Aluminum Alloy Panels Research Articles

Influences of residual stresses and initial distortion on springback prediction of 7B04-T651 aluminium plates in creep-age forming

Springback behaviour of surface-machined 7B04-T651 aluminium plates of 3–8mm thicknesses, creep-age formed under a single curvature bending radius of 1200mm, has been experimentally investigated. The surface residual stress levels of the plates have been measured and the typical residual stress distribution and values in 7000-series aluminium alloys have been reviewed and analysed. Finite element models have been developed to simulate creep-age forming (CAF) processes and predict the amount of springback in the physical CAF test. Machining-induced residual stresses and distortion have been considered in the finite element models. This paper presents a first attempt to study the effect of machining-induced residual stresses on the errors of springback prediction in CAF of 7000-series aluminium alloy panel components. It has been found that the effect of machining-induced residual stresses on the accuracy of springback prediction is related to the initial loading stress levels in CAF. In addition, when the initial distortion conforms to the loading curvature, an opposing effect on the material’s deformation is observed.

Experimental study of CFRP patches bonded on a cracked aluminum alloy panel

An experimental study was conducted to arrest a crack in 1.0-mm-thick 6061-T6 aluminum single-edge-notch tension specimen. The specimen was repaired by symmetrical bonding of carbon fiber-reinforced polymer patches made of several plies. Although J-integral was reduced drastically, the specimen failed at one of the leading edges through separation at the interface. The separation was suppressed by tapering all the four leading edges of the patches. The specimens did not fail in the patched area when loaded up to the far field stress of 272 MPa, which was higher than the yield stress, 265 MPa of the aluminum plate.

Imaging and Information Processing of Pitting-Corroded Aluminum Alloy Panels with Surface Metrology Methods

It has been established that corrosion is one of the most important factors causing structural deterioration, loss of metal, and ultimately decrease of product performance and reliability. Corrosion monitoring, accurate detection and interpretation are recognized as key enabling technologies to reduce the impact of corrosion on the integrity of critical aircraft and industrial assets. Interest in corrosion measurement covers a broad spectrum of technical approaches including acoustic, electrical and chemical methods. Surface metrology is an alternative approach used to measure corrosive rate and material loss by obtaining surface topography measurement at micrometer levels. This paper reports results from an experimental investigation of pitting corrosion detection and interpretation on aluminum alloy panels using 3D surface metrology methods, image processing and data mining techniques. Sample panels of AA 7075-T6, an aluminum alloy commonly used in aircraft structures, were coated on one side with a corrosion- protection coating and assembled in a lap-joint configuration. Then, a series of accelerated corrosion testing of the lap-joint panels were performed in a cyclic corrosion chamber running ASTM G85-A5 salt fog test. Panel surface characterization was evaluated with laser microscopy and stylus-based profilometry to obtain global and local surface images/characterization. Promising imaging and surface features were extracted and compared between the uncoated and coated panel sides, as well as on the uncoated sides under different corrosion exposure times. In the evaluation process, image processing, information processing and other data mining techniques were utilized. Information processing involves the steps of feature or Condition Indicator extraction and selection. The latter step addresses the problem of selecting those features that are maximally correlated with the actual corrosion state, for the purpose of corrosion detection, localization, quantification and state estimation. The results, verified by mass loss data, confirmed the contention that pits at the panel surfaces formed as a result of electrochemical corrosion attack, and showed that deteriorating pitting corrosion attack correlates with increasing corrosion exposure times. This study is a first step in the process of understanding, assessing and responding to the pitting corrosion and ultimately preventing material failure to insure aircraft structural integrity.

Research on Formability of Aluminum Alloy Automotive Panels by Using Viscous Pressure Forming

The viscous pressure forming (VPF) was applied to form an automotive aluminum alloy panel in this paper. The formability of the panel was investigated. Further, the springback characteristics of the panel were analyzed. The results show that the viscous medium is helpful to improve the flow of the sheet blank. The positive and negative springback occur for the panel formed by VPF. It is different from the panel formed by rigid punch forming, in which only the positive springback occurs. Furthermore, the springback value of the panel formed by VPF is smaller than the latter.

Experimental and Numerical Post‐buckling Analysis of Thin Aluminium Aeronautical Panels under Shear Load

ABSTRACTIn this work, a novel multi‐hinged test fixture, which can apply a pure shear load to thin panels, is presented. The new device has been used to perform several experimental tests on the shear behaviour of aluminium alloy panels. Four different test configurations have been considered: stiffened and not stiffened panels, with and without a central rectangular cut‐out. The specimens, made of Al 6082 T6, are tested under shear load boundary conditions up to buckling and post‐buckling occurrence. Furthermore, finite element analyses have been performed in order to simulate the four panel configurations tests. The new test fixture has shown the capability to buckle a panel attaining very large out‐of‐plane displacements, if compared to the outcomes of a traditional picture frame test fixture, under the same external load. Numerical and experimental results are finally compared, showing a satisfactory agreement.

Shielding performances of the designed hybrid laminates impacted by hypervelocity flyer

With increasing threat of man-made debris in outer space, a shielding panel designed for spacecraft or satellite should withstand hypervelocity impact of debris, with speed up to 9.0km/s. In this study, shielding performances of 2024 aluminum alloy panel, carbon fiber reinforced polymer composite (CFRP) panel and their hybrid laminates with different stacking configurations were investigated by ballistic tests of the materials impacted by Mylar flyer at 9.0km/s. The results are supposed to guide the design of shielding laminate panel. It was found that the stacking combination of CFRP and aluminum alloy significantly reduced the peak shock pressure induced by hypervelocity impact, and the increase of the layer number enhanced the shielding performance of the hybrid laminate. The five-layered aluminum alloy/CFRP laminates resisted the impact of the flyer without perforation. Furthermore, the extent of the damage of an impacted laminate was related to the velocity profile on its free surface. The planar plate impact testing employing a Doppler laser velocity interferometer is a feasible approach to quantitatively evaluate the shielding performance of structural material.

Corrosion Mechanism and Characteristic of 7075-T6 Aluminum Alloy Panel on Airline Aircraft

In order to provide an insight into the corrosion mechanisms of civil aircraft structural components,extensive investigation is performed to characterize the corrosion morphology,crack characteristics and corrosion productions of 7075-T6 aluminum alloy aircraft panel.Furthermore,the corrosion mechanisms are analyzed systematically.Microstructures and composition of coating surface on complete coating and coating corroded without peeling are analyzed respectively via scanning electron microscopy(SEM) and energy disperse spectrometer(EDS).Cross-section metallographic samples on aircraft panel are observed by optical microscope(OM),and X-ray diffraction(XRD) analysis of surface corrosion residue is also conducted.It has been found that for the surface coatings in the vicinity of rivet holes of 7075 aluminum alloy panel,bubble and cracking are easily to occur accompanied by stratiform exfoliation corrosion.Surface corrosion morphology is mainly made up of microcracks and chaps,and the major corrosion products are hydroxides.In the neighboring areas of rivet holes,a large number of discontinuous and stair-step paralleled stress corrosion cracks are induced under the surface coating.The inflation caused by crack expansion is a key factor determining surface coating desquamation.Moreover,Cl–and SO42– in corrosive solution can accelerate the atmospheric corrosion of aluminum alloy aircraft panel obviously.

Finite Element Analysis of Formability of Aluminum Alloy Double-Curved Panel by VPF

To investigate the formability of aluminum alloy panel during the viscous pressure forming (VPF), the stress state and plastic flow of the aluminum alloy double-curved panel during VPF are analyzed by the commercial software ANAYS/LS-DYNA. The influence factors of thickness reduction are also investigated. The results show that the viscous medium is helpful to improve the uniform flow of the sheet blank. From the panel centre to the flange edge, the stress of the sheet blank turns from tensile-tensile stress to tensile-compressive stress. The stress state boundary lies at the edge of the double-curved panel. The formability of the sheet blank can be improved by controlling the blank holder pressure and drawbead height.

Impact Analysis of Aluminum-Fibre Laminated Composite

Impact analysis of an advanced laminated composite comprising aerospace 2024-T3 aluminum alloy and resin impregnated E-glass fiber layers with different stacking sequences is presented in the paper. The panel under study is subjected to load-time history equivalent to the low velocity crash using FEM software ANSYS 12. Stress values in the laminate are found to be discontinuous at the interfaces whereas the displacements are continuous across the interfaces. The maximum stress and displacement values in the laminate are comparable with those in monolithic aluminum alloy panel of same thickness.

Scanning and Modeling of Large Thin-Walled Curved Surface Part

The large sandwich structure composed of thin-walled aluminum alloy panels, and variable thickness of honeycomb or Polymethacrylimide (PMI) foam core is usually manufactured by pre-bonded forming process, that is pre-forming panels and sandwich core, and then curing adhesive them to be sandwich structure. Welding process of large thin-walled panel causes the panel surface to be irregular and have greater errors relative to the design surface. Simply CNC machining the sandwich core according to the design surface cannot guarantee an exact match sandwich core consistent with the panels. The actual topography of the panels must be scanned. It is proposed that the use of a new hand-held laser scanner, Handyscan to scan large thin-walled curved surface parts, of Geomagic software to handle the acquired point clouds and construct the surface model.

Manufacturing Process of Large Scale Sandwich Structure with Variable Thickness of PMI Foam Core

Composite sandwich structures are extensively used in the aerospace, wind power, sports equipment, shipbuilding, automotive and train locomotive industries in order to improve structure rigidity and reduce weight. The molding process of sandwich structure using glass cloth and fibre materials as panels has been reported in many literatures. However, few researches are found relative to the molding process of large scale sandwich structure with the characteristics of thin-walled aluminum alloy panels and variable thickness of Polymethacrylimide (PMI) foam cores. This paper describes a preformed molding process that consists of thermoforming foam core blocks, assembling blocks into a whole sandwich core, CNC machining the sandwich core according to surface models of the thin-walled aluminum alloy panels, and finally, bonding and curing panels and sandwich cores.

Residual Stress Modeling and Simulating for the Panel with Interference-Fit Riveting

The interference-fit riveting technique is widely used in aircraft assembly, which can generate residual stress around the wall of hole and significantly improve the fatigue life of the structure. The residual stress is usually determined by experiment or numerical simulation, locking of theoretical support. So this paper researches the residual stress modeling of the panel in a riveting process. Firstly, based on the axisymmetric upsetting theory, dividing the riveting process into two stages, the formulas for calculating the contract pressure caused by the rivet around the hole of panel are built. Secondly, under the action of contact pressure, the residual stress distribution expressions are developed by elastic-plastic mechanics. Finally, in ABAQUS system, a three-dimensional instance made up of one flush rivet and two panels of aluminum alloy is used to verify the theoretical formulas. The feasibility of the theoretical formulas about the residual stress of the panel is validated by comparing the theoretical calculation and finite element simulation results.

Validation of a modified material model for use with shell elements to improve the predictive accuracy of the thickness distribution in superplastic forming of sheet metals

The automotive industry has recently begun using the superplastic forming (SPF) process to fabricate complex aluminum and magnesium alloy panels that cannot be formed at room temperature due to insufficient formability. One of the manufacturing problems encountered during SPF is excessive thinning in the form of a localized neck; which can lead to fracture. Localized necking can be predicted with the use of continuum elements in finite element analysis (FEA); however, the use of these elements in simulating SPF of large automotive panels is computationally intensive and often computationally prohibitive due to convergence issues. This paper examines the use of a modified material model (developed by engineers at Livermore Software Technology Corporation (LSTC) that can be used with conventional Belytschko–Tsay shell elements. This model considers normal stresses during SPF, which is needed to predict necking locations. The paper reports the results on investigating means for improving computational efficiency with this new formulation (i.e. element size, mass scaling, and adaptive meshing) and compares the performance of the normal stress element formulation with that of Belytschko–Tsay shell element in simulating the SPF process. The findings indicate that the newly developed formulation can be used for predicting localized thinning under SPF conditions.

Using natural cellular materials to improve blast-wave protection

Energy absorbing materials and systems incorporating materials such as foams or honeycomb structures are highly relevant in the design of blast protection systems. Systems with high strength-to-weight ratio and excellent energy absorption capacity under dynamic loading play an important role in defence systems. Sandwich structures, namely their cores, have the ability to sustain large deformations (strains) under constant load enabling them to absorb energy. After reaching their yield stress cellular materials exhibit a region of constant stress for increasing strain until densification is reached, allowing them to absorb considerable amounts of energy relative to their density. The authors incorporate a Micro-Agglomerate Cork (MAC) compound core between two aluminium alloy panels in order to combine the properties of light-weight materials with their high energy absorption capability. It is shown that the inclusion of the micro-agglomerate cork core increases the energy absorption capability of the protection structure under blast loading.

An experimental study of contact pressure distribution in panel stamping operations

This paper presents an analysis of the dynamic contact pressure distribution at the sheet metal-tooling interface in normal and defective panel stamping operations. Specifically, the stamping of a 1.27 × 20 × 15 cm rectangular aluminum alloy panel has been simulated using finite-element modeling methods. The modeling results are leveraged to determine the forces acting on sensors embedded in the die structure. Based on these results, a customized stamping test bed has been designed, which incorporates an array of force sensors embedded into the stamping die to enable online measurement of the forces in the die. The force measurements are processed by the thin plate spline numeric surface interpolation method to evaluate a continuous contact pressure distribution map of the die cavity. The pressure distribution in the die cavity in normal and defective scenarios has been presented along with a suggested volume thresholding method for automated defect detection. By leveraging the numeric surface generation method to analyze experimental data, the present study establishes the basis for intelligent tooling and in-process monitoring for improved observability in sheet metal stamping.

The Effect of Ultrasonic Impact Treatment on the Fatigue Resistance of Friction Stir Welded Panels

In this work, the results of an experimental study for assessing the effects of Ultrasonic Impact Treatment on the fatigue resistance of Friction Stir Welded aluminum alloy panels are presented. Although the significant compressive residual stress introduced on the material by ultrasonic impact treatment (UIT) was expected to cause retardation in the crack growth rate, this was only noted at low initial ΔΚ values. At high ΔΚ values, the effect of UIT practically diminishes. The phenomenon was attributed to the relaxation/redistribution of the residual stresses with fatigue damage. This provides an alarming situation where damage tolerance design relies on models where only the initial residual stress profile is taken into account without knowledge of the potential re-distribution of the residual stresses caused by the fatigue damage accumulation. The findings of this work also indicate that any FCG tests performed can only be considered as case-specific and conclusions can only be drawn for the case studied.

Harvesting Vibration Energy for Structural Health Monitoring in Aircraft

The concept of harvesting energy is not a new one: there has been an interest in this area for around 10 years. Devices typically use either vibration (rigid body motion) or thermal gradients and can harvest sufficient energy to power telemetry, small devices or to charge a battery or capacitance device. However, for the new generation of aircraft, (both fixed wing and rotating) there is now an urgent need to develop energy harvesting systems in order to provide localised power for sensors in structural health monitoring systems (SHM). By implementing SHM, aircraft manufacturers can benefit from improved safety, reduced maintenance and extended aircraft life. The work presented examines the feasibility of designing an energy harvesting system powered by the vibrations of aircraft panels generated in flight. PZT (lead zirconate titanate) harvesters are bonded to an aluminium alloy panel, representative of an aircraft wing panel which is vibrated across a range of amplitudes (up to + 0.2mm) and frequencies (up to 300Hz). By recording voltage and current outputs from each harvester, generated power is calculated which when normalised for area and mass indicates values of up to 7.0 Wm-2 and 2.5Wkg-1 respectively, representing mechanical to electrical energy conversion efficiencies of up to 35% dependant on frequency of vibration. From these values it is estimated that a harvester area of down to 71cm2 or mass of as little as 20g is necessary to meet the current minimum power requirements of SHM systems of 50mW. With predicted reductions in sensor power consumption indicating system power requirements in the order of 0.1-1mW, this work shows that piezoelectric energy harvesting has future potential for powering aerospace SHM systems.

Fail-safe design of integral metallic aircraft structures reinforced by bonded crack retarders

This paper presents an investigation on the effectiveness of crack growth retarders bonded to integral metallic structures. The study was performed by both numerical modelling and experimental tests. It focuses on aluminium alloy panels reinforced by bonded straps made of carbon-epoxy, glass-epoxy composite materials or a titanium alloy. The goal was to develop a fail-safe design for integrally stiffened skin-stringer panels applicable to aircraft wing structures. The modelling strategy and finite element models are presented and discussed. The requirements that the models should meet are also discussed. The study has focused on establishing the extent of crack retarder benefits, in terms of fatigue crack growth life improvement, by numerical simulation and experimental tests of various crack retarders. The results of predicted fatigue crack growth retardation have been validated by tests of laboratory samples. This study concludes that by bonding discrete straps to an integral structure, the fatigue crack growth life can be significantly improved.

Residual Strength of Stiffened LY12CZ Aluminum Alloy Panels with Widespread Fatigue Damage

Experimental and analytical investigations on the residual strength of the stiffened LY12CZ aluminum alloy panels with widespread fatigue damage (WFD) are conducted. Nine stiffened LY12CZ aluminum alloy panels with three different types of damage are tested for residual strength. Each specimen is pre-cracked at rivet holes by saw cuts and subjected to a monotonically increasing tensile load until failure is occurred and the failure load is recorded. The stress intensity factors at the tips of the lead crack and the adjacent WFD cracks of the stiffened aluminum alloy panels are calculated by compounding approach and finite element method (FEM) respectively. The residual strength of the stiffened panels with WFD is evaluated by the engineering method with plastic zone linkup criterion and the FEM with apparent fracture toughness criterion respectively. The predicted residual strength agrees well with the experiment results. It indicates that in engineering practice these methods can be used for residual strength evaluation with the acceptable accuracy. It can be seen from this research that WFD can significantly reduce the residual strength and the critical crack length of the stiffened panels with WFD. The effect of WFD crack length on residual strength is also studied.

Formability of 6k21-T4 car panel sheet for viscoelastic–plastic flexible-die forming

The service performance of a car body is obviously improved by using aluminum alloy panels. Because of the lower hardness and elastic modulus of aluminum alloy sheets, the car panels are easily scraped to reduce the surface quality, and the dimensional accuracy of sheet parts is affected due to higher spring-back after forming. In order to overcome forming problems of aluminum alloy panels, the viscoelastic–plastic flexible-die forming method is proposed to form the aluminium alloy panels in this paper. The scrape defect and the spring-back are improved by making use of the characteristics of the viscoelastic–plastic flexible-die. The forming limit curve of 6k21-T4 sheet is obtained through viscoelastic–plastic flexible-die bugling experiments. The car outer door with scale-down simulating dimensions is chosen as experimental parts for panels, and the viscoelastic–plastic flexible-die forming experiments are carried out to analyze the effect of the relationship between viscoelastic–plastic flexible-die pressure and BHF on the formability of aluminum panels. Experimental results indicate that the viscoelastic–plastic flexible-die forming can meet the formability of the aluminium alloy panels by using the proper BHF.