Dislocation Structure of VT22 Titanium Alloy in Cyclic Loading with Various Loading Frequencies

We study the dislocation structure of a Tindash;5percnt;Alndash;5percnt;V alloy upon cyclic-strength and crack-propagation resistance tests with symmetrical tensionndash;compression loading with frequencies of 100 Hz, 500 Hz, 3 kHz, and 10 kHz. The identity of the test conditions has allowed us to carry out a comparative analysis of the influence of the loading frequency on the evolution of the dislocation structure in the material main volume during the fatigue-damage accumulation and in the fracture zone during the fatigue-crack propagation. It is shown that at these two loading stages the plastic-deformation micromechanisms adapt themselves to the loading rate. In the first case, this is due to the fact that a decrease in the activity of the work of the Frankndash;Read sources in high-frequency loading is compensated for by a more pronounced deformation of the alpha;-phase as a result of the formation of stacking faults. In the second case, the high level of local stresses activates the cross sliding and the formation of a honeycomb structure in the alpha;- and beta;-phases, and the elements of this structure decrease in size with increasing loading frequency. The incompleteness of relaxation processes in high-frequency cyclic loading is offset by the deformation of boundary volumes which are initially present in the alpha;-phase of twins.     

Regularities in the Main-Crack Propagation and Dislocation Structure Evolution in the Fracture Zone of VT22 Alloy at Various Frequencies of Cyclic Loading

We have studied the relationship between the dislocation structure in the fracture zone and fractographic features of the main-crack propagation in a Ti–5%Al–5%V alloy tested for cyclic crack growth resistance in symmetrical tension–compression at frequencies of 140, 600, 3000, and 10000 Hz. It is demonstrated that the prevailing Types of dislocation structure are cellular over the near-threshold K range and of band-Type structure for the remaining values of the stress intensity factor range. For these Types of structure of the alloy studied at all loading frequencies, the characteristic micromechanism of fracture is the formation of fatigue striations. In the region of low K values, the above-mentioned Types of substructure, and thus fatigue striations, are most commonly formed along certain crystallographic planes and directions. As the K values grow, the crack sensitivity to crystallographic orientation decreases. The effect of the loading frequency on the regularities and mechanisms of fatigue crack growth is governed by two main factors: the processes of plastic deformation at the crack tip during the pre-fracture period and the interaction between the crack front and the initial and formed structural and substructural elements. The appearance of the brittle-fracture elements with increasing loading frequency is due to a rather high sensitivity of the -phase to the loading rate.     

Fatigue Fracture in VNS-25 Stainless Steel

The paper covers the experimental study of the influence of cycling frequency ranging from 20 Hz to 10 kHz and the cycle asymmetry on the crack growth resistance of VNS-25 stainless steel. It is demonstrated that as the cycling frequency is raised, the crack growth rate decreases monotonically, while threshold values of the stress intensity factor K_th grow. The latter is due to the fact that at high cycling frequencies the plastic deformation processes remain incomplete and the contribution of the fracture mechanisms that facilitate greater energy intensity of fatigue crack growth processes becomes more significant. A comparison of K_th and fatigue life values as a function of cycling frequency for smooth and notched specimens demonstrates that if represented in relative coordinates these functions are described by a common relationship. This allows a quantitative assessment of the effect of the strain cycling rate on the above-mentioned properties of a material from the results of tests of one type only. It has been found that the values K_th in asymmetrical loading cycles are related to those in symmetrical cycles, as well as to a coefficient of material's sensitivity to the static component, and linearly depend on the load ratio.     

Damageability and Structural Inhomogeneity of VT14 Titanium Alloy under Cyclic Loading

On the basis of the statistical analysis of physicomechanical characteristics of the surface layer of specimens of VT14 titanium alloy obtained by the acoustic method, we study distinctive features of the kinetic dependences of an empirical parameter connected with the structural inhomogeneity of the metal. This enables us to identify typical stages of the process of fatigue damage.     

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。     

18CrNiMo7-6合金钢的弯曲微动疲劳特性

对18CrNiMo7-6合金钢进行弯曲微动疲劳实验,建立弯曲微动疲劳S-N曲线,并对实验结果进行分析。结果表明:该合金钢的弯曲微动疲劳S-N曲线不同于中碳钢材料,也不同于常规弯曲疲劳,而是呈"ε"型曲线特征。随着弯曲疲劳应力的增加,微动运行区域由部分滑移区向混合区和滑移区转变,损伤区的磨损机制以剥层、磨粒磨损和氧化磨损为主。在混合区内,裂纹最易萌生和扩展,且裂纹均萌生于材料接触区次表面。受接触应力和弯曲疲劳应力影响,弯曲微动疲劳裂纹的萌生和扩展可分为三个阶段:初期,在接触应力控制下,裂纹萌生于次表面;随后,裂纹受接触应力和弯曲疲劳应力共同控制,转向更大角度方向扩展;最后,裂纹完全受弯曲疲劳应力控制而垂直于接触表面扩展,直至断裂失效。     

孔强化对TC18钛合金疲劳寿命的影响

为提高TC18钛合金带孔零件的疲劳寿命,使用基体和焊缝上开孔的TC18钛合金试样,研究孔挤压和孔喷丸强化前后的表面残余应力,孔强化工艺对试样疲劳寿命的影响以及试样疲劳断口.研究表明,对基体和焊缝上的孔进行喷丸强化处理后,孔表面残余压应力值达到-300MPa以上,由于残余压应力和表面完整性的作用,孔喷丸强化效果比挤压强化...     

The Influence of the Cyclic Loading Rate on the Plastic Zone Depth in VNS-25 Alloy

We have studied the plastic zone depth at the fatigue crack tip in VNS-25 (03Kh12N10MT) alloy specimens, previously subjected to the cyclic fracture toughness tests under symmetrical push-pull loading with a frequency of 20, 170, 600 Hz, 3 and 10 kHz. For the same values of the stress intensity factor, an increase in the loading frequency is shown to slow down the fatigue crack growth and reduce the plastic zone depth under the fracture surface. However, the dependence of the plastic zone depth on the crack growth rate is invariant with respect to the loading frequency.     

TA15钛合金两类组织对疲劳性能和断裂韧度的影响

研究了TA15钛合金片状和双态两种典型组织对疲劳性能和断裂韧度的影响,结果表明:在S-N曲线的高应力区,双态组织的疲劳强度高于片状组织;在低应力区,情况则相反,且片状组织的疲劳极限(656MPa)高于双态组织(565MPa).片状组织的疲劳裂纹扩展速率低于双态组织,且断裂韧度K1C高于双态组织,即片状组织的损伤容限性能优于双态组织.     

Ti60钛合金中富钕稀土相颗粒对叶片室温振动疲劳性能的影响

采用振动疲劳实验及SEM断口分析等方法,研究了含稀土元素Nd的Ti60高温钛合金稀土相颗粒对叶片振动疲劳裂纹萌生的影响.结果表明:叶片振动疲劳寿命与稀土相颗粒的尺寸和分布位置具有密切关系.稀土相颗粒尺寸越大,对叶片振动疲劳裂纹的萌生作用也越大,疲劳寿命降低;外露于或镶嵌于叶片表面的稀土相颗粒明显促进了叶片振动疲劳裂纹的萌生.     

TA15钛合金的高周疲劳性能和断裂特征

研究了双态组织的TA15钛合金的高周疲劳性能和疲劳断裂特征,结果表明,β转变组织中次生α相的数量和形态对疲劳性能有显著影响,次生相α相的球化显著降低了合金抗裂纹扩展的能力,而大量的片状次生α相则通过造成疲劳裂纹的分枝有效地降低了疲劳裂纹的扩展速度,提高了疲劳极限.     

再次喷丸周期对TC18钛合金疲劳寿命的影响

研究了再次喷丸对于经过预先喷丸的TC18钛合金残余应力和室温疲劳寿命的影响。使用X射线衍射仪和旋转弯曲疲劳机测定了再次喷丸后的合金的表面残余应力以及总疲劳寿命。结果表明,疲劳试验会使预先喷丸的TC18钛合金疲劳试样表面残余压应力松弛30%～50%,而再次喷丸可以使由于疲劳而松弛的表面残余压应力回复到疲劳试验前试样的50%～70%。此外,相比未经过再次喷丸的试样,选择合适的再次喷丸周期可使TC18钛合金的总疲劳服役寿命提高了75%。     

基于小裂纹理论的铸造钛合金ZTC4疲劳寿命预测

铸造钛合金ZTC4在飞机和航空发动机上应用日益广泛.深入研究ZTC4疲劳全寿命预测方法,旨在为航空构件的损伤容限设计和寿命预测探索新的途径.本文以宏观和微观结合的手段,采用板材试样的高周疲劳试验、中心裂纹试样的长裂纹扩展试验和扫描电子显微镜(SEM)的断口分析等三种试验,研究了ZTC4在室温恒幅载荷条件下的疲劳断口特征和裂纹扩展行为;对引起疲劳失效的主要原因-材料初始缺陷(夹杂或气孔)进行了定量表征;基于Newman裂纹闭合模型建立了ZTC4长裂纹的(da/dN)-△Keff基线数据;通过对平板内埋椭圆裂纹的断裂力学分析,从基于微观结构和断口分析统计确定的初始缺陷尺寸出发,对ZTC4在恒幅载荷条件下两种应力比的疲劳全寿命进行了预测和实验验证,得到了具有较好学术意义和工程应用价值的研究结果.     

Microcrack initiation and growth in heat-resistant 15Kh2MFA steel under cyclic deformation

This paper presents the results of investigation of a nuclear reactor pressure vessel steel 15Kh2MFA of two strength levels under cyclic loading. The mechanism of microcrack formation on the surface and in the bulk of 15Kh2MFA steel under cyclic deformation was investigated. Analysis of the specimen surfaces has shown that microcracks are caused by cyclic sliding in grains most favourably oriented with respect to the direction of the maximum shear stresses. Transmission electron microscope investigations show that microcracks in the material inside the grains are formed mainly along the band‐type dislocation structure parallel to the dislocation subboundary. During cyclic deformation, the dislocation density on the subboundaries increases, in the local areas the dislocation density becomes limiting and it reaches the plasticity limit and causes microcrack formation. The interrelation of the average length of microcracks and their surface density with the energy density of inelastic deformation has been found.     

A stochastic damage accumulation model for crack initiation in high-cycle fatigue

A stochastic damage accumulation model for crack initiation in high-cycle fatigue is proposed. It is assumed that the fatigue damage is accumulated in the form of dislocations under the repeated stress and that the slip band crack is initiated when the strain energy due to a local pile-up of dislocations exceeds a critical value. The size of an initiating crack is the cell size, derived from a probabilistic argument and its depth is determined in relation to the stored dislocation energy. Our theoretical results are compared with the experimental data from a low-carbon steel S20C in order to examine the consistency of our model.     

Fatigue analysis of offshore platforms subject to sea wave loadings

The fatigue damage calculation for random loading on offshore platforms takes the form of a rainflow analysis of the dynamic response of individual members to various sea states. This procedure is lengthu and consequently this paper attempts to provide a theoretical method for determining random load fatigue damage. This dynamic response for many joints leads to a broad band random loading but despite this, previous theoretical methods have simplified the loading to narrow band. This has not been done in the present case; instead, an analysis based on broad band random loading has been produced. This theoretical approach gives a fatigue life estimate which is slightly (6.6%) more conservative, for a typical example, than a rainflow analysis.     

Fatigue crack growth behaviour of gamma base titanium aluminides under different loading conditions

Static and cyclic fatigue crack growth behaviour of gamma base titanium aluminides with three different microstructures were investigated. Influence of cyclic test frequency on fatigue crack growth behaviour was also studied at room temperature under a controlled humidity condition. The crack growth behaviour both under static and cyclic loading was strongly influenced by the microstructure. The threshold stress intensity and crack growth behaviour under cyclic loading were much inferior than that under static loading indicating the ‘true-cyclic fatigue’ effect exhibited in gamma base titanium aluminides. No significant effect of test frequency on the crack growth behaviour was observed for the equiaxed and duplex microstructure materials.     

Fatigue strength and damage progression in a circular-hole-notched GRP composite under combined tension/torsion loading

Fatigue strength and failure processes in a hole-notched glass-fiber-reinforced plastic (GRP) material under combined tension/torsion cyclic loading have been investigated. Thin-walled tubular specimens were used in the experiment. The reinforcing fibers (plain-weave glass cloth) were aligned parallel to the longitudinal and circumferential axes of the specimen. The damage state at each biaxiality stress ratio (: the ratio of normal stress to shear stress) was observed macroscopically and microscopically.

The fatigue damage progression is dependent on the combined stress ratio. In the case of = 1/0, damage spreads radically from the hole, but covers an area of only π/4 on either side of the circumferential axis. Damaged areas are symmetrical and opposite each other. When = 0/1, damage progresses radially along both longitudinal and circumferential axes. These areas of damage progression are called ‘fatigue bands’. Macro- and microscopical observations reveal that the damage state under combined loading has mixed features from the damage states at = 1/0 and = 0/1.     

Fatigue of SIC-based ceramics at ambient temperature

The effects of static and cyclic loading on monolithic sintered silicon carbide (SiC) and SiC reinforced with 16 vol.% particulate titanium diboride (SiC/16vol.% TiB₂) have been studied. Tests were carried out at ambient temperature in air on precracked specimens loaded in three- or four-point bending. No crack growth under cyclic loading has been observed in the monolithic silicon carbide. There is an additional cyclic contribution to crack growth after static crack growth has arrested in the composite material. Observations suggest that damage to the titanium diboride particles ahead of the crack tip occurs prior to crack extension through the SiC matrix.     

低合金高强度钢的微观组织和疲劳裂纹扩展行为

用透射电镜观察了３０ＣｒＭｎＳｉＮｉ２Ａ钢等温的微观组织，疲劳裂纹扩展行为、裂纹尖端塑性区和位错结构，结果表明，等温状态组织由马氏体和贝氏体组成。在一个奥氏体晶粒内一般存在四个板条领域、裂纹尖端的塑性区内存在主位错带，疲劳断裂的基本组织单元为板条晶或板条束。裂纹遇到板条束界时方向发生较大偏斜。