Variable amplitude cyclic deformation and fatigue behaviour of stainless steel 304L including step,periodic, and random loadings

This paper discusses cyclic deformation and fatigue behaviours of stainless steel 304L and aluminium 7075‐T6 under variable amplitude loading using strain‐controlled as well as load‐controlled tests. Load sequence effects were investigated in step tests with high‐low and low‐high sequences. For stainless steel 304L, strong hardening induced by the first step of the H‐L sequence significantly affects the fatigue behaviour, depending on the test control mode used. For periodic overload tests of stainless steel 304L, hardening due to the overloads was progressive throughout life and more significant than in H‐L step tests. For aluminium 7075‐T6, no effect on deformation behaviour was observed due to periodic overloads. However, the direction of the overloads was found to affect fatigue life, as tensile overloads led to longer lives, while compressive overloads led to shorter lives. Deformation and fatigue behaviours under random loading were also studied for the two materials. To correlate a broad range of fatigue life data for a material with strong deformation history effect, such as stainless steel, it is shown that a damage parameter with both stress and strain is required. The Smith‐Watson‐Topper parameter as such a parameter is shown to correlate the data reasonably well under different control modes and loading conditions.     

Low‐cycle fatigue life behaviour of BS 460B and BS B500B steel reinforcing bars

This paper investigates the low‐cycle fatigue resistance of BS 460B and BS B500B steel reinforcing bars and proposes models for predicting their fatigue life based on plastic‐strain (?_ap) and total‐strain (?_a) amplitudes. Constant‐amplitude, strain‐controlled low‐cycle fatigue tests were carried out on these bars under cyclic load with a frequency of 0.05 Hz. The maximum applied axial strain amplitude (?_s,max) ranges from 3 to 10% with zero and non‐zero mean strains. The strain ratios (R = ?_s,min/?_s,max) used are R =?1, ?0.5 and 0. Hysteresis loops were recorded and plastic and total strain amplitudes were related to the number of reversals (2N_f) to fatigue failure and models for predicting the number of reversals to fatigue failure were proposed. It is concluded that the predicted fatigue life of these bars is very accurate when compared with the measured experimental fatigue life results for wide range of values of strain ratios. It is also observed that based on plastic‐strain amplitude, BS B500B consistently has a longer life (higher number of cycles to failure) than those of BS 460B for all R values; however, at low plastic‐strain amplitudes they tend to behave similarly, irrespective of R value. Other observations and conclusions were also drawn.     

Dauerschwingfestigkeit und zyklisches Werkstoffverhalten

Fatigue Limit and Cyclic Material Behaviour Fatigue tests to determine the fatigue limit are very timeconsuming. Therefore a lot of possibilities have been proposed to estimate the fatigue limit, especially the so-called correlation formulae. This paper shows for carbon and low alloy steel, that the correlation between fatigue limit σ_W (unnotched specimen, tension-compression, mean-stress equal zero) and cyclic yield strength R′_p0.2 is better than between fatigue limit and static tensile strength R_m or static yield strength R_p0,2. An additional improvement of this correlation is possible by fixing the fatigue limit on the level of the cyclic stress for a plastic strain amplitude ε_{a, p} = 0,026%. These results are based on 24 test series taken from the data collection [7].     

In-plane anisotropy in low cycle fatigue properties of and bilinearity in Coffin-Manson plots for quaternary AI-Li-Cu-Mg 8090 alloy plate

Abstract

Low cycle fatigue (LCF) behaviour as a function of test direction was studied for a quaternary Al–Li–Cu–Mg alloy. The analysis of the variation in fatigue life with plastic strain amplitude ??_p/2 or with average stress amplitude ?σ/2 or with plastic strain energy per cycle ?W_p, revealed bilinear power law relationships in all the test directions. The transition strain values in the Coffin–Manson plots were seen to match closely with those obtained in the cyclic stress response as well as with the cyclic stress–strain relationships of the alloy. The observed bilinear behaviour in these LCF properties was attributable to a change in deformation as well as to the deformation assisted fracture mode. The alloy revealed significant in plane anisotropy in the LCF properties. The observed anisotropy was found to result from the combined effects of strong crystallographic texture and grain fibering.

MST/3016     

Fatigue crack growth with overloads/underloads: Interaction effects and surface roughness

The generalization of damage tolerance to variable amplitude fatigue is of prime importance in order to maintain the reliability of structures and mechanical components subjected to severe loading conditions. Engineering spectra usually contain overloads and underloads which distribution may not be random. However for predicting the life of a structure, a simplified spectrum is usually determined from the real one, in order to reduce testing periods on prototypes. Therefore it is thus important to know which cycles can contribute to crack growth and which can be neglected. This paper presents an analysis of fatigue crack growth on M (T) specimens made of a medium carbon steel DIN Ck45. The specimens are subjected to repeated blocks of cycles made up of one or several (1, 2, 6 or 10) overloads (or underloads) separated by a variable number (10, 1000 or 10 000) of baseline cycles. The main objective of this study is to better understand the mechanisms at the origin of interactions effects due to the presence of overloads (or underloads) at different locations of each block loading. Under constant amplitude loading, single variables ΔK and K_max are required in crack growth relationships. The transferability of fatigue laws, obtained under constant amplitude loading to variable amplitude fatigue, requires at least an additional variable, whose evolution with crack length accounts for the interactions effects between cycles of different types. Results have shown that the interaction effects in fatigue crack growth are closely related to the mechanisms of crack growth: cyclic plastic behaviour of the material and fracture surface roughness. Measurements of roughness of the surface fracture were carried out in both constant amplitude and variable amplitude tests. The roughness characterization helped to determine the importance of the mechanisms on variable amplitude fatigue crack growth and determine the influence of overloads/underloads on fatigue crack growth.     

Comparative analysis of fatigue life predictions in central notched specimens of Al–Mg–Si alloys

The fatigue behaviour of an Al–Mg–Si alloy was studied using notched specimens. Fatigue tests were conducted at two stress ratios R= 0 and R= 0.4 on thin plates with a central hole. Constant and block variable loading amplitudes were applied to the specimens using a servo‐hydraulic machine. The applicability of the local strain approach method to the prediction of the fatigue life was investigated for this type of discontinuity. Two methods, the equivalent strain energy density approach and a modified stress–strain intensity field approach, were used to predict the fatigue strength. For the second one an elastic–plastic finite element analysis was carried out in order to obtain the local strain and stress distributions near the notch root. Based on Miner's rule an equivalent stress was used to correlate the fatigue lives for the variable amplitude histories. The experimental results were compared with the predicted results obtained by the two methods investigated and better agreement was found with the stress–strain field intensity approach, while the strain energy approach gave more conservative results. Miner's rule gives a good correlation between the variable amplitude and constant amplitude results.     

ELEVATED TEMPERATURE FATIGUE IN Ni3Al-BASED ALLOYS

Both high-cycle and low-cycle fatigue properties of hot-extruded powders of a Ni₃Al-based alloy, IC218, have been evaluated. High cycle fatigue measurements were performed under stress controlled conditions at temperatures ranging from 25°C to 850°C. Tests were made in both laboratory air and vacuum environments. Low cycle fatigue tests were conducted under total strain control in a laboratory air environment at 650°C. In high cycle fatigue, high ratios of the fatigue limit (Δσ at 10⁶ cycles) to monotonic yield strength (σ_ys), of approximately Δσ/σ_ys～1, were obtained in the powder extruded IC218 alloy for temperatures ranging from 25°C to 650°C. In low cycle fatigue, a substantial decrease in fatigue life occurred at 650°C, compared to results obtained previously at 25°C. High cycle fatigue performance at low stress/strain amplitudes is better than expected when compared to precipitation strengthened superalloys. The improved performance is explained in terms of the cyclic hardening behavior of the alloy.     

Fatigue life prediction: A Continuum Damage Mechanics and Fracture Mechanics approach

Continuum Damage Mechanics (CDM) approach is used to predict crack initiation life and Fracture Mechanics approach predicts crack growth life. Strain controlled fatigue life of a ferrous alloy, EN 19 steel, has been determined using CDM and Fracture Mechanics approach. By combining these two approaches, life could be predicted with damage value in the material. All inputs required for the models have been determined by conducting monotonic, cyclic and fracture tests. Predicted life is also compared by conducting strain controlled fatigue tests. Predicted life in the strain amplitude range of 0.3–0.7% (fatigue life range of 10²–10⁵), compares well with the experimental results. All tests have been conducted at specimen level, stress ratio of −1 and at room temperature. The variation of crack initiation and crack propagation life with strain amplitude shows that maximum life is consumed by crack growth process at higher strain amplitude and at lower strain amplitudes, maximum life is spent for crack initiation process.     

Low‐cycle fatigue behavior of a newly developed cast aluminum alloy for automotive applications

The drive for increasing fuel efficiency and decreasing anthropogenic greenhouse effect via lightweighting leads to the development of several new Al alloys. The effect of Mn and Fe addition on the microstructure of Al‐Mg‐Si alloy in as‐cast condition was investigated. The mechanical properties including strain‐controlled low‐cycle fatigue characteristics were evaluated. The microstructure of the as‐cast alloy consisted of globular primary α‐Al phase and characteristic Mg₂Si‐containing eutectic structure, along with Al₈(Fe,Mn)₂Si particles randomly distributed in the matrix. Relative to several commercial alloys including A319 cast alloy, the present alloy exhibited superior tensile properties without trade‐off in elongation and improved fatigue life due to the unique microstructure with fine grains and random textures. The as‐cast alloy possessed yield stress, ultimate tensile strength, and elongation of about 185 MPa, 304 MPa, and 6.3%, respectively. The stress‐strain hysteresis loops were symmetrical and approximately followed Masing behavior. The fatigue life of the as‐cast alloy was attained to be higher than that of several commercial cast and wrought Al alloys. Cyclic hardening occurred at higher strain amplitudes from 0.3% to 0.8%, while cyclic stabilization sustained at lower strain amplitudes of ≤0.2%. Examination of fractured surfaces revealed that fatigue crack initiated from the specimen surface/near‐surface, and crack propagation occurred mainly in the formation of fatigue striations.     

Deformation und Versagen von StE460 und AlMg4,5Mn bei mehrachsig-proportionalen Beanspruchungen mit konstanten und variablen Amplituden

Deformation and failure behaviour of FeE460 and AlMg4.5Mn under multiaxial proportional loading with constant and variable amplitudes To calculate the fatigue life-to-crack initiation of engineering components under combined cyclic loading, experimentally secured knowledge on the cyclic deformation and failure behaviour of the materials used under the certain multiaxial cyclic stress and strain conditions are required. To obtain this, strain-controlled fully reversed experimental tests at tensional, torsional and combined loading with constant and variable amplitudes have been conducted using thin-walled tube specimens of FeE460 and AlMg4.5Mn. Experimental tests on standard uniaxially loaded hourglass specimens have also been conducted to study specimen form effects. Cyclic deformation behaviour can be uniformly described by the stabilised cyclic σ-ε-curve, if stresses and strains are expressed as equivalent values according to the von Mises criterion. Failure behaviour at constant and variable amplitude loading is characterized by the initiation and growth of short cracks at right angle to the direction of the greatest principal stress (mode I) in the case of tensional or combined loading and by short crack growing in both shear stress directions (mode II+III) in the case of torsional loading. At fully reversed constant amplitude loading, all three types of load can be described by one constant amplitude strain life-to-crack initiation curve. At variable amplitude loading (notch strain simulation with gaussian spectrum, H₀=10⁵), the experimental fatigue life-to-crack initiation values are lower than estimated values based on Miner-calculations using an equivalent stress-strain supported P_SWT-N-curve. The question of mean stresses and their evaluation is discussed.     

Effect of strain ratio on cyclic deformation and fatigue of extruded AZ61A magnesium alloy

Strain-controlled fatigue experiments were conducted on an extruded AZ61A magnesium alloy at three strain ratios (R_ɛ = −∞, −1, 0) using smooth tubular specimens. As the strain ratio decreased, stronger cyclic hardening, more asymmetric hysteresis loop, smaller stress amplitude, lower mean stress, and higher initial plastic strain amplitude were observed. These phenomena were associated with twinning in the compressive phase and detwinning in the tensile phase during cyclic deformation. At the same strain amplitude, fatigue life increased with decreasing strain ratio. The strain-fatigue life curve at each strain ratio exhibited a distinguishable kink. Such a kink point represents a demarcation point above which persistent twinning–detwinning occurs under cyclic loading. Two Smith, Watson, and Topper (SWT) fatigue criteria can predict the fatigue lives of the material at all strain ratios satisfactorily.     

Fatigue Behavior of Ti-6Al-4V

Low-cycle-fatigue texts in vacuum and air were performed. Under cyclic loading the Ti-6Al-4V showed both cyclic hardening and cyclic softening depending on heat treatment, stress amplitude, and microstructure. Plastic deformation of the β-phase in the unaged condition due to stress induced martensitic transformation caused cyclic hardening. Cyclic softening was observed if the α-phase hardened by coherent Ti₃Al particles was plastically deformed. Equiaxed microstructures exhibited a stronger cyclic softening than lamellar structures. This behavior could be explained by the pronounced texture of the equiaxed microstructures, whereas the lamellar structures were texture-free. The fatigue life was influenced by the cyclic softening process mainly in the low-cycle-fatigue regime. The fatigue life at normalized stress amplitude (σ_a/σ_y) was shorter for microstructures with strong cyclic softening as compared to microstructures with lower cyclic softening.     

Experimental results and fatigue life evaluation of magnesium laserbeam‐welded joints under proportional and non‐proportional multiaxial fatigue loading with variable amplitudes

Fatigue life of magnesium laserbeam‐welds (AZ31 and AZ61 alloys) was assessed experimentally under variable amplitude loadings. The specimens were subjected to load‐controlled cyclic loadings. The tests were carried out using a Gauss‐distributed amplitude sequence of length L_S = 5 · 10⁴ cycles and loading ratio R = –1 under pure axial, pure torsion as well as in‐phase and out‐of‐phase combined loadings. The notch stresses were obtained from a linear‐elastic FE‐model using the reference radius approach with r_ref  = 0.05 mm. The stress‐based hypotheses were applied: Effective equivalent stress hypothesis (EESH), shear stress intensity hypothesis (SIH), Findley, and modified Gough‐Pollard. A non‐proportionality factor is introduced and steps required for computing are presented in order to improve fatigue life assessment under non‐proportional loadings.     

Loading sequence effect on fatigue life of Type 316 stainless steel

The change in fatigue life due to variable cyclic loading was investigated experimentally in order to consider the loading sequence effect in fatigue damage assessment for a component design, and the reason for the change was discussed. Strain-controlled fatigue tests, that is, two-step, surface removal two-step, repeating two-step and periodical overload tests were conducted using Type 316 stainless steel specimen in a room temperature laboratory environment. The high-low loading amplitude sequence for the two-step test, and the repeating two-step and periodical overload tests showed a shorter fatigue life than that predicted by the linear damage accumulation rule. On the other hand, the low–high loading amplitude sequence for the two-step test exhibited a longer fatigue life. The reduction in the fatigue life was mainly attributed to the change in effective strain amplitude. The fatigue life reduction due to the loading sequence effect could be assessed conservatively by determining the allowable number of cycles for effective stress amplitude. Namely, by assuming the crack mouth was fully opened in the assessment, predicted fatigue life became shorter than the experimental results. It was concluded that the margin of 1.3–2.3 should be considered in the design fatigue curve in order to take account of the reduction in fatigue life due to the loading sequence effect.     

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

The 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 and high‐cycle fatigue properties of an ultrahigh‐strength TRIP bainitic steel

The scope of this study is to characterize the mechanical properties of a novel Transformation‐Induced Plasticity bainitic steel grade TBC700Y980T. For this purpose, tensile tests are carried out with loading direction 0, 45 and 90° with respect to the L rolling direction. Yield stress is found to be higher than 700 MPa, ultimate tensile strength larger than 1050 MPa and total elongation higher than 15%. Low‐cycle fatigue (LCF) tests are carried out under fully reverse axial strain exploring fatigue lives comprised between 10² and 10⁵ fatigue cycles. The data are used to determine the parameters of the Coffin–Manson as well as the cyclic stress–strain curve. No significant stress‐induced austenite transformation is detected. The high‐cycle fatigue (HCF) behaviour is investigated through load controlled axial tests exploring fatigue tests up to 5 × 10⁶ fatigue cycles at two loading ratios, namely R = ?1 and R = 0. At fatigue lives longer than 2 × 10⁵ cycles, the strain life curve determined from LCF tests tends to greatly underestimate the HCF resistance of the material. Apparently, the HCF behaviour of this material cannot be extrapolated from LCF tests, as different damage, cyclic hardening mechanisms and microstructural conditions are involved. In particular, in the HCF regime, the predominant damage mechanism is nucleation of fatigue cracks in the vicinity of oxide inclusions, whereby mean value and scatter in fatigue limit are directly correlated to the dimension of these inclusions.     

Effect of Sc addition on low-cycle fatigue properties of extruded Al-Zn-Mg-Cu-Zr alloy

ABSTRACT

The influence of minor Sc addition on the low-cycle fatigue (LCF) properties of hot-extruded Al-Zn-Mg-Cu-Zr alloy with T6 state was investigated through performing the LCF tests at room temperature and air environment. The results indicate that two alloys show cyclic stabilisation, cyclic hardening and cyclic softening during fatigue deformation. The addition of Sc can significantly enhance the cyclic stress amplitude of the alloy. Al-Zn-Mg-Cu-Zr-Sc alloy shows higher fatigue lives at lower strain amplitudes, while has lower fatigue lives at higher strain amplitudes. For the two alloys, the density and movability of dislocations are related to the change of cyclic stress amplitudes. The existence of Al₃(Sc,Zr) phase can inhibit the appearance of cyclic softening phenomenon in the Al-Zn-Mg-Cu-Zr-Sc alloy.     

A new model for describing a stable cyclic stress–strain relationship under non-proportional loading based on activation state of slip systems

This paper proposes a new simple model for cyclic incremental plasticity based on activation states of slip systems describing stable cyclic stress–strain relationships under non‐proportional loading. In the model, the magnitude and the direction of incremental plastic strain are estimated by (1+αf_NP) and Q , respectively. Here, α is the constant related to the dependence of material on additional hardening and f_NP the intensity factor expressing the severity of non‐proportional loading. Q is the second‐order tensor describing the activation states of slip systems in polycrystalline metals and is given by the calculation using a virtual specimen. The model was examined by application to the prediction of the stable cyclic stress–strain relationship in extensive non‐proportional low cycle fatigue tests for type 304 stainless steel and 6061 aluminium alloy. The simulated results showed that the model gave a satisfactory prediction of the stable cyclic stress–strain relationship under complex non‐proportional multiaxial loadings for the two materials.     

The influence of yield strength and negative stress ratio on fatigue crack growth delay in 4140 steel

Single tensile overloads were applied to AISI 4140 compact steel specimens with three yield strengths of 110, 165 and 205 ksi and three stress ratios R of 0, –1/2, and –1. Fatigue crack growth delay was significantly longer for the low yield strength steel with R=0. However, at R=–1 only slight differences in delay occurred for all three yield strengths. Thus delay from single tensile overloads with R0 can be quite misleading when compared with negative stress ratio delay results. This implies that small compressive cyclic stresses, which often have only a small influence on constant amplitude crack growth behavior, may have significant influence in spectrum loading due to reduction of delay from high tensile overloads. This reduction in delay may be attributed to cyclic relaxation of both crack region residual stresses and crack closure. Under constant amplitude testing, fatigue crack growth rates were more dependent upon yield strength than upon R ratio.