Papers by Keyword: Delamination

Abstract: Carbon fiber reinforced plastics (CFRP) has been widely used in various aircraft structural components. However, it is difficult for conventional methods such as drilling and helical milling to meet the requirements on high quality and efficient holes creation. Hence a so-called tilt helical milling (THM) method has been proposed. This new method is performed by replacing the revolving motion of the tool in conventional helical milling (CHM) with a conical pendulum motion, in which the tool axis is tilted towards the hole axis at a certain angle. As a step toward the establishment of the new method, in this work, the fundamental drilling characteristics of CFRP by the THM is elucidated by experimentally investigating the effects of tilt angle on thrust force and delamination factor. The obtained experimental results demonstrated that thrust force and delamination factor can be reduced with THM technique. In addition, THM can achieve better hole surface finish than CHM.

Abstract: A new numerical method for homogenization of elastic properties of dispersedly-reinforced composites was presented. The method takes into account special model of adhesive contact. Homogenization of properties was performed by averaging the solutions of boundary value problems on representative volume cell (RVC) using the finite element method (FEM). A new approach of calculation of components of effective tensor of elastic moduli was proposed. A heterogeneous finite element model with elements of two types was built: three-dimensional tetrahedron elements for every phases and spring element with zero-length for adhesion layer with zero-thickness. The results of homogenization of elastic properties of dispersedly-reinforced composites with variable stiffness of the adhesive layer between phases were obtained and analyzed. The homogenization results were compared with the available experimental data.

Abstract: A test campaign has been carried out to determine the interlaminar tensile strength (ILTS) in AS4/8552 composite laminates with different thicknesses, following AITM 1-0069 and ASTM D6415/D6415M-06a standards. Results show that L-shaped samples with low thicknesses present lower values of the ILTS. Numerical finite element (FE) models of the samples have been carried out and the results of the numerical analyses have been compared with the experimental measurements, showing that a loss of stiffness has taken place in the samples. Optical inspection of the samples has shown up a significant misalignment of the fibers in the curved part that justifies the loss of stiffness in all samples and the loss of strength in the thinner samples.

Abstract: It is well known that delamination is one of the most critical mechanism of failure of laminated composite materials. It supposes an important load capacity reduction, it is difficult to see and his evolution modify the failure of the component. Composite delamination depends on their fracture toughness. On the other hand, impacts are the most dangerous loads for those materials due to the important deformation rate induced in the material. This work analyses the influence of load velocity in the fracture toughness, for modes I and II, in textile carbon/epoxy, up to an impact velocity of 0,190 m/s. For that range, results show that the mode I fracture toughness decrease with velocity, while for mode II it remains nearly constant. However, the load velocities analyzed are yet far from those induced in a low speed impact. We propose to continue this research by increasing deformation rates using drop tower impact techniques, to observe if the trend observed so far is maintained on increasing speed.

Abstract: This paper presents experimental and numerical results on the deformations and failureof integral composite T-joints subjected to a realistic combined tensile and bending (mixed mode)load case. For this reason, standard pull-off and mixed mode load cases are experimentally studiedby means of a novel test fixture which keeps the force angle constant to the T-joint’s base and allowsfor repositioning of the specimen in order to minimize constraining forces. Two types of specimenswith varying deltoid radius are investigated. Additionally, kinematically nonlinear numerical simulationsare performed to locate damage onset and deformations of the specimens. It is found that thesimulations are in good agreement with the experimental results.

Abstract: The mechanical behavior of cross-ply laminates loaded under in-plane compression containing matrix cracks and delaminations is investigated in order to study their influence on the structural stability behavior. This is done by employing a semi-analytical modelling approach which comprises an analytical framework for a structural stability analysis of damageable structures and the Equivalent Constrained Model for deriving reduced stiffness properties of the cracked layers. Cross-ply laminates with varying delamination depths as well as varying matrix crack densities are studied.

Abstract: The downtime problem of wind turbine increases due to fatigue damage of wind turbine blades, which is even more crucial in the larger blades. One of the critical failure modes is the blade trailing edge failure, which can result in the trailing edge joint cracked. In this paper, we experienced that abnormal sound was happened in the trailing edge at the cross-section in the max chord during fatigue testing of a 2 MW full-scale wind turbine blade according to IEC61400-23. Through the conditional monitoring of the trailing edge, the delamination between GFRP and balsa wood is caused by stress concentration. The abnormal sound is happened due to GFRP beat the balsa wood when the blade vibrates in the edgewise direction. Moreover, the sound is amplified because the introduction of air due to the delamination. The local stress distribution and stability factors are computed through FEM methods, the program that increasing the core materials in the trailing edge is proposed. Therefore the structure reliability in the trailing edge is improved.

Abstract: One method that has shown great potential in visualising and characterising the interaction of guided waves with damage in composites is Laser Vibrometry. A Laser Doppler Vibrometer (LDV) can be used to produce 2D wavefield images of guided Lamb waves but a single scan is very time consuming and normally multiple scans are required at various frequencies in order to determine the best input signal. This paper demonstrates the use of a chirp excitation method requiring only a single scan and a post-processing algorithm to obtain results corresponding to any narrowband signal within the frequency range of the chirp signal. The method was used on an artificially delaminated composite panel and showed that the S₀ mode, dominant at higher frequencies, mainly caused mode conversions whilst the A₀ mode, dominant at lower frequencies, mainly caused a change in phase and amplitude across the delaminationOne method that has shown great potential in visualising and characterising the interaction of guided waves with damage in composites is Laser Vibrometry. A Laser Doppler Vibrometer (LDV) can be used to produce 2D wavefield images of guided Lamb waves but a single scan is very time consuming and normally multiple scans are required at various frequencies in order to determine the best input signal. This paper demonstrates the use of a chirp excitation method requiring only a single scan and a post-processing algorithm to obtain results corresponding to any narrowband signal within the frequency range of the chirp signal. The method was used on an artificially delaminated composite panel and showed that the S₀ mode, dominant at higher frequencies, mainly caused mode conversions whilst the A₀ mode, dominant at lower frequencies, mainly caused a change in phase and amplitude across the delamination.

Abstract: In this paper, the damage behaviour of aerospace composite structures has been studied. The analysed structure is an all-composite wing with inter-laminar manufacturing induced damage. The manufacturing induced delaminations are located at the leading edge on the extrados and in the area near the fuselage. Different analyses have been performed to correctly predict the pre-existent damages evolution under service loading conditions. Preliminary linear buckling analyses have been performed to check for instability of the delaminations under service loads; then the risk of delaminations’ propagation has been evaluated by means of a Virtual Crack Closure Technique based approach.

Abstract: In many present applications is requested decreasing of weight of components and increasing of their strength. Often unique properties are required. These properties could be solved by using of composite materials. However, different material properties of matrix and reinforcing material cause issues during machining, such as rapid tool wear and delamination of composite. Therefore there is afford to enhance machining process by different ways to decrease tool wear as well as delamination of composite. This article deals with comparison of conventional milling and ultrasonic assisted milling of glass fibre reinforced polymer (GFRP) composite material by special designed cutting tool.