316L不锈钢焊接接头高温低周期疲劳显微结构变化和断裂特征

本研究对316L奥氏体不锈钢母材和焊缝分别进行高温低周疲劳试验,对试样的微观结构和裂纹扩展形貌进行观察,分析母材和焊缝在高温低周疲劳循环应力响应下的位错结构和损伤机制。结果表明,316L奥氏体不锈钢母材在试验过程中由于位错增殖和位错湮灭导致发生循环硬化和循环稳定,在焊缝中由于位错湮灭导致发生循环软化。母材和焊缝在连续低周疲劳试验中裂纹主要以穿晶方式扩展,焊接接头处孔洞的连接是最终导致焊接接头疲劳断裂的主要机制。     

氢处理对铸造钛合金低周疲劳寿命及断裂韧性的影响

本文研究了氢处理对铸造钛合金的低周疲劳寿命及断裂韧性的影响,发现经氢处理后铸造钛合金粗大的魏氏组织转变成细小的等轴组织,其应变低周疲劳寿命大大延长。在低周疲劳过程中,应变量较小时,试样出现循环硬化现象;而在应变量较大时,出现循环软化现象。氢处理后材料的断裂形式由脆性断裂转为韧性断裂;断裂韧性提高。     

氢处理对铸造钛合金低周疲劳寿命及断裂韧性的影响

本文研究了氢处理对铸造钛合金的低周疲劳寿命及断裂韧性的影响,发现经氢处理后铸造钛合金粗大的魏氏组织转变成细小的等轴组织,其应变低周疲劳寿命大大延长,在低周疲劳过程中,应变量较小时,试样出现循环硬化现象,而在应变量较大时,出现循环软化现象,氢处理后材料的断裂形式由脆性断裂转为韧性断裂;断裂韧性提高。     

TA5钛合金大厚板低周疲劳特性研究

本文给出了ＴＡ５钛合金的低周疲劳性能试验数据。研究了主冷凝器在循环载荷下，材料循环硬化或软化特性、应力与应变曲线以及应变与寿命的关系，并预示了低周疲劳寿命。此外，对疲劳断口进行了分析对比。     

沉淀硬化不锈钢弹簧低周疲劳性能研究

研究了Ｃｒ１２Ｍｎ５Ｎｉ４Ｍｏ３Ａｌ沉淀硬化不锈钢弹簧低周疲劳性能和疲劳断口，指出在５２０℃时效时，由δ铁素体析出的大片状Ｊ相和α＇相诱发疲劳裂纹是降低疲劳寿命的主要因素；害３００℃时效处理可大幅度提高疲劳寿命。     

AZ91D室温环境棘轮及其低周疲劳行为

通过AZ91D室温环境应力控制下的低周疲劳试验,对铸造镁合金棘轮及其低周疲劳行为进行了研究,讨论了室温环境下材料的应力循环特性、棘轮行为、塑性应变范围、全应变范围等疲劳参量随载荷水平和加载历史的变化规律,同时基于平均应力修正对材料的应力-寿命曲线进行了讨论。研究结果表明:AZ91D在室温环境下的应力循环呈循环硬化,材料的棘轮行为和塑性应变范围、全应变范围等疲劳参量依赖于载荷水平和加载历史,另外考虑平均应力修正后的应力-寿命曲线预测效果有明显改观。     

316不锈钢原位质子辐照疲劳过程中的声频内耗

３１６不锈钢原位质子辐照下疲劳过程中的声频内耗用弯摆在３３３－５７３Ｋ温区和应变振幅为１０＾－＾４量级下进行了研究。结果表明，有质子辐照的疲劳过程中的内耗在较大庆振幅下是与振幅有关的内耗明显小于质子辐照疲劳中的内耗。     

在多轴载荷下45钢的循环特性

通过多轴疲劳试验，研究了在多轴加载条件下45钢的循环特性变化规律，分析了非比例附加强化、多轴循环软化/硬化特性及疲劳寿命对加载路径参数的依赖性，结果表明，相位角主要影响非比例附加强化程度，幅值比主要影响多轴循环软化/硬化特性，二者都影响多轴疲劳寿命。     

淬火回火低碳合金钢的疲劳形变与断裂的电镜观察

应用透射电子显微术研究了淬火并400℃和600℃回火钢中疲劳位错结构随循环加载周数的增加所发生的变化。钢中出现具有循环显微硬化与循环显微软化的位错结构形式。前者主要是形成位错缠结,后者主要是粗大疲劳变形带的萌生和扩展,在扩展过程中萌生显微疲劳裂纹。     

316L不锈钢表面氮离子注入后的疲劳及腐蚀疲劳行为

３１６Ｌ（Ｆｅ－１７．５Ｃｒ１３．５Ｎｉ－２．５Ｍｏ）不锈钢注入Ｎ离子后，其自由腐蚀电位Ｅ＿０、孔蚀再钝化电位Ｅ＿ｐ及孔蚀电位Ｅ＿ｒ均大大提高。在大应变量下，疲劳寿命下降；在小应变量下，疲劳性能改善。在电位Ｅ＿０作用下，其腐蚀疲劳寿命较高子注入前有所降低。在施加电位Ｅ＿０－１００ｍＶ时，其疲劳寿命较离子注入前提高。表面离子注入层的存在提高材料的循环应力。     

The effect of temperature on low-cycle fatigue behavior of prior cold worked 316L stainless steel

Low-cycle fatigue tests on cold worked 316L stainless steel were carried out at various temperatures from room temperature to 650 °C and tensile tests were conducted on the cold worked and solution-treated materials. At all test temperatures, the cold worked material showed the tendency of higher strength and lower ductility. Following initial cyclic hardening for a few cycles, cyclic softening behavior was observed to dominate until failure occurred during low-cycle fatigue deformation. The softening behavior strongly depends on temperature and strain amplitude. Several life prediction models were examined and it was found that it is important to select a proper life prediction parameter since stress and strain depend strongly on temperature. A phenomenological fatigue life prediction model is proposed to account for the influence of temperature on life. The model is correlated with the experimental results.     

Cyclic deformation behavior of austenitic Cr–Ni-steels in the VHCF regime: Part II – Microstructure-sensitive simulation

In Part I – Experimental study, the cyclic deformation behavior of two austenitic stainless steel grades (AISI 304, AISI 316 L) were experimentally investigated at low stress amplitudes in the very high cycle fatigue (VHCF) regime. The observations indicate that during VHCF the metastable austenitic stainless steel (304 grade) performs a pronounced localization of plastic deformation in shear bands followed by a deformation-induced martensitic phase transformation. The 316 grade undergoes only a very limited local plastic deformation in shear bands with almost no phase transformation. Consequently, both materials exhibit distinctly different cyclic softening and hardening characteristics during VHCF. In order to provide a more detailed knowledge about the individual deformation mechanisms and their effect on the cyclic softening and hardening behavior the experimental study is extended by microstructure-sensitive modeling and simulation. Two-dimensional (2-D) microstructures consisting of several grains are represented using the boundary element method and plastic deformation within the microstructure is considered by a mechanism-based approach. Specific mechanisms of cyclic plastic deformation in shear bands and deformation-induced martensitic phase transformation – as documented by experimental results and based on well-known model approaches – are defined and implemented into the simulation. The fatigue behavior at low stress amplitudes observed in experiments can be well represented in simulations so that the underlying model helps to understand the cyclic deformation behavior of austenitic stainless steels at low stress amplitudes in the regime of VHCF strength. In a comparative study based on the resonant behavior the effect of certain deformation mechanisms on the global cyclic softening and hardening characteristics is pointed out for both materials.     

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.     

Torsional fatigue with axial constant stress of oligo‐crystalline 316L stainless steel thin wire

A series of symmetric torsional fatigue with axial constant stress tests, a kind of multiaxial fatigue test, was conducted on oligo‐crystalline 316L stainless steel thin wire, which was less than 3.5 grains across diameter of 200 μm. The material presents significant cyclic hardening under symmetric torsion cycling, and hardening is more obvious with the increasing shear strain amplitude. However, symmetric torsional cycle with constant axial stresses tests characterize rapid initial hardening and then gradually softening until fatigue failure. The axial stress has a great effect on torsional fatigue life. Fractography observation shows a mixed failure mode combined torsional fatigue with tensile strain because of axial tensile stress. A newly proposed model with axial stress damage parameter is used to predict the torsional fatigue life with constant axial stress of small scale thin wire.     

A simple energy‐based model for nonproportional low‐cycle multiaxial fatigue life prediction under constant‐amplitude loading

From the literature concerning the traditional nonproportional (NP) multiaxial cyclic fatigue prediction, special attentions are usually paid to multiaxial constitutive relations to quantify fatigue damage accumulation. As a result, estimation of NP hardening effect decided by the entire history path is always proposed, which is a challenging and complex task. To simplify the procedure of multiaxial fatigue life prediction of engineering components, in this paper, a novel effective energy parameter based on simple material properties is proposed. The parameter combines uniaxial cyclic plastic work and NP hardening effects. The fatigue life has been assessed based on traditional multiaxial fatigue criterion and the proposed parameter, which has been validated by experimental results of 316 L stainless steel under different low‐cycle loading paths.     

Comparative evaluation of strain controlled low cycle fatigue behaviour of solution annealed and prior cold worked 316L(N) stainless steel

The effect of 20% prior cold work on low cycle fatigue (LCF) behaviour of type 316L(N) stainless steel (SS) was studied at 873 K by conducting total axial strain controlled tests in air with strain amplitudes in the range ±0.25% to ±1.0%. The cyclic deformation behaviour of 20% prior cold worked (PCW) material was compared with the LCF response of solution annealed (SA) alloy tested under similar conditions. The cyclic stress response (CSR) of 316L(N) SS in the PCW condition was characterized by a short period of hardening followed by prolonged softening prior to failure, whereas SA material exhibited a significant hardening regime followed by stress saturation. Interrupted tests on PCW material were carried out at different stages of CSR in order to determine the underlying mechanisms as reflected in substructural changes. The fatigue life in the solution annealed condition was similar to that of the PCW material at higher strain amplitudes of testing (≥±0.5%) while at lower strain amplitudes, the PCW material exhibited longer life.     

Influence of prior cyclic hardening on high temperature deformation and crack growth in type 316L (N) stainless steel

For power generating equipment subjected to cyclic loading at high temperature, crack growth could arise from the combinations of fatigue and creep processes. There is potential for the material to undergo hardening (or more generally changes of material state) as a consequence of cyclic loading. Results of an experimental study to examine the influence of prior cyclic hardening on subsequent creep deformation are presented for type 316L(N) stainless steel at 600°C. Experiments were also carried out to explore creep crack growth at constant load, and crack growth for intermittent cyclic loading. For the as-received material there is substantial primary creep (hardening) at constant load, while for the cyclically hardened material at constant load the creep curves show recovery, and increasing creep rate with increasing time. Specimens subjected to prior cyclic hardening were also used for a series of creep and creep-fatigue crack growth tests. These tests demonstrated that there was accelerated crack growth compared to crack growth in as-received material.     

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.     

Effects of temperature on the low cycle fatigue behaviour of nitrogen alloyed type 316L stainless steel

Strain-controlled low cycle fatigue tests have been conducted in air between 298–873 K to ascertain the influence of temperature on LCF behaviour of nitrogen-alloyed type 316L stainless steel. A strain amplitude of ± 0.60% and a symmetrical triangular waveform at a constant strain rate of 3 × 10⁻³ s⁻¹ were employed for all tests. Crack initiation and propagation modes were evaluated, and the deformation and damage mechanisms which influence the cyclic stress response and fatigue life identified. The cyclic stress response at all temperatures was characterized by an initial hardening to the maximum stress, followed by gradual softening prior to attaining saturation. Temperature dependence of fatigue life showed a maximum in the intermediate temperature range. The drastic reduction in fatigue life at elevated temperatures has been ascribed primarily to the combined influence of dynamic strain ageing effects and oxidation-enhanced crack initiation, while the lower life at room temperature is attributed to detrimental effects associated with deformation-induced martensite.     

Micromechanical methodology for fatigue in cardiovascular stents

A finite element based micromechanical methodology for cyclic plasticity and fatigue crack initiation in cardiovascular stents is presented. The methodology is based on the combined use of a (global) three-dimensional continuum stent-artery model, a local micromechanical stent model, the development of a combined kinematic–isotropic hardening crystal plasticity constitutive formulation, and the application of microstructure sensitive crack initiation parameters. The methodology is applied to 316L stainless steel stents with random polycrystalline microstructures, based on scanning electron microscopy images of the grain morphology, under realistic elastic–plastic loading histories, including crimp, deployment and in vivo systolic–diastolic cyclic pressurisation. Identification of the micromechanical cyclic plasticity and failure constants is achieved via application of an objective function and a unit cell representative volume element for 316L stainless steel. Cyclic stent deformations are compared with the J₂-predicted response and conventional fatigue life prediction techniques. It is shown that micromechanical fatigue analysis of stents is necessary due to the significant predicted effects of material inhomogeneity on micro-plasticity and micro-crack initiation.