High Strain Amplitudes Research Articles

Investigation on low-cycle fatigue behaviors of wrought superalloy GH4742 at room-temperature and 700 °C

Low-cycle fatigue (LCF) behavior of wrought superalloy GH4742 was studied at room-temperature (RT) and 700 °C. Scanning electron microscope (SEM) and Transmission electron microscope (TEM) were used to study the development of cracks and deformation mechanism. An obvious decrease in fatigue life occurred as the temperature was increased from RT to 700 °C and as the total strain amplitude was raised from 0.5% to 1.0%. The Halford-Marrow relationship based on plastic strain energy density was used to predict the fatigue life, which is in good accordance with experimental data. At RT, for all investigated samples, initial cycle hardening is followed by gradual cycle softening especially at higher strain amplitude (∆εt≥0.8%). Detailed TEM observation revealed that γ′ phase was sheared by dislocations. The shearing mechanism, reducing the ordering degree and diameter of γ′ phase, should be responsible for fatigue softening. Differently, at 700 °C, the tested samples exhibited continuous cycle hardening. A sharp drop in the number of stacking faults occurred. Lots of dislocation loops existed around tertiary γ′ phase and paired dislocations moved in secondary γ′ phase, indicating that Orowan and shearing mechanism have occurred. Fatigue cracks usually initiated from the surface. SEM investigation showed that the failure mode changed from transgranular to mixed transgranular and intergranular modes with increasing temperature.

Strain controlled low cycle fatigue properties of stepped aging heat treated AA5754

Abstract In present work strain controlled low cycle fatigue (SC-LCF) properties of stepped ageing heat treated(SAHT) AA5754 alloy is evaluated at three different strain amplitudes (0.4–0.6)% at fixed strain rate of 0.001 sec-1. SAHT process significantly improves the mechanical and fatigue life of the as-received alloy. The material shows cyclic hardening behavior at all the strain amplitudes. The hysteresis loop areas for SAHTed alloys are found to be significantly higher at corresponding elapsed cycles at higher strain amplitudes. Finite element simulation using Chaboche cyclic plasticity is utilized to predict the cyclic response of the alloy. Chaboche kinematic hardening material parameters are determined from stabilized hysteresis loop. Numerical results agree well with experimentally obtained results.

On the phase angle role in the shear response of ZK60 Mg alloys under multiaxial fatigue

Proportional and non-proportional multiaxial fatigue tests are conducted on the closed-die forged ZK60 extrusion. The shear strain amplitude was kept constant at 0.5% for all the tests, while two different axial strain amplitudes of 0.4% and 0.7% were considered. At the higher strain amplitude (0.7%) significant difference was observed between the torque amplitudes of proportional and non-proportional tests, whereas the axial load amplitude responses remained the same regardless of the phase angle shifts. It is likely that as the phase angle changes from 0-90, the twin volume fraction at the peak shear strain decreases resulting in higher torque responses. On the other hand, at the lower strain amplitude, i.e. 0.4%, where twinning is not active, phase angle does not show any effect on the shear response. An energy-based fatigue model is employed that effectively explains the different damage contributions by the axial and torsional loadings at different strain amplitudes, and accurately predicts the proportional and non-proportional multiaxial fatigue lives.

Ultrafine versus coarse grained Al 5083 alloys: From low-cycle to very-high-cycle fatigue

The fatigue performance of coarse and ultrafine-grained (UFG) 5083 Al alloy were compared, from low to very high cycle fatigue. The UFG alloy exhibited cyclic hardening and predominant kinematic hardening. At high plastic strain amplitude (and only in this regime), it showed easier crack initiation and a lower fatigue resistance. Its resistance to HCF was hardly better than that of its coarse grained counterpart until 2.106 cycles, but 43% higher in VHCF, until 5.108 cycles. Beyond that point, internal crack initiation occurred, and the fatigue resistance of the UFG material decreased, which was explained using Fracture Mechanics.

Multiscale mechanical fatigue damage of stainless steel investigated by neutron diffraction and X-ray microdiffraction

Mechanical fatigue behavior of AL6XN stainless steel as a typical type of planar slip alloy was investigated by in situ neutron diffraction and synchrotron-based X-ray microdiffraction methods. Under cyclic loading at a high strain amplitude (±0.8%), the fatigue damage originated mainly from the accumulation of statistical stored dislocations, as clearly evidenced from a continuous increase in diffraction peak width with increasing the number of load cycles. However, under cyclic loading at a low strain amplitude (±0.3%), the density of statistical stored dislocations became saturated just after a hundred loading cycles and the fatigue damage was mainly dominated by the accumulation of persistent Lüders bands (PLBs) and the complex interactions among various PLBs as evidenced through X-ray microdiffraction measurements. It was further found that there exists obvious grain-orientation-dependent local damage in the low-strain-amplitude fatigued sample. In particular, fatigued grains orientated with [001] paralleling the loading direction are subjected to compressive stress and contain a large number of broad PLBs in boundaries arraying the edge dislocation pile-ups, which generate a large stress gradient leading to local plastic instability. The highly localized stress field at PLBs in the cyclically-deformed sample at a low strain amplitude may explain the obvious cyclic stress softening.

Sustainability Study of Inherent Damping in Additively Manufactured Nickel Alloy

The laser powder bed fusion additive manufacturing process has been used to fabricate unique specimens with inherent damping capability for suppressing vibratory responses associated with high cycle fatigue. The innovative concept of the unique specimen is that it is manufactured with internal geometries, densely packed with unfused powder, and during vibration, the powder interaction dissipates energy via friction. Studies of this manufacturing and design approach have demonstrated the capability of suppressing vibration by 10X of a fully fused part. A particle motion mechanism has been identified, and association with vibratory behavior produces a criterion for damping optimization. Despite the accuracy of the criterion, the sustainability (or degradation) of initial/inherent damping performance after subjection to millions of cycles at high strain amplitudes has not been extensively scrutinized. This Paper investigates damping performance sustainment by assessing the endurance, repeatability, and recovery of the inherent damping provided by unfused powder within a uniquely designed specimen. Endurance and repeatability assessments, respectively, identify a limitation in achieving optimal damping and a steady-state damping performance after millions of duty cycles at high strain amplitudes. Though the optimal damping performance recovery attempt is negligible, the inherent damping of the unique specimen after rigorous usage is greater than the results from fully fused parts.

The deformation-induced martensite and dynamic strain aging during cyclic deformation in AISI 321

To investigate the deformation-induced martensite and dynamic strain aging (DSA) during cyclic deformation in AISI 321, fatigue tests were carried out at strain amplitude 0.5% and 0.9% at room temperature (RT), 550 °C and 650 °C. Optical microscope and scanning electron microscope were used to observe the crystal grains and fracture surface microstructure respectively; x-ray diffraction was used to detect the phase transformation. Cyclic stress response curves exhibited different combinations of cyclic hardening, saturation and cyclic softening. Deformation-induced martensite was detected by x-ray diffraction and the amount increased with strain amplitude at RT, which is responsible for the rapid secondary hardening. DSA’s manifestation only showed up at 650 °C and enhanced at higher strain amplitude, resulting in higher hardening at 650 °C than 550 °C. Fatigue life reduction at RT is due to rapid crack propagation caused by deformation-induced martensite hardening. The DSA hardening leads to a reduction of fatigue life at elevated temperatures by ways of multiple crack initiation and rapid crack propagation.

Microstructure analysis and low-cycle fatigue behavior of spray-formed Al–Li alloy 2195 extruded plate

Microstructure and low-cycle fatigue behavior of spray-formed Al–Li alloy 2195 extruded plate were investigated in this work. The spray-formed alloy after hot extrusion experiment was treated with solid solution treatment and artificial aging. Microstructure analysis indicated the aged plate was dominated by elongated unrecrystallized grains, and had a rolling-type texture along extrusion direction with the highest intensity at Brass component. The existence of T1 phase strengthened the alloy crucially, but δ′ phase was basically absent. Then, the fully-reversed strain-controlled low-cycle fatigue tests were conducted at total strain amplitudes ranging from 0.4% to 1.0% for samples along two orthogonal directions. The stress-strain hysteresis loops were acquired, and the cyclic stress response curves were derived. At low strain amplitudes (0.4–0.5%), the initial cyclic hardening was slight and followed by a cyclic stability, while at higher strain amplitudes (0.6–1.0%), the alloy merely presented a continuously increasing cycle hardening behavior. Moreover, the fatigue life model based on the total strain energy was built and found to be suitable to predict life. Finally, the fatigue fractography observation showed that the fatigue source is relatively concentrated and the fracture surface had typical fatigue striations at 0.5% strain amplitude, while multiple cracks originated on the sample surface and the final fracture zone showed a ductile characteristic at 1.0%. The deformed microstructure near fracture surfaces were observed, and it was found that the cyclic hardening and stability were closely associated with the interaction between moving dislocations and obstacles including (sub)grain boundaries and secondary phase particles against them.

Fourier-transform rheology of unvulcanized styrene butadiene rubber filled with increasingly silanized silica

ABSTRACTSilica as one the most important fillers for rubber material is routinely modified by silane to improve its compatibility with the rubber matrix. Silanization of the silica particle affects both the linear and nonlinear rheological behaviors of the compounds. Their rheological nonlinearity, however, is mostly analyzed in an indirect way from linear rheological parameters, e.g. G′(ω1, γ0) and G″(ω1, γ0), which lose their physical meaning in the nonlinear viscoelastic regime. In the present work, the nonlinearity is directly quantified by the Fourier-transform rheology (FT-Rheology) technique in terms of I3/1(ω1, γ0), the third relative higher harmonic, for unvulcanized styrene butadiene rubber compounds filled with a fixed amount of silica, but varying dosages of silane. With the proposed model for I3/1(γ0), the contributions toward nonlinearity from the filler networks at a low strain amplitude and the one from the polymer networks at high strain amplitude can be successfully separated for filled systems. The utmost nonlinearity contribution from the filler networks decreases with the silane content, which is assigned to the weakening interparticle interaction of the filler. With increasing silanization of silica, the utmost nonlinearity contribution from the polymer networks is found to increase. This nonlinear mechanical response is attributed to the enhanced interfacial interaction between the filler and polymer.

Corrosion effects on fatigue behavior of dissimilar friction stir welding of high-strength aluminum alloys

This paper examines the effects of corrosion defects on the low-cycle fatigue performance of dissimilar friction stir welded AA6061-to-AA7050 aluminum alloys. Corrosion defects were produced on the crown surface of the weld by static immersion in 3.5% NaCl for various exposure times. Results revealed localized corrosion damage in the thermo-mechanically affected and heat-affected zones. The corrosion damage featured general pitting, pit clustering, and exfoliation that revealed increasing depth with increasing exposure time. The higher corrosion attack was measured in the AA7050 compared to the AA6061. Results demonstrated a decrease in the fatigue life with evidence of crack initiation at the corrosion defects; however, the fatigue life was nearly independent of the exposure time. This can be attributed to total fatigue life dominated by incubation time. Furthermore, two types of failure were observed: crack propagation in the AA6061 side at high-strain amplitudes (> 0.3%) and crack propagation in the AA7050 side at low-strain amplitudes (< 0.2%). This can be attributed to the cyclic strain hardening evolution and the localized high-stress field at the tip of the corrosion defect. Lastly, a microstructure-sensitive fatigue model was employed to capture the effect of corrosion defects for the life prediction of the dissimilar friction stir welded AA6061-to-AA7050.

Cyclic stress responses of a newly developed nickel-base superalloy at elevated temperatures

Total strain-controlled low cycle fatigue tests were conducted on the newly designed nickel-base superalloy M951G under different testing conditions; the relationship among cyclic stress responses, microstructural degradations, deformation mechanisms and testing conditions has been established. Results show that both the cyclic hardening and softening behaviors are dependent on the testing temperature and strain amplitude. As the strain amplitude increases both at 900 and 1000 °C, M951G alloy exhibits cyclic hardening under low strain amplitudes and cyclic softening under higher strain amplitudes. At 900 °C, the initial cyclic hardening is related to the coherent γ/γ′ interface, parallel dislocation arrays and dislocation bypassing the tiny γ′ particles. At higher strain amplitudes, the initial cyclic softening is due to the higher density of shearing dislocations in γ′ precipitates. At 1000 °C, plenty of parallel dislocation arrays present in γ channels, which reduces the possibility of dislocation interactions from different slip systems and results in initial cyclic hardening. Under higher strain amplitudes, apart from microstructural degradations, dislocations shearing into γ′ precipitates and formation of dislocation networks, the dislocation annihilation and partial loss of coherency of γ′ precipitates are also responsible for the initial cyclic softening at 1000 °C.

Wall slip and multi-tier yielding in capillary suspensions

Impact of wall slip on the yield stress measurement is examined for capillary suspensions consisting of cocoa powder as the dispersed phase, vegetable oil as the continuous primary fluid, and water as the secondary fluid using smooth and serrated parallel plates. Using dynamic oscillatory measurements, we investigated the yielding behavior of this ternary solid-fluid-fluid system with varying particle volume fraction, ϕ, from 0.45 to 0.65 and varying water volume fraction, ϕw, from 0.02 to 0.08. Yield stress is defined as the maximum in the elastic stress (G′γ), which is obtained by plotting the product of elastic modulus (G′) and strain amplitude (γ) as a function of applied strain amplitude. With serrated plates, which offer minimal slippage, capillary suspensions with ϕ ≥ 0.45 and a fixed ϕw = 0.06 showed a two-step yielding behavior as indicated by two peaks in the plots of elastic stress as a function of strain amplitude. On the other hand with smooth plates, the capillary suspensions showed strong evidence of wall slip as evident by the presence of three distinct peaks and lowered first yield stresses for all ϕ and ϕw. These results can be interpreted based on the fact that a particle-depleted layer, which is known to be responsible for slip, is present in the vicinity of the smooth surfaces. The slip layer presents itself as an additional “pseudo-microstructure” (characteristic length scale) besides the two microstructures, aqueous bridges and solid particle agglomerates, that may occur in the system. With serrated plates, both the yield stresses (σ1σ2) and storage moduli plateau at lower strain (before the first yield point) and at higher strain (before the second yield point) (G $^{\prime }_{p1}$ , G $^{\prime }_{p2}$ ) were found to increase with ϕ (at a fixed ϕw = 0.06) following power-law dependences. Similarly with increasing ϕw (0.02 – 0.08) at a fixed ϕ = 0.62, the system behaved as a solid-like material in a jammed state with particles strongly held together as manifested by rapidly increasing σ1 and σ2. The usage of smooth surfaces primarily affected σ1 which was reflected by an approximately 70–90% decrement in the measured σ1 for all values of ϕ. By contrast, σ2 and G $^{\prime }_{p2}$ were found to be unaffected as shown by close agreement of values obtained using serrated geometry due to vanishing slip layers at higher strain amplitudes.

Effect of high strain amplitude and pre-deformation on damping property of Fe-Mn alloy

In order to investigate the effect of high strain amplitude (maximum 1.6 × 10−3) and large pre-deformation (maximum 15%) on the damping properties of Fe-Mn alloys, the damping properties of the alloys were measured by a computer controlled automatic inverted torsion pendulum, microstructures were observed by SEM and EBSD, and phases were identified by XRD. The results show that the damping performance decreases at first and then increases when the pre-deformation amount is higher than 10%, and the valley value is reached at about 12%. When the pre-deformation amount is more than 12%, the content of ε phase decreases, while the amount of γ austenite and α′ martensite increases, the ε martensite phase of the alloy is divided and appears finer, ε martensitic phase and γ austenite have better parallelism, the phase interfaces of γ/ε significantly increase, and more damping sources with better slippage are provided, resulting in gradually increasing of damping properties of Fe-Mn binary alloy. When the strain amplitude is greater than 1 × 10−3, the damping properties and strain amplitudes of the pre-deformation alloys do not always maintain an approximately linear relationship, which closely related to different characteristics of damping sources due to stress-induced phase transition and formation of different alloy microstructures.

Superior fatigue life of Fe-15Mn-10Cr-8Ni-4Si seismic damping alloy subjected to extremely high strain amplitudes

The fatigue life of a Fe–15Mn–10Cr–8Ni–4Si seismic damping alloy, undergoing cyclic deformation-induced martensitic transformation from face-centered-cubic austenite to hexagonal-close-packed martensite, is investigated at extremely high strain amplitudes at which common steels exhibit extremely low cycle fatigue (ELCF) with a fatigue life (Nf) less than 102 cycles. Hourglass-shaped specimens were subjected to diametral strain-controlled cyclic tension–compression loading at room temperature with axial strain amplitudes ranging from 4 × 10−2 to 1 × 10−1. The alloy exhibits superior Nf, for example, 112 cycles at the axial strain amplitude of 8 × 10−2, at which a carbon steel is reported to fail at less than 10 cycles. To the best of our knowledge, the Nf of the alloy is the highest ever among ferrous alloys and steels at extremely high strain amplitudes.

Effect of Overaging on the Cyclic Deformation Behavior of an AA6061 Aluminum Alloy

The present work encompasses the effect of overaging on the strain-controlled low-cycle fatigue (LCF) behavior of an extruded AA6061 aluminum alloy at varying strain amplitudes. During the T7 aging treatment, the size of precipitates increased from 60 nm under T6 conditions to 220 nm after aging for 48 h at 200 °C, leading to a decrease in the monotonic tensile strength. During the LCF tests, nearly symmetrical hysteresis loops can be observed in the mid-life cycle under all test conditions, whereas the first-cycle hysteresis loops were moderately inflected under long-aging conditions. With increasing aging time, the cyclic peak stresses decreased and the plastic strain increased. Nearly ideal Masing behavior was exhibited under T6 conditions, while it was lost under T7 overaging conditions. The cyclic stress responses were similar under all tested conditions, involving stabilization at low strain amplitudes and softening at high strain amplitudes, with initial hardening for the first few cycles. Compared to the T6 condition, the fatigue life increased with increasing T7 aging time. Various LCF parameters were estimated based on the Coffin-Manson and Basquin relationships and on the LCF experimental results. The relationship between the fatigue life, strength, and microstructure of the investigated AA6061 aluminum alloy under various aging conditions was discussed.

Comparative study of the effects of surface mechanical attrition treatment and conventional shot peening on low cycle fatigue of a 316L stainless steel

In this work, the effects of surface mechanical attrition treatment (SMAT) and conventional shot peening (CSP) are comparatively studied on the surface characteristics and low cycle fatigue (LCF) properties of a 316L stainless steel. For this purpose, surface topography, residual stress and work hardening are evaluated on the treated samples. Both CSP and SMAT are performed with the same Almen intensity and the same coverage. Total strain controlled LCF tests are carried out in order to compare the effects of these two techniques on the fatigue properties of the studied steel. The results show that the samples treated by SMAT and CSP exhibit similar trends of cyclic stress amplitude evolution, and higher mechanical strength than the untreated samples. However, the samples treated by CSP manifest higher fatigue life under low strain amplitude (±0.5%), but lower fatigue life under high strain amplitude (±1.25%), compared to the SMATed samples. The fatigue life results are discussed based on the analysis of different characteristics including surface topography, microstructure, residual stress and its relaxation, as well as fracture surfaces.

Tuning Low Cycle Fatigue Properties of Cu-Be-Co-Ni Alloy by Precipitation Design

As material for key parts applied in the aerospace field, the Cu-Be-Co-Ni alloy sustains cyclic plastic deformation in service, resulting in the low cycle fatigue (LCF) failure. The LCF behaviors are closely related to the precipitation states of the alloy, but the specific relevance is still unknown. To provide reasonable regulation of the LCF properties for various service conditions, the effect of precipitation states on the LCF behaviors of the alloy was investigated. It is found that the alloy composed fully of non-shearable γ′ precipitates has higher fatigue crack initiation resistance, resulting in a longer fatigue life under LCF process with low total strain amplitude. The alloy with fine shearable γ′I precipitates presents higher fatigue crack propagation resistance, leading to a longer fatigue life under LCF process with high total strain amplitude. The cyclic stress response behavior of the alloy depends on the competition between the kinematic hardening and isotropic softening. The fine shearable γ′I precipitates retard the decrease of effective stress during cyclic loading, causing cyclic hardening of the alloy. The present work would help to design reasonable precipitation states of the alloy for various cyclic loading conditions to guarantee its safety in service.

Microstructural evolution in the low strain amplitude multi-axial compression (LSA-MAC) after equal channel equal pressing (ECAP) of aluminum

Low strain amplitude multi-axial compression (LSA-MAC) increases the work hardening capacity and uniform ductility of Aluminum after Severe Plastic Deformation (SPD) through Equal Channel Angular Pressing (ECAP). The mechanisms associated with this processing after ECAP are analyzed. LSA-MAC after 1 ECAP pass in Aluminum softens the material and is connected to the stabilization of the dislocation structures induced by ECAP into a configuration displaying an increased fraction of High Angle Grain Boundaries (HAGB). This occurs for a lower total imposed strain than that through a sequence of high strain ECAP passes or high strain amplitude multi-axial compression (HAS-MAC).

Effects of Semi-solid Isothermal Heat Treatment on Microstructures and Damping Capacities of Fly Ash Cenosphere/AZ91D Composites

The fly ash cenosphere/AZ91D composites were successfully prepared and isothermally heat-treated at different temperatures for different time. The effects of semi-solid isothermal heat treatment on the microstructures and damping capacities of fly ash cenosphere/AZ91D composites were investigated. With the increase in isothermal temperature or holding time, the small liquid droplets within grains increased in size but decreased in quantity. The average size and shape factor of Mg2Si particles increased with the rise of isothermal temperature. The damping capacities of the composites were improved by isothermal heat treatment. At room temperature, the composites after heat treatment at 520 and 550 °C had a higher damping capacity due to interface damping when the strain amplitude was lower than about 8.8 × 10−5, and the composite after heat treatment at 580 °C had a better damping capacity because of the dislocation damping under the condition of high strain amplitude. The damping capacities of the composites increased with the rise of the test temperature, and the damping mechanisms varied depending on different test temperatures. The interface damping played an important role when the test temperature was below about 100 °C, and the dislocation damping and grain boundary damping took effect with the rise of test temperature.

Experimental analysis and constitutive modelling of cyclic behaviour of 316L steels including hardening/softening and strain range memory effect in LCF regime

Cyclic behaviour prediction of materials and structures is always of great interest to industry. In this work, total strain controlled low cycle fatigue (LCF) tests of two 316L austenitic stainless steels (named 316L-A and 316L-B) were conducted under strain amplitudes ranging from ±0.3% to ±1.5%. The analysis of the experimental results shows that both of the studied steels undergo an initial hardening, followed by a long range of softening, especially under lower strain amplitudes (from ±0.3% to ±0.5%). For 316L-B, obvious secondary hardening can be observed under higher strain amplitudes (from ±0.8% to ±1.25%). In addition, the two steels exhibit a significant strain range memory effect during cyclic loading. It is revealed that the observed hardening/softening behaviour as well as the strain range memory effect are mainly attributed to the back stress, while the isotropic hardening variation under different strain amplitudes has a similar trend.From these experimental results, a cyclic constitutive model is developed and implemented to describe the cyclic behaviour of 316L-A including the hardening/softening and the strain range memory effect. First, a nonlinear kinematic hardening rule is proposed based on the classical decomposed Armstrong and Frederick's description, by introducing a kinematic hardening coefficient ϕ to incorporate the hardening/softening effect in the back stress. Second, a functional relation between the kinematic hardening coefficient ϕ and the accumulated plastic strain is established according to the analysis of cyclic hardening/softening behaviour. The strain range memory effect is considered by incorporating the effect of the memory surface in the parameters of the kinematic hardening coefficient ϕ. An identification procedure is developed to determine all the material parameters of the cyclic constitutive model. With the constitutive model implemented, a series of strain controlled cyclic simulations are performed using a cycle jumping method. The comparison between the modelling and the experimental results indicates that the developed cyclic constitutive model is efficient to precisely describe the cyclic behaviour of the 316L-A steel in the LCF regime.