Scarf Repair Research Articles

Repair effects of offcut-carbon-fiber-powder-modified PU-PMMA copolymer on flexure-after-impact behaviors of composite laminate.

Polymer matrix composites are popular for their lightweight and high strength. Poly (methyl methacrylate) (PMMA), known for its transparency, can be toughened with polyurethane (PU) to expand its applications. This study further strengthened PU-PMMA by adding carbon fiber powder from offcut fabrics (oCFP), enhancing mechanical and adhesion properties. Tensile, flexural, and lap-shear tests were conducted to investigate the effects of the oCFP on the mechanical and adhesion properties of the PU-PMMA resin. The oCFP-modified PU-PMMA was then used in scarf repair to assess its effectiveness on the impacted composite laminates. Tensile and flexural tests showed that 1 wt% oCFP improved tensile strength from 38 MPa to 43.31 MPa and flexural strength from 56.02 MPa to 82.21 MPa, while further addition of oCFP led to reductions in these strengths. Although oCFP-modified PU-PMMA did not significantly restore the impacted laminates' flexural strength, the repairing effect was still more pronounced than that of a commercial epoxy adhesive, demonstrating potential as a repair or structural adhesive.

Design and evaluation of a novel variable-length stepped scarf repair technique using a cohesive damage model

The advantages of Carbon Fibre Reinforced Polymers (CFRPs) are well established, but repairing CFRP components remains difficult and costly, posing challenges for industries like aerospace. This paper explores the design, modelling, inspection, and testing of a Variable Length Stepped Scarf (VLSS) repair scheme for highly loaded composite structures. A fully nonlinear 2D Finite Element Model (FEM) is used to design the VLSS repair, predict failure loads and modes, and model adhesive cohesion and delamination. The model incorporates a validated progressive damage model, general contact, and both force and geometric nonlinearities. Two manufacturing techniques involving hard repair patches and glass beads to maintain a constant bond line are employed. A 3D FEM validated against repaired composite coupons under uniaxial tension shows excellent agreement with experimental data. The static strength repair efficiency is approximately 80 % of a pristine sample, with failure displacements at 87 %, and Hooke's stiffness at 102 % of pristine laminates. Cohesive failure at adhesive overlap edges is identified as the cause of stiffness degradation, confirming experimental observations. This study contributes to both composite repair modelling and repair design optimisation.

Cure-induced residual stresses and viscoelastic effects in repaired wind turbine blades: Analytical-numerical investigation

During scarf repair of wind turbine blades, the difference in coefficients of thermal expansion and chemical shrinkage between the original part and the repair patch leads to the development of residual stresses. These residual stresses are detrimental when the repaired composite structures are subjected to operational cyclic loads and affect their post-repair lifetime. This paper uses a hybrid analytical-numerical model to evaluate the residual stresses in a scarf-repaired composite panel. A Prony series-based viscoelastic model is used to describe the material behaviour of the composite undergoing cure to replicate real-life effects more closely. Experiments on the repaired composite samples and numerical simulations on a model of the same are performed to study the post-repair mechanical behaviour. It is found that the damage initiates at the adhesive interface between the scarf patch and the base composite. The resulting debonding and damage to the base composite leads to the failure of the repaired section.

Experimental investigation of buckling and post-buckling behaviour of repaired composite laminates under in-plane shear loading

The performance of Ramped Scarf (RS) and Stepped Scarf (SS) repair schemes for highly loaded composite structures has been extensively studied under compressive, tensile, and flexural loadings. However, there is a lack of research on the buckling and post-buckling behaviour of RS and SS repairs under in-plane shear loading, which is critical for aerostructures. This paper addresses this gap by investigating the buckling and post-buckling performance of these repair schemes. Pristine (P) laminates, RS repairs with a scarf angle of 3°, and SS repairs with an overlap step length of 1/60 were manufactured and tested. The quality of the scarf repairs was inspected using artificial intelligence-based machine vision. Mechanical in-plane shear testing was conducted to evaluate the buckling and post-buckling behaviour of P, RS, and SS laminates. The experimental results indicate that RS repairs demonstrated an average of 35 % higher maximum displacement and 12 % higher failure load compared to pristine laminates. SS repairs showed 19 % higher maximum displacement, and 5 % higher failure load compared to pristine laminates. RS repairs excelled in all key mechanical performance indicators, including buckling load, Hooke's stiffness, maximum displacement, and failure load, compared to SS repairs. The failure modes of P, RS, and SS laminates were similar. However, RS repairs exhibited greater susceptibility to repair patch detachment compared to SS repairs.

Parametric investigation on stepped-lap scarf repair of wind turbine blade spar cap

Parametric investigation on stepped-lap scarf repair of wind turbine blade spar cap

Repair Analysis of Overlay Woven Fabric CFRP Laminates

The increase in aerospace composites usage for structural components demands advanced repair analysis. Overlay repairs of carbon fiber-reinforced polymer laminates offer an alternative that is easier to perform and less time-consuming to produce than the widely used tapered scarf repair and stepped lap. Composite specimen manufacturing was based on both twill carbon/epoxy prepreg and wet lay-up. The repair was performed with both prepreg and wet extra plies to the parent prepreg structure. However, the design of overlay joints must be carefully investigated to avoid generating stress concentration regions at free edges. This study examined specific extra ply terminations' impact on peak stresses in the adhesive bond line. Linear finite element analysis was performed to conduct a maximum principal stress study with a focus on three joint design parameters: ply material, overply effect, and stacking sequence. FEA accurately predicted experimentally observed responses and provided further insight into the failure behavior of the structure. Results showed that overlay joints have a strong sensitivity to ply material type, the number of overply, and stacking sequence. The introduction of overplies provided protection and stiffness at joint tips, and an overply material behavior was identified. The location of 0̊ plies in the composite laminates was highlighted as an important factor. The analysis was then extended to three-dimensional FE models for verification. In conclusion, results showed that high-stress concentration in overlay joints can be mitigated with the introduction of overplies and appropriate changes in joint design parameters to reduce stress peaks at joint tips and corners.

Repair-Based Design of Composite Structures: Scarf Repair

Repair-Based Design of Composite Structures: Scarf Repair

Study on the low‐velocity impact response of the repaired carbon fiber‐reinforced plastic laminates: Bonded versus scarf repair

AbstractExperimental and simulation studies of the response of bonded‐repaired and scarf‐repaired carbon fiber‐reinforced polymer laminates under the low‐velocity impact (LVI) were carried out. The center‐ and eccentric‐repaired laminates were prepared for experiments. The experimental results verify the validity of the finite element model in terms of the force response and different modes of damage. The effect of impact energy and the variation of repair position on the impact response of the laminate was investigated based on the benchmark model. The bonded repaired laminates have higher peak impact forces and lower energy absorption than the scarf‐repaired laminates; the damage of the bonded repaired laminate is mainly intralaminar damage and delamination damage of the patch; the adhesive of the scarf repair is prone to failure, leading to poor impact resistance. The relative positions of the impact point and the patch determine the impact response of a laminate repaired at different locations. The impact point at the edge of the patch has a detrimental effect on the repaired laminate. The impact point vertical position near the bottom hole edge of the parent plate in the scarf‐repaired case harms the impact resistance of the laminate.Highlights The low‐velocity impact response of bonded versus scarf‐repaired laminates was compared. Bonding repairs provide superior impact resistance to damaged laminates than scarf repairs. The risk of adhesive failure in scarf repair is significantly higher than in bonded repair. The relative position of the damage (repair) to the impact point critically influences the impact response. Impacts near the edge of the initial damage hole can cause severe damage to scarf‐repaired laminates.

Study on Shear Fatigue Performance of Scarf Repaired Honeycomb Sandwich Structure

The shear static and shear fatigue tests of the honeycomb sandwich composite were carried out to study the effects of three different repair methods on shear fatigue performance: the unilateral panel scarf repair, the unilateral panel and core scarf repair, and the bilateral panel and core scarf repair. The stress ratio R=0.1 was selected. The results show that the failure mode of the honeycomb sandwich structure is the destruction of the honeycomb core. The structure is torn into two halves from the core, and the core tends to squeeze to one side. There is no debonding between the panel and the core, and no delamination in the panel. The static strength, fatigue limit, fatigue life and residual strength of the specimens are significantly decreased by the scarf repair. The rank of reduction from most to least is: the unilateral panel scarf repair, the bilateral panel and core scarf repair, and the unilateral panel and core scarf repair.And the latter two are remarkably similar.

Structural behaviors of edge-closed titanium honeycomb sandwich panels with a repaired penetrating damage under a unidirectional tensile load

This paper deals with penetrating damage experimental and numerical investigation on edge-closed titanium honeycomb sandwich panels under uniaxial tension. Three damage state specimens were designed and manufactured for tests including intact, prefabricated penetrating damage, and repaired. The test results show the diameter of penetrating hole has a great impact on failure mode and load, and the failure mode of repaired specimens is consistent with the intact. A parametric numerical methodology containing structural damage details was established. Numerical analysis successfully simulated the failure process of all damage states, which was consistent with the experimental observations. Scarf and inlay repair technique can be used to restore the mechanical properties of edge-closed titanium honeycomb sandwich panels.

Effect of Scarf Repair Geometry on the Impact Performance of Aerospace Composites.

This experimental study investigates the effect of scarf geometry in restoring the impact response of scarf-patched 3 mm thick glass-fiber reinforced polymer (GFRP) matrix composite laminates. Traditional circular along with rounded rectangular scarf patch configurations are considered repair patches. Experimental measurements revealed that the temporal variations of force and energy response of the pristine specimen are close to that of circular repaired specimens. The predominant failure modes were witnessed only in the repair patch which includes matrix cracking, fiber fracture, and delamination, and no discontinuity in the adhesive interface was witnessed. When compared with the pristine samples, the top ply damage size of the circular repaired specimens are larger by 9.91%, while that of the rounded rectangular repaired specimens is larger by 434.23%. The results show that circular scarf repair is a more suitable choice of repair approach under the condition of a 37 J low-velocity impact event even though the global force-time response is similar.

In Situ Thermal Ablation Repair of Delamination in Carbon Fiber-Reinforced Thermosetting Composites

Repairing delamination damage is critical to guarantee the structural safety of carbon fiber-reinforced thermosetting composites. The popular repair approaches, scarf repair and injection repair, can significantly restore the in-plane mechanical performance. However, the out-of-plane properties become worse due to the sacrifice of fiber continuity in these repairing processes, leading to the materials being susceptible under service loads. Here, we propose a novel in situ delamination repair approach of controllable thermal ablation in damage removal, achieving a high repair efficiency without impairing the fiber continuity in carbon fiber/epoxy panels. The epoxy resin in the delaminated region was eliminated under the carbonization temperature in a few minutes, allowing the carbon fiber frame to retain its structural integrity. The healing agent, refilled in the damaged region, was cured by the Joule heating of designed electrodes for 30 min at 80 °C, yielding the whole repair process to be accomplished within one hour. For the delaminated carbon fiber/epoxy panels with thicknesses from 2.5 to 6.8 mm, the in-plane compression-after-impact strength after repair could recover to 90.5% of the pristine one, and still retain 74.9% after three successive repair cycles of the 6.8 mm-thick sample. The simplicity and cost-saving advantages of this repair method offer great potential for practical applications of prolonging the service life of carbon fiber-reinforced thermosetting composites.

Effects of repair techniques and scarf angles on mechanical performance of composite materials

Today, composite materials, which have the advantage of strength and lightness ratio, have gained great importance especially in the aeronautical industry and automotive sector. With widespread use of composites, the repair of damages caused by external factors has also become an important research topic. In this study, the effects of different repair methods and scarf angles on the mechanical performance of the material were investigated. Scarf angles of 20°, 30°, and 45° have been selected in order not to create too many scarf areas and to find a quick repair method. Also, a comparison of single scarf and double scarf repair was made to find a more robust solution. The samples were produced from carbon fiber prepreg and with the [45/0/90/0/45]2s fiber orientation. Tensile, compression, and flexural (3-point bending) tests according to ASTM standards were applied to composite samples prepared with single scarf and double scarf repair configurations at different angles. The stress–strain curves obtained as a result of the tests showed that the specimens repaired at 20° had the highest strength. In addition, it has been determined that the samples repaired with double scarf withstand higher forces compared to the samples repaired with single scarf at the same angle.

Improvement of scarf repair patch shape for composite aircraft structures

ABSTRACT Scarf patching is the preferred technique for repairing physically damaged aircraft composite structures. The commonly used patch design is a circular conical frustum with a 3°optimized scarf angle. However, for this repair patch design the volume of material removed from the repaired composite panel can be quite large. This paper examines the effects of the different parameters on the stress state in the patch adhesive, such as ply orientation, stacking sequence, ply thickness and scarf angle. It explores an adaptive design of the repair patch for orthotropic materials and composite laminates, using 3D Finite Element Analysis. The design optimization approach is based on the evaluation of a Failure Criterion Index (FCI) and the minimization of its Relative Standard Deviation (RSD). An optimized scarf shape, which takes into account biaxial tensile loading conditions, fiber direction, and stacking sequence, is an elliptical conical frustum that induces a variable scarf angle along the sweeping angle. This design showed that less parent material is removed than that of the circular shape (decreased by up to 41%) and that the shear stress dispersion in the adhesive was significantly reduced (RSD decreased by up to 18%). The optimization method and the improved shapes are presented and discussed with respect to the influencing parameters.

A comprehensive 2 Dimensional and 3 Dimensional FEM study of scarf repair for a variety of common composite laminates under in-plane uniaxial and equibiaxial loadings

Due to their potential to recover strength and stiffness with a minimum impact on aerodynamic performance of a damaged structure, scarf repairs are boosted as a viable repair option for primary aero-structures. So far, most of the experimental and numerical studies have been limited to joint specimens and their equivalent 2 Dimensional models and a few number of studies attempted to examine the results using real scarf repair geometry. Suggestions previously made to justify the difference between scarf joint and scarf repair strength and possible solution to diminish the difference are challenged in current work. Here, it is tried to investigate the strengths and shortcomings of 2D scarf joint modelling and their influence on the associated results by promoting 3 Dimensional Finite Element Modelling of complete geometry of the scarf repair. A method to assign material properties to a planar geometry of composite material has been developed that provides the possibility of plane strain, plane stress, and generalized plane strain modelling options of a scarf repair cross section under uniaxial load. Besides, accuracy of scarf joint specimen as a representative to scarf repair to predict load carrying capacity of a circular scarf repair is investigated. The study mainly focuses on 2D and 3D FEM simulation results discrepancies considering the effect of angle ply, cross ply and various stacking sequences of quasi-isotropic laminate under uniaxial and equi-biaxial loads. Results show that the laminate lay-up angles and stacking sequence substantially affect the 2D and 3D simulations agreement. Also, the observed discrepancies in scarf repair and scarf joint results are not limited to the effects of plastic deformation of the adhesive, but it seems the modelling shortcomings greatly affect the results. In overall, the current work demonstrates that for a particular laminate, joint specimen does not represent reasonably a scarf repair. In fact, the load carrying capacity estimated based on joint specimen data can mislead decision making procedure in a way that results in rejection of an eligible repair. To avoid inaccurate strength estimation, a 3D simulation of a scarf repair especially for a load other than uniaxial tension or compression is strongly recommended.

Design, novel quality check and experimental test of an original variable length stepped scarf repair scheme

The performance of well-established stepped scarf repair schemes for highly loaded composite structures has been studied in previous works. However, none of the proposed repair schemes appear to minimise healthy material removal. Thus, this paper proposes a Variable Length Stepped Scarf (VLSS) scheme, which does just that. In addition, various standard schemes with overlap step length (1/60, 1/45 and 1/30) are studied. Both experimental and simulation (FEA) investigations are undertaken and for the first time, the quality of scarf is inspected by artificial-intelligence based machine vision. The experimental results show the VLSS scheme is comparable to the other repair designs in restoring structural stiffness of the intact structure. The VLSS shows the ability to restore ≈ 95 % stiffness of the pristine structure compared to 91 % of the largest repair scheme (overlap step length = 1 / 60 ). However, the VLSS scheme falls short in restoring the static strength of the structure, with an efficiency of 64 % . By contrast, the largest repair scheme shows a superior strength repair efficiency ≈ 77 % and demonstrates a desirable failure response, i.e. fibre fracture in both repair patch and parent laminate. • A novel minimum size Variable Length Stepped Scarf (VLSS) and machine vision technique for quality control of scarfed laminates are introduced and performance of VLSS is compared against well-established stepped scarf repair schemes for highly loaded composite structures. • The VLSS shows the ability to restore ≈ 95 % stiffness of the pristine structure compared to 91.4 % of the largest repair scheme (overlap step length ≈ 1 / 60 ). • The VLSS scheme falls short in restoring the static strength of the structure, with an efficiency of 64 % . • The largest repair scheme shows a superior strength repair efficiency ≈ 77 % . • The largest repair scheme shows a desirable failure response, i.e. fibre fracture in both repair patch and parent laminate.

Efficient cocured scarf repair of composite structures through rheology modeling

To address the need for increased efficiency and high-quality in-field repair of composite structures, a vacuum bag only (VBO) semi-preg was produced, modeled, and evaluated against a conventional resin and format commonly used for repairs. The semi-preg featured a vinyl hybrid resin formulated for rapid processing with a discontinuous distribution of resin on the fiber bed. The format imparted high through-thickness air permeability relative to conventional out-of-autoclave (OoA) prepregs by virtue of abundant air evacuation pathways with short breathe-out distances. A model was developed to describe the rheological behavior of the resin, and then flow number analysis was employed to assess model accuracy and to guide the design of efficient cure cycles. A custom-built scarfed repair tool featuring an in situ observation window was employed to analyze the resin flow and cure process during a scarf repair. Microstructural quality and interlaminar shear strength were compared across the epoxy/vinyl hybrid and conventional/semi-preg panels. The results demonstrated that fast-cure resins can be used in conjunction with flow number analysis and semi-preg formats to design efficient VBO cure cycles that consistently yield patch repairs with low defect contents in repair environments.

Failure and reliability analysis of scarf-repaired CFRP: effect of hot–wet environment

Scarf repair method which can get uniform stress distributions and smooth surfaces has emerged as an adequate technique to repair damaged aerospace composite components. However, a prominent problem with the use of scarf-repaired structures is their susceptibility to environmental conditions. This research deals with the failure and reliability analysis of scarf-repaired carbon fiber-reinforced polymer composites (CFRP) before and after aged in hot–wet environment. Scarf-repaired CFRP with different laminate thicknesses was tested to investigate the failure strength and damage modes. The results indicate that the strength reduction of scarf-repaired CFRP varies with the laminate thickness (13.4–7.9%) because of different moisture contents. Fracture observations reveal that the decrease in load-bearing capacity of scarf-repaired CFRP is mainly due to the degradation of adhesive layer. Subsequently, theoretical models based on the distribution function of experimental results were established to predict the safe design strength at different reliability and confidence levels. It is shown that reliability has a greater effect on the safe design strength of both unaged and aged specimens than confidence level. A higher penalty paid to get high-reliability strength (R = 0.99, γ = 0.99) is observed for the unaged specimen, which has a larger coefficient of variation.

Effect of bonded stepped and scarf repair on impact damaged glass fibre reinforced polymer composite properties

The objective of the present work is to investigate the effect of bonded stepped and scarf repair methods on the tensile strength restoration of glass fibre epoxy composites. The composites were fabricated using hand lay-up technique and the specimens were subjected to impact at two different energies of 41 J and 44.3 J. The damage was assessed using dye penetration along with CAD image scaling technique. Damage repaired specimens were characterised for the tensile test which was carried out on a cut portion of the plate having repair identical to the parent laminate. Tensile strength restoration for 41 J and 44.3 J impact energies of scarf repaired laminates was 69.26% and 64.19%, respectively, whereas for step repaired laminates it was 63.46% and 59.74% respectively.

Effect of bonded stepped and scarf repair on impact damaged glass fibre reinforced polymer composite properties

The objective of the present work is to investigate the effect of bonded stepped and scarf repair methods on the tensile strength restoration of glass fibre epoxy composites. The composites were fabricated using hand lay-up technique and the specimens were subjected to impact at two different energies of 41 J and 44.3 J. The damage was assessed using dye penetration along with CAD image scaling technique. Damage repaired specimens were characterised for the tensile test which was carried out on a cut portion of the plate having repair identical to the parent laminate. Tensile strength restoration for 41 J and 44.3 J impact energies of scarf repaired laminates was 69.26% and 64.19%, respectively, whereas for step repaired laminates it was 63.46% and 59.74% respectively.