Finite Element Analysis of Adhesively Bonded Lap Joints

Adhesive bonding is a preferred method of joining aerospace structural components, since it provides fewer points of stress concentration compared to fastener joints. Geometrically nonlinear analysis of adhesively bonded lap joints is presented in this paper using both linear and non-linear material properties of the adhesive. The numerical results show beneficial effects of material non-linear behaviour of the adhesive which decrease the stress concentration at the ends of the lap Length. This paper also presents the estimation of strain-energy-release rate components in the presence of debond in the adhesive. The studies have relevance in structural integrity assessment of the joints

Failure prediction of adhesively bonded lap joints between metal and composite adherends

Most of the modern civilian or military aircrafts use advanced composite materials for their primary structural components, in addition to metals. The components are joined together by using either fastener or adhesively bonded joints. But with the introduction of composite materials in aircraft industries, adhesively bonded lap joints are most preferred. This is due to the fact that they develop smooth load transfer and fewer points of stress concentration as compared to fastener joints. The failure prediction of such joints is extremely important, to avoid catastrophic failures during aircraft service period. In the present investigation, an adhesively bonded lap joint between metal-composite (i.e., Al 2024-T3/CFRP) adherends bonded with Redux 319-A adhesive has been analyzed using finite element method considering geometric non-linearity and incorporating adhesive material nonlinear behavior. The failure has been predicted using plastic zone size criterion of adhesive material, which is innovative approach of this study. Also, experimental program is carried out on such joints to correlate with the predicted failure load obtained from numerical model. In this study, the failure of joint is assumed to take place due to adhesive failure only. Plastic zone size in adhesive at failure load of joint is taken as 15 % of the lap length as established from the previous work of the authors. It is observed that the failure load of the adhesively bonded lap joint between composite-metal adherends as obtained from numerical model is well compared with that obtained from experimental study. Results are discussed

User's manual for GAMNAS: Geometric and Material Nonlinear Analysis of Structures

GAMNAS (Geometric and Material Nonlinear Analysis of Structures) is a two dimensional finite-element stress analysis program. Options include linear, geometric nonlinear, material nonlinear, and combined geometric and material nonlinear analysis. The theory, organization, and use of GAMNAS are described. Required input data and results for several sample problems are included

Behaviour of Bi-material Interface Cracks in the Presenceof Material Nonlinearity of Adherends

The well known features of crack face interpenetration/contact at the tip of an interfacecrack is re-examined using finite element analysis and assuming material nonlinear properties forthe adherends. It was assumed in literature that the crack tips are fully open at all load levelsin the presence of material nonlinearity of the adherends.  Analysis for the case of remote tensionshows that even in the presence of material nonlinearity, crack tip closes at small load levelsand opens above a certain load level. Mixed-mode fracture parameters are evaluated for thesituation when the crack tips are fully open.  Due to the presence of nonlinearity, the mixed-modefracture parameters are measured with the symmetric and anti-symmetric components of J-integral.The present analysis explains the sequence of events at the interface crack tip with progressivelyincreasing remote tension load for the case of adherends with material nonlinear behaviour

Aerodynamic and Structural Optimisation of Maritime Patrol Radar System Radome using Evolutionary Algorithms

Airborne early warning systems are deployed for collecting surveillance information on airborne enemy targets in real-time. The Maritime Patrol Radar system is used for surveillance of sea surface for various types of ships and low flying aircraft. Radio Detection And Ranging system, or RADAR, in short, is an Electromagnetic sensor integrated on such airborne platforms. An antenna of this radar system is generally mounted under the belly of the aircraft and protected by a cover called a radome. This radome is installed to protect the radar antenna from environmental disturbances. Due to the installation of the radome, increased drag is experienced by aircraft during its flight due to resistance to the flow of the oncoming air. Radome design is a multidisciplinary effort involving structural, aerodynamics, and electromagnetic disciplines. In this study, the multi-disciplinary design of the maritime patrol aircraft radome for optimality in terms of structural strength and aerodynamic performance is carried out by integrating both disciplinary analyses on an optimisation software platform. The utopia point in terms of these two disciplines is found

Aero structural and Electromagnetic Design Optimisation of Maritime Patrol Aircraft Radome Using Direct Search Algorithms

Airborne surveillance systems such as Maritime Patrol Aircraft are deployed by armed forces to collect surveillance information on airborne and sea surface enemy targets. Airborne Electronically Scanned Array Radar is an electromagnetic sensor integrated on this aircraft. The antenna of this radar is installed generally in belly of a turboprop aircraft. An electro-magnetically transparent cover, called radome, protects this antenna to protect itfrom various environmental effects, like rain, dust, etc. Installation of the radome results in additional drag, weightand electromagnetic signal loss. The Pareto optimality involving three design disciplines of structure, erodynamicsand electromagnetics is attempted with direct search optimisation algorithm NSGA II

Geometrically nonlinear analysis of adhesively bonded joints

A geometrically nonlinear finite element analysis of cohesive failure in typical joints is presented. Cracked-lap-shear joints were chosen for analysis. Results obtained from linear and nonlinear analysis show that nonlinear effects, due to large rotations, significantly affect the calculated mode 1, crack opening, and mode 2, inplane shear, strain-energy-release rates. The ratio of the mode 1 to mode 2 strain-energy-relase rates (G1/G2) was found to be strongly affected by he adhesive modulus and the adherend thickness. The ratios between 0.2 and 0.8 can be obtained by varying adherend thickness and using either a single or double cracked-lap-shear specimen configuration. Debond growth rate data, together with the analysis, indicate that mode 1 strain-energy-release rate governs debond growth. Results from the present analysis agree well with experimentally measured joint opening displacements

Strength Prediction of Adhesively Bonded Joints using Plastic Zone Size Criterion

The prediction of the strength of adhesively bonded joints has been an issue of considerable interest in literature. This exercise requires numerical techniques combined with experimental programs and matching the two to arrive at a viable criterion. The configurations used for the study are single lap adhesively bonded joints between (i) aluminium (Al) – aluminium (Al) and (ii) carbon fibre reinforced composite (CFRP) and aluminium adherends with Redux-319A epoxy. Geometric and material non-linear finite element analysis was conducted using the NASTRAN software package to establish the proposed plastic zone size (PZS) failure criterion. On the same configuration both experimental program for joint strength and numerical analysis were conducted. The plastic zone size corresponding to failure load was initially estimated from Al-Al joints. The same value was used to predict failure load for CFRP-Al bonded joint. The average experimental value and numerical predictions for CFRP-Al joints matched within 7%. This study suggests an alternative method of strength prediction of adhesively bonded single lap joint in presence of inelastic behaviour of adhesive material

Multiple Interacting and Coalescing Semi-Elliptical Surface Cracks in Fatigue-Part-I: Finite Element Analysis

Abstract: Damage tolerance analysis is essential for evaluating life extension of aged structures which are in service beyond originally stipulated life. The major issue for such an analysis for aged aero-engines is the effect of multiple threedimensional flaws on the structural integrity. In this paper, an improved modified virtual crack closure integral technique is applied to investigate the interaction of twin coplanar semi-elliptical cracks in a finite body under both tension and bending. The specific configuration analysed and presented here is two semi-elliptical surface cracks for combinations of various aspect ratios, thickness ratios and combination of interspacing. The interaction effects are studied for both interacting and coalescing phases as observed to occur during the growth of multiple surface cracks under fatigue load. Empirical equations are formulated to estimate interaction factors which could be used in simulation of fatigue crack growth of three-dimensional multi-site damage

Stochastic modeling of functionally graded double-lap adhesive joints

Perturbation(s) in the adhesive’s properties originating from the manufacturing, glue-line application method and in-service conditions, may lead to poor performance of bonded systems. Herein, the effect of such uncertainties on the adhesive stresses is analyzed via a probabilistic mechanics framework built on a continuum-based theoretical model. Firstly, a generic 2D plane stress/strain linear-elastic model for a composite double-lap joint with a functionally graded adhesive is proposed. The developed model is validated against the results obtained from an analogous finite element model for the cases of bonded joints with metal/composite adherends subjected to mechanical and thermal loadings. Subsequently, the proposed analytical model is reformulated in probabilistic mechanics framework where the elastic modulus of the adhesive is treated as a spatially varying stochastic field for the cases of homogeneous and graded adhesives. The former case represents stochastic nature of conventional joints with a homogeneous bondline while the later case showcases the perturbation in the properties of functionally graded joints. To propagate the uncertainty in the elastic modulus to shear and peel stresses, we use a non-intrusive polynomial chaos approach. For a standard deviation in the elastic modulus, the proposed model is utilized to evaluate the spatial distribution of shear and peel stresses in the adhesive, together with probability and cumulative distribution functions of their peaks. A systematic parametric study is further carried out to evaluate the effect of varying mean value of the adhesive’s Young’s moduli, overlap lengths and adhesive thicknesses on the coefficient of variation/standard deviation in peak stresses due to the presence of a random moduli field. It was observed that the joints with stiffer adhesives and longer bondlengths show smaller coefficient of variation in peak stresses. The findings from this study underscore that the predictive capability of the proposed model would be useful for the stochastic design of adhesively bonded joints