Delamination In Composite Materials Research Articles

Dual-scale numerical analysis of delamination in composite materials with varied fiber orientations at the interface

What is the relationship between the fiber orientation at delamination interface and the interlaminar fracture toughness of the composite? The answer is inconclusive based on experimental results. The conflicting observations stem from inconsistencies and inaccuracies in characterization techniques. As an alternative approach to circumvent the experimental challenges, the present study proposes a numerical dual-scale model for delamination, incorporating both micro- and meso-scale features. The model investigates two different fiber orientations at the mid-thickness interface: 0°/0° and 90°/90°. Comparative analyses demonstrate the importance of including a refined region with micromechanics to accurately predict the crack path during delamination. The results exhibit agreement with the literature models and enable estimation of early-initiation fracture toughness, considering delamination interacting with the microstructure while simulating a double cantilever beam specimen with reduced width. The approach provides a deeper understanding of delamination behavior and fracture toughness estimation, contributing to improving composite characterization techniques and design strategies.

Understanding and mitigating delamination in composite materials: A comprehensive review

Delamination significantly undermines the integrity and performance of composite materials, impacting their application in engineering structures. This comprehensive review synthesizes current research on the mechanisms of delamination in composite materials, with a focus on identifying critical factors contributing to this failure mode. It also examines recent advances in detection, prediction, and mitigation strategies. Key findings include the identification of manufacturing processes, moisture absorption, and mechanical loading as primary initiators of delamination. Additionally, innovative methodologies for delamination prediction and prevention are highlighted, showcasing advancements in experimental techniques and computational modeling. This review underscores the importance of understanding delamination to enhance the durability and reliability of composite structures. This analysis aims to guide future research directions and inform the development of more resilient composite structures.

Numerical Investigation of the R-Curve Effect in Delamination of Composite Materials Using Cohesive Elements

This paper presents a numerical investigation of the R-curve effect in delamination propagation in composite materials. The R-curve effect refers to the phenomenon whereby resistance to crack propagation increases with the advancement of the delamination, due to toughening mechanisms, such as fiber bridging. Numerical models often neglect this effect assuming a constant value of the fracture toughness. A numerical approach based on cohesive elements and on the superposition of two bilinear traction-separation laws is adopted here to accurately predict the R-curve effect in skin-doubler composite specimens subjected to three-point bending tests. The carbon-epoxy material presents two different sensitivities to the fiber bridging phenomenon resulting in two different R-curves. Comparisons with literature experimental data, in terms of load and delaminated area vs. applied displacement, and ultrasonic C-scan images show the effectiveness of the adopted approach in simulating the R-curve effect. The predicted numerical stiffness aligns with the experimental scatter, although the maximum load is slightly underestimated by approximately 15% compared with the average experimental results. The numerical model accurately predict the R-curve effect observed in the experimental data, demonstrating a 31% increase in the maximum load for the material configuration exhibiting greater sensitivity to fiber bridging.

Tailoring the thermomechanical behaviour of epoxy with the incorporation of bamboo and graphite filler

The present study is committed to overcome the problem associated with fiber orientation, delamination in composite material. Micro-sized bamboo particulate and graphite is selected as potential filler incorporation material in view of its potential application in thermal management in electronic, automobile parts, electronic packaging and consumer goods. Chemical treatment of natural micro filler is conducted to improve the binding strength among graphite and epoxy matrix. To develop the hybrid composite with varied bamboo filler 2.5 wt% to 12.5 wt% and graphite is incorporated in matrix material. Composite is developed by using NaOH treated micro bamboo particulate almost of 75 µm and graphite filler incorporated in the polymer matrix. The characteristics of specific ‘grade epoxy’ matrix are investigated through uniaxial tensile test, flexural test, dynamic mechanical analysis (DMA), thermal conductivity and wear performance. Results exhibited that with the addition of 0.2 wt% graphite filler (GF) in hybrid composite material enhanced the thermomechanical properties. The developed bamboo filler and graphite based hybrid composite will help to boost the utilization of waste natural filler for greener environment.

Technological Assurance of Fiberglass Composites Surface Layer Quality during End Milling

Introduction. Today fiberglass is one of the most common composite materials. Therefore, its mechanical processing continues to be the subject of many studies. In many scientific publications, the influence of cutting modes and structural and geometric parameters of the tool on the roughness of the machined surface, cutting forces and wear of the cutting tool has been established. The purpose of this work is to study the effect of machining modes on delamination and roughness of fiberglass composites during end milling, as well as testing the hypothesis about the effect of torque on the delamination. The relevance of the study is due to the fact that delamination, along with roughness, has a significant impact on the quality of processing and subsequent assembly of the finished product. A criterion is proposed for assessing the magnitude of the delamination of composite materials during its machining. The results of experimental studies of the torque on the cutter, the relative coefficient of delamination and surface roughness from cutting conditions are presented. Methods: factorial experiment using an experimental assembly developed by the authors based on a piezoelectric torque sensor. The installation allows real-time recording of the change in torque during the milling process, depending on the modes of operation. Results and Discussion. A comparative analysis of the obtained dependences showed that the torque is directly related to delamination. To reduce the delamination, the depth of cut should be decreased, and in order to ensure the specified productivity, the feed and the rotational speed of the cutter should be increased. The presented results confirm the prospects of the developed approach aimed at machining new classes of composite materials.

Crack propagation in CFRP laminates under mode I monotonic and fatigue loads: A methodological study

Delamination in composite materials is often accompanied by fiber bridging that increases resistance to crack growth. Here, delamination of two unidirectional carbon/epoxy systems was studied under monotonic and displacement-controlled fatigue loading. One material (CP004) presented a random distribution of fiber clusters and matrix-rich zones while the other (SE-84) had an ordered microstructure. The results showed that monotonic and fatigue delamination of CP004 presented similarities, with smooth crack growth and significant bridging. The SE-84 showed no bridging but matrix cohesive failure related to stick–slip growth under monotonic load and adhesive/cohesive failure with limited fiber bridging in fatigue. Data from embedded strain sensors showed that bridging under monotonic and fatigue were similar in CP004. The identified traction-separation relation under monotonic loads was used in a cohesive model to approximate the traction-separation at lower subcritical displacement, corresponding to crack arrest in fatigue. The results demonstrated that monotonic and fatigue delamination may have important similarities (or differences) depending on the microstructure and material properties.

Numerical analysis of the delamination in CFRP laminates: VCCT and XFEM assessment

This document develops a critical analysis of the capabilities offered by well-known numerical approaches such as eXtended Finite Element Method (XFEM) and Virtual Crack Closure Technique (VCCT) to predict delamination in composite materials. Despite several computational analyses having been performed so far, the study of the adequacy of using different modelling approaches in the delamination of composites is still limited. This paper addresses this matter, confronting the advantages and disadvantages offered by VCCT, a well-established numerical approach, and XFEM, a promising and relatively novel modelling technique. For this purpose, the delamination of carbon fibre reinforced polymer (CFRP) laminates is investigated with the simulation of three common tests: Double Cantilever Beam (DCB), End-Notch Flexure (ENF) and Mixed-Mode Bending (MMB). Numerical results are validated with experimental data, taken from other publications, for both modelling approaches analysed. Consistency is maintained for all finite element (FE) simulations carried out in this work to draw meaningful comparisons between XFEM and VCCT. Several interesting conclusions are extracted from this work. For instance, VCCT simulations overall have high accuracy and low computational time, while XFEM shows high capabilities to predict Mode I fracture.

Damage Analysis and Experimental Study of Composite Structures with Initial Delamination

Composite laminates are prone to delamination during hot pressing and cooling, this delamination is one of the most important defects that affect the properties of composite structures, the formation and propagation of delamination in composite materials can significantly reduce the structural strength of composite materials and even lead to catastrophic failure. In this paper, the two parameters of the initial stratified position and the initial stratified diameter are selected to research the effects of these two parameters on the mechanical properties of composite laminates through numerical simulation and experiment, and concludes that the influence of initial delamination along the thickness direction on the average compression modulus and strain of the linear segment is small; the influence of delamination on the ultimate bearing capacity increases with the decrease of the distance between the delamination and the center plane. When the diameter of the delamination is in the range of 8%~20% of the effective area(18~44 mm), the influence on the average compression modulus, ultimate bearing capacity and strain of the linear segment is not obvious. When the diameter is increased to 27% of the effective length(60 mm), the above properties have changed greatly. And then, this paper introduces the bilinear controlled cohesive elements into the progressive failure method to calculate the ultimate load capacity of composite laminates, and demonstrate the accuracy of the simulation method by experiments.

Effects of moisture absorption on the different modes of carbon/epoxy composites delamination

It is essential that moisture attack be considered when characterising interlaminar fracture toughness. However, limited work has been reported on the correlation between interlaminar fracture toughness of mixed mode I/II ratios and moisture content. Therefore, the present study aims to characterise the delamination of carbon/epoxy composites, immersed in distilled water at constant 70 °C. The specimens are tested at various water absorption levels using mode I, mode II and mixed-mode I/II loading configurations. The results revealed that mode I fracture toughness is insensitive to the moisture attack. However, approximately 50% drop in mode II and mixed-mode I/II fracture toughness are observed at high moisture content levels. Fibre bridging is also found in all wet specimens with an increment of up to 60–100% in fracture energy being observed for the tested water absorption levels. Upon water exposure, fractured surfaces are also found to possess a more porous matrix with a more significant fibre/matrix interface debonding. Through the study, a new three-dimensional criterion is proposed to characterise the variation of fracture toughness with both mode ratio and moisture content. The criterion is expected to allow for a better fundamental understanding with respect to the extent of moisture attack towards delamination of composite materials.

Formulation of a mixed-mode multilinear cohesive zone law in an interface finite element for modelling delamination with R-curve effects

A constitutive model for an interface finite element is proposed to enable simulation of delamination in composite materials with R-curve effects. The constitutive model is formulated in the framework of cohesive zone modelling (CZM). In essence, a multilinear CZ law with an arbitrary number of line segments is developed. The CZ law seeks to enable constitutive modelling of failure mechanisms on multiple scales within the fracture process zone and reduce conventional a priori assumptions regarding the shape of the CZ law. The CZ law relies on damage mechanics, an equivalent one-dimensional formulation, and criteria for mode interactions to simulate delamination under mixed-mode loading. Special emphasis is put on the derivation of interpolation formulas and a constitutive tangent stiffness tensor for the multilinear formulation. The constitutive model is implemented in the commercial FE program ANSYS Mechanical, for implicit finite element analysis (FEA), using user-programmable features. The implementation is verified through single interface element numerical studies, and its applicability is demonstrated by simulating an experiment of quasi-static delamination showing large-scale fiber bridging in pure mode I DCB glass-fiber epoxy specimens. Experimental measurements and simulation outputs using the novel cohesive element is compared to those of the conventional bi- and trilinear CZ laws.

An extrinsic interface developed in an equilibrium based finite element formulation

The phenomenon of delamination in composite material is studied in the framework of hybrid equilibrium based formulation with extrinsic cohesive zone model. The hybrid equilibrium formulation is a stress based approaches defined in the class of statically admissible solutions. The formulation is based on the nine-node triangular element with quadratic stress field which implicitly satisfy the homogeneous equilibrium equations. The inter-element equilibrium condition and the boundary equilibrium condition are imposed by considering independent side displacement fields as interfacial Lagrangian variable, in a classical hybrid formulation.The hybrid equilibrium element formulation is coupled with an extrinsic interface, for which the interfacial separation is zero for a sound interface. The extrinsic interface is defined as a rigid-damage cohesive zone model (CZM) in the rigorous thermodynamic framework of damage mechanics and is defined as embedded interface at the hybrid equilibrium element sides.

Numerical Study of Vibrational Processes in Composite Material for the Development of a Delamination Control System

Composite materials, due to their properties (high specific strength and low weight), are currently among the most demanded materials used in creating objects for many purposes. The strict modern safety standards require timely monitoring of the occurrence and development of defects, in particular, delaminations. In this respect, considerable attention is paid to the development and improvement of methods of flaw detection. This paper presents a numerical study of the possibility of detecting and localizing delaminations in structures made of laminated composite materials by using vibrational approaches. The approach proposed is based on the excitation of vibrations with an increased amplitude in the region of the defect. This is possible due to the rise of natural vibration frequencies, the maximum amplitude of which is localized exactly in this place. In the first stage of numerical experiments, it was found that such natural frequencies depend weakly on the location of the defect but are strongly correlated with the size of the delamination. In the next stage, for an adequate description of the vibrational processes in the structure, forced steady-state vibrations were simulated taking into account the necessary dissipative parameters of the composite material. The frequency of the external action was chosen in accordance with the natural vibration frequency corresponding to the defect obtained from the modal analysis. The calculation results showed a significant increase in the vibration amplitude in the delamination region in comparison with the defectless structure if the frequency of the external action is chosen correctly. The effectiveness of the approach depending on the distance from the place of the application of the forced oscillation to the delamination region was tested. The method of defect monitoring proposed in the paper makes it possible to justify the possibility of developing, based on the vibtrational processes, a system for detecting delaminations in composite materials and to determine the main parameters of such a system.

A gradient-based optimization approach for the detection of partially connected surfaces using vibration tests

The integrity of engineering structures is often compromised by embedded surfaces that result from incomplete bonding during the manufacturing process, or initiation of damage from fatigue or impact processes. Examples include delaminations in composite materials, incomplete weld bonds when joining two components, and internal crack planes that may form when a structure is damaged. In many cases the areas of the structure in question may not be easily accessible, thus precluding the direct assessment of structural integrity. In this paper, we present a gradient-based, partial differential equation (PDE)-constrained optimization approach for solving the inverse problem of interface detection in the context of steady-state dynamics. An objective function is defined that represents the difference between the model predictions of structural response at a set of spatial locations, and the experimentally measured responses. One of the contributions of our work is a novel representation of the design variables using a density field that takes values in the range [0,1] and raised to an integer exponent that promotes solutions to be near the extrema of the range. The density field is combined with the penalty method for enforcing a zero gap condition and realizing partially bonded surfaces. The use of the penalty method with a density field representation leads to objective functions that are continuously differentiable with respect to the unknown parameters, enabling the use of efficient gradient-based optimization algorithms. Numerical examples of delaminated plates are presented to demonstrate the feasibility of the approach.

Analysis of the partially closed interface crack in orthotropic materials

The determination of fracture parameters such as mode I and II stress intensity factors, KI and KII, is of considerable practical importance in the analysis and failure prediction of various types of structures. For example, interfacial cracks may develop and propagate in bonded joints or lead to delaminations in composite materials. The finite element method is a powerful technique for analyzing complex structures; however, the singular behavior near crack tips is poorly approximated by conventional finite elements and convergence theorems cease to be applicable at those locations. In a previous paper, a combined analytical and numerical model based on frictionless crack face contact was used to obtain fracture parameters at interfacial cracks in composite laminates in cylindrical bending and subject to transverse loads. In this article, we further consider the method and extend the previous results by considering a variety of shear and normal loads on a cracked bimaterial plate with isotropic or orthotropic materials. Furthermore, additional validation of the method is provided by comparing the results with closed form solutions in the literature for the special case of isotropic materials in a combined tensile and shear field, and good agreement is found. Overall, we conclude that the method provides a practical way to analyze a variety of engineering structures with interfacial cracks.

A half-analytical elastic solution for 2D analysis of cracked pavements

This paper presents a half-analytical elastic solution convenient for parametric studies of 2D cracked pavements. The pavement structure is reduced to three elastic and homogeneous equivalent layers resting on a soil. In a similar way than the Pasternak's modelling for concrete pavements, the soil is modelled by one layer, named shear layer, connected to Winkler's springs in order to ensure the transfer of shear stresses between the pavement structure and the springs. The whole four-layer system is modelled using a specific model developed for the analysis of delamination in composite materials. It reduces the problem by one dimension and gives access to regular interface stresses between layers at the edge of vertical cracks allowing the initial debonding analysis. In 2D plane strain conditions, a system of twelve-second order differential equations is written analytically. This system is solved numerically by the finite difference method (Newmark) computed in the free Scilab software.The calculus tool allows analysis of the impact of material characteristics changing, loads and locations of cracks in pavements on the distribution of mechanical fields. The approach with fracture mechanic concepts is well suited for practical use and for some subsequent numerical developments in 3D.

Detecting the Honeycomb Sandwich Composite Material's Moisture Impregnating Defects by Using Infrared Thermography Technique

Many composite materials are used in the aerospace industry because of their excellent mechanical properties. However, the nature of aviation exposes these materials to high temperature and high moisture conditions depending on climate, location, and altitude. Therefore, the molecular arrangement chemical properties, and mechanical properties of composite materials can be changed under these conditions. As a result, surface disruptions and cracks can be created. Consequently, moisture-impregnating defects can be induced due to the crack and delamination of composite materials as they are repeatedly exposed to moisture absorption moisture release, fatigue environment, temperature changes, and fluid pressure changes. This study evaluates the possibility of detecting the moisture-impregnating defects of CFRP and GFRP honeycomb structure sandwich composite materials, which are the composite materials in the aircraft structure, by using an active infrared thermography technology among non-destructive testing methods. In all experiments, it was possible to distinguish the area and a number of CFRP composite materials more clearly than those of GFRP composite material. The highest detection rate was observed in the heating duration of 50 mHz and the low detection rate was at the heating duration of over 500 mHz. The reflection method showed a higher detection rate than the transmission method.

Cohesive zone and level set method for simulation of high cycle fatigue delamination in composite materials

This paper deals with high cycle fatigue delamination in composite materials. The cohesive zone approach along with the level set method is used to simulate fatigue-driven delamination growth. The cohesive zone method is used for calculation of the energy release rate at the crack front because of its superiority over the virtual crack closure technique (VCCT) for bi-material interfaces and non self-similar crack growth. Evolution of the crack front in 3D during fatigue growth is handled with the level set method. The damage variable in the cohesive zone formulation is changed according to the updated level set field. Benchmarks are used to evaluate the performance of the proposed approach in simulation of 3D delamination growth under fatigue loading.

Assessment of delamination in composite materials: A review

The perennial challenge for the industry is to make parts, assemblies and machinery lighter and more efficient. This has led to progressively wider application of composites due to their excellent strength, stiffness and corrosion resistance properties. Composite components are assembled predominantly by fastening, which makes drilling a common machining process for such components. Delamination of the component surface at entry and exit of the drill is a drilling defect frequently encountered. Assessment of the severity of the delamination is necessary for correction and improvement of the performance of the parts and assemblies. Over a period of time, several factors have been suggested as the index of delamination for comparison and control. Also, various techniques are being adopted to measure the extent and severity of delamination so as to calculate the assessment factor. This study aims to present and review the different means and methods for the assessment of delamination.

Energy dissipation during delamination in composite materials – An experimental assessment of the cohesive law and the stress-strain field ahead of a crack tip

This paper presents detailed experimental information on mode-II delamination development in fibre/epoxy composite materials and provides observations about the process zone in the vicinity of the crack tip. It is shown that the energy dissipated in delamination propagation is spent on two ways (i) creating new fracture surfaces (delamination) and (ii) nonlinear shear deformation in the composite plies adjacent to the delaminating interface. Therefore, the nonlinear process zone is not restricted to the resin-rich interface between the layers, but also extends into the fibre/epoxy composite layers. This is different from the conventional assumption in modelling delamination using cohesive elements where the fibre/epoxy layers are fully linear-elastic and the process zone is lumped at the plane of fracture. Based on the accurately measured displacement field around the crack tip, the experimental traction-separation relation at the interface is found to be trapezoidal which is again different from the conventional bilinear cohesive law.

A novel method for fatigue delamination simulation in composite laminates

This paper presents a FEM model for the simulation of delamination in composite materials under fatigue loads using extended cohesive interface elements. A virtual fatigue damage variable is proposed to continually locate the crack tip elements with local element information only. The proposed method allows for a fully automatic identification of crack tip elements without any global information. Once crack tip elements are identified, the fatigue damage accumulation is determined by a modified Paris-law with a strain energy release rate correction method to reduce mesh sensitivity. The predicted delamination growth rates of different mesh sizes were compared with experimental results from open literature using a mode-I Double Cantilever Beam specimen and a mode-II 4-point End Notch Flexure specimen respectively. In all cases good agreement was obtained with weak mesh dependency.