Fatigue and damage tolerance behaviour of corroded 2024 T351 aircraft aluminum alloy

The fatigue and damage tolerance behaviour of pre-corroded 2024 T351 aluminum alloy specimens has been investigated and compared to the behaviour of the uncorroded material. The experimental investigation was performed on specimens pre-corroded in exfoliation corrosion environment and included the derivation of S–N and fatigue crack growth curves as well as measurements of fracture toughness. The fatigue crack growth tests were performed for different stress ratios R. To obtain reference material behaviour all mechanical tests were repeated under the same conditions for uncorroded specimens. For the corroded material an appreciable decrease in fatigue resistance and damage tolerance was obtained. The results of the experimental investigation were discussed under the viewpoint of corrosion and corrosion-induced hydrogen embrittlement of the 2024 aluminum alloy. The need to account for the influence of pre-existing corrosion on the material’s properties in fatigue and damage tolerance analyses of components involving corroded areas was demonstrated.     

Fatigue and static strength of notched and unnotched aluminum-alloy and steel specimens

This paper describes a method of presentation of fatigue data on three commonly used aircraft materials, 2024-T3 and 7075-T6 aluminum alloys and normalized SAE 4130 steel, such that variations in fatigue strength with stress-concentration factor can be shown. Comparisons of the fatigue strengths of 2024-T3 and 7075-T6 aluminum are made for the most useful range of stress-concentration factors. Static-strength results of notched and unnotched specimens of the three materials are presented to show how the strength varies with some parameters of the stress concentration. Comparison of the data with one theory for the strength of cracked specimens was made.     

Crack growth in bi-material laminates

Growth rates of fatigue cracks have been measured in laminates fabricated by adhesively bonding layers of 2024-T351 and 7075-T6 aluminum alloys. The laminates had either two or four layers, with equal thicknesses and numbers of layers of each alloy. Fatigue-crack-propagation tests were performed with through-cracks, giving a crack-divider geometry, the results being compared to those for the two alloys tested in monolithic form. Crack-propagation rates in the bi-material laminates were intermediate between those of the monolithic alloys, with the slower growth in 2024-T351 tending to dominate over a portion of the growth-rate range. Fracture toughnesses of the laminates are also discussed.     

磷氮型极压抗磨添加剂对钢—铝摩擦副摩擦磨损性能的影响

采用ＳＲＶ摩擦磨损试验机评价了磷酸三甲酚酯（ＴＣＰ）、磷酸二丁酯（ＤＢＰ）、十二胺（ＤＡ）和磷酸胺盐（ＰＡＳ）抗磨添加剂在液体石蜡中对钢－铝摩擦副摩擦磨损性能的影响。采用Ｘ射线光电子能谱仪分析铝试磨痕表面边界润滑膜中的Ｐ和Ｎ元素的化学状态。结果表明,含Ｐ抗磨添加剂可以有效提高铝合金的耐磨性,而其中以磷氮剂的效果最好。其它含磷添加剂也可以有效地降低磨损。ＸＰＳ分析表明,在铝合金磨损表面形成了含磷酸铝     

An investigation about interference fit effect on improving fatigue life of a holed single plate in joints

In this article, the effect of interference fit on fatigue life of holed plate of mechanical joints was investigated both experimentally and numerically. In the experimental part, fatigue tests were carried out on the holed specimens of Al 7075-T6 alloy in which oversized steel pins were force fitted to them. These fatigue tests were conducted on open hole specimen and specimens with 1, 1.5, 2 and 4% nominal interference fit sizes at different cyclic longitudinal loads. From these tests the stress-life (S–N) data for different interference fit sizes were obtained. The results show that interference fit increases fatigue life compared to open hole specimens. In the numerical part of the investigation, 3D finite element simulations have been performed to obtain stress (or strain) histories and distributions around the hole due to interference fit and subsequent cyclic longitudinal loading using FEM package. The stress history from finite element (FE) simulation was used to explain the reason for fatigue life improvement in the interference fitted specimens.     

J resistance curves in aluminum SEN specimens using Moiré interferometry

TheU _y -displacement field obtained by white-light moiré interferometry were used to estimate the approximate far- and near-fieldJ-integral values associated with the subcritical crack growths in fatigue precracked 7075-T6 and blunt notched and fatigue precracked 2024-0 and 5052-H32 aluminum, single-edged notch (SEN) specimens. The initial phases of theJ-resistance curves for the somewhat brittle 7075-T6 and the two ductile 2024-0 and 5052-H32 aluminum SEN specimens are presented. Paper was presented at the 1987 SEM Spring Conference on Experimental Mechanics held in Houston, TX on June 14–19.     

Protective role of local Al cladding against corrosion damage and hydrogen embrittlement of 2024 aluminum alloy specimens

The present investigation aims to inquire whether Al cladding of 2024 aluminum alloy specimens could provide, additionally to the expected protection against corrosion damage, also a protection against the corrosion induced hydrogen embrittlement of the alloy. The latter is observed when bare 2024 material is subjected to laboratory exfoliation corrosion exposure also in the absence of mechanical loading. Furthermore, the study aims to ponder on the question whether local Al cladding at small regions of the specimen surface might suffice for protecting the specimen against corrosion damage and hydrogen embrittlement. The work comprises the results of an extensive experimental investigation including tensile tests on precorroded 2024 specimens protected through both complete and local surface Al cladding, metallographic and fractographic analyses as well as measurements of the hydrogen uptake during the corrosion process.     

比例超载对铝合金圆孔薄板试件低周拉-拉疲劳寿命的影响

对铝合金圆孔薄板试件进行了超载和常辐的对比实验,取六种超载比,对每一超载比各进行了不同超载周次的实验,从这些实验结果看出,对每一超载比,超载试件的疲劳寿命和常辐疲劳试件的寿命比值先是随超载周数的增加而较快的增加,之后在一个相当大的超载周数范围内,超载寿命比的变化不大。当超载周数超过一定值时,超载寿命比随超载周数的增加而略有减少。本文的实验中,在超载60％的情况下,超载周数从20周到90周的范围内,超载试件的平均寿命比常辐疲劳试件的寿命约高3.5倍。     

Low-cycle fatigue of 7075-T651 aluminum alloy in biaxial bending

The uniaxial and biaxial low-cycle-fatigue life of 7075-T651 aluminum alloy in completely reversed bending is investigated. Uniaxial data is obtained from a cantilever specimen, and round and elliptical simply supported plate specimens are used for nominal strain biaxialities of 1∶1, 1∶075 and 1∶0.50 in the plane of the specimen. Experimental data is correlated on the basis of the total octahedral shearing strain range in which the dependent component strains are calculated using the “effective value” of Poisson's ratio for elastic-plastic loading. Good agreement is obtained oetween the uniaxial and biaxial data when the total octahedral shearing-strain range is plotted against the number of cycles to failure in logarithmic coordinates. Also, it is shown that, in anisotropic materials, the directional uniaxial and biaxial low-cycle properties can be predicted from fatigue properties in any one direction if the anisotropy of fracture ductility is known.     

TC4钛合金超高周次轴向振动疲劳试验

应用基于压电超声疲劳试验技术开发的20kHz轴向振动疲劳试验系统,完成了室温下TC4钛合金超高周疲劳试验,获得了TC4合金在107～109周次范围内的轴向振动疲劳寿命曲线(S-N曲线);运用C.Paris推导公式预测了TC4合金材料的寿命,得到各应力水平下破坏率为50％、95％、99％的安全寿命.结果表明:在疲劳循环大于107周次时,试件仍会发生疲劳断裂,疲劳强度随循环次数的增加而下降,并不存在明显的疲劳极限.TC4合金的S-N曲线在107～109周次的范围内呈连续下降型.在轴向振动超高周疲劳试验中,试件的裂纹扩展寿命只占其在50％破坏率下疲劳安全寿命的一小部分,其疲劳寿命主要由试件的裂纹萌生寿命决定.     

DYNAMIC PROPERTIES OF AL-ALLOY FOAM BEAM DAMAGED BY COMPRESSIVE FATIGUE

The permanent residual strain in aluminum （Al） alloy foams induced by compressive fatigue gradually increases with the increasing number of loading cycles. Consequently, the progressive shortening of Al-alloy foam degrades the dynamic material performance by the failure and ratcheting of multi-cells in the foam. In this paper, the dynamic properties of Al-alloy foams damaged by compressive fatigue were studied. The beam specimens with various residual strains were made by cyclic compression-compression stress. The dynamic bending modulus and loss factor were evaluated by using a beam transfer function method. As a result, the dynamic bending stiffness of Al-alloy foam turned out to be decreased due to damage while the loss factor was improved because of the increasing energy dissipation of such factors as cracked cell walls formed during the shortening process of the foam. The loss factor shows a manifest dependence on the fatigue residual strain.     

CTOA and crack-tunneling measurements in thin sheet 2024-T3 aluminum alloy

The stable tearing behavior of 2.3-mm thick sheets of 2024-T3 aluminum alloy was experimentally investigated for middle crack tension specimens having initial flaws that were: (a) flat fatigue cracks (low fatigue stress) and (b) 45-deg through-thickness slant cracks (high fatigue stress). The critical CTOA values during stable tearing were measured by two independent methods, optical microscopy and digital-image correlation. Results from the two methods agree well.The CTOA measurements and observations of the fracture surfaces have shown that the initial stable tearing behavior of low and high fatigue stress tests is significantly different. The cracks in the low fatigue stress tests underwent a transition from flat-to-slant crack growth, during which the CTOA values were high and significant crack tunneling occurred. After crack growth equal to about the thickness (a>B), CTOA reached a constant value of 6 deg and after crack growth equal to about twice the thickness (a>2B), crack tunneling stabilized. The cracks in the high fatigue stress tests reach the same constant CTOA value after crack growth equal to about the thickness, but produced only slightly higher CTOA values during initial crack growth. The amount of tunneling in the high fatigue stress tests was about the same as that in the low fatigue stress tests after the flat-to-slant transition.This study indicates that stress history has an influence on the initial portion of the stable tearing behavior. The initial high CTOA values, in the low fatigue crack tests, coincided with large three-dimensional crack front shape changes due to a variation in the through-thickness crack-tip constraint. The measured CTOA reached a constant value of 6 deg for crack growth of about the specimen thickness. This coincided with the onset of 45-deg slant crack growth and a stabilized, slightly tunneled (about 20 percent of the thickness) crack-front shape. For crack growth on the 45-deg slant, the crack front and local field variables are still highly three dimensional.     

Prediction of fatigue crack growth life under variable-amplitude loading using finite element analysis

This article presents an elastic-plastic study aiming at predicting the fatigue crack growth (FCG) of 2024-T3 aluminum alloys under variable-amplitude loading. The proposed analysis needs the estimation of the residual stress distribution ahead of the crack tip during propagation. An elastic-plastic FE analysis has been implemented for modeling FCG using Chaboche's model. The FE study has been carried out through consideration of the loading history effect using the memory rules. Three different loading spectra have been applied in this work. The obtained results have been compared to the experimental ones and it has been proved that the suggested model has a better prediction of the FCG lives of cracked 2024-T3 aluminum alloy structures subjected to variable-amplitude loading.     

碳含量对腐蚀条件下低碳高合金钢冲击磨损性能的影响

在冲击、腐蚀磨损条件下,对不同碳含量的新型衬板用低碳高合金钢的磨损行为进行比较,采用金相显微镜、扫描电子显微镜和X射线衍射仪观察分析合金组织、其亚表层金相组织及冲击腐蚀磨损表面形貌,对合金组织和在腐蚀环境下的冲击磨损机制进行探讨,结果表明：与含碳量为0．16％、0．25％的低碳高合金钢相比,碳含量为0.21％的低碳高合金钢在腐蚀环境下具有最佳的抗冲击磨损性能,其磨损机制以疲劳剥层为主．     

成分和组织对衬板钢在腐蚀料浆环境下的冲击磨损性能与机理的影响

利用改造后的MLD-10型冲击磨损试验机研究了3种冶金矿山湿磨衬板钢在铁矿石酸性矿浆中的冲击腐蚀磨损行为;采用扫描电子显微镜观察了试样磨损表面形貌,用光学显微镜分析了垂直于试样磨损表面的亚表层金相组织.结果表明:低碳高合金钢的耐冲击腐蚀磨损性能优于高锰钢及中碳合金钢;低碳高合金钢的冲击腐蚀磨损机制主要为挤出硬化棱剥落、轻微的腐蚀磨损及浅层疲劳剥落,高锰钢的主要冲击腐蚀磨损机制为较深层的累积变形疲劳剥落和严重的腐蚀磨损,而中碳合金钢的主要冲击腐蚀磨损机制为深层脆性剥落和严重的腐蚀磨损.     

Surface hardness increase of 2024 aluminum alloy subjected to cyclic loading

Constant amplitude fatigue tests at R = 0.1, conducted on the aircraft aluminum alloy 2024 T3, have revealed an appreciable surface hardness increase of the alloy at the nano- and meso-scale during fatigue. The observed surface hardness changes could be monitored with confidence by means of nanoindentations. The degree of hardening increases with increasing number of fatigue cycles following exponential relations. With increasing fatigue stress level degree of hardening increases as well. The observed results provide a basis for developing concepts to early detect and also monitor fatigue damage accumulation in aluminum aircraft structures based on measurements of the material’s hardness changes by means of nanoindentations.     

Three-dimensional effects on the dynamic fracture determination of Al 7075-T651 using TPB specimens

Here we present a numerical analysis of three-dimensional effects on the dynamic three-point-bending fracture tests on Al 7075-T651 alloy specimens performed in a modified Split Hopkinson Pressure Bar. Using the Finite Element Method implemented in a commercial code the whole experimental device has been modeled. Different thicknesses and initial crack-to-width ratio specimens are simulated at different impact velocities to study the possible effect on the Crack Mouth Opening Displacement and, using a local stress fracture criterion, in the critical Stress Intensity Factor. The numerical results are compared with experimental results found by the authors.     

TC25钛合金低周疲劳特性与断口分析

对TC25钛合金进行不同应力幅值下的低周疲劳试验,测试钛合金的低周疲劳寿命.在分析钛合金的低周疲劳应力应变迟滞回线的基础上,利用试验特性和应力应变本构关系,推导简化的Manson-Coffin公式,获取钛合金的应力-寿命曲线.最后,应用SEM对钛合金试样断口进行形貌观测,分析低周疲劳的断裂机理.结果表明,TC25钛合金的低周疲劳塑性变形,主要产生在加速断裂阶段.钛合金试样断面存在多处剪切唇,瞬时断裂由剪应力主导.钛合金的低周疲劳断裂以解理形核形成疲劳源,裂纹扩展存在多种模式,瞬断为准脆性断裂.     

Evaluation of the probability distribution of pitting corrosion fatigue life in aircraft materials

Corrosion and fatigue properties of aircraft materials are known to have a considerable scatter due to the random nature of materials, loading, and environmental conditions. A probabilistic approach for predicting the pitting corrosion fatigue life has been investigated which captures the effect of the interaction of the cyclic load and corrosive environment and all stages of the corrosion fatigue process (i.e. the pit nucleation and growth, pit-crack transition, short- and long-crack propagation). The probabilistic model investigated considers the uncertainties in the initial pit size, corrosion pitting current, and material properties due to the scatter found in the experimental data. Monte Carlo simulations were performed to define the failure probability distribution. Predicted cumulative distribution functions of fatigue life agreed reasonably well with the existing experimental data. The project supported by SRF for ROCS, State Education Ministry of China, and the Society for the Promotion of Science (JSPS) of Japan (JSPS-P02042)     

常幅载荷下结构元件断裂可靠度估算的应力强度因子模型

给出了一个估算结构元件疲劳可靠度的应力强度因子模型,系统阐述了元件在常幅载荷下疲劳可靠性的分析方法。该模型研究了常幅载荷作用下材料瞬时裂纹长度和应力强度因子的分布形式,建立了应力强度因子与断裂韧性之间的干涉关系。对7075-T7351铝合金中心裂纹试件试验数据分析的结果表明：裂纹的瞬时扩展长度和可靠度的预测结果均与试验结果符合很好,本文给出的基于应力强度因子的可靠性分析模型是合理的。