Fatigue crack growth analysis of stiffened cracked panel repaired with bonded composite patch

Fatigue crack growth behavior in a stiffened thin 2024-T3 aluminum panel repaired with one-sided adhesively bonded composite patch was investigated through experiments and analyses. The patch had three plies of unidirectional boron/epoxy composite. 2024-T3 aluminum stiffeners were riveted as well as bonded on the panel. Stiffeners were oriented in the loading direction and were spaced at either 102 mm or 152 mm with a crack centered between them. Also, un-repaired cracked panel with and without stiffeners were studied. Experiment involved tension-tension fatigue at constant amplitude with maximum stress of 120 MPa and stress ratio of 0.05. Bonded composite patch repair increased fatigue life about five-fold in the case of stiffened panels while it increased about ten fold in the case of un-stiffened panels. Fatigue life also increased with decrease of the distance between the stiffeners for both repaired and un-repaired panels. A three-dimensional finite element method was used to analyze the experiments. Residual thermal stresses, developed during patch bonding, requires the knowledge of temperature at which adhesive becomes effective in creating a bond between the structure and patch in the analysis. A simple method to estimate the effective curing temperature range is suggested in this study. The computed stress intensity factor versus measured crack growth relationships for all panel configurations were consistent and in agreement with the counterpart from the test material. Thus, the present approach provides a means to analyze the fatigue crack growth behavior of stiffened structures repaired with adhesively bonded composite patch.     

Experimental fatigue crack growth and crack-front shape analysis of asymmetric repaired aluminium panels with glass/epoxy composite patches

In this study, we investigate the experimental fatigue crack-growth behaviour of centrally cracked aluminium panels in mode-I condition which have been repaired with single-side composite patches. It shows that the crack growths non-uniformly from its initial location through the thickness of the single-side repaired panels. The propagated crack-front shapes are preformed for various repaired panels with different patch thicknesses. It is shown that there are considerable differences between the crack-front shapes obtained for thin repaired panels with various patch thicknesses. However, the crack-front shapes of thick repaired panels are not significantly changed with various patch thicknesses. Furthermore, effects of patch thickness on the crack growth life of the repaired panels are investigated for two typical thin and thick panel thicknesses. It shows that the crack growth life of thin panels may increase up to 236% using a 16 layers patch. However, for thick panels, the life may extended about 21–35% using a 4 layers patch, and implementing 8 and 16 layers patches has not a significant effect on the life extension with respect to the 4 layers patch life.     

Experimental investigations on fatigue crack growth of repaired thick aluminium panels in mixed-mode conditions

In this paper, experimental fatigue crack growth of thick aluminium panels containing a central inclined crack of 45° repaired with single-side glass/epoxy composite patch are performed. It is shown that, the technique of single-side repair using glass/epoxy composite patch is effective in the crack growth life extension of the thick panels in mixed-mode conditions. It is also shown that the crack-front of the propagated cracks of the repaired panels has a curvilinear shape which is the effect of the existed out-of-plane bending due to the asymmetry conditions in the single-side repaired panels. It is indicated that the crack propagation path at patched surface is different from the un-patched surface of the panels. In the primary stages of the crack growth, the crack surfaces through the thickness, in the vicinity of the mid-plane propagate without surface twisting. There are considerable differences between the obtained crack growth path at patched and un-patched surfaces of the panels which mean that the crack propagation surfaces have three-dimensional patterns. Using the various thin patch lay-ups has minor effects on the crack re-initiation life of the repaired thick panels. It is shown that using various four layers patch lay-up configurations, the crack propagation life of the cracked panels may increase by the order of 30–85%. The most fatigue crack growth life extension belongs to the repaired panel with the patch lay-up of [90]₄.     

Numerical simulation study of fatigue crack growth behavior of cracked aluminum panels repaired with a FRP composite patch using combined BEM/FEM

A combined boundary element method and finite element method (BEM/FEM) is employed to investigate the fatigue crack growth behavior of cracked aluminum panels repaired with an adhesively bonded fiber-reinforced polymer (FRP) composite patch. Numerical simulation of crack growth process of a cracked aluminum panel repaired with a FRP composite patch under uniaxial cyclic loading has been carried out. The curve of crack length on unpatched side of the cracked panel versus the number of cyclic loading is determined by the numerical simulation, and it agrees well with experimental data. Furthermore, the crack front profiles of the cracked panel during fatigue crack growth and the distributions of stress intensity factors along crack fronts are also numerically simulated.     

A comparison of fatigue crack growth performance of two aerospace grade aluminium alloys reinforced with bonded crack retarders

To improve the fail‐safety performance of integral metallic structures, the bonded crack retarder concept has been developed in recent years. This paper presents an experimental investigation on the effectiveness of bonded crack retarder on fatigue crack growth life in two aerospace aluminium alloys: 2624‐T351 and 7085‐T7651. M(T) specimens bonded with a pair of straps made of GLARE fibre‐metal laminate were tested under the constant amplitude load. Although the bonded crack retarders increased the crack growth life in both alloys, the magnitude of life improvement is very different between them. Compared to unreinforced specimens, application of crack retarders has resulted in 90% increase in fatigue life in AA7085, but only 27% increase in AA2624. The significant difference in fatigue life improvement is owing to the material's intrinsic fatigue crack growth rate property, ie, the Paris law constants C and n. Value of n for AA7085 is 1.8 times higher than that for AA2624. Therefore, AA7085 is much more sensitive to reductions in the effective stress intensity factor brought by the crack retarders, hence better life improvement.     

Mixed-mode fatigue crack growth of thin aluminium panels with single-side repair using experimental and numerical methods

In this paper, experimental and numerical fatigue crack growth of thin aluminium panels containing a central inclined crack of 45° with single-side glass/epoxy composite patch are performed. Effects of patch lay-up configuration on the restarting crack growth (crack re-initiation) life and crack growth rate of the repaired panels are investigated. The obtained experimental results are compared with those predicted using finite element analysis based on both mid-plane and unpatched surface fracture parameters. In the finite elements analyses, it is assumed that the crack-front remains perpendicular to the panel's surfaces during its propagation. It is shown that the finite element crack re-initiation and propagation lives predictions using the unpatched surface results are too conservative. However, the finite element mid-plane results lead to a non-conservative life prediction. It is experimentally shown that, the most effective patch lay-up configurations to retard the crack growth of the repaired panels is [−45/+45]₂; however, the most life extension including the crack propagation cycles belongs to the patch lay-up of [90₄]. It is also shown that using the asymmetric patch lay-up configuration similar to [90₂/0₂] with a proper bonding process may lead to a very slow crack growth rate, even slower than the patch lay-up of [90₄].     

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.     

Micromechanisms of fatigue crack growth in aluminium alloys

The fatigue crack growth characteristics of high-strength aluminium alloys are discussed in terms of behaviour during mechanical testing and fracture surface appearance. For a wide range of crack growth rates, the crack extends both by the formation of ductile striations and by the coalescence of micro-voids. Dimples are observed at stress intensities very much less than the plane strain fracture toughness, and this is explained in terms of the probability of inclusions lying close to the crack tip. The striation formation process is described as a combination of environmentally-enhanced cleavage processes and plastic blunting of the crack tip.     

Shear mode fatigue crack growth in 1050 aluminium

The shear mode crack growth mechanism in 1050 aluminium was investigated using pre‐cracked specimens. A small blind hole was drilled in the centre section of the specimens in order to predetermine the crack initiation position, and a push–pull fatigue test was used to make a pre‐crack. Crack propagation tests were carried out using both push–pull and cyclic torsion with a static axial load. With push–pull testing, the main crack grew by a mixed mode. It is thus apparent that shear deformation affects the fatigue crack growth in pure aluminium. In tests using cyclic torsion, the fatigue crack grew by a shear mode. The micro‐cracks initiated perpendicular and parallel to the main crack's growth direction during the cyclic torsion tests. However, the growth direction of the main crack was not changed by the coalescence of the main crack and the micro‐cracks. Shear mode crack growth tends to occur in aluminium. The crack growth behaviour is related to a material's slip systems. The number of slip planes in aluminium is smaller than that of steel and the friction stress during edge dislocation motion of aluminium is lower than many other materials. Correlation between the crack propagation rate and the stress intensity factor range was almost the same in both push–pull and cyclic torsion with tension in this study.     

Mixed-mode fatigue crack growth behaviour in aluminium alloy

Fatigue crack propagation tests in compact mixed-mode specimens were carried out for several stress intensity ratios of mode I and mode II, K_I/K_II, in AlMgSi1-T6 aluminium alloy with 3 mm thickness. The tests were performed in a standard servo-hydraulic machine. A linkage system was developed in order to permit the variation of the K_I/K_II ratio by changing the loading angle. Crack closure loads were obtained through the compliance technique. A finite element analysis was also done in order to obtain the K_I and K_II values for the different loading angles. Crack closure increases under mixed-mode loading conditions in comparison to mode-I loading due the friction between the crack tip surfaces. Moreover, the crack closure level increases with the K_I/K_II ratio decrease. Correlations of the equivalent values of the effective stress intensity factor with the crack growth rates are also performed. Finally, an elastic–plastic finite element analysis was performed to obtain the plastic zones sizes and shapes and model the effect of mixed-mode loading on crack closure.     

Fracture toughness analysis of inclined crack in cylindrical shell repaired with bonded composite patch

Adhesively bonded composite patch repair has been widely used to restore or extend the service life of cracked structural components due to its efficiency and cost-effectiveness compared to mechanical repair technique. Current available knowledge on patch repair mainly focus on flat damaged structures and the corresponding analysis methods and empirical databases are computationally efficient. In contrast, only limited work has contributed to studying patch repair to curved damaged structures. Authors have developed an adhesive element in conjunction with a shell element to investigate the effect of curvature on the adhesive stresses and mode I fracture toughness of the cracked host shell in the curved repairs. In this paper, this technology is again employed to model an adhesively bonded composite patch repair to a cylindrical shell embedded with an inclined through-thickness crack. The total strain energy release rate (SERR), calculated by the modified virtual crack closure technique (VCCT), is used to evaluate the mix-mode fracture toughness of the damaged structure and further to estimate the efficiency of patch repair. An automatic mesh generation scheme is proposed to conduct a quick parametric analysis, which can also be used to structural optimization design of composite patch repair. The numerical results are presented to show the effect of curvature and inclined angle of the through-thickness crack on fracture toughness of the repaired structure subject to different loads.     

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.     

Suppression of fatigue crack growth in aluminium alloy sheets

The effect of crack bridging reinforcing members on the growth of a transverse fatigue crack in an aluminium alloy sheet has been examined. The specimens have been subjected to zero tension cyclic loading applied in the reinforcement direction. In the case of some specimens initial tensile stresses were developed in the substrate aluminium sheet by the reinforcing members. In other cases initial compressive stresses were developed. Fatigue crack growth rates in the unreinforced aluminium alloy sheet were also determined. In the case of both types of reinforced structures the reinforcing elements suppressed the growth of fatigue cracks compared with the unreinforced material. This effect was particularly marked in the case where the substrate aluminium sheet was prestressed in compression-the crack growth rates approaching zero as the peak applied tensile strain approached the value of the initial compressive pre strain.The mechanisms whereby fatigue crack growth in the substrate aluminium sheet is suppressed are discussed and their effectiveness related to the behaviour of the unreinforced material by comparing crack growth rates in terms of effective stresses intensity factors.The compressive stresses were developed in the substrate sheet using two part core/tube reinforcing members. The cores were held in residual tension with the rest of the structure in corresponding compression. The prestress condition appears to be achievable with negligible weight penalty in structures of appreciable size where the engineering requirement is to limit the deformation of a structure under load. Where increased elastic deflections and correspondingly higher stresses are acceptable, reductions in structure weight together with an improved resistance to fatigue crack growth compared with the unreinforced material appear possible.The cores are designed not to be fracturable as a consequence of tensile deformations of any magnitude applied to the composite structure. Hence, they provide a residual load bearing capability and also inhibit crack extension in the fracturable part of the structure under all conditions of tensile loading.

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Résumé On a examiné l'effet de pontage d'une fissure occasionné par des éléments de renforcement, dans le cas de la croissance de fissure de fatigue transversale dans une tôle d'alliage d'aluminium. Les éprouvettes étaient soumises à une sollicitation de traction cyclique pulsée appliquée dans la direction du renforcement. Dans le cas de certaines éprouvettes, des contraintes initiales de traction étaient développées dans la tôle d'aluminium servant de substrat directement par l'élément de renforcement. Dans d'autres cas des contraintes initiales de compression étaient développées. La vitesse de croissance de la fissure de fatigue de la tôle d'alliage d'aluminium non renforcée a également été déterminée. Dans les deux types de structure renforcée qui ont été examinés les éléments de renforcement suppriment la croissance de fissure de fatigue, quand on se compare au matériau non renforcé. Cet effet a été particulièrement marqué dans le cas d'une tôle d'aluminium pré-comprimée: dans ce cas, les vitesses de croissance de fissuration approchaient de 0 lorsque la déformation appliquée en traction approchait la valeur de la pré-déformation de compression initiale.On discute les mécanismes par lesquels la croissance d'une fissure de fatigue dans un tôle d'aluminium peut être supprimée ainsi que l'efficacité de leur comportement dans le matériau non renforcé en comparant les vitesses de croissance de fissure en termes de facteur d'intensité de contrainte.Les contraintes de compression étaient introduites dans la tôle de base en utilisant des éléments de renforcement constitués d'un tube et d'un noyau. Le noyau était maintenu en tension résiduelle tandis que le reste de la structure était amené en compression. Les conditions de précontrainte apparaissent réalisables avec une faible pénalisation en poids dans des structures d'une dimension déjà appréciable lorsque les exigences de la conception limitent la déformation de la structure sous contrainte. Lorsqu'on admet des déflexions élastiques plus importantes et des contraintes correspondantes plus élevées, il apparait possible d'obtenir des réductions dans le poids de la structure ainsi qu'une résistance accrue à la croissance d'une fissure de fatigue, par rapport à une structure non renforcée.Les noyaux sont conçus de manière à résister aux déformations de traction appliquées à la structure composite. Dès lors, ils apportent un complément de résistance aux charges appliquées et empêchent l'extension d'une fissure dans la partie de la structure susceptible de se rompre, sous toutes les conditions de mise en charge.

     

Multiscale fatigue crack growth modelling for welded stiffened panels

The influence of welding residual stresses in stiffened panels on effective stress intensity factor (SIF) values and fatigue crack growth rate is studied in this paper. Interpretation of relevant effects on different length scales such as dislocation appearance and microstructural crack nucleation and propagation is taken into account using molecular dynamics simulations as well as a Tanaka–Mura approach for the analysis of the problem. Mode I SIFs, K_I, were calculated by the finite element method using shell elements and the crack tip displacement extrapolation technique. The total SIF value, K_tot, is derived by a part due to the applied load, K_appl, and by a part due to welding residual stresses, K_res. Fatigue crack propagation simulations based on power law models showed that high tensile residual stresses in the vicinity of a stiffener significantly increase the crack growth rate, which is in good agreement with experimental results.     

Numerical and experimental fatigue crack growth analysis in mode-I for repaired aluminum panels using composite material

Crack-front shape is an important parameter influencing the stress intensity factor and crack propagation rate in asymmetric repaired panels. In this study, the numerical and experimental fatigue crack growth behaviour of centrally cracked aluminum panels in mode-I condition repaired with single-side composite patches are investigated. It is shown that the crack growths non-uniformly from its initial location through the thickness of a single-side repaired panel. There is a good agreement between the propagated crack-front shapes obtained from finite element analysis with those obtained from the experiments for various repaired panels with different patch thicknesses. Furthermore, effects of plate and patch thickness on the crack growth life of the repaired panels are investigated. The experimental results show that the crack growth life of thin panels may increase up to 236% using a 16 layers patch. However, for thick panels, the life may extend about 21–35% using a 4 layers patch. Implementing of 8 and 16 layers patches has not a significant effect on the life extension of thick panels with respect to the 4 layers patch life.     

Fatigue crack growth simulation in adhesively bonded composite joints

A numerical model has been developed for simulating fatigue crack growth (debonding) in adhesively bonded composite joints subjected to mode‐I, mode‐II, and mixed‐mode I + II loading conditions. The model employs a cohesive zone model described by a modified bilinear traction‐separation law. Fatigue damage in the composite adherends is not considered in the model. To account for crack divergence and reduce sensitivity of numerical results on mesh density, a crack front detection algorithm based on the effective element's length was employed. The model is implemented as a user‐defined subroutine (UMAT) in the commercial FE code LS‐DYNA. The model's input parameters, in the form of a modified Paris law, and the validation data were obtained from experimental tests conducted by the authors. It was found that the model is able to successfully simulate crack growth in the regime of the experimental data.     

Identification of crack and disbond fronts in repaired aircraft structural panels with bonded FRP composite patches

Structural health monitoring of fatigue-cracked aircraft structural panels repaired with bonded FRP composite patches for extending the service life of aging aircraft has received wide attention. In this paper a method for identifying the locations and shapes of crack and disbond fronts in aircraft structural panels repaired with bonded FRP composite patches is presented. The identification is performed by minimizing the residual norm between the measured in-plane strain range on a strain measurement plane in the FRP composite patches and the calculated in-plane strain range. Several numerical examples of identification of the locations and shapes of crack and disbond fronts are examined. The effects of the number of strain measurement points, position of the strain measurement plane, and measurement errors of the in-plane strain ranges on the identification results are discussed. The validity of this identification method is verified by comparing the identification results with the exact ones.     

Micromechanisms of fatigue crack growth in cast aluminium piston alloys

The fatigue crack growth behaviour in as-cast and hot isostatically pressed (HIP) model cast aluminium piston alloys with hypoeutectic Si compositions of 6.9 wt% and 0.67 wt% has been investigated. The HIP alloys showed slightly improved fatigue crack growth resistance. Analysis of the crack path profiles and fracture surfaces showed that the crack tends to avoid Si and intermetallic particles at low ΔK levels up to a mid-ΔK of ∼7 MPa√m. However, some particles do fail ahead of the crack tip to facilitate crack advance due to the interconnected microstructure of these alloys. At higher levels of ΔK, the crack increasingly seeks out Si and intermetallic particles up to a ΔK of ∼9 MPa√m after which the crack preferentially propagates through intermetallic particles in the 0.67 wt%Si alloy or Si and intermetallics in the 6.9 wt%Si alloys. It was also observed that crack interaction with intermetallics caused crack deflections that led to roughness-induced crack closure and possibly oxide-induced crack closure at low to mid-ΔK. However, crack closure appears unimportant at high ΔK due to the large crack openings and evidenced by the fast crack growth rates observed.     

Microstructure dependent fatigue crack growth in aged hardened aluminium alloys

Fatigue crack propagation tests in constant amplitude loading, as well as with single peak overloads, have been performed in AlMgSi1-T6 aluminium alloys with different Mn and Cr contents. Crack closure was monitored in all tests by the compliance technique using a pin microgauge. A moderate stress ratio and a strong material dependence effects on the fatigue crack growth were observed. These effects are discussed in terms of the different dominant closure mechanism (plasticity-induced closure or roughness-induced closure). Roughness-induced closure dominates crack closure in the alloys with higher contents of Mn and Cr elements. In the alloy with a lower content of these elements, plasticity-induced closure is dominant. When roughness-induced closure is the prime pre-overload closure mechanism, the retardation effect is decreased in comparison to when plasticity-induced closure is dominant.     

Short fatigue crack growth in aluminium alloy 6082-T6

Fatigue crack growth tests on 6082-T6 aluminium specimens have been carried out. The single edge notched tension specimens had a spark eroded 0.2 mm deep starter notch. In order to measure the crack growth, a special direct current potential drop method was used. The experiments were carried out at four different stress ratios. The crack growth data validate the long crack growth data given in Eurocode 9 [CEN. ENV 1999-2: Eurocode 9: Design of aluminium structures. Part 2: Structures susceptible to fatigue. European Committee for Standardisation, 1998.] and some indication of accelerated crack growth for short cracks was found. An effective stress intensity factor, which is a generalisation of an approach proposed by El Haddad et al. [El Haddad MH, Topper TH, Smith KN. Prediction of nonpropagating cracks. Eng Fract Mech 1979; 11:573–84], has been applied to correct for short crack growth behaviour. It has been shown that the potential drop technique can be successfully used to measure crack growth in aluminium specimens. The mean stress dependence was found to be less pronounced than for the 7075-T6 and 2024-T3 aluminium alloys.