Modelling the tensile fracture behaviour of CFRP scarf repairs (original) (raw)
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Buckling Behaviour of Carbon–Epoxy Adhesively-Bonded Scarf Repairs
Journal of Adhesion Science and Technology, 2009
The present work is dedicated to the experimental and numerical study of the buckling behaviour under pure compression of carbon-epoxy adhesively-bonded scarf repairs, with scarf angles varying from 2 to 45 • . The experimental results were used to validate a numerical methodology using the Finite Element Method and a mixed-mode cohesive damage model implemented in the ABAQUS ® software. The adhesive layer was simulated using cohesive elements with trapezoidal traction-separation laws in pure modes I and II to account for the ductility of the adhesive used. The cohesive laws in pure modes I and II were determined with Double Cantilever Beam and End-Notched Flexure tests, respectively, using an inverse method. Since in the experiments interlaminar and transverse intralaminar failures also occurred, cohesive laws to simulate these failure modes were also obtained experimentally following a similar procedure. Good correlations were found between the numerical predictions and experimental results for the elastic stiffness, maximum load and the corresponding displacement, plateau displacement and failure mode of the repairs. by an accident (e.g., tool impact during maintenance). Since this kind of damage significantly reduces the structures' strength, replacement or repair must be followed. Repair of these structures is more efficient from economical and ecological points of view, since composite materials are difficult to recycle. Repair by adhesive bonding is a valid option due to its numerous advantages over the conventional bolting or riveting methods, e.g., more uniform stress distributions, reduced weight penalty, minimal aerodynamic disturbance, and fluid sealing characteristics. If a full or significant strength recovery is required, or if a repair without aerodynamic perturbation is needed, a scarf repair should be used. The higher efficiency of this repair method, compared with the easy-execution strap repairs, is due to the larger bond areas and the reduction of stress concentrations at the bond edges due to the adherend tapering effect.
Tensile failure of adhesively bonded CFRP composite scarf joints
Materials Science and Engineering B-advanced Functional Solid-state Materials, 2006
Adhesively bonded scarf joints, comprising unidirectional carbon fibre-reinforced epoxy adherends and AF-163-2 film adhesive of 0.15 mm thick, are tested under uniaxial tensile loading. Test results revealed that failure occurred into two modes, namely fibre fracture and pull-out (Mode A) and cohesive shear failure of the adhesive film (Mode B). Mode A failure was observed for scarf angles less than about 2 • , while Mode B failure was observed for scarf angles more than 2 • . The knockdown in tensile strength was most prominent for scarf angles less than about 1 • . For the largest scarf angle of 5 • , the tensile strength dropped by 89% in comparison to the strength values of neat composites with no joints. The knockdown in tensile strength with increasing scarf angle was well described by our FEM predictions, which modeled the unidirectional composite as a homogeneous orthotropic continuum with Hashin-Lee failure criterion. Finally, the effect of adhesive bond line thickness on the joint strength is investigated by finite element simulations.
Using a cohesive damage model to predict the tensile behaviour of CFRP single-strap repairs
International Journal of Solids and Structures, 2008
This work addresses both experimental and numerical analyses regarding the tensile behaviour of CFRP single-strap repairs. Two fundamental geometrical parameters were studied: overlap length and patch thickness. The numerical model used ABAQUS ® software and a developed cohesive mixed-mode damage model adequate for ductile adhesives, and implemented within interface finite elements. Stress analyses and strength predictions were carried out. Experimental and numerical comparisons were performed on failure modes, failure load and equivalent stiffness of the repair. Good correlation was found between experimental and numerical results, showing that the proposed model can be successfully applied to bonded joints or repairs.
Strength Prediction of Adhesively-Bonded Scarf Repairs in Composite Structures under Bending
Materials Science Forum, 2010
This work reports on the experimental and numerical study of the bending behaviour of two-dimensional adhesively-bonded scarf repairs of carbon-epoxy laminates, bonded with the ductile adhesive Araldite 2015 ® . Scarf angles varying from 2 to 45º were tested. The experimental work performed was used to validate a numerical Finite Element analysis using ABAQUS ® and a methodology developed by the authors to predict the strength of bonded assemblies. This methodology consists on replacing the adhesive layer by cohesive elements, including mixed-mode criteria to deal with the mixed-mode behaviour usually observed in structures. Trapezoidal laws in pure modes I and II were used to account for the ductility of the adhesive used. The cohesive laws in pure modes I and II were determined with Double Cantilever Beam and End-Notched Flexure tests, respectively, using an inverse method. Since in the experiments interlaminar and transverse intralaminar failures of the carbon-epoxy components also occurred in some regions, cohesive laws to simulate these failure modes were also obtained experimentally with a similar procedure. A good correlation with the experiments was found on the elastic stiffness, maximum load and failure mode of the repairs, showing that this methodology simulates accurately the mechanical behaviour of bonded assemblies.
Taper Angle Optimization of Scarf Repairs in Carbon-Epoxy Laminates
The increasing use of Carbon-Fibre Reinforced Plastic (CFRP) laminates in high responsibility applications introduces an issue regarding their handling after damage. The availability of efficient repair methods is essential to restore the strength of the structure. The availability of accurate predictive tools for the repairs behaviour is also essential for the reduction of costs and time associated to extensive tests. This work reports on a numerical study of the tensile behaviour of three-dimensional (3D) adhesively-bonded scarf repairs in CFRP structures, using a ductile adhesive. The Finite Element (FE) analysis was performed in ABAQUS ® and Cohesive Zone Models (CZM's) was used for the simulation of damage in the adhesive layer. A parametric study was performed on two geometric parameters. The use of over-laminating plies covering the repaired region at the outer or both repair surfaces was also tested as an attempt to increase the repairs efficiency. The results allowed th...
Numerical evaluation of three-dimensional scarf repairs in carbon-epoxy structures
International Journal of Adhesion and Adhesives, 2010
The widespread employment of carbon-epoxy laminates in high responsibility and severely loaded applications introduces an issue regarding their handling after damage. Repair of these structures should be evaluated, instead of their disposal, for cost saving and ecological purposes. Under this perspective, the availability of efficient repair methods is essential to restore the strength of the structure. The development and validation of accurate predictive tools for the repairs behaviour are also extremely important, allowing the reduction of costs and time associated to extensive test programmes. Comparing with strap repairs, scarf repairs have the advantages of a higher efficiency and the absence of aerodynamic disturbance. This work reports on a numerical study of the tensile behaviour of threedimensional scarf repairs in carbon-epoxy structures, using a ductile adhesive (Araldite s 2015). The finite elements analysis was performed in ABAQUS s and Cohesive Zone Modelling was used for the simulation of damage onset and growth in the adhesive layer. Trapezoidal cohesive laws in each pure mode were used to account for the ductility of the specific adhesive mentioned. A parametric study was performed on the repair width and scarf angle. The use of over-laminating plies covering the repaired region at the outer or both repair surfaces was also tested as an attempt to increase the repairs efficiency. The obtained results allowed the proposal of design principles for repairing composite structures.
Tensile Failure Prediction and Measurement in Composite Scarf Repair (Preprint)
2007
: Oversized quasi-isotropic tensile specimens were manufactured from IM6/3501-6 graphite/epoxy prepreg. Seven specimens were scarfed in the center of the panel, and four of the panels were subsequently repaired. The repair patch consisted of a ply-by-ply replacement of the removed material with a FM-300M095 film adhesive placed between the repair patch and the scarfed specimen. The patch and adhesive were then co-cured. The repaired and unrepaired specimens were strain gaged and tested to failure. A three-dimensional failure analysis was performed. The strength prediction was based on the state of stress in the 0(exp 0) plies by taking into account the redistribution of stress due to adhesive failure. The performed analysis accurately predicted both the strength of the scarfed and repaired panels based solely on properties characterized by testing unnotched standard coupons.
Optimization of a composite scarf repair patch under tensile loading
Composites Part A: Applied Science and Manufacturing, 2009
Mechanics of the composite repair under tensile loading with and without overlay plies was examined for nontraditional patch ply orientations. Three-dimensional nonlinear analysis was performed for repair failure prediction and good baseline comparison for open hole scarfed panels and panels repaired by using standard ply-by-ply replacement patch composition was achieved. Multidimensional optimization was performed to calculate the repair patch ply orientations which minimize the von Mises stresses in the adhesive. These optimal stacking sequences achieved significant reduction of the stress levels and resulted in predicted up to 85% and 90% strength restoration for flush and single ply thickness overply repair. These results are intended to illustrate additional design variables available for efficient composite repair design, namely the composition of the repair patch.
Application a direct/cohesive zone method for the evaluation of scarf adhesive joints
Applied Adhesion Science
The use of adhesive bonds in the aerospace, aeronautical and automotive industries, among others, has assumed a high preponderance to the detriment of conventional joining methods such as riveting, bolting, brazing and welding. In fact, adhesive bonds offer several advantages such as the reduction of stress concentrations, good response to fatigue stresses, ability to bond dissimilar materials and lightness of structures. However, they also present some limitations, namely the difficulty of disassembly, high cure times (in some cases) and limited temperature and humidity conditions [1]. The strength and behaviour of adhesive bonds depends on several factors, namely the type of adhesive used, the material of the adherends, the joint configuration and dimensional factors, such as the overlap length (L O ) and the adhesive and adherends' thickness (t A and t P , respectively). There are several types of joint geometries, although the most common are single-lap, double-lap and scarf . Scarf joints are modified butt joints that are
Composite Structures, 2019
The repair of composite materials has received much attention in recent decades because these materials are used in a wide range of industrial applications owing to their excellent mechanical properties. In this work, the failure of scarf patch-repaired composite laminates subjected to bending loads was experimentally and numerically investigated. Thirty repaired laminates with a scarf angle of 5.7° or 3.8° were prepared and tested under different bending loads. Three-dimensional finite element models implementing cohesive elements were created to examine crack initiation and propagation in the specimens during loading. Experimental results revealed that a combination of adhesive, intralaminar, and interlaminar failure occurred in the bonding surface area. Compared to the failure of repair under tensile loading, additional failure occurred in the patch. Simulations reproduced these failure modes well with cohesive elements. The failure loads were also accurately predicted with a maximum prediction error of 9.7%.