Showing papers in "International Journal of Structural Integrity in 2011"

Design of a smart leading edge device for low speed wind tunnel tests in the european project sade

Abstract: Purpose – The purpose of this paper is to describe the pre‐design and sizing of a smart leading edge section which is developed in the project SADE (Smart High Lift Devices for Next Generation Wings), which is part of the seventh framework program of the EU.Design/methodology/approach – The development of morphing technologies in SADE concentrates on the leading and trailing edge high‐lift devices. At the leading edge a smart gap and step‐less droop nose device is developed. For the landing flap a smart trailing edge of the flap is in the focus of the research activities. The main path in SADE follows the development of the leading edge section and the subsequent wind tunnel testing of a five meter span full‐scale section with a chord length of three meters in the wind tunnel T‐101 at the Russian central aero‐hydrodynamic institute (TsAGI) in Moscow.Findings – The presented paper gives an overview over the desired performance and requirements of a smart leading edge device, its aerodynamic design for the ...

Surface layer properties and fatigue behavior in Al 7075‐T73 and Ti‐6Al‐4V: Comparing results after laser peening; shot peening and ball‐burnishing

Abstract: Purpose – The purpose of this paper is to investigate the influence of mechanical surface treatments on the surface layer properties and the fatigue performance of the aircraft alloys Al 7075‐T73 and Ti‐6Al‐4VDesign/methodology/approach – Laser peening without coating (LPwC), shot peening (SP), ultrasonic shot peening (USP) and ball burnishing (BB) were applied and the resulting changes in surface roughness and residual stress‐depth profiles were evaluated. Fatigue performance of both alloys was tested in rotating beam loading (R=−1) on hourglass‐shaped specimens and the results were compared with the electrolytically polished (EP) reference conditions.Findings – All studied mechanical surface treatments led to pronounced increases in fully reversed fatigue lives and fatigue strengths in both Al 7075‐T73 and Ti‐6Al‐4V.Originality/value – To the authors' knowledge, this is the first paper that compares fatigue performance of a wide variety of mechanically surface treated conditions in two aircraft alloys.

Laser cladding as a potential repair technology for damaged aircraft components

Abstract: Purpose – The purpose of this paper is to demonstrate the preliminary work on using laser cladding technology for the restoration of structural integrity.Design/methodology/approach – The primary methodology used in this research is to develop a laser cladding‐based metal deposition technique to articulate restoration of structural geometry affected by corrosion damages. Following from this method, it is planned to undertake further work to use the laser cladding process to restore geometry and the associated static/fatigue strength.Findings – This work has found that it is possible to use laser cladding as a repair technology to improve structural integrity in aluminium alloy aircraft structures in terms of corrosion reduction and geometrical restoration. Initial results have indicated a reduction of static and fatigue resistance with respect to substrate. But more recent works (yet to be published) have revealed improved fatigue strength as measured in comparison to the substrate structural properties.O...

Unified approach to the characterization of coupled composite laminates: Hygro‐thermally curvature‐stable configurations

Abstract: Purpose – The purpose of this paper is to demonstrate new design concepts for 24 classes of laminate, which have been derived as part of an ongoing study on the development of a unified approach to the characterization of coupled laminates. The paper presents a description of each class of coupled laminate.Design/methodology/approach – The paper gives an overview of the desired performance and requirements of a smart leading edge device, its aerodynamic design for the wind tunnel tests and the structural pre‐design and sizing of the full‐scale leading edge section which will be tested in the wind tunnel.Findings – Coupled laminates have potential applications in the design of aero‐elastic compliant rotor blades or aircraft wing structures, by introducing tailored extension‐twist and/or shear‐extension coupling at the laminate level; or in the design of thermally activated morphing structures, by exploiting more complex coupling behaviour.Practical implications – These laminates contain standard cross‐ply ...

Surface modification of laser‐ and shot‐peened 6082 aluminium alloy: Laser peening effect to pitting corrosion

Abstract: Purpose – The purpose of this paper is to investigate the effect of shock waves and strain hardening effect of laser and shot peening on precipitation‐hardened aluminium alloy AA 6082‐T651.Design/methodology/approach – The hardened layer was evaluated by means of surface integrity with optical microscopy, scanning electron microscope (SEM), energy dispersive spectroscopy, analysis of microhardness and residual stress profiles. Corrosion anodic polarization tests in a 3.5 per cent NaCl water solution were carried out to express a pitting potential and the degree of pitting attack, which was verified on SEM and with 3D metrology.Findings – Research results indicated significant differences between two treatment techniques which had an important influence on the final condition of the surface layer. Potentiodynamic polarization tests inferred that laser peening enabled shift of the pitting potential to more positive values, which ensures higher corrosion resistance.Originality/value – Results confirmed that ...

Application of laser peening without coating on steel welded joints

Abstract: Purpose – Laser peening without coating (LPwC) is an innovative surface enhancement technology for introducing compressive residual stress in metallic materials. The purpose of this study is to examine the characteristic at the laser‐peened welded zone and the fatigue lives of the welding joints.Design/methodology/approach – LPwC conditions for 490 MPa grades of structural steels were selected. By using the conditions, the characteristic at the laser‐peened welded zone, residual stresses, hardness and roughness of welding toes were examined. Moreover, the fatigue lives of the toes of box‐welded joints and butt welded joints pre‐treated by LPwC were compared to the fatigue lives of those that were not pre‐treated by LPwC.Findings – The main results are: LPwC conditions for 490 MPa grade steels were established; residual stresses, Vickers hardness and roughness at the laser‐peened welded zone were revealed; and LPwC can dramatically extend the fatigue life of welded joint.Originality/value – The effects of ...

Application of laser shock peening for spinal implant rods

Abstract: Purpose – The current industry standard rigid spinal implants suffer fatigue failures due to bending and torsion loads. The purpose of this program was to design novel prototype flexible titanium alloy spinal implant rod with machined features, and then apply the laser shock peening (LSP) process to restore the fatigue strength debit due to these features.Design/methodology/approach – A flexible prototype rod was designed with flat section at the center of the rod. The flat section was laser shock peened. Static compression tests were conducted as per American Society of Testing Materials standards for three‐ and four‐point bending tests and “vertebrectomy” constructs. Finite element models were developed to aid in the design of LSP and also to guide the experiments.Findings – The test results indicated a ∼3X improvement in flexibility and a reduction in fatigue load ratio, defined as applied load divided by the yield load; from 72 to 68 percent. This rod was LSP's on the flat sections, and tested again. ...

Improvement of fatigue limit by shot peening for high‐strength steel containing a crack‐like surface defect

Abstract: Purpose – The paper's aim is to investigate the effects of shot peening (SP) on the bending fatigue limit of high‐strength steel containing an artificial semi‐circular slitDesign/methodology/approach – SP and stress SP (SSP) were conducted on the specimens containing an artificial semi‐circular slit with a depth of a=01, 02 and 03 mm Then, bending fatigue tests were conducted on the specimensFindings – The fatigue limit was improved by SP and SSP In the case of SP and SSP specimens, the specimens with a semi‐circular slit under a=02 mm fractured outside the slit, and they had considerably high fatigue limits Therefore, a semi‐circular slit with a depth of under a=02 mm could be rendered harmless by SP or SSP It was found that the fatigue limit of specimens with a semi‐circular slit that received SP or SSP was determined by the threshold condition for non‐propagation of fatigue cracks that emanated from outside the slit Whether the semi‐circular slit is rendered harmless or not is decided by th

Influence of spectrum loading on fatigue resistance of AA6082 friction stir welds

Abstract: Purpose – Welded components are often subjected to variable amplitude service loads, increasing the uncertainty of fatigue life due to material strength, notch geometries, defect content and residual stresses. In the case of friction stir welding (FSW) of aluminium alloys no data were found available concerning fatigue behaviour under variable amplitude loading. The purpose of this paper is to determine the fatigue strength of friction stir welds in AA6082‐T6 under constant and variable amplitude loading and analyse the validity of Miner's rule for these specific welding conditions.Design/methodology/approach – Fatigue tests were carried out in a servo‐hydraulic testing machine using a stress ratio of R=0. Typified Gassner amplitude spectra were considered, using four shape exponent values. Microhardness tests were performed to characterize the Vickers hardness profile in the vicinity of the weld area. Relatively to the base material (BM), the FSW process leads to a decrease of the static mechanical prope...

Experimental validation of simulated fatigue life estimates in laser‐peened aluminum

Abstract: Purpose – The purpose of this paper is to develop and implement a structural fatigue life estimation framework that includes laser‐peened (LP) residual stresses and then experimentally validates these fatigue life estimations.Design/methodology/approach – A three‐dimensional finite element analysis of an Al 7075‐O three‐point bending coupon being LP was created and used to estimate the fatigue life when loaded. Fatigue tests were conducted to validate these estimations.Findings – The framework developed for fatigue life estimation of LP‐processed coupons yielded estimates with goodness‐of‐fit between the log‐transformed experimental and analytical data of R2=0.97 for the baseline coupons and R2=0.94 for the LP‐processed coupons.Research limitations/implications – Approximated e‐life fatigue parameters were used to calculate the fatigue life resulting from the complex residual stress fields due to the simulated LP process.Originality/value – A fatigue life estimation framework that considers LP residual st...

Laser shock processing with two different laser sources on 2050‐T8 aluminum alloy

Abstract: Purpose – The purpose of this paper is to conduct a comparative study of the surface modifications induced by two different lasers on a 2050‐T8 aluminum alloy, with a specific consideration of residual stress and work‐hardening levels.Design/methodology/approach – Two lasers have been used for Laser shock peening (LSP) treatment in water‐confined regime: a Continuum Powerlite Plus laser, operating at 0.532 mm with 9 ns laser pulses, and near 1.5mm spot diameters; a new generation Gaia‐R Thales laser delivering 10 J‐10 ns impacts, with 4‐6mm homogeneous laser spots at 1.06 mm. Surface deformation, work‐hardening levels and residual stresses were analyzed for both LSP conditions. Residual stresses were compared with numerical simulations using a 3D finite element (FE) model, starting with the validation of surface deformations induced by a single laser impact.Findings – Similar surface deformations and work‐hardening levels, but relatively lower residual stresses were obtained with the new large 4‐6 mm impa...

Laser shock peening to improve the fatigue resistance of AA7050 components

Abstract: Purpose – Laser shock peening (LSP) is a process capable of introducing compressive residual stresses into a metallic component. The residual compressive stress field can extend deeper below the treated surface than that produced by conventional shot peening (SP). The effect of such deep compressive stress profile is expected to result in a significantly greater benefit in fatigue resistance after LSP compared to SP. The purpose of this paper is to examine this further.Design/methodology/approach – Residual stress profiles have been determined by X‐ray diffraction and incremental centre hole drilling. They have been correlated with the respective LSP process parameters and the obtained fatigue behavior.Findings – A significant improvement of the fatigue life was found for an R ratio of 0.1. SP leads to a fatigue improvement of about 15 percent. For the same specimen geometry, a fatigue life improvement of about 25‐35 percent, depending on the load level, can be obtained after LSP. However, not only for th...

Thermomechanical modelling of stress fields in metallic targets subject to laser shock processing

Abstract: Purpose – Laser shock peening (LSP) is mainly a mechanical process, but in some cases, it is performed without a protective coating and thermal effects are present near the surface. The numerical study of thermo‐mechanical effects and process parameter influence in realistic conditions can be used to better understand the process.Design/methodology/approach – A physically comprehensive numerical model (SHOCKLAS) has been developed to systematically study LSP processes with or without coatings starting from laser‐plasma interaction and coupled thermo‐mechanical target behavior. Several typical results of the developed SHOCKLAS numerical system are presented. In particular, the application of the model to the realistic simulation (full 3D dependence, non‐linear material behavior, thermal and mechanical effects, treatment over extended surfaces) of LSP treatments in the experimental conditions of the irradiation facility used by the authors is presented.Findings – Target clamping has some influence on the re...

Development of flexible matrix composites (FMC) for fluidic actuators in morphing systems

Abstract: Purpose – This paper seeks to focus on material combinations for flexible matrix composites (FMCs) and the production methods thereof. These materials enable a high flexibility in one direction while being very stiff in the other.Design/methodology/approach – Tested were rubber, silicone and thermoplastic elastomer matrices with carbon fibers using different production methods. These tests focused on the impregnation of the fibers with the different matrices and the orthotropy of the produced materials.Findings – In the paper, a production capability for large quantities of easy to use off‐the‐shelf material was developed. The produced material handles similar to prepreg material known from “classical” composite materials. Test specimens were manufactured and characterized for mechanical properties using tensile tests.Originality/value – These FMC materials are envisaged for a new pneumatic actuation system for an aircraft's droop nose to replace the electro‐mechanical system designed in the SADE and Smar...

Influence of mosaic patterns on the structural integrity of filament wound composite pressure vessels

Abstract: Purpose – Filament wound pressure vessels have a characteristic pattern observed in their helical layers. These are mosaic‐shaped patterns and affect the layer structural behavior. The present research aims to focus on the influence of mosaic patterns on stress‐strain field and structural design of thin‐walled internally pressurized filament wound pressure vessel. The widely used stress analysis procedures and the commercially available finite element tools usually neglect the effect of the mosaic patterns. The present work seeks to deal with the modeling and stress analysis of complete pressure vessel, incorporating mosaic patterns.Design/methodology/approach – The incorporation of the mosaic effect provides more realistic modeling of the real stress distribution and the stress values compared to the conventional analyses (the effect would depend on the shell structure, i.e. number of plies, relative thicknesses, etc.). The structural analysis is performed using commercial finite element analysis (FEA) t...

Retained residual stress profiles in a laser shock‐peened and shot‐peened nickel base superalloy subject to thermal exposure

Abstract: Purpose – The purpose of this paper is to analyze the residual stress relaxation in laser shock‐peened and shot‐peened IN100 subject to thermal exposure.Design/methodology/approach – Shot peening (SP) is a commonly used surface treatment that imparts compressive residual stress into the surface of components. The shallow depth of compressive residual stresses, and the extensive plastic deformation associated with SP, has been overcome by modern approaches such as laser shock peening (LSP). LSP surface treatment produces compressive residual stress magnitudes that are similar to SP that extend four to five times deeper, and with less plastic deformation. Retention of compressive surface residual stresses is necessary to retard initiation and growth of fatigue cracks under elevated temperature loading conditions.Findings – Results indicated that the LSP processing retains a higher percentage of the initial residual stress profile over that of SP.Originality/value – The retained residual stresses after therm...

Innovative streamline‐flow preserving actuation strategies for wing airfoil nose

Abstract: Purpose – The purpose of this paper is to focus on a morphing architecture, conceived to produce droop nose effect, thus preserving high lift performance and laminar flow.Design/methodology/approach – A numerical approach was adopted. On the base of preliminary aerodynamic requirements, the main aspects of the actuation architecture were defined and then assessed through a genetic approach.Findings – Two different working modalities of mentioned architecture were identified: the former implying the use of an actuator, the latter taking advantage of a tailored elastic element, able to actuate morphing under the action of aerodynamic loads, without the aid of actuators.Research limitations/implications – The research presented in this work refers to an optimisation process currently tailored on preliminary aerodynamic requirements (leading edge vertical displacement maximisation, leading edge radius increase).Originality/value – The research shows the possibility of producing morphing on the leading edge zo...

Stability of residual stress induced by laser peening under cyclic mechanical loading

Abstract: Purpose – The purpose of this paper is to investigate the behavior of compressive residual stress induced by laser peening under external loading on an age‐hardened high‐strength aluminum alloy A2024‐T3, a low‐carbon austenitic stainless steel SUS316L (Type 316L) and a nickel‐based alloy NCF600 (Alloy 600).Design/methodology/approach – The surface residual stress was measured intermittently by X‐ray diffraction during cyclic uniaxial loading.Findings – The compressive residual stress due to laser peening significantly decreased during the first few cycles at stress ratio of 0.1 with the maximum loading stress exceeding the 0.2 per cent yield stress. No remarkable decrease was observed afterward until the end of the loading cycles.Originality/value – Under symmetric loading at the stress ratio of −1 to A2024‐T3, a major decrease took place in the compression side of the first loading cycle. The surface residual stresses remained in compression within all the extent of the present experiments, even if the m...

A numerical simulation of the residual stresses in laser‐peened friction stir‐welded aluminum 2195 joints

Abstract: Purpose – The purpose of this paper is to present an analytical framework for predicting the residual stresses that result from the laser shock peening of a friction stir‐welded 2195 aluminum alloy sample, using the finite element software LS‐DYNA.Design/methodology/approach – The pressures resulting from the laser peening are directly applied in an explicit transient analysis as forces. At the completion of the transient analysis, an implicit springback analysis is performed to determine the final residual stresses. This cycle is repeated for the appropriate number of peen applications, including the appropriate overlap of application areas. To validate the analytical framework, a comparison of residual stresses between analysis and a test specimen is made using laser‐peened base material which was not friction stir‐welded. Friction stir welding (FSW) causes residual stresses and material property variations. In this work, the varying material properties regions are simplified and defined as discrete, se...

Use of an orthogonal array based on the Kriging model to maximize the fatigue life of a turbine blade

Abstract: Purpose – Approximation techniques were used instead of expensive computing analysis in a traditional parametric design optimization of a complex system. A Kriging meta‐model was utilized, which enabled the fit of approximated design characteristics for a complex system such as turbine blades that incorporate a large number of design variables and non‐linear behaviors. This paper aims to discuss these issues.Design/methodology/approach – The authors constructed a Kriging meta‐model with a multi‐level orthogonal array for the design of experiments, which were used to optimize the fatigue life of turbine blades under cyclic rotational loads such as centrifugal force. By combining a seven‐level orthogonal array with the Kriging model, the non‐linear design space of fatigue life was explored and optimized.Findings – A computer‐generated multi‐level orthogonal array provided a good representation of the non‐linear design space information. The results show that not only was the fatigue life of the leading edge...

The quantification of fatigue crack initiators in aluminium alloy 7050‐T7451 using quantitative fractography

Abstract: Purpose – This paper aims to present some aspects associated with the life prediction of structures with fatigue cracks growing from small natural discontinuities in aluminium alloy (AA)7050‐T7451 for a surface condition that is present in F/A‐18 A/B aircraft critical structure.Design/methodology/approach – Fatigue results are presented for thick section AA7050 plate coupons loaded with a representative fighter aircraft wing root bending moment loading spectrum. Detailed quantitative fractography (QF) was used to gain a deeper understanding of issues relevant to an improved fatigue life predictive capacity for this material by using the QF results to investigate the “effectiveness” of the fatigue initiating discontinuities.Findings – Estimates of the “effectiveness” of the fatigue initiating discontinuities as quasi pre‐existing fatigue cracks (“equivalent pre‐crack size” (EPS) here) were made with the aid of a simple crack growth model. This model, based on experience, was found to be valid for the appli...

Transient thermo‐elastic disturbances in a visco‐elastic semi‐space due to moving internal heat source

Abstract: Purpose – This paper aims to investigate the transient disturbances created by an internal line heat source that suddenly starts moving uniformly inside a visco‐elastic half‐space.Design/methodology/approach – Generalised theory of thermo‐elasticity with relaxation time proposed by Lord‐Shulman is applied. The material of the semi‐infinite medium is an isotropic visco‐elastic solid of Kelvin‐Voight type. Fourier and Laplace transform techniques are used.Findings – Applying the Fourier and Laplace transform techniques, expressions for displacement components in the transformed domain are found. These expressions prove the existence of three waves – a modified thermal wave, a visco‐elastic wave of defused nature and a transverse visco‐elastic wave.Research limitations/implications – Surface displacement components were evaluated on the boundary for only a short time.Originality/value – The paper provides numerical results that are illustrated graphically to highlight the variations of surface displacement c...

Predicting the fatigue life of pristine and pre‐corroded friction stir welded joints

Abstract: Purpose – Friction stir welding (FSW) is simple, clean and cost effective joining technology which allows high‐quality joining of materials that have been traditionally troublesome to weld conventionally without distortion, cracks or voids such as high‐strength aluminium alloys. Since FSW has been identified as “key technology” for primary aerospace structures, the recent FAR regulations for damage tolerance and fatigue evaluations of aircraft structures require fatigue life predictions for this specific joint type also in the presence of corrosion. The purpose of this paper is to give an overview of the prediction of small coupon fatigue lives of thin section friction stir welded butt and T‐joints.Design/methodology/approach – Particularly, as a special application, widespread fracture mechanics software will be used to predict the fatigue life of FSW joints and to obtain SN curves. The engineering approach will start from an easy definition of the damage affecting the fatigue life of any of the previous...

Aeroelastic analysis of remotely controlled research vehicles with numerous control surfaces

Abstract: Purpose – The purpose of this paper is to find an influence of the reduced stiffness of actuators, located on the most outer parts of ailerons, flaperons, rudders, elevators and elevons on the excitation of flutter. This phenomenon is especially important for unmanned aerial vehicles because they continuously use all these control surfaces for trimming and stabilisation and on the other hand, the numerous statistics show that failure of elements of flight control systems are still the most probable reasons of aircraft critical failure.Design/methodology/approach – Flutter calculations were performed by use of the classical modal approach. The normal vibrations of the free aircraft were measured in the ground vibration test (GVT). Test results were used either for verification of the FEM model of the structure – in this case for flutter calculation the MSC.Nastran software was used, or directly for flutter calculation. Based on the flutter analysis, the control surfaces critical for flutter were determined...

Crack propagation in buckling plates: test results and a simplified numerical approach

Abstract: Purpose – The purpose of this paper is to present the first results of tests where cracks lie in the tension field of a shear forced aluminium panel. The paper's main focus is on the crack propagation behavior and possible 3D‐effects caused by the bending of the plate. A simplified numerical approach is presented to confirm the observed phenomena.Design/methodology/approach – Experiments have been performed to investigate the influence of buckling on accidental damages. A simplified numerical approach is presented and compared to the experimental results.Findings – It can be shown that the crack propagates due to buckling of the plate. The principal stress of the neutral axis of the plate has significant influence on the crack propagation.Originality/value – Investigations of stability problems and damage tolerance behavior of metallic structures have been realized but mainly separately. This paper shows that cracks propagate due to buckling and that both phenomena influence one another considering accide...

Finite element analysis and comparison of laser shock peening and shot peening processes

Abstract: Purpose – The aim of this paper is to give a simple and accurate tool for prediction and comparison of residual stresses in laser shock peened and shot peen treated materials.Design/methodology/approach – This work applies finite element code ABAQUS in order to compare the residual stress state and plastic deformation in specimens in aluminium alloy 7050‐T7451, treated with shot peening (SP) and laser shock peening (LSP) processes. Both processes are simulated using the Hugoniot elastic limit (HEL) of the material in question, and the processes are modelled using same input parameters (pressure on the surface of the specimen and the duration of contact between the material and the peening medium).Findings – By using the same approach in both the analyses, a sound comparison of two technologies can be made, by comparing the obtained residual stress profiles. In addition, surface pressure and contact time can be varied easily in a parametric analysis, allowing the calibration of the numerical results.Resear...

Bending fatigue strengths/limits of smooth specimens with and without surface‐enhanced layer of aeronautical metallic alloys

Abstract: Purpose – The purpose of this paper is to estimate the bending fatigue strengths/limits of smooth specimens and to quantitatively analyze the effects of residual stresses caused by surface enhancements.Design/methodology/approach – Rotating bending and three‐point bending fatigue tests were employed to investigate the effects of surface enhancements including shot peening and laser peening on fatigue strengths/limits of smooth specimens. The fatigue sources were determined by scanning electron microscopy (SEM).Findings – The two new concepts of surface fatigue strength/limit and subsurface fatigue strength/limit were proposed based on experimental results and the relationship of surface fatigue strength/limit to subsurface fatigue strength/limit was built.Research limitations/implications – This paper provides the estimation that fatigue strengths/limits of surface‐enhanced specimens is 1.34‐1.42 times that of un‐surface‐strengthened specimens for smooth ones without stress concentration.Originality/value...

Automated overlays for laser peening

Abstract: Purpose – Sacrificial opaque overlays used in laser peening provide optimal processing and protect the surface of the part being processed from thermal damage from the laser pulses. Traditional solid film overlays for laser peening often require several applications and the running of multiple partial laser peening sequences in order to completely process the desired surface. This paper aims to discuss an automated overlay system that eliminates these issues.Design/methodology/approach – LSP Technologies' (LSPT') patented RapidCoater™ automated overlay system provides optimal laser processing and surface protection by providing a conformal opaque layer that is automatically refreshed between each laser pulses. PLC control provides precise timing of the application of the process overlays in synchronization with the laser pulse.Findings – Use of the RapidCoater system has been shown to reduce processing time by up to five times when compared to using tape overlays. Cost reductions of about 40 percent are a...