Large Strain Amplitudes Research Articles

Dynamic behavior of geocell-reinforced rubber sand mixtures under cyclic simple shear loading

Due to the attributes of low shear modulus and high initial damping, rubber sand mixtures (RSM) can be used as a soil alternative to reduce ground motions when seismic loads are of great concern. However, when RSM is used as a vibration isolation material for geotechnical seismic isolation systems, it suffers from a lack of load-bearing capacity. To overcome this, a three-dimensional interconnected geocell (one type of geosynthetics) is placed within RSM to increase the vertical confinement of the system. In this technical note, to investigate the shear modulus and damping ratio of geocell-reinforced RSM, large-scale cyclic triaxial tests were conducted on specimens prepared with four granulated rubber contents (by weight) and sheared under different cyclic shear strain amplitudes. The results show that Geocell reinforcement can restrict the development of local shear bands in the specimen, weaken the anti-S-shaped characteristics of hysteresis loops and enhance the damping ratio of RSM at large strain amplitudes. The tuck net effect causes an increase in the normal stress of both contact particles, and then leads to the degradation rate of the maximum shear modulus of RSM varying with the rubber content and vertical pressure. The normalized shear modulus degradation curves show the influence range of geocell reinforcement, and demonstrate that a rubber content of 20% for reinforced specimen may be an optimal value from the perspective of the stability of dynamic properties. Additionally, the quantitative analysis of the effect of geocell reinforcement on the mechanical behavior of RSM can provide a reference for subsequent theoretical research and engineering applications.

Linear and nonlinear viscoelastic parameters of asphalt binders modified with softening agents

Asphalt binder modification using various chemically processed synthetic or bio-based additives can be used to reduce cracking potential in asphalt concrete (AC), especially with recycled materials. These modifiers affect rheological properties of asphalt binders. State-of-the-practice parameters obtained within the linear viscoelastic range (LVER) have not successfully predicted their performance. Using a large-strain amplitude and/or novel small-strain parameters at extended aging conditions (double and triple pressure aging vessel cycles) could help. In this study, parameters for unmodified and modified asphalt binders were studied at various aging conditions. The discussed data supports that: (1) Extended aging allows binder distinction beyond performance grading (2) Intermediate-temperature small-strain rheological parameters of binders may be predicted using low-temperature small-strain parameters, and vice versa. (3) Stiffness-dependent parameters must be interpreted based on temperature and frequency of testing. (4) Δ|G*|peak τ is not correalted to small-strain parameters.

Quantitative investigation on the nonlinear viscoelasticity of magnetorheological gel under large amplitude oscillatory shear

Magnetorheological gel (MRG) is a kind of magneto-sensitive smart composite whose nonlinear rheological behavior is highly controllable by external magnetic fields. For filling the gap that most publications studied the nonlinear property of MR materials in a qualitative manner, this work adopts a quantitative analysis method, called Fourier-transform rheology (FTR), to investigate the nonlinear viscoelasticity of MRG under large amplitude oscillatory shear (LAOS) tests. The influences of magnetic field, strain amplitude and frequency on the nonlinearity of MRG are thoroughly discussed. Results indicate that MRG, under a relative smaller magnetic field, appears higher degree of nonlinearity in the high-frequency region. The magneto-mechanical coupling mechanism of microstructures in MRG is also proposed to explain the various nonlinear phenomena under different loading conditions. Furthermore, for remedy the limitation that higher-order harmonics in FTR lack of clear physical meanings, a geometrical nonlinear parameter, S factor, is utilized to disclose the intra-cycle strain stiffening characteristic of MRG. The effects of strain amplitude, frequency and magnetic field on S factor are similar with that on the third-order harmonic in FTR. Moreover, the energy dissipation density is calculated based on Lissajous curve (i.e. hysteretic stress-strain loop) to characterize the damping property or capacity to dissipate energy of MRG. It is found that frequency only has a great effect on energy dissipation density of MRG under relatively large strain amplitude.

Oscillatory behavior with fiber orientation in the flow–vorticity plane for fiber suspension under large amplitude oscillatory shear flow

In fiber suspensions, fibers with three-dimensional orientation states are dramatically flow-oriented in the flow–flow gradient plane when the flow starts. In contrast, large strains are required for flow orientation in the flow–vorticity plane. Under oscillatory shear flow, when the strain amplitude is small, the flow orientation in the flow–vorticity plane is weakly induced, unlike that in the flow–flow gradient plane. The orientation in the flow–vorticity plane increases with the strain amplitude. At large strain amplitudes, fibers are oriented in the flow–flow gradient plane; thus, the rotational motion of fibers in the flow–flow gradient plane is dominant, i.e., fibers are almost flow-oriented in the flow–vorticity plane. However, contributions of the oscillatory behavior of fiber orientation in the flow–flow gradient and flow–vorticity planes to complex viscosity are unclear. In this study, the objective is to determine the contributions of fiber orientation in each plane to complex viscosity with the two initial orientation states (random and flow-oriented states) for the strain sweep test. The two initial orientation states were controlled by manipulating the upper disk of the parallel disk flow path. The initial random state was induced by sliding the upper disk up and down. The initial flow-oriented state was induced by rotating the upper plate. Furthermore, phase transition behaviors from the random to flow-oriented state in the flow–vorticity plane with increasing strain amplitude were qualitatively estimated as the orientation angle via visualization. Consequently, when the initial orientation was random, the fibers gradually vibrated in a medium strain amplitude region, and complex viscosity was higher than that of the initial flow-oriented state. In the conclusion, we divided the complex viscosity behavior of the strain sweep test into five strain amplitude regions and clarified the dominant orientation state in each region. It is conceivable that this result will help in understanding the relationship between fiber orientation and dynamic viscoelastic properties of fiber suspension like fiber-reinforced thermoplastics.

Study seismic performance of duplex stainless steel under large strain amplitude by cyclic loading test

The existing experimental data on stainless steel are limited to a small strain range (<1.50%), which is not enough to analyze the response of stainless steel structures in large plastic strain under a strong earthquake. This paper presents cyclic loading tests containing constant and variable strain amplitude up to 5.0% to study the low-cycle fatigue properties of duplex stainless steel S2205. The cyclic skeleton curve and the fatigue life equation are established and the hysteretic behavior and cyclic hardening-softening characteristics are discussed. The analyzed results show that steel S2205 exhibits a nonlinear stress-strain relationship with high yield strength and good ductility. The hysteresis loops of the material are plump and the equivalent viscous damping ratio is about 0.5 at strain of 5.0%, indicating a good energy dissipation capacity. Under constant amplitude cyclic loading, steel S2205 exhibits cyclic hardening in the first few fatigue cycles and then undergoes prolonged cyclic softening until fracture. The plastic strain energy-life model is found to be appropriate for fitting the low-cycle fatigue life equation. Furthermore, the parameters of the cyclic constitutive model are derived, providing basic data for seismic analysis of duplex stainless steel structures under large plastic strain values.

High-performance, low-cost nanoporous alloy actuators by one-step dealloying of Al-Ni-Cu precursors

Nanoporous metal-based actuators show advantages over conventional piezoelectric ceramics and conducting polymers, but are mainly limited to noble metals. Herein, we design a series of ternary Al-Ni-Cu alloys with variable Ni/Cu ratios to investigate their dealloying behaviors as well as electrochemical actuation performances. The change of the Ni/Cu ratio influences the microstructure and phase constituent of the as-cast Al-Ni-Cu alloys. Specially, an Al3(Ni, Cu)2 phase is identified in all the Al-Ni-Cu alloys, and its amount and composition alter with the Ni/Cu ratio. Furthermore, the corrosion-resistant Al3(Ni, Cu)2 phase significantly affects the dealloying behaviors of the ternary Al-Ni-Cu precursors. Additionally, the as-dealloyed Al-Ni-Cu alloys show good electrochemical actuation performance, which is associated with the existence of efficient nanoporous (np) components (np-Cu and np-Ni). The largest strain amplitude and strain rate could reach up to 0.9% and 6.6 × 10−4 s−1, respectively. Compared with noble metal-based actuators, the large strain amplitude and strain rate, as well as the lower material cost, make nanoporous Al-Ni-Cu alloy a potential actuation material for practical applications.

Why the Cox–Merz rule and Gleissle mirror relation work: A quantitative analysis using the Wagner integral framework with a fractional Maxwell kernel

In this work, we mathematically derive the conditions for which empirical rheometric relations such as the Cox–Merz rule and Gleissle mirror relationship are satisfied. We consider the Wagner integral constitutive framework, which is a special limiting case of the Kaye–Bernstein Kearsley Zapas (K-BKZ) constitutive equation to derive analytical expressions for the complex viscosity, the steady shear viscosity, and the transient stress coefficient in the start-up of steady shear. We use a fractional Maxwell liquid model as the linear relaxation modulus or memory kernel within a non-linear integral constitutive framework. This formulation is especially well-suited for describing complex fluids that exhibit a broad relaxation spectrum and can be readily reduced to the canonical Maxwell model for describing viscoelastic liquids that exhibit a single dominant relaxation time. To incorporate the nonlinearities that always become important in real complex fluids at large strain amplitudes, we consider both an exponential damping function as well as a more general damping function. By evaluating analytical expressions for small amplitude oscillatory shear, steady shear, and the start-up of steady shear using these different damping functions, we show that neither the Cox–Merz rule nor the Gleissle mirror relation can be satisfied for materials with a single relaxation mode or narrow relaxation spectrum. We then evaluate the same expressions using asymptotic analysis and direct numerical integration for more representative complex fluids having a wide range of relaxation times and nonlinear responses characterized by damping functions of exponential or Soskey–Winter form. We show that for materials with broad relaxation spectra and sufficiently strong strain-dependent damping the empirical Cox–Merz rule and the Gleissle mirror relations are satisfied either exactly, or to within a constant numerical factor of order unity. By contrast, these relationships are not satisfied in other classes of complex viscoelastic materials that exhibit only weak strain-dependent damping or strain softening.

Experimental study on cyclic hardening characteristics of structural stainless steels

This paper studies the cyclic hardening characteristics of commonly used structural stainless steel types and grades including austenitic stainless steel grades S30408 and S31608 (equivalently to EN 1.4301 and EN 1.4401) as well as duplex stainless steel grade S22053 (equivalently to EN 1.4462) under tension-compression cyclic loading. A total of twenty-four monotonic loading tests and thirty-six cyclic loading tests were performed and two different cyclic loading protocol types were selected, namely cyclic ascend (CA) loading and cyclic constant (CC) loading with strain amplitudes ranging from ±0.5% to ±5%. Specimens with plate thicknesses of 6 mm and 8 mm were tested. The test results indicate that the cyclic hardening/softening behavior of the three grades of structural stainless steel generally shows obvious strain amplitude dependence and unsaturation before fracture under large strain amplitudes, which are significantly different from carbon steel. Meanwhile, there is a huge difference between the cyclic skeleton curves and the monotonic constitutive curves for different stainless steel grades. The effects of strain amplitude, stainless steel grade and loading history on the cyclic hardening/softening characteristics have been thoroughly studied. It should be noted that the seismic performance of stainless steel structures is greatly affected by the loading history. From the viewpoint of the seismic design of stainless steel structures, this paper proposes an equation to predict the maximum cyclic hardening degree at various strain amplitudes under constant amplitude loading.

Correlation between plastic rearrangements and local structure in a cyclically driven glass.

The correlation between the local structure and the propensity for structural rearrangements has been widely investigated in glass forming liquids and glasses. In this paper, we use the excess two-body entropy S2 and tetrahedrality ntet as the per-particle local structural order parameters to explore such correlations in a three-dimensional model glass subjected to cyclic shear deformation. We first show that for both liquid configurations and the corresponding inherent structures, local ordering increases upon lowering temperature, signaled by a decrease in the two-body entropy and an increase in tetrahedrality. When the inherent structures, or glasses, are periodically sheared athermally, they eventually reach absorbing states for small shear amplitudes, which do not change from one cycle to the next. Large strain amplitudes result in the formation of shear bands, within which particle motion is diffusive. We show that in the steady state, there is a clear difference in the local structural environment of particles that will be part of plastic rearrangements during the next shear cycle and that of particles that are immobile. In particular, particles with higher S2 and lower ntet are more likely to go through rearrangements irrespective of the average energies of the configurations and strain amplitude. For high shear, we find very distinctive local order outside the mobile shear band region, where almost 30% of the particles are involved in icosahedral clusters, contrasting strongly with the fraction of <5% found inside the shear band.

Fatigue-free Cu films on wrinkled soft substrates

The fatigue behavior of nanocrystalline Cu films on polydimethylsiloxane (PDMS) substrates was studied. It is revealed that the Cu films on wrinkled PDMS substrates exhibit ultrahigh fatigue resistance without electrical failure. The electrical resistance either remains constant during cyclic loading or slightly increases and rapidly reaches saturation, which is attributed to the buffering effect of wrinkle structures on crack initiation and the suppression effect of surface undulations on crack propagation. The film thickness-dependence of fatigue resistance is sensitive to the amount of the strain amplitude. The fatigue resistance increases with decreasing film thickness at small strain amplitudes because of higher strength and smaller local strain concentration, while the fatigue resistance first increases and then decreases as the film thickness increases at large strain amplitudes, which is related to the extrinsic deformation capability of Cu films.

Anomalous dynamic response of nematic platelets studied by spatially resolved rheo-small angle x-ray scattering in the 1–2 plane

Dispersions of colloidal platelets in the nematic phase display strong wall anchoring, which competes with the reorientational motion of the director when the system is subjected to flow. We show that the mechanical response to large amplitude oscillatory strain and stress depends on the confinement of the system due to this competition. We elucidate the underlying structural response by deflecting a x-ray beam vertically along the vorticity direction of a Couette geometry, such that the structure can be probed throughout the gap with an unprecedented spatial resolution while recording in situ the mechanical response. We observe strong inhomogeneities in terms of the orientation of the nematic director, depending on the extent of the system's yield during an oscillation. At small strain amplitudes, we observe a small region where the director oscillates between wall anchoring and the Leslie angle, while in the bulk, the director tilts out of the flow–flow gradient plane. At large strain amplitudes, the oscillations of the director are symmetric, close to the wall, and propagate into the bulk. Here, a twinning is observed where the director rotates out-of-plane in two opposite directions. Using the sequence of physical process method to analyze the LAOStrain response for both the mechanical and structural response, we locate the yielding in a small time-window around flow reversal and identify that the bulk is the main contributor to the mechanical response. The structural response to LAOStress is much less pronounced even when the stress amplitude causes significant shear thinning.

Spectral method for time-strain separable integral constitutive models in oscillatory shear

The time-strain separable Kaye–Bernstein–Kearsley–Zappas model (tssKBKZM) is a popular integral constitutive equation that is used to model the nonlinear response of time-strain separable materials using only their linear viscoelastic properties and damping function. In oscillatory shear, numerical evaluation of tssKBKZM is complicated by the infinite domain of integration, and the oscillatory nature of the integrand. To avoid these problems, a spectrally accurate method is proposed. It approximates the oscillatory portion of the integrand using a discrete Fourier series, which enables analytical evaluation of the resulting integrals for the Maxwell model. The spectral method is generalized for arbitrary discrete and continuous relaxation spectra. Upper bounds for quadrature error, which can often be driven to machine precision, are presented. The Doi–Edwards model with independent-alignment approximation (DE-IA) is a special case of tssKBKZM; for DE-IA, the spectral method is compared with trapezoidal rule to highlight its accuracy and efficiency. The superiority of the proposed method is particularly evident at large strain amplitude and frequency. For continuous relaxation spectra, the spectral method transforms the double integral corresponding to the tssKBKZM to a single integral. Solutions computed to a specified level of accuracy using standard numerical libraries show that the spectral method is typically two to three orders of magnitude faster. Extensions to fractional rheological models, materials with nonzero equilibrium modulus, stretched exponential models, etc., are also discussed.

Strain rate effect on static and dynamic behaviors of eastern Canada fine-grained soils

The assessment of the strain rate effect on the geotechnical properties of soils constitutes an important step toward a more accurate analysis of their response. This study presents the experimental results of monotonic and cyclic simple shear tests performed to examine the strain rate ([Formula: see text]) effect on the behavior of eastern Canada soils. Nine natural soils sampled from different locations in eastern Canada were used in this study. The tests were performed on a simple shear device using a strain-controlled mode. In addition to the obtained experimental results, published data in the literature were used to draw the conclusions of this study. Analysis of the data indicates that the undrained shear strength (τf) increases proportionally with the strain rate by approximately 6%–17% per log cycle of [Formula: see text]. The results also show that the secant shear modulus G increases with the strain rate, especially at large strain amplitudes. Moreover, the analysis of the data revealed that the extent of the strain rate effect seems to be correlated with the shear strain amplitude (γc) and plasticity index (Ip). A practical application of the outcomes on the backbone curves is given, illustrating the influence of Ipand γcon the effect of strain rate.

YOUNG’S MODULUS OF PEAT SOIL UNDER CYCLIC LOADING

Peat soil classified as high organic content with diverse range of fibers, distinctive intrinsic properties with low shear strength. The behaviour of peat soils under dynamic loading conditions has been studied. Therefore, a series laboratory of cyclic triaxial test on peat soil carried out to determine the effect of cyclic loading to the peat soil behavior after subjecting to cyclic loading is presented. In this study, the frequencies applied for the dynamic testing on the peat soil samples were focused and simulated on traffic vehicle loading frequencies, earthquakes and machine operations. Peat soil sample used from Parit Nipat, Malaysia (PNpt) with natural moisture content, m = 603% and liquid limit of wl = 231%. A series of undrained cyclic triaxial test were performed on undisturbed peat soil sample under isotropically consolidated specimens. In addition, the strain of amplitude applied is 0.1% to investigate the effectiveness of large strain amplitude response by allowing the generation of cyclic pore pressure and developed stress-strain cycle during cyclic loading. Correspondingly, the undrained Young’s modulus of the undrained shear strength subjected to cyclic loading in this study in the range of 60 to 70 for hemic peat soil. The specimen loaded into specific frequency causes in reduction of Young’s modulus related to stiffness and more pronounced in softening behaviour. The hysteresis-loops profile with regard to E parameters from 1st cycle to 100th cycles of Young’s modulus, E (MPa) for PNpt degrades precisely to an applied effective stress, 0.4 MPa (25 kPa) and 0.4 MPa (50 kPa). Cyclic loading frequencies resulted in decreases of the Young’s Modulus of peat soil that related to stiffness and more pronounced in softening behaviour. The result indicates that a reduction effect in the stress-strain cycle occurs in a peat soil from the initial stage of cyclic loading towards the end of 100th cycles due to the repeated loading application.

Payne effect of carbon black filled natural rubber nanocomposites: Influences of extraction, crosslinking, and swelling

Rubber nanocomposites experiencing dynamic shears at large strain amplitudes (γ) exhibit the nonlinear Payne effect featured by decays of storage and loss moduli (G′ and G″) or by G′ decay accompanied with G″ overshoot near a critical strain amplitude. The occurrence of the Payne effect has been assigned to damages of “filler network” and rubber-filler interfacial interactions for a long time and to Rouse dynamics of rubber chains recently. To solve the dispute, influences of extraction, crosslinking, and paraffin swelling on the Payne effect of carbon black filled natural rubber nanocomposites are investigated systematically. Master curves of G′ as a function of γ could be always created, and overshoot of G″ in the filled vulcanizates weakens with increasing filler content and intensifies by dilution via paraffin swelling, suggesting that the Payne effect is not mainly rooted in the “filler network” and rubber-filler interfacial interactions. The filler reduces the onset strain amplitude of the Payne effect by amplifying microscopic strain amplitude of the rubber phase, irrespective of whether the matrix is crosslinked or not and whether the crosslinked matrix is swollen or not. Partial removal of bound rubber by compounding the paraffin swollen compounds could lower modulus and eliminate G″ overshoot of the deswollen vulcanizates without influence on the mechanism of G′ decay accompanying Payne effect. The overshoot is found to be closely related to the overall viscous characteristic of the vulcanizates in the linear viscoelastic regime. Provided herein are new insights for recognizing the important roles of the viscoelastic rubber phase on the Payne effect of the nanocomposites.

Polysaccharide-based skin scaffolds with enhanced mechanical compatibility with native human skin

We report a custom-made technique to synthesize process-convenient skin scaffolds by tuning the mechanical properties of hydrogels based on a few naturally occurring polysaccharides to match the rheological properties of previously established benchmarks, i.e., the ex vivo native human skins. We studied the mechanical parameters using oscillatory shear rheology. At small strain amplitudes, the intrinsic elastic modulus showed an almost linear dependence in the middle and a changing rate profile at the two ends with concentration of the principal hydrogel component variant, i.e., kappa (κ)-carrageenan. At large strain amplitudes, the hydrogels demonstrated intercycle strain-softening behavior, the onset of which was directly proportional to the κ-carrageenan concentration. We observed a concentration match for the intrinsic elastic modulus of the benchmark within this sigmoidal curve fit. Contextually, we need to explore other potent polymeric hydrogel systems to achieve mechanical affinity in terms of multiple rheological parameters derived from both strain amplitude and angular frequency sweeps. Additionally, we carried out diffusion experiments to study caffeine permeation attributes. The hydrogels show improved barrier features with increasing κ-carrageenan concentration. In terms of the penetration flux and total cumulative amount of permeated caffeine, this enhanced mechanical adherence demonstrates comparable penetration features with the commercial 3D skin model.

Modeling of Nonlinear Dynamics and Temperature Stability of Doped Silicon Microresonators

Silicon is widely used as the device material for many micro resonators applied in timing and frequency referencing. One key disadvantage of silicon resonators compared to quartz resonators is their high thermal sensitivities. Doping silicon is a promising approach for temperature stability. Doped resonators operating at large deformation and finite strain amplitude often go to nonlinear regimes; therefore, nonlinear dynamics must be considered for adequately predicting the system behavior. In this article, the nonlinear vibration analysis is given for rectangular resonators operating in the Lame mode, incorporating both the second- and third-order elastic constant (SOEC and TOEC) components. This article presents an analytic demonstration for the linear and nonlinear lumped mass system equivalent spring constants explicitly in terms of SOEC and TOEC. We show that, for a rectangular resonator in the Lame mode, the first-order nonlinear spring constant would be an explicit expression in terms of TOEC components, which, for a square resonator, will be nullified. We show that there exist optimal doping levels where the anharmonic stiffness coefficient is minimized, implying the most dynamic stable vibrations. Furthermore, this article shows that there exists a tradeoff between dynamical and temperature–frequency stability in terms of the doping level.

Cumulative deformation capacity of structural steel subjected to extremely large amplitude strain histories

This study investigated deformation behavior of structural steel subjected to large strain. When structures experience powerful earthquakes, strain amplitudes at the fracture locations can be quite large, and the material fractures after few loading cycles. The Manson-Coffin relationship is commonly used to predict failure of metals subjected to cyclic loadings. However, the traditional Manson–Coffin relationship does not necessarily hold for some materials under extremely large strain amplitude. There are few relevant experiments, largely because it is difficult to reproduce structural steel behavior under large amplitude cyclic stains in the laboratory due to buckling. In this study, 71 steel plate specimens of structural steel were loaded until fracture under diverse cyclic axial strain loadings and monotonic tension. The relationship between cumulative strain and its skeleton portion was obtained based on the analysis of the hysteresis loops. Due to the limitation of test setup, the largest compression strain amplitude is 6%, which is still not sufficient to study the material behavior of structural steel under severe earthquakes. Therefore, numerical simulations of single steel elements subjected to cyclic strain were conducted to complement the material behavior under extremely large constant strain amplitudes. The cumulative deformation capacity of structural steel subjected to extremely large strain amplitudes was evaluated through a strain-life curve with the experimental and analytical results. In the final part of this study, experimental results from former research were introduced to validate the proposed strain-life curve.

RETRACTED ARTICLE: Research on application of flexible strain sensor in human motion monitoring

The flexible wearable device is a kind of flexible electronic device, which can be installed on the skin of the human body, follow the activities of human body joints and tissues to produce a variety of deformation, such as extending the body, bending and turning activities, and respond to the movement of the wearer. Composite conductive polymer has excellent extensibility and mechanical properties, as well as very strong flexibility. It can change the resistance in a wide range and under the action of external force, so it can maintain sustainable development in human movement, electronic skin, robot and other fields. Using these new polymer materials as matrix, flexible polymer materials such as graphene and carbon nanotubes as conductive fillers, RGO/TPU strain sensors and PDA/CNTs/EB strain sensors with excellent strain sensitivity were fabricated. It can be found that PDA/CNT/EB has the advantages of large strain amplitude, wide range, high sensitivity, fast reaction rate and high durability/EB conductive composite can monitor people's daily muscle and joint activities, and perform human–computer interaction, which can show very good safety and high accuracy, so that wearable flexible sensor has better ideas and requirements in the design and development process, and wearable sensor has good practical application effect.

Deformation capacity of 400 N/mm2 class structural steel under extremely large strains

To provide qualitative database for further studies regarding the evaluation of the cyclic deformation capacities of 400 N/mm2 class structural steel up to the maximum strength as well as the complete fracture, strain-based cyclic loading tests were performed in the low and extremely low-cycle fatigue regimes. Thirty-nine specimens were employed and tested with strain amplitudes in the range from ±2% to ±12%. By using three different specimen configurations made from three different lots of steel materials, and by performing axial cyclic tests (starting with tensile and compressive loads), the influences of different lots, specimen configurations, and different initial loading directions on the strain-life curve could be examined. The experimental results indicated that cyclic hardening behaviors of all specimens occurred in the first several cycles, while specimens tested with smaller strains (3% or below) stabilized until failure. However, specimens fractured in a ductile manner without achieving stabilization when they were subjected to large strain tests. The strain-life curve (up to failure) exhibited a linear relationship with strain amplitudes up to 10%. At larger strain amplitudes, the cyclic deformation capacity of the steel material reached its peak and subsequently became nonlinear. Nevertheless, the strain-life curves up to peak cyclic stress of all specimens show a continuous linear relationship with strain amplitudes up to 12%.