Low Strain Amplitudes Research Articles

The ultrahigh damping capacity of Mg-Sn-Y alloy

High temperature heat treatment Mg-0.69Sn-0.69Y alloy showed a high damping capacity (the damping value is 0.201 at ε=1 × 10−3) and a very low critical strain amplitude at room temperature (εcr1=2.97 × 10−5), which can be attributed to the co-effects of purified matrix phase and interfaces of subgrains. The substructure is unstable and the damping capacity decreases with the increasing temperature.

Small and large oscillatory shear behaviors of gelatin/starch system regulated by amylose/amylopectin ratio

This work explores the influence of starch amylose/amylopectin ratio on the gel-related features of gelatin/starch system using small and large amplitude oscillation shear methods. The higher self-aggregation and self-entanglement ability of amylose reduced the miscibility of gelatin and starch, and increases zero-shear viscosity (ŋ0) and reduces sol-gel transition temperature (Tgel) by enhancing the difficulty of molecule movement and thus the difficulty of gel formation. The gelatin/amylopectin-rich system has higher gel hardness and wider linear viscoelastic region, which are related to the higher integrity and uniformity of network structure. The gelatin/amylose-rich system shows enhanced ability to resist deformation against external shear forces (suggested by higher storage and complex modulus) under small amplitude oscillatory shear (SAOS), which is ascribed to the larger rigidity and entanglement degree of amylose. At strain amplitude up to 50% (middle amplitude oscillatory shear, MAOS), gelatin/amylose-rich system exhibits reduced structure recovery ability (reflected by larger semi-minor axis of ellipse, and lower strain amplitude for inflexion of elastic stress) under MAOS, but enhances restructuring capacity at strains of 500% and 1000% (large amplitude oscillatory shear, LAOS) (indicated by occurrence of secondary loop) than gelatin/amylopectin-rich system. Those seemingly contradictory phenomena are correlated with the preference of hydrogen bond forming ability among amylose rather than with gelatin. This weakens interactions between amylose and gelatin and thus inhibits recovery of bulk structure under MAOS, but is favor for the formation of amylose aggregations, which is helpful for the local structure restructuring and thus enhances structure restructuring capacity under LAOS. The results help us comprehensively understand the gel-related properties of gelatin/starch system and are valuable for rational design of gelatin/starch systems with improved gel-related features.

Cyclic stress response and microcrack initiation mechanism of modified 9Cr‐1Mo steel under low cycle fatigue at room temperature and 350°C

AbstractTo investigate the effect of temperature on microcracking mechanisms of modified 9Cr‐1Mo steel, low cycle fatigue tests were performed at 350°C and room temperature (RT). At RT, the cyclic stress response at low strain amplitude (<0.5%) is controlled by friction stress while dominated by back stress at high strain amplitude. By comparison, the disappearance of sub‐grain boundaries induced by dislocation annihilation occurs at 350°C, which contributes to the decrease in back stress, resulting in more significant effects of back stress compared to RT. As a result of the accumulated fatigue damage, the distance between two parallel extrusions at 350°C is larger than that at RT, which is ascribed to the disappearance of low angle sub‐grain boundaries due to dislocation annihilation at 350°C. Due to the disappearance of small‐angle grain boundaries at 350°C, microcracks initiate along high‐angle grain boundaries at 350°C instead of along low‐angle grain boundaries at RT.

Low cycle fatigue properties assessment and damage influence on DP 500/800 steel sheet

A dual phase (DP500/800) steel sheet (1.6 mm thickness) was evaluated by means of low cycle fatigue (LCF). The fatigue properties of the material were assessed by means of the cyclic elastic modulus determination at half-life as well as by the plastic strain amplitude measurements. Both methods show a good correlation and high reliability, however, the plastic strain amplitude approach tends to fit better to the experimental results. From a parametric analysis performed, it was observed that the plastic strain amplitude approximation could be more suitable to assess the fatigue properties for LCF of the material due to an improved correlation of the computed cyclic elastic modulus in comparison to the cyclic elastic modulus measurements at half-life. Both methods showed that, for strain amplitudes higher than εa = 0.004, the cyclic elastic modulus tends to be lower than 210 GPa, meanwhile for lower strain amplitudes, the cyclic elastic modulus tends to increase. An evaluation by X-ray Diffraction technique allowed to determine that the increment of residual equivalent strain contributes to the cyclic elastic modulus diminution.

Role of crystallographic orientation on the deformation behaviour of Ti-6Al-4V alloy during low cycle fatigue

ABSTRACT Low cycle fatigue (LCF) tests were carried out on Ti6Al4V alloy at room temperature for different strain amplitudes ranging from 0.5% to 2%. The microstructure and texture evolutions as a function of strain amplitude have been investigated in the present study. Cyclic softening behaviour was observed at all strain amplitudes, and it was found to increase with increasing the strain amplitude. The fatigue transition life was observed to be at 1.25% strain amplitude. Only type tensile twins were formed during the deformation, and this was found to be significant at higher strain amplitude of 2%. The grains along were appeared to be the twinning parent, and the twinned products were appeared along [0001]. The grains were further found to have substantial cyclic deformation than the other grains, such as and [0001]. The dominant texture was found along during the LCF tests. However, the sample at higher strain amplitude of 2% had also prominent [0001] texture along with texture. Fractographic examination revealed a dimple fracture surface at lower strain amplitude (0.5%), whereas a quasi-cleavage type fracture was observed at higher strain amplitudes (1% & 2%).

Mechanisms of intergranular fatigue crack initiation in polycrystalline materials

Fatigue failure in polycrystalline materials occurs by initiation and propagation of fatigue cracks. Fatigue cracks can initiate transgranularly or intergranularly. While considerable progress has been achieved in the explanation of the mechanism of transgranular fatigue crack initiation intergranular initiation is still the subject of controversy. A recent study of the crack initiation in superaustenitic Sanicro 25 steel [1] revealed the important role of persistent slip markings (PSMs) in the grains adjoining the grain boundary. A novel mechanism of intergranular fatigue crack initiation has been proposed. Provided the cyclic slip cannot proceed from one grain to the neighbour one, extrusions and intrusions arise not only on the surface but also on the grain boundary. Growing intrusions represent void-like defects and growing intrusions push neighbour grain apart. A systematic study of intergranular cracking has been performed on polycrystalline copper cycled with high and low strain amplitudes. SEM observations of the early stages of fatigue crack initiation on the grain boundaries, EBSD study of grains orientation, FIB cutting of the grain boundaries and simultaneous documentation of the effect of extrusions and intrusions on the grain boundary cohesion are reported simultaneously with the conditions favourable for the initiation of the fatigue cracks.

An exact expression of three-body system for the complex shear modulus of frictional granular materials.

We propose a simple model comprising three particles to study the nonlinear mechanical response of jammed frictional granular materials under oscillatory shear. Owing to the introduction of the simple model, we obtain an exact analytical expression of the complex shear modulus for a system including many monodispersed disks, which satisfies a scaling law in the vicinity of the jamming point. These expressions perfectly reproduce the shear modulus of the many-body system with low strain amplitudes and friction coefficients. Even for disordered many-body systems, the model reproduces the results by introducing a single fitting parameter.

Effect of texture on damping behaviour in ZK60 magnesium alloy

ABSTRACT Effect of texture on the damping capacity of ZK60 magnesium alloy is investigated through compression experiments with a deformation of 10% at room temperature. It is found that both the dislocation networks, twins and the basal texture intensity affect the damping capacity. At the low strain amplitudes, the dislocation networks and twins are dominant and the damping capacity decreases with the increase in the dislocation networks and twins. At the high strain amplitudes, the basal texture intensity is dominant and the damping capacity decreases with the increase in the basal texture intensity. Based on the anisotropy of the damping capacity of the wrought magnesium alloys, this research provides provide a direction for the study of high-strength and high-damping magnesium alloys.

Investigation of Low Cycle Fatigue Behaviors of Inertia-Friction-Welded Joints of the TC21 Titanium Alloy

As a new highly damage-tolerant structural material, the TC21 titanium alloy has been widely used in aerospace applications. Inertial friction welding (IFW) is a form of pressure welding technology with less welding parameters and high welding joint performance, which is especially suitable for the connection of rotors of aero-compressors and engines. In this paper, inertia friction welding of TC21 titanium alloys was successfully carried out, and the microhardness, tensile properties and low cycle fatigue (LCF) behaviors of IFW joints were studied. Based on the mechanical parametric results of the tensile test, the true stress–strain curves of the IFW joint of TC21 titanium alloys are obtained by further calculation. Based on the LCF test results under different strain amplitudes, life prediction of IFW joints was investigated. The results of the LCF test show that there is no obvious cyclic hardening and cyclic softening of the IFW joints. Moreover, the fracture morphology of LCF samples under high strain amplitude (0.9%) and low strain amplitude (0.6%) was observed. The results show that the fatigue cracks initiate and propagate at multiple points in the LCF samples, and the transient fracture zone is larger under high strain amplitude. However, under low strain amplitude, a fatigue crack nucleates and propagates at a single point, and the crack propagation zone is larger.

Microstructural evolution and stability during strain-controlled fatigue in a multiphase microalloyed steel

Multiphase ferrite-bainite-martensite (F-B-M) microstructure processed through rolling and two-step cooling in a V-microalloyed steel exhibited excellent stability under strain-controlled fatigue. The steel exhibited a steady state response and even cyclic hardening up to a total strain amplitude of 0.6% and displayed a cyclic softening response at total strain amplitudes >0.7%. To understand and determine the degradation mechanisms, a detailed transmission electron microscopic investigation was carried out in fatigue tested samples as a function of total strain amplitude. At low strain amplitudes (<0.4%), microstructural constituents such as the bainite/martensite colonies, and the inter-lath films of retained austenite remained stable. The deformation occurred only in the polygonal ferrite and the lamellar retained austenite, which was sheared into fine, smaller strips. Only some boundaries of bainite/martensite colonies were subjected to shearing. At an intermediate strain amplitude (0.6%), all the coarse bainite/martensite colonies exhibited shear deformation with very fine retained austenite segments distributed over large regions. The bainite/martensite lath interiors were filled with dislocation substructures and sheared inter-lath austenite layers. Austenite grain rotation was noticed, and the grain interior exhibited sheared or segmented layers. Selective inter-lath austenite layers transformed into acicular martensite. At higher amplitudes (0.8%), the F-B-M microstructure displayed homogeneous deformation and cyclic softening. Carbides and lath debris were distributed throughout the matrix and consisted of decomposed bainite and martensite. Formation of acicular and twinned martensite was present due to the TRIP effect. The F-B-M microstructure comprising small colonies of bainite/martensite with fine lath morphology, polygonal ferrite (∼20%), retained austenite with lamellar morphology and inter-lath films (∼5%) along with a fine dispersion of carbonitride precipitates in the matrix appears to be highly resistant to fatigue damage. Hot rolling processes aimed at promoting these microstructural features in the steel, which is also accompanied by texture strengthening, seem to be effective against cyclic softening in fatigue.

Effect of carbide precipitation on Coffin–Manson relationship of a polycrystalline nickel‐based superalloy

AbstractThis study explores the dual‐slope Coffin–Manson (C–M) behavior of a polycrystalline nickel‐based superalloy, EA, used in turbine engine applications with an emphasis on discerning the micro‐mechanisms responsible for it. The motivation for distinguishing the micro‐mechanisms responsible for bi‐linear C–M behavior stemmed from the earlier evolved comprehensions that state that the fatigue life estimation based on the extrapolation of any single line gives inaccurate results. Transmission electron microscopy (TEM) investigations of fatigue fractured specimens at low strain amplitudes, Δε/2, revealed that dislocations are homogeneously distributed in the γ‐channels and occasionally form networks at γ/γ′ interface. Whereas deformation is heterogeneous at high Δε/2 owing to the complex dislocation reactions. Cr23C6 carbides are precipitated during the high Δε/2 fatigue tests, which act as obstacles for dislocations. The deformation heterogeneity resulting from the dislocation–γ′ precipitate interactions and the dislocation–M23C6 carbide interactions accounts for the dual‐slope C–M behavior.

Low Cycle Fatigue Behavior of Plastically Pre-Strained HSLA S355MC and S460MC Steels.

Cold roll forming used in the manufacturing of lightweight steel profiles for racking storage systems is associated with localized, non-uniform plastic deformations in the corner sections of the profiles, which act as fatigue damage initiation sites. In order to obtain a clearer insight on the role of existing plastic deformation on material fatigue performance, the effect of plastic pre-straining on the low cycle fatigue behavior of S355MC and S460MC steels was investigated. The steels were plastically deformed at different pre-strain levels under tension, and subsequently subjected to cyclic strain-controlled testing. Plastic pre-straining was found to increase cyclic yield strength, decrease ductility, and induce cyclic softening, which, in S460MC, degrades fatigue resistance compared to the unstrained material. In unstrained conditions, the materials present a cyclic softening to hardening transition with increasing plastic strain amplitude, which in S355MC occurs at lower strain amplitudes and degrades its fatigue resistance with regard to the pre-strained material. Pre-straining also leads to a reduction in transition life from low to high cycle fatigue. SEM fractography, performed following the onset of crack initiation, revealed that plastic pre-straining reduces the fatigue fracture section as well as striation spacing, predominantly in the S355MC steel.

Deformation behavior of a newly-developed T4-treated Al–Si die cast alloy

The aim of this study was to identify the monotonic and cyclic deformation behavior of a high-pressure die cast Silafont®-36 alloy in a naturally-aged T4 state. The T4 heat treatment led to a significant microstructural change from the as-cast coralloid-like eutectic Si particles to spheroidal Si particles embedded in the Al matrix, while the prior Fe-, Mn-rich inclusions remained. Although the strength became lower, the ductility of the T4 alloy was higher. Low cycle fatigue life was observed to be longer than its as-cast state at lower strain amplitudes. The T4 alloy exhibited strong cyclic hardening, where the degree of cyclic hardening was found to increase linearly with increasing strain amplitude along with the absence of Masing behavior, due to the strong particle-dislocation interactions during deformation. Strain energy density-based model by incorporating the microstructural aspects in the form of intrinsic fatigue toughness can be used to predict the fatigue life of the T4 alloy.

Relaxation dynamics in amorphous alloys under asymmetric cyclic shear deformation

The influence of cyclic loading and glass stability on structural relaxation and yielding transition in amorphous alloys was investigated using molecular dynamics simulations. We considered a binary mixture cooled deep into the glass phase and subjected to cyclic shear deformation where strain varies periodically but remains positive. We found that rapidly cooled glasses under asymmetric cyclic shear gradually evolve towards states with lower potential energy and finite stress at zero strain. At the strain amplitude just below a critical value, the rescaled distributions of nonaffine displacements converge to a power-law decay with an exponent of about -2 upon increasing number of cycles. By contrast, more stable glasses yield at lower strain amplitudes, and the yielding transition can be delayed for hundreds of cycles when the strain amplitude is near a critical value. These results can be useful for the design of novel thermo-mechanical processing methods to improve mechanical and physical properties of metallic glasses.

Comparing the fatigue performance of Ti-4Al-0.005B titanium alloy T-joints, welded via different friction stir welding sequences

Ti–4Al-0.005B titanium alloy T-joints were produced with two different friction stir welding (FSW) sequences, and its effect on the low-cycle fatigue performance was investigated. Results show that hysteresis loop became a straight line with no significant fatigue damage occurring at low strain amplitudes (0.2% and 0.4%). As the strain amplitude increased to 0.6%, the area enclosed by the hysteresis loop increased for both T-joints and the base material (BM) due to fatigue damage accumulation. As the stress amplitude decreased gradually with increasing number of cycles, the fatigue life followed this decreasing trend. The fatigue life of single-weld T-joints is close to that of the double-weld T-joints, with a cyclic strain hardening index of the BM being in-between of the two T-joints. However, cyclic strength coefficient of BM is the lowest, and that of the double-weld T-joint is larger than that of the single-weld T-joint. The single-weld T-joint breaks at the heat affected zone (HAZ) on the advancing side, where the double-weld T-joint breaks at the HAZ of the second weld.

Low cycle fatigue behavior of Cu‐Cr‐Zr alloy with different cold deformation

AbstractThe present study addressed the cyclic deformation behavior and fatigue properties of Cu‐0.69Cr‐0.07Zr alloy with different cold deformation (ε = 64%, 75%, and 84%) using low cycle fatigue test. Low cycle fatigue tests were conducted under fully‐reversed conditions at different total strain amplitudes. The microstructure changes and fatigue fracture characteristics were analyzed by scanning electron microscope (SEM) and transmission electron microscope (TEM). The main findings suggest that the Bauschinger effect was significantly stronger with larger deformation at low total strain amplitude. And it was proved that the relationship between the total strain amplitude and the low cycle fatigue life of Cu‐Cr‐Zr alloy with different deformation can be expressed by the Manson–Coffin–Basquin formula. Further, the reason for the fatigue life was shorter and the cyclic softening rate decreased faster at high applied total strain amplitude was that the dislocation density decreased due to the rearrangement of the dislocations.

Low cycle fatigue behaviour study of a nano precipitate strengthened ferrite-bainite 780 steel

Low-cycle fatigue behaviour of a newly developed ferrite-bainite 780 steel with the ferrite phase, strengthened by nanosized precipitates was investigated. The LCF tests were conducted in fully reversed strain-controlled mode to determine the cyclic stress response and strain life. The steel showed predominantly cyclic softening behaviour at lower strain amplitudes (up to 0.25%). However, at higher strain amplitudes (≥ 0.35%), cyclic hardening was observed with increase in the number of loading cycles. EBSD study done on the fatigue tested samples revealed grain fragmentation and texture weakening during cyclic deformation. TEM study elucidated the evolution of dislocation structure responsible for cyclic softening/hardening behaviour during cyclic deformation. At lower strain amplitude (0.2%), concurrent presence of random arrangement of dislocations and dislocation tangles were observed with some grains having weakly developed persistent slip bands (PSBs) and veins. At high strain amplitude (0.55%), subgrain structure was found to be the dominant feature along with occurrence of PSBs and veins at few locations. Further to this the role of nano-precipitates in influencing the fatigue behaviour was also investigated.

Numerical and Analytical Studies of Low Cycle Fatigue Behavior of 316 LN Austenitic Stainless Steel

Abstract Mechanical components are frequently subjected to severe cyclic pressure and/or temperature loadings. Therefore, numerical and analytical low cycle fatigue methods become widely used in the field of engineering to estimate the design fatigue lives. The primary aim of this work is to evaluate the accuracy of the most commonly used numerical and analytical low cycle fatigue life methods for specimens made of 316 LN austenitic stainless steel and subjected to fully reversed uniaxial tension–compression loading, in the room temperature condition. It was found that both maximum shear strain and Brown–Miller criterions result in a very conservative estimation for uniaxially loaded specimens. However, maximum shear strain criteria provide better results compared to the Brown–Miller criteria. The total strain energy density approach was also used, and both the Masing and non-Masing analysis were adopted in this study. It is found that the Masing model provides conservative fatigue lives, and non-Masing model results in a more realistic fatigue life prediction for 316 LN stainless steel for both low and high strain amplitudes. The fatigue design curves obtained from the commonly used analytical low cycle fatigue equations were reexamined for 316 LN SS. The obtained design curves from Langer model and its modified versions are nonconservative for this type of material. Consequently, the authors suggest new optimized parameters to fit the given test data. The obtained curve using the currently suggested parameters is in better agreement with the experimental data for 316 LN SS.

On the cyclic hardening/softening behavior of a rare earth-containing magnesium alloy during strain-controlled fatigue

The origin of cyclic hardening under low strain amplitude (with N = 25284 cycles to failure) followed by pseudo-plateau response under high strain amplitude (with N = 2671 cycles to failure) during fully reversal (R = −1) strain-controlled pull–push fatigue of Mg-8.2Gd-3.6Y-1.6Zn-0.5Zr (wt.%) at room temperature condition was explored. The (i) dislocations trapping between LPSO (long period stacking order) lamellas, (ii) substructure development, and (iii) deformation twinning and tangling/rearrangement of partials within twin band were known as the main factors contributing to the room temperature cyclic hardening response. The pseudo-plateau behavior and cyclic stabilization were also attributed to the co-formation of the veins structure in the vicinity of 18R LPSO and reversion of 14H LPSO at higher strain amplitudes.

Study on high damping L12/D03 biphase Fe-27Ga alloys with near-zero magnetostriction and its damping mechanism

A novel L12/D03 biphase Fe-27Ga alloy with both high damping (0.007, equivalent to a specific damping capacity (SDC) of 0.04) and the near-zero (∼−4 ppm) magnetostriction has been prepared by using an appropriate heat treatment process: annealing at 480 °C for 1 h. Here the L12 phase proportion is about 44.8%. More importantly, the damping mechanism of L12/D03 biphase Fe-27Ga alloy has been deeply studied based on the experiments of amplitude-dependent IF measurements with and without magnetic field, combined with the magnetostriction measurements. All the results indicate that for the L12/D03 biphase Fe-27Ga alloy, D03 phase with a magnetic damping plays a dominant role under a lower strain amplitude while L12 phase with a non-magnetic damping plays a main role under a higher strain amplitude. At the same time, the L12/D03 biphase Fe-27Ga alloy with more L12 phase has a higher damping capacity under a higher strain amplitude, which is mainly the non-magnetic damping contribution originated from the twin boundaries movement. This can provide a new way and design concept for high damping Fe-Ga alloys to satisfy more application requirements, such as micro-vibration field, strong vibration field, magnetic or non-magnetic field.