Axial Strain Control Research Articles

Low cycle fatigue behavior of JLF-1 steel at elevated temperatures

Reduced activation ferritic/martensitic (RAF/M) steels have been developed for structural application in potential fusion energy systems. In this paper, the low cycle fatigue behavior of JLF-1 RAF/M steel at 673 K in vacuum condition under fully reversed push–pull triangular wave with an axial strain control were studied using engineering-sized round bar specimens with 8 mm in diameter. Total strain range is selected from 0.6% to 2.4% at 673 K, and the strain rate was 0.1%/s. The fatigue life at 673 K is almost the same as that at room temperature when the life is plotted against the total strain range. JLF-1 steel demonstrates monotonic cyclic softening at 673 K. Serration was observed on the stress–strain hysteresis curve, which is attributed to the dynamic strain ageing (DSA) effect.

Low-cycle fatigue behavior of HAYNES ® HR-120 ® alloy

Low-cycle fatigue behavior of the HAYNES HR-120 alloy in the temperature range from 24°C to 982°C and total strain range from 0.4% to 2.3% was investigated under an axial total strain control mode in laboratory air. It was noted that increasing temperature generally led to a substantial decrease in the fatigue life of the alloy. It was found that the alloy could exhibit cyclic strain hardening or softening, which was closely related to the imposed total strain range and testing temperature. The microstructural evolution due to low-cycle fatigue deformation was characterized using optical, scanning-electron, and transmission-electron microscopy. It was observed that the precipitation of second-phase particles would occur at or above 761°C. The presence of these precipitates could be taken into account the greater the cyclic-hardening behavior and the reduction of fatigue life, with increasing temperature. In addition, strain fatigue parameters were determined at different temperatures, based on Coffin–Manson and Holloman equations.

Low cycle fatigue properties of a low activation ferritic steel (JLF-1) at room temperature

To investigate fatigue properties of a low activation ferritic steel (9Cr–2W steel, JLF-1), low cycle fatigue tests were performed in air at room temperature under axial strain control for a complete push–pull condition. The strain rate was 0.4% s−1. Cyclic strain-hardening was observed within the initial 20 cycles, and then cyclic strain-softening occurred gradually until the final failure, though the plastic strain range did not change significantly. Tensile peak stresses in hysteresis curves measured at around half the number of cycles to failure depended on the total strain range. The drop in the peak stress by the cyclic strain-softening increased with decreasing total strain range. The regression curve of the total strain range against the fatigue life was formulated using the Manson–Coffin equation and the fatigue life of JLF-1 steel was compared with that of 8Cr–2W steel.

An assessment of three creep–fatigue life prediction methods for nickel‐based superalloy GH4049

High‐temperature low‐cycle fatigue tests with and without a 10‐s strain hold period in a cycle were performed on a nickel base superalloy GH4049 under a fully reversed axial total strain control mode. Three creep–fatigue life prediction methods are chosen to analyse the experimental data. These methods are the linear damage summation method (LDS), the strain range partitioning method (SRP) and the strain energy partitioning method (SEP). Their ability to predict creep‐fatigue lives of GH4049 at 700, 800 and 850 °C has been evaluated. It is found that the SEP method shows an advantage over the SRP method for all the tests under consideration. At 850 °C, the LDS and SEP methods give a more satisfactory prediction for creep–fatigue lives. At the temperatures of 700 and 800 °C, the SRP and SEP methods can correlate the life data better than the LDS method. In addition, the differences in predictive ability of these methods have also been analysed. The scanning electron microscopy (SEM) examination of fracture surfaces reveals that under creep–fatigue test conditions crack initiation mode is transgranular, while crack propagation mode is either intergranular plus transgranular or entirely intergranular, dependent on test temperature.

Room-temperature low-cycle fatigue behaviour of Ti-27Al-15Nb alloy

Low-cycle fatigue (LCF) behaviour of the alloy Ti-27Al-15Nb, in (α 2+B2) heat-treated condition was studied in total axial strain control mode at different total strain amplitudes (Δɛ 1/2) from ±0·65 to ±1·0% and room temperature. While there was little hardening of the material at the lowest strain amplitude (Δɛ t/2: ±0·65%), pronounced hardening was observed at the higher strain amplitudes (Δɛ t/2⩾0·83%). The cyclic stress, at high strain amplitudes, continuously increased from the beginning till fracture of the specimens. The LCF resistance of the material was found to be low and this was due to its poor ductility at room temperature. Dual slope was observed in the Coffin-Manson plot, with less slope of the upper segment than that of the lower one, as observed in several alloys. The fracture behaviour pointed to brittleness and faceted features were observed.

A review of fatigue crack growth in steels under mixed mode I and II loading: Bold, P. E., Brown, M. W. and Allen, R. J. Fatigue Fract. Eng. Mater. Struct. (Oct. 1992 15 (10) 965–977

The effects of hold condition (tension-only, compression-only and tension-plus-compression holds of 1 min duration) and hold time (up to 10 min in tension) on the low-cycle fatigue behaviour of type 304 stainless steel base metal and type 308 stainless steel all-weld metal were investigated at 923 K. All the tests were performed in total axial strain control in air, employing a strain range of 1.0%. The welds were prepared by a submerged metal arc welding process. The microstructure of the base metal and the weld consisted of gamma phase and duplex gamma-delta ferrite respectively. The results clearly indicated a reduction in continuous cycling as well as creep-fatigue interaction life of type 308 SS weld metal compared with type 304 SS base metal under identical testing conditions. Hold times displayed an effect on life that was dependent not only on material condition but also on the position of the hold in the cycle. Fatigue lives recorded for type 308 SS weld in the 1 min hold-time tests were in the order: compression hold → continuous cycling → tension-plus-compression hold → tension hold. Fatique lives of 304 SS were in the order: continuous cycling → compression hold → tension-plus-compression hold → tension hold. A significant reduction in the life of the weld metal was noted on increasing the duration of tension hold time to 10 min. 308 SS weld metal exhibited cyclic softening whereas 304 SS base metal showed rapid initial hardening followed by a saturation stage. The observed variations in life are explained on the basis of crack initiation and propagation modes and microstructural changes that occured during low-cycle fatigue and creep-fatigue interaction testing.

Low cycle fatigue damage in nickel-base superalloy single crystals at elevated temperature

Low cycle fatigue tests on AM1 nickel-base superalloy single crystals were conducted under axial strain control at 650, 950 and 1100 °C. The behaviour of the [001] orientation was investigated at the three temperatures, that of the [111], [101] and [213] specimens was studied at the two lower temperatures. The orientation dependence of fatigue life-total strain range curves was mainly due to variations in Young's modulus with orientation. Most cracks grow in stege II mode whatever the temperature. Cracks nucleate at micropores and in the interior of specimens at low temperatures; surface cracks induced by oxidation are dominant at high temperatures and low strain ranges. Most of fatigue life is spent in microcrack growth.

Creep-fatigue interaction behaviour of type 308 stainless steel weld metal and type 304 stainless steel base metal

The effects of hold condition (tension-only, compression-only and tension-plus-compression holds of 1 min duration) and hold time (up to 10 min in tension) on the low-cycle fatigue behaviour of type 304 stainless steel base metal and type 308 stainless steel all-weld metal were investigated at 923 K. All the tests were performed in total axial strain control in air, employing a strain range of 1.0%. The welds were prepared by a submerged metal arc welding process. The microstructure of the base metal and the weld consisted of gamma phase and duplex gamma-delta ferrite respectively. The results clearly indicated a reduction in continuous cycling as well as creep-fatigue interaction life of type 308 SS weld metal compared with type 304 SS base metal under identical testing conditions. Hold times displayed an effect on life that was dependent not only on material condition but also on the position of the hold in the cycle. Fatigue lives recorded for type 308 SS weld in the 1 min hold-time tests were in the order: compression hold → continuous cycling → tension-plus-compression hold → tension hold. Fatique lives of 304 SS were in the order: continuous cycling → compression hold → tension-plus-compression hold → tension hold. A significant reduction in the life of the weld metal was noted on increasing the duration of tension hold time to 10 min. 308 SS weld metal exhibited cyclic softening whereas 304 SS base metal showed rapid initial hardening followed by a saturation stage. The observed variations in life are explained on the basis of crack initiation and propagation modes and microstructural changes that occured during low-cycle fatigue and creep-fatigue interaction testing.

Influence of orientation on the low cycle fatigue of MAR-M 200 single crystals at 650 °C II: Cyclic stress-strain behaviour

The cyclic stress-strain behaviour of MAR-M 200 single crystals was studied at 650 °C (923 K) under total axial strain control, using either low cycle fatigue tests under constant-strain-range or cyclic tests conducted under stepwise-increasing strain. Specimens having crystallographic orientation near [001], [111], [213] and [101] were tested under fully reversed strain. The cyclic stress-strain curves were found to be strongly orientation dependent and plastic strains were accommodated mainly by slip bands above a critical plastic strain threshold. Cyclic shear stress-strain curves for [111]〈1 1 0〉 slip exhibit a significant orientation dependence. Mean stresses were found to occur, the sign of which depends upon the crystallographic orientation. The nature of slip systems was analysed using slip band markings on the gauge length as well as accurate measurements of cross-section of tested specimens. The cyclic behaviour was then discussed using these observations and previous experimental data on γ-γ′ alloys.

Influence of orientation on the low cycle fatigue of MAR-M 200 single crystals at 650 °C I: Fatigue life behaviour

Low cycle fatigue tests on MAR-M 200 single crystals were conducted at 650 °C under total axial strain control. Specimens having crystallographic orientations near [001], [111], [213] and [101] were tested under fully reversed strain. Fatigue life-total strain range curves were strongly orientation dependent but this behaviour was mainly due to variations in Young's modulus with orientation. Crack initiation occurs at MC particles at high plastic strain ranges where slip bands formed in the overall specimen gauge length. At longer lives, cracks initiate at subsurface micropores. The most part of fatigue life is spent in stage II crack propagation as shown by plastic replicas and metallographic observations of specimens. Fatigue life was thus predicted using a local approach to fracture and crack growth data from conventional compact tension fracture mechanics specimens.

Influence of neutron irradiation at 430°C on the fatigue properties of SA 316L steel

Abstract Fatigue tests have been carried out at 430°C on hour-glass shaped specimens of the CEC reference heat of SA 316L stainless steel. The tests were performed under constant total axial strain control with a triangular fully reversed wave shape at frequencies of 0.5, 0.05 and 0.005 Hz. Specimens irradiated at 430°C to doses of 9–12 dpa and helium contents of 80 to 145 appm showed a fatigue life reduction by about a factor of two, compared to unirradiated specimens. The cyclic stress is found to be strongly increased by the irradiation. The test frequency influences the fatigue hardening slightly but it does not affect the fatigue life.

Cryogenic Fatigue Properties of 304L and 316L Stainless Steels Compared to Mechanical Strength and Increasing Magnetic Permeability

Abstract For superconducting magnet applications, the 4 K fatigue life curves of the cryogenic structural materials 304L and 316L stainless steel were obtained using axial-strain control conditions. These curves were compared with others at 77 and 300 K and with data for other 300-series stainless steels in the literature. Increasing magnetic permeability with monotonic tensile and cyclic straining, due to strain-induced martensitic transformation, was also investigated, since 304L and 316L are metastable austenitic stainless steels. 4 K fatigue life curves relating magnetic permeability effects were obtained. Fatigue tests relating magnetic permeability at 77 and 300 K were also conducted for comparison. Finally, a cryogenic fatigue design criterion for preventing mechanical failure and increasing magnetic permeability was considered.

Variation in ratio of diametral strain to axial strain during multistep cycling and cycling with compression hold of Cr–Mo–V steel at elevated temperature

Cr–Mo–V rotor steel has been studied using multistep tests, in axial strain control using steps of increasing amplitude, with and without holds in tension, and in one constant amplitude test with holds in compression. The results of the multistep tests confirm that the variation with cycles in the ratio of diametral strain to axial strain (a ‘Poisson's ratio’ in low-cycle fatigue) is insignificant at low strain amplitudes, but considerable at higher amplitudes. The results emphasize particularly the decline of this ratio at high amplitude in the early stages of tests with or without tensile holds. The absence of this decline in the test with compression demonstrates that the decline in the ratio for other waveforms is induced by damage peculiar to tensile deformation mechanisms, which eventually lead to easily observable cavitation.MST/204

Changes in morphology and composition of carbides during cyclic deformation at room and elevated temperature and their effect on mechanical properties of CrMoV steel

Cr-Mo-V rotor steel has been cycled in total axial strain control at room and elevated temperature with and without holds in tension. We have studied by TEM/EDS techniques the changes in carbide morphology and composition produced by these exposures. The carbide morphology has been found to change, even at room temperature, from anisotropic morphology to more equiaxed forms, and the compositions of the carbides have become enriched with Mo and V, at rates which are orders of magnitude greater than would be observed in conventional tempering processes. Naturally the effects of high temperature and tests with holds have been to accelerate these changes. This applies particularly to the redistribution of Mo which can be associated with a possible fatigue-induced decline in ductility. One explanation of this behavior is the elimination of the impurity-scavenging action of the Mo as it is absorbed into the previously existing carbides and as it forms new carbides.

An Experimental Technique for Obtaining Complete Stress-Strain Curves for High Strength Concrete

Abstract For a rational design of concrete structures, it is necessary to know both the ascending and the descending parts of the stress-strain curve of concrete subjected to uniaxial compression. It is difficult to determine the descending part experimentally because of the interaction between the testing system and the specimen, especially for concrete of high compressive strength. The influence of the testing machine depends on the stiffness of the machine, the frequency response of the system, the length and diameter of the specimen, the strength of concrete, the rate of loading, and the inherent brittleness of the material. For a closed-loop testing machine, the specimen-machine interaction also depends on the parameter chosen to control the loading. It is shown that the descending part of the curve can be obtained by controlling the circumferential strain even when the axial strain control produces an unstable failure. A method of testing specimens at a controlled rate of circumferential strain increase is described. Stress-strain curves of concrete subjected to uniaxial compression using both the axial and the lateral strain as a control were compared for compressive strengths ranging between 70 to 90 MPa (10 000 to 13 000 psi).

A Procedure for Strain-Controlled High-Cycle Fatigue Testing at Elevated Temperatures

Abstract A procedure for conducting high-cycle fatigue tests under indirect axial strain control is presented. Stroke control is maintained throughout the tests. However, the frequency of cycling is periodically reduced so that the axial strain can be measured directly, and the stroke signal is adjusted to correct any drift in the axial strain signal. Tests have been conducted to demonstrate the viability of such a procedure.