相位角加载条件下2A12铝合金多轴疲劳失效行为

采用SDN100/1000电液伺服拉扭复合疲劳试验机对2A12铝合金进行不同相位角加载条件下多轴疲劳试验研究,通过加载循环曲线和微观断口形貌分析失效机理,对不同损伤累积模型的预测效果进行评价,修正Manson损伤曲线模型以期达到更好的预测效果。结果表明:单级加载条件下,随相位角正弦值的增加疲劳寿命线性递减,当相位角为0°时,轴向硬化、软化交替出现,切向出现循环硬化,90°加载下轴向和切向单独作用效果明显;两级累积路径下,随一级加载周次的增加多轴疲劳寿命延长,0°加载阶段轴向和切向都出现循环硬化现象,两种路径下断口都呈现出多裂纹源特征,在裂纹源区附近观察到台阶状形貌,扩展区存在大量划痕和鳞片状花样;修正后的Manson损伤曲线模型预测误差均在15%以内。     

2A12铝合金的多轴加载疲劳行为

采用SDN100/1000电液伺服拉扭复合疲劳试验机对2A12铝合金进行多关键参数的多轴疲劳性能研究,通过对断口的微观分析探究疲劳失效机理。结果表明:等效应力加载条件下,随拉扭相位差的增加疲劳寿命降低,0°相位差下断面裂纹源区能观察到轮胎状、鱼骨状以及钟乳石状的特殊形貌,裂纹扩展区存在二次裂纹和模糊的疲劳条带;分别改变拉、扭平均应力,多轴疲劳寿命均降低,裂纹源区能看到白色絮状的氧化物,瞬断区存在二次裂纹和剪切型韧窝;不同加载波形条件下,正弦波对应最长的多轴疲劳寿命,三角波次之,方波时最短且体现出最大的结构耗能。低-高两级加载条件下,材料产生"锻炼效应"。     

7075-T651铝合金薄壁管件多轴低周疲劳行为及寿命预测

针对航空铝合金多轴疲劳失效问题,对7075-T651铝合金薄壁管件进行不同加载条件下的拉扭复合疲劳实验。结果表明:随等效应力幅的降低,多轴疲劳寿命增加;等效应力恒定时,寿命随应力幅比的升高而增加;拉扭相位差对寿命影响较小。高应力幅下材料在轴向和扭向以软化为主,低应力幅下硬化和软化交替出现。宏观断口平台区随应力幅比的增加而逐渐减小,微观断口呈现管壁外侧的多裂纹源特征,扩展区可以观察到疲劳条带和二次裂纹,瞬断区出现混合型韧窝。提出基于Basquin准则的改进模型,得到较好的寿命预测效果,寿命预测值均位于两倍分散带内。     

加载路径对Sr变质A319铝合金疲劳行为的影响

研究了Sr变质A319铸造铝合金在0.2%应变幅不同加载路径条件下的疲劳性能,包括循环应力响应特征及疲劳寿命,并分析了失效试样的断口特征以及Si颗粒的破坏方式。结果表明:在不同加载路径下材料发生循环硬化程度和速率从大到小排序是:圆形加载、比例加载和单轴加载;疲劳寿命随着加载路径的变化与材料循环硬化程度和速率随着加载路径的变化相对应。断口分析结果表明,宏观断口在比例路径下表现为"人"字形的两条主裂纹,且从单轴、比例到圆形路径,裂纹源区逐渐不明显,裂纹源区和稳定扩展区尺寸也变小;在单轴加载条件下裂痕的断面基本上与主轴平行,而在多轴加载条件下裂痕的分布较为分散。     

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。     

拉-压及拉-扭载荷下6063合金的疲劳行为

对6063锻造铝合金进行了不同应力幅值下拉-压及拉-扭复合疲劳试验,并用透射电子显微镜观察了疲劳失效试样的位错结构。结果表明:在循环加载过程中,循环硬化占据主要地位,循环硬化的速率和程度对应力幅值和加载路径有依赖性;相同应力幅值下,试样在拉-扭复合加载下失效比拉-压失效形成更为复杂的位错结构,且位错密度更高;位错之间及位错与析出相的交互作用是材料发生循环硬化的主要原因,循环硬化程度越高,疲劳寿命越短。     

拉弯扭比例加载下50CrVA弹簧钢的多轴疲劳寿命及损伤特征

用MTS 809拉-扭电液伺服材料试验机研究50CrVA弹簧钢标准试样以及各偏心试样在比例加载下的多轴疲劳寿命,并用扫描电子显微镜(SEM)对疲劳断口形貌进行观察。结果表明:比例加载下当弯曲应力幅值不超过等效应力的11%时,附加弯矩对试样的疲劳寿命无影响。各试样断口在疲劳扩展前期可以观察到轮胎花样;疲劳裂纹的扩展方向有周向和径向两种。断口上的疲劳特征为综合应力作用的结果;沿径向和周向分布的疲劳条带分别为轴向应力作用和扭转切应力作用的结果。     

交变应力下A356铝合金疲劳行为及微结构演变

对处于交变应力下A356铝合金的单轴疲劳寿命以及变形行为进行了研究,并与单一应力加载下的加载情况进行了对比.发现先进行高应力加载后换用低应力加载将会显著延长合金的疲劳寿命.合金的循环应变值主要与合金的循环加载应力幅值有关.此外,利用透射电镜的方法观察了在不同循环加载历史条件下合金中微观结构的变化情况,尤其是位错以及位错带的演变规律.并发现了沉淀物附近的位错塞积现象.     

循环荷载下1-3型水泥基压电复合材料的力电响应

利用MTS810万能材料试验机,对1-3型水泥基压电复合材料开展了准静态单轴压缩试验和循环加载试验。结果表明:在循环荷载下,试件输出电压与施加荷载之间呈现出一一对应的响应特性;在较高应力比下,随循环周次增加,试件输出电压幅值无明显变化;在较低应力比下,随循环周次增加,试件输出电压幅值呈现递减规律,加载前期电压幅值下降较快,后期逐渐趋于平稳;相同应力比时,随循环周次增加不同加载频率下,试件输出电压幅值变化规律与输出电压幅值随应力比的变化规律一致。     

热等静压对圆形加载路径下A319铝合金多轴疲劳特性的影响

采用MTS809拉扭复合疲劳试验机、扫描电镜(SEM)研究了热等静压(Hot isostatic pressing,HIP)处理前后,圆形加载路径下,A319铝合金多轴疲劳特性。结果表明,HIP处理后,材料中的孔洞缺陷数量减少,疲劳失效过程中产生的微裂纹的数量减少且尺寸减小。相同等效应变幅值下产生的轴向应力幅值、切向应力幅值、等效应力幅值均显著增加。材料轴向上表现为先产生循环硬化而后循环软化,切向则表现为先产生循环硬化后趋于循环稳定,HIP处理前后循环软化硬化趋势大致相同。轴向应力应变滞后回线、切向扭矩扭角滞后回线面积有所降低,附加强化效果增强。     

Multiaxial notch fatigue life prediction based on pseudo stress correction and finite element analysis under variable amplitude loading

A new computational methodology is proposed for fatigue life prediction of notched components subjected to variable amplitude multiaxial loading. In the proposed methodology, an estimation method of non‐proportionality factor (F) proposed by authors in the case of constant amplitude multiaxial loading is extended and applied to variable amplitude multiaxial loading by using Wang‐Brown's reversal counting approach. The pseudo stress correction method integrated with linear elastic finite element analysis is utilized to calculate the local elastic‐plastic stress and strain responses at the notch root. For whole local strain history, the plane with weight‐averaged maximum shear strain range is defined as the critical plane in this study. Based on the defined critical plane, a multiaxial fatigue damage model combined with Miner's linear cumulative damage law is used to predict fatigue life. The experimentally obtained fatigue data for 7050‐T7451 aluminium alloy notched shaft specimens under constant and variable amplitude multiaxial loadings are used to verify the proposed methodology and equivalent strain‐based methodology. The results show that the proposed methodology is superior to equivalent strain‐based methodology.     

Study on the dislocation sub-structures of Al–Mg–Si alloys fatigued under non-proportional loadings

In this article, the fatigue dislocation sub-structures of Al–Mg–Si alloys were studied under the same equivalent stress amplitude of 93 MPa with multi-axial non-proportional loading paths, which were circle, ellipse, rectangle, and square loading paths. After fatigue test, the fatigue dislocation sub-structures of the failure specimens were also observed under different equivalent stress amplitudes, which were 47, 70, 90, 140, and 163 MPa with the transmission electron microscopy method. The results indicate that fatigue lives of the alloy are different under different loading paths with the same equivalent stress amplitude, and the fatigue life decreases with the increase in equivalent stress amplitude from 47 to 163 MPa. Cyclic additional hardening is found above medium effective stress amplitude of 93 MPa. The dislocation sub-structures strongly depend on equivalent stress amplitudes and loading paths. The dislocation sub-structures of parallel slip band, slight small dislocation cell (or labyrinth), and remarkable labyrinth feature are found under the equivalent stress amplitudes of 47, 93, and 140 MPa, respectively. The movability of the dislocation and the stress concentration degree are closely related to fatigue life and cyclic hardening of the alloy.     

Non-proportional multiaxial whole-life transformation ratchetting and fatigue failure of super-elastic NiTi shape memory alloy micro-tubes

Experimental observations are performed to investigate the non-proportional multiaxial whole-life transformation ratchetting and fatigue failure of superelastic NiTi SMA micro-tubes in stress-controlled loadings at human-body temperature (310 K). The effects of axial mean stress and stress hold on the whole-life transformation ratchetting and fatigue life are investigated with uniaxial, torsional and five different multiaxial loading paths. The results show that the stress holds on the upper or lower transformation plateaus will both promote forward and reverse transformation, and lead to shorter fatigue life. The multiaxial fatigue lives of NiTi shape memory alloy depend significantly on loading paths and applied stress levels.     

Fatigue behavior and dislocation substructures for 6063 aluminum alloy under nonproportional loadings

In this paper, the fatigue behavior and dislocation substructures of 6063 aluminum alloy were studied under several nonproportional path loadings, which were circle, ellipse, rectangle and square paths. After fatigue test the micro-structure especially the dislocation substructures of the failure materials was carefully observed with the transmission electron microscope (TEM) method. Under the same 93 MPa equivalent stress amplitude loading, the alloy has the shortest life and the most severe cyclic additional hardening with circle path loading among all the loading paths. This attributes to the complicated dislocation substructures and severe stress concentration of the alloy during the cycling process. While under the ellipse path loading, the alloy has a comparably long life and light cyclic additional hardening. The deformation of the alloy and the morphology of the dislocation substructures determine the fatigue behavior of 6063 alloy under the same equivalent stress amplitude loading. Under the circle path loading, the fatigue life decreases while the cyclic strain increases as the loading stress amplitude increases from 47 MPa to 163 MPa. The dislocation evolution of 6063 alloy during the cycling process under circle path loading was examined with TEM. It was found that the dislocation merges with each other and changes from single lines to crossed bands. The movability of dislocation reduces and the stress concentration degree rises during the cycling process.     

Biaxial fatigue for proportional and non‐proportional loading paths

In order to study the use of a local approach to predict crack‐initiation life on notches in mechanical components under multiaxial fatigue conditions, the study of the local cyclic elasto‐plastic behaviour and the selection of an appropriate multiaxial fatigue model are essential steps in fatigue‐life prediction. The evolution of stress–strain fields from the initial state to the stabilized state depends on the material type, loading amplitude and loading paths. A series of biaxial tension–compression tests with static or cyclic torsion were carried out on a biaxial servo‐hydraulic testing machine. Specimens were made of an alloy steel 42CrMo4 quenched and tempered. The shear stress relaxations of the cyclic tension–compression with a steady torsion angle were observed for various loading levels. Finite element analyses were used to simulate the cyclic behaviour and good agreement was found. Based on the local stabilized cyclic elastic–plastic stress–strain responses, the strain‐based multiaxial fatigue damage parameters were applied and correlated with the experimentally obtained lives. As a comparison, a stress‐invariant‐based approach with the minimum circumscribed ellipse (MCE) approach for evaluating the effective shear stress amplitude was also applied for fatigue life prediction. The comparison showed that both the equivalent strain range and the stress‐invariant parameter with non‐proportional factors correlated well with the experimental results obtained in this study.     

High cycle fatigue and fracture mode analysis of 2A12–T4 aluminum alloy under out-of-phase axial–torsion constant amplitude loading

Fatigue experiments with solid cylindrical bar specimens are carried out to investigate the effects of stress amplitude ratios and phase angles on the axial–torsion high-cycle fatigue failure of 2A12–T4 aluminum alloy. Macro- and micro-analysis of the specimen fracture appearance is conducted in order to obtain the fracture characteristics under combined axial–torsion loading with different stress amplitude ratios and different phase angles. The soft factor of fatigue loading is introduced to estimate the failure mode under the combined axial–torsion loading. The result indicates that the estimation about the soft factor of fatigue loading is consistent with the macro- and micro-analysis.     

渗碳Cr-Ni高强硬度合金钢超高周疲劳寿命预测

为了给渗碳合金钢提供一种有效可行的超高周疲劳寿命预测方法,在应力比为0和0.3两种情况下,对渗碳Cr-Ni高强硬度合金钢展开疲劳试验研究.通过对试样断口的微观组织观测,发现渗碳层与基体材料中均有非金属夹杂的存在;通过对裂纹萌生位置和疲劳断口形貌的观察,将疲劳失效分为带有细晶粒区(Fine Granular Area,F...