Papers by Keyword: Delamination

Abstract: Composite fiber products and components are widely used in various industries, from highly stressed structural elements in the aerospace industry to sports equipment. In order to achieve the desired final shape, these materials are often subjected to various machining methods. Due to the inhomogeneous structure of composites and the different physical and mechanical properties of the matrix and reinforcement, specific problems arise during machining, such as delamination, intensive tool wear, increased temperature in the cutting area, or poor surface finish.This work deals with the observation of delamination size, wear, and cutting forces when drilling holes in carbon composites with tools with different rake angles. The result of this work is a recommendation for the geometry of tools for drilling this type of carbon composite.

Abstract: The research is concerned with obtaining basic knowledge in the field of machining biocomposite materials with hemp fibers and a matrix in the form of a mixture of polyester and methyl methacrylate resin in a secret ratio. The research was focused on milling technology, or rather side milling. For the needs of the research, 3 milling tools were selected with which experimental measurements were performed. Each tool was different in its type of sharpened geometry, both standard and specialized, including one coated. The experimental measurements focused on the size and course of wear of the cutting edge of the tools, the roughness value of the machined surface and the size and type of delamination of the upper and lower layers of the biocomposite material under investigation. The obtained results helped to evaluate the machinability of the selected hemp biocomposite and at the same time determined the future direction of research with regard to the design of a suitable cutting geometry of the tool and the overall optimization of the machining process during side milling.

Abstract: Composite sandwich panels are extensively used in aerospace, automotive, and construction applications due to their exceptional strength-to-weight ratio and structural efficiency. However, local surface deviations, such as waviness and dents, often develop during manufacturing and operation, potentially leading to adhesion failures and delamination between the composite skin and the core. This study aims to establish acceptable defect size limits that can be corrected through technological pressing, ensuring structural integrity of composite material while minimizing the negative impact on load-bearing capacity of sandwich panels. An analytical approach was adopted to assess the stress behavior of composite skins with waviness and elliptical dent defects. The analysis was based on beam and plate theory, incorporating the effects of flexural rigidity, material anisotropy, and applied technological pressure. The Hill strength criterion was applied to define permissible defect limits, considering variations in structural criticality levels. The study determined the maximum allowable sizes for waviness and dents in composite sandwich panels, factoring in the responsibility level of the panel, expressed as the maximum stress intensity coefficient. The results show that the acceptable defect size decreases with increasing structural criticality. It was also found that forced compression of dents induces pre-stress zones within the composite skin, potentially altering its stress distribution and reducing its long-term load-bearing capacity. The proposed methodology provides a quantitative framework for evaluating acceptable defect limits, supporting manufacturing quality control and repair optimization. The results offer practical insights for enhancing the reliability and durability of composite structures, ensuring that local surface deviations remain within permissible limits without compromising structural performance.

Abstract: In this paper, the stress behaviors of the single lap joint having orthotropic inclusion in the adhesive layer under tensile load were analyzed. In these analyses, a more realistic approach was taken by considering the heterogeneous nature of the adhesive layer. In the study, there are three different orthotropic inclusion materials in the adhesive layers of the connections: glass-epoxy, graphite-epoxy and boron-epoxy. The effects of the selected inclusion material on the stresses in the connections were analyzed using the finite element method. When single lap connections are examined in terms of material, the stresses have the highest value for boron-epoxy inclusion and the lowest value for glass-epoxy inclusion Additionally, the effect of both the adherend material and the thickness of the inclusion material on the stresses was examined. According to this, in single lap connections, the highest peeling, shear and von-Mises stress values were observed on the adhesive layer when the t_o/t_i value was 0.5. The possibility of delamination near the inclusion area has been taken into account.

Abstract: Glass Fiber Reinforced Composites (GFRPs) are widely used in various industries due to their exceptional mechanical properties. To ensure their structural integrity, it is crucial to understand the behavior of GFRPs under impact loading. This study investigates the influence of impact energy distribution on damage accumulation in GFRPs laminates under low velocity impact loading. Experimental tests were conducted with different impact scenarios, including single impacts of 20 J, cumulative damage from two impacts of 10 J each, and mixed impacts of 8 J and 12 J. Through careful analysis of the experimental data, including load-time, load-displacement, and energy-time curves, the deformation and damage evolution were examined. The results revealed that the distribution of impact energy had a significant impact on damage accumulation in GFRPs. The single impact of 20 J caused substantial damage, highlighting the severity of high-energy impacts. However, cases involving cumulative damage from two impacts displayed varying damage patterns and levels. The distribution of energy between the impacts influenced the damage evolution and propagation, resulting in differences in the delaminated area and fracture process zone within the plates.

Abstract: This research paper investigates the impact of Kevlar fiber in Glass Fiber fiber-reinforced polymer (GFRP) composite laminates for aerospace applications. The study focuses on a hybrid combination of glass and Kevlar fibers developed through the Vacuum Bagging process. Low-velocity impact tests were conducted according to ASTM D7136 using a 12.7 mm hemispherical impactor at energy levels 10J and 20J with a constant mass of 4.1 kg. The study's objective is to explore the behavior of different impact scenarios resulting from symmetric stacking sequences, including damage mechanisms, peak force, displacement, and energy absorption. The influence of Kevlar on top, followed by Glass at the bottom and vice versa is studied for the different parameters. In conclusion, this study emphasizes the potential advantages of integrating Kevlar fibers into GFRP composite laminates for aerospace applications, enabling improved impact performance and facilitating the choice of appropriate configurations based on desired performance characteristics. The findings contribute to developing lightweight and robust materials for the aerospace industry, ensuring enhanced safety and efficiency for various applications.

Abstract: This article addresses the burning test conducted on two different configuration of composite sandwich panel. Same flame-retardant material core but different face sheet material. The aim is to identify the heat resistance capability of plant fiber sandwich panel with flame-retardant core compare with traditional carbon fiber sandwich. There are researches of heat resistance behavior for carbon fiber sandwich panel with flame-retardant core [1] [2], but few on plant fiber. As plant fiber has been take a role in composite field, the heat resistance behavior of flame-retardant plant fiber sandwich panel deserves further investigated [3]. The test result indicates that, plant fiber with flame-retardant core sandwich has similar heat resistance behavior in the first stage of burning test with carbon fiber sandwich. However, after burning for 40 seconds, the heat resistance of plant fiber specimen attenuated dramatically than carbon fiber specimen. In addition, the heat induced delamination of two different specimens were also observed. Hence, decided the residual mechanical properties of the burnt specimens.

Abstract: Composite structures, such as glass fiber reinforced polyether-ether-ketone (PEEK) and polyamide (PA66), usually undergo drilling operations for subsequent assembly. A typical problem with these composites is damage around the drilled surface due to a possible non-homogeneous cutting of the fibers. In this context, the delamination is evaluated after a cryogenic drilling. Thus, the objective of this paper is to determine the feasibility of cryogenic drilling considering surface damage after cryogenic machining, at hole the entry and exit. Experimental test were carried out in a machining center at a temperature close to -130 °C using liquid nitrogen, LN₂, as cooling environment. The diameter of the drill is 6 mm and the drill tip is polycrystalline diamond (PCD). The plate material is PEEK-GF30 and PA66-GF30. The delamination factor was obtained using a three-dimensional measurement device with an optical sensor and a focus-variation device. The results obtained are favorable regarding the potential use of cryogenic machining.

Abstract: In this work, an experimental test series was carried out in order to evaluate the influence of the geometry of the specimen and stacking sequence on macroscopic behavior and the failure modes. A CFRP prepreg unidirectional was used to perform the tested specimens according to ASTM D790 standard. Five main lay-up configurations have been analyzed: [0]₁₂, [±30]_3s, [±45]_3s, [±80]_3s, and [0/90]_3s subjected to flexural loading. The macroscopic behavior was followed by an MTS machine equipped with a bending fixture. A digital microscope is used to follow the microscopic failure modes during loading. These laminates exhibit a more complex behavior due to coupling effects and the combination of different failure modes. However, the most predominant damage observed is delamination accompanied with matrix cracking. Furthermore, the present work has shown a linear behavior of [0]₁₂, [±80]_3s, [0/90]_3s, and a distinctive behavior of [±45]_3s and [±30]_3s laminate under flexural loading due to its pseudo ductile behavior.

Abstract: This paper deals with the issue of detection of internal defects and failures of concrete structures with emphasis on the use of non-destructive methods. All parts of the structure that reduce its service life or bearing capacity can be considered as internal defects and failures. These are, for example, gravel nests, cracks, delamination, caverns etc. Due to the prevalence of concrete structures, the development of this part of diagnostics is very important both for the safety of their users and to reduce the economic costs of their future repairs.