Low-cycle impact fatigue of mild steel and austenitic stainless steel

A study has been made of the low-cycle impact fatigue behaviours of low-carbon steel and austenitic stainless steel including cyclic stress-strain behaviour, cyclic hardening and fatigue life. Similarities and dissimilarities in cyclic deformation and fatigue fracture between low-cycle impact fatigue and ordinary low-cycle fatigue of the materials are discussed. This gives evidence for the presence of a considerable strain rate effect in low-cycle fatigue.     

TENSILE AND LCF PROPERTIES OF AISI 316LN SS AT 300 AND 77 K

Tensile and low-cycle fatigue tests were performed on a 316LN austenitic stainless steel at 300 and 77  K. The tensile and low-cycle fatigue properties were obtained and analysed in terms of the influence of temperature on the plastic deformation process and the formation of strain-induced martensite. The martensite content was evaluated using measurements of magnetic saturation. No α'-martensite was detected at 300  K under either monotonic or cyclic straining. On the contrary, at 77  K, strain-induced martensitic transformation is responsible for the higher elongation in tension and the secondary hardening observed on hardening/softening curves in low-cycle fatigue. The induced martensite content in tensile tests is a function of strain which deviates from Angel's model. In low-cycle fatigue, it is a function of the strain level and the accumulated plastic strain. At a given total strain amplitude, the decrease of temperature from 300 to 77  K results in the decrease of plastic strain amplitude and homogenization of plastic strain distribution, and thus in the prolongation of fatigue life. The cyclic over-stress at 77  K, due to an intermediate ageing at 300  K, is related to pinning of initially free dislocations resulting from nitrogen diffusion during isothermal holding at room temperature. This results in a reduced fatigue life.     

A Method for the Evaluation of Fatigue Life under Nonproportional Low-Cycle Loading

We consider a method for the evaluation of fatigue life under multiaxial nonproportional low-cycle loading based on the concept of equivalent strains. The expression for the equivalent strain range is a function only of the strain path and contains a constant depending on the additional hardening of the material under nonproportional loading. We propose a new parameter of the material based on the work of plastic strains in a cycle. This parameter is universal when applied to materials with both low and high degrees of additional hardening. It is in good agreement with the results of testing of 08Kh18N10T stainless steel and VT9 titanium alloy under nonproportional low-cycle loading.     

LOW-CYCLE FATIGUE UNDER NON-PROPORTIONAL LOADING

A series of strain-controlled, low-cycle fatigue experiments have been conducted on 42CrMo steel under various loading paths including circular, square, cruciform, and rectangular paths. Present experiments have shown that there is additional hardening under non-proportional cyclic loading. Non-proportional cyclic additional hardening also results in a shorter life for multiaxial low cycle fatigue. A non-proportionality measure of strain path based on both a physical basis and macromechanical phenomena is proposed. The loading path effect on additional hardening is also described well. Low-cycle fatigue damage accumulation and the evolution process under non-proportional loading is analysed via the Continuum Damage Mechanics Model of Chaboche. A non-proportinality measure is introduced in the damage evolution equation and a modified Coffin-Manson type formula is derived. A novel fatigue life prediction approach based on the critical-plane concept of Brown and Miller is proposed.     

Stress–strain time-dependent behavior of A356.0 aluminum alloy subjected to cyclic thermal and mechanical loadings

This article presents the cyclic behavior of the A356.0 aluminum alloy under low-cycle fatigue (or isothermal) and thermo-mechanical fatigue loadings. Since the thermo-mechanical fatigue (TMF) test is time consuming and has high costs in comparison to low-cycle fatigue (LCF) tests, the purpose of this research is to use LCF test results to predict the TMF behavior of the material. A time-independent model, considering the combined nonlinear isotropic/kinematic hardening law, was used to predict the TMF behavior of the material. Material constants of this model were calibrated based on room-temperature and high-temperature low-cycle fatigue tests. The nonlinear isotropic/kinematic hardening law could accurately estimate the stress–strain hysteresis loop for the LCF condition; however, for the out-of-phase TMF, the condition could not predict properly the stress value due to the strain rate effect. Therefore, a two-layer visco-plastic model and also the Johnson–Cook law were applied to improve the estimation of the stress–strain hysteresis loop. Related finite element results based on the two-layer visco-plastic model demonstrated a good agreement with experimental TMF data of the A356.0 alloy.     

含Zr、Sc的Al-Zn-Mg-Cu合金的低周疲劳行为

利用透射电子显微镜和低周疲劳试验机研究了单级时效状态及回归再时效状态两种含Zr、Sc的Al-Zn-Mg-Cu合金的微观组织和低周疲劳性能。结果表明:单级时效基体析出相以η′相为主,晶界析出连续分布平衡相,并伴有晶间无析出带;回归再时效基体析出相略有长大,晶界析出相长大明显,无析出带变宽。低周疲劳加载条件下,合金在0.4%~0.7%外加总应变幅范围内表现出循环稳定性;在0.8%的应变幅下,呈现先软化后硬化。在0.4%~0.6%较低的外加总应变幅范围内,回归再时效合金表现出较高的低周疲劳寿命。两种状态合金的塑性应变幅和弹性应变幅与载荷反向周次之间均成直线关系,并可分别用Coffin-Manson公式和Basquin公式来描述。两种状态的合金的疲劳裂纹均萌生于试样表面,并以穿晶方式扩展。     

低周疲劳变形过程中Fe-33Mn-4Si合金钢的微观组织演变EI北大核心CSCD

借助X射线衍射和电子背散射衍射,研究低周疲劳变形过程中Fe-33Mn-4Si合金钢的微观组织演变及其对力学行为的影响。结果表明:实验用钢的原始微观组织由奥氏体和热诱发ε马氏体两相组成。原始组织通过影响变形过程中ε马氏体相变来影响实验用钢的低周疲劳变形行为。在变形初期(100周次内),随循环周次增加,ε马氏体含量迅速增加并且马氏体不同变体之间频繁相互交叉作用,使实验用钢的平均峰值应力和循环加工硬化程度快速增加;随后至疲劳断裂,ε马氏体成为变形微观组织中主要组成相,ε马氏体含量和马氏体不同变体的交叉频次随循环周次的增加而增速放缓,导致平均峰值应力和循环加工硬化程度的增速也明显减缓。     

Low-cycle fatigue life of SiC-particulate-reinforced Al-Si cast alloy composites with tensile mean strain effects

The low-cycle fatigue behaviour of a SiC-particulate-reinforced Al-Si cast alloy with two different volume fractions has been investigated under strain-controlled conditions with and without tensile mean strains. The composites and the unreinforced matrix alloy showed cyclic hardening behaviour. The composite having a higher volume fraction of the SiC particles exhibited a more pronounced strain-hardening rate. For the tensile mean strain tests, the initial high tensile mean stress relaxed to zero for the ductile Al-Si alloy, resulting in no influence of the tensile mean strain on the fatigue life of the matrix alloy. However, tensile mean strain for the composite caused tensile mean stresses and reduced the fatigue life. The pronounced effects of mean strain on the low-cycle fatigue life of the composite compared to the unreinforced matrix alloy were attributed to the initial large prestrain causing non-relaxing high tensile mean stress in the composite with limited ductility and cyclic plasticity. Fatigue damage parameter using strain energy density accounted for the mean stress effects quite satisfactorily. Predicted fatigue life using this damage parameter correlated fairly well with the experimental life within a factor of 3. Moreover, the fatigue damage parameter indicated the inferior life in the low-cycle regime and superior life in the high-cycle regime for the composite, compared to the unreinforced matrix alloy.     

Low-cycle fatigue behavior of AA2618-T61 forged disk

The cyclic stress–strain response and the low-cycle fatigue life behavior of an aluminum alloy AA2618-T61 forged disk were studied. Fully reversed strain-controlled tests were performed at 200 °C in air at a constant total strain rate and under the total strain ranges of 0.5–0.9%. Specimens cut from longitudinal direction of disk displayed cyclic hardening or softening which was dependent on the total strain range. The variation of low-cycle fatigue life with plastic strain amplitude followed a single-slope Coffin–Manson power-law relationship. Fracture of the samples was predominantly ductile fracture of high density microdimples.     

Deformation and Life of Titanium Alloy VT9 under Conditions of Nonproportional Low-Cycle Loading

Results of experimental investigation of low-cycle fatigue of titanium alloy VT9 under nonproportional loading are presented. Tensile-torsional low-cycle fatigue tests were carried out on thin-walled tubular specimens under proportional and nonproportional cyclic deformation at room temperature. It has been found that cyclic hardening in a steady state at the same value of the equivalent von Mises strain range is the same in circular cycles and in torsion (or tension–compression) cycles. In the case of nonproportional loading of specimens, the life of titanium alloy VT9 shortens in comparison with proportional loading, but not as much as for stainless steels.     

Effectiveness of Using Cycle Nonproportionality Coefficients in the Determination of Strain Hardening

The effectiveness of predicting the strain hardening of metallic materials at low-cycle fatigue under nonproportional biaxial loading conditions has been analyzed. The cycle nonproportionality coefficient, which characterizes the geometry of a deformation cycle path, is used as the determining parameter. A comparative analysis of the effectiveness of using the most known nonproportionality coefficients to determine the levels of additional hardening of the material in the state of stabilization of its cyclic properties has been carried out.     

Understanding the influence of particle size on strain versus fatigue life,and fracture behavior of aluminum alloy composites produced by spray deposition

The strain versus fatigue life and fracture behavior of spray-formed Al–Si composites reinforced with SiC particles of two different sizes were studied under total strain amplitudes. Both composites exhibit short low-cycle fatigue (LCF) which follows a Coffin-Manson relationship, and display cyclic hardening at all strain amplitudes. The LCF endurance of the composite with large particles is higher than that of composite containing small particles in the high strain amplitudes, however, at low strains the difference in fatigue endurance between the two composites decreased. Moreover, the decrease in particle size results in a higher degree of hardening at low and middle strains, but reduces the magnitude of hardening at highest strain. Fractographic analysis reveals that particle/matrix debonding is the main mechanism of failure in composite with small particles, while fracture and debonding of SiC particle are predominant in the large particle reinforced composite.     

Kurzzeitschwingfestigkeit von duktilen Stählen unter mehrachsiger Beanspruchung

Low-Cycle Fatigue of Ductile Steels under Multiaxial Deformations To investigate the fatigue behaviour of cyclically softening and hardening steels under multiaxial elastic-plastic strains, axial strain and shear strain controlled fatigue tests under constant amplitude loading were carried out. S-N curves under axial strain and torsional pure shear as well as under combined axial strain and shear, in and out of phase, were obtained for the cyclically softening tempered steel 30 CrNiMo 8 (similar to AlSI-Type 4340) and the cyclically hardening quenched stainless steel X 10 CrNiTi 189 (AISI-Type 321) in the region of low-cycle fatigue. For both steels, used in the design of vessels, pipings, shafts, etc. the fatigue life to crack initiation is reduced by an out of phase (δ = 90°) shearing of the strained specimens in comparison to the in phase loading. The decrease of fatigue life under out of phase strains is caused by changing direction of principal strains resulting in an interaction of the deformations in all directions of the surface. This interaction is taken into account by a calculation procedure deriving an equivalent strain and predicting the fatigue life under combined strain on the base of S-N curves for unaxial strain.     

Effect of surface roughness on low-cycle fatigue life of Cr---Mo---V steel at 550 °C

Low-cycle fatigue tests have been carried out on Cr---Mo---V steel specimens with two different modes of surface roughness at 550 °C. The fatigue life of specimens with a rough surface is approximately half of that of specimens with a smooth surface over the plastic strain range investigated. From interrupted tests, it is seen that low-cycle fatigue behaviour is largely influenced by the crack initiation process even for a high strain range and the decrease of fatigue life in specimens with a rough surface is mainly due to a reduction in the number of cycles for crack initiation.     

Cyclic stress response, strain resistance and fracture behavior of modified 1070 steel

A combined experimental and finite element study of fatigue crack closure in modified 1070 steel has been conducted. In this paper, the material property evaluations required for this study are presented. The monotonic and cyclic stress-strain properties, cyclic stress response, cyclic strain resistance, low cycle fatigue life and fracture behavior are examined. The low cycle fatigue tests were conducted using tension-compression cycling, under total strain amplitude control, over a wide range of strain levels. The material was found to possess medium strength and high ductility; while displaying a strain level dependent combination of cyclic strain softening and hardening behavior. The observed softening behavior is attributed to the rearrangement of dislocations produced by processing, formation of slip bands on the specimen surface and the formation of microcracks. The observed hardening behavior is ascribed to contributions from synergistic influences of dislocation multiplication, dislocation-dislocation interactions and dislocation-microstructural feature interactions. The material followed the strain-life relationships attributed to Basquin and Coffin-Manson. The fracture surfaces of the fatigue specimens showed distinct regions of crack initiation, microscopic-macroscopic crack growth and sudden fracture. The low-cycle fatigue characteristics and fracture behavior are discussed in the light of competing and mutually interactive influences of cyclic strain amplitude, concomitant response stress, intrinsic microstructural effects and dislocation-microstructure interactions during cyclic straining.     

Calculating the stress-strain behaviour of uniaxial metallic composites with strain-hardening components

An algorithm is developed which models the response of uniaxial composites to non-monotonic loading. Experimental information on strain hardening and Bauschinger effect of the components is taken into account. By comparison of composite stress-strain curves generated in this way with experimental ones, residual stresses can be determined quantitatively. The algorithm seems suitable for modelling low-cycle fatigue of ductile composites.     

高强度合金抗疲劳应用技术研究与发展

评述了超高强度钢、高强度Al合金和Ti合金表面完整性抗疲劳应用技术的研究和发展。高强度合金疲劳性能对应力集中敏感，不适当的加工工艺和切削热等造成的表面损伤和高拉应力使其疲劳和应力腐蚀性能损失殆尽。先进的表面完整性加工尤其是表面改性可显著提高疲劳性能，如激光冲击使7475-T761拉-拉疲劳寿命提高约89％，7075-T6裂纹扩展速率降低到原来的1／1500；超声喷丸使超高强度钢低周疲劳强度提高约50％，Ti7Al4Mo合金高周疲劳强度提高约15％；表面超硬化可使Vasco X-2M齿轮钢接触疲劳寿命提高30～35倍等。     

Cyclic stress-strain response during isothermal and thermomechanical fatigue

Isothermal high-temperature low-cycle fatigue and in-phase and out-of-phase thermomechanical fatigue tests were carried out on 316L austenitic stainless steel specimens controlled by computer. A non-linear kinematic hardening model with internal variables was used to simulate the cyclic stress-strain behaviour of isothermal fatigue. This model was modified by considering thermal cyclic effects in order to describe the cyclic stress-strain behaviour of thermomechanical fatigue (TMF) using only isothermal fatigue data and the material performance data. A very good approximation of the hysteresis loops was obtained by comparing with experiments of both in-phase and out-of-phase cases. The thermomechanical fatigue behaviour described by isothermal fatigue data gives the possibility of developing the TMF lifetime prediction technique.     

Low-cycle fatigue in Ni3Al-based alloys

The low-cycle fatigue properties of hot-extruded powders of a Ni₃Al-based alloy, IC 218, with nominal composition Ni-16.5Al-8.0Cr-0.4Zr-0.1B (at %) have been evaluated at room temperature. Tests were conducted under total strain conditions in a laboratory air environment. Results indicate that the low-cycle fatigue performance of the PM processed IC 218 nickel aluminide is superior to other structural alloys especially at higher strain amplitudes. These results are explained in terms of the high ductility of the fine-grain material and good crack growth propagation resistance in these alloys. Stress response curves for annealed IC 218 alloys indicate considerable cyclic hardening followed by cyclic softening. The onset of cyclic softening is found to occur at a constant cumulative plastic strain. The critical cumulative plastic strain criteria are verified for step-loaded IC 218 nickel aluminide coupons.     

4.5Ni钢表面裂纹的低周疲劳扩展行为研究

采用悬臂弯曲加载方式，以总应变范围作为受检和控制参数，分析了高强度４．５Ｎｉ钢表面裂纹的低周疲劳扩展行为，给出了裂纹扩展速率ｄ（２ａ）／ｄＮ与总应变范围ΔεＴ的关系式及关系曲线。同时对弯曲加载条件下低周疲劳损伤断口微观形貌进行了观察分析。指出４．５Ｎｉ钢的低周疲劳裂纹扩展方式主要是穿晶，疲劳辉纹为晶体学延性辉纹，疲劳裂纹扩展属于塑性钝化模型机制。