Delamination In Composite Materials Research Articles

Past, Present, and Future of Polymer Science at BAM

Past, Present, and Future of Polymer Science at BAM

Effect of Cutting Revolution Speed on Cutting Temperature in Helical Milling of CFRP Composite Laminates

CFRP (Carbon Fiber Reinforced Plastics) has higher specific strength, modulus of elasticity and so on. In the application for aviation industries, CFRP are remarkably used for many parts of airplane structures. It will be also widely applied for automobile parts in the future. CFRP is, however, well known as difficult-to-cut material which has very strong physical and mechanical characteristics. Drilling technique of CFRP which is one of the most important cutting operations is currently carried out in industrial factories. These manufactured parts have containing many precision holes for various purposes such as rivet holes. There are many typical problems in the precision drilling processes of CFRP plates which are burrs, chippings and delamination of composite materials, rapid wear of drilling tools and so on In our researches, it is found that drilled holes on CFRP plate by ball-nose end mill have good quality without any defections. By using the helical method with cemented-carbide ball end mill, high quality holes on the CFRP plates are obtained without burrs, chippings, and so on. However, cutting heat have negative effect on the strength of CFRP when it transcend heatresistant temperature of epoxy resin. In these researches, it is considered that the effect of the revolution speed to cutting heat on helical cutting of CFRP plates to improve the cutting processes. It is found that the cutting heat become lower by increasing revolution speed. Besides, because of shortening the cutting time, it is restrained the heat from transferring to the workpiece.

Three-dimensional finite element simulation of closed delaminations in composite materials

Early stage delaminations in composite materials tend to be closed at rest. Inspection with traditional linear ultrasonic techniques generally fails to diagnose and locate such imperfections. However, if undetected and left untreated, incipient defects may gradually grow within the material and eventually lead to failure of the component. Kissing bonds or clapping contacts inherently demand a non-linear diagnostic method, applying a finite excitation amplitude that is able to overcome an activation threshold to open and close the contact. In order to obtain a better understanding and analysis of the macroscopic non-linear behavior that can be observed at the component level, we developed and investigated the results of a finite element model for a composite material containing a single circular delamination. The model makes use of local node splitting and the non-linear constitutive behavior is implemented by means of spring-damper elements at the delamination interface. The results of this parametric study allow a better insight in the behavior of the excited delamination in experimental conditions, including the appearance of localized subharmonics and harmonics of the excitation frequency. Based on the developed model, two different detection and localization techniques (using either a single frequency or a sweep excitation) were demonstrated to determine position, shape, depth and orientation of one or multiple delaminations.

Monitoring the initiation and growth of delamination in composite materials using acoustic emission under quasi-static three-point bending test

Control and optimization of machining processes are important issues affecting the development of productivity. Monitoring systems have become indispensable in the evaluation of materials during machining. In this paper, the path toward the delamination-free drilling of glass/epoxy composite material is established using some novel methods based on acoustic emission features. Acoustic emission monitoring with three techniques, sentry function, acoustic emission energy distribution, and acoustic emission count distribution, are developed to detect and realize the critical force at the onset of delamination process on glass/epoxy composite materials. The three-point bending test simulates thrust force, the most effective factor in delamination, throughout the process of drilling without backup plate. Sentry function is regarded as a new method based on the combination of AE information and mechanical behavior of composite materials. The sentry function was used to study the initiation and growth of delamination process. Two types of specimen with different layups, woven [0, 90]s and unidirectional [0]s, leading to different levels of damage evolution, were studied. Results show that AE parameters and sentry function method are useful tools for the examination of initiation and the growth of delamination during drilling process and can help to avoid delamination damage while drilling.

Acoustic Emission based on sentry function to monitor the initiation of delamination in composite materials

Delamination is the most common failure mode in composite materials, since it will result in the reduction of stiffness and can grow throughout other layers. Delamination is consisted of two main stages including initiation and propagation. Understanding the behavior of the material in these zones is very important, hence it has been thoroughly studied by different methods such as numerical methods, Acoustic Emission (AE), and modeling. Between these two regions initiation is a more vital stage in the delamination of the material. Once initiation occurs, which normally requires greater amount of force, cracks can easily propagate through the structure with little force and cause the failure of the structure. A better knowledge of initiation can lead to better design and production of stronger materials. Additionally, more knowledge about crack initiation and its internal microevents would help improve other parameters and result in higher strength against crack initiation. AE is a suitable method for in situ monitoring of damage in composite materials. In this study, AE was applied to test different glass/epoxy specimens which were loaded under mode I delamination. A function that combines AE and mechanical information is employed to investigate the initiation of delamination. Scanning electron microscope (SEM) was used to verify the results of this function. It is shown that this method is an appropriate technique to monitor the behavior of the initiation of delamination.

New finite elements with embedded strong discontinuities in the finite deformation range

This paper develops new finite elements with embedded strong discontinuities for the modeling of the failure of solids in the finite deformation range. The cases of interest include cracks, shear bands, delamination in composites and similar failures modeled by a propagating discontinuity in the deformation mapping that incorporates a cohesive law in terms of the associated displacement jumps. The new elements accommodate the discontinuity in their interiors with a linear interpolation of the displacement jumps in contrast to the existing elements with constant jumps. A general methodology is proposed for the development of such finite elements, but the final focus is on quadrilateral elements for plane problems where a constant jump interpolation leads to a spurious transfer of stresses through the discontinuity (or stress locking) in certain common configurations. The key aspect of the new elements is the proper enhancement of the deformation gradient that can represent correctly the kinematics of linearly separating discontinuities, including a relative finite rotation across the discontinuity, and in a frame indifferent manner. The enhanced parameters modeling the displacement jumps are introduced independently for each element, allowing for their static condensation at the element level and, hence, to arrive at a global system of equations with the same number and connectivity of the original global degrees of freedom for the underlying finite element mesh. This situation leads to a particularly simple implementation of the proposed approach in an existing finite element code. The performance of the new elements is illustrated with several numerical simulations, including delamination in composite materials and propagating shear bands in a finitely deforming elastoplastic solid.

Computerized Tomography and C-Scan for Measuring Drilling-Induced Delamination in Composite Material Using Twist Drill and Core Drill

Delamination is one of the most concern defects in drilling of composite material. Delamination depends on the factors such as feed rate, tool geometry and wear. The mechanics of drilling composite materials has been examined along with the quality of the hole and the effect of tool design parameters. The capacity of computerized tomography (CT) showing sample cross-section in a nondestructive way made it successful in measuring the drilling-induced delamination. In the experiment, the correlation between thrust force and the measured delamination extent in use of twist drill and core drill is illustrated and compared with the known ultrasonic C-scan. It is compared with the ultrasonic technique and is demonstrated a feasible and an effective tool for the evaluation of drilling-induced delamination.

Measuring delamination in carbon/epoxy composites using a shadow moiré laser based imaging technique

Delamination is considered one of the most serious problems associated with the machining of fiber-reinforced composite materials. The evaluation of the drilling-induced delamination damage in these materials is rather difficult, particularly for the carbon fiber-based composites. This is attributed to their color that makes the current visual inspection techniques extremely difficult. This paper presents a non-contact nondestructive technique for measuring and determining the extent of delamination in composite materials. The approach developed consists of utilizing a shadow moiré laser based imaging technique. This technique allows the assessment of the degree of delamination resulting from the machining processes necessary for final set-up and assembly. Mechanical drilling has been implemented on carbon fiber reinforced epoxy (CFRE) composites. The effects of cutting variables (feed and speed) on the delamination size in mechanical drilling of CFRE composites have been investigated. The discussed work should improve reliability and economy in fabrication and assembly of carbon fiber-based composites. Moreover, the results show that the proposed technique is easily adaptable to typical manufacturing environments and possesses promising advantages with respect to accuracy and simplicity.

Prevention from Delamination of Composite Material during Drilling Using Ultrasonic Vibration

Composite materials are used for many structures because of its high strong-to-weight ratio and easy formation. Secondary machining processes such as drilling and sawing are required to assemble and join composite materials to other structures. When composite materials are machined, delamination occurs at adhesive layer between laminated layers. It is well known that it is possible to cut hard metals and to improve precision of machined surface by using ultrasonic vibration. This method is applied to machining composite materials in this paper. Drilling of composite materials is focused on. First, this method is examined experimentally. Next, experimental results are examined by analytical method. Using condition of experiment, prevention of delamination can be demonstrated from the proposed analytical method.

Failure mode and strength of uni-directional composite single lap bonded joints with different bonding methods

Failure process, mode and strength of unidirectional composite single lap bonded joints were investigated experimentally with respect to bonding methods, that is, co-curing with or without adhesive and secondary bonding. The co-cured joint specimen without adhesive had the highest failure strength. Progressive failures along an adhesive layer occurred in the secondary bonded specimen. In the co-cured specimen with adhesive film, delamination failure occurred and the joint strength was lower than that of secondary bonded specimens. Delamination failure did not occur in the secondary bonded specimen because of early crack growth and progressive failure in the adhesive layer. Therefore, The failure strength of composite bonded joint is not always proportionate to adhesion strength of adhesive due to the weakness of delamination in composite materials. The influences of surface roughness, bondline thickness and fillets in the secondary bonded specimens were also studied.

Nonlinear acoustic scattering by a partially closed surface-breaking crack

A theoretical model describing the nonlinear scattering of acoustic waves by surface-breaking cracks with faces in partial contact is presented. The nonlinear properties of the crack are accounted for by suitable boundary conditions that are derived from micromechanical models of the dynamics of elastic rough surfaces in contact. Both linear and nonlinear responses of the crack are shown to be largest for a shear vertical wave incident on the surface containing the crack at an angle just above the critical angle for longitudinal waves. These findings question the fitness for the purpose of a conventional inspection method, which utilizes shear vertical waves at 45 degrees of incidence to search for surface-breaking cracks in many engineering components. For angles of incidence proximal to the critical angle of longitudinal waves, the efficiency of the second harmonic's generation appears to be the highest. Thanks to the increased sensitivity to surface-breaking cracks, this configuration seems to offer a solution to the localization problem, a task that has eluded nonlinear techniques operating under other circumstances. Finally, this model suggests a simple interpretation of the highly localized nonlinear response of delaminations in composite materials.

複合材中のはく離欠陥に対する受動型電気ポテンシャルCT法の適用性に関する実験的検討(OS1-3 はく離モニタリング,OS1 構造ヘルスモニタリング)

複合材中のはく離欠陥に対する受動型電気ポテンシャルCT法の適用性に関する実験的検討(OS1-3 はく離モニタリング,OS1 構造ヘルスモニタリング)

Theoretical description of a delamination mechanism in fibrous micro- and nanocomposites

Theoretical analysis and numerical modeling of unidirectional fibrous micro- and nanocomposites carried out based on the theory of two-component mixture testify that the second mode of a transverse wave propagating along and polarized across the fibers may produce, at high frequencies, a kinematical pattern critical for the strength of the composite material. While the wave propagates, the components (matrix and fibers) of the mixture oscillate in antiphase. This fact may be critical because such oscillations generate forces separating the matrix and fibers, which is typical for the delamination of composite materials.

Mixed‐Mode Delamination – Experimental and Numerical Studies

Abstract: A fracture energy approach for modelling mixed‐mode delamination of composite materials and other bonded structures is introduced. The model is incorporated within an explicit finite element (FE) code and ties layered shell elements together via a stiffness condition, a failure criterion and post‐failure damage law. The procedure for predictive modelling of delamination using the approach is described and the set of required input parameters is presented. A benchmark test comparing experimental results for a continuous filament random E‐glass/polyester composite and explicit FE simulations for standard fracture toughness tests for a range of mode mixities is included.

Numerical Simulation of Mixed-Mode Progressive Delamination in Composite Materials

A new decohesion element with the capability of dealing with crack propagation under mixed-mode loading is proposed and demonstrated. The element is used at the interface between solid finite elements to model the initiation and non-self-similar growth of delaminations in composite materials. A single relative displacement-based damage parameter is applied in a softening law to track the damage state of the interface and to prevent the restoration of the cohesive state during unloading. The softening law is applied in the three-parameter Benzeggagh-Kenane mode interaction criterion to predict mixed-mode delamination propagation. To demonstrate the accuracy of the predictions, steady-state delamination growth is simulated for quasi-static loading of various single mode and mixed-mode delamination test specimens and the results are compared with experimental data.

425 複合材料におけるはく離欠陥導出に対する受動型電気ポテンシャル CT 法の適用性に関する検討

425 複合材料におけるはく離欠陥導出に対する受動型電気ポテンシャル CT 法の適用性に関する検討

Modelling and simulation of the delamination in composite materials

Delamination is a phenomenon characterized by the loss of adhesion between two adjacent laminae. This is a damage process frequently observed in composite materials and it may cause either loss of structural stiffness or total failure of the laminate. This contribution presents a model to describe composite delamination. The proposed model considers a laminate with a finite thickness interlayer. Interlaminar stresses are evaluated from a modified lamination theory. This result is used as input in the constitutive adhesion model which describes the damage evolution of the interlayer. An iterative numerical procedure is developed, solving the model equations separately. This work considers numerical simulations of a laminated tube and a laminated bar as applications of the proposed general formulation. Numerical results are capable of capturing the general behaviour of experimental data.

Non-destructive characterization of delaminated areas at interfaces between plies in carbon fiber/epoxy laminates with Foucault currents

Carbon fiber epoxy laminates are extensively used in the aerospace industry as they offer a combination of high performance with relatively low weight. However, damage may occur in these materials, either at the manufacturing level or during use, which may affect their structural integrity. Among the type of damage which may occur, delamination is a common defect which causes a reduction in compressive strength and thus limits the life of laminated composites. If undetected, delamination may propagate during use due to mechanical loads and in the worst case may cause fracture of the material. Therefore, delamination in composite materials needs to be efficiently detected and accurately quantified for safety and economic reasons. Because of their heterogeneity, multi-layered structure and anisotropy, the inspection of composite materials remains difficult with conventional NDT methods. A method making use of Foucault currents, and relying on the principle of electromagnetic induction, can be adapted for the detection and characterization of damage in composite materials. The use of Foucault currents in characterizing delaminated areas at interfaces between plies in carbon fiber/epoxy laminates is addressed in this paper. It is demonstrated that Foucault currents can accurately detect and quantify delamination in CFRP materials. Furthermore, the potential of this technique for monitoring defect propagation in composite materials is discussed.

Ultrasonic evaluation of matrix damage in impacted composite laminates

Conventional ultrasonic inspection methods are largely used for detection of delaminations in composite materials while only recently new techniques have been proposed to identify matrix cracks in simple tension loaded coupon specimens. In this study delaminations and matrix cracking caused by low-energy impacts on quasi-isotropic carbon/PEEK laminated plates are examined by means of different pulse-echo techniques: conventional time-of-flight and amplitude C-scans at normal incidence are used to check for the presence of delaminations, while backscattering C-scans (in which the transducer is set at an angle to the laminate plane) allow the detection of matrix cracks through the laminate thickness. Selected results from full waveform ultrasonic analysis of impacted carbon/PEEK laminates are discussed and compared with X-ray data in order to demonstrate the efficiency of the proposed inspection technique.

Predicting Progressive Delamination of Composite Material Specimens via Interface Elements

This article describes the application of interface elements to the prediction of progressive delamination in composite materials. The work has been implemented in two separate commercial finite-element computer packages, ABAQUS [1] and LUSAS [2]. In the former case, this has been achieved via a "user subroutine", while in the latter case the elements are embedded directly within the system, either in the standard "release version" of the code or in a research version, to which the authors have access and in which they have implemented various algorithms directly. Comparisons are made with a range of experimental results for composite material specimens. In addition to the finite-element solutions, closed-form results are also presented that are based on a corrected beam theory.