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.     

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.     

Tensile and energy density properties of 2024, 6013, 8090 and 2091 aircraft aluminum alloy after corrosion exposure

Evaluation was made for the corrosion susceptibility of aircraft structure aluminum alloys 2024 T351, 6013 T6, 8090 T81 and 2091 T84. Tensile and energy density data were obtained. Stereoscopic and metallographic corrosion analysis were made as well. The specimens were pre-corroded using accelerated laboratory corrosion tests or out-door atmospheric conditions before testing. Noticeable decrease of yield and ultimate tensile stress were detected when the specimen surface was corroded. Dramatic volumetric embrittlement was observed even after short exposure times that were not sufficient for the appreciable development of surface corrosion attack. Observed material degradation behavior is attributed to hydrogen penetration and absorption.     

Modèle de comportement mécanique du béton armé corrodéMechanical model of reinforced concrete damaged by steel corrosion

This article deals with an analytical mechanical model of reinforced concrete damaged by steels corrosion. It is based on the taking into account of local bond between steel and the tensile concrete located between the cracks. Then, this model allows to quantify the coupled effect of steel cross section reduction and loss of bond strength due to steel corrosion on mechanical behaviour of reinforced concrete. To cite this article: A. Castel et al., C. R. Mecanique 330 (2002) 45–50     

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.     

Fatigue properties of 2024-T3, 7075-T6 aluminum alloys modified using plasma-enhanced ion beams

This paper presents experimental results on the application of Microsecond Plasma Opening Switch (MPOS) technology to Al alloy surface modification. The main objective of the experiments presented here was to study the change in the tensile and fatigue properties of the MPOS-treated Al2024, Al7075 alloy samples. The bending fatigue test was carried out both in air and in corrosive media. The measurements indicate significant improvement of fatigue properties for the treated 7075 alloy in corrosive media (1.5 times higher in fatigue limit). For the 2024 alloy the enhancement in fatigue lifetime for higher stresses was measured. Anodic polarization curve measurements were carried out at various values of fatigue cycles.     

Effects of inter-fibre spacing on damage evolution in unidirectional (UD) fibre-reinforced composites

A three dimensional (3D) micromechanical study has been performed in order to investigate local damage in UD composite materials under transverse and longitudinal tensile loading. In particular, the influence of non-uniform distribution of fibres in RVEs (representative volume element) with a hexagonal packing array and the effects of thermal residual stresses has been investigated. To examine the effect of inter-fibre spacing and residual stress on failure, a study based on the Maximum Principal Stress failure criterion and a stiffness degradation technique has been used for damage analysis of the unit cell subjected to mechanical loading. Results indicate a strong dependence of damage onset and its evolution from the fibres position within the RVE. Predicted mechanical properties, damage initiation and evolution are also clearly influenced by the presence of residual stress.     

A PREDICTIVE APPROACH TO THE IN-PLANE MECHANICAL PROPERTIES OF STITCHED COMPOSITE LAMINATES

This contribution attempts to model the alteration of the in-plane elastic properties in laminates caused by stitching,and to predict the in-plane effective tensile strength of the stitched composite laminates.The distortion of in-plane fibers is considered to be the main cause that affects the in-plane mechanical properties.A fiber distortion model is proposed to characterize the fiber misalignment and the fiber content concentration due to stitching.The undistorted region,the fiber distortion region,the resin-rich pocket and the through-thickness reinforcement section are taken into account.The fiber misalignment and inhomogeneous fiber content due to stitching have been formulated by introducing two parameters,the distortion width and maximum misalignment.It has been found that the ply stress concentration in stitched laminates is influenced by the two concurrent factors,the stitch hole and inhomogeneous fiber content.The stitch hole brings about the stress concentration whereas the higher fiber content at the local region induced by stitching restrains the local deformation of the composite.The model is used to predict the tensile strength of the [0/45/0/-45/90/45/0/-45]_(2s) T300/QY9512 composite laminate stitched by Kevlar 29 yarn with different stitching configurations,showing an acceptable agreement with experimental data.     

Mechanical behavior of 2024 Al alloy specimen subjected to paint stripping by laser radiation and plasma etching

Paint removal is required in a series of aeronautical procedures such as maintenance and repair. Today's paint stripping processes which are based on application of chemicals and abrasion are inadequate for modern aircraft structures in addition to environmental contamination. several alternative techniques are in progress. However, the aspect of material property degradation when developing novel, alternative paint stripping techniques has not been properly faced up to present. The influence of two novel paint stripping processes on the mechanical properties of the substrate 2024 7351 aluminium alloy has been investigated. The paint stripping processes included laser radiation with excimer, CO₂, TEA-C0₂ and YAG laser sources as well as plasma etching. These processes have been applied for the removal of polyurethane coating which is a typical aeronautical paint system. The results indicated no significant degradation in yield strength and ultimate tensile strength. However a significant degradation in tensile ductility and toughness is observed with the application of all paint stripping processes, the highest degradation being associated with the ultraviolet excimer laser and plasma etching. On the other hand there is a considerable extension in fatigue life, which depends on the paint stripping process and the applied stress amplitude. At high stress there is no appreciable effect while at low stress there is an order of magnitude life extension associated with CO₂ laser paint stripping. At moderate stresses, there is an up to sixfold life extension associated with the excimer laser processing.     

Tensile testing of microsamples

Structural analysis has advanced rapidly to the point where lack of knowledge about the local inhomogeneity and anisotropic nature of the materials prevents the accurate prediction of the behavior; models can incorporate local material properties provided they can be experimentally determined. Conventional tensile techniques are unable to test speciments from submillimeter-sized regions, and micro-hardness measurements do not reveal directional variations. A system has been developed to perform tensile tests on microsamples that are 3.1 mm long with a gage cross section that is 0.2 mm square. The volume of the test region is roughly 500,000 times smaller than a standard half-inch diameter specimen. Tensile stress-strain curves are measured, and the yield strength, ultimate strength, modulus and elongation are calculated for each sample. Samples cut from the weld metal of undermatched welds in HY-100 plate showed a large variation in properties from the center of the weld to the outer edge, as well as anisotropy at some locations. The average mechanical properties of a group of microsamples cut from the outer region of the weldment compared favorably with measurements made using full-sized specimens; the variation in the microsample measurements reflect local variations in the material that are not measured with full-sized specimens.     

锈蚀钢筋混凝土偏心受压构件实验及其承载力计算方法研究

大量锈蚀钢筋混凝土偏压构件的实验研究表明,随着纵向受力主筋锈蚀率的增大,偏心受压构件的承载力及其刚度均有不同程度的降低。并且由于钢筋锈蚀,钢筋混凝土偏心受压构件延性也随之降低,脆性性质明显,从而可能使得钢筋混凝土偏压构件的破坏形态发生变化。在对预制钢筋混凝土柱进行快速锈蚀实验85研究的基础上,对锈蚀偏心受压构件的结构性能退化机理和破坏特征进行了分析;在现行计算理论和实验85结果分析的基础上,通过拟合出的锈蚀钢筋与混凝土间的应变不协调系数,对未锈蚀构件的相对界限受压区高度进行修正,得到了锈蚀偏心受压构件相对界限受压区高度的修正公式;考虑锈蚀钢筋截面的削弱、钢筋屈服强度的降低以及钢筋和混凝土之间粘结性能退化的影响,提出锈蚀偏心受压构件正截面承载力计算方法,为今后混凝土结构耐久性评估和可靠性鉴定提供了科学依据。     

A model for predicting the retardation effect following a single overload

Many engineering components are subjected to variable amplitude loading history. It is well known that retardation in fatigue crack growth occurs due to application of single overloads in a constant amplitude loading block. Many models have been proposed to capture this counter intuitive phenomenon which has resulted in improved understanding of retardation effect following tensile overloads and consequently resulting in better life prediction models. The proposed study is focused on to evaluation of retardation in fatigue life due to application of a single overload. A model for prediction of crack growth and crack growth rate following single overloads is presented. Several modifications to Wheeler’s growth idea are proposed, which incorporate a consideration for effective stress intensity factor, based on Elber’s concept of crack closure, relationship between overload ratio and the Wheeler’s exponent, and fatigue growth rate calculations. The results presented here show that plastic zone interaction following overload and the consideration of crack closure explain retardation effect following a single overload. Correlation between analysis and experimental data obtained from several sources in literature show that the scheme, is robust and provides an insight into the nonlinear aspect of crack growth results. The model has been tested for 2024-T3 aluminum alloy and 6061-T6 aluminum alloy and thorough calibrations performed, established the fidelity of the program.     

A multi-fiber approach for modeling corroded reinforced concrete structures

Taking into account the specific behavior of the steel/concrete interface is of primary importance to predict properly the structural response of RC structures. Several constitutive models have been proposed in the literature within the framework of nonlinear finite element method (2D and 3D). Such approaches usually lead to high computational costs due to the large number of degrees of freedom. In the present paper, a multifiber-based model including the steel/concrete interface behavior is proposed. Despite the fact that the kinematics of the multi-fiber approach is based on the theory of beams, this simplified strategy accounts for local phenomena such as the relative sliding between concrete and steel. Furthermore, this steel/concrete interface constitutive model can be extended to model the loss of bond properties due to corrosion. The numerical implementation aspects are described and local responses at the Gauss point level are exposed in the cases of monotonic loadings with and without corrosion. The efficiency and the reliability of the proposed approach are tested on structural case studies which highlight a good agreement between numerical and experimental results. This multifiber-based model provides a pertinent tool for the engineers concerns with the structural assessment of degraded reinforced concrete structures.     

Cyclic behaviour of short glass fibre reinforced polyamide: Experimental study and constitutive equations

Polymer matrix composites are widely used in the automotive industry and undergo fatigue loadings. The investigation of the nonlinear cyclic behaviour of such materials is a required preliminary work for a confident fatigue design, but has not involved many publications in the literature. This paper presents an extensive experimental study conducted on a polyamide 66 reinforced with 35 wt% of short glass fibres (PA66 GF35), at room temperature. The material was tested in two conditions: dry-as-moulded (DAM) and at the equilibrium with air containing 50% of relative humidity (RH50).An exhaustive experimental campaign in tensile mode has been carried out, including various strain or stress rates, complex mechanical histories and local thermo-mechanical recordings. Such an extended database allowed us to highlight several complex physical phenomena: viscoelastic effects at different time scales, irrecoverable mechanisms, non-linear kinematic hardening, non-linear viscous flow rule, cyclic softening.Taking into account this advanced analysis, a constitutive model describing the cyclic behaviour is proposed. As the experimental database only includes uniaxial tensile tests, the general 3D anisotropic frame is reduced to an uniaxial model valid for a specific orientation distribution. The robust identification process is based on tests which enable the uncoupling between the underlined mechanical features. This strategy leads to a model which accurately predicts the cyclic behaviour of conditioned as well as dry materials under complex tensile loadings.     

Q420钢管大气腐蚀后的抗侧撞性能

为研究大气腐蚀对Q420钢管构件服役期内耐撞性的影响,提出了计及腐蚀损伤的材料模型。引入损伤因子（ω）修正Voce模型,推导随腐蚀程度变化的低合金钢材本构方程,并通过加速腐蚀试验结果回归相应参数。利用ABAQUS软件定义构件材料特性,建立起受腐蚀的Q420钢管仿真模型,采用显式动力算法分析多种初始状态下,撞击体与不同腐蚀程度钢管的冲击响应规律。开展预腐蚀Q420钢管的落锤试验,将试验结果与数值计算结果进行对比,验证所建模型的合理性。结果表明:大气腐蚀导致材料名义强度降低,对Q420钢管抗撞击能力影响显著;随着腐蚀程度增加,冲击力峰值减小,撞击时间和深度增加;Q420钢管受腐蚀后抗冲击刚度减小,构件整体变形耗能增加,表明大气腐蚀使其抗冲击性能下降;同等动能增量下,增大撞击体初速度比增加初始质量获得的冲击力峰值增幅更大,而所得到的接触时间增幅更小。     

高温升率下LY12铝合金拉伸破坏及其机理的研究

使用Gleeble1500热力模拟实验系统研究了温升率及其历史对LY12铝合金拉伸破坏的影响。对采用不同温升率加热到给定温度后的试件的拉伸试验表明,相同工作温度下经历较高温升率历史的试件的强度较低;以不同温升率对不同预载应力水平的试件快速加热直至破坏,发现预载应力相同时经历较高温升率试件的失效温度较低．对试件断口附近材料金相组织的分析表明,当工作温度或预载应力水平相同时,较高温升率下材料的微缺陷明显增加．高温升率造成的局部热失配及材料微观组织结构的损伤,不同温度、应力和时间下材料的再结晶程度等对材料的宏观本构行为及失效具有重要影响．     

Numerical study of strength properties for a composite material with short reinforcing fibers

A numerical approach to the determination of strength properties for concrete with short reinforcing fibers on the basis of the finite element method is proposed. The mathematical model takes into account various processes of nonlinear deformation of the concrete matrix under compressive and tensile loads, the possibility of developing the inelastic strains in the concrete matrix and reinforcing fibers and the nonlinear interaction between them. The effect of fiber concentration, various loading surfaces for the material matrix, and the bonding type on the deformation of a composite material is analyzed. Numerical examples of strength analysis are given.     

Strain tunability of dielectric and ferroelectric properties in epitaxial lead titanate thin films

Many distinguished properties of epitaxial ferroelectric thin films can be tunable through the misfit strain. The strain tunability of ferroelectric and dielectric properties in epitaxial lead titanate ultrathin films is numerically investigated by using a phase field model, in which the surface effect of polarization is taken into account. The response of polarization to the applied electric field in the thickness direction is examined with different misfit strains at room temperature. It is found that a compressive misfit strain increases the coercive field and the remanent polarization while a tensile misfit strain decreases both of them. The nonlinear dielectric constants of the thin films with tensile misfit strains are much larger than those of the thin films without misfit strains, which are attributed to the existence of the a/c/a/c multiple domains in the thin films under tensile misfit strains.     

A Finite Element Analysis of a Tapered Flat Sheet Tensile Specimen

A uniaxial tension sheet metal coupon with a tapered instead of a straight gage section has been used for centering the location of diffuse neck and for measuring sheet stretchability in a non-uniform strain field. A finite element analysis of such a tensile coupon made of automotive steel sheet metals has been carried out to assess the effect of the tapered gage section geometry and material plastic strain hardening characteristics on the development of local plastic deformation pattern and local stress state, especially beyond the onset of diffuse necking but before localized necking. In particular, the finite element analysis was used in this study to evaluate the accuracy and reliability of an experimental data analysis method for estimating the post-necking effective plastic stress-strain curve based on the direct local surface axial plastic strain measurements for base metal, heat-affected zone, and weld metals of a dual-phase steel DP600. It is concluded that the estimated lower and upper bounds of the effective stress-strain curve at large strains are not satisfactory for low strain-hardening materials such as heat-affected zone and weld metals with the tapered tension coupons. A simple correction method utilizing only the additional local surface strain measurement in the transverse direction is proposed and it is shown to be effective in correcting the estimated effective stress-strain curve of dual-phase steel weld metals obtained for two tapered gage section geometries.     

Experimental Study of Bending Behaviour of Reinforcements

In composite reinforcement shaping, textile preform undergo biaxial tensile deformation, in plane shear deformation, transverse compaction and out-of-plane bending deformations. Bending deformations have been neglected in some simulation codes up to now, but taking into account them would give more accurate simulations of forming especially for stiff and thick textiles. Bending behaviour is specific because the reinforcements are structural parts and out of plane properties cannot be directly deduced from in-plane properties, like for continuous material. Because the standard tests are not adapted for stiff reinforcements with non linear behaviour a new flexometer using optical measurements has been developed to test such reinforcements. This new device enables to carry out a set of cantilever tests with different histories of load. A series of tests has been performed to validate the test method and to show the capacities of the new flexometer to identify non linear non elastic behaviour.