激光冲击强化对TC4 钛合金缺口叶片疲劳强度的影响

目的 提高航空发动机叶片抗外物损伤的性能。方法 采用薄壁件激光冲击强化工艺,对某型发动机TC4钛合金叶片包含一阶弯曲振动节线区域的表面进行处理,随后在叶片前缘一阶弯曲振动节线位置设计不同应力集中系数的缺口。参考有限元仿真软件分析结果和相关标准要求,预制应力集中系数Kt为3.2的缺口。通过力值校核和有限元仿真之间的多次迭代,明确应力测试位置与缺口危险点应力之间的关系。通过振动疲劳试验对激光冲击强化效果进行评价。通过扫描电子显微镜观察疲劳断口的形貌,采用残余应力仪对梯度残余应力进行测试,并提取相应位置的半峰全宽值,对激光冲击强化提升缺口叶片疲劳强度的原因进行分析。结果 经激光冲击强化处理后的钛合金缺口叶片在107次循环下的疲劳强度提升了63.2%;残余压应力层深度可达1.5 mm,且表层位错密度提升了67.5%;经激光冲击强化处理后钛合金缺口叶片裂纹萌生于近表面。结论 激光冲击强化引入的表层梯度残余压应力和位错增殖是缺口叶片疲劳强度提升的主要原因。     

Notch Sensitivity and Failure Behavior of TiAl and K418 Alloys

The notch sensitivity of tensile specimens of TiAl and K418 alloys has been investigated, and the notch strength has been quantitatively analyzed. The fracture surface of the specimens has also been observed and analyzed by scanning electron microscopy (JEOL-6700F). By comparing notch rod tensile specimens and special notch rod tensile specimens, it was found that the basic nature, fracture driving force, and fracture criteria of the brittle TiAl alloy and the ductile K418 alloy are different. The final fracture of the K418 alloy is controlled by strain and not by stress, and the specimens do not exhibit notch sensitivity. However, the final fracture of the TiAl alloy is controlled by stress, and the specimens exhibit small notch sensitivity. For the special notch rod tensile specimens, the K418 specimens do not exhibit notch sensitivity. However, for the TiAl alloy, when the notch depth reaches 10% of the specimen diameter, the tensile strength decreases, and when the notch depth reaches 20% of the specimen diameters, the tensile strength sharply decreases.     

The influence of aging treatments on sulfide stress corrosion cracking of PH 13-8 Mo steel welds

Slow displacement rate tensile tests were carried out to study sulfide stress corrosion cracking (SSCC) of PH 13-8 Mo stainless steel welds in a saturated H₂S solution. The welds aged in the temperature range of 482-593 °C were susceptible to SSCC; the fracture surfaces revealed mainly quasi-cleavage fractures after notched tensile tests. However, the SSCC susceptibility in terms of the percentage loss of the notched tensile strength (NTS) of the welds was dependent on the aging treatment. The SSCC resistance and the austenite content of the welds increased with the aging temperatures. The presence of greater amounts of austenite, mainly reverted austenite, in the W1100 specimen (the weld aged at 1100 °F or 593 °C) than that in other aged specimens could account for its lower hardness and better SSCC resistance. On the other hand, the AW (as-welded) specimen containing a small amount of retained austenite films in a soft matrix exhibited a slightly improved SSCC resistance than that in the W1100 specimen. The lower hardness of the AW specimen was owing to the absence of fine coherent precipitates, leading to a reduced local stress and an enlarged plastic zone located in front of the notch in the test. With lower hardness, the local stress would also be lower and less likely to exceed some critical stress for failure in the saturated H₂S solution. For the aged specimens, the hardness/strength level and the amount of reverted austenite were the important factors that affect SSCC susceptibility.     

7075铝合金在液体金属Ga中的脆断行为

用WOL恒位移试样研究了7075铝合金在液体金属Ga中的脆断行为金相跟踪观察表明,Ga吸附后首先促进局部塑性变形,在缺口前方形成四下的塑性区,然后脆性激裂纹形核．液体金属脆断的临界应力强度因子．反向加载（位移V＜0）时,液体金属脆性激裂纹在远离缺口顶端的拉应力区中形核,微裂纹向拉应力区扩展而中止于压应力区反向加载时,试作断裂的名义应力强度因子为．所有断裂试样均为沿晶断口．     

Damage and fracture mechanism of 6063 aluminum alloy under three kinds of stress states

To study the damage and fracture mechanism of 6063 aluminum alloy under different stress states,three kinds of representative triaxial stress states have been adopted,namely smooth tensile,notch tensile,and pure shear.The results of the study indicate the following.During the notch tensile test,a relatively higher stress triaxiality appears in the root of the notch.With the applied loading increasing,the volume fraction of microvoids in the root of the notch increases continuously.When it reaches the critical volume fraction of microvoids,the specimen fractures.During the pure shear test,the stress triaxiality almost equals to zero,and there is almost no microvoids but a shear band at the center of the butterfly specimen.The shear band results from nonuniform deformation constantly under the shear stress.With stress concentration,cracks are produced within the shear band and are later coalesced.When the equivalent plastic strain reaches the critical value（equivalent plastic fracture strain）,the butterfly specimen fractures.During the smooth tensile test,the stress triaxiality in the gauge of the specimen remains constant at 0.33.Thus,the volume of microvoids of the smooth tensile test is less than that of the notch tensile test and the smooth specimen fractures due to shearing between microvoids.The G-T-N damage model and Johnson-Cook model are used to simulate the notch tensile and shear test,respectively.The simulated engineering stress-strain curves fit the measured engineering stress-strain curves very well.In addition,the empirical damage evolution equation for the notch specimen is obtained from the experimental data and FEM simulations.     

近全层组织γ-TiAl基合金的室温拉伸断裂机理

通过对直缺口近全层组织的扫描电镜原位拉伸实验以及相应的断裂表面观察，结合有限元计算了TiAl基合金近全层组织拉伸的断裂机理。研究表明：许多裂纹在塑性变形前沿着层间起裂和扩展，断裂过程的驱动力是拉应力。在直缺口试样中，许多裂纹直接起裂于缺口根部，并且沿着层间扩展。随着拉应力的增加，主裂纹和新裂纹也可以通过障碍晶粒的穿层解理断裂来连接。通过有限元计算得沿层断裂强度大约为50MPa，穿层断裂强度大约为120MPa。     

Effect of triaxiality on void growth and coalescence in model materials investigated by X-ray tomography

Void growth and coalescence/linkage, which play significant roles during ductile fracture processes, are strongly influenced by stress triaxiality in a deforming solid. The stress state can be changed by cutting notches in a tensile sample. In the current paper, void growth and linkage of an artificial void array embedded in a notched model material was studied by X-ray computed tomography, coupled with in situ tensile deformation. The cross-sectional shape of the tensile specimens was square, and a pair of notches was cut along only one direction. Thus, the lateral principal stress does not have an isotropic distribution: the principal stress along the notch direction is considered to be higher. This technique allowed us to explore the entire process of growth and linkage events of a void array embedded in a metal matrix. The notch effect creates a marked acceleration in void growth, leading to a large reduction in the linkage strains, as compared with similarly fabricated unnotched samples. The standard models for coalescence could not provide consistent predictions of the measured notch effect.     

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 本文通过拉伸试验研究了铸造TiAl合金单一取向层片组织在缺口应力集中系数分别为2、3和4时的缺口敏感性。发现,该合金在沿层片界面方向的应力作用下,对应各Kt水平的缺口试样均具有大于光滑试样的抗拉强度,且其缺口敏感系数相当;并根据该合金具有较好的拉伸塑性和缺口拉伸断口上存在裂纹萌生和稳态扩展区域的特征,分析讨论了其形成缺口强化效应的机制。     

Effect of hydrogen environment on the notched tensile properties of T-250 maraging steel annealed by laser treatment

In the present work, slow displacement rate tensile tests were performed to find out the influence of ageing condition and hydrogen-charging on the notched tensile strength and fracture characteristics of T-250 maraging steel aged at various conditions. The influence of embrittling species in the environment on the notched tensile strength was accessed by comparing the measured properties in air, gaseous hydrogen and H₂S-saturated solution. The hydrogen diffusivity, permeation flux and apparent solubility of various specimens determined by electrochemical permeation method, were correlated well with the microstructures and mechanical property. The results indicated that the peak-aged (H900) specimen was highly sensitive to hydrogen embrittlement even in gaseous hydrogen. In contrast, the microstructures of over-aged (H1100) specimen comprising of reverted austenite and incoherent precipitates could trap large amount of hydrogen atoms, resulting in decreased hydrogen permeability and hydrogen embrittlement susceptibility. The solution-annealed specimen had the highest diffusion coefficient and the lowest quantity of trapped hydrogen among the specimens, showing high susceptibility to sulfide stress corrosion cracking. In the presence of notches, hydrogen atoms were prone to segregate and trap at grain boundaries, resulting in the formation of intergranular fracture.     

缺口对GH3536镍基高温合金蠕变性能的影响

针对GH3536镍基高温合金无应力集中和带典型应力集中圆棒试样在160,200,240 MPa应力条件下的高温(750℃)蠕变性能进行了试验和有限元分析,研究了缺口的存在对材料蠕变性能的影响。采用扫描电镜分析了试样断口的形貌,确定了材料的断裂机制。结果表明:缺口的存在对GH3536镍基高温合金试件的蠕变性能影响较大,其降低了试件的最大蠕变应变及在蠕变各个阶段的蠕变速率。缺口对试件的蠕变寿命存在硬化作用,缺口根部轴向应力的再分配和松弛可简单认为是导致缺口对该合金蠕变寿命起硬化作用的影响因素。断口形貌分析表明由于缺口的存在,2种试件的表面形貌虽有不同,但蠕变断裂机理都是由微孔洞不断增大与聚集引起的。     

dc Potential Drop Technique in Creep Stress Rupture Testing

The dc potential drop technique can be used in conventional creep tests for high sensitivity measurements of deformation and damage. More importantly, it can be used in creep tests of notched specimens to: measure load creep deformation and damage at the root of the notch; detect crack initiation at the root of notches; and measure crack propagation rates, when the crack grows axisymmetrically. This test methodology is well-suited for testing new alloys. The effects of heat treatment, environment, temperature, and specimen geometry on deformation rates, crack initiation, and crack propagation, can now be assessed in a quantitative way.     

Deformation behaviour of closed-cell aluminium foams in tension

The mechanical behaviour of commercially available ALPORAS aluminium foam with two different densities was studied under tension loading. In addition to the common stress–strain measurements, local deformation, notch-opening displacement and damage evolution were determined. The deformation characteristics deviated from those observed in aluminium foams under compression. No deformation bands or plastic instabilities could be observed in tension, which are very frequent in compression of metallic foams. Four regimes were evident in the stress–strain curves and deformation maps: the linear elastic regime, the plastic regime with no significant crack initiation and propagation, the regime of formation of a fracture process zone and, finally, the regime of fracture, where a main crack propagates through the specimen and leads to failure. The fracture strain was only a few per cent, with the higher-density foam showing a larger fracture strain, and the plastic Poisson's ratio was about 0.35. The notched specimens showed increasing fracture strengths in terms of the net section stress with increasing notch depth. It is suggested that a change in stress state, caused by a non-vanishing Poisson's ratio, in front of the notch tip creates an increase of the fracture strength similar to the behaviour in ductile bulk metals.     

DUCTILE FRACTURE OF FERRITE NODULAR GRAPHITE CAST IRON UNDER MULTIAXIAL TENSILE STRESS

为研究多轴应力下铁素体球铁延性断裂机制,采用三种不同曲率半径(ρ=2,4,10mm)的缺口圆柱试样进行拉伸试验,并用大弹塑性变形有限元分析法计算试样缺口部位的应力、应变分布。在中断拉伸试样上进行金相观察,跟踪石墨球与基体界面处微空洞的形核和长大。研究结果表明,当石墨球与基体界面处的应力到达650MPa时,微空洞形核,空洞长大和聚合导致延性断裂。     

Notch sensitivity of ductile metallic foams: A computational study

The role of notches in the fracture strength of metal foams has been studied using a multiscale model based on a two-dimensional Voronoi representation of the cellular architecture. The effect of the crack length to the specimen width ratio on the net section strength of double edge notch (DEN) specimens and central crack panels (CCP) has been investigated in two ways: (i) by fixing the number of cells in the width and (ii) by fixing the number of cells in the ligament. Notch strengthening has been observed in DEN specimens in excellent agreement with experiments. Analysis of the strain fields suggests that in the DEN configuration, the constraint imposed on the rotation fields in the notch tip zones of the ligament enhances the net section strength. In CCP specimens, stress concentration and free-edge effects produced a small level of notch weakening. Our results show that both the ligament length as well as the notch length (normalized by the cell size) influence the notch behaviour in two-dimensional metal foams.     

两种Al2O3结构陶瓷断裂强度的统计分析

研究了结构陶瓷材料的统计分布规律。首先通过实验测试了两种Ａｌ２Ｏ３陶瓷光滑试件和不同缺口半径试件的断裂强度;对实验数据用正态分布和Ｗｅｉｂｕｌｌ分布分别进行了分析,发现对于缺口件,用正态分布描述其断裂强度分散性要比Ｗｅｉｂｕｌｌ分布好一些。这也说明在对陶瓷材料结构件进行设计时,应考虑用正态分布表述其断裂强度的分散性和存活概率。     

Environmentally assisted cracking of 18%Ni maraging steel

Slow strain rate testing of notched cylindrical specimens of 18Ni2400 maraging steel has been carried out in air with 30% relative humidity and synthetic seawater environments. Peak-aged condition has been chosen, considering the relevance to engineering applications. Studies have also been carried out with different notch geometries to understand the effect of stress concentration factor. It is concluded from the study that (i) degree of stress concentration at the notch influences the notched tensile strength (ii) mild hydrogen embrittlement seems to occur in air environment, (iii) synthetic seawater environment drastically brings down the notched tensile strength and time to fracture (iv) environmentally assisted cracking occurs in air tests in quasicleavage and microvoid coalescence modes and in seawater tests in intercrystalline mode.     

NUMERICAL STUDY OF THE NOTCH EFFECT ON THE CREEP BEHAVIOR AND LIFE OF NICKEL-BASE SINGLE CRYSTAL SUPERALLOYS

Numerical calculations of creep damage development and life behavior of circular notched specimens of nickel-base single crystal had been performed. The creep stress distributions depend on the specimen geometry. For a small notch radius, von Mises stress has an especial distribution. The damage distribution is greatly influenced by the notch depth, notch radius as well as notch type. The creep crack initiation place is different for each notched specimen. The characteristics of notch strengthening and notch weakening depend on the notch radius and notch type. For the same notch type, the creep rupture lives decrease with the decreasing of notch radius. A creep life model has been presented for the multiaxial stress states based on the crystallographic slip system theory.     

The influence of cracks,notches and holes on the tensile strength of cellular solids

Finite element simulations of two-dimensional cellular structures with defects were performed to study the influence of defect size and cell size on the tensile strength of the cellular solid. Three types of defects were considered: circular holes, sharp cracks and notches. The net section strength, defined as the peak tensile load divided by the remaining intact area in the section with the defect, was determined for a range of defect sizes and cell sizes. For the circular holes, the net section strength was found to be independent of hole size, and equal to the tensile strength of the intact honeycomb. For cracks and notches the net section strength showed a notch-strengthening effect, exceeding the tensile strength of the intact honeycomb and increasing with crack or notch depth. The results were sensitive to the cell size for the crack defects but not for the hole or notch defects.     

STRESS CORROSION CRACKING OF AN ALUMINIUM ALLOY UNDER COMPRESSIVE STRESS

用WOL缺口压缩试样研究LC4铝合金在3.5％NaCl水溶液中的压应力腐蚀.有限元计算指出,试样加压缩位移时缺口附近有压应力集中.实验结果表明,当压缩位移大于临界值后产生压应力腐蚀裂纹. 实验测出压应力腐蚀裂纹的门槛值为K_(ISCC*)=27.6MPam~(1/2),等于拉应力腐蚀门槛值的3.5倍.在相同K_I下压应力腐蚀裂纹的孕育期比拉应力高一个数量级.压应力腐蚀时获得带有平行条纹花样的准解理断口,这和拉应力腐蚀的沿晶断口完全不同。     

Contribution to the Interpretation of the Formation of the Active Path in Stress Corrosion Cracking of Aluminium Alloys: I. Indirect evidence from electrolytically notched specimens

Abstract

With the object of interpreting the formation of the active path in stress corrosion cracking (s.c.c.) of metals and alloys, a new, electrolytic, method of preparing notched specimens was employed. Double notched specimens of Al–2 · 5 wt- % Mg alloy were so prepared, using several different current densities to prepare the notches. Their behaviour was then compared with that of mechanically notched specimens under accelerated laboratory stress corrosion conditions in M NaCl with differing impressed current densities. In some instances the notches were removed mechanically before testing (‘denotched’ specimens).

For each category of specimen, applied stress vs specimen life (time to cracking) was established. Results obtained from this procedure indicated that, for this alloy, a sensitisation takes place during the first step of stress corrosion cracking, i.e., localised anodic dissolution (pitting) of the stressed zone. In the second part of this work, based on further measurements (to be published) it will be shown that this sensitisation corresponds to active path formation, attributable to de-cohesion as a result of fast solid diffusion of mobile (reversibly disordered) Al³⁺.