Showing 23 results for Delamination
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Volume 13, Issue 1 (4-2013)
Abstract
In this research, initiation and propagation of delamination are investigated using finite element analysis and existing theories for isotropic and composite double cantilever beam (DCB) specimens. These theories work based on the well-known traction-separation laws such as linear, bilinear and exponential laws. In addition, the effects of cohesive zone parameters, i.e., critical strain energy release rate and maximum interfacial stress, transverse shear deformations and fiber bridging law are studied. The results show that the introduced theories and finite element analysis based on bilinear cohesive law are not capable to predict initiation and propagation of delamination in unidirectional composite specimen with fiber bridging effect and neglecting this region in CZM cause significant error in prediction of delamination growth. For this purpose, bilinear CZM considering bridging law is modified and implemented in 3D finite element analysis. Comparing numerical results with available experimental data in the literature shows that finite element models based on modified CZM can predict initiation of delamination as well as propagation accurately.
Parinaz Belalpour Dastjerdi, Mohammad Fotouhi, Sakine Fotouhi, Mahdi Ahmadi,
Volume 14, Issue 3 (6-2014)
Abstract
Delamination is one of the main defects in composite materials. Studying the initiation and propagation of delamination is useful for the design and production of high quality resistant materials. Therefore understanding the behavior of delamination damage and having enough knowledge about that will be helpful. In this study, generated acoustic emission signals from two types of glass/epoxy composite specimens at different layups, [0,90]5s and [0]10, is recorded. After that signals were analyzed by applying strain energy function. The results indicate that specimens at different layups have different stiffness behavior toward damage initiation and growth. Using strain energy function method initiation and growth of delamination can be evaluated. The obtained results provide useful information for the design and production of high quality resistant composite materials.
Milad Saeedifar, Mohammad Fotouhi, Reza Mohammadi, Mehdi Ahmadi Najafabadi, Hossein Hosseini Toudeshky,
Volume 14, Issue 4 (7-2014)
Abstract
Delamination is the most common failure mode in composite materials. It takes place in different modes, i.e. mode I, mode II or the combination of these modes. The present study is concerned with an investigation of mechanical and acoustic emission behavior of delamination. In this work, various lay-ups of glass/epoxy composite laminates have been used to study the delamination behavior when subjected to mode I, mode II and the mixed-mode I/II tests. First, the characterization of load-displacement curves of the specimens is done based on the AE parameters and mechanical responses and the curves were divided into three parts. The crack growth in the mode I was stable state and in the mixed-mode and mode II was unstable. In the next, interlaminar fracture toughness of the specimens, Gc, were measured using standard methodologies and acoustomechanical methodologies which is based on the mechanical behavior and AE information. It was found that the acoustomechanical method presents the lower limit of the interlaminar fracture toughness and agrees with the results that obtained from standard. The images were captured with Scanning electron microscope (SEM) from damage surfaces verifies the results that obtained from Acoustic emission.
Milad Saeedifar, Mohammad Fotouhi, Reza Mohammadi, Mehdi Ahmadi, Milad Hajikhani,
Volume 14, Issue 6 (9-2014)
Abstract
Sandwich composites are widely used in structural applications because of their appropriate mechanical properties and low strength/weight ratio. Delamination is common failure mode in these structures that lead to a reduction in strength and stiffness of composite. In this paper, using acoustic emission, initiation and propagation of delamination in sandwich composite specimens was investigated. The specimens were loaded under mode I loading. Then the characteristics of the signals related to different damage mechanisms were specified. The acoustic emission signals were classified based on their frequency ranges. Then the acoustic emission signals were recorded during the test specimens were processed using wavelet transform. Thus the percentage of energy in each components of the acoustic emission signal was specified. Each of these components has a certain frequency range corresponding to a damage mechanism. Thus the percentages of different damage mechanisms in each specimen were specified. The Scanning Electron Microscopy (SEM) was also employed to verify the results which were obtained from acoustic emission and wavelet transform method. The results showed acoustic emission is efficient tool for identification and separation of different damage mechanisms in sandwich structures.
Bijan Mohammadi, Davood Salimi-Majd, Mohammad Hossain Ali-Bakhshi,
Volume 14, Issue 10 (1-2015)
Abstract
Due to high strength and stiffness in comparison with their weights, laminated composite materials are widely used in many structures such as aerospace and naval structures. Therefore, the understanding of their failure mechanisms to predict their mechanical response is of high importance. One of the major aforementioned mechanisms is the delamination which commonly occurs in skin/stiffener joints. In the present paper, a comparative study on the delamination in composite skin/stringer structures under 3 point and 4 point bending loads is performed by the finite element method (FEM) employing the cohesive elements. The detailed effects of stacking sequence on the damage of structure are investigated. A user defined interface element has been implemented in the Ansys software in continuum damage mechanics framework based on the bilinear cohesive zone model. The advantage of this method is the modeling of delamination growth without any requirements to the presence of initial crack and remeshing. Comparison of the obtained results from FEM with that of experiment justifies the capability of the employed model to predict the delamination initiation and propagation. The results indicate that in the 3 point bending load, the damage initiates from the adhesive between skin and stringer, while in 4 point bending load it initiates from the interface elements between skin layers near the adhesive bond. Finally, in order to increase the strength of skin/stringer structures, the results strongly recommends preventing the use of 45 and 90 degrees plies near each other around the adhesive bond.
Amin Farrokhabadi, Maryam Aghaebrahimi,
Volume 15, Issue 1 (3-2015)
Abstract
In the present study, using a precise shear lag parameter in an extended shear lag model, by considering the effects of out of plane shear stresses, the stress fields distribution as well as strain fields and displacement distributions will be obtained for a typical [0m/90n]s cross ply composite laminate containing a specified matrix cracking density. Then, the stiffness degradation due to existence of matrix cracking in these cross-ply composite laminates will be evaluated and specific damage parameters, which affect the stiffness matrix of composite ply, will be defined. Furthermore, using the concept of fracture mechanics by applying two different criteria including the maximum stress and strain energy release rate, the matrix cracking initiation and evolution as well as induced delamination propagation will be studied. Finally, a closed form relation will be presented which predicts the evolution of matrix cracking under uniaxial loading conditions in cross-ply composite laminates. At last, the obtained results by present study will be compared with available semi-analytical and experimental results. The obtained results reveal that the proposed closed form relations by the authors have a less difference with experimental results in comparison with the previous semi analytic results.
Hossein Movahhedi Aleni, Gholam Hossein Lighat, Mohammad Hossein Pol, Ali Afrouzian,
Volume 15, Issue 3 (5-2015)
Abstract
In this research, effect of adding silica nano-particles on the mode II interlaminar fracture toughness of epoxy matrix composites reinforced with glass fibers was experimentally studied. Hand lay-up method has been used to manufacture nanocomposites with18 layers of 2D woven glass fibers with 40% fiber volume fraction. The nano-epoxy resin system is made of diglycidyl ether of bisphenol A (epon 828) resin with jeffamine D400 as the curing agent. Nanosilica particles are dispersed with 0, 0.5, 1 and 3 wt.% (of epoxy resin) to study the effect of nanosilica content on fracture toughness. Also a series of nanocomposites with 1 wt.% nanosilica content and contained 55 vol.% glass fibers were fabricated to investigate the effect of fiber volume fraction on results. End notch flexure (ENF) test was adopted for the measurement of mode II interlaminar fracture toughness. The results show that high loading of nanosilica has no significant effect on the interlaminar fracture toughness of nanocomposites while the addition of 0.5 wt% nanosilica enhanced the interlaminar fracture toughness about 36% compared to the neat composites. Decreasing fiber volume fraction improved interlaminar fracture energy.
Mir Mohammad Mousavi Nasab, Milad Saeedifar, Mehdi Ahmadi Najafabadi, Hossein Hosseini Toudeshky,
Volume 15, Issue 8 (10-2015)
Abstract
Despite the fact that fiber reinforced plastic composites have excellent mechanical properties, various failure mechanisms can be occurred in these materials. Delamination is the most common failure mode in laminated composites that can be occurred under quasi-static and fatigue loading conditions. The present study is concerned with the investigation of mechanical and Acoustic Emission (AE) behavior of delamination in glass/epoxy composites under mode I quasi-static and fatigue loading conditions. First, the unidirectional and woven specimens were subjected to mode I quasi-static loading. The behavior of the delamination in the specimens was investigated and interlaminar fracture toughness of the specimens was calculated. Then, according to the information that obtained from quasi-static loading, the similar specimens were subjected to the fatigue loading. The mechanical and AE behavior of the delamination under fatigue loading was investigated. A linear relationship was established between cumulative AE energy and fatigue crack growth and fatigue crack growth curve was predicted using the AE method. Then, energy release rate variations curve and fatigue crack growth rate diagram were predicted using AE method. The predicted results by AE have a good compatibility with the visually based data that recommended by standard. The results indicate that, the AE method has good applicability for health monitoring of composite structures that subjected to quasi-static and fatigue loading conditions.
Morteza Ahmadi Najafabadi, Mojtaba Sedighi, Manouchehr Salehi, Hossin Hossini Toudeshky,
Volume 15, Issue 9 (11-2015)
Abstract
In this study the delamination behavior of FMLs loaded under mode I and II conditions is investigated by using numerical modeling and acoustic emission (AE) data analysis. Test samples is made of prepreg (glass/epoxy composite) and aluminum 2024-T3 (chromic acid anodized). Detection of delamination initiation moment is required for calculation of interlaminar fracture toughness in mode I and II which this moment is detected by using AE technic. Initiation and propagation of delamination is modeled by Abaqus software by using cohesive element. Load-displacement curve, progressive debonding and delamination face are the results taken from FEM and are compared with test results. Signal frequency processing is done for identifying delamination propagation and classification of fracture mechanism. Delamination mechanism is validated by Scanning electron microscope (SEM) images.
Morteza Ahmadi Najafabadi, Hossein Hosseini Toudeshky, Mojtaba Sedighi,
Volume 16, Issue 2 (4-2016)
Abstract
In this paper, acoustic emission monitoring of repaired aluminum 2024-T3 sheet with FML patch is studied. For the experimental investigation, 12 samples were made and classified into 4 categories according to the crack angle (zero and 45 degrees), and repaired or unrepaired state. To reduce manufacturing errors, composite prepreg is used for producing patches, aluminum surfaces is anodized and curing is done in an autoclave. In fatigue crack initiation process by using Acoustic Emission data acquisition, crack initiation moment is detected. By using Acoustic Emission signal cumulative energy parameter onset of delamination, growth of delamination and critical delamination growth is identified. SEM image and investigation of failure surface are used for detecting of failure mechanism. By introducing one frequency analysis method tried to classify frequency range of failure mechanism signals. Because of frequency range intersection of matrix cracking, fiber/matrix separation and delamination of patch from aluminum sheet, force-displacement curve is divided to 3 zone and frequency analysis is done in each zone that occurrence possibility of certain failure mechanism is higher than the others. Signal frequency range of aluminum plasticity and crack growth is in the range of 440-480 kHz, and signal frequency range of delamination is in the range of 100-150 kHz and 200-220 kHz.
Amirreza Shahani, Razieh Abolfathitabar,
Volume 16, Issue 5 (7-2016)
Abstract
The unidirectional composite DCB specimen is considered as two finite length Timoshenko beams, attached together along a common edge except at the initial delamination length. Because of symmetry, only one half of the specimen is considered, which is partly free and partly resting on an elastic foundation. The problem is analytically solved by considering Timoshenko beam resting on Winkler and Pasternak elastic foundations and fracture toughness is generally derived. In the prior researches on this specimen using Timoshenko beam theory, the effect of the ligament length on the energy release rate was ignored. This research presents the solution for finite ligament length. Besides, the effect of ligament length on energy release rate and its minimum value that makes the energy release rate independent of the ligament length, is presented. For the special case when the ligament is large compared with the beam thickness, a closed form solution is derived for Timoshenko beam resting on Winkler elastic foundation. The analytical results are compared to prior researches on this subject and a good agreement is observed. The fracture toughness and compliance obtained by Timoshenko beam resting on Winkler elastic foundation predicts more accurate results with respect to experimental results.
Vahid Zal, Hassan Moslemi Naeini, Ahmad Reza Bahramian, Javad Shahbazi Karami,
Volume 16, Issue 9 (11-2016)
Abstract
Due to the polar functional groups of PVC thermoplastic and its good adhesiveness to the metals, production and roll forming of PVC/ aluminuim/ glass fiber FMLs were investigated in this research. At the first, flexural strength and bonding quality between PVC matrix and aluminuim layer in the FMLs were studied by doing three point bending tests according to ASTM D790 standard. In the following, FMLs with dimension of 12×80 cm and two layups including [0/90, 0/90, Al]s and [45/-45, 45/-45, Al]s were produced using film stacking and hot pressing procedure. The FMLs were rollformed into 30, 45 and 60º channel section profiles at 160ᵒC using a single stand rollforming process and geometrical decects including profile bowing, edge wrinkling, spring back and also aluminuim/composite layers delamination of the resulted profiles were evaluated. The FMLs also were roll formed into 86º channel section profiles using a multi stand roll forming process and the effects of multi stand roll forming on the defects stacking were evaluated. Finally, it was concluded that more than 45º bend angle increase in a rollforming stand results in composite/ aluminum delamination. Also, placement of the reinforcing fibers in the longitudinal direction of the profiles reduces the profile bowing and edge wrinkling defects significantly.
Mohammadjavad Mahmoodi, Araz Mohammad Fakhri Inchehborun,
Volume 17, Issue 5 (7-2017)
Abstract
In this study, error back-propagation neural network is used for fault detection of composite plate with delamination damage. At the first step of the fault detection process, a free vibration analysis of laminated composite plates is performed based on numerical finite element method and the natural frequencies of individual modes is obtained for different delamination models (size, geometry and location of the delamination region). Then natural frequencies extracted from the model are considered as the input parameter and the size, geometry and location of the delamination region are also considered as the output parameters of the neural network. 8-layers composite plate is modeled based on the three-dimensional elasticity theory and considering hexagonal brick elements. So, transverse shear deformations effect is taken into account in the modeling of composite plate. ABAQUS software capabilities are used for modeling because of the complexity of process governing on the composite plate with delamination. The numerical results obtained by the finite element method are compared and validated with available numerical and experimental data. Two training methods including Levenberg - Marquardt and Error propagation flexible algorithm are used to train the neural network and compare responses. Predicted results by Levenberg – Marquardt training method are in very good agreement with the values obtained by the finite element method. After training the neural network, the model generalization is used for predicting and detecting of the damage in composite plate.
Mehdi Ganjiani, Hossein Orui, Mahdi Ganjiani,
Volume 17, Issue 8 (10-2017)
Abstract
Delamination is one the most important defect that reduces strength of part. Many researchers have been studied delamination in drilling of composite materials and they tried to formulate this phenomenon by developing of analytical, numerical and experimental models. In this models up to now, only effect of modeΙ and modeΙΙΙ of crack propagation is considered and effect of modeΙΙ is neglected. The goal of this research is determination of modeΙΙ effect on propagation of interlaminar cracks during drilling of multilayered composites by means of finite element analysis (FEA).Thus the numerical analysis of delamination of unidirectional Carbon/epoxy composite during drilling is performed by modification of previous numerical models. The numerical method which is used for determination of strain energy release rates in modeΙ and modeΙΙ is Virtual Crack Closure Technique (VCCT). This analysis is performed for crack propagation under chisel edge when the drill have not come out from the workpiece and for crack propagation under cutting edges when the drill have come out from the workpiece. By determination of strain energy release rates in modeΙ and modeΙΙ and comparing with critical values G_Ιc andG_ΙΙc, the critical thrust force that causes to delamination is determined and contribution of each crack propagation mode in delamination is discussed. At the end of this research, it was found that the effect of modeΙ is more than modeΙΙ in all of cases such that more than 95percent of crack propagation parameter in power law criteria is due to strain energy release rates in modeΙ.
Amin Farrokhabadi, Hamid Reza Madadi,
Volume 17, Issue 10 (1-2018)
Abstract
Studying the behavior of composite materials reveals that various types of failure modes occur when material experiences different loading conditions, which may have a significant impact on performance and properties of a structure. In this research, we study the mechanical response of orthogonal multi-layers by considering different failure modes at micro-scale and their development in macro-scale. For this purpose, the effect of the emergence and growth of fiber separation and subsequent formation of matrix cracks are investigated in the micro-scale. Furthermore, interlayer separation caused by leaving the matrix are studied in macro-scale. To model the separation of fiber matrix which is the first dominant failure mode, the sticky area method is used. The model verification and obtained results are compared with the previous research. Then, XFEM method is used to take into account the failure mode of matrix. Finally, using of the sticky area method, we are able to simulate the separation of matrix layers. The FE-program Abaqus via its user scripting interface (Python) are employed in this research for modeling of fibers embedded into matrix.
Mahdi Habibi, Jalal Yousefi, ,
Volume 18, Issue 4 (8-2018)
Abstract
Delamination is one of the major failure modes of the laminated composite material, which is responsible for the stiffness degradation of these materials. Hence, it is necessary to investigate this damage mechanism in these types of materials in order to distinguish their behaviors and their effects on the residual strength of the composite laminates. In this paper, a very capable procedure is proposed to assess delamination using Acoustic Emission (AE) method in composite laminates. Firstly, a novel procedure was established to decompose the fundamental Lamb wave modes in small size specimens. The damage mechanisms in End Notched Flexure (ENF) in woven and unidirectional specimens were then distinguished using Fuzzy Clustering Method (FCM). Subsequently, the crack-arrest phenomenon was inspected in each specimen. Next, experimental and Cohesive Zone Modeling (CZM) methods were done to characterize the delamination using ENF specimens. The results displayed how, it is possible to effectively reduce the effect of propagating media such as attenuation of AE signals using the new proposed procedure. In conclusion, the results of this research could lead to proficiently distinguishing different damages in laminated composite using AE Lamb-based technique.
Hossein Ipakchi, Mitra Oleiaei, Masoud Esfandeh, Mohammad Yousefi, Amir Masoud Rezadoust,
Volume 18, Issue 6 (10-2018)
Abstract
Glass-reinforced phenolic laminates show a low resistance to delamination. Toughening of the matrix resin with a polymeric interlayer is among the method used to improve the delamination strength. In this research Polyvinyl butyral(PVB) nanoweb with the fiber diameter of 300-600 nanometer were used as an interlayer in a 14-layer glass reinforced phenolic composite. A hybrid nanoweb consists of PVB nanoweb reinforced with pyrolytic carbon and carbon nanotube (CNT) were also prepared and used as the interlayer. Mode I and Mode II delamination tests were conducted on the samples according to the related ASTM standard test method. The results showed that PVB interlayer improves the delamination strength of the composites by 13.6% and 13.8%. for mode I and Mode II, respectively. Also, with the hybrid nanoweb, better improvement in the fracture toughness was achieved. In the hybrid nanowebs, CNTs at the optimum amount has a greater effect on the Mode I fracture (49% improvement in GIc), while the pyrolytic carbon mainly affected the Mode II fracture toughness by 38% improvement in GIIc. Morphological studies carried out by SEM microscopy showed that crack deviation is the dominant mechanism for toughening of the polymeric matrix which results in the delay in fracture initiation and increase of the crack length and in doing so enhances the fracture toughness of the laminates.
Mohammad Amin Ghasemi, ,
Volume 18, Issue 8 (12-2018)
Abstract
Coatings are used in various industries in order to improve the surface properties of materials. Delamination of coatings from their substrate, at the root of channel cracks, is one of the common failure modes in these structures. In this paper, discrete element method is used in order to simulate the initiation and propagation of damages, caused by the mismatch between the thermal expansion coefficients of coating and substrate. Coating and substrate are considered to be brittle elastic in which, substrate is stiffer than the coating, but the thermal expansion coefficient of coating is considered to be much greater than substrate. The interface properties are also considered to be the geometric average between the coating and substrate. Temperature reduction is applied to the whole structure as loading. The effect of elastic mismatch and coating thickness was investigated. The results showed that, by increasing the elastic mismatch and decreasing the coating thickness, the temperature reduction, need to delamination initiation at the interface, increased. Also, changing in the damage propagation pattern was happened by changing in the elastic mismatch. In coatings with high elastic mismatch, damage propagation was happened inside them but by increasing the stiffness, damage propagation happened at the interface.
M. Habibi, J. Yousefi, M. Ahmadi,
Volume 19, Issue 12 (12-2019)
Abstract
Delamination or interlayer cracking is one of the most important imperfections in composite materials. The existence of this defect in a structure reduces the strength and, as a result, disables the structure. To analyze the effective factors in interlayer separation, it is necessary to analyze the effective loading parameters. In this paper, the effect of the change in loading rate on the failure mechanism in I failure mode was analyzed using an acoustic emission for unidirectional samples made of glass fiber/epoxy resin. At first composite, samples were made according to standard and placed at different rates of displacement under loading. Force data, displacement and crack growth rate for different loading rates were used to calculate the exact strain energy release rate. In addition to the extensometer, the Dino camera was used. In this paper, a high-reliability method was proposed to evaluate the separation between the layered composites using acoustic emission method. By comparing mechanical data and acoustic emission signals, the mechanical behavior obtained for each loading rate was determined so that the mechanical behavior of the composite material varied with the change in loading rate. The results show that, with increasing loading rates, the resin lost its elastic properties, and the specimen exhibited a more rigid behavior and is quite rigorous so that the fracture failure process is changed. The failure processes and crack growth rate was validated by use of acoustic emission signals. There was good agreement between the fracture toughness of accretion of acoustic emission signals with the experimental values.
M. Baraheni , A. Tabatabaeian , A. Ghasemi, S. Amini,
Volume 20, Issue 7 (6-2020)
Abstract
Thermal fatigue is one of the most important issues in different engineering fields. The importance of this phenomenon is its application in aerospace industries and considerable effects on the material properties. In this research, the effect of thermal fatigue on the machining quality of polymeric CNT-reinforced composites is studied. To follow this aim, initially the composite specimens with eight layers in symmetrical and unsymmetrical layups are fabricated and subjected to thermal cycling. Then, two different machining processes including conventional drilling and ultrasonic vibration assisted drilling are carried out and the thermal fatigue effects are experimentally studied. Additionally, the effects of various parameters including “addition of multi wall carbon nanotube”, “machining process” and “layup method” on machining quality of composites under thermal fatigue condition is investigated in order to obtain the least delamination. The results indicated that addition of multi-walled carbon nanotubes enhances the machining quality up to 13%. It was also revealed that the implement of ultrasonic assisted drilling could reduce the delamination damage up to 10%.
