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Delamination is one of the most feared failure modes in laminated composites and yet there is a lack of well established procedures to find the ply configuration with the highest delamination resistance for a given geometry of the component and a set of elastic and strength constraints. This paper presents a methodology for determining the quasi-isotropic ply sequence more resistant to delamination under off-axis uniaxial tension. Delamination is considered to be triggered by interlaminar stresses at free-edges or by matrix cracks. Two different ply thicknesses (standard and thin) and two sets of ply orientations (multiples of π /4 or π /8) are considered. The results show that the use of thin plies enlarges the design domain by up to 70% and generates a practically isotropic safe space. The methodology presented is also suitable for optimizing the delamination resistance of other load cases with stress singularities. [ABSTRACT FROM AUTHOR]

Adhesive bonding is usually used to fabricate composite structures that are hard to manufacture in one piece, however, their lightweight advantage is usually impaired by low failure strength. For high performance composite structures, bonding properties of joints dominate the failure performance and commonly are the primary target of structural optimization. Both experimental and numerical studies of failure behavior of single-lap joints with three-dimensional braided composite laminate adherends are presented in this paper. First, tensile failure tests were performed on braid-laminates single-lap joints bonded with epoxy resin. Compared with the laminates-laminates single-lap joints, the failure load of the braid-laminates single-lap joints increased by 18.4%. Then, the Finite Element Method (FEM) coupled with cohesive zone models (CZM), considering different value of overlap length (L), was used to perform the detail stress distribution of the overlap sections of SLJs. Further, damage initialization and crack growth of single-lap joints are analyzed in detail to fully characterize the failure process, and both experimental and numerical results lead to the same conclusion. Lastly, the effect of three-dimensional braided adherends' braiding angle on braid-laminates single-lap joints' performance was investigated, which provides suggestions for the design and optimization for adhesive bonded composite structures. [ABSTRACT FROM AUTHOR]

This paper investigates the tensile and fatigue properties of a novel fiber-metal laminate (FML) based on a titanium alloy and carbon fiber-reinforced poly-ether-ether-ketone (PEEK). Tensile tests on unidirectional unnotched laminates have shown that their mechanical properties follow the predictions offered by a simple law of mixtures approach. Tension-tension fatigue tests on notched unidirectional FMLs have shown that these laminates offer fatigue lives up to fifty times greater than those offered by a notched monolithic titanium alloy. An examination of the failed FMLs highlighted the presence of delamination between the titanium alloy and the fiber-reinforced composite. The experimental evidence suggests that this form of damage is harmful during fatigue loading conditions. It has also been shown that delamination is more widespread in FMLs based on thick composite layers than in laminates containing thin composite layers. [ABSTRACT FROM AUTHOR]

The applications of carbon fiber reinforced plastic materials have increased widely in the fields of aerospace, automotive, maritime, and sports equipment because of their excellent mechanical properties. Machining of carbon fiber reinforced plastics has a considerably more complex effect on drilling qualities than machining of conventional metals and their alloys due to the nonlinear, inhomogeneous, and abrasive nature of CFRPs. This article addresses the methodologies that have been adopted to minimize the material damages in drilling of polymeric composite materials. Key papers are reviewed with respect to tool types, materials, geometry and coatings, back-up plate, coolants, environment, unconventional machining, and high-speed drilling methodologies, which influence the hole qualities of delamination, burr, surface roughness, cylindricity, diameter error, and thermal damage with the effect of cutting variables (spindle speed and feed rate). In addition, some deburring strategies are also reviewed and discussed. [ABSTRACT FROM AUTHOR]

Glass fiber reinforced plastics are used in variety of industrial and constructional applications for their superb mechanical and physical properties. Delamination is one of the most important and prevalent damages in machining of glass fiber reinforced plastics. Diamond core drills have the capability of simultaneously drilling and grinding of fiber reinforced plastics, which results in hole quality improvement. Rotary ultrasonic-assisted drilling is the new machining process to drill holes on fiber reinforced plastics and has attracted increasing attention in recent years. In this paper, thrust force and delamination in rotary ultrasonic-assisted drilling of glass fiber reinforced plastics using diamond core drill have been investigated. A proper ultrasonic system for a core drill in ABAQUS was designed and fabricated. Influence of ultrasonic vibration and then tool grain size, fiber-volume fraction and machining parameters including spindle speed and feed rate on thrust force and delamination was investigated. Higher thrust force induces more delamination. As a result of the study, less delamination was achieved in ultrasonic-assisted mode by using a tool with larger grain size onto lower fiber-volume fraction composites. Additionally, experiments showed that lower spindle speed and higher feed rate induce more delamination. [ABSTRACT FROM AUTHOR]

Carbon fiber reinforced plastic composites are used in many applications due to their superior properties. Drilling is the most frequently used machining process due to the need for assembly of carbon fiber reinforced plastic parts in mechanical structures. Rotary ultrasonic machining has been successfully used in drilling carbon fiber reinforced plastic composites. Reported investigations on rotary ultrasonic machining of carbon fiber reinforced plastic cover several output variables (including cutting force, torque, surface roughness, material removal rate, fiber delamination, tool wear, and power consumption). However, there are no reported studies on cutting temperature in rotary ultrasonic machining of carbon fiber reinforced plastic. This paper presents an experimental study on cutting temperature in rotary ultrasonic machining of carbon fiber reinforced plastic using two measurement methods (thermocouple and fiber optic sensor). Comparisons between these two methods are made and relations between input variables (ultrasonic power, tool rotation speed, and feedrate) and cutting temperature are experimentally determined. [ABSTRACT FROM PUBLISHER]

Machining processes of composites are common operations in industry involving elevated risk of damage generation in the workpiece. Long fiber reinforced polymer composites used in high-responsibility applications require safety machining operations guaranteeing workpiece integrity. Modeling techniques would help in the improvement of machining processes definition; however, they are still poorly developed for composites. The aim of this paper is advancing in the prediction of damage mechanisms involved during cutting, including out-of-plane failure causing delamination. Only few works have focused on three-dimensional simulation of cutting; however, this approach is required for accurate reproduction of the complex geometries of tool and workpiece during cutting processes. On the other hand, cohesive interactions have proved its ability to simulate out-of-plane failure of composites under dynamic loads, as impact events. However, this interlaminar interaction has not been used up to date to model out-of-plane failure induced during chip removal. In this paper, both a classical damage model and cohesive interactions are implemented in a three-dimensional model based on finite elements, in order to analyze intralaminar and interlaminar damage generation in the simplified case of orthogonal cutting of carbon LFRP composite. More realistic damage predictions using cohesive interactions were observed. The strong influence of the stacking sequence on interlaminar damage has been demonstrated. [ABSTRACT FROM AUTHOR]

Drilling is involved in many applications of carbon fiber–reinforced plastic composite. Twist drilling is widely used in industry. Rotary ultrasonic machining has been successfully tested to drill holes in carbon fiber–reinforced plastic. However, there are no reports on comparisons between rotary ultrasonic machining and twist drilling of carbon fiber–reinforced plastic. This paper compares rotary ultrasonic machining and twist drilling of carbon fiber–reinforced plastic in six aspects (cutting force, torque, surface roughness, delamination, tool life, and material remove rate). Experimental results show that rotary ultrasonic machining is superior in almost all these aspects. [ABSTRACT FROM PUBLISHER]

In this work a 3D dynamic finite element method was developed to calculate and study the low-velocity impact damage characteristics of stitched laminates. It was found that through-thickness reinforcement in the form of transverse stitching can significantly improve the resistance to delamination of composite laminates in low-velocity impact. Analytical predictions show that the area of delamination of stitched laminates is about 15% less than that of unstitched laminates, and the numerical results are comparable with the test results. It is shown that this method is very efficient. [ABSTRACT FROM AUTHOR]

This paper presents the effect of stiffeners on the free vibration response of delaminated composite shallow cylindrical shells employing the finite element method. An eight-noded isoparametric shell element based on the first-order shear deformation theory is combined with a three-noded isoparametric curved beam element in the present formulation. The stiffeners follow the nodal lines of the shell wherein the stiffness and mass of the stiffeners are lumped at the corresponding nodal points of the shell elements considering curvature and eccentricity. The generalized dynamic equilibrium equation is derived from Lagrange’s equation of motion, wherein Coriolis effect for moderate rotational speeds is neglected. The multi-point constraint algorithm has been used to model delamination at the desired locations wherein the compatibility of deformation and equilibrium of stress resultants are ensured at the delamination crack front. Numerical results are presented for cantilevered long, intermediate and short cylindrical shells as defined by Aas-Jakobsen’s parameters, and the influence of important parameters like location of delamination, twist angle, rotational speed, number of layers and eccentricity of the stiffeners is studied. The mode shapes for a typical composite un-stiffened and stiffened long cylindrical shell at different rotational speeds and twist angles are also presented. [ABSTRACT FROM AUTHOR]

In this paper, the phenomenon of delamination under static and dynamic loading of a composite made of an epoxy matrix and carbon fiber reinforcement has been studied, analyzing its fracture behavior under mixed mode I/II loading employing an asymmetric double cantilever beam test. Under static loading, some of the most representative formulations for calculating the energy release rate were analyzed, finding a good agreement between the results obtained by means of the different formulations. Under dynamic loading, the number of cycles necessary for the crack onset was determined (determination of ΔG − N fatigue curves and number of cycles necessary for the delamination onset for a given energy release rate). As regards the experimental results, apparent fatigue limits of the order of 38% of the critical fracture energy were obtained for an asymmetry coefficient of 0.1. Subsequent statistical analysis of the results enabled the fatigue limit to be defined more accurately. This was found to be 15% for this material, indicating the need to use these tools for the actual determination of the infinite fatigue life of the material. Finally, an optical study of the fracture surfaces was carried out which confirmed the presence of mixed mode fracture typologies. [ABSTRACT FROM AUTHOR]

Carbon fiber-reinforced plastics (CFRP) have been widely applied in aerospace industry as structural components due to their excellent mechanical and physical properties. Meanwhile, the drilling process is indispensable for machining the assembly holes of CFRP. However, in the conventional drilling process of CFRP, it is prone to produce the defects including delamination, spalling, fuzzing, and tool wear. In recent years, the rotary ultrasonic-assisted drilling (RUAD) with diamond core drill, as a novel machining method, has been employed to reduce the defects. But this is few reported investigations on chip adhesion of tool surface and machined rod jamming into core drill tool during RUAD of CFRP. Therefore, this paper detailedly reported a study on removal analyses of chip and rod in RUAD of CFRP using core drill under no cooling condition for the first time. To begin with, the principle analysis on RUAD of CFRP was presented to illustrate the removal process of chip and rod. And then, the experiment analysis on RUAD of CFRP was carried out to observe the removal effects of chip and rod. The experimental results indicated that compared with the common drilling of core drill, when the vibration amplitude reached 5.0 and 7.5 µm in RUAD, the cutting ability of core drill tool was greatly enhanced, excellent removal effects of chip and rod were obtained, which obviously reduced the chip adhesion, rod jamming, rod fragmentation, thrust force, cutting temperature, and surface roughness, improved the dimensional accuracy of machined hole and rod diameter, prolonged the tool life, as well as acquired superior surface integrity of machined hole and flat fibers fracture surfaces. Furthermore, the experimental results also validated the accuracy of the principle analysis. [ABSTRACT FROM AUTHOR]

This paper presents a new non-destructive techniques method based on modal flexibility curvature matrix to detect the delamination in composite structures. The natural frequencies and the mode shapes of the composite beams are obtained using the experimental and finite element modal analysis. It has been found that the calculated modal flexibility curvature matrix FDM can precisely identify the location and size of the delaminations in composite beam. Simultaneously, a new identification theory of delaminated composite plate is discussed, based on flexibility curvature matrix. However, Finite element analysis shows that flexibility curvature matrix, Fc can accurately identify the location and size of either single or multiple delaminations in composite plate. [ABSTRACT FROM AUTHOR]

This paper aims to investigate the delamination issues of laminated glass plates and the effect of boundary conditions on the delamination of laminated glass plates. Delamination of laminated glass which may be defined as reduction or total loss in the perfect bond between the glass sheets and the PVB interlayer is one of the most important failure modes. Due to delamination, the load-carrying capacity and structural stiffness of laminated glass unit can change significantly. The author conducted experimental and numerical studies about the delamination of laminated glass plate problem based on the results of digital image processing. [ABSTRACT FROM AUTHOR]

Delamination is the most common failure mode in laminated composites and it leads to loss of structural strength and stiffness. In this paper, acoustic emission monitoring was applied on the carbon/epoxy laminated composites when subjected to mode I, mode II, and mixed-mode I and II loading conditions. The main objective is to investigate delamination behavior and to predict propagation curve of the delamination in different GII/GT modal ratio values by the acoustic emission. First, a combination of acoustic emission and mechanical data (sentry function) is used to characterize the propagation stage of the delamination. Next, the crack tip location during propagation of the delamination in the specimens is identified using two methods. In the first method, by determining the velocity of the acoustic emission waves in the specimens, the position of the crack tip can be estimated throughout the tests. In the second method, the cumulative energy of the acoustic emission signals is utilized for localization of the crack tip. Agreement between the predicted crack length and the actual crack length verifies the presented procedures. It can be concluded from the results that the acoustic emission method is a powerful approach to investigate the delamination behavior and to estimate the crack tip position in the composites. [ABSTRACT FROM AUTHOR]

This paper is a novel application of acoustic emission (AE) technique to investigate the actual occurring modes of delamination damage (i.e. mode I, mode II and the combination of these pure modes) in glass/epoxy composites. AE parameters and mechanical information associated with some novel methodologies were used to develop new AE-based approaches for studying delamination damage. The inter-laminar fracture energy for the initiation of delamination, Gc, was also measured using the novel introduced methods and standard methodologies. The results indicate that different interface lay-ups and different GII/GT modal ratio values indicate different AE signals and mechanical behaviors. Scanning electron microscope was also used to observe the induced damage mechanisms. It was found that the presented methods of employing AEs to characterize delamination damage and to obtain Gc were successful, especially in mode II and mixed-mode conditions where unstable crack growth and difficult crack growth monitoring prohibit a rigorous measurement of Gc. The results of this study could lead to improve automatic techniques for health monitoring of the real composite structures. [ABSTRACT FROM AUTHOR]

In this paper, an effort has been made to investigate the incorporation of carbon nanotubes in structural composites in order to improve damage characteristics, such as delamination. The nanocomposite material is introduced in the damage-prone regions of complex aerospace stiffener sections; the methodology proposed is an alternative to traditional approaches used to suppress delamination in composites, such as the use of metallic fittings. Numerical simulations are conducted using a multiscale modeling framework. The effective properties of the nancomposites are computed using a micromechanics-based approach and the results are compared with those obtained using a Kalman filter algorithm. The information is then used to analyze the structural response of a hat stringer using detailed finite element models. The stringer is analyzed under different loading conditions and varying levels of defects in the structure. Results obtained indicate that the use of nanocomposites improves the structural performance by improving the initial failure load. It is anticipated that the use of carbon nanotubes during the manufacturing process will help delay the onset of initial damage and damage growth, which can ultimately lead to a more robust structural design with enhanced performance against unique composite failure modes. [ABSTRACT FROM PUBLISHER]

Modern day aerospace, automotive, marine, mechanical and civil structures rely on the advanced composites for their added benefits over conventional metallic structures. The complex damage process in composites involves various failure modes such as matrix cracking, fibre-matrix debonding, fibre fracture and delamination. This paper presents an acoustic emission technique for the failure characterisation of top-hat stiffener specimens using conventional piezoelectric acoustic emission sensors and modern fibre Bragg Gratings. Strain insensitive fibre Bragg Grating sensors are used in this experimental investigation to compare the performance of piezoelectric sensors for composite structures. This work has contributed to the development of an acoustic emission sensing system based on fibre Bragg Gratings. Main objectives of this work are to compare the sensing characteristics simultaneously in a composite structure with a surface-bonded acoustic emission–fibre Bragg Grating sensor and acoustic emission–piezoelectric sensor. The feasibility of the system is demonstrated in typical applications of in-situ structural health monitoring based on acoustic emission techniques. [ABSTRACT FROM PUBLISHER]

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. [ABSTRACT FROM PUBLISHER]

Free-edge stress analysis and the onset of delamination at the free-edges of FRPC laminates have been studied in the present paper. The stress analysis has been performed using a layer-wise 3-D mixed finite element model developed by the authors. Because the mixed FE formulation appropriately models the kinetics and the kinematics of laminated structures, the high stress gradients at the free-edge region of composite laminates have been appositely mapped. The onset of delamination failure has been computed using the tensor polynomial (quadratic) failure criterion. Results have been compared with the theoretical and experimental results available in the literature. It has been shown that the present results are in good agreement with the experimental observations. [ABSTRACT FROM AUTHOR]

This paper presents experimental investigations on impact response of fiberglassaluminum (FGA) composites. A number of test specimens were subjected to increasing impact energies until complete perforation of the target was achieved. The damage process of fiber-glass aluminum (FGA) composites is examined by comparing the load-deflection curves, energy profile diagram (showing the correlation between impact energy and absorbed energy) and images of damaged specimens. The failure processes in the impact-loaded FGAs were initially investigated by examining the front and rear surfaces of the damaged samples. After initial examination, damage mechanisms at the interior layers were ascertained through destructive analysis, i.e., sectioning and de-plying, of samples. Careful examination of the damaged FGA panels highlighted a number of failure mechanisms such as permanent plastic deformation, tearing and shear fracture in the aluminum layers as well as fiber fracture in glass-epoxy layers, and delamination between adjacent layers, i.e., at composite/composite and composite/metal interfaces. Along with the cross-sections, images of several impacted samples for varied impact energies are provided for discussion. [ABSTRACT FROM AUTHOR]

This paper deals with the finite element analysis of the low-velocity impact of multiple masses on laminated FRP composites. Eight-noded, layered solid elements have been developed for the finite element modeling of FRP laminates. The Newmark-β method along with the Hertzian contact law has been used for transient dynamic finite element analysis [2] and implemented for the impact of multiple masses at different times. An appropriate delamination criterion has been used to assess the location and extent of delamination due to multiple impacts. The present method is quite general in the sense that it can incorporate any number of impactors of different masses striking with different velocities, at different points of time and at arbitrary locations on the top surface of the laminated plates. It has been observed that the contact force magnitude as well as delamination at the interface are greatly influenced by the time interval between successive multiple impacts. [ABSTRACT FROM AUTHOR]

In the present paper, a finite element method for a quasi-three-dimensional problem is described. A finite element program developed by the authors is used to analyze in detail the interlaminar stress distributions of composite laminates and further to predict the delamination locations of the studied composite laminates. The predicted are in agreement with those measured experimentally. [ABSTRACT FROM AUTHOR]

The effects of the strip delamination width on the buckling loads of the simply supported carbon/epoxy woven laminated composite plates have been investigated. For this purpose, 3D finite elements models of the square carbon/epoxy laminated plates have been established. Each of these models possesses four layers and different delamination width between second and third layers. Firstly, the harmony between theoretical and finite element solutions results of the plate without delamination has been shown. Then, the buckling loads have been determined for each model and different fiber orientation angles. The results show that important decrease in the buckling loads occur after a certain value of the delamination width. The changing ratios of the results of the symmetric or antisymmetric cases are approximately the same for each angle. For the high values of orientation angles, the values and changing ratios of the buckling loads are also higher than those of the low angles. [ABSTRACT FROM AUTHOR]

Results are presented from a study to determine the most appropriate crack length measurement and data reduction techniques for use with the four-point bend end-notched flexure test. Using two different graphite/epoxy materials, the variability in crack length measurements due to different observers and/or measurement techniques, including visual observation and ultrasonic inspection, and their effects on the perceived values of the mode II delamination toughness, GIIc, were evaluated for two different data reduction techniques. It was found that a technique using pretest compliance calibrations is the only approach that will produce accurate non-precracked and precracked values of GIIc regardless of the material being tested or the nature of crack advance. For those materials where GIIc values, as obtained by the more common approach that utilizes compliance values obtained during the test, agree with those obtained by compliance calibration, the results from the more common approach may be applied to obtain resistance curve toughnesses. [ABSTRACT FROM AUTHOR]

This study is based on the statistical analysis of interlaminar fracture toughness of various laminated polymer composites used in aerospace applications through parametric and non-parametric tests. Tukey's, Dunkan's, two-sample t -test, and Kolmogorov–Smirnov tests are used to analyze tensile mode interlaminar fracture toughness of various fiber-reinforced polymer composites obtained by beam theory, modified beam theory, and modified compliance calibration method. Among the studied composite samples, modified compliance calibration method provided the highest average interlaminar fracture toughness, whereas the modified beam theory showed the lowest one. Room temperature cured carbon fiber-reinforced composite samples exhibited higher interlaminar fracture toughness than the autoclave cured samples. Two-sample t -test show that all methods are found coherent with each other in terms of being significantly different. On the contrary, Kolmogorov–Smirnov test revealed no significant difference. Besides, Tukey's and Duncan's tests exhibited almost the same results in regard to significant differences except those obtained by the modified compliance calibration method. Two-sample t -test method should have to be performed in order to observe significant relations rather than Kolmogorov–Smirnov test, since the results of Tukey's and Duncan's tests are only consistent with each other for the beam theory and modified beam theory method. [ABSTRACT FROM AUTHOR]

The progressive failure analysis based on a modified stiffness degradation model was proposed to predict mechanical response of sandwich L-joints for ships. A representative volume element was established for the derivation of the modified material degradation model for two-dimensional orthogonal woven fabric structures based on micromechanical theory. The modified Shokrieh failure criterion and six field variables were applied in ABAQUS user subroutines. Three different coordinate systems were used to assign reasonable material orientation. The predictions of ultimate load, stress distribution, load-displacement curve and failure patterns were compared with experimental results. The good consistence indicates the effectiveness of the proposed stiffness degradation model. The FE results show the edge of the arc bracket is the stress concentration area due to the shear lag effect, and the declined slope of load–displacement curve is caused by delamination failure due to the stiffness difference of laminates and foam core. [ABSTRACT FROM AUTHOR]

Current manufacturing processes using resin transfer molding or low-pressure prepreg curing may result in different defects and interfacial properties. The effect of autoclave pressure on the delamination behavior of T800/X850 composite laminates is explored. Cohesive zone model was used to model the delamination of unidirectional composite laminates under short-beam bending. Composites with various interlaminar properties were manufactured using autoclave under cure pressure from 0 MPa to 0.6 MPa. Cohesive zone model was validated using the material parameters of the composite cured under 0.6 MPa. The effect of cohesive zone model parameters including cohesive strength, mode I fracture toughness ( GIc), and mode II fracture toughness ( GIIc) on the delamination behavior and load–displacement response was investigated. Parametric study shows that interlaminar cohesive strength and mode II fracture toughness dominated the initiation of yield and post-yield region, respectively. The correlation between autoclave pressure and mode II fracture toughness was predicted, which is mainly affected by void content. [ABSTRACT FROM AUTHOR]

Bio-degradability and sustainability have become major objectives in the field of materials engineering. Due to this, the utilisation of natural reinforcement is gaining immense attention among composite researchers. Among the different composites, polymeric composites are used as structural components. In the present day investigations, bio-elements are majorly utilized as reinforcements due to their properties and bio-degradability. Although polymeric components are manufactured in the shape of the final component, they need secondary processing to make it suitable for assembly. Hence, a review on machining and associated investigations on bio-composites would help the composites industries and upcoming researcher to know the scope of research and for selection of apt machining conditions. This review gives a detailed insight on the various machinability factors and their measurement, influence of machining parameters on the machinability and optimum conditions during machining of bio-composites. In addition, this review also presents about the importance of bio-fibre under various situations of machining and untraditional machining methods used for bio-composites. [ABSTRACT FROM AUTHOR]

A finite strip method is formulated in order to extract the stability characteristics of laminated composite cylindrical panels containing embedded square delamination region. The material properties are assumed to vary along the panel axial length of any lamina according to a linear fiber orientation variation. The effects of existence of a square delamination region are studied on the stability characteristics of the curved panel. The friction effects, contact conditions, and delamination growth phenomenon at delaminated interfaces and edges are overlooked. The natural frequencies of the panel have been investigated based on two composite micromechanical schemes. The dynamic behavior of the delaminated panel has been extracted through using an enhanced B-spline finite strip method formulated based on the principle of virtual work. A third-order Reddy-type shear deformation theory is utilized in order to bring the effects of moderately thick laminate in to account besides a Koiter–Sanders strain–displacement theory of shallow shells. The effects of material volume fractions, layup, micromechanical model, delamination size, and location on dynamic stability characteristics of the structure are studied. In order to demonstrate the capabilities of the developed method in predicting the structural dynamic behavior, some representing results are obtained and compared with those available in the literature. [ABSTRACT FROM AUTHOR]

The progressive damage simulation and strength prediction of open-hole fibre-reinforced composite laminates subjected to compressive loading is quite difficult due to numerical stability issues. Here, we proposed a progressive damage model for open-hole fibre-reinforced composite laminates subjected to compressive loading based on open-hole fibre-reinforced composite laminates under a tensile load. The proposed model is divided into two parts: the in-plane damage and delamination are modelled by continuum shell elements and cohesive elements, respectively, and the damage mechanics of sublaminate-scaled laminates with ply sequence [45/0/–45/90]ns and ply-level scaled laminates with ply sequence [45m/0m/−45 m/90 m]s are investigated by our proposed model. The complete failure process and mode of notched composite laminate under compressive load are also modelled in our model. The numerical results show good agreement with the experimental results. [ABSTRACT FROM AUTHOR]

A new practical procedure is presented for delamination detection in beam-like composite structures. This technique identifies the delamination axial location and its length accurately based on the only first natural frequency. An additional simply support condition besides the boundary conditions is defined and it is moved along the beam length. When this support is located on delamination, especially its tips, the frequency reduction will be noticeable. Simulating this idea in ABAQUS finite element software shows that the location and size of delamination can be detected accurately. To verify the numerical results, some experiments are conducted and a new fixture is designed and manufactured for simulating the additional moving support along the length of the beam. Both finite element and experimental results show the ability of the proposed method in delamination detection for different boundary conditions. Also, this new simple and applicable technique can be used even for small delamination lengths by monitoring only the first natural frequency, unlike the available methods in the literature. [ABSTRACT FROM AUTHOR]

Critical energy release rate for delamination initiation in composites as a material property, supposed to be independent from non-material variables. However, a thorough literature review presented in this study shows that in many cases it may vary with the variation of layup configuration or geometrical and dimensions. This study is aimed to investigate the effect of interface layers orientation on fracture toughness by eliminating the other influential parameters such as stacking sequence, by selecting the anti-symmetric layup configuration of Double Cantilever Beam, [±θ/010/±θ]as, in which θ will be 0°, 30°, 45° and 60°. The energy release rates data have been calculated using different criteria and techniques to obtain the load and displacement at initial crack growth and the results were compared with the standard methods. The damage zone near the crack tip is also illustrated before and after the crack propagation by microscopic images of delamination front, and discussed for all investigated interface fiber angles. Experimental results show that the effect of interface layers orientation on fracture toughness of the investigated layup configurations based on the nonlinear technique as a standard procedure is negligible while other techniques show a considerable changes in the calculated energy release rate with the increase of interface layers angle from zero to 60 degrees. [ABSTRACT FROM AUTHOR]

The formation of longitudinal splitting alleviates the extremely high stress concentration at notch tips of fiber-reinforced composites under remote tension. Theoretical foundation is provided to show that the true stress field along the potential splitting routes can be accurately modelled by inserting cohesive zones in finite element models, such that strength-based criteria can be used to predict damage initiation. Progressive failure analyses are performed to study the in-plane size effect of double-notched quasi-isotropic composite laminates. To capture the stress relief effect of longitudinal splitting during the loading process, surface-based cohesive contacts are introduced along the fiber directions to model the longitudinal splitting. To predict the possible delamination, interface cohesive contacts are also inserted between plies with different fiber orientations. The advantage of the surfaced-based cohesive contact method over the conventional cohesive element method is that it allows different mesh configurations for plies with different fiber orientations. Failure patterns and failure loads predicted by finite element analyses of three scaled composite laminates were compared with experimental results from open literature. [ABSTRACT FROM AUTHOR]

An experimental investigation was performed to study the mechanical performance of 3D carbon/phenolic composites subjected to bending and impact forces. T300-3 k dry preforms were tufted using carbon threads of 396, 800, and 1600 tex. Also, the tufting densities varied from sparse density (tufted 16 × 16: areal density = 0.37 cm−2) to moderate density (tufted 11 × 11: areal density = 0.82 cm-2) and high density (tufted 5.5 × 5.5: areal density = 3.30 cm−2). The tufted preforms were then infiltrated by phenolic resin through the vacuum pressure infusion process. The three-point bending tests results revealed that the flexural strength loss of tufted composites ranged from 17 to 34%. In untufted composites, the specified crack grew quickly along the composite while through-the-thickness reinforcements stopped the crack growth and led to crack branching along the thickness of composite. With the presence of through-the-thickness yarns, damage behavior changed and the tufted composites exhibited a slower rate of decrease in the level of stress. These composites also showed higher levels of after-failure stress values. The experimental results of the impact test demonstrated that tufting had different effects on the energy absorption of composites. In a number of tufted composites, the energy absorption increased by 14.6% and in the other ones, it reduced by 8%. [ABSTRACT FROM AUTHOR]

This study explored the use of J-integral approach for characterizing mode I, mode II and mixed-mode I/II interlaminar fracture toughness of composites materials. Delamination tests were conducted to measure the fracture toughness and the resistance curve (R-curve) for double cantilever beam, end-notched flexure and mixed-mode bending specimens. The J-integral approach and the well-established ASTM standard methods based on LEFM were compared in this work. The results obtained from both methods are in very good agreement. However, the J-integral method has the advantages of being applicable to materials with large fracture process zone (e.g., composites with fiber bridging) and provides a simpler experimental procedure with fewer inputs. More importantly, the presented method avoids the ambiguity in visual measurements of delamination length required by the ASTM standard methods. In this study, an image-processing program based on Hough transform was developed to obtain the rotation angles of the specimen. The method provides good accuracy and has lower requirements for image resolution, light and material surface compared to previous methods. [ABSTRACT FROM AUTHOR]

The effects of seawater exposure on Mode II fatigue delamination growth of a woven E-glass Bismaleimide composite are investigated by end-notched flexure tests in this study. Dynamic mechanical analysis shows a strong plasticization effect of seawater immersion on the matrix material. Static tests of specimens after 2500 h of immersion in 50℃ seawater reveal that the critical strain energy release rate (GIIC) is increased by 15.4%. This is mainly due to seawater effects of plasticization of the matrix, extension of the damage zone and enhancement of the ductility of the material. In fatigue tests, a practical formulation based on the Paris law is proposed to model the stable delamination growth rate for both dry and wet specimens. At a given normalized strain energy release rate, the delamination growth was found to be slower in wet specimens than in dry specimens. The examination on fracture surfaces by scanning electron micrographs also reveals evidence of the plasticization effect of seawater which improves the GIIC of wet specimens. [ABSTRACT FROM AUTHOR]

Curved section is regarded as the weak point of composite structures because of the delamination caused by the stress concentration. In the present work, L-shaped specimens made of randomly oriented short fiber-reinforced polypropylene were prepared to investigate the effect of the radius of the curved section of composite structures on the strength and the damage modes. The results of tensile tests and finite element analysis indicated that the radius greatly affects on the stress distribution and the damage modes, and showed good agreement with former researches using different composites. The strengths of the materials were extrapolated from the results of tensile tests and finite element analysis in this research. Finally, allowable radii of the curved section of the composite structures of two kinds of materials were given respectively. [ABSTRACT FROM PUBLISHER]

Impact-induced delamination propagation on both tufted and untufted carbon fibre-reinforced plastics composite laminates was investigated. High-velocity impacts were performed on a ballistic gas gun. Initial and residual velocities were monitored with optical sensors especially developed for this study. Energy-absorption abilities were compared for different tufting parameters. The perforated specimens were inspected with an original technique allowing the observation of interlaminar delamination area for each interface. The existence of preferential propagation direction is highlighted. It is concluded that tufting helps to contain delamination and provides improved energy absorption depending on the tufting pitch. [ABSTRACT FROM PUBLISHER]

A computationally efficient technique for the analysis of delamination in composite structures with the explicit finite element method is proposed. In the approach, the in-plane and the out-of-plane stiffness of a composite laminate are represented by different material phases. The laminate is seen as a stack of sub-laminates modeled by 2D elements that carry the in-plane stress components. These sub-laminates are connected together with three-dimensional elements that carry the out-of-plane stress components and account for the transverse shear stiffness. Cohesive zone models are adapted to represent the interlaminar damage within the connection elements. The principal advantage of this hybrid modeling technique is that interlaminar damage can be modeled without introducing zero-thickness cohesive elements, which are characterized by means of very large penalty stiffnesses. Consequently, the material stiffness is specified according to physical considerations and, therefore, stable time steps in explicit analyses are not affected by non-physical stiffnesses. The theoretical aspects of the modeling technique and the assessment of a laminate’s model are introduced. Then, three different test cases are considered to confirm that the correct kinematic response is achieved and to demonstrate that sequences of complex delamination events can be predicted accurately and efficiently. [ABSTRACT FROM AUTHOR]

This article aims to obtain damage-tolerant designs with minimum weight for a laminated composite structure using genetic algorithm. Damage tolerance due to impacts in a laminated composite structure is enhanced by dispersing the plies such that too many adjacent plies do not have the same angle. Weight of the structure is minimized and the Tsai–Wu failure criterion is considered for the safe design. Design variables considered are the number of plies and ply orientation. The influence of dispersed ply angles on the weight of the structure for a given loading conditions is studied by varying the angles in the range of 0°–45°, 0°–60° and 0°–90° at intervals of 5° and by using specific ply angles tailored to loading conditions. A comparison study is carried out between the conventional stacking sequence and the stacking sequence with dispersed ply angles for damage-tolerant weight minimization and some useful designs are obtained. Unconventional stacking sequence is more damage tolerant than the conventional stacking sequence is demonstrated by performing a finite element analysis under both tensile as well as compressive loading conditions. Moreover, a new mathematical function called the dispersion function is proposed to measure the dispersion of ply angles in a laminate. The approach for dispersing ply angles to achieve damage tolerance is especially suited for composite material design space which has multiple local minima. [ABSTRACT FROM PUBLISHER]

This study aims to investigate drilling process in carbon-fiber reinforced plastic (CFRP) composites with multilayer TiAlN/TiN PVD-coated tungsten carbide drill. The effect of process parameters have been investigated in drilling of Hexcel M21-T700GC. Thrust force and torque were measured online throughout the drilling experiments. Delamination were observed using optical microscope and analyzed via a developed algorithm based on digital image processing technique. Surface roughness of each hole was measured using a surface profilometer. In addition, the progression of tool wear in various surfaces of drill was observed using tool microscope and measured using image software. Our results indicate that the thrust force and torque increased with the increasing cutting speed and feed rate. Delamination and average surface roughness that rose with the increase in feed rate, however, decreased with the increasing cutting speed. The average surface roughness tended to increase with the increase in feed rate and decrease with the increasing cutting speed in drilling of carbon-fiber reinforced plastic (CFRP). Feed rate was found as the predominant factor on the drilling outputs. Abrasive wear was observed on both flank and relief surfaces, which created edge wear on cutting edges. No sign of chipping or plastic deformation has been observed on the surfaces of drills. [ABSTRACT FROM PUBLISHER]

This article deals with the dynamic response of thin circular clamped GLARE fiber—metal laminates subjected to low velocity impact by a lateral hemispherical impactor. Using a spring-mass model, the differential equations of motion corresponding to loading and unloading stages of impact are derived and solved numerically. Internal damage due to delamination is taken into account. Previously published analytical formulas1,2 concerning the indentation of circular GLARE plates are used during the loading stages of impact. In this study, an equation for the unloading path is derived and used during the unloading impact stage. The load—time, position—time, velocity—time, and kinetic energy—time history responses are calculated. In this regard, the position where delamination occurs, the maximum plate deformation and the position where the impact load becomes zero are predicted. Also, the maximum impact load and the total impact duration are determined. The derived differential equations of motion are applied for GLARE 4-3/2 and GLARE 5-2/1 circular plates subjected to low velocity impact. The predicted load—time history response is compared with published experimental data and a good agreement is found. No other solution of this problem is known to the authors. [ABSTRACT FROM AUTHOR]

This article presents the mixed-mode I/II prestressed end-loaded split specimen for the fracture testing of composite materials. The system is based on the fact that one of the two fracture modes is provided by the prestressed state of the specimen, and the other one is increased up to fracture initiation using a testing machine. The novel beam-like specimen is able to provide any combination of the mode-I and mode-II energy release rates. A simple closed-form solution is developed using beam theory as a data reduction scheme and for the calculation of the energy release rates in the new configuration. The applicability and the limitations of the novel fracture mechanical test are demonstrated using unidirectional glass—polyester composite specimens. If only crack propagation onset is involved, then the prestressed mixed-mode beam specimen can be used to obtain the fracture criterion of transparent composite materials in the G I—GII plane. [ABSTRACT FROM AUTHOR]

A finite element analysis of composite conoidal shells with delamination damage is carried out to explore the bending characteristics of such shell configurations. Civil and structural engineers have always preferred conoidal shell configuration, which is a ruled and aesthetically pleasant shape. The introduction of laminated composite as the structural material has provided the impetus to explore the different behavioral aspects of composite conoids. The laminated composites are susceptible to delamination damage, occurring due to manufacturing defects or over loadings. Keeping the above points in mind, a finite element analysis using isoparametric shell bending element is presented here, incorporating a multipoint constraint algorithm to take care of compatibility of deformation and equilibrium of forces and moments at the delamination crack front. The formulation is validated through solution of benchmark problems, and a number of delaminated conoidal shells with different boundary conditions are solved for different extents of delamination damage and stacking sequences under uniformly distributed load. The truncation ratio, and hence curvature values, are also varied keeping in mind the industrial requirements of both full and truncated conoids. Results are carefully analyzed to arrive at a set of meaningful conclusions. [ABSTRACT FROM PUBLISHER]

This article presents results for mode II delamination resistance of fiber-reinforced polymers (FRP) using a recently developed internal notched flexure (INF) test. The study showed that the decrease of the compliance with the growth of delamination in the INF test was much less than that predicted by analytical or finite element analyses of the same configuration. The difference was mainly due to pronounced shear force interaction generated by bridging fibers and stitching threads between fracture surfaces. A new data-deducing method, named direct method with correction (DMC), was developed for establishing a delamination resistance curve (R-curve), which can take into account the effects of shear force interaction but does not require in situ measurement of crack growth length. The DMC was further examined using results from a series of INF tests with variation in test set-up configurations and types of glass fiber preform. [ABSTRACT FROM AUTHOR]

The bending behavior of delaminated composite shallow cylindrical shells subjected to uniformly distributed load with corner-point supported, simply supported, and clamped boundary conditions is analyzed by the finite element method using an eight-noded isoparametric shell element. To ensure compatibility of deformation and equilibrium of forces and moments at the delamination crack front, a multipoint constraint algorithm is incorporated, which leads to an unsymmetrical stiffness matrix. Lamination, curvature and extent of delamination area are varied to compare the performances of delaminated cylindrical shells with those with no damage. The results are carefully observed and a set of conclusions is presented at the end of the article. [ABSTRACT FROM AUTHOR]

This article presents an experimental and numerical investigation into the influence of transverse stitching on failure of composite L-joints under tensile loading. Six unstitched and six stitched L-joint specimens were manufactured and tested under quasi static tensile loading. It was observed that the average measured failure load and the associated crosshead displacement for the stitched L-joint specimens are increased significantly compared to those for the unstitched specimens. Full 3D and 2D plane strain finite element (FE) models were developed to simulate both stitched and unstitched L-joints with an implemented stitch element. The load-displacement curves and results predicted via FE models compare favorably with the experimental results. For the stitched L-joints, it is shown that the observed delamination in the elbow region of the flange can be modeled by using a softening model for epoxy layer. [ABSTRACT FROM AUTHOR]

The impact response and the impact-induced damage in a curved composite laminate subjected to transverse impact by a metallic impactor are studied using a three-dimensional finite element method. Several example problems of a graphite/epoxy cylindrical shell are considered and effects of impactor parameter (impactor velocity and impactor mass) and laminate characteristics (shell curvature and fiber orientation of plies) are established. Impact-induced damages (matrix cracking and delamination) are predicted using appropriate three-dimensional stress-based failure criteria. In order to take account of degradation of material due to damage during the impact, the stiffness matrix of the failed region of the laminate is reduced as the solution progresses. [ABSTRACT FROM AUTHOR]

Woven glass fabric epoxy composite laminates were subjected to low velocity impacts in an uninstrumented drop weight tester, and the corresponding delamination areas traced. The compression strengths of the composite before and after the impact event were measured adopting a novel technique, consisting of tests on a scaled down version of the standard Boeing-CAI specimen, using a custom designed compression test fixture, in order to facilitate tests within the capacity of an available 100 kN testing machine. Experimental data showed that, as the incident impact energy (Ein) was increased from 10 J to 25 J, the delamination area (A del) increased from 369 mm² to 640 mm², and the compression strength retained was in the range of 62.73% to 43.17% of its virgin strength. Further, a concept of "Area Retention Factor" has been introduced to explain the phenomenon causing the compression strength degradation (or its retention) in impact damaged composites. [ABSTRACT FROM AUTHOR]