Dynamic Tensile Fracture Behaviours of Selected Aluminum Alloys under Various Loading Conditions

Experiments investigating dynamic tensile fracture were performed on the extruded rods of 2024‐T4 and 7075‐T6 aluminum alloys under varying loading conditions. The initial yield stress and fracture strain of 7075‐T6 alloy obtained in spilt Hopkinson tension bar tests are higher than that of 2024‐T4 alloy. But the initiation fracture toughness and spall strength of 2024‐T4 alloy are higher than those of 7075‐T6 alloy in three‐point bending and plate impact experiments, which indicates that 2024‐T4 alloy has better crack initiation tolerance and stronger spall failure resistance. Based on metallurgical investigations by using optical and scanning electron microscopes, it is revealed that the microstructure has a profound effect on the dynamic tensile fracture mechanism of each aluminum alloy. The 2024‐T4 alloy is relatively brittle due to voids or cracks nucleated at many coherent CuMgAl₂ precipitate phases in the grain interiors, and the fracture mode is predominantly transgranular. The 7075‐T6 alloy exhibits relatively ductile fracture because voids or cracks growth is partly intergranular along the grain boundaries and partly transgranular by void formation around coarse intermetallic particles. The obvious differences of damage distribution and void coalescence mechanisms for 2024‐T4 and 7075‐T6 alloys under plate impact are also discussed.     

Ductile rupture in thin sheets of two grades of 2024 aluminum alloy

The damage and rupture mechanisms of thin sheets of 2024 aluminum alloy (Al containing Cu, Mn, and Mg elements) are investigated. Two grades are studied: a standard alloy and a high damage tolerance alloy. The microstructure of each material is characterized to obtain the second phase volume content, the dimensions of particles and the initial void volume fraction. The largest particles consist of intermetallics. Mechanical tests are carried out on flat specimens including U-notched (with various notch radii), sharply V-notched and smooth tensile samples. Stable crack growth was studied using “Kahn samples” and pre-cracked large center-cracked tension panels M(T). The macroscopic fracture surface of the different specimens is observed using scanning electron microscopy. Smooth and moderately notched samples exhibit a slant fracture surface, which has an angle of about 45° with respect to the loading direction. With increasing notch severity, the fracture mode changes significantly. Failure initiates at the notch root in a small triangular region perpendicular to the loading direction. Outside this zone, slant fracture is observed. Microscopic observations show two failure micromechanisms. Primary voids are first initiated at intermetallic particles in both cases. In flat regions, i.e. near the notch root of severely notched samples, void growth is promoted and final rupture is caused by “internal necking” between the large cavities. In slanted regions these voids tend to coalesce rapidly according to a “void sheet mechanism” which leads to the formation of smaller secondary voids in the ligaments between the primary voids. These observations can be interpreted using finite element simulations. In particular, it is shown that crack growth occurs under plane strain conditions along the propagation direction.     

预拉伸厚铝板内 部残余应力 与晶粒取向均匀性的研究

目的消减预拉伸铝板内部残余应力。方法利用短波长X射线衍射仪(SWXRD),分别对某公司国产25 mm厚2024-T351预拉伸铝板,以及美国铝业公司(ALCOA)20 mm厚7075-T651预拉伸铝板的内部残余应力、内部织构及其沿板厚的分布,进行了无损测定。结果 ALCOA的20mm厚7075预拉伸铝板内部残余应力小于25 MPa,其内部晶粒取向沿板厚均匀分布;而某公司原工艺生产的25 mm厚2024预拉伸铝板,内部残余应力高达100 MPa左右,其内部晶粒取向沿板厚分布很不均匀。结论源自于轧制的内部织构沿板厚分布的不均匀性,使得以消减残余应力为目的的预拉伸处理中的铝板塑性变形不均匀,导致某公司国产预拉伸铝板内部残余应力的消减效果差,在其后续加工中容易产生加工变形超差的问题,需要抑制强剪切织构的产生,减小织构在整个板材厚度上的不均匀分布程度。     

Fatigue property of semisolid A357 aluminum alloy under different heat treatment conditions

Effect of heat treatment conditions on fatigue property of a semisolid A357 aluminum alloy under cyclic tensile loading was investigated. Comparison of the fatigue property of the semisolid A357 under T5 and T6 heat treatment conditions with other aluminum alloys including conventional casting A357-T6 alloy and four wrought aluminum alloys: 2024-T4, 7075-T6, 5052-T6 and 6061-T6 was made. It is found that the fatigue strength of the semisolid A357 under both heat treatment conditions is much higher than that of the casting A357-T6 alloy, comparable to that of the 6061-T6, but lower than that of the 2024-T4 and 7075-T6. Two-parameter Weibull distribution of fatigue data for the semisolid A357 under the two heat treatment conditions was constructed to show the statistical significance in fatigue lifetime. Fatigue fracture surface of the semisolid A357 under T5 and T6 heat treatment conditions was examined using scanning electron microscope (SEM). In the stable crack propagation region, the semisolid A357-T5 shows fatigue damage species of severely deformed grains, void coalescence, striations and ridgelines, while the A357-T6 displays less plastic deformation as revealed by the fatigue damage features of intergranular cracks, and transgranular cleavage patterns.     

Korrosionsverhalten der Legierung Al-Li-Cu-Mg

Corrosion Behaviour of the Al-Li-Cu-Mg Alloy The localized corrosion behaviour of the Al-Li-Cu-Mg alloy 8090-T651 (25 mm thick plate) was investigated using salt spray tests, EXCO tests and tests according to the MIL-H-6088F specification. These accelerated laboratory tests indicate that the material is susceptible to intergranular and exfoliation corrosion. The stress corrosion cracking behaviour was studied under constant load, constant deformation and slow strain rate conditions. The material was found to be resistant against stress corrosion cracking in the long transverse direction. In the short transverse direction a threshold stress below 50 MPa was determined. The same values were measured for the conventional alloys 2014-T351 and 2024-T351. The fracture surfaces of specimens failed due to stress corrosion cracking were examined using a scanning electron microscope. Fractographic features attributed to corrosion were discussed.     

搅拌铸造SiCp/2024复合材料热挤压-轧制变形组织及性能

利用搅拌铸造-热挤压-轧制工艺制备SiCp/2024复合材料薄板。通过金相观察(OM)、扫描电镜(SEM)及力学测试等手段研究了该复合材料在铸态、热挤压态及轧制态下的显微组织及力学性能,分析了材料在塑性变形过程中显微组织及力学性能的演变。结果表明,该复合材料铸坯主要由80～100μm的等轴晶组成,粗大的晶界第二相呈非连续状分布,SiC颗粒较均匀地分布于合金基体中;热挤压变形后,晶粒沿挤压方向被拉长,SiC颗粒及破碎的第二相呈流线分布特征;轧制变形后,基体合金组织进一步细化,晶粒尺寸为30～40μm,SiC颗粒破碎明显,颗粒分布趋于均匀,轧制变形对挤压过程中形成的SiC颗粒层带状不均匀组织有显著的改善作用。数学概率统计指出,塑性变形有利于提高颗粒分布的均匀性。力学测试表明,塑性变形后,复合材料的抗拉强度、屈服强度和延伸率显著提高。SiCp/2024铝基复合材料主要的断裂方式为:合金基体的延性断裂、SiC颗粒断裂及SiC/Al界面脱粘。     

原位自生TiB2颗粒增强2024-T4铝基复合材料断裂行为数值模拟

利用SEM结合原位观测技术观察了颗粒体积分数为4.17%的原位自生TiB₂颗粒增强2024-T4铝基复合材料（TiB₂/2024-T4）的损伤断裂行为。试验结果表明,TiB₂颗粒偏聚带中的铝合金基体比颗粒稀疏区域中的铝合金基体率先发生断裂。根据这一试验现象建立了三种含随机颗粒偏聚带的二维体胞有限元模型,并施加拉伸载荷和周期性边界条件,推导了平面应力状态下的径向返回算法,结合Rice-Tracey局部失效准则模拟了颗粒偏聚带中微裂纹的萌生及扩展过程。数值分析结果表明：就单个颗粒来说,颗粒两极附近基体损伤最严重。颗粒偏聚导致损伤在颗粒附近基体中迅速累积,并发展成为基体微裂纹,且随着颗粒偏聚程度加剧,材料断裂应变下降。另外,体胞模型应力-应变曲线的非线性部分低于实测曲线,说明除了本文模型反映的载荷传递强化机制外,还需要进一步考虑颗粒对基体的间接强化机制。     

2024-T3和2524-T34铝合金疲劳裂纹的萌生机制

通过2024-T3和新型2524-T34铝合金的疲劳实验和对试样表面及疲劳断口的观测,研究了材料的微观结构和疲劳裂纹萌生机制.实验在室温下完成,应力比为0.1、加载频率为15 Hz.结果表明:实验材料呈现了再结晶的层状晶粒结构,晶粒沿着轧制方向被拉长,并较为平坦.2024铝合金中二相粒子的分布更为密集无序,且粗大、不规...     

Ductile fracture initiation and propagation modeling using damage plasticity theory

Ductile fracture is often considered as the consequences of the accumulation of plastic damage. This paper is concerned with the application of a recently developed damage plasticity theory incorporates the pressure sensitivity and the Lode angle dependence into a nonlinear damage rule and the material deterioration. The ductile damaging process is calculated through the so-called “cylindrical decomposition” method. The constitutive equations are discussed and numerically implemented. An experimental and numerical investigation for three-point bending tests is reported for aluminum alloy 2024-T351. Crack initiation and propagation in compact tension specimens are also studied numerically. These simulation results show good agreement with experiments. The present model can successfully predict slant fracture as well as the formation of shear lips.     

Numerical prediction of slant fracture with continuum damage mechanics

Ductile specimens always exhibit an inclined fracture surface with an angle relative to the loading axis. This paper reports a numerical study on the cup-cone fracture mode in round bar tensile tests and the slant fracture in plane-strain specimens based on continuum damage mechanics. A combined implicit-explicit numerical scheme is first developed within ABAQUS through user defined material subroutines, in which the implicit solver: Standard, and the explicit solver: Explicit, are sequentially used to predict one single damage/fracture process. It is demonstrated that this numerical approach is able to significantly reduce computational cost for the simulation of fracture tests under quasi-static or low-rate loading. Comparison with various tensile tests on 2024-T351 aluminum alloy is made showing good correlations in terms of the load-displacement response and the fracture patterns. However, some differences exist in the prediction of the critical displacement to fracture.     

2024-T3铝合金动力学实验及其平板鸟撞动态响应分析

通过电子万能试验机和分离式霍普金森拉杆（SHTB）拉伸试验分别获得2024-T3铝合金材料准静态和高应变率两种应变率下的应力-应变曲线。铝合金材料的本构关系由能够反映材料硬化效应和应变率强化效应的Johnson-Cook材料模型描述,方程中的4个参数通过不同应变率下的应力-应变曲线拟合得到。基于瞬态动力学软件PAM-CRASH,结合材料动态力学性能试验所获得的2024-T3铝合金Johnson-Cook模型方程,耦合光滑粒子流体动力学（SPH）方法和有限元（FE）方法建立2024-T3铝合金平板的鸟撞数值模型,数值计算所得动态响应与鸟撞试验结果吻合较好,表明建立的鸟撞数值计算模型是合理、可靠的,整个分析流程从材料动态力学性能试验、鸟撞数值计算到最终的鸟撞试验验证为飞机结构的抗鸟撞设计与分析提供了有力的参考。     

In situ investigation of small fatigue crack growth in poly-crystal and single-crystal aluminium alloys

Abstract   In situ scanning electron microscope observations of short crack growth in both a poly-crystal and a single-crystal alloy revealed that fatigue cracks may grow in a shear decohesion mode over a length that is several times the grain size, far beyond the conventional stage I regime. In the poly-crystal aluminium alloy 2024-T351, fatigue cracks were found to continue to grow along one shear band even after two mutually perpendicular shear bands had formed at the crack tip. For the single-crystal alloy specimen with the loading axis being nearly perpendicular to its main shear plane, mode I fatigue cracks were found to grow along the shear band. These two types of fatigue crack growth pose a significant challenge to the existing fatigue crack growth correlating parameters that are based on crack-tip opening displacement. In particular, it has been found that the cyclic crack-tip opening displacement, which accounts for both large-scale yielding and the lack of plasticity-induced crack closure, is unable to unify the growth rates of short and long cracks in aluminium 2024-T351, suggesting a possible dependence of crack growth threshold on crack length.     

定量研究2024-T351铝合金搅拌摩擦焊过程中的时效相行为

采用差示扫描量热法研究了2024-T351铝合金搅拌摩擦焊接过程中时效相回溶或粗化的相对值,给出了一种定量研究搅拌摩擦焊接过程中时效相行为的方法.结果表明:相对于母材,焊缝不同区域时效相的回溶或粗化值可通过公式f=Sx/SBM-1来计算;当搅拌摩擦焊搅拌头转速为700r·min-1,焊接速度为200mm·min-1时,...     

Limits to the applicability of LEFM in notch fatigue

A comparison is made between elastic-plastic, and linear-elastic, fracture mechanics parameters, for notch fatigue problems. It is shown that in high-strength aluminium alloy 2024-T351, at normal working stress levels, the errors involved in using LEFM for any crack detectable during non-destructive testing, is likely to be small.The applicability of the method to structural steels is also discussed, and some problems identified.     

A MODEL FOR THE FATIGUE LIMIT AND SHORT CRACK BEHAVIOUR RELATED TO SURFACE STRAIN REDISTRIBUTION

A model based on surface strain redistribution and the reduced closure stress of short cracks is shown to successfully predict the fatigue limit and short crack growth behaviour for aluminium alloy 2024-T351. Using this approach, the length of non-propagating cracks can be anticipated. The local stress intensity range may be resolved into two components (first the linear elastic fracture mechanics component and the second is due to surface strain concentration). Consequently, the local stress intensity range of aluminium alloy 2024-T351 is a maximum at a depth of approximately half a grain diameter and a minimum at a depth slightly in excess of three grain diameters. The reduced closure stress for short cracks coupled with the increased applied stress intensity caused by surface strain redistribution accounts for the variation of the effective stress intensity parameter as a function of crack depth. This parameter is a maximum for the smallest possible crack (3 μm) and decreases as crack length increases.     

Fracture of laminates combining 2024-T3 and 7075-T6 aluminum alloys

Crack growth resistance curves have been determined for crack-divider laminates in which layers of 2024-T3 aluminum alloy are adhesively bonded to layers of 7075-T6 alloy. Results are compared with the fracture resistance of laminates consisting wholly of each material, the layer thickness being the same (1.54 mm) in all cases. The initial portions of the resistance curves are similar for both alloys; however those for 2024-T3 have steeper slopes at longer effective crack lengths. As a result, laminates consisting entirely of 2024-T3 alloy exhibit greater amounts of stable crack extension and higher toughnesses at instability. This is attributed in part to the greater strain hardening rate in 2024-T3 material. Laminates combining 2024-T3 and 7075-T6 layers are intermediate between those consisting entirely of one or the other alloy.     

Fatigue of aluminium alloy 2024-T351 in humid and dry air

Reversed bending fatigue tests conducted on specimens of aluminium alloy 2024-T351 in dry and humid air at stress levels of 248, 276, 290, 317 and 359 MPa showed that at low stress amplitude humid air reduces the fatigue life by as much as 21%. Mirco-hardness tests showed that the reduction in fatigue life is primarily attributed to localized hydrogen-induced overageing. SEM analysis and microhardness data were combined with past studies to propose a mechanism for environmentally induced fatigue in aluminium alloy 2024-T351 over a wide range of stress levels.     

Variation of the cyclic strain-hardening exponent in advanced aluminium alloys

The cyclic strain-hardening exponents for five fatigue-resistant aluminium alloys were determined throughout the fatigue life to study the degree of cyclic stability of these alloys. Data were compared with results for 2024-T4 aluminum and for two high-pressure steels. The strain-hardening exponent increased logarithmically in all cases except 2024-T4, although the increase was small and did not exceed 33% over the fatigue life. 7475-T351 aluminium alloy was found to be entirely stable, and 7075-T7351 almost so. These were followed in order of rising sensitivity by 2014-T6, 7050-T73651, and 2124-T851 aluminium alloys, and 28NiCrMo7.4 and 30CrNiMo8 steels. 2024-T4 aluminum alloy demonstrated a strong decrease in strain-hardening exponent with fatigue life.     

Characterization of inertia-friction welds between a rapidly-solidified Al-Fe-Mo-V alloy and IM 2024-T351

The structures, mechanical properties and fracture behaviour of inertia-friction welds produced between rapidly-solidified/powder metallurgy (RS/PM) Al-9Fe-3Mo-1V (wt %) and ingot metallurgy (IM) 2024-T351 aluminium were investigated. Visual examination showed the axial displacement experienced by the specimens during welding and the degree of metal expulsion from the weld interface (i.e. flash) to increase with an increase in axial force. The weld flash was observed to originate principally from the IM 2024-T351, which was consistent with the lower elevated-temperature strength of this precipitation-hardened alloy. Although the weld interface region remained nearly flat in welds produced using low axial force, this surface became increasingly curved (concave into the Al-9Fe-3Mo-1V alloy) with an increase in axial force. Microstructure analysis using both light and analytical electron microscopy characterized the heat- and deformation-affected zones (HDZs) in each of the base metals and the weld interface regions. The HDZ directly adjacent to the weld interface in the IM 2024-T351 exhibited fine, recrystallized alpha aluminium grains and an absence of S precipitates present in the base metal microstructure. The HDZ directly adjacent to the weld interface in the Al-9Fe-3Mo-1V exhibited fine alpha grains and fine, spherical and acicular dispersoids, which in part originated from the plastic deformation and fracture of coarse base metal dispersoid particles. The extent of this dispersoid-refined region was greatest at the centre of the weld as opposed to the outer periphery, and in the low rather than the high axial force weld. At the weld interface in the vicinity of the axial centre line, the occurrence of highly localized mechanical mixing between the two alloys was determined using both light and electron microscopy and electron-microprobe analysis techniques.Microhardness traverses showed relatively little variation in hardness across the weld interface and an absence of hardness degradation at any location relative to the unaffected base metals. Room-temperature transverse-weld tensile testing showed tensile strengths to range between 85 and 90% of the RS/PM base metal, with fracture occurring in the Al-9Fe-3Mo-1V HDZ remote from the weld interface. Three-point guided bend testing also revealed fracture to occur in the Al-9Fe-3Mo-1V HDZ. SEM fractographic analysis of the fracture surfaces found fracture in the Al-9Fe-3Mo-1V to involve microvoid formation at dispersoid/alpha aluminium interfaces and subsequent ductile rupture in the alpha aluminium matrix.     

Damage characterization and modeling of a 7075-T651 aluminum plate

In this paper, the damage-induced anisotropy arising from material microstructure heterogeneities at two different length scales was characterized and modeled for a wrought aluminum alloy. Experiments were performed on a 7075-T651 aluminum alloy plate using sub-standard tensile specimens in three different orientations with respect to the rolling direction. Scanning electron microscopy was employed to characterize the stereology of the final damage state in terms of cracked and or debonded particles. A physically motivated internal state variable continuum model was used to predict fracture by incorporating material microstructural features. The continuum model showed good comparisons to the experimental data by capturing the damage-induced anisotropic material response. Estimations of the mechanical stress–strain response, material damage histories, and final failure were numerically calculated and experimentally validated thus demonstrating that the final failure state was strongly dependent on the constituent particle morphology.