently proposed nonlinear framework for electroelastieity and the associated linear incremental theory, the basic equations governing the axisymmetric wave motion in the cylinder, which is subjected to homo- geneous pre-stretches and pre-existing axial electric displacement, are presented when the elec- troactive material is isotropic and incompressible. Exact wave solution is then derived in terms of （modified） Bessel functions. For a prototype model of nonlinear electroactive material, illus- trative numerical results are given. It is shown that the effect of pre-stretch and electric biasing field could be significant on the wave propagation characteristics.