Total Strain Amplitude Control Research Articles

Effect of Al on the low cycle fatigue deformation behavior and microstructure evolution of Fe–22Mn-0.6C TWIP steel

The dependence of low cycle fatigue (LCF) behavior and microstructure evolution of Fe–22Mn-0.6C twinning-induced plasticity (TWIP) steel on the alloying element Al has been investigated under fully reversed total strain amplitude control with a nominal strain rate of 0.006 s−1 and strain amplitudes ranging from 0.002 to 0.010 at room temperature. The results indicate a strong dependence of LCF response and evolved microstructure on the addition of Al, owing to its effect on the stacking fault energy (SFE). At all strain amplitudes, the 3Al steel with a higher SFE exhibits a visibly lower cyclic hardening and a higher cyclic softening, resulting in a lower cyclic yield strength than the 0Al steel. As compared with the 0Al steel, the relatively higher SFE of the 3Al steel causes higher accumulated plastic strain, decreased slip planarity and weaker reversibility of dislocations, thereby decreasing the fatigue life. The optical microstructure is featured by increase in slip band density with increasing strain amplitude for both steels. However, the slip band density of the 3Al steel is higher than that of the 0Al steel. Moreover, cyclic loading promotes the formation of dislocation cells, rough labyrinths as well as planar dislocation arrays. Deformation twins are observed only in the 0Al steel with a visible lower critical twinning stress due to its low SFE as compared to the 3Al steel.

Integrated TEM/transmission-EBSD for recrystallization analysis in nickel-based disc superalloy

Turbine discs in gas-turbine engines always experience high-temperature and high-stress environment. Extensive investigations have been performed towards achieving high performance. Deformation behaviors and degradation factors above normal service temperature are important to be comprehensively understood for further development and optimization of turbine disc superalloys. In this study, the low cycle fatigue behaviors were investigated under total strain amplitude control in U720Li disc superalloys. The results revealed that with the increasing strain, the number of dislocation increased and dislocation configuration transferred from pile-up at the matrix/γ′ precipitates interfaces to penetrating into γ′ precipitates. The great extent of recrystallization was found above the normal service temperature through transmission-EBSD combined with TEM analysis. The low cycle fatigue property above the normal service temperature was weakened to a large extent after recrystallization of small-angle grains formed by the dislocation substructures and movements. Large extent of local recrystallization could also provide much more sites for the crack initiation and propagation.

Microstructure and mechanical properties of friction stir welded AA6092/SiC metal matrix composite

There is a need for improved understanding on the effects of friction stir welding (FSW) on the metallurgical and mechanical properties of aluminium matrix composite (AMC). In this study, AA6092/SiC/17.5p-T6 AMC joints were produced by using FSW with varying tool rotation and traverse speeds. The microstructural characterisation by scanning electron microscopy equipped with electron backscattered diffraction (EBSD) system revealed a substantial grain refinement and a homogeneous distribution of reinforcement particles in the nugget zone. The grain size of the nugget zone was greatly influenced by weld pitch, as a key indicator to control the amount of heat input, exposure time and cooling rate. Vickers microhardness profile across the welding zone revealed a significant difference in microhardness among the base metal, heat affected zone, thermo-mechanically affected zone and nugget zone. The tensile strength of the cross-weld specimens showed a high joint efficiency of about 75% of the base metal combined with relatively high ductility. Low-cycle fatigue properties were investigated in the axial total strain-amplitude control mode (from 0.3% to 0.5%) with R=εmin/εmax=−1. The results indicate that the fatigue life of the cross-welded joints varies with grain size in the nugget zone and it is lower than that of the base metal. A significant improvement of fatigue life is found to be related to the finer equiaxed grains dominated by high angle grain boundaries in the nugget zone.

Low cyclic fatigue behavior of electron-beam-welded Ti–6Al–4V titanium joint

The low cycle fatigue (LCF) tests were carried out using symmetrical cyclic loading under total strain amplitude control conditions. The present paper is devoted to investigating the cyclic deformation response of Ti–6Al–4V titanium and the electron-beam-welded (EBW) joint in the following aspects, i.e., cyclic deformation behavior, fatigue life and fatigue fracture behavior. The results show that the softening of the joint is significant at larger strain ranges, while not obvious at smaller strain ranges. The joint shows shorter fatigue life at larger strain ranges and equivalent fatigue life at smaller strain ranges compared with Ti–6Al–4V base metal. A fatigue crack of the joint not only originates at the surface or subsurface, but also at defects in the fusion zone (FZ). The crack propagation zone of Ti–6Al–4V base metal shows ductile fracture mechanism, while the joint shows brittle fracture mechanism. In all the fatigue fracture zones many dimples appear, showing the typical ductile fracture.

Low Cycle Fatigue Behavior of AA2219-T87 at Room Temperature

Abstract The low cycle fatigue behavior of AA2219-T87 was studied at ambient temperature using fully reversed total strain controlled fatigue tests in longitudinal and transverse orientations. Tension-compression cyclic tests were carried out under total strain amplitude control with strain ratio Rε = −1 and at a frequency of 0.5 Hz. The total strain amplitude was varied from ±0.4 to ±1.5 %. The low cycle fatigue results were analyzed to determine the cyclic stress strain curves. The cyclic strain hardening parameters were evaluated from the cyclic stress–strain curves. Important fatigue parameters namely fatigue ductility coefficient, fatigue ductility exponent, and fatigue strength coefficient were evaluated. The material exhibited cyclic strain softening in both longitudinal and transverse directions. Analysis of the experimental results showed that the stress–strain hysteresis loops exhibited inflections in the first few cycles and the plastic strain amplitude versus fatigue-life plots exhibited bi-linear relationship. The geometry of hysteresis loops and the occurrence of Masing behavior are analyzed and found that the alloy in peak aged condition exhibited tendency towards Masing behavior.

Low cycle fatigue properties of CLAM steel at room temperature

The low cycle fatigue (LCF) properties and the fracture behavior of China Low Activation Martensitic (CLAM) steel have been studied over a range of total strain amplitudes from 0.2 to 2.0%. The specimens were cycled using tension-compression loading under total strain amplitude control. The CLAM steel displayed initial hardening followed by continuous softening to failure at room temperature in air. The relationship between strain and fatigue life was predicted using the parameters obtained from fatigue test. The factors effecting on low cycle fatigue of CLAM steel consisted of initial state of matrix dislocation arrangement, magnitude of cyclic stress, magnitude of total strain amplitude and microstructure. The potential mechanisms controlling the stress response, cyclic strain resistance and low cycle fatigue life have been evaluated.

Low-cycle fatigue behavior of a high manganese austenitic twin-induced plasticity steel

The monotonic tensile properties and deformation mechanisms of Fe–Mn–C twinning-induced plasticity (TWIP) steels have been extensively studied; however, the low-cycle fatigue (LCF) properties of this series of advanced steels have not been well understood. The present paper addresses the cyclic deformation behavior and the deformed microstructure of an as-annealed TWIP steel. Fully reversed push–pull LCF tests were performed at room temperature under total strain amplitude control with a strain rate of 0.006s−1 and strain amplitudes ranging from 0.002 to 0.01. The results show initial rapid cyclic hardening within the initial 10% of the fatigue life at all strain amplitudes, and demonstrate an obviously enhanced cyclic yield strength. Different types of cyclic stress responses were revealed, which are featured by initial cyclic hardening followed by cyclic saturation, or followed by cyclic softening and saturation, or followed by cyclic softening without saturation till the final fracture, depending on the strain amplitude applied. The microstructure prior to and after fatiguing were examined by means of optical and transmission electron microscopy. The typical optical microstructure of fatigued samples is characterized by increases in slip band density with increasing strain amplitude or number of cycles at a given strain amplitude applied. The substructures of the deformed samples are featured by the formation of stacking faults and vein/labyrinth dislocation structures, while fine twins and cell or wall dislocation structures, besides those generated at lower strain amplitudes, are formed at high strain amplitudes.

The role of shear banding on the fatigue ductility of ultrafine-grained aluminium

To study the effect of deformation mode used for grain refinement on the low-cycle-fatigue (LCF) behaviour of ultrafine-grained (UFG) materials, a commercially pure Al was subjected to cryorolling, and room temperature symmetric and asymmetric rolling. The LCF test was conducted under fully reversed total strain amplitude control. Despite its lower tensile ductility, the cryorolled Al had significantly higher fatigue ductility. This has been attributed to a higher density of shear bands in the cryorolled Al.

Strain rate effect on the cyclic deformation response of UFG Al alloys

► We studied the cyclic deformation response of UFG Al to strain rate change. ► The materials considered for this study were commercially pure Al and 2024 Al alloy. ► The cyclic softening rate in UFG pure Al was not strain rate sensitive. ► However, the appearance of shear bands was found strain rate sensitive. ► The stable behaviour of UFG 2024 Al alloy was also found insensitive to strain rate. Commercially pure aluminium and 2024 Al alloy were cryo-rolled and annealed to produce ultrafine grained (UFG) microstructures. Both materials were cyclically deformed under a fully reversed total strain amplitude control condition at different frequencies to study strain rate effects. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used to identify microstructural changes and explain the mechanical properties. The cyclic stress response showed significant softening in commercial purity Al that was due to shear band formation and grain coarsening within these shear bands. While there was no strain rate sensitivity for commercial purity Al in terms of the mechanical response, the shear bands became noticeably more defined at lower strain rates. 2024 Al alloy had a cyclically stable response and microstructure that was not affected by strain rate.

Characterizations of Fatigue Deformation and Fracture Behavior of Commercially Pure Iron with Grain Boundary Micro-Voids

Fatigue deformation behavior of the commercially pure iron containing micro-voids at grain boundaries (GBs) were investigated under total strain amplitude control, and fatigue fractures were quantitatively characterized by fractal analyses. The cyclic response curves of the CP iron show an initial softening stage within early several cycles followed by a continuous cyclic hardening. No stress saturation phenomenon was found. Pre-existence of micro-voids at GBs spurs intergranular cracking to become a common damage mode besides transgranular cracking along slip bands. Quantitative analyses of fracture surface demonstrate that the value of fractal dimension D of the scanning profile in the crack growth zone is the largest as compared to those in the crack source zone and final rapid fracture area, almost regardless of the applied strain amplitude. This phenomenon was discussed in terms of the tortuosity of crack propagation path.

Dynamic Strain Aging Induced by Cyclic Deformation in Two Magnesium Alloys

Cyclic deformation for two magnesium alloys AZ91 and AM50 with different processing status has been performed under total strain amplitude control mode and at room temperature. A serrated flow can be observed in both tensile and compressive directions of the stress-strain hysteresis loop for as-extruded AZ91 and AM50 magnesium alloys. It means that the so-called dynamic strain aging occurs during cyclic deformation. In addition, the dynamic strain aging phenomenon can also be observed in two extruded magnesium alloys subjected to aging treatment as well as the AZ91 alloy subjected to solution treatment. However, the dynamic strain aging seems not to take place in the extruded AM50 alloy subjected to solution treatment because there exists no significant serrated flow behavior in either compressive or tensile direction of the stress-strain hysteresis loop. It is suggested that the occurrence of the dynamic strain aging is associated with collective behavior of many mobile dislocations as well as interactions between solute atoms and dislocations.

Ultrasonic Linear and Nonlinear Parameters in Cyclically Deformed Cu and Cu-35Zn Alloy

The cyclic deformation in Cu and Cu-35Zn alloy were studied using ultrasonic nondestructive evaluation (NDE) technique to measure the ultrasonic velocity, attenuation coefficient and a nonlinear parameter. These materials were cyclically deformed under total strain amplitude control to investigate their ultrasonic reaction to different dislocation substructures, depending on the stacking fault energy difference. The microstructure evolution was observed using a transmission electron microscope, and the ultrasonic NDE parameter was measured after several cycles of fatigue deformation, in order to clarify the relationship between them. In both materials, the ultrasonic velocity was observed to decrease as the fatigue life fraction increased, which was attributed to the increasing dislocation density caused by the cyclic deformation. In the case of Cu, with its cell structure evolving during cyclic deformation, the rate of increase of the ultrasonic attenuation coefficient and the ultrasonic nonlinear parameter was higher than that observed in the case of the Cu-35Zn alloy with its planar array structure. This result implies that the dislocation cell structure is more sensitive to the ultrasonic parameter changes than the planar array structure formed during cyclic deformation.

The low-cycle fatigue, deformation and final fracture behaviour of an austenitic stainless steel

The low-cycle fatigue (LCF) behaviour of SUS304-HP austenitic stainless steel was investigated systematically using tension-compression cycling under fully reversed total strain amplitude control conditions at room temperature in laboratory air. In addition to tests at constant strain amplitudes, incremental step tests (IST) were also carried out. Cyclic stress response, during companion specimen tests (CST), revealed combinations of a variable cyclic hardening, stable behaviour and softening, depending on the applied cyclic strain amplitude, while during incremental step tests it exhibited cyclic hardening character at all strain levels. Microstructure observations using optical and transmission electron microscopy (TEM) revealed that with increasing total strain amplitudes the slip band density increased and the dislocation structure changed from a planar array to a more cellular-like structure. Cyclic deformation-induced austenite/martensite transformation was observed at higher cyclic strain amplitudes. The change in microstructures during cycling is responsible for the fatigue hardening/softening behaviour of the material. The SEM micrographs revealed that at low-strain amplitudes the inclusion-type nucleation occurred near the surface, while at the higher strain amplitudes crack initiation characterized by cleavage cracking occurred not only near the surface but also in the interior of the specimen. Linear or single-slope behaviour was seen both in cyclic stress–strain and Coffin-Mason plots. Masing cyclic stress–strain behaviour was presented only in the IST method but not in the CST method.

Low-cycle fatigue behavior of two magnesium alloys

Magnesium alloys possess many excellent performances, such as low density, high specific strength, excellent thermal conductivity, and machinability. Magnesium alloys, including Mg-Al alloy family, are increasingly considered for structural components, especially in automotive industry. AZ91 alloy with about 9 wt.% Al and 1 wt.% Zn is the most common magnesium die-casting alloy. It has excellent castability and good room-temperature mechanical properties. AE42 alloy with about 4 wt.% Al and 2.5 wt.% rare earth elements shows good creep resistance to 150 ◦C, and is often used for automotive engine components. For both AZ91 and AE42 alloys, few low-cycle fatigue data are available even though their monotonic tensile and creep properties have been well documented [1–6]. In this paper, the low-cycle fatigue behavior of two magnesium alloys including AZ91 and AE42 were investigated. The rectangular fatigue samples with dimensions 14 × 5 × 5 mm in gauge section were directly machined from die-cast plate of each alloy (designated F). Some fatigue samples were subjected to solution treatment in argon at 415 ◦C for 12 hr (designated T4). Fully-reversed pull-push fatigue tests were performed at room temperature in laboratory air under total strain amplitude control mode using Shimadzu servohydraulic testing machine. All tests were run to final separation of sample, and the corresponding cycling number was defined as low-cycle fatigue life. The fatigue life data (total strain amplitude, et/2, versus number of cycles to failure, Nf) of two alloys with F and T4 status are shown in Fig. 1a and b, respectively. It can be seen that at most of total strain amplitudes except for the lowest strain amplitude of 0.3%, the AZ91-T4 alloy gives greater resistance on strain cycling than the die-cast AZ91 alloy. On the other hand, the AE42 alloy subjected to solution treatment exhibits longer fatigue life than the die-cast AE42 alloy at all total strain amplitudes imposed in this investigation. The above-mentioned fact implies that solution treat-

The strain amplitude-controlled cyclic fatigue, defomation and fracture behavior of 7034 aluminum alloy reinforced with silicon carbide particulates

Abstract In this paper, the cyclic stress response and stress versus strain response characteristics, cyclic strain resistance and low-cycle fatigue life, and mechanisms governing the deformation and fracture behavior of aluminum alloy 7034 discontinuously reinforced with silicon carbide particulates are presented and discussed. Specimens of the metal–matrix composite were cyclically deformed, using fully-reversed tension–compression loading under total strain amplitude control, at both ambient and elevated temperatures for the under aged and peak aged microstructural conditions. Under fully-reversed total strain amplitude control cycling, specimens of the composite exhibited combinations of cyclic hardening and cyclic softening to failure. The cyclic stress response and stress versus strain response characteristics, cyclic strain resistance, low-cycle fatigue (LCF) life, and final fracture behavior of the composite, for both the under aged and peak aged microstructures, at the two temperatures, are compared and observed differences rationalized in light of the mutually interactive influences of cyclic strain amplitude and concomitant response stress, intrinsic composite microstructural effects, deformation characteristics of the composite constituents, and macroscopic aspects of fracture.

Low cycle fatigue behavior of nickel-based superalloy GH4145/SQ at elevated temperature

Low-cycle fatigue (LCF) behavior of nickel-based precipitation-strengthened superalloy GH4145/SQ was investigated systematically under fully reversed total strain amplitude control conditions at 538degreesC in laboratory air. It was shown that the alloy exhibited a pronounced initial fatigue hardening followed by continuous fatigue softening to failure at high strain amplitudes, while at low strain amplitudes the initial hardening was followed by a well-defined saturation stage. Microstructure observations using optical and transmission-electron microscopy revealed that slip band density increased with increasing total strain amplitudes and the size of the y' precipitates tended to reduction due to repeated shearing by the dislocations with continuous cyclic straining. The above changes in microstructures during cycling were essentially responsible for the fatigue hardening-softening behavior of the alloy. Both optical and SEM micrographs indicated that at low strain amplitudes crack propagation was basically transgranular, while at high strain amplitudes crack propagation was essentially intergranular. Bilinear behavior with a change of slope at a plastic strain amplitude of about 0.2 pct was observed in cyclic stress-strain (CSS), Coffin-Manson (C-M) and plastic strain energy-life plots. A change in the number of operating slip systems and the fracture modes with the strain amplitude employed was used to explain this two-slope characteristic. In both the companion specimens test (CST) and the incremental step test (IST) methods the alloy exhibited non-masing type behavior. It was suggested that, for the first approximation, the half-life plastic strain energy could be calculated directly according to the equation without reckoning in the non-masing behavior. (C) 2004 Elsevier B.V. All rights reserved.

The tensile deformation, cyclic fatigue and final fracture behavior of dispersion strengthened copper

The tensile deformation, cyclic stress response characteristics, cyclic stress–strain response, cyclic strain resistance and fatigue properties of an oxide dispersion strengthened copper alloy were studied over a range of cyclic plastic strain–amplitudes. The specimens were cycled using tension-compression loading under total strain–amplitude control. In the as-drawn condition, the dispersion strengthened copper alloy displayed continuous softening to failure at all cyclic plastic strain amplitudes. The rolled plus heat-treated microstructure displayed initial hardening followed by rapid softening to failure. The strain–fatigue life relationship is predicted using the parameters obtained from the fatigue test. Potential mechanisms controlling the stress response and concomitant cyclic strain resistance and cyclic fatigue life are also evaluated, and rationale for observed behavior is discussed in light of concurrent and interactive influences of initial state of the matrix dislocation arrangement, magnitude of cyclic strain amplitude, response stress and intrinsic microstructural effects.

An investigation of the cyclic fatigue and fracture behavior of aluminum alloy 7055

In this research paper, the cyclic stress- and strain-amplitude-control fatigue response of aluminum alloy 7055 is presented and discussed. Specimens of the alloy in the T7751 temper were cyclically deformed over a range of stress amplitudes at both ambient and elevated temperatures. An increase in test temperature was found to have a detrimental influence on cyclic fatigue life of the transverse-oriented specimens and little influence on the longitudinally oriented counterparts. Under total strain-amplitude control, the alloy showed evidence of softening to failure at the two test temperatures. The degree of cyclic softening increased with increasing temperature. For both stress- and strain-amplitude control fatigue, the macroscopic fracture mode was essentially identical, regardless of the orientation of the test specimen with respect to the wrought rolled plate. Cyclic fatigue fracture on a microscopic scale revealed features reminiscent of locally ductile and brittle mechanisms. The plausible mechanisms governing damage and failure of the alloy are discussed in terms of the competing influences of intrinsic microstructural effects, matrix deformation characteristics arising from a combination of mechanical and microstructural interactions, the magnitude of cyclic stress and cyclic strain amplitudes and the test temperature.

Cyclic stress response characteristics of an aluminum–magnesium–silicon alloy

In this paper, the cyclic stress response characteristics of aluminum alloy 6061 is presented. Test specimens of the alloy were cyclically deformed using fully-reversed tension–compression loading under total strain amplitude control. The alloy showed evidence of initial hardening followed by progressive softening to failure. The stress response, deformation characteristics and fracture behavior of the alloy is interactively discussed in light of the competing influences of intrinsic microstructure contributions and deformation behavior arising from a combination of mechanical and microstructural contributions.

Mechanisms governing cyclic deformation and failure during elevated temperature fatigue of aluminum alloy 7055

The cyclic stress response, deformation and damage characteristics of aluminum alloy 7055 was studied at ambient and elevated temperatures. Specimens of the alloy were cyclically deformed using tension-compression loading under total strain-amplitude control. The alloy showed evidence of softening to failure at the ambient and elevated test temperatures. The degree of cyclic softening increased with an increase in test temperature. The presence of deformable matrix strengthening precipitates and particles in the T7751 microstructure resulted in a local decrease in resistance to the motion of “mobile” dislocations, causing a progressive loss of strengthening contributions to hardening. At the elevated temperatures, the occurrence of localized oxidation and embrittlement at the grain boundaries are promoted by the applied cyclic stress and play an important role in accelerating crack initiation and exacerbating stable crack propagation. The damage response of the alloy is discussed in terms of competing influences of intrinsic microstructural effects, matrix deformation characteristics arising from a combination of mechanical and microstructural interactions, cyclic plastic strain amplitude and concomitant response stress, and test temperature.