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Delamination is one of the main defects observed from the drilling of composite materials, and can be an important limiting factor for the use of composite materials. In this study, the delamination area factor for GFRP (Glass Fiber Reinforced Plastic) drilling was proposed to quantify the practical delamination zone via digital imaging of the damage zone around the hole and by calculating the damage zone by pixel. From the tests, it was concluded that a higher drill feed rate reduced the delamination zone and the cutting speeds have no influence on the delamination zone. Also, the proposed delamination area factor was more suitable and useful for delamination zone evaluation than the existing delamination factor. [ABSTRACT FROM PUBLISHER]

The joining of composite materials has traditionally been achieved by adhesive bonding or mechanical fastening. Mechanical fastenings require little or no surface preparation, and are easy to inspect for joint quality. However, they require machining of holes that interrupt the fiber continuity and may reduce the strength of the adherend. We proposed the new composite key joint to minimize the fiber discontinuity and strength degradation of adherend. Mechanical key joints with three different d/t (slot depth to thickness) ratios and two different e/w (edge length to width) ratios were manufactured and tested. Carbon-epoxy unidirectional prepreg (USN125) and plain weave prepreg (HPW193) were used for the composite key joints. Tensile tests were performed on the composite key joints and their failure modes were evaluated. Finite element analyses for the mechanical key joints were performed and the stresses around the key slot were calculated. From the tests, it could be concluded that the failure load of the key joints was about two times larger than those of a mechanical joint for the investigated composite joints. [ABSTRACT FROM PUBLISHER]

The reinforcement of composite structures in the through-thickness direction has been developed through a new stitching process. This reinforcement is called I-fiber stitching process due to the final shape of the reinforced fiber. Following mode I results from a previous research, the present study proposes a mode II analysis in order to define the cohesive parameters through a cohesive law that will allow to predict the behavior of reinforced composite structures under this specific fracture mechanism. Several stitching patterns have been tested with two kinds of dry carbon fiber yarn stitched as reinforcement, the first group with 1 × 6 and 2 × 6 for 6 k yarn and the second group with 1 × 3 and 2 × 3 patterns for 12 k yarn. A parametric study was performed after test a single stitch unit cell specimen for the two carbon fiber yarns to obtain the cohesive parameters that have been used after in a macro-scale model with the reinforced stitching pattern. Using the finite element modeling, the numerical analyses were compared with the test results and have showed a great concordance with the experiments, these results have allowed to fix the stitching parameters through the cohesive law for mode II fracture mechanism. [ABSTRACT FROM AUTHOR]

This study presents a broadband radar-absorbing foam-based sandwich structure (RAS) composed of a foam core, glass fiber-reinforced polymer (GFRP), and electroless nickel-plated glass fabric. The nickel-coated fiber (NCF) for electromagnetic wave absorption is prepared using electroless nickel plating on glass fabric with discussing the detailed electromagnetic wave absorption mechanisms relationship between the plating thickness and the skin depth in terms of lossy dielectric properties. To design the broadband NCF RAS, transmission line theory linked with a genetic algorithm is used to optimize the absorption performance. The EM wave absorption performance of fabricated broadband NCF RAS was measured using by using a free-space measurement system operating from 5.8 GHz to 18 GHz. From the measurement and the tolerance analysis, the proposed NCF RAS works well in the designed frequency range and is very robust for the fabrication tolerance. In addition, a flatwise tensile test was performed according to ASTM C297 to check the influence between the NCF and the foam core materials in terms of interfacial behavior. It has been indicated that they have no significant effect on interfacial properties. [ABSTRACT FROM AUTHOR]

In this study, a microwave absorbing structure consisting of a periodic pattern coated glass fabric (PPCF) and a glass-fiber/epoxy composite as the spacer is presented using a practical approach for application in aircraft. To fabricate a PPCF, a carbon-based conducting material bar coating device was used. To test the performance of the PPCF, an X-band (8.2–12.4 GHz) microwave absorbing structure was fabricated using an autoclave and measured using a free-space measurement system. The return loss of the fabricated PPCF microwave absorbing structure was −27 dB at the resonance frequency, and the measurement result was well matched with the simulation result. Since the pattern was implemented on an uneven surface, the performance stability of the distribution of sheet resistance on the surface, pattern spreading phenomena, and waviness of patterns was verified using a simulation method. The simulation result showed good performance stability. To verify the mechanical properties, an interlaminar shear strength (ILSS) test was performed. Since the pattern on the glass fabric can act in a similar way to a defect, there is a high probability of mechanical property degradation. However, the ILSS test results of the PPCF specimen was comparable with a pure glass-fiber/epoxy composite. [ABSTRACT FROM AUTHOR]

Abstract The delamination failure of a composite laminate under a flexural load was repaired by micro-bolts and adhesive resin. The specimens were made by autoclave processing, and in the prepreg-stacking step, a Teflon release film was inserted to mimic the delamination in the specimens. Bolts made with brass or steel were used in the micro-bolt repair method, and the diameters of bolts were 0.6, 1.0, and 1.2 mm. For the resin injection repair method, the EA9396, a low-viscosity and room-temperature curing epoxy adhesive, was used as the repair material. In the case of the micro-bolt repair method, although the larger bolt resulted in higher bending strength, the difference in bending strength of the repaired specimens, depending on bolt diameter, was insignificant. The difference in bending strength was only 21% when 0.6-mm and 1.2-mm diameter bolts were installed. On the other hand, the bending strength of the specimens, repaired by the resin injection method, was recovered, up to 93% of the pristine specimen. From the results of this study, it was confirmed that the resin-injection repair, which injects resin into the delamination to bond the entire separated surface area, is more effective than using micro-bolts if the flexural load is predominant. [ABSTRACT FROM AUTHOR]

Abstract Repair of a composite laminate with delamination using micro-bolts is studied parametrically. The effects on compressive buckling load of delamination crack length, bolt material, bolt diameter and arrangement, and bolt–laminate clearance are considered. For 45- and 65-mm crack lengths, buckling load recovery rates using brass bolts are 90% and 73%, respectively. Using steel increases the rate by 6.6% compared to the intact specimen. Tests are performed fixing the number of steel bolts for bolt diameters of 0.6, 1.0, and 1.2 mm. The buckling load recovery rate using 1.2- versus 0.6-mm bolts increases by 6.8%. Finite element analysis confirms the effect of the bolt–laminate gap: 27% larger buckling load for no clearance than for a 0.1-mm gap. 18% higher buckling load, which is 97% of intact-specimen buckling load, is obtained for a 1.2-mm-diameter bolt repair by gluing bolts and holes and then tightening them compared with using bolts alone. [ABSTRACT FROM AUTHOR]

Abstract Resin transfer molding (RTM) is a mass production process that can replace autoclave processes, and composite lap joints are extensively used in composite structures. When a tensile load is applied to a single-lap joint, both shear and tensile peel stresses are generated owing to the eccentric load effect. Reinforcing the composite joint in the thickness direction can reduce the shear and peel stresses generated in the joint, which can contribute considerably to the increase in the strength of the composite joint. Several reinforcing methods have been developed to improve the directional properties along the material's thickness. Accordingly, a stitching process is typically used. However, the conventional stitching process has disadvantages because a) it requires complex equipment, and b) it cannot use highly elastic brittle fibers, such as carbon fibers. Recently, we proposed an I-fiber stitching process to minimize the bending of carbon fibers to prevent their fracture. In this study, composite, single-lap joint specimens were fabricated with RTM using an I-fiber stitching process, and their strengths were evaluated. The strengths of composite joint specimens fabricated at different stitching intervals and with the use of different patterns were compared with those of specimens fabricated without the use of the stitching process. [ABSTRACT FROM AUTHOR]

This study proposes a method for repairing delaminated composite plates using micro-bolts. We first conducted a test to examine the effect of the hole size on the tensile strength of composite laminates. The test results showed that, among laminates with the same total area of holes, those with multiple small holes had a higher tensile strength than those with only a single large hole. Based on this result, brass bolts with a 0.6 mm diameter were used to repair the delaminated composite plates. Buckling tests and analysis were conducted for an intact laminate without delamination, an unrepaired laminate with delamination, a repaired laminate using one bolt, three bolts with two types of layout, and nine bolts. The test results indicate that the unrepaired specimen demonstrates local buckling at 68% of the buckling load of an intact specimen. In contrast, the buckling load was recovered to 82% of that of an intact specimen when only a single brass bolt was fastened. When nine bolts were used, the buckling load recovery rate increased to 90%. Finite element analysis produced the same results as the test, and predicted that the use of nine steel bolts will increase the buckling load to 93%. [ABSTRACT FROM AUTHOR]

Delamination-based failure is commonly observed in curved composite structures owing to the significant curvature-induced through-thickness stresses present therein. To gain further insights on this failure mode, we investigated the initiation/propagation of delamination in C-shaped composite beams under an opening load and determined their failure load utilizing a three-dimensional finite element simulation based on a cohesive zone model (CZM). Several cohesive parameters (e.g., initial interface stiffness, cohesive zone length, and interface strength) were examined to construct an optimal CZM for the chosen loading conditions, with all parameters except for mode II fracture toughness (directly obtained from end-notched flexure test results) determined by an extensive literature survey. The predicted beam failure loads were compared with experimental results, with the obtained maximum prediction error of 8.4% indicating good agreement. Finally, the predicted position of delamination initiation was found to be heavily dependent on the ratio of the shear strength to the normal strength of the interlayer. [ABSTRACT FROM AUTHOR]

When a laminated composite structure with curvature is subjected to a bending load, the most important failure mode is due to delamination in curved regions. In this study, the reinforcement effect of grooved Z-pins is investigated for preventing delamination of curved beams. Through a four-point bending test, the effect of stainless-steel pin diameter and areal density on bending failure of curved beams is studied. For areal pin densities of 0.5, 1.0, and 2.0%, pin diameters of 0.3 and 0.5 mm result in average pin strength increases of 21, 27, and 42% and 8.7, 12, and 32%, respectively. When the pin size is small, the pin is inserted closer to 90° from the mid-plane, a smaller resin-rich zone is generated, and there is less laminate damage. [ABSTRACT FROM AUTHOR]

Carbon–epoxy composite laminate recovery rates are analyzed with respect to their static and fatigue strengths following local-damage repair using the scarf-patch bonding method. The tensile strength recovery rate is tested for 1.9–11.3° scarf angles (scarf ratios: 1/30–1/5). Static tests are performed for three external-ply overlap lengths and two defect sizes. The fatigue characteristics are identified via fatigue testing of a defect-free specimen and two specimens with different scarf angles. The static test results indicate that the strength recovery rate is significantly lower for scarf ratios of 1/10 or greater. Conversely, 1/20 and 1/30 scarf-ratio specimens exhibit high recovery rates of 79.1% and 83.6%, respectively, compared to the defect-free specimen. The external-ply overlap length and defect-size effects are negligible. For a million-cycle fatigue test, the fatigue strength recovery rates of the 1/20 and 1/30 specimens are 39.7% and 52.1%, respectively, being lower than the static strength recovery rate. [ABSTRACT FROM AUTHOR]

The curved region in composite structures is vulnerable and easily experiences delamination under opening load due to the localized transverse stresses. This failure occurred in a composite primary part, e.g. spars and bulkheads, of an aircraft can intensively reduce its rigidity and strength. In this study, various moderately thick composite laminates with a C-section, which were cut from a carbon–epoxy C-spar, were tested with an opening load for different thicknesses and heights. On the basis of the test results, a three-dimensional finite element model in conjunction with Ye-delamination failure criteria was proposed to investigate the failure behavior as well as to predict the failure load of the structures. The finite element models were verified using strain results at specific locations. Failure loads were predicted using the damage zone method (DZM). Damage volume and damage volume ratio were used to evaluate damage zone in the model. Analysis results show deviations of less than 0.7–9.0% of the experimental results in the case of using damage volume ratio. The proposed damage zone method was also examined for laminated beams with different corner radius and good agreement was obtained. [ABSTRACT FROM AUTHOR]

The repair of composite materials has received much attention in recent decades because these materials are used in a wide range of industrial applications owing to their excellent mechanical properties. In this work, the failure of scarf patch-repaired composite laminates subjected to bending loads was experimentally and numerically investigated. Thirty repaired laminates with a scarf angle of 5.7° or 3.8° were prepared and tested under different bending loads. Three-dimensional finite element models implementing cohesive elements were created to examine crack initiation and propagation in the specimens during loading. Experimental results revealed that a combination of adhesive, intralaminar, and interlaminar failure occurred in the bonding surface area. Compared to the failure of repair under tensile loading, additional failure occurred in the patch. Simulations reproduced these failure modes well with cohesive elements. The failure loads were also accurately predicted with a maximum prediction error of 9.7%. [ABSTRACT FROM AUTHOR]