Increase In Prestrain Research Articles

Estimation of mechanical strength for pre-strained 316L austenitic stainless steel by small punch test

During pre-strain process of 316L austenitic stainless steel, the increase in dislocation density and the appearance of mechanical twinning lead to the variation of mechanical behavior with pre-strain. In this study, the effect of pre-strain on strength parameter and deformation energy was discovered by small punch test. The results show that yield strength and saturated elastic energy increase with the increase in pre-strain. But fracture energy decreases with the increase in pre-strain, and maximum load remains invariable. Load method and energy method were compared to estimate mechanical strength of pre-strained material. It is found that, energy method without artificial factor and with physical meaning is more suitable to estimate yield strength than load method, while load method is more suitable to estimate ultimate tensile strength than energy method. Moreover, the effects of pre-strain and temperature were coupled in the energy method to estimate yield strength of pre-strained specimens at different temperatures.

Negligible effect of twin-slip interaction on hardening in deformation of a Mg-3Al-1Zn alloy

Extruded magnesium alloys with a strong basal texture present a strong tension-compression asymmetry in deformation due to the fact that dislocation slips dominate plastic deformation in tension along the extrusion direction (ED) whereas twinning dominates plastic deformation in compression along the ED. The deformation characteristics allows for a study of the twin-slip interaction by introducing dislocations in the material before activating twinning. In the current work, dislocations of different densities are first introduced by prestraining in tension an extruded Mg-3Al-1Zn alloy along the ED to 5% and 10% of total strain, respectively. Then the loading direction is reversed to compression such that deformation twinning is activated. The results show that the hardening rate after different prestrains remains nearly unchanged, indicating that twin-slip interaction produces negligible effect on strain hardening. The yield stress in the reversed compression increases only slightly with increasing prestrain: ~20 MPa at every 5% increase in prestrain. Electron backscatter diffraction results show that prestraining can retard twin nucleation at the very early stage of plastic deformation, but it has little effect on twin growth.

Investigation of forming process of the third-generation automotive medium-Mn steel part with large-fractioned metastable austenite for high formability

With the increasing requirement of lightweight and safety of automotive industry, the third-generation automotive medium-Mn (TAMM) steel with large-fractioned metastable austenite has been newly developed in order to enhance the strength and guarantee the formability. In this paper, the effect of plastic deformation condition, such as strain direction and strain rate, on mechanical properties and microstructure of TAMM steel is firstly investigated. The results show that the TAMM steel holds isotropic characteristic and the effect of strain direction and strain rate on mechanical properties can be neglected. Furthermore, the two-step forming process of TAMM steel is studied step by step to understand the effect of pre-forming on final forming. On the one hand, the martensitic transformation and work hardening during pre-deformation increase the strength but decrease the uniformity of mechanical properties, which decreases the final formability. On the other hand, with the increase of pre-strain, the enhanced yield strength, lengthened yield platform and undulating work hardening rate during final forming increase the probability of the crack. The simulated and experimented results prove that one-step forming process is suitable for the TAMM steel part with complex shape because the TRIP effect during one-step forming contributes to high plasticity.

Metadynamic and static recrystallization softening behavior of a bainite steel

The metadynamic recrystallization (MDRX) and static recrystallization (SRX) softening behavior of a bainite steel was investigated by two-pass isothermal compression experiments at temperatures of 1173, 1273, 1373, and 1473 K and strain rates of 0.01, 0.1, 1, and 10 s−1 with inter-pass times of 1, 5, 10, and 30 s on a Gleeble-1500 thermo-mechanical simulator. Kinetic equations were developed to evaluate the softening fractions caused by MDRX and SRX. A comparison between the experimental and predicted softening fractions showed that the proposed kinetic equations can provide a precise estimation of the MDRX and SRX behavior of the studied steel. The results based on the kinetic equations indicated that the MDRX and SRX softening fraction increases with the increase in strain rate, deformation temperature, inter-pass time, and pre-strain; the activation energy of MDRX is much smaller than that of SRX; and the no-recrystallization temperature of the investigated steel is 1179.4 K.

Investigation on strain dependence of metadynamic recrystallization behaviors of GH4169 superalloy

The metadynamic recrystallization (MDRX) behaviors of a solid-solution GH4169 superalloy were studied by two-pass hot compression tests. The effects of pre-strain (deformation degree of the first pass) on MDRX behaviors were analyzed. Results show that the MDRX behaviors intensely depend on the pre-strain. The MDRX fraction first increases and then drops with the increased pre-strain. This is because a relatively large pre-strain can facilitate the nucleation of dynamic recrystallization (DRX) grains, which promotes the subsequent MDRX behaviors. However, due to the decreased stored deformation energy at relatively high pre-strain, the number of DRX grains cannot continuously increases. Thus, the MDRX fraction is reduced with the further increase of pre-strain. The segmented MDRX kinetics models were proposed and validated to describe the significant strain dependence of MDRX behaviors.

Characterization of ductile damage in polyethylene plate using ultrasonic testing

An experimental study was conducted to explore the possibility of using ultrasound to quantify deformation-induced damage in polyethylene (PE) plate. Specimens of two gauge lengths were machined from PE plates with thickness in the range from 1.5 to 10 mm. The specimens were first stretched monotonically to various prestrain levels to vary the extent of damage introduced by the stretch. Ultrasonic testing in the through transmission mode was then conducted on the prestrained specimens to determine the time of flight, based on which ultrasonic velocity was determined. The results show that the ultrasonic velocity, normalized by the speed in the virgin plate of the same thickness, decreases with the increase of prestrain. The study also shows that, with the correction of density change by the prestrain, the normalized ultrasonic velocity can be used to determine the dependence of damage level on the prestrain which, for specimens with long gauge length, is consistent with the damage determined by mechanical testing. The study concludes that ultrasonic testing can be used as a non-destructive means to quantify deformation-induced damage evolution in PE plates.

The Effects of Prestrain and Subsequent Annealing on Tensile Properties of CP-Ti

The aim of the present work is to investigate the effects of prestrain and subsequent annealing on tensile properties of commercial pure titanium (CP-Ti). According to tensile test results, yield strength and ultimate tensile strength increase with the increase of prestrain. Elongation and uniform strain decrease linearly with prestrain. In the case of prestrain that is higher than 3.5%, the macro-yield of specimens changes from gradual yielding to discontinuous yielding. It is supposed that considerable numbers of dislocations introduced into the material lead to the appearance of yield plateau. The quantitative analysis of the contribution of dislocation hardening to the strain hardening shows that dislocation-associated mechanisms play an important role in strain hardening. Moreover, a modified Fields-Backofen model is proposed to predict the flow stress of prestrained CP-Ti at different strain rates. Both strain rate sensitivity and strain hardening exponent decrease with prestrain. Fracture surfaces of the specimens show that fracture mechanism of all tested specimens is dimple fracture. The more ductile deformation in prestrained CP-Ti after annealing indicates that its ductility is improved by annealing.

Martensitic anelasticity in 40Mn60Cu alloy

The martensite anelasticity effects such as internal friction and the shape memory effect in 40Mn60Cu alloy were studied. The kinetics of spinodal decomposition in the 40Mn60Cu alloy after during aging (400, 440 and 540°C) was studied by the mechanical spectroscopy methods. It is shown that the temperature of the martensitic transformation depends on temperature and time of aging within the metastable miscibility gap. The characteristics of the shape memory effect were determined at different pre-strains. It is shown that the extent of shape recovery K decreases exponentially with the increase of pre-strain (γr=1…5%). This is due to the increase of the proportion of irreversible slip deformation.

Tensile and compressive behaviors of prestrained single-layer black phosphorus: a molecular dynamics study.

The effect of prestrain on tensile and compressive behaviors of single-layer black phosphorus (SLBP) in both armchair and zigzag directions is investigated by using molecular dynamics simulations. Prestrain is carried out by stretching or compressing SLBP in an orthogonal in-plane direction. The results show that the overall mechanical properties of SLBP, including Young's modulus, tensile strength, compressive strength, and yield strength are enhanced with an increase in compressive prestrain but are reduced with an increased tensile prestrain. With the same value of prestrain, SLBP exhibits unidirectional-homogeneous characteristics for tensile and compressive deformation. It is demonstrated that the armchair-oriented prestrain leads to a more significant improvement in the overall mechanical properties than the zigzag-oriented prestrain, indicating the anisotropic deformation behavior due to the characteristic puckers in SLBP. This work also reveals the mechanisms of prestrain underlying the mechanical behaviors of SLBP at the nanoscale, suggesting that the application of prestrain is an effective way to modify the mechanical properties of two-dimensional materials.

Low temperature cross-rolling to modify grain boundary character distribution and its effect on sensitization of SS304

Sensitization of austenitic stainless steels is well-known in the temperature range of 400–800°C. This effect can be lowered by the introduction of low-Σ coincident site lattice (CSL) boundaries in the material. In present work, 304 stainless steel was thermo-mechanically processed to obtain microstructures containing varied fractions of low-Σ CSL boundaries. Processing consisted of multi-step cross rolling at −41°C followed by a short heat treatment at 1000°C. In order to understand the effect of grain boundary character distribution (GBCD) on sensitization behavior, firstly, heating condition corresponding to the maximum degree of sensitization (DOS) was found by comparing double loop electro-chemical potentiodynamic reactivation (DL-EPR) plots for all the samples, which were given sensitization heat treatment at 600°C for various time. Later, a relation between the GBCD and DOS was established by conducting DL-EPR test on all the processed samples. Low-temperature cross rolling was found to be effective in keeping the fraction of low angle grain boundaries (LAGBs) low and ensuring equi-axed grain structure. The sample, which was given low strain followed by short heat treatment, showed highest fraction of CSL boundaries among all other conditions. Grain size was found to decrease with increase in pre-strain. The results indicated that sensitization was inversely related to the fraction of CSL boundaries and grain size, while it was directly related to the fraction of random high angle grain boundaries (HAGBs). It was also found that sensitization was weakly related to the fraction of LAGBs indicating that these boundaries did not show “special” behavior, as they do in many other grain boundary related phenomenon.

Effect of pre-strain on creep aging behavior of 2524 aluminum alloy

As an advanced manufacturing technology, creep age forming (CAF) has been developed to form large aluminum panels. However, industrial processes often involve complex thermo-mechanical treatments. For instance, it is necessary to impose a plastic strain after solution heat treatment to relieve quench stresses. In this work, the effect of tensile pre-strain before CAF on creep aging behavior of 2524 aluminum alloy is investigated by constant-stress creep aging test, Transmission Electron Microscope (TEM) and mechanical properties test. It is found that the magnitude of creep strain is greatly increased with the increase of pre-strain. Moreover, the peak strength of the creep aged AA2524 is enhanced and the time to reach the peak strength is decreased with the increases of pre-strain. TEM observations revealed that the precipitation process of AA2524 is accelerated due to pre-strain treatment. With the increase of pre-strain, the average size of S (Al2CuMg) phases is reduced. Meanwhile, the density and homogeneity of S phases are improved. Thus, it can be concluded that the pre-strain can evidently improve the formability, mechanical properties and microstructures of AA2524 in creep age forming process.

Effect of pre-deformation on microstructures and mechanical properties of high purity Al–Cu–Mg alloy

The effects of pre-deformation following solution treatment on the microstructure and mechanical properties of aged high purity Al–Cu–Mg alloy were studied by tensile test, micro-hardness measurements, transmission electron microscopy and scanning electron microscopy. The micro-hardness measurements indicate that compared with un-deformed samples, the peak hardness is increased and the time to reach peak hardness is reduced with increasing pre-strain. Additionally, a double-peak hardness evolution behavior of cold-rolled (CR) samples was observed during aging. The results of TEM observation show that the number density of S′(Al2CuMg) phase is increased and the size is decreased in CR alloy with increase of pre-strain. The peak hardness and peak strength of the CR alloy are increased because of quantity increasing and refinement of S′ phase and high density dislocation.

Effect of cathodic hydrogen-charging current density on mechanical properties of prestrained high strength steels

The present work investigated the effect of cathodic hydrogen-charging current density on mechanical properties of prestrained high strength steels. This was done by tensile tests on both hydrogen-charged and -uncharged prestrained specimens at a cross-head displacement speed of 0.03mm/min. The influence of prestrain on hydrogen behavior was also studied using an electrochemical permeation technique. The results show that the relationship between ultimate tensile strength of the hydrogen-charged specimens (UTS-H) and prestrain depends on current density. The UTS-H decreases with increasing current density independent on prestrain. With an increase in prestrain the diffusion coefficient of hydrogen gradually decreases, which is attributed to increasing dislocations density acted as hydrogen traps. SEM fractograph reveals that hydrogen charging causes a change from ductile to brittle failure.

A numerical method for predicting O-forming gap in UOE pipe manufacturing

Large diameter welded pipes used in oil and gas pipelines are primarily manufactured by the UOE process, which consists of U-forming, O-forming, and expansion procedures. The formation of O-forming gap in the O-forming stage is a key processing factor and it involves complex plastic deformation and springback of the steel plate. In this paper, a numerical method is proposed for predicting geometric configuration and O-forming gap of steel plate after C–U–O-forming. A combined hardening model for characterizing the loading–unloading–reverse loading deformation of pipe steels is developed based on the Chaboche hardening model. The yield plateau phenomenon and the variation of elastic modulus with the increase of pre-strain are incorporated. Industrial experiments are carried out, and the proposed method is validated through comparison between experimental and calculated results. Furthermore, the influence of material and processing parameters on O-forming gap is numerically investigated. The results reveal that U-forming radius is the most important factor in C- and U-forming stages to determine O-forming gap, and the opening gap width increases with increasing of compression ratio in O-forming stage. Finally, the established numerical method is combined with a genetic algorithm and applied to optimize UOE process parameters for obtaining an ideal opening gap. The proposed method provides an effective and efficient tool for UOE process design.

Effect of prestrains on swelling-induced buckling patterns in gel films with a square lattice of holes

We investigate the effect of prestrains on swelling-induced buckling patterns in polymeric films with a square lattice of holes. To reproduce experiments conducted by Zhang et al. (2008), poly(dimethylsiloxane) (PDMS) films are pre-strained in in-plane uniaxial tension in a lattice direction, and subsequently swelled by toluene. Finite element analysis is performed using an inhomogeneous field theory of polymeric gels in equilibrium, and with the aid of artificial damping. Periodic units consisting of 2×2 and 10×10 unit cells are analyzed under a generalized plane strain assumption. Analysis of the 10×10 unit cell shows that the resulting buckling pattern depends on the increase in prestrain of ε=0, 0.2, 0.4 and 0.6, evolving as a diamond plate pattern (ε=0), a slightly distorted diamond plate pattern (ε=0.2), a binary pattern of circles and lines (ε=0.4), and a monotonous pattern of ellipses (ε=0.6). These predictions are in very good agreement with experiments. The 2×2 unit cell reveals that these different patterns appear continuously as transitional states during transformation into diamond plate patterns; prestrains in uniaxial tension delay the onset of the pattern transformation and equilibrium swelling interrupts the progress of the transformation. Parametric studies demonstrate that the pattern dependence on prestrains originates from intrinsic swelling features, and is a consequence of a particular combination of the selected polymer and solvent.

Effects of pre-strain and baking parameters on the microstructure and bake-hardening behavior of dual-phase steel

In a typical process, C-Mn steel was annealed at 800°C for 180 s, and then cooled rapidly to obtain the ferrite-martensite microstructure. After pre-straining, the specimens were baked and the corresponding bake-hardening (BH) values were determined as a function of pre-strain, baking temperature, and baking time. The influences of pre-strain, baking temperature and baking time on the microstructure evolution and bake-hardening behavior of the dual-phase steel were investigated systematically. It was found that the BH value apparently increased with an increase in pre-strain in the range from 0 to 1%; however, increasing pre-strain from 1% to 8% led to a decrease in the BH value. Furthermore, an increase in baking temperature favored a gradual improvement in the BH value because of the formation of Cottrell atmosphere and the precipitation of carbides in both the ferrite and martensite phases. The BH value reached a maximum of 110 MPa at a baking temperature of 300°C. Moreover, the BH value enhanced significantly with increasing baking time from 10 to 100 min.

The effects of compressive pre-deformation on yield asymmetry in hot-extruded Mg–3Al–1Zn alloy

The effects of compressive pre-deformation along extrusion direction (ED) on yield asymmetry between tension and compression in hot-extruded Mg–3Al–1Zn (AZ31) alloy was investigated in this paper. {10–12}-type twinning occurred when samples were subjected to compressive pre-deformation. The compressive yield strength of sample with compressive pre-deformation increased gradually with the increase in compressive prestrain. The tensile yield strength of the sample decreased rapidly with the increase in compressive prestrain from zero to ~1.7%, but it was nearly unaltered when compressive prestrain was higher than ~1.7%. Detwinning occurred in the twinned regions played roles in the subsequent tensile process so that the yield stress was lower than sample without prestrain. Yield asymmetry between tension and compression decreased gradually when compressive prestrain was lower than ~1.1%, but it began to increase after that. It suggests that yield asymmetry can be effectively controlled by appropriate pre-deformation.

Effect of Pre-Strain on Fatigue Behaviour of AZ31 Mg Alloy

Mg Alloys Are the Lightest Structural Alloys with Excellent Castability and Machinability as Well as Highest Specific Strength and Stiffness. According to their Hexagonal close Packed Crystal Lattice there Is an Urgent Requirement of Mechanical Property Evaluation Method for Industrial Application, Particularly to Wrought Mg Alloys. Cyclic Loading Is a Very Popular Mode for Most Structural Application Situations. Recent Development of Fatigue Examination of Extruded Profile Has Shown that Mg Alloys Show Not so Ideal Fatigue Property. so that the Further Detailed Study on Cyclic Loading for Plastic Deformed Mg Alloys Is Needed. Tension-Tension Fatigue Tests Were Conducted on Pre-Strained AZ31 Mg Alloys that Produced by Rolling and Extruding Procedures. the Results Show that the Compressive Plastic Deformation Leads to Reduction of Fatigue Life/limit Significantly. SEM Analysis on Fatigue Fracture Surfaces Indicates that the Fatigue Cracks Initiate in the Surface or Sub-Surface of Dramatically Deformed Zones. the Microstructure Analysis Reveals that the Pre-Strain Brings More Abundant Twinning Bands with the Increase in Pre-Strain. the Decrease in Fatigue Life Demonstrates Also some Critical Feature with the Pre-Strain Level.

Postbuckling of Shape Memory Alloy Reinforced Cross-Ply and Angle-Ply Laminated Plates

ABSTRACTThe effect of shape memory alloy (SMA) on the postbuckling behavior of rectangular cross-ply and angle-ply plates by varying the SMA fiber spacing was investigated using the Finite Element Method. The formulation of the location-dependent linear, nonlinear stiffness matrices due to non-homogeneous material properties and the temperature-dependent recovery stress stiffness matrix were derived. Numerical results show that the increase of SMA fiber volume fraction and prestrain may generate more recovery stress, and increase the stiffness of SMA reinforced composite laminate. Therefore, the postbuckling deflections of the plate may be decreased significantly. The buckling mode that plate will buckle into is dependent on the fiber orientation of the angle-ply laminates. When the SMA fibers are concentrated in the center of the plate, the postbuckling deflections of the plate will be decreased considerably.

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 °C 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 (Al 2CuMg) 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.