Delamination Resistance Curve Research Articles

Damage history in composite laminates: Matrix cracks leading to delaminations

Strain energy release rate calculations for various cases of delaminations emanating from matrix cracks are developed and used to predict the onset of delaminations and their growth size as a function of applied tension and shear loads in composite laminates. The method determines the matrix crack spacing, the delamination onset load, the delamination size at onset and, through the use of a newly proposed delamination resistance curve, the size of delaminations as they grow under load. The method can be applied to any symmetric laminate. Comparisons to test results in the literature for a variety of layups and materials shows very good agreement with the exception of cases where significant edge delaminations appear before delaminations caused by matrix cracks.

Experimental and numerical analysis of mode-I interlaminar fracture of composite pipes

A common industrial production process for composites is filament winding, widely used for the production of axially symmetric components. In these composite components, delamination is a predominant failure mechanism. The current work focuses on investigating experimentally the effect of the initial crack and fiber bridging length on the mode-I delamination resistance curve (R-curve) behavior of various double cantilever beam (DCB) specimens. For this purpose, the magnitudes of initiation and propagation fracture toughness (GIC-init and GIC-prop) and the compliance C are calculated. DCB specimens with a stacking sequence of [± 50]6 and various initial crack lengths of a0 = 33, 37, 59 and 70 mm are manufactured from a real composite pipe using filament winding process. In order to evaluate the critical energy release rate in mode-I, various fracture tests are conducted on these specimens. The greatest bridging zone length dues to good penetration of two adjacent layers of the delamination interface. Moreover, the results indicate that the fiber bridging length has a significant effect on the GIC and the largest value of fiber bridging causes a large fracture toughness value. Finally, numerical simulations are performed using finite element (FE) method and GIC-init measurements obtained experimentally are compared to the numerical findings. The numerical displacements and GIC-init, calculated from the numerical displacements, are found to be in good agreement compared to the experimental results.

Delamination propagation criterion including the effect of fiber bridging for mixed-mode I/II delamination in CFRP multidirectional laminates

Large-scale fiber bridging developed in the CRFP multidirectional (MD) laminates upon mixed-mode I/II fracture has a significant effect on the delamination behavior. Without considering the effect of fiber bridging, traditional delamination propagation criteria fail to evaluate the delamination behavior in MD laminates. To solve this problem, an improved criterion is proposed here, which accounts for the delamination resistance curve (R-curve) induced by fiber bridging. Systematic delamination tests under mode I, II and mixed I/II loadings were carried out to determine the unique parameter in the criterion and a numerical delamination model based on the cohesive zone model (CZM) was established with implementation of the improved criterion. Initial and ultimate damage loads from numerical predictions and experiments show good agreements, verifying the accuracy of the improved criterion on evaluating the mixed-mode I/II delamination behavior in MD laminates. Additional mixed-mode I/II delamination test was conducted, which was used to validate the applicability of the improved criterion on evaluating the delamination behavior with any other mode-mixity ratios (φ-ratios).

On the mixed mode I/II delamination R-curve of E-glass/epoxy laminated composites

The main effort of the current study is to propose a procedure for prediction of delamination resistance curve (R-curve) of laminated composites under mixed mode I/II loading conditions based on the experimental observations. Hence, two material systems, i.e., E-glass/epoxy and E-glass/MWCNTs-toughened epoxy were tested at a wide range of mixed mode I/II delamination to present R-curves. Each R-curve was characterized by fiber-bridging zone (FBZ) length, critical strain energy release rate at delamination initiation (GTC) and stable propagation (GTR) states. It was manifested the magnitude of the FBZ length is constant at different mixed mode I/II loading states for a specific laminate system. Then, using the Benzeggagh and Kenane (B-K) criterion, two relations for computing the GTC and GTR were derived. Finally, according to these findings a relation was obtained to predict the delamination behavior of laminated composites (from delamination initiation to propagation states) under arbitrary mixed mode I/II loading condition.

Mixed-Mode Interactions Between Graphene and Substrates by Blister Tests

Many of the attractive properties of graphene will only be realized when it can be mass produced. One bottleneck is the efficient transfer of graphene between various substrates in nanomanufacturing processes such as roll-to-roll and transfer printing. In such processes, it is important to understand how the ratio of shear-to-tension at the interface between graphene and substrates affects the adhesion energy. With this in mind, this paper examines the mixed-mode adhesive interactions between chemical vapor deposition (CVD) grown graphene that had been transferred to copper or silicon substrates. The approach that was taken was to use blister tests with a range of graphene backing layer materials and thicknesses in order to provide a wide range of the shear-to-tension ratio or fracture mode-mix at the interface. Raman spectroscopy was used to ensure that graphene had indeed been delaminated from each substrate. Measurements of pressure, top surface deflection, and blister diameter were coupled with fracture mechanics analyses to obtain the delamination resistance curves and steady state adhesion energy of each interface. The results showed that the adhesive interactions between graphene and both substrates (Cu and Si) had a strong dependence on the fracture mode-mix. In the absence of plasticity effects, the most likely explanation of this effect is asperity locking from the inherent surface roughness of the substrates.

Applicability of standard delamination tests (double cantilever beam and end notch flexure) for 5HS fabric-reinforced composites in weft-dominated surface

Currently, the standard delamination tests established by ASTM (available for mode I and mix mode) are limited to unidirectional composites. Although some researchers have conducted delamination tests in woven composites, their information is still limited. In order to understand the propagation behavior and the value of fracture toughness of woven composites, this article evaluated a 5HS carbon/epoxy composite with a weft-dominated surface. Tests were conducted in double cantilever beam and end notch flexure configurations using an energy-based approach for data reduction in modes I and II, respectively. The results were assessed in terms of delamination resistance curves (R-curves). Both delamination modes showed consistent behaviors for extending the application of the standard procedures, as the energy variation can describe well the crack growth dependence of the irregular surface caused by the crimp, which was more pronounced for mode I.

Effect of data reduction and fiber-bridging on Mode I delamination characterization of unidirectional composites

Reliable delamination characterization data for laminated composites are needed for input in analytical models of structures to predict delamination onset and growth. The double-cantilevered beam specimen is used to measure fracture toughness, GIc, and strain energy release rate, GImax, for delamination onset and growth in laminated composites under Mode I loading. The current study was conducted as part of an ASTM Round Robin activity to evaluate a proposed testing standard for Mode I fatigue delamination propagation. Static and fatigue tests were conducted on specimens of IM7/977-3 and G40-800/5276-1 graphite/epoxies, and S2/5216 glass/epoxy double-cantilevered beam specimens to evaluate the draft standard “Standard Test Method for Mode I Fatigue Delamination Propagation of Unidirectional Fiber-Reinforced Polymer Matrix Composites.” Static results were used to generate a delamination resistance curve, GIR, for each material, which was used to determine the effects of fiber-bridging on the delamination growth data. All three materials were tested in fatigue at a cyclic GImax level equal to 90% of the fracture toughness, GIc, to determine the delamination growth rate. Two different data reduction methods, a two-point and a seven-point fit, were used and the resulting Paris Law equations were compared. Growth rate results were normalized by the delamination resistance curve for each material and compared to the non-normalized results. Paris Law exponents were found to decrease by 5.7 to 47.6% due to normalizing the growth data. Additional specimens of the IM7/977-3 material were tested at three lower cyclic GImax levels to compare the effect of loading level on delamination growth rates. The IM7/977-3 tests were also used to determine the delamination threshold curve for that material. The results show that tests at a range of loading levels are necessary to describe the complete delamination behavior of this material.

Delamination R-curve as a material property of unidirectional glass/epoxy composites

It is still questionable to think of delamination resistance of a double cantilever beam (DCB) as a material property independent of the specimen size and geometry. In this research, the effects of initial crack length and DCB specimen thickness on the mode I delamination resistance curve ( R-curve) behavior of different unidirectional glass/epoxy DCB specimens are experimentally investigated. It is observed that the magnitudes of initiation and propagation delamination toughness ( G Ic-init and G Ic-prop) as well as the fiber bridging length are constant in a specific range of the initial crack length to the DCB specimen thickness ratios of 8.5 < a 0/ h < 19. Finally, a mathematical relationship is proposed for prediction of mode I delamination behavior (from the initiation to propagation) of E-glass/epoxy DCB specimens.

Mode II Delamination Toughness in Glass Fiber-Reinforced Polymers with Bridging Fibers and Stitching Threads

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.

Manufacturing Influence on the Delamination Fracture Behavior of the T800H/3900-2 Carbon Fiber Reinforced Polymer Composites

‘Torayca’ T800H/3900-2 is the first material qualified on Boeing Material Specification (BMS 8-276) which utilizes the thermoplastic-particulate interlayer toughening technology. Two manufacturing processes, the autoclave process and the fast heating rated Quickstep™ process, were employed to cure this material. The Quickstep process is a unique composite production technology which utilizes the fast heat transfer rate of fluid to heat and cure polymer composite components. The manufacturing influence on the mode I delamination fracture toughness of laminates was investigated by performing double cantilever beam tests. The composite specimens fabricated by two processes exhibited dissimilar delamination resistance curves (R-curves) under mode I loading. The initial value of fracture toughness G IC–INIT was 564 J/m2 for the autoclave specimens and 527 J/m2 for the Quickstep specimens. However, the average propagation fracture toughness G IC–PROP was 783 J/m2 for the Quickstep specimens, which was 2.6 times of that for the autoclave specimens. The mechanism of fracture occurred during delamination was studied under scanning electron microscope (SEM). Three types of fracture were observed: the interlayer fracture, the interface fracture, and the intralaminar fracture. These three types of fracture played different roles in affecting the delamination resistance curves during the crack growth. More fiber bridging was found in the process of delamination for the Quickstep specimens. Better fiber/matrix adhesion was found in the Quickstep specimens by conducting indentation-debond tests.

Surface modification of carbon/epoxy prepreg using oxygen plasma and its effect on the delamination resistance behavior of carbon/epoxy composites

It was shown in previous study that the fracture toughness of carbon/epoxy laminated composites could be significantly improved by modifying the surface of the prepreg using Ar + irradiation in an oxygen environment. In this study, the surface of carbon/epoxy prepreg was modified using an oxygen plasma to improve the delamination resistance behavior of carbon/epoxy laminated composites. The variation of the contact angle on the prepreg surface was determined as a function of the modification time, in order to determine the optimal modification time. An XPS analysis was conducted to investigate the chemical changes on the surface of the prepreg caused by the plasma modification. Mode I delamination resistance curves of the composites with and without surface modification were plotted as a function of the delamination increment. The results showed that the contact angle varied from ∼64° to ∼47° depending on the modification time and reached a minimum for a modification time of 30 min. The XPS analysis showed that the hydrophilic carbonyl C O group was formed by the oxygen plasma modification. The results also showed that the delamination resistance behavior was significantly improved by the plasma modification of the prepreg. This improvement was caused by the better layer-to-layer adhesion as well as increased interfacial strength between the fibers and matrix.

A Study on the Fracture Characteristics of Plasma-Treated Graphite/Epoxy Laminated Composites

For a present study, the surfaces of graphite/epoxy prepregs were modified using plasma treatment to improve the delamination resistance behavior of graphite/epoxy laminated composites. The optimal treatment time was determined by measuring the change of contact angle with treatment time. Unidirectional DCB (double cantilever beam) specimens were used in the mode I delamination fracture tests. The delamination resistance curve of regular (untreated) specimen was compared with that of plasma-treated specimen in order to determine the effect of prepreg treatment on the resistance behavior. It was found that contact angle was changed from ~64° to ~47° depending on the treatment time. The contact angle was a minimum for a 30 min treatment time. It was also found that delamination resistance behavior of graphite/epoxy composites was improved about 20%.

Prepreg Treatment with Ar+ Ion Irradiation to Improve GIcof Unidirectional Graphite/Epoxy Laminated Composites

Currently, many methods are used to improve the fracture toughness of graphite/epoxy composites. However, no work has been done on the prepreg treatment with Ar+ ion irradiation. In the present work, the effect of surface treatment of prepreg by Ar+ ion irradiation on the delamination resistance of unidirectional graphite/epoxy laminated compositeswas studied. The surface of the prepregwas treated with Ar+ ion irradiation under oxygen environment, and the change of contact angle with surface-treatment was measured. Two kinds of [06//06] DCB (double cantilever beam) specimens were made: (1) DCB specimens made of graphite/epoxy prepregs that had been surface-treated with Ar+ ion irradiation under oxygen environment (Laminate A) and (2) DCBspecimens made of regular, untreated graphite/epoxy prepregs (Laminate B). The delamination resistance curve of Laminate A was compared with that of Laminate B so that the effect of prepreg treatment on the resistance behavior could be determined.

Delamination resistance behaviour in unidirectional graphite-epoxy composites

According to the energy criterion for fracture, crack growth occurs when the crack driving force or the energy release rate is at least equal to the crack resistance which is de®ned as the energy consumed in crack propagation. It is well known that, for a cracked metal structure under the plane-strain loading condition, the crack resistance is independent of the crack length. In the case of plane stress, the crack resistance varies with the crack increment. Although a considerable amount of work has been performed to characterize the resistance curve for isotropic materials [1±3], the application of the resistance curve to composites has only recently been performed. Ratwani and Deo [4] applied the resistance curve approach to characterize the delamination growth of AS4=3501-6 composites under mode I loading. They used double-cantilever-beam (DCB) specimens and obtained the delamination growth resistance curve over a wide range of delamination extensions. They found that the resistance increased monotonically with increasing delamination extension, and the delamination growth resistance curve was path independent. Mai and Hakeem [5] noted a resistance-type fracture behaviour of cellulose ®bre cements. They used grooved DCB specimens and observed that the resistance increased monotonically with increasing crack length to a maximum and then dropped to a steady-state value. Foote et al. [6] developed a theoretical model for the crack growth resistance (KR) curve for a strain-softening material with a power-law stress± displacement relationship. They applied the model to a cellulose±asbestos-®bre-reinforced motar and obtained comparable results with the experimental data. Currently, the delamination resistance curve plays an important role in characterizing the delamination growth for unidirectional laminated composites particularly under mode I loading. However, the delamination resistance behaviour of graphite±epoxy composites is not fully characterized at present. In the present work, delamination resistance curves of unidirectional graphite±epoxy laminated composites were constructed as a function of delamination increment, and the effect of initial delamination length on the resistance curve was investigated. DCB specimens made by using graphite±epoxy prepregs (CU125NS) manufactured by Hankuk Fiber Group were used for the resistance curve tests. The DCB specimen was 08 unidirectional and 24 plies thick (3.8 mm) in which delamination was located at the midplane. Initial delaminations applied were 50 and 90 mm, and the specimen length was 230 mm. The edge of the specimen was painted with a white typewriter correction uid and was marked regularly in order to measure the delamination increments. The delamination length was measured using the travelling microscope. Aluminium blocks were adhesively bonded to the ends of the specimen arms to provide the loading points. The experiments were conducted under room-temperature dry environmental conditions, and a displacement-controlled loading of rate 3.5 mm miny1 was applied. After the initial delamination increased by a certain amount, the applied displacement was reduced and then increased to cause delamination growth. This process was repeated at least ten times until the delamination increased to a length of 170 mm. For each specimen, curves of load, P, against displacement, a, during slow delamination growth were obtained where the delamination length was marked against each loading and unloading line. A typical loading±unloading P±a diagram of initial delamination length 50 mm is shown in Fig. 1. As shown in the ®gure, the fracture behaviour is linear elastic. However, it was found that unloading curves do not return to zero displacement although the amount of residual displacement is very small. The reason is that ®bre bridging occurred behind the delamination tip during delamination growth, which prevents the delamination surfaces from closing up

ＣＦ／エポキシ複合材多方向強化積層板の層間はく離のメカニズム

The unidirectional CFRP has a weak point in interlaminar strength and therefore, delamination resistance has been evaluated on many types of unidirectional composites and the evaluation method is now being established. In these studies, however, unidirectional reinforced CFRP, in which the stacking sequence is [0]n, was used. Thus, the measured delamination resistance is not applicable to evaluate the delamination resistance of actual composite structures made from multidirectionally reinforced CFRP. Moreover, the delamination micromechanism of the multidirectionally reinforced CFRP has not been understood yet. Therefore, the material design to improve the delamination resistance of the actual composite structures has not been conducted. In this study, the objective was to examine the delamination resistance curves of multidirectional reinforced CF/Epoxy composites and to investigate problems on the testing method. Additionally, the micromechanism was modeled based on the observations by a scanning acoustic microscope and a scanning electron microscope. As the results, it was shown that the delamination crack propagating between 90° laminae has the highest resistance, and the crack deflection and branching are most effective for increasing the delamination resistance.

Role of matrix resin in delamination onset and growth in composite laminates

The effect of the matrix resin on the onset and growth of delamination in composite laminates has been investigated in this work. Two kinds of graphite/epoxy composite materials (T300/648-BF 3/MEA and T300/634-DDS) with quite different matrix properties have been used. The study was done on two different layups, [(±30) 3/90 2] s and [(±45) 2/O 2/90 2] s . Out-of-plane moiré interferometry and diiodomethane-enhanced X-radiography were used to detect delamination. A strength criterion for the onset of delaminatoin is proposed and an assessment made of the effect of matrix properties on delamination onset. A modified energy release rate model is presented for characterization of delamination growth emphasis being placed on assessing the behavior of delamination resistance curves and delamination growth rate. The results indicate that enhancement of matrix strength and ductility increases the critical loads for delamination onset and delamination resistance in the composite laminates under static loading, and significantly reduces the delamination growth rate under cyclic loading.