Uniaxial Prestrain Research Articles

Influence of uniaxial and equi-biaxial tensile pre-straining on the high cycle and notch fatigue behaviour of AA2024-T4 aluminium alloy

The present study investigates the impact of different tensile pre-straining (equi-biaxial and uniaxial) on the tensile, high cycle and notch fatigue behaviour of AA2024-T4. EBSD examination of the pre-strained specimen shows more substantial lattice distortion than the material with 0% pre-strain. Pre-strained specimens exhibit increased ultimate tensile strength, yield stress, and reduced ductility, irrespective of pre-straining paths. Moreover, a significant enhancement in fatigue performance is observed, with a 13% and 21% improvement in fatigue strength for uniaxial and equi-biaxial pre-strain, respectively, at an optimal fatigue lifetime of approximately 1 × 106 cycles. The theory of critical distance is utilized to predict fatigue life with double-edge semi-circular notches for each pre-strained condition, and predicted life falls within a scatter band ± 2.

Influence of pre-strain on tensile response of extra deep drawing (EDD) steel under varying strain rates and crash performance

This study explores the impact of pre-strain on the uniaxial tensile stress-strain behavior of extra deep drawing (EDD) steel at varying strain rates. The EDD steel blanks were subjected to a uniaxial pre-strain of 15 % in either the transverse or rolling direction. Subsequently, uniaxial tensile tests were conducted at different strain rates, both orthogonal and parallel to the initial pre-straining direction. The study also investigates the alterations in yield stress (YS), ultimate tensile strength (UTS), uniform elongation (UEL), total elongation (TEL), and strain energy absorption (SED, represented by the area under the tensile stress-strain curve) at diverse strain rates after pre-straining. The experimental findings indicate that pre-straining is advantageous for enhancing YS, UTS, and SED. However, this improvement comes at the expense of UEL or TEL. A square crush tube, which shares similarities with automotive components, was examined to assess the crash performance of several automotive structural parts during crash events. A validated constitutive model was used to predict the square tube's energy absorption and crush performance under high-velocity conditions for different pre-straining scenarios.

Study on the Effect of Pre-strain on Fatigue Properties of Dual-phase Steel

The uniaxial tensile test investigated the fatigue performance of DP780 steel without pre-strain and with 5% and 10% uniaxial pre-strain. Topography of fractured surfaces was observed to study the relationship between pre-strain and fatigue performance. The results showed that pre-strain the specimens would affect the fatigue performance of DP780 steel. It would cause an increase in fatigue life when the stress was comparatively high and the fatigue life was less than 100,000 cycles. However, the fatigue life was reduced when the stress level was comparatively low, and the fatigue life was more than 100,000 cycles. Pre-strain reduced the fatigue strength of samples. Fatigue strength of specimens without pre-strain and 5% and 10% pre-strain were 318 MPa, 310 MPa, and 303 MPa, respectively. The sample fracture was a brittle fracture with a pre-strain of 5%, while it was a ductile fracture with a pre-strain of 10%.

Effects of Prestrain and Baking on the Tensile and Fatigue Properties of Fe–0.1C–5Mn Transformation‐Induced Plasticity Steel

This study aims to investigate the effects of uniaxial tensile prestrain (PS) and bake hardening (BH) on tensile and force‐controlled (R = 0.1) high‐cycle fatigue (HCF) properties of Fe–0.1C–5Mn medium Mn transformation‐induced plasticity (TRIP) steel. Stress‐life data of the as‐received (AR: 0% prestrain) and 10%, 15%, and 20% prestrained (PS10, PS15, PS20) samples have been generated under both baked and unbaked conditions. It is manifested that prestraining increases the HCF life of the steel. The addition of BH further increases the HCF life. The fatigue limit is greatly improved from 433 MPa (AR) to 532 MPa (BH20), and the maximum incremental value is as great as 99 MPa. It is attributed to the austenite to martensite transformation (progressive TRIP effect) that occurs during the prestraining and cyclic loading processes.

The influence of equi-biaxial and uniaxial tensile pre-strain on the low cycle fatigue performance of the AA2024-T4 aluminium alloy

In the present investigation, the low cycle fatigue performance of the aluminium alloy AA2024-T4 is examined for different tensile pre-straining histories. The equi-biaxial tensile pre-strained specimens revealed identical fatigue life to the 0% pre-strained specimens, while all the uniaxial tensile pre-strained specimens showed improved fatigue life. As-received specimens exhibit noticeable cyclic hardening at all the strain amplitude. The pre-strained specimen shows an almost stable cyclic stress–strain response for each strain amplitude. The electron backscattered diffraction examination of the without pre-strained and pre-strained specimens revealed large lattice distortion in the equi-biaxial tensile pre-strained case in comparison with the other two cases.

Effect of Uniaxial Pre-strain on Room Temperature Tensile and Creep Behavior of Aluminum Alloy AA2219-T87 for Propellant Tanks of Satellite Launch Vehicles

Effect of Uniaxial Pre-strain on Room Temperature Tensile and Creep Behavior of Aluminum Alloy AA2219-T87 for Propellant Tanks of Satellite Launch Vehicles

Electrically Tunable, Fully Solid Reflective Optical Elements

AbstractElectrochromic devices have seen widespread adoption in the automotive and aerospace industries and are increasingly considered for applications in the built environment, such as smart windows. Here, a distinctive approach is reported to realizing electrochromism in solid, polymeric materials. The optical elements are based on liquid crystalline elastomers (LCEs) that retain the cholesteric phase (CLCEs). By integrating flexible, optically transparent, and conductive electrodes the CLCEs are actuated as dielectric elastomer actuators. Application of an electric field causes a significant change in the reflection wavelength of fully solid CLCE. The electromechanical response (Maxwell stresses) is reversible. Both uniaxial and biaxial prestrain are shown to enhance and differentiate the stimuli‐response in the photonic device.

Multi-strain path deformation behavior of AA6016-T4: Experiments and crystal plasticity modeling

The development of backstresses that occur during a strain path change when forming sheet metal renders final part geometry prediction, after springback, difficult using conventional models. Most deformation models do not explicitly account for the influence of these stresses. A more recently developed elasto-plastic self-consistent (EPSC) model incorporates the backstresses to influence the activation of slip systems for more accurate simulations of the complex material response. The current study assesses the performance of the EPSC model for deformation response of AA6016-T4 via multiple biaxial, plane-strain, and uniaxial tension strain paths. The response to complex strain paths was examined by first pre-straining under uniaxial, biaxial, and plane-strain tension, then by loading in uniaxial tension. The EPSC model predictions closely matched experimental results. The model correctly predicted the highest yield stress and sharpest transition from elastic to plastic deformation for uniaxial tension after an initial uniaxial pre-strain. Lower yield stress for uniaxial tension after first pre-straining in biaxial and plane-strain tension is also correctly predicted, along with a smoother transition from elastic to plastic behavior. A linear geometrically necessary dislocation (GND) development, with strain, was observed using high-resolution electron backscattered diffraction (HREBSD) while a quadratic statistically stored dislocation (SSD) development was predicted by the model. The comparison revealed an expected transition from kinematic to isotropic hardening at higher strains. Finally, at higher strain levels the backstress accounted for around 15% of the total subsequent flow stress in all pre-strain cases.

Enhanced biaxial stretchability of wrinkled SiO2 thin films for stretchable encapsulation

Enhanced biaxial stretchability of highly impermeable SiO2 encapsulation thin film with wrinkle structures is demonstrated. Submicron-thick SiO2 films are grown on single-crystalline Si wafers to endure local strain on wrinkled film, and their high elastic deformation limit stabilizes wrinkle structure. SiO2 films are transferred onto prestrained elastomer, and two types of wrinkle structures are formed by removal of the prestrain: aligned wrinkles by uniaxial prestrain and random wrinkles by equibiaxial prestrain. The stretchability of the large area film is correlated with the unwrinkling of individual wrinkles, and a mechanics model is suggested to predict stretchability with parameters of wrinkle morphology as well as elastic properties of the encapsulation film and the elastomer substrate.

Influence of uniaxial and biaxial pre-straining on the notch fatigue performance of DP590 steel

The influence of the 10% uniaxial and equi-biaxial tensile pre-strain on the notch fatigue performance of DP590 steel is investigated in the present study. Comparison has been made between smooth and notched specimens to recognize the effect of pre-strain on notch fatigue performance. The material’s fatigue strength improves from 240 MPa to 270 MPa, 294 MPa for the smooth specimen and from 91 MPa to 97 MPa, 120 MPa for the notched specimen under the uniaxial and equi-biaxial tensile pre-strain conditions. 10% uniaxial and equi-biaxial tensile pre-strain has little influence on notch fatigue life. Finite element analysis highlights stress-concentration around notch tip rise and cyclic plastic deformation zone decrease with pre-straining. An increase in stress-concentration in the notched specimen nullifies improvement of tensile yield stress and fatigue strength due to pre-strain; thereby a small enhancement of notch fatigue life is noticed after pre-straining.

Influence of uniaxial and biaxial pre-straining on the high cycle fatigue performance of DP590 steel

Present work aims to assess tensile and high cycle fatigue (HCF) performance of dual-phase (DP590) steel subjected to uniaxial and equi-biaxial pre-strain. Pre-strain increases material’s strength, reduces ductility and simultaneously enhances HCF performance, quantified by increased material’s tensile yield strength 12% & 21%, ultimate tensile strength 6% & 7%, endurance limit 11% & 18%, after 10% uniaxial and equi-biaxial pre-strain. The rotation of one maximum shear stress plane during uniaxial re-loading after equi-biaxial pre-strain results in increased fatigue life. Finite element analysis suggests an alteration in the location of deformation concentration zone during uniaxial fatigue cycling of equi-biaxial pre-strain material.

Experimental and theoretical plasticity analyses of steel materials deformed under a nonlinear strain path

In this study, the macroscopic mechanical behavior of extra-deep drawing quality (EDDQ) and transformation-induced plasticity (TRIP1180) steels subjected to nonlinear-strain-path modes of deformation was investigated. For the nonlinear-strain-path experiments conducted on steels, an optimized experimental approach was proposed for deforming the material using two linear loading segments with a single abrupt change in the strain path. A specific medium-scale uniaxial tension specimen was designed for the pre-strain stage. The advantage of this approach over existing methods was that it enabled testing of various strain-path changes for steels with strengths greater than 1 GPa within a reasonable load range. In addition, this approach produced a more uniform strain field, which is important during the pre-strain stage. After the uniaxial pre-strain, various specimens were extracted from the uniformly deformed area for use in experiments involving changes in the strain path. To predict the material behavior under nonproportional loading, the homogeneous anisotropic hardening (HAH) distortional plasticity model was selected. The predictions of the HAH model were compared with the corresponding experimental results to assess the suitability of the constitutive model to accurately reproduce the observed effects of strain-path changes.

Formability and fracture in deep drawing sheet metals: Extended studies for pre-strained anisotropic thin sheets

In this study, experimental and numerical investigations were conducted to predict the formability and fracture behavior of as-received and pre-strained sheet materials during deep drawing process. In this context, various laboratory scale experimental setups were developed to impart different types and amounts of pre-strain such as 5% and 10% equi-biaxial pre-strains (5% EBP and 10% EBP), 10% plane strain pre-strain (10% PSP), and 10% uni-axial pre-strain (10% UP) on the extra deep drawing (EDD) steel and aluminum alloy (AA5052) sheets of 1.2 mm thickness. Further, all the pre-strained sheet samples were deformed using a cylindrical deep drawing setup. The forming limit diagrams (FLDs) of as-received sheets were predicted by the Marciniak–Kuczyński (MK) model incorporating different anisotropic yield functions such as Hill48 models identified based on r-values (Hill48-r), and yield stresses (Hill48-σ), and the non-quadratic plane stress Yld2000-2d model. Also, the Bao–Wierzbicki (BW) fracture curve was calibrated using different anisotropic yield functions. Subsequently, the formability in terms of limiting drawing ratio (LDR) was predicted using the MK-FLD. The BW fracture curve was incorporated into the generalized incremental stress state dependent damage model (GISSMO) platform in LS-Dyna software and the fracture behavior was predicted in terms of failure location and cup height at the onset of fracture. It was also found that the incorporation of Barlat Yld2000-2d yield function into the FE simulation efficiently predicted the necking and fracture behavior of as-received sheets. Furthermore, the concept of path independent polar effective plastic strain (PEPS) based failure model was used to predict LDR, thinning profile and fracture cup height of all the different pre-strained sheets. Finally, the strain paths and experimental fracture strains were plotted in 3D fracture locus to get insight into the deformation behavior during the deep drawing experiments.

Non-linear strain path experiment and modeling for very high strength material

The goal of this work is to develop experimental methods and modeling of very high strength steel sheets subjected to non-linear strain paths. A 1.6 mm thick TRIP steel sheet with a tensile strength of 1180 MPa was investigated. A suitable test for the uniaxial tension pre-strain was developed. A constraint was to achieve an as uniform as possible strain field over the entire gauge area. The pre-strain was conducted on a 500kN MTS tensile testing machine. The geometry of a new specimen was optimized through finite element simulations and experiments on the TRIP1180 material. Then, different types of subsequent specimens were extracted from the pre-strained material by electro-discharge machining (EDM) for the second deformation step leading to a strain path change. The subsequent plastic behavior was measured and modeled using the HAH distortional plasticity model. Comparisons between the experimental and predicted behavior are discussed.

Low cycle fatigue performance of DP600 steel under various pre-straining paths

The present work concerns the assessment of low cycle fatigue performance of dual phase steel subjected to change in uniaxial pre-strain path. An uniaxial pre-strain of 12.5% in the rolling or transverse direction were imposed on the dual phase steel blanks, thereafter fully reversed strain controlled fatigue tests were conducted parallel or orthogonal to the initial pre-straining direction. As received specimens depict mild cyclic hardening in the initial cycles followed by gradual cyclic softening until failure. However, all pre-strained materials exhibit continuous cyclic softening with higher magnitude and rate throughout the life as compared to the as received specimen. Relaxation of mean stresses were observed for the fatigue tests with pre-straining. Rotation of principal axis due to orthogonal reloading (i.e. low cycle fatigue) after initial pre-straining introduces non-proportionality, thereby leading to significant reduction of fatigue lives. In-grain misorientation and lattice rotation analyses are noticed during electron backscattered diffraction investigation, and which is due to the non-proportional loading effect.

Influence of uniaxial tensile pre-strain on forming limit curve by using biaxial tensile test

This paper focused on the effect of pre-strain on forming limit curves (FLC) of 5754-O aluminum alloy sheet through utilizing biaxial tensile approach. Based on Swift model and Yld2000-2d yield criterion, the dimensions of cruciform specimen was optimized through applying finite element method for increasing the strain at specimen center. After that, with the recommended specimen size, the cruciform specimen was tested under various stroke ratios to experimentally characterize the limit strains under different pre-strain levels. Subsequently, the biaxial tensile tests were simulated by Abaqus to obtain the limit strains and validate the material models. It can be observed in both experiments and simulations that the pre-strained uniaxial tension followed by plane tension or equi-biaxial tension can improve the formability of sheet metals. Besides, the strain path change affects the trend of first derivative of strain rate difference between neighboring points with respect to time. An early increase occurred and then fell back to the stable value, the steady evolution continued until to a new increase reaching the critical value. The M–K prediction approach was simulated to verify the influence of pre-strain on FLC. It can be found that the early increase peaks of the major strain incremental ratio rose with the amplitude of pre-strain. Finally, the phenomenon of pseudolocalization caused by the strain path change was explained through evolution of stress state inside the groove.

Characterization of Plastic Anisotropy of AA5182-O Sheets During Prestraining and Subsequent Annealing

This paper described the effects of prestraining and annealing on plastic anisotropy (r-value) of aluminum alloy 5182-O sheets including two prestrain paths and two annealing conditions. During the prestraining and annealing processes, r-value changed depending on prestrain paths and annealing conditions. Although there were slight changes of the normal anisotropy coefficient, r¯, during prestraining and annealing processes, the planar anisotropy coefficient, Δr, increased significantly, especially for the uniaxial prestrain condition. This could accelerate the development of earing during a sheet forming operation. Also, the corresponding sheet textures in rolling direction (RD)/TD plane after prestraining and annealing processes were observed through electron backscatter diffraction (EBSD) analysis to explain the r-value changes, where the viscoplastic self-consistent (VPSC) model was used to correlate the determined texture to measured r-values. It is found that the sheet texture also had significant changes relating to the prestrain paths and annealing conditions resulting in varied r-values.

Formability analysis of pre-strained AA5754-O sheet metal using Yld96 plasticity theory: Role of amount and direction of uni-axial pre-strain

Automotive industries are very much interested in formability of different pre-strained aluminum alloy sheets in the context of multistage stamping to fabricate complex components. In the present work, different uni-axial pre-strains of 6.4% and 12.2% were induced in AA5754-O aluminum alloy both along rolling direction (RD) and transverse direction (TD). The true stress-strain response, limiting dome height (LDH) and strain based forming limit diagram (ε-FLD) of as received and all pre-strained materials were evaluated experimentally. The anisotropy constitutive material model was developed using the Yld96 plasticity theory in-conjunction with the Hollomon isotropic hardening law to predict the yield strength evolution of the pre-strained materials. Also, it was found that the limiting strains in ε-FLD shifted significantly depending on the amount and direction of uni-axial pre-strain. Hence, the limiting strains of the as-received materials were transposed into stress space to estimate the stress based forming limit diagram (σ-FLD) using the anisotropy constitutive material model. Further, the dynamic shifts of ε-FLDs of four different pre-strained materials were predicted by successfully decoupling the σ-FLD of as-received materials within root mean square error of 0.008. Finite element models of both uni-axial pre-straining and subsequent LDH tests were developed, and the forming behavior of the pre-strained materials were predicted implementing the Yld96 plasticity model and estimated σ-FLD. It was found that LDH was significantly influenced by the amount of pre-strain, and the maximum thinning location shifted close to pole in the case of 12.2% pre-strained materials. However, the effect of uni-axial pre-strain direction on both LDH and maximum thinning location in AA5754-O material was very negligible.

Elastic wave dispersion in pre-strained negative stiffness honeycombs

A doubly periodic array of curved beams, known as a negative stiffness (NS) honeycomb, has recently been shown to exhibit tunable impact isolation by exploiting nonlinear mechanical behavior [Correa et al., Rapid Prototyping J., 21 (2), 2015]. The primary benefit of NS honeycombs over standard honeycomb materials lies in their ability to return to their original shape after experiencing an impact. The recoverable nature of their response is a result of the curved beam structure which has a single stable configuration but experiences one or more buckling events when loaded. The complex nonlinear elastic response of NS honeycombs lead to large variations in mechanical stiffness, which makes these materials compelling candidates for tunable control of elastic waves. The present work investigates linear elastic wave propagation of a representative NS honeycomb lattice at varying levels of pre-strain. Eigenfrequency finite element analysis is performed on the unit cell subjected to finite uniaxial deformation. The longitudinal and transverse phase speeds are shown to be anisotropic and highly dependent on uniaxial pre-strain. This is especially true near points of instability, where one observes very different functional dependence of longitudinal and transverse wave motion on pre-strain.

Uniaxial Pre-strain and Free Recovery (UPFR) as a Flexible Technique for Nitinol Characterization

The measurement of phase transformation temperatures of superelastic (SE) and shape memory (SM) NiTi alloy products and components was studied in this work. The transformation temperatures of a set of twenty different 300 μm NiTi superelastic wires were measured by two well-established and standardized techniques, namely differential scanning calorimetry (DSC) and bend and free recovery (BFR) and then compared with the results from the Uniaxial Pre-Strain and Free Recovery (UPFR) test. UPFR is a tension-based test, whose aim is to overcome the limitations associated with BFR testing. Within this work, a test procedure has been set up and validated. UPFR is found to be the only method showing a very strong correlation with the mechanical properties measured using the standard uniaxial tensile test method for superelastic NiTi alloy. Further, UPFR has been validated as a robust technique for measuring the R-phase and austenitic transformation temperatures in specimens of various sizes, composition, and of different geometries. This technique overcomes the limitations of BFR and DSC which cannot be used for testing products such as 25 μm SM wire, a 50 μm SE strip, and different springs and microsprings for actuation.