Pre-strain Level Research Articles

Pre-strain-dependent natural ageing and its effect on subsequent artificial ageing of an Al-Cu-Li alloy

The effects of pre-strain and natural ageing (NA) on the precipitation hardening behavior in an Al-Cu-Li alloy have been systematically investigated. Applying pre-strain significantly restricts the solute clustering-induced formation of fine GPI zones (single Cu-layer) and thus hardness increase during NA. NA before artificial ageing (AA) has a marginally positive effect on the age hardening in samples without pre-strain, giving rise to a strength improvement of 15–20 MPa without loss of ductility. NA slightly enhances the precipitation of T1 phase during AA while shows no obvious effect on θ′, σ, δ′ and X phases. The X phase hasn't been reported and is isostructural to the θ′ phase, but the habit plane is (011)Al instead of (001)Al. With the increase of pre-strain level, the T1 plates become dominant and finer. However, NA after pre-strain has no appreciable effect on the age hardening and microstructure of the alloy. The analysis by complementary methods suggests that the low stability of NA clusters and their inhibition by pre-strain are responsible for the observed NA effect on AA in Al-Cu-Li alloy.

Analytical layerwise solution of nonlinear thermal instability of SMA hybrid composite beam under nonuniform temperature condition

Thermal buckling and post-buckling behavior of composite beam reinforced with shape memory alloy (SMA) wires under nonuniform temperature distribution is explored. Thermo-mechanical behavior of SMA wires is formulated by using the one-dimensional Brinson SMA model. Considering von Karman strain–displacement relation, corresponding nonlinear governing equations are obtained and solved analytically. Heat conduction equation is employed and through-the-thickness temperature distribution is obtained by discretization scheme of layerwise method. Influence of SMA-wire positioning across the thickness, temperature distribution, SMA wire pre-straining level and volume fraction of SMAs upon the thermal buckling and post buckling of reinforced beam are examined and discussed in detail.

Effect of pre-strain on cyclic plastic behaviour of 7050-T6 aluminium alloy

This paper aims at studying the pre-strain effect on cyclic plastic behaviour of the 7050-T6 aluminium alloy. In order to meet this goal, different series of tests with various pre-strain histories are performed under fully-reversed strain-controlled conditions. In a second stage, fracture surfaces are examined by scanning electron microscopy to identify the main fracture damage micro-mechanisms associated with the pre-strain scenarios. Overall, the results show that higher tensile pre-strain levels lead to lower fatigue lives. This reduction of the fatigue life expectancy tends to increase for higher cyclic strain amplitudes.

The effect of pre-twinning on the mechanical behavior of free-end torsion for an extruded AZ31 magnesium alloy

In this paper, the effect of pre-twinning on the mechanical behavior of free-end torsion (FET) for an extruded AZ31 Mg alloy was investigated via pre-compression along extruded direction with 1.5%, 3.5%, 5.5% and 6.5% plastic strain levels. After pre-compression, torsion was performed at room temperature. The yield strength was enhanced remarkably during FET test after 3.5% pre-compression. However, the change of yield strength is small during FET with pre-compression (PRC) greater than 3.5%. Refinement hardening induced by twinning contributes to the enhancement of yield strength for PRC 3.5 specimen. With pre-strain increasing, the change of yield strength is ascribed to dislocation hardening. The stage II and stage III of strain hardening were observed during pure torsion and FET test after 1.5% pre-compression. In contrast, the stage II is suppressed and the stage III only occurs during FETs after 3.5%, 5.5% and 6.5%. The pre-twinning favor the activation of prismatic slip under torsion, which enhances multiple slip and dynamic recovery. The transition of deformation mechanisms from basal slip to multiple slip leads to the change of strain hardening during FET after different pre-strain levels.

The effects of induction quenching and pre-strain levels on twin behavior in extruded ZK21 magnesium alloy

In the present study, pre-compressive test, re-compressive test or reverse tensile test were carried out on the extruded ZK21 alloy to investigate the twinning and detwinning behaviors after induction quenching. The influence of quenching temperature and the pre-strain levels on mechanical behavior was systematically addressed. It is found that the growth and shrinkage of the previous twins generated by pre-compression are impeded for all samples after quenching and the extent of twin growth or shrinkage is closely related to the pre-strain levels. Induction quenching has little influence on twin nucleation during recompression, while the number of new nucleated twins decreases dramatically with the increase of pre-strain levels for all the samples. Meanwhile, our results also show that quenching temperature and pre-strain levels generate quite different effects on the re-compressive yield strength and the reverse tensile yield strength. Correspondingly, the influence of twin behavior on mechanical properties was discussed.

Multistate and On-Demand Smart Windows.

Composite films consisting of wrinkles on top of the elastomeric poly(dimethylsiloxane) film and a thin layer of silica particles embedded at the bottom is prepared as on-demand mechanoresponsive smart windows. By carefully varying the wrinkle geometry, silica particle size, and stretching strain, different initial optical states and a large degree of optical transmittance change in the visible to near infrared range with a relatively small strain (as small as 10%) is achieved. The 10% pre-strain sample has shallow wrinkles with a low amplitude and shows moderate transmittance (60.5%) initially and the highest transmittance of 86.4% at 550 nm when stretched at the pre-strain level. Stretching beyond the pre-strain level leads to a drastic decrease of the transmittance at 550 nm, 39.7% and 70.8% with an additional 10% and 30% strain, respectively. The large drop of optical transmittance is the result of combined effects from the formation of secondary wrinkles and nanovoids generated around the particles. The 20% pre-strain sample has wrinkles with a moderate amplitude, showing 36.9% transmittance in the initial state, and the highest transmittance of 71.5% at 550 nm when stretched to the pre-strain level. Further stretching leads to increased opacity similar to that seen from the 10% pre-strain sample.

The effect of tensile pre-straining on fatigue crack initiation mechanisms and mechanical behavior of AA7050 friction stir welds

This work examines the plastic strain gradients developed by pre-straining AA7050 friction stir welds (FSW), the damage to intermetallic particles caused by this strain, and the effect of these fractured particles on the subsequent fatigue performance of the weld. After pre-straining FSW samples to 1% and 3%, the local plastic strain distribution was measured along the crown (top) and root (bottom) of the weld, and the maximum local plastic strain was found to be 3.34 and 1.83 times higher than the average plastic strain. Metallography conducted on pre-strained welds showed cracked intermetallic particles in both the 1% and 3% levels which may have influenced fatigue crack initiation. Pre-straining was found to decrease the fatigue life at high cycles for the 1% pre-strain; however the 3% pre-strain level showed essentially the same fatigue life as the 0% pre-strained welds. Fractography showed that increasing pre-strain was correlated with increasing prevalence of Fe-rich particles at the fatigue crack initiation sites. When corrected for maximum local strain, the fatigue results for non-pre-strained AA7050 FSWs were shown to match the base material, AA7050-T7451.

Effect of α′ Martensite Content Induced by Tensile Plastic Prestrain on Hydrogen Transport and Hydrogen Embrittlement of 304L Austenitic Stainless Steel

Effects of microstructural changes induced by prestraining on hydrogen transport and hydrogen embrittlement (HE) of austenitic stainless steels were studied by hydrogen precharging and tensile testing. Prestrains higher than 20% at 20 °C significantly enhance the HE of 304L steel, as they induce severe α′ martensite transformation, accelerating hydrogen transport and hydrogen entry during subsequent hydrogen exposure. In contrast, 304L steel prestrained at 50 and 80 °C and 316L steel prestrained at 20 °C exhibit less HE, due to less α′ after prestraining. The increase of dislocations after prestraining has a negligible influence on apparent hydrogen diffusivity compared with pre-existing α′. The deformation twins in heavily prestrained 304L steel can modify HE mechanism by assisting intergranular (IG) fracture. Regardless of temperature and prestrain level, HE and apparent diffusivity ( D app ) increase monotonously with α′ volume fraction ( f α ′ ). D app can be described as log D app = log ( D α ′ s α ′ / s γ ) + log [ f α ′ / ( 1 − f α ′ ) ] for 10 % < f α ′ < 90 % , with D α ′ is diffusivity in α′, s α ′ and s γ are solubility in α′ and austenite, respectively. The two equations can also be applied to these more typical duplex materials containing both BCC and FCC phases.

Effect of pre-strain on fatigue crack growth behavior for commercial pure titanium at ambient temperature

In this paper, the effects of pre-strain on fatigue crack growth rate are investigated considering wide ranges of load amplitudes and load ratios for commercial titanium alloy (CP-Ti). Considering the influences of load ratio and load amplitude, the effect of pre-strain on the strain energy and crack tip plastic zone size, and on the fatigue fracture mechanism have been clearly demonstrated. Besides these, how the pre-strain affects fatigue crack propagation, and therefore affects whole fatigue life have also been clearly stated. Results demonstrate that pre-strain may be more sensitive to the enhancement of fatigue crack growth resistance under high load ratio and high peak load conditions. Fatigue lives increase with increasing in the pre-strain level. With the increase of load ratio and load amplitude, pre-strain has more and more significant influence on the plastic strain energy of specimen. A high pre-strain level of 8% and a high load ratio of R = 0.8 are more sensitive to the fatigue life. Static load or static damage has the most significant contribution to the crack tip plastic deformation than fatigue load. The production of twin crystal and dislocation structure after pre-strain process will greatly ensure the enhancement of fatigue crack resistance under all loading conditions. This research will provide support to the insight of pre-strain effect on fatigue crack growth behavior of CP-Ti.

Flexural Response of Reinforced Concrete Beams Strengthened with Near-Surface-Mounted Fe-Based Shape-Memory Alloy Strips

This paper proposes an advanced near-surface-mounted (NSM) technique with an Fe-based shape-memory alloy (Fe-SMA) strip which can solve issues of low workability and reduced ductility of reinforced concrete (RC) beams strengthened with an NSM technique using prestressed fiber-reinforced polymer (FRP) strips in the concrete tension section. The flexural behavior of the RC beam strengthened by the NSM technique with the Fe-SMA strip was investigated. A total of seven RC beams were tested by four-point bending tests under displacement control. The type of reinforcements, the quantity of Fe-SMA strips, and the pre-straining level of the Fe-SMA strips were considered as experimental variables. Cracking load, yielding load, and ultimate load increased, respectively, with larger quantities of Fe-SMA strip. In addition, activation of embedded Fe-SMA in the concrete by electrical resistance heating effectively induces a prestressing force on the concrete beam, resulting in a cambering effect. The introduced prestressing force to the RC beam by activation of the Fe-SMA increased the crack and yielding loads, and did not decrease the ductility of the RC beam compared to the RC beam with non-activated Fe-SMA. It can be concluded from the test results that the strengthening technique using the recovery stress of the Fe-SMA strip as the prestressing force solves the various problems of the existing prestressing strengthening systems, meaning that Fe-SMA can be used as a substitute for conventional prestressing strengthening systems.

Tunable wave propagation by varying prestrain in tensegrity-based periodic media

This paper investigates the dynamic properties of one, two and three-dimensional tensegrity-based periodic structures introduced in Rimoli and Pal (2017), which are here termed as tensegrity beams, plates and solids, respectively. We study their linear wave propagation properties and show that in each case, these properties can be significantly altered by the prestrain in the cables. As the prestrain is varied, we observe jumps in the wave velocities at two critical prestrain values, which define transitions between the three distinct phases of these structural assemblies. At low cable prestrains, the wave speeds are zero as the lattices have zero effective stiffness. At moderate prestrains, the wave speed is nonzero and finally, at prestrain levels where the bars buckle, the wave speeds change to a near constant value. Dispersion analysis on these beams, plates and solids reveal unique properties such as very low wave velocities compared to their constituent material and the existence of flat bands at low frequencies. Furthermore, we find that shear waves travel faster than longitudinal waves in tensegrity solids in a range of cable prestrains. Finally, we verify the key observations through detailed numerical simulations on finite tensegrity solids.

Compressive yield stress improvement using thermomechanical treatment of extruded Mg-Zn-Ca alloy

Thermomechanical treatment consisting of pre-compression and a subsequent isothermal aging was used to improve the compressive properties of extruded Mg-Zn-Ca (ZX10) alloy. Isothermal aging at 150 °C and 200 °C was performed on as-extruded samples for various times to find peak aged conditions. To achieve a different twin volume fraction in the alloy, samples were pre-compressed up to 2%, 3% and 4% along the extrusion direction. Afterwards, the peak-aged conditions were applied to the pre-compressed samples to enhance their mechanical properties. It was found that after the 4 h@150 °C thermomechanical treatment, deformation curves exhibit a distinctive yield plateau independently on the pre-strain level. Using the 2 h@200 °C condition, the yield plateau occurs only after pre-strain of 3%. Higher compressive yield strength values were observed for the thermomechanically treated samples at the lower temperature.

Evaluation of Fe-Based Shape Memory Alloy (Fe-SMA) as Strengthening Material for Reinforced Concrete Structures

This paper aims to evaluate potential of an Fe-based shape memory alloy (Fe-SMA) for strengthening civil structures. Mechanical properties of the Fe-SMA were investigated with a direct tensile test, which showed the stress-induced transformation, stress at fracture of the Fe-SMA, and modulus of elasticity. Heating temperature ranging from 110 ℃ to 220 ℃ and pre-straining level ranging from 2% to 8% of the Fe-SMA were considered as variables to provoke a shape memory effect (SME), which generates a recovery stress. The recovery stresses ranged from 207.59 MPa to 438.61 MPa, which plays a role in introducing a pre-stressing force to concrete members. Bonding behavior of the Fe-SMA embedded into a groove with a cement-based mortar filler was investigated to determine the required bonding length to fully develop the pre-stressing force of the Fe-SMA with a near-surface mounted (NSM) strengthening technique. All the tested specimens showed slippage failure and suggested a minimum bonding length of 600 mm. The pre-stressing force applied on the concrete can be calculated with the recovery stress of the Fe-SMA. Based on those test results, the Fe-SMA shows sufficient potential to be used as strengthening material for civil structures.

Understanding the compressive behavior of shape memory alloy (SMA)-confined normal- and high-strength concrete

This paper presents the first experimental study on the axial compressive behavior of high-strength concrete (HSC) confined by shape memory alloy (SMA) wire. Concrete cylinders that were prepared using two different grades of concrete, namely normal-strength concrete (NSC) and HSC, were confined with Nitinol (Ni-Ti) SMA spirals having a pitch spacing of 36 and 20 mm, respectively. Preliminary material tests were performed on SMA wires to investigate the tensile strength and strain of SMA in martensitic and austenitic phases and the effect of the temperature on the recovery stress. The confining pressure was applied on concrete cylinders by SMA spirals that were prestrained at 0, 5.5, and 9.5%. The material test results show that an increase in the prestrain level from 5.5% to 9.5% leads to an increase in the recovery stress of SMA wire at temperatures higher than the austenitic finish temperature (90 °C). The compression test results of SMA-confined concrete specimens show that an increase in the prestrain level leads to an increase in the peak axial stress and corresponding axial strain of SMA-confined concrete. Confinement of NSC and HSC specimens by 9.5% prestrained SMA spirals results in a 38.1% and 23.6% higher peak axial stress and a 333% and 346% higher corresponding axial strain, respectively, compared to those of unconfined specimens. It is also shown that because of the more brittle behavior of HSC specimens, SMA-confined HSC fails at a lower axial strain compared to SMA-confined NSC. The lower ultimate axial strain of HSC specimens is also attributed to the lower confinement ratio of the HSC specimens of the current study compared to that of NSC specimens. The promising findings of this study point to the significant potential for the use of SMA spirals in the construction industry for the development of high-performance composite structural members and for strengthening and retrofit of existing concrete members.

Negative stiffness honeycombs as tunable elastic metamaterials

Acoustic and elastic metamaterials are media with a subwavelength structure that behave as effective materials displaying atypical effective dynamic properties. These material systems are of interest because the design of their sub-wavelength structure allows for direct control of macroscopic wave dispersion. One major design limitation of most metamaterial structures is that the dynamic response cannot be altered once the microstructure is manufactured. However, the ability to modify wave propagation in the metamaterial with an external stimulus is highly desirable for numerous applications and therefore remains a significant challenge in elastic metamaterials research. In this work, a honeycomb structure composed of a doubly periodic array of curved beams, known as a negative stiffness honeycomb (NSH), is analyzed as a tunable elastic metamaterial. The nonlinear static elastic response that results from large deformations of the NSH unit cell leads to a large variation in linear elastic wave dispersion associated with infinitesimal motion superposed on the externally imposed pre-strain. A finite element model is utilized to model the static deformation and subsequent linear wave motion at the pre-strained state. Analysis of the slowness surface and group velocity demonstrates that the NSH exhibits significant tunability and a high degree of anisotropy which can be used to guide wave energy depending on static pre-strain levels. In addition, it is shown that partial band gaps exist where only longitudinal waves propagate. The NSH therefore behaves as a meta-fluid, or pentamode metamaterial, which may be of use for applications of transformation elastodynamics such as cloaking and gradient index lens devices.

Influence of the stacking fault energy and temperature on the prestrain memory effect of face centered cubic metal submitted to cyclic loadings

In this paper, the effect of a monotonic prehardening in tension on the subsequent cyclic stress strain curves is investigated as a function of the deformation mechanisms. To this aim, three materials are employed, copper for wavy dislocation slip, Ni20Cr alloy for planar slip and AISI 316L for intermediate deformation mechanisms. Samples were first prestrained in tension and then submitted to incremental strain cyclic tests at room temperature, and also 200°C for the 316L. Based on the analysis of the flow stress components (backstress and effective stress), the results show that all materials are sensitive to a prehardening depending on the prestrain level and cyclic test characteristics (amplitude and temperature).

A numerical analysis of the effects of manufacturing processes on material pre-strain in offshore wind monopiles

The majority of offshore wind turbines in Europe are supported by monopile type foundation structures. Monopiles are made of large thickness steel plates which are longitudinally welded to fabricate “cans” and these cans are subsequently welded around the circumference to manufacture a monopile. Monopile structures can have diameters of 4-10m, with wall thicknesses of 40-150mm. To achieve the cylindrical shape in individual cans, large thickness steel plates are typically cold formed via the three-roll bending process. During forming of these plates, the material is subjected to plastic pre-strain, which subsequently influences the fracture and fatigue properties of monopile structures. In this study, a finite element model has been developed to predict the pre-straining levels in monopiles of different dimensions. To determine the influence of numerous manufacturing practices, a sensitivity analysis of different factors has been conducted. These include fabrication dependent variables such as the influence of friction coefficient and bending force, and geometry dependent factors such as plate thickness, length, and distance between rollers. From the numerical results, a range of expected material pre-strain levels have been identified and presented in this paper.

Confinement of NORMAL- AND HIGH-STRENGTH CONCRETE by Shape Memory Alloy (SMA) Spirals

This paper presents the results of an experimental study on the axial compressive behaviour of normal- and high-strength concrete (NSC and HSC) confined by shape memory alloy (SMA) spirals. A spiral pitch space of 36 and 20 mm was used for SMA confinement of NSC and HSC columns, respectively. The confining pressure was applied on the concrete cylinders by SMA spirals that were prestrained at 0, 5.5, and 9.5%. The compression test results on the SMA-confined specimens indicate that the prestrain level of SMA significantly affects the axial compressive behaviour of both NSC and HSC. An increase in the level of prestrain leads to an increase in the peak axial stress and corresponding strain of SMA-confined concrete.

Effects of manufacturing plastic prestrains found on calendered and UOE pipes and pressure vessels on structural integrity assessments regarding fatigue crack growth and LBB

High responsibility components operating under cyclic loading can have their resistance against initiation and growth of fatigue cracks highly influenced by previous thermomechanical processing. Within the interest of the present work, different manufacturing processes (such as the fabrication of pipes using UOE) or installation procedures (such as pipe reeling) can lead to nonuniform plastic prestrain fields thus affecting lifetime and safety. Previous studies conducted by the authors revealed that prestraining up to 14% considerably reduced fatigue crack growth rates (da/dN-ΔK) for a hot-rolled (1/2”) ASTM-A36 steel as a result of strain hardening and its effects on plastic zone sizes. In addition, results also revealed two interesting trends: i) the larger is the imposed prestrain, the greater is the growth rate reduction in a nonlinear asymptotic relationship; ii) the larger is imposed ΔK, the more pronounced is the effect of prestraining. Within this scenario, this work explores the effects of those results on more realistic lifetime predictions applicable to pipelines and pressure vessels. First, a nonlinear model based on the experimental results was developed to predict crack growth rates (da/dN) as a function of ΔK and varying plastic prestrain levels. Second, refined nonlinear FE models were developed to quantify plastic prestrain fields caused by manufacturing sequence of the studied pressure vessels and pipes. Such fields proved to be remarkably nonuniform, in accordance to the literature. Finally, considering an idealized cyclic internal pressure and the obtained fields, a MatLab algorithm was implemented to predict fatigue crack growth rates until LBB (Leak Before Break) condition takes place. Best practices for implementing such evaluations are presented and results revealed that effects of plastic prestrains are not negligible and should be taken into account on recommended practices and structural integrity assessments to avoid excessive conservatism or lack of safety in applications.

Effect of shape memory alloy wires on high-velocity impact response of basalt fiber metal laminates

Fiber metal laminates are commonly used materials in industrial applications due to low density and excellent mechanical properties such as impact resistance. In this study, the effect of shape memory alloy wires on the impact properties of fiber metal laminates is evaluated and discussed. The volume fraction of wires and the pre-strain of wires were selected as the variables of the experiments. The samples were fabricated using hand lay-up method and their behavior was evaluated under high-velocity impact test. Based on the results, a significant change in the amount of absorbed energy was obtained using embedded wires. However, with increasing the volume fraction of wires, the absorbed energy decreases due to the discontinuity in mechanical properties of the specimens. On the other hand, applying pre-strain leads to residual stresses in samples. The observed residual stress in specimens with four embedded wires and 2% pre-strain level shows the most absorbed energy.