双相钢的疲劳行为（下）

评述了近年来国内外有关双相钢疲劳行为方面的文献资料,并提出了尚待解决的问题。     

双相钢疲劳断口的分形分析

本文研究了双相钢疲劳门槛值与门槛值附近断口形貌和分形维数的关系。初步实验结果表明:分形维数基本上能反映断口粗糙度。发现门槛值随分形维数呈线性变化,门槛值、分形维数和粗糙度都随马氏体含量呈抛物线型变化。     

13SiMNiCrMov钢切口疲劳裂纹萌生门槛值

本文研究在１３ＳｉＭｎＮｉＣｒＭｏＶ结构钢中用尖裂纹的应力强度因子来反映三点弯曲缺口试样的疲劳裂纹萌生规律。当Ｒ＝０．１ｆ＝１００Ｈｚ，试样尺寸Ｂ＊Ｗ＊Ｌ＝１２．５＊２５＊１１７ｍｍ时，缺口名义应力幅的门槛值为△σｔｈ＝３３１５ρ＾０．３５２ＭＰａ，０．５ｍｍ≤ρ≤５ｍｍ，△Ｋｔｈ＝２５０ρ０．３５２ＭＰａｍ门槛值与ρ有关。同时得到缺口裂纹萌生的循环次数Ｎｉ与名义应力幅△σ，缺口曲率半径ρ的定量关     

热轧双相钢的发展现状及高强热轧双相钢的开发

介绍了热轧双相钢的发展现状及存在问题,指出低成本热轧双相钢、高延伸凸缘型铁素体+贝氏体热轧双相钢(F-B热轧双相钢)及高强度热轧双相钢的开发及应用,将促进我国热轧双相钢的发展,推动汽车工业的"以热代冷"进程。同时,探讨了纳米析出强化型热轧双相钢的强化机理及工艺控制原理,并在实验室进行了中试,开发出铁素体基体析出强化型的热轧双相钢,其抗拉强度达770~830 MPa,屈强比0.75,组织为铁素体+马氏体,且铁素体基体中存在大量细小的纳米级尺寸的TiC过饱和析出和相间析出。     

双相钢冲击拉伸变形行为本构方程的建立

利用旋转盘式冲击拉伸试验装置，对马氏体含量为３０％，４２％，４８％，５０％和６０％的２０Ｃｒ双相钢进行３个应变率的冲击拉伸试验，得到了这些材料在不同高应变率下 绝热应力应变曲线；发现２０Ｃｒ双相钢的屈服应力随马氏体的含量和应变率的增加而增加，且马氏体的含量对应变率的强化系数影响不大。     

循环荷载作用下钢桥面铺装疲劳损伤失效行为研究

针对钢桥面铺装工程中普遍采用的改性沥青(Stone Matrix Asphalt,SMA)、浇筑式沥青(Guss asphalt,GA)、环氧沥青(Epoxy asphalt,EP)混合料双层铺装结构,进行了循环车载作用下钢桥面与沥青混凝土铺装疲劳损伤特性理论分析与试验研究。基于疲劳损伤度,研究了钢桥面铺装疲劳损伤失效行为和疲劳开裂过程中损伤场、应力和应变场动态演变机制,推导出疲劳失效时的损伤场、应力和应变场计算表达式,并给出钢桥面铺装疲劳寿命理论公式。以三座钢箱梁桥桥面铺装(润扬长江大桥2005,南京长江三桥2005,苏通大桥2008)为例,对不同铺装结构组合方案下的复合梁进行疲劳试验分析和使用寿命理论预测。实例研究结果表明,钢桥面铺装疲劳损伤失效行为预估模型合理可行;相较于改性沥青、浇筑式沥青,环氧沥青混合料具有较强高的强度低变形能力,更适合于大跨径钢桥面铺装抗疲劳的设计要求;由环氧沥青混合料组合而成的“双层环氧沥青混凝土”和“浇注式沥青混凝土(下层)+环氧沥青混凝土(上层)”的抗疲劳性能优于其它沥青混合料铺装结构组合方案,同等厚度组合情况下疲劳使用寿命可延长1倍~2倍以上;“双层环氧沥青混凝土”已应用于润扬长江大桥、南京长江三桥和苏通长江大桥钢桥面工程,并已成功运行10年以上,其跟踪观测结果良好。     

中碳合金钢双相组织疲劳裂纹扩展的研究

研究了４２ＣｒＭｏ钢亚温淬火及二次淬火两种双相组织的疲劳裂纹扩展行为。结果表明，亚温淬火双相组织可显著提高裂纹萌发生抗力，降低扩展速率；较高的疲劳性能与裂纹尖端闭合应力，断口表面粗糙度以及裂纹扩展路径弯折程度有关。断裂力学分析表明，粗糙度诱发裂纹闭合效应及裂纹路径变折效应是提高裂纹扩展抗力的主要因素，同时得到上述两种效应对裂纹扩展速率影响的表达式。     

GH34钢的热—机械疲劳性能和等温低周疲劳性能

本文研究了珠光体耐热钢GH34的热—机械疲劳性能。热—机械疲劳试验在最大应变与最高温度同相位和逆相位两种情况下进行。控制波形为三角波,频率为0.008Hz,温度范围为200(?)550℃。GH34在淬火+高温回火状态下,经受热—机械循环,表现出循环软化的特性。热—机械疲劳时的循环应变硬化指数,大于上限温度的等温循环应变硬化指数。同相位和逆相位热—机械疲劳抗力相差不大,但都稍高于550℃等温循环时的疲劳抗力。用550℃等温低周疲劳寿命代替200(?)550℃的热—机械疲劳寿命是可行的,且结果偏于安全。     

温度对GH34钢低周疲劳性能的影响

本文用应变控制的低周疲劳试验方法,研究了室温、350、450和550℃下,燃气轮机叶轮用钢GH34的低周疲劳性能。试验表明,各温度下的循环应力——应变特性均为循环软化。试验温度提高,低周疲劳性能下降。由于蓝脆性的发展,450℃的低周疲劳性能迅速下降,接近550℃的低周疲劳性能。COFFIN—MANSON方程中的指数α随试验温度提高而增大。高温低周疲劳裂纹的扩展,主要为Ⅱ阶段扩展,在所采用的试验条件下,没有发现蠕变引起的治晶疲劳断裂。     

In‐phase and out‐of‐phase thermomechanical fatigue behavior of 4Cr5MoSiV1 hot work die steel cycling from 400 °C to 700 °C

The hysteresis loops, stress and strain behavior, lifetime behavior and fracture characteristic of 4Cr5MoSiV1 hot work die steel at a wide range of mechanical strain amplitudes (from 0.5% to 1.3%) during the in‐phase (IP) and out‐of‐phase (OP) thermomechanical fatigue (TMF) tests cycling from 400 °C to 700 °C under full reverse strain‐controlled condition were investigated. Stress‐mechanical strain hysteresis loops of 4Cr5MoSiV1 steel are asymmetric, and stress reduction appears at high‐temperature half cycles owing to a decrease in strength with increasing temperature. 4Cr5MoSiV1 steel always exhibits continuous cyclic softening for both types of TMF tests, and the cyclic softening rate is larger in OP loading condition. OP TMF life of 4Cr5MoSiV1 steel is approximately 60% of IP TMF life at the same mechanical strain amplitude and maximum temperature. Lifetime determined and predicted in both types of TMF tests is adequately described by the Ostergren model. Fracture surfaces under IP TMF loading display the striation and tear ridge, showing quasi‐cleavage characteristics, and the cracks are less but longer. However, fracture surfaces under OP TMF loading mainly display the striation and dimple characteristics, and the cracks are more and shorter.     

High-cycle Fatigue Fracture Behavior of Ultrahigh Strength Steels

The fatigue fracture behavior of four ultrahigh strength steels with different melting processes and therefore different inclusion sizes were studied by using a rotating bar two-point bending fatigue machine in the high-cycle regime up to 107 cycles of loading. The fracture surfaces were observed by field emission scanning electron microscopy （FESEM）. It was found that the size of inclusion has significant effect on the fatigue behavior. For AtSI 4340 steel in which the inclusion size is smaller than 5.5 μm, all the fatigue cracks except one did not initiated from inclusion but from specimen surface and conventional S-N curve exists. For 65Si2MnWE and Aermet 100 steels in which the average inclusion sizes are 12.2 and 14.9 μm, respectively, fatigue cracks initiated from inclusions at lower stress amplitudes and stepwise S-N curves were observed. The S-N curve displays a continuous decline and fatigue failures originated from large oxide inclusion for 60Si2CrVA steel in which the average inclusion size is 44.4 pro. In the case of internal inclusion-induced fractures at cycles beyond about 1×10^6 for 65Si2MnWE and 60Si2CrVA steels, inclusion was always found inside the fish-eye and a granular bright facet （GBF） was observed in the vicinity around the inclusion. The GBF sizes increase with increasing the number of cycles to failure Nf in the long-life regime. The values of stress intensity factor range at crack initiation site for the GBF are almost constant with Nf, and are almost equal to that for the surface inclusion and the internal inclusion at cycles lower than about 1×10^6. Neither fish-eye nor GBF was observed for Aermet 100 steel in the present study.     

Concurrent ratcheting–fatigue damage analysis of uniaxially loaded A‐516 Gr.70 and 42CrMo Steels

This study intends to investigate the concurrent interaction of fatigue damage and ratcheting strain in two commonly used steel alloys of (American Society for Testing and Materials) ASTM A‐516 Gr.70 and 42CrMo, respectively for pressure vessels and high grade machinery parts over uniaxial stress cycles. Ratcheting extension and fatigue damage progress were both characterized cycle‐by‐cycle over life cycles of tested materials. The interaction of ratcheting and fatigue damage was defined based on mechanistic parameters involving the effects of mean stress, stress amplitude and cyclic softening/hardening response of materials. The extent of ratcheting effect was defined by product of average ratcheting strain per cycle, and maximum stress value during a cycle, while fatigue damage was analysed based on earlier developed energy‐based models of Xia–Ellyin, and Smith–Watson–Topper. Overall damage due to ratcheting and fatigue was calibrated through a weighting factor at various mean/ cyclic amplitude stresses. An algorithm was developed to evaluate overall damage due to ratcheting and fatigue stress cycles of materials subjected to various mean and amplitude stresses. The estimated lives at different mean stresses and stress amplitudes for ASTM A‐516 Gr.70 and 42CrMo samples showed good agreements as compared with those of reported experimental data.     

600MPa级冷轧双相钢的试制与研究

在实验室试制了600MPa级低成本C-Mn系冷轧双相钢,比较了热轧态普通组织和退火后双相组织与力学性能的关系,分析了双相组织的强化机理,讨论了热轧卷取温度对最终力学性能的影响,分析并且优化了连续退火工艺参数.采用X衍射和电子背散射EBSD观察了试制双相钢组织的宏观与微观取向.     

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。     

超细晶粒钢的疲劳性能

采用常规疲劳试验方法分别对400MPa级和500MPa级两种热轧态超细晶粒钢进行了疲劳试验,分析了这两种超细晶粒钢的疲劳性能,并研究了超细晶粒钢疲劳裂纹萌生的机制。结果表明：由于500MPa级超细晶粒钢的晶粒尺寸相比400MPa级钢的更细小,因此其疲劳性能优于400MPa级钢的;在应力比R=-1、正弦波加载、循环周次基数N=5×10^6次的条件下,500MPa级和400MPa级超细晶粒钢的条件疲劳强度分别为270MPa和220MPa;两种超细晶粒钢的疲劳裂纹均在试样表面形核,且呈单源特征。