Compressive Prestrain Research Articles

Time-varying compressive properties and constitutive model of EPDM rubber materials for tunnel gasketed joint

The stress relaxation of ethylene propylene diene monomer (EPDM) rubber material in a compressed state significantly increases the risk of water leakage at tunnel precast segmental joints. Additionally, EPDM rubber gaskets typically experience a complex precompression strain state, further complicating the prediction of long-term stress relaxation. Therefore, it is imperative to propose a novel constitutive model that realistically reflects the stress relaxation of rubber with arbitrary precompression strain, providing a reference for the long-term waterproof design of shield tunnels. In this study, the time-varying properties of EPDM rubber at material level were studied in detail. At first, a comprehensive set of accelerated aging tests were conducted on EPDM rubber samples with varying precompression strains. The hardening effect of rubber without precompression and the stress relaxation effect of rubber with precompression were explored. The time-varying law and mechanism governing rubber properties under these two effects were analyzed. Subsequently, a conversion relationship between the EPDM rubber test conditions and the actual service time was established. The predicted hardening coefficient for uncompressed EPDM after 100 years is 1.468, whereas the relaxation coefficient for compressed EPDM is 0.359. Comparative analysis revealed that the compression set index do not consider the beneficial effects of hardening, resulting in a 54.1% overestimation of the design water pressure. Finally, the analytical study on the constitutive model of EPDM rubber material was carried out. The error in compression curve for unaged sample obtained from empirical formulas exceeded three times that of the analytical model, underscoring the need to study time-varying constitutive models. An interpolation method was employed to extend the test curve, leading to the development of a new constitutive model that represents the stress relaxation characteristics of EPDM materials under arbitrary precompression strain states. By combining the proposed constitutive model with the timetemperature conversion relationship, the time-varying expressions of the constitutive parameters for relaxation and hardening effects during the service life were derived. In comparison with existing models, the proposed model demonstrates wider applicability, effectively captures the time-varying characteristics of EPDM materials under arbitrary precompression strains, offering an efficient approach for studying the long-term stress relaxation prediction of rubber gaskets in shield tunnels under complex precompression strain states.

On the determination of viscoelastic properties of EPP foam in dependence on pre-strain and loading direction

Understanding the viscoelastic properties of EPP foams is of crucial importance for their use in industrial applications. However, mechanical testing of polymeric foams is challenging due to the difficulties that arise from their morphology. For this reason, here currently available DMA methods using a single drive and a multi-drive rheometer as well as multiple loading directions are investigated in detail. Experimental challenges are highlighted and discussed with regard to foam-specific properties. Special focus is laid on the influence of compressive pre-strain. Based on the results, guidelines to perform reproducible DMA tests on the investigated type of material are derived. In addition, the results give deeper insight into the deformation behavior of bead foams at small strain levels and reveal an early onset of the transition from linear to plateau regime.

Enhancing the thermal stability and recoverability of ZrCu-based shape memory alloys via interstitial doping

ZrCu-based shape memory alloys (SMAs) are regarded as emerging smart materials with high phase transformation temperatures. However, unfavorable thermal stability and poor strain recovery rate hinder commercial applications. Herein, these issues were addressed by doping interstitial B atoms, and the impacts of B content on microstructure, martensitic transformation behavior and properties of ZrCu-based SMAs were systematically investigated. It was shown that (Zr50Cu25Ni7.5Co17.5)99.98B0.02 SMAs possessed excellent thermal stability and shape memory effect. The change of martensitic transformation temperature for this alloy was 15.7 °C after several cycles. The shape recovery rate reached 92.5 % at 8 % compressive pre-strain, which was attributed to the de-twinning of (0 0 1) compound twins and the appearance of (1 1 1) type I twins. Furthermore, it was elucidated for the first time by first-principles calculations that B atoms were covalently bonded with Zr and Cu atoms occupying the octahedral interstitial positions in the B19′ and Cm phases. This study offers an available pathway to exploit efficient and advanced products.

Atomistic study of internal stress effect on scratching behavior in titanium single crystal

The internal stress effect on the scratching behavior in titanium single crystal is investigated by molecular dynamics simulations. It is introduced by applying the compressive/tensile pre-strain in this work. The friction force in the model with compressive pre-strain is higher than that with tensile pre-strain, which is related to the atomistic spacing and the distribution of pile-up. The dislocation mechanism is systematically illustrated that three types of <a> dislocations of edge dislocation loop, prism edge dislocation and screw dislocation dominate the plastic deformation. The coupling of dislocation loop and screw dislocation in the model with tensile pre-strain causes much more defects to remain in the matrix. However, when the model is applied by small compressive pre-strain, the dislocations left in the matrix are obviously decreased, reducing the internal crystal damage. Overall, the compressive/tensile internal stress has a distinguishing effect on the scratching behavior, which can be used for altering the friction and wear property of titanium.

An anomalous compression-induced softening behavior of AA6014-T4P during cyclic loading

This paper investigated the cyclic plasticity of AA6014-T4P with the tension-compression (T-C) and compression-tension (C-T) cyclic loading tests. A new compression-induced softening behavior is found and modeled. Specifically, the material was softened permanently during the tensile load when a compressive pre-strain was applied, whereas such an abnormal phenomenon was not observed when the loading sequence is reversed. Moreover, the magnitude of the permanent softening scale linearly with the compressive pre strain. Therefore, we term this phenomenon as compression-induced softening in this paper. Based on this compression-induced softening phenomena, a modified Y–U model is formulated by introducing a correction factor that modifies the hardening rate of the boundary surface to describe the permanent softening effect, which can predict the compression-induced softening well. Our work reveals a new phenomenon for the cyclic loading of metal sheets and stimulates further efforts on the path-dependence of metal plasticity.

Snap-through of graphene nanowrinkles under out-of-plane compression

Nanowrinkles (i.e. the buckled nanoribbons) are widely observed in nano-devices assembled by two-dimensional (2D) materials. The existence of nanowrinkles significantly affects the physical (such as mechanical, electrical and thermal) properties of 2D materials, and thus further, impedes the applications of those devices. In this paper, we take the nanowrinkle formed in a monolayer graphene as a model system to study its deformation behaviours, especially the configuration evolution and the snap-through buckling instabilities, when subjected to the out-of-plane compression. By performing molecular dynamics simulation, the graphene nanowrinkles with or without self-adhesion (which are notated as ‘clipped’ state or ‘bump’ state, respectively) are obtained depending on the geometric size and the applied axial compressive pre-strain. The elastica theory is employed to quantify the shape of ‘bump’ nanowrinkles, as well as the critical condition of the transition between ‘clipped’ and ‘bump’ states. By applying out-of-plane compression to the generated graphene nanowrinkle, it flips to an opposite configuration via snap-through buckling. We identify four different buckling modes according to the configuration evolution. An unified phase diagram is constructed to describe those buckling modes. For the cases with negligible van der Waals interaction getting involved in the snap-buckling process, i.e. without self-adhesion, the force–displacement curves for nanowrinkles with same axial pre-strain but different sizes can be scaled to collapse. Moreover, the critical buckling loads can also be scaled and predicted by the extended elastica theory. Otherwise, for the cases with self-adhesion, which corresponds to the greater axial pre-strain, the van der Waals interaction makes the scaling collapse break down. It is expected that the analysis about the snap-through buckling of graphene nanowrinkles reported in this work will advance the understanding of the mechanical behaviours of wrinkled 2D materials and promote the design of functional nanodevices, such as nanomechanical resonators and capacitors.

Measurement of Irreversible External-Compressive Strain at RT and Enhancing Strain Tolerance at RT on MgB2 Multifilament Wire

The strain tolerance of magnesium dioboride (MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ) wire should be enhanced at room temperature for fabricating MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> coils by using the react and wind method. The irreversible external-compressive strain of MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> filaments was measured by evaluating the critical current degradation of bent sample wires. From the results, MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> filaments can accept -1.77% strain, and the pre-compressive strain should be -0.88% at room temperature to minimize its reversible bending radius. A stainless-steel reinforced 18-filament MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> wire was fabricated and its strain tolerance was evaluated through bending and tensile tests at room temperature. The irreversible strain of this wire is 0.60%, and this wire seems to be suitable for the react and wind method. The filling factor of MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> of this wire is 17%, so this wire has both suitable strain tolerance for the react and wind method and higher engineering current density.

Folded graphene reinforced nanocomposites with superior strength and toughness: A molecular dynamics study

• Folded graphene is designed and generated with the aid of hydrogenation and pre-compression. • Current molecular dynamics simulation model is experimentally and numerically validated. • The designed folded graphene leads to superior strength and toughness simultaneously in copper nanocomposites. • Pre-compressive strain of folded graphene further improves the strength and toughness of nanocomposites. • Nanocomposites reinforced with folded graphene with more wave and peak numbers exhibit better toughness. Toughness and strength are important material parameters in practical structural applications. However, it remains a great challenge to achieve high toughness and high strength simultaneously for most materials. Here, we report a folded graphene (FG) reinforced copper (Cu) nanocomposite that overcomes the long-standing conflicts between toughness and strength. Intensive molecular dynamics simulations show that the 10% pre-strain-induced four-wave-patterned FG (1.09 wt%) reinforced Cu nanocomposite exhibits simultaneous enhancement in toughness (∼13.59 J/m 2 ), ductility (∼32.38%), and strength (∼9.52 GPa), corresponding to 38.53%, 58.88%, and 2.26% increase, respectively when compared with its counterpart reinforced by pristine graphene (PG). More importantly, the mechanical properties of FG/Cu nanocomposites can be effectively tuned by changing the pre-compressive strain, wave number, and peak number of FG. The toughening and strengthening mechanisms are applicable to other metal materials reinforced by other 2D nanomaterials, opening up a new avenue for developing tough and strong metal nanocomposites.

Mesoscopic numerical study on ductile fracture in TRIP steel under reverse loading

The aim of this study is to elucidate the fracture mechanism of transformation-induced plasticity (TRIP) steel under reverse loading using a ductile damage simulation method. First, the fracture limit of a TRIP steel under the reverse path is experimentally investigated and compared with the results of a dual-phase (DP) steel without a retained austenite. In both steels, the reduction in fracture strain by compressive pre-strain is smaller than that by tensile pre-strain. The difference between the effects of tensile and compressive pre-strain is even larger in the TRIP steel than in the DP steel. These results are analyzed via numerical calculations using a model that extends the ductile damage theory proposed by Ohata et al. to account for the microstructural changes caused by martensitic transformation. The results indicate that the development of damage in the TRIP steel strongly depends on the fraction and distribution of the hard martensite generated by the transformation. Moreover, when TRIP steels are subjected to compressive pre-strain, martensitic transformation occurs less likely than when they are subjected to tensile pre-strain. This leads to less heterogeneity and less void formation within the microstructure. As a result, the fracture strain decrease in TRIP steel due to compressive pre-strain is smaller than that in DP steel.

Experimental Study on Recentering Behavior of Precompressed Polyurethane Springs.

Traditional seismic design has a limitation in that its performance is reduced by significant permanent deformation after plastic behavior under an external load. The recentering characteristics of smart materials are considered to be a means to supplement the limitations of conventional seismic design. In general, the recentering characteristics of smart materials are determined by their physical properties, whereas polyurethane springs can regulate the recentering characteristics by controlling the precompression strain. Therefore, in this study, 160 polyurethane spring specimens were fabricated with compressive stiffness, specimen size, and precompression strain as design variables. The compression behavior and precompression behavior were studied by performing cyclic loading tests on a polyurethane spring. The maximum stress and maximum strain of the polyurethane spring showed a linear relationship. Precompression and recentering forces have an almost perfect linear relationship, and the optimal level of precompression at which residual strain does not occur was derived through regression analysis. Additionally, a prediction model for predicting recentering force based on the linear relationship between precompression and recentering force was presented.

Deformation dynamics and pre-compression effects on Mg-3Al-1Zn alloy: An in situ synchrotron-based multiscale study

In situ synchrotron diffraction is used to investigate deformation dynamics during unloading and reverse loading (tension) in Mg–3Al–1Zn alloy pre-compressed to different strains. The specimens are compressed along the transverse direction (TD) to −4.8% and −9.8% true strains and then stretched along TD to failure. Bulk-scale stress−strain curves, X-ray diffraction and total strain field mapping via digital image correlation are obtained simultaneously. Electron backscatter diffraction characterization is performed as a complement. Detwinning occurs immediately after unloading, and dominates the deformation upon unloading and reverse tension, and more grains are involved in the detwinning with the increase of stress, accelerating the detwinning rate. The large pre-compression strain (−9.8%) boosts the twin boundary-dislocation interaction, which impedes twin boundary motion and reduces the detwinning rate and proportion. When deformation twins are almost exhausted via detwinning, dislocation motion dominates the plastic deformation, and the transition of deformation modes leads to variations in strain hardening rate. • Deformation dynamics of AZ31 during compression, subsequent unloading and tension • In situ X-ray multiscale diagnostics with complemented EBSD characterization • Negligible effects of pre-compression strain levels on onset of detwinning • Twin boundary-dislocation interaction reduces detwinning rate and proportion.

Tunable low-frequency bandgaps of a new two-dimensional multi-component phononic crystal under different pressures, geometric parameters and pre-compression strains

For the control of low-frequency noise in complex external-pressure environment, a new two-dimensional phononic crystal plate which is made of the lead stub with rubber coatings connected by narrow epoxy bars is proposed and the bandgap characteristics are simulated tentatively by the finite element method. The bandgap generation mechanism of the designed model is investigated by analyzing corresponding vibration modes. Furthermore, the bandgap adjustability under different pre-compression strains and geometric parameters is also investigated. Results show that obvious bandgaps tunability with small pre-compression strains or hydrostatic pressure is realized by changing geometric parameters and applying pre-compression strain to the matrix.

Boundary curvature effect on the wrinkling of thin suspended films

A relation between the boundary curvature κ and the wrinkle wavelength λ of a thin suspended film under boundary confinement is demonstrated. Experiments were performed with nanocrystalline diamond films of approximate thickness 184 nm grown on glass substrates. By removing portions of the substrates after growth, suspended films with circular boundaries of radius 30–811 μm were fabricated. Due to residual stresses, the portions of the film bonded to the substrate are of approximate compressive prestrain 11×10−4 and the suspended portions of the film are azimuthally wrinkled at their boundary. Measurements show that λ decreases monotonically with κ, and a simple model that is in line with this trend is proposed. The model can be applied to design devices with functional wrinkles and can be adapted to gain insight into other systems such as plant leaves. A method for measuring residual compressive strain in thin films, which complements standard strain characterization methods, is also described.

How pre-strain affects the chemo-torsional response of the intervertebral disc

BackgroundThe role of the axial pre-strain on the torsional response of the intervertebral disc remains largely undefined. Moreover, the chemo-mechanical interactions in disc tissues are still unclear and corresponding data are rare in the literature. The paper deals with an in-vitro study of the pre-strain effect on the chemical sensitivity of the disc torsional response. MethodsFifteen non-frozen ‘motion segments’ (two vertebrae and the intervening soft tissues) were extracted from the cervical spines of mature sheep. The motion segments were loaded in torsion at various saline concentrations and axial pre-strain levels in order to modulate the intradiscal pressure. After preconditioning with successive low-strain compressions at a magnitude of 0.1 mm (10 cycles at 0.05 mm/s), the motion segment was subjected to a cyclic torsion until a twisting level of 2 deg. at 0.05 deg./s while a constant axial pre-strain (in compression or in tension) is maintained, the saline concentration of the surrounding fluid bath being changed from hypo-osmotic condition to hyper-osmotic condition. FindingsAnalysis of variance shows that the saline concentration influences the torsional response only when the motion segments are pre-compressed (p < .001) with significant differences between hypo-osmotic condition and hyper-osmotic condition. InterpretationThe combination of a compressive pre-strain with twisting amplifies the nucleus hydrostatic pressure on the annulus and the annulus collagen fibers tensions. The proteoglycans density increases with the compressive pre-strain and leads to higher chemical imbalances, which would explain the increase in chemical sensitivity of the disc torsional response.

Effect of Compressive Prestrain on the Anti-Pressure and Anti-Wear Performance of Monolayer MoS2: A Molecular Dynamics Study.

The effects of in-plane prestrain on the anti-pressure and anti-wear performance of monolayer MoS2 have been investigated by molecular dynamics simulation. The results show that monolayer MoS2 observably improves the load bearing capacity of Pt substrate. The friction reduction effect depends on the deformation degree of monolayer MoS2. The anti-pressure performance of monolayer MoS2 and Pt substrate is enhanced by around 55.02% when compressive prestrain increases by 4.03% and the anti-wear performance is notably improved as well. The improved capacities for resisting the in-plane tensile and out-of-plane compressive deformation are responsible for the outstanding lubrication mechanism of monolayer MoS2. This study provides guidelines for optimizing the anti-pressure and anti-wear performance of MoS2 and other two-dimension materials which are subjected to the in-plane prestrain.

Effect of texture evolution on corrosion resistance of AZ80 magnesium alloy subjected to applied force in simulated body fluid

The electrochemical corrosion behavior of a rolled AZ80 magnesium alloy after pre-compressive strain along different orientation in a simulated body fluid (SBF) was investigated by immersion test for 48 h. The results showed that the corrosion resistance is related to the crystal texture of the material. The texture evolution induced by plastic deformation changed the corrosion behavior of the magnesium alloy. Slip and twin associated with plastic deformation changed the orientation of crystallographic planes. The surface containing highly concentrated orientation of (0002) basal plane has higher corrosion resistance. The appearance of tension twinning could improve the corrosion resistance of magnesium alloy with specific orientation. However, the coexistence of tension twinning with high volume fraction and slip with high density dislocation reduced the corrosion resistance and corrosion anisotropy of the magnesium alloy.

Post-forming Room Temperature Brittle Fracture in a High-Strength Low-Alloy Steel Sheet After Various Forming Modes

The effects of various sheet forming modes on the post-forming room temperature fracture behavior of a 550-MPa yield strength high-strength low-alloy (HSLA) steel are investigated. In-plane biaxial stretching, plane strain (cold rolling), uniaxial tension, cylindrical cup drawing, and in-plane compression (IPC) are examined up to a von Mises effective prestrain eeff = 0.7. Most of the results pertain to sub-size Charpy-type impact specimens prepared from the prestrained material. After sufficiently large prestrains, the fracture behavior is highly anisotropic. Low-energy fracture modes, namely cleavage and intergranular fracture, occur after all modes of prestraining along plane directions that are perpendicular to the principal compressive prestrain. These planes are parallel to the sheet surface after biaxial stretching and cold rolling and correspond to brittle splits extending into the primary, ductile fracture surface. After cup drawing, the brittle planes are oriented perpendicular to the circumferential compressive prestrain. This results in very low energy fractures propagating in the length direction of a fully drawn cup. After in-plane compression, brittle fracture occurs along planes that are perpendicular to the compression direction. The fracture toughness for a crack propagating along such a plane after IPC to eeff = 0.38 was very low (12.7 MPa √m), much lower than in the undeformed condition (247 MPa√m). Changes in grain shape and in crystallographic texture caused by the various prestraining modes, as well as the microstructural damage at non-metallic inclusions and carbides, are examined to try to understand the fracture behavior.

Matrix-induced pre-strain and mineralization-dependent interfibrillar shear transfer enable 3D fibrillar deformation in a biogenic armour

The cuticle of stomatopod is an example of a natural mineralized biomaterial, consisting of chitin, amorphous calcium carbonate and protein components with a multiscale hierarchical structure, and forms a protective shell with high impact resistance. At the ultrastructural level, cuticle mechanical functionality is enabled by the nanoscale architecture, wherein chitin fibrils are in intimate association with enveloping mineral and proteins. However, the interactions between these ultrastructural building blocks, and their coupled response to applied load, remain unclear. Here, we elucidate these interactions via synchrotron microbeam wide-angle X-ray diffraction combined with in situ tensile loading, to quantify the chitin crystallite structure of native cuticle – and after demineralization and deproteinization – as well as time-resolved changes in chitin fibril strain on macroscopic loading. We demonstrate chitin crystallite stabilization by mineral, seen via a compressive pre-strain of approximately 0.10% (chitin/protein fibre pre-stress of ∼20 MPa), which is lost on demineralization. Clear reductions of stiffness at the fibrillar-level following matrix digestion are linked to the change in the protein/matrix mechanical properties. Furthermore, both demineralization and deproteinization alter the 3D-pattern of deformation of the fibrillar network, with a non-symmetrical angular fibril strain induced by the chemical modifications, associated with loss of the load-transferring interfibrillar matrix. Our results demonstrate and quantify the critical role of interactions at the nanoscale (between chitin-protein and chitin-mineral) in enabling the molecular conformation and outstanding mechanical properties of cuticle, which will inform future design of hierarchical bioinspired composites. Statement of SignificanceChitinous biomaterials (e.g. arthropod cuticle) are widespread in nature and attracting attention for bioinspired design due to high impact resistance coupled with light weight. However, how the nanoscale interactions of the molecular building blocks – alpha-chitin, protein and calcium carbonate mineral – lead to these material properties is not clear. Here we used X-ray scattering to determine the cooperative interactions between chitin fibrils, protein matrix and biominerals, during tissue loading. We find that the chitin crystallite structure is stabilized by mineral nanoparticles, the protein phase prestresses chitin fibrils, and that chemical modification of the interfibrillar matrix significantly disrupts 2D mechanics of the microfibrillar chitin plywood network. These results will aid rational design of advanced chitin-based biomaterials with high impact resistance.

Twin nucleation, twin growth and their effects on annealing strengths of Mg–Al–Zn–Mn sheets experienced different pre-compressive strains

We combine pre-compressive test, annealing, re-compressive test, quasi-in-situ optical microscopy (OM), electron backscattered diffraction (EBSD) and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) to systematically investigate the macroscopic yield strength differences (ΔASs) of Mg-1.5Al-1.0Zn-0.3Mn (AZM110, wt.%) sheets with different pre-compressive strains 2%, 4% and 6% after re-compression with annealing, respectively. We observe that with increasing the annealing time, ΔAS firstly increased and then decreased. Moreover, with the increase of pre-compressive strains, ΔAS peak value gradually decreased. Quasi-in-situ OM images reveal that the contribution of new twin nucleation increased and initial twin growth decreased for pre-compressed AZM110 sheets after re-compression with annealing. With the pre-compressive strains, the contribution of new twin nucleation gradually is weakened and initial twin growth is enhanced, which leads to the alteration of ΔAS. By the observation of HAADF-STEM, twin boundaries are stabilized by the segregation of Al and Zn solutes and nano-Al8Mn5 phases pinning, creating pre-compressive annealing strengthening (PCAS). With the annealing time, the disappearance of back stress and dislocation tangle decreases the PCAS effect. Pre-compressive strain also strongly influences the PCAS effect. The increasing volume fraction and width of twins observed from EBSD accelerate twin boundary motion (TBM) with pre-compressive strains from 2% to 4%, decreasing the PCAS effect after re-compression with annealing. Although increasing pre-compressive strain to 6% can put off TBM, low new twin nucleation rate still gives rise to the decreasing PCAS effect.

Effect of stress history on compressive and rheological behaviors of EPS geofoam

This paper investigates the effect of stress history on the compressive and rheological behaviors of EPS geofoam. Short-time, constant-load and constant-strain precompression loadings were applied to impose different types of stress history on EPS samples. These samples were thereafter subjected to rapid compression, creep and stress relaxation tests to reveal their compressive (i.e. stress-strain relationship) and rheological (i.e. creep and stress relaxation) behaviors. Test results showed that (i) the yield strain of EPS increases as the maximum past strain level (precompression strain) increases and the stress-strain behavior of the EPS in the post-yielding range is not affected by its stress history; and (ii) the accumulated plastic strain during precompression decreases the rate of creep and stress relaxation. Based on the observed influence of stress history on the EPS, suggestions on the use of pre-loaded EPS were provided in this study.