Analyzing objects in images for estimating the delamination influence on load carrying capacity of composite laminates

The use of fiber reinforced plastics has increased in the last decades due to their unique properties. Advantages of their use are related with low weight, high strength and stiffness. Drilling of composite plates can be carried out in conventional machinery with some adaptations. However, the presence of typical defects like delamination can affect mechanical properties of produced parts. In this paper delamination influence in bearing stress of drilled hybrid carbon+glass/epoxy quasi-isotropic plates is studied by using image processing and analysis techniques. Results from bearing test show that damage minimization is an important mean to improve mechanical properties of the joint area of the plate. The appropriateness of the image processing and analysis techniques used in the measurement of the damaged area is demonstrated

Evaluation of delamination damages on composite plates using techniques of image processing and analysis and a backpropagation artificial neural network

Nowadays, drilling of carbon/epoxy laminates is extremely frequent in manufacturing and assembling processes and is normally carried through using standard drills, like twist or Brad drills. However, it is always necessary to have in mind the need to adapt properly the drilling operations and/or the drilling tools used as the risk of delamination occurrence in the laminates involved, or other kind of damages, is very high. Moreover, delamination can be critical because the mechanical properties of the produced parts can be severely affected. Thus, the production of higher quality holes, with damage minimization, is a key challenge to everyone related with composites industry to develop adequate methodologies to delamination characterization and assessment. In this paper, the delamination caused on laminates by drilling machining operations is analytically evaluated by processing and analyzing enhanced conventional radiography images of the laminates involved. In resume, in order to evaluate the delamination damage in laminates plates caused by drilling operations, the radiography images acquired are processed using a computational methodology that uses techniques of image processing and analysis and a backpropagation artificial neural network. Experimental results show that the proposed methodology can be successfully used to measure and characterize the delaminated area. Hence, using our methodology, the damage evaluation on laminates can become more accurate, efficient and simple

Damage assessment of drilled hybrid composite laminates

Hole drilling operations are common in fibre reinforced plastics - FRP’s - to facilitate fastener assembly to other parts in more complex structures. As these materials are non-homogeneous, drilling causes some damages, like delamination, for example. Delamination can be reduced by a careful selection of drilling parameters, drill material and drill bit geometry. In this work two types of laminates are drilled using different machining parameters and comparing drill geometries. Results show the importance of a cautious selection of these variables when composites’ drilling is involved

Recent advances in drilling of carbon fiber–reinforced polymers for aerospace applications: a review

Drilling is considered as one of the most challenging problems in aerospace structures where stringent tolerances are required for fasteners such as rivets and bolts to join the mating parts for final assembly. Fiber-reinforced polymers are widely used in aeronautical applications due to their superior properties. One of the major challenges in machining such polymers is the poor drilled-hole quality which reduces the strength of the composite and leads to part rejection at the assembly stage. In addition, rapid tool wear due to the abrasive nature of composites requires frequent tool change which results in high tooling and machining costs. This review intended to give in-depth details on the progress of drilling of fiber-reinforced polymers with special attention given to carbon fiber–reinforced polymers. The objective is to give a comprehensive understanding of the role of drilling parameters and composite properties on the drilling-induced damage in machined holes. Additionally, the review examines the drilling process parameters and its optimization techniques, and the effects of dust particles on human health during the machining process. This review will provide scientific and industrial communities with advantages and disadvantages through better drilled-hole quality inspection

Optimal Numerical/Experimental Assessment on GFRP for Wind Turbine Blades in Repairing Process utilizing Photo Polymerizable Resin

Fiber reinforced plastics are drawing significant attention to the renewable energy sectors in terms of their excellent specific strength, modulus and stiffness properties as well as outstanding processability, corrosion, and chemical resistance. Wind turbine blades, which inherent GFRP main bodies, are the representative practice for generating renewable energy. While the wind turbine blades may inevitably be damaged due to the harsh service environments like low temperature, acidic moisture, and intense UV irradiation. Remediations on a damaged wind turbine are indispensable to preserve the integrity, guarantee the strength, and expanse the service life. The photo polymerizable resins are featured by less energy consumption, equipment requirement and curing time compared to conventional thermal polymerizable resins. During this dissertation, the photo polymerizable resins were employed to conduct patch remediation on pre-damaged GFRP specimens. The stress and damage distributions of repaired GFRP specimens were obtained through numerical/experimental assessments to validate the remediation feasibility; properties in low temperature were investigated, which was in consideration of the relative cold service environment; durability of repaired GFRP specimens in long-term UV and acid ageing were evaluated to represent the typical service environments of wind turbine blades.List of Tables iv List of Figures v Abstract x 1. Introduction 1 1.1 Research background 1 1.2 Research objectives 6 1.3 Chapter overview 6 2. Literature Review 8 2.1 Fiber reinforced composite materials 8 2.2 Fabrication techniques of composites 9 2.3 Repair methods of composites 16 3. Experimental and Numerical Analysis of UV repaired GFRP 22 3.1 Introduction 22 3.2 Experimental works 24 3.2.1 Materials 24 3.2.2 Fundamental specimens fabrication and remediation 24 3.2.3 Characterizations 28 3.2.4 Experimental results and discussion 28 3.3 Numerical works 32 3.3.1 Material properties determination 32 3.3.2 Geometry and material properties 36 3.3.3 Intralaminar damage model 40 3.3.4 Interlaminar damage model 42 3.3.5 Simulation results of tensile test 46 3.3.6 Simulation results of bending test 55 3.4 Conclusions 64 4. Effect of Low Temperature Environment on UV repaired GFRP 66 4.1 Introduction 66 4.2 Experimental works 68 4.3 Results and discussion 69 4.3.1 Tensile and bending strength 69 4.3.2 Mold II fracture toughness 74 4.3.3 Interlaminar shear strength 76 4.3.4 Microscopic observations 77 4.4 Conclusion 81 5. Durability of UV repaired GFRP among Acidic Atmosphere 82 5.1 Introduction 82 5.2 Experimental works 84 5.2.1 Materials 84 5.2.2 Fundamental specimens fabrication 85 5.2.3 Specimens remediation and ageing conditioning 85 5.2.4 Characteristics 88 5.3 Results and discussion 89 5.3.1 Tensile and bending strength 89 5.3.2 Mode I fracture toughness 91 5.3.3 Interlaminar shear strength 95 5.3.4 Thermal analysis 96 5.3.5 SEM observations 98 5.4 Conclusions 101 6. Performance of UV repaired GFRP during UV ageing Process 103 6.1 Introduction 103 6.2 Experimental works 105 6.2.1 Materials 105 6.2.2 Specimens remediation and ageing conditioning 105 6.2.3 Characteristics 107 6.3 Results and discussion 108 6.3.1 Tensile and bending strength 108 6.3.2 Mode I fracture toughness 111 6.3.3 Interlaminar shear strength 113 6.3.4 Thermal analysis 114 6.3.5 SEM observations 117 6.4 Conclusions 120 7. Summary and Conclusions 121 Acknowledgements 125 References 126Docto

Out-of-plane fiber waviness in composite materials: origins, detection and mechanical evaluation

Out-of-plane fiber waviness, also referred to as wrinkling, is considered one of the most significant effects that occur in composite materials. It significantly affects mechanical properties, such as stiffness, strength and fatigue and, therefore, dramatically reduces the load carrying capacity of the material. Fiber waviness is inherent to various manufacturing processes of fiber-reinforced composite parts. They cannot be completely avoided and thus have to be tolerated and considered as an integral part of the structure. Because of this influenceable but in many cases unavoidable nature of fiber waviness, it might be more appropriate to consider fiber waviness as effects or features rather than defects. Hence, it is important to understand the impact of different process parameters on the formation of fiber waviness in order to reduce or, in the best case, completely avoid them as early as possible in the product and process development phases. Mostly depending on the chosen geometry of the part and the specific manufacturing process used, different types of fiber waviness result. Fiber-reinforced composite materials allow for a significant mass reduction due to the comparably low density (c.f. 4-5 times less than steel) and, in addition, fibers can be aligned in accordance with the load paths. This possibility of alignment allows the fibers to be placed at the exact position where they are needed to provide the component with the required stiffness and strength. However, this can lead to a load path-optimized composite structure, which is not necessarily easy to produce and free of defects. The placement of the fibers or semi-finished textile products is still often carried out by hand-lay-up, especially in the aviation industry. This allows a diverse draping of the unidirectional (UD) layers, woven textiles or non-crimped fabrics (NCF) onto the production tool. However, manufacturing effects such as fiber waviness, porosity, delamination and distortion cannot be completely avoided. The increased demand for composite components and their production process stability for the aviation and automotive industries requires a transition to at least partially automated manufacturing processes. Those systems come with a higher deposition rate and ensure reproducible quality, but also imply production effects, e.g. fiber waviness. This necessitates a sophisticated understanding of those implicit effects on the mechanical properties of the manufactured structure. The decision as to whether these unwanted irregularities are considered as manufacturing features (effects), or as defects, depends on the size, number and location in the component. Those allowance limits depend on the strength and stiffness reserve at the location of the feature, as well as on functional requirements, e.g. water tightness. The assessment of manufacturing effects further depends on the industry. In the aviation industry, the allowance limits for defects are very restricted, while in the automotive industry the need for short cycle times leads to a trade-off between robust processes and tolerated manufacturing imperfections. To this point, there is still no generally accepted approach to quantitatively support accept/reject/repair-decisions and make a consistent assessment of wavy layers in composites. If the effect is termed to be a defect, typically a deviation from design must be requested in the aviation industry and an individual decision must be made on "use as is", repair or reject entirely. In some cases, experiments on representative test samples are performed at the subcomponent-level on a statistical basis. However, this is both time consuming and cost intensive. It is necessary to strive for a fiber-oriented and in particular a manufacturing-oriented design and construction of composite components. Towards this goal, design and production engineers aim to expand the permissible margin of safety by assessing the effect on stiffness and strength of those production effects, i.e. fiber waviness, porosity, delamination etc. Additionally, they aim to reduce or, in the best case, avoid them on the process side, increasingly with the help of finite element based process simulations. In this thesis, numerous mechanisms of wrinkling were analyzed, leading to several recommendations to prevent wrinkle formation not only during composite processing, but also at an earlier design stage, where generally several influencing factors are defined. Based on that, an overview of typically occurring wave shapes is presented and a classification scheme based on ten characteristic features is suggested for categorization purposes. The assessment of out-of-plane fiber waviness in composite materials is strongly dependent on the accuracy of detection and quantification of the wave parameters such as amplitude, wavelength and position in the laminate. In the aviation industry, ultrasonic testing (UT) is the preferred method for the evaluation of composite materials. The evaluation of the ultrasound signal from different manufacturing effects is difficult and it often cannot be clearly determined whether there are actually wavy regions in the laminate or not. In this thesis, different non-destructive testing (NDT) methods, such as infrared thermography (IRT), digital shearography, eddy current testing (ET) and X-ray computed tomography (CT) have been used to assess their potential for the detection and characterization of embedded out-of-plane fiber waviness in composite materials. These methods were applied on test plates with artificially embedded waviness with varying amplitudes, wavelengths and positions in the laminate and evaluated with respect to their ability of detecting the wrinkle morphology. The experimental non-destructive procedures of infrared thermography and digital shearography were simulated using the Finite Element Method (FEM) to gain a deeper understanding on the influence of fiber waviness on the measured results. To understand the complex failure behaviour of composite materials containing out-of-plane fiber waviness under compressive and tensile loading, numerous experimental tests have been carried out. Digital image correlation (DIC), passive thermography (IRT) and acoustic emission (AE) test methods have been used to investigate damage initiation and propagation on specimen level. In addition to that, an extensive material characterization on planar specimens was also performed. Composite materials exposed to harsh environmental conditions, i.e. hot-wet, show considerably reduced mechanical properties, governed by a degrading matrix. To investigate the effect of fiber waviness on the mechanical properties at both room temperature and after 12 months hot-wet conditioning at 70°C and 85% relative humidity, mechanical tests (compressive and tensile loading) were conducted. The basic strategies for the assessment of fiber waviness are briefly described. In engineering practice several approaches are used, i.e. empirical, generic, and semi-empirical. These include experimentally obtained knockdown factors, simplified simulations or extensive testing on subcomponent level, both experimentally and numerically. A developed micromechanical model is implemented in a MATLAB GUI to determine the effective elastic properties as well as the resulting complex stress state of uniform and graded fiber waviness. The well-established Puck failure criterion was implemented and applied on the calculated stresses to predict local ply failure and determine the strength of wavy plies. The mechanical behavior of out-of-plane fiber waviness is investigated for both unidirectional and quasi-isotropic laminates by numerically simulating damage initiation and propagation. A nonlinear material model was implemented in ABAQUS/Explicit as a material user-subroutine, which is able to capture the material behavior including shear nonlinearities, failure initiation and propagation in unidirectional laminates reasonably accurate

DISSERTATION TITLE: DRILLING BEHAVIOUR OF COMPOSITES AND HYBRID FIBER COMPOSITES AT VARYING DRILLING PARAMETERS

Nowadays, fiber composites and hybrid fiber composites are widely used in aerospace industry to replace conventional materials due to weight to strength ratio and resistance to corrosion. These attractive properties resulted in increased machining fiber composites and hybrid fiber composites such as drilling. However, drilling fiber composites and hybrid fiber composites are very problematic compared to that with conventional materials because they are non-homogenous material. The problems encountered during drilling fiber composites and hybrid fiber composites include damage at hole interface, and damage at the drilled holes wall. The aim of this research was to investigate the influence of drilling fiber composites and hybrid fiber composites at various cutting speed, feed rate, different types and thickness of composites materials on damage factor (Fd), surface roughness (Ra), and surface microstructure. The fiber composites used in this research were glass fiber reinforced polyester (GRP) and glass fiber reinforced epoxy (GRE). The hybrid fiber composites used in this research were glass fiber and carbon fiber reinforced polyester (GCRP) and glass fiber and carbon fiber reinforced epoxy (GCRE). Fiber composites and hybrid fiber composites were fabricated using hand lay-up technique. Each composite was fabricated in two different thicknesses (3 mm and 10 mm). 55% fiber volume fraction (FVF) was used to fabricate the fiber composites and hybrid fiber composites to expedite the wear process of the drill bit. In order to maintain the properties of the composites for two different thicknesses, stepped structure design was used to fabricate the fiber composites and hybrid fiber composites. In this research the drilling parameters used to drill fiber composites and hybrid fiber composites were cutting speed (from 1000 rpm to 3000 rpm) and feed rate (from 0.05 mm/rev to 0.2 mm/rev). The damage of the drilled holes was evaluated based on three different evaluations; damage factor (Fd), surface roughness of the drilled holes (Ra) and surface microstructure. The damage factor (Fd) for GCRP (hybrid) fiber composites was lower than GRP fiber composites for 3 mm and 10 mm thickness. The damage factor (Fd) of vii GCRE (hybrid) fiber composites was lower than GRE fiber composites for 3 mm thickness and 10 mm thickness. The surface roughness of GCRP (hybrid) fiber composites was lower than GRP fiber composites for 3 mm and 10 mm thickness. The surface roughness (Ra) of GCRE (hybrid) fiber composites was lower than GRE fiber composites for 3 mm thickness. The surface roughness (Ra) of GRE fiber composites was lower than GCRE (hybrid) fiber composites due to matrix smearing occurred during drilling of GRE fiber composites for 10 mm thickness. Scanning electron microscopy (SEM) evaluation showed that the damage occurred during drilling of fiber composites and hybrid fiber composites were due to fiber pull-out, fiber-matrix debonding and delamination. It can be concluded that drill GCRP (hybrid) fiber composites at 3 mm thickness with lowest cutting speed and feed rate were more suitable compare to GRP fiber composites. On the other hand, it was more suitable to drill GRE fiber composites at 10 mm thickness with lowest cutting speed and feed rate compare to GCRE (hybrid) fiber composites

A new mixed model based on the enhanced-Refined Zigzag Theory for the analysis of thick multilayered composite plates

The Refined Zigzag Theory (RZT) has been widely used in the numerical analysis of multilayered and sandwich plates in the last decay. It has been demonstrated its high accuracy in predicting global quantities, such as maximum displacement, frequencies and buckling loads, and local quantities such as through-the-thickness distribution of displacements and in-plane stresses [1,2]. Moreover, the C0 continuity conditions make this theory appealing to finite element formulations [3]. The standard RZT, due to the derivation of the zigzag functions, cannot be used to investigate the structural behaviour of angle-ply laminated plates. This drawback has been recently solved by introducing a new set of generalized zigzag functions that allow the coupling effect between the local contribution of the zigzag displacements [4]. The newly developed theory has been named enhanced Refined Zigzag Theory (en- RZT) and has been demonstrated to be very accurate in the prediction of displacements, frequencies, buckling loads and stresses. The predictive capabilities of standard RZT for transverse shear stress distributions can be improved using the Reissner’s Mixed Variational Theorem (RMVT). In the mixed RZT, named RZT(m) [5], the assumed transverse shear stresses are derived from the integration of local three-dimensional equilibrium equations. Following the variational statement described by Auricchio and Sacco [6], the purpose of this work is to implement a mixed variational formulation for the en-RZT, in order to improve the accuracy of the predicted transverse stress distributions. The assumed kinematic field is cubic for the in-plane displacements and parabolic for the transverse one. Using an appropriate procedure enforcing the transverse shear stresses null on both the top and bottom surface, a new set of enhanced piecewise cubic zigzag functions are obtained. The transverse normal stress is assumed as a smeared cubic function along the laminate thickness. The assumed transverse shear stresses profile is derived from the integration of local three-dimensional equilibrium equations. The variational functional is the sum of three contributions: (1) one related to the membrane-bending deformation with a full displacement formulation, (2) the Hellinger-Reissner functional for the transverse normal and shear terms and (3) a penalty functional adopted to enforce the compatibility between the strains coming from the displacement field and new “strain” independent variables. The entire formulation is developed and the governing equations are derived for cases with existing analytical solutions. Finally, to assess the proposed model’s predictive capabilities, results are compared with an exact three-dimensional solution, when available, or high-fidelity finite elements 3D models. References: [1] Tessler A, Di Sciuva M, Gherlone M. Refined Zigzag Theory for Laminated Composite and Sandwich Plates. NASA/TP- 2009-215561 2009:1–53. [2] Iurlaro L, Gherlone M, Di Sciuva M, Tessler A. Assessment of the Refined Zigzag Theory for bending, vibration, and buckling of sandwich plates: a comparative study of different theories. Composite Structures 2013;106:777–92. https://doi.org/10.1016/j.compstruct.2013.07.019. [3] Di Sciuva M, Gherlone M, Iurlaro L, Tessler A. A class of higher-order C0 composite and sandwich beam elements based on the Refined Zigzag Theory. Composite Structures 2015;132:784–803. https://doi.org/10.1016/j.compstruct.2015.06.071. [4] Sorrenti M, Di Sciuva M. An enhancement of the warping shear functions of Refined Zigzag Theory. Journal of Applied Mechanics 2021;88:7. https://doi.org/10.1115/1.4050908. [5] Iurlaro L, Gherlone M, Di Sciuva M, Tessler A. A Multi-scale Refined Zigzag Theory for Multilayered Composite and Sandwich Plates with Improved Transverse Shear Stresses, Ibiza, Spain: 2013. [6] Auricchio F, Sacco E. Refined First-Order Shear Deformation Theory Models for Composite Laminates. J Appl Mech 2003;70:381–90. https://doi.org/10.1115/1.1572901