Modeling and validation of composite patch repair to cracked thick and thin metallic panels

A two-dimensional finite element analysis is presented to predict crack growth behavior of cracked panels repaired with bonded composite patch. Fatigue experiments were conducted with precracked aluminum specimens of two thicknesses (1 and 6.35 mm), with and without debond, and repaired asymmetrically. Fatigue lives of thick and thin repaired panels extended four and ten times relative to unrepaired cases, respectively. The predicted fatigue crack growth rates were in agreement with experimental values at the unpatched face but not at the patched face. Thus, the present analysis provides a conservative assessment of durability and damage tolerance of repaired thin and thick panels.     

3-D modelling of cyclically loaded composite patch repair of a cracked plate

A three-dimensional finite element model is utilized to capture the effects of plasticity-induced crack closure (PICC) on an adhesively bonded patch repair of a cracked plate. To accurately capture the PICC process the choice of material model employed is of significant importance, therefore this paper considers a relatively new model, the Ellyin–Xia model. The paper shows that the PICC phenomenon is beneficial to the repair.     

Nonlinear analysis of bonded composite patch repair of cracked aluminum panels

Analyses of adhesively bonded composite patches to repair cracked structures have been the focus of many studies. Most of these studies investigated the damage tolerance of the repaired structure by using linear analysis. This study involves nonlinear analysis of the adhesively bonded composite patch to investigate its effects on the damage tolerance of the repaired structure. The nonlinear analysis utilizes the three-layer technique which includes geometric nonlinearity to account for large displacements of the repaired structure and also material nonlinearity of the adhesive. The three-layer technique uses two-dimensional finite element analysis with Mindlin plate elements to model the cracked plate, adhesive and composite patch. The effects of geometric nonlinearity on the damage tolerance of the cracked plate is investigated by computing the stress intensity factor and fatigue growth rate of the crack in the plate. The adhesive is modeled as a nonlinear material to characterize debond behavior. The elastic-plastic analysis of the adhesive utilizes the extended Drucker-Prager model. A detailed discussion on the effects of nonlinear analysis for a bonded composite patch repair of a cracked aluminum panel is presented in this paper.     

The stress intensity factor for a cracked stiffened sheet repaired with an adhesively bonded composite patch

The problem of a cracked, stiffened metallic sheet adhesively bonded by a composite patch is analyzed. The composite patch is assumed to be either an infinite orthotropic sheet or an infinite orthotropic strip normal to the crack. Due to the high stress concentration around the crack and on the interface, an elliptical disbond is assumed to exist around the crack. The crack is asymmetric with respect to the stiffener's locations as well as to the patch's center. The effect of thermal stresses in curing process is also considered. The fracture problem is solved by the displacement compatibility method, using the complex variable approach and the Fourier integral transform method.The problem is dealt with in two steps. First, starting with an uncracked, patched stiffened sheet, the stress at the prospective location of the crack is determined in a closed-form solution. The second step is to introduce a crack into the stiffened patched sheet. The multivalue of the analytical formulation is treated in detail to ensure proper implement in the computer. The results show that the effect of the stiffeners on the stress intensity factor is not significant for a crack fully covered by a patch.For the repairs by Boron/Epoxy patches, the difference in KI between the infinite sheet patch and the infinite strip model is only minor (less than 5 percent) in the absence of the curing thermal stresses and it becomes more pronounced when these stresses are taken into consideration. The stress intensity factor for a crack repaired by an infinite composite strip also can be estimated with a good or reasonable accuracy via a simplified analysis in which the patch is considered as an infinite strip in the first step and is treated as an infinite sheet in the second step of the solution procedure mentioned above.The latter simplified analysis is based on the approach originally proposed by Rose for a relatively simple repair configuration. For most cases, that approach seems to work well for the repair of a stiffened sheet by an infinite composite strip with the effects of thermal stresses and a disbond included. It should be emphasized that the present methodology can apply to the problem of a crack in a metallic stiffened sheet growing beyond the patch's boundary and also to the repairs by an infinite adhesively bonded composite strip parallel to the crack.     

Analysis of cracked plates with a bonded patch

The problem of a cracked plate repaired by an adhesively bonded patch is studied. A shear spring model is adopted to reduce the problem to the analysis of a cracked plate and a patch subjected to external loads and interacting adhesive shear. While the patch is treated by the finite element method, the cracked plate is analyzed by the boundary element method, in which a special fundamental solution satisfying the boundary condition on the crack surface is introduced. The present formulation provides comparable results on the stress intensity factor of the patched crack with less computational effort.     

Nonlinear adhesive behavior effects in a cracked orthotropic sheet stiffened by a semi-infinite orthotropic sheet

The stress-intensity factors are determined for a cracked orthotropic sheet adhesively bonded to an orthotropic stringer where the adhesive layer is modeled with a nonlinear stress-strain curve. Since the stringer is modeled as a semi-infinite sheet, the solution is most appropriate for a crack tip located near a stringer edge. By the use of Green's functions and the complex variable theory of orthotropic elasticity developed by Lekhnitskii, a set of integral equations is obtained. The integral equations are replaced by an equivalent set of algebraic equations, which are solved to obtain the shear stress distribution in the adhesive layer. With these adhesive stresses, the crack-tip stress-intensity factors are found.When the adhesive was modeled with a nonlinear stress-strain curve, the peak shear stresses in the adhesive were considerably reduced in comparison to the solution for the linear elastic adhesive. This resulted in increases in the stress-intensity factors for the nonlinear adhesive solution compared to the linear adhesive solution. The nonlinear adhesive did not have a significant effect on the stress-intensity factor unless the near crack tip was beneath the stringer. The present investigation assumes that the adhesive bond remains intact. Onset of adhesive failure is predicted to occur at decreasing levels of applied stress as the crack propagates beneath the stringer.     

Effect of a riveted wide patch on stresses in a cracked plate

We study the stress-strain state and limiting equilibrium of a thin plate with curvilinear cracks reinforced by a wide patch. The patch is arbitrarily located relative to the cracks and attached to the plate with elastic rivets. The boundary-value problem is reduced to a system of singular integral and integro-algebraic equations and this system is solved by the method of mechanical quadratures. Numerical analysis is performed for the case of a plate with one curvilinear or rectilinear crack reinforced by an elliptic patch. The stress intensity factors formed in this reinforced cracked plate and ultimate loads are determined for various geometric and physical parameters of the plate, crack, patch, and rivets. Karpenko Physicomechanical Institute, Ukrainian Academy of Sciences, L'viv. Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 34, No. 1, pp. 37–46, January–February, 1998.     

Design and validation of composite patch repairs to cracked metallic structures

This paper describes an approach to a patch design for a crack repair, based largely on the simple closed-form analyses. For simplicity, the design approach does not include the effect of the out-of-plane bending nor polygon-shaped patch. However, once the basic design was brought out by that design approach, it can be re-analysed by using other advanced analytical models to account for those latter complexities. These advanced analytical methods are therefore also reviewed briefly in the paper. In addition, test data to validate the mentioned design and analysis methods are also presented and discussed.     

The effects of size and stacking sequence of composite laminated patch on bonded repair for cracked hole

The performance of a bonded repair for cracked holes has been studied using the three dimensional finite element method, linear elastic fracture mechanics and strain energy density theory. Increasing the composite patch size reduces the strain energy level at the crack tip; increasing the patch length normal to the crack is a better choice. The stacking sequences of the laminated patch have little influence on the strain energy distribution in the vicinity of the crack. To repair the cracked holes of aircraft components subjected to variable direction loading during flight, the orientations of the patch ply, 90° and ±45° with respect to the crack direction, are the optimum selection in bonded repairs.     

Modelling of composite sheet forming: a review

The use of composite materials in sheet forming applications is gaining popularity with the rise of consumer demands and specific mechanical properties. In addition to unidirectional (UD) fibres, the use of textile reinforcements such as woven fabric and knitted fabric has been shown to be feasible in recent years. This paper gives a survey on the modelling of composite sheet forming for both UD fibre and textile composites. Two broad approaches are reviewed here—the mapping approach and the mechanics approach. Mapping approaches for UD fibre composites, woven fabric composites and knitted fabric composites are elucidated on the basis of their fibre geometry. For the mechanics approach both the viscous fluid models and elastic solid models, as a means of describing the constitutive properties, are reviewed. Various updating methods for modelling large deformation found in sheet forming are then described. Finally, a guideline for the choice of modelling techniques for various types of fibre/fabric reinforcements and suggestions for future work are given.     

Limiting state of a thin cracked plate under biaxial tension

The results of investigation of the limiting state of cracked plates formed from aluminum alloy D16AT under biaxial tension are presented. The ratio of the corresponding displacements of the plate's boundary and the ratio of nominal stresses at the crack tip are adopted as parameters characterizing the stress-strain state of the plate's material. A relation is established between these parameters and the limiting state of the plate.Translated from Problemy Prochnosti, No. 12, pp. 21–26, December, 1992.     

Buckling behavior of a central cracked thin plate under tension

The buckling characteristics of cracked plates subject to uniaxial tensile loads are analysed by the aid of the finite element method. Owing to the fact that crack buckling behavior is affected by the in-plane stress distribution around a crack, to get more accurate results, pre-buckling in-plane stress fields are analysed by the finite element method. For the critical loads calculation, the finite element approach adopted is based on Von Karman's linearize theory for buckling of plates subjected to pre-buckling state of plane stress. Several singular elements based on the Willian series are used in this plate bending approach. In this study, the effect of crack length, the effect of boundary condition, the effect of Poison's ratio and the effect of biaxial force on critical loads are analysed and discussed. Furthermore, the effect of initial imperfection is also discussed. There is a good agreement between other researcher's work and present results.     

Expansion of cracked fastener holes as a measure for extension of lifetime to repair

This paper presents results of experimental and theoretical investigations about the influence of cold expansion on crack-propagation life of pre-cracked fastener holes in 2024T3 aluminium alloy. Three different types of test specimens with different levels of load transfer and secondary bending and different pre-crack configurations were used. It turned out that these parameters as well as the type of loading (constant amplitude or flight-by-flight) and the initial crack length have a considerable influence on the possible gain in crack-propagation life.     

Reinforcement of a thin cracked plate by a system of parallel stringers

The stress-strain state of an infinite plate with curvilinear cracks reinforced by a system of parallel stringers is investigated. The stringers are attached to the plate by riveting. It is assumed that the cracked plate with the system of stringers is in a generalized plane stress state. The boundary-value problem is reduced to a system of singular integral equations, which is solved by the method of mechanical quadratures. Numerical results are presented for a reinforced plate with a rectilinear or curvilinear crack (along a parabolic arc).Karpenko Physicomechanical Institute, Ukrainian Academy of Sciences, L'viv. Translated from Fiziko-Khimicheskaya Mekhanika Materialov, Vol. 30, No. 1, pp. 96–105, January–February, 1994.     

Analysis of cracked steel members reinforced by pre-stress composite patch

Pre‐stress bonded composite patch is a promising technique to reinforce steel member damaged by fatigue. The effectiveness of this technique was verified by fatigue tests on notched steel plates. Results showed that the application of carbon fibre reinforced plastic (CFRP) strips and, eventually, the introduction of a compressive stress by pretension of the CFRP strips prior to bonding produced a significant increment of the remaining fatigue life. In this paper, the stress intensity factor in the notched plates is computed by a two‐dimensional finite element model in connection with the three‐layer technique in order to reduce the computational effort. Due to high stress concentration at the plate crack tip, debond is assumed at the adhesive–plate interface. The goal is to illustrate the influence of some reinforcement parameters such as the composite strip stiffness, the pre‐stress level, the adhesive layer thickness and the size of the debonded region on the effectiveness of the composite patch reinforcement.     

Thermoelastic solutions for anisotropic cracked thin films

Thermal stress intensity factors for cracks in anisotropic thin films subjected to uniform heat fluxes are investigated. Based on the method of analytic continuation associated with the alternating technique, the general solution of an anisotropic thin film/substrate under thermal loading is derived. Rapidly convergent series solutions for both the temperature and stress fields are obtained in terms of the corresponding homogeneous potentials. Using the technique of superposition and the Cauchy integral, the solution of the crack problem with arbitrary locations and angles is solved. Some typically numerical results are discussed, showing that a stiffer substrate can efficiently reduce the thermal stress intensity factors of the cracked thin films. The result also shows that the solution is accurate and rapidly converges as the thin film/substrate degenerates to a homogeneous medium.     

Bolted patch repair of composite panels with a cutout

The present investigation concerns the analysis of bolted patch repairs of flat composite panels by using a complex potential–variational method. The validity of the current analysis predictions is established by comparison against experimental measurements and previous predictions. The experimental investigation used two patch repairs, with different bolt patterns, of a cutout in an aluminum skin under uniaxial loading. The previous predictions were made for a patch-repaired composite skin with 16 bolts under uniaxial loading. The same patch repair configuration is analyzed here under more complex loading conditions and with two bolts missing, leading to a non-symmetric bolt arrangement. Also, the influence of patch geometry and bolt pattern on the effectiveness of the repair is investigated by considering an elliptical cutout in the skin.     

Extended finite element fracture analysis of a cracked isotropic shell repaired by composite patch

An extended finite element method (XFEM) is developed to study fracture parameters of cracked metal plates and tubes that are repaired on top of the crack with a composite patch. A MATLAB® stand‐alone code is prepared to model such structures with eight‐noded doubly curved shell elements in the XFEM framework. Crack trajectory studies are performed for a diagonally cracked panel under fatigue loading. Verification studies are investigated on different shell type structures such as a cracked spherical shell and cracked cylindrical pipe with different crack orientations. The effects of using patch repairs with different fibre orientations on the reduction of stress intensity factors (SIFs) is also studied which can be useful for design purposes. XFEM is selected as any crack geometry can be embedded in the finite element mesh configuration with no need to coincide the crack geometry with meshed elements and so re‐meshing with fine mesh generation is not needed in the current method.