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2 Hand layup steps (A) Building the dam structure (B) Layup the fibers and the resin (C) Placing the Teflon insert at the mid-plane of the panel; on top of two layers (D) Continue the layup on top of the Teflon insert; two more layers (E) Fiber and resin ready to be subjected to the vacuum (F) Vacuum bagging for 24 hours (G) Panels after vacuum bagging-hard (H) Final shape of panels after being cured and cut.
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![Fig.2.7 (A) Idealized flow over an airfoil [41] (B) Schematic...](profile/Nisrin-Abdelal/publication/287295419/figure/fig3/AS:368208876982283@1464799245717/A-Idealized-flow-over-an-airfoil-41-B-Schematic-illustration-of-the-multiscale_Q320.jpg)
This research describes the effect of voids on static and fatigue interlaminar fracture behavior under mode I and mode II loading of glass fiber composites for wind turbine blades. Samples with different void volume fractions in the 0.5%-7% range were obtained by varying the vacuum in the hand layup vacuum bagging process. Void content was characte...
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Composite materials have become materials of choice for wind turbine blade manufacturing due to their high specific stiffness, strength and fatigue life. Glass fiber composites are used extensively in light-weight structural components for wind turbines, aircrafts, marine craft and high performance automobile because glass fiber is inexpensive and...
Recently, woven composites are of high acceptance and demand because of their high resistance against impact loads in comparison to unidirectional composites in many fields such as aerospace industry. Although woven composites lamination quality is very delicate toward shear, cracks and delamination mainly because of interlaminar stresses. Because...
![Figure 5. Mode II –ENF static and fatigue test setup [32]](profile/Nisrin-Abdelal/publication/316845902/figure/fig3/AS:511012795551744@1498846352902/Mode-II-ENF-static-and-fatigue-test-setup-32_Q320.jpg)
This research describes the effect of voids on static and fatigue interlaminar
fracture behavior under mode I and mode II loading of glass fiber composites for
wind turbine blades. Samples with different void volume fractions in the 0.5%-7%
range were obtained by varying the vacuum in the hand layup vacuum bagging
process. Void content was characte...
Citations
... Mechanical properties decrease as the porosity content increases; for example, interlaminar shear strength values decrease by about 34% for carbon/epoxy fabric laminates when the void content increases from 0.55% up to 5.60% [3]. One of the most important properties of fiber composite that can be affected by porosity is the composite resistance to delamination, as measured by interlaminar fracture toughness. ...
... Hand lay-up vacuum bagging set up shown in Figure 1 was used to fabricate Figure 1. VB setup for fabricating the S1-HM/Epoxy composite panels [3]. samples of carbon fiber-reinforced polymer composite with three different vacuum levels, indicated High (−686 mmHg), Moderate (−330 mmHg) and Poor (0 mmHg) in order to vary porosity content since it can be adjusted to induce different porosity levels in the resulted composite material. ...
... In order to make the sample surface conductive, the fractured surface of each sample was sputtered by gold-palladium using a Denton Desk II Sputter coater. The fractographic images of the fractured surfaces of each sample were obtained at several magnification levels from 30× to 80,000× [3] [10]. ...
Fiber-reinforced polymer composite materials have become materials of choice for manufacturing application due to their high specific stiffness, strength and fatigue life, low density and thermal expansion coefficient. However, there are some types of defects such as porosity that form during the manufacturing processes of composites and alter their mechanical behavior and material properties. In his study, hand lay-up was conducted to fabricate samples of carbon fiber-reinforced polymer composites with three different vacuum levels in order to vary porosity content. Nondestructive evaluation, destructive techniques and mechanical testing were conducted. Nondestructive evaluation results showed the trend in percentages of porosity through-thickness. Serial sectioning images revealed significant details about the composite’s internal structure such as the volume, morphology and distribution of porosity. Mechanical testing results showed that porosity led to a decrease in both Mode I static interlaminar fracture toughness and Mode I cyclic strain energy release rate fatigue life. The fractographic micrographs showed that porosity content increased as the vacuum decreased, and it drew a relationship between fracture mechanisms and mechanical properties of the composite under different modes of loading as a result of the porosity effects. Finally, in order to accurately quantify porosity percentages included in the samples of different vacuum levels, a comparison was made between the parameters and percentages resulted from the nondestructive evaluation and mechanical testing and the features resulted from fractography and serial sectioning.
The effect of porosity in composite materials has been studied for years due to its deleterious effects on mechanical properties, especially matrix dominated properties. Currently there is an increasing use of composites in infrastructure worldwide, for example bridge components, residential and building structures, marine structures such as piers and docks, and large industrial chemical tanks. Most of these applications use fiberglass composites. Unfortunately, most of the published literature has focused on carbon fiber composites, in which fiber diameter and gas-fiber interactions are different than fiberglass composites. Therefore, the present study was undertaken to revisit the effect of porosity but specifically in fiberglass composites. The goal of this experimental study was to implement and evaluate various methods for creating porosity in fiberglass composites in a controlled manner in terms of obtaining repeatable void content, morphology, and location within the laminate. The various methods included using different amounts of autoclave pressure, adding a small amount of water between prepreg layers, and using dry fabric layers to starve the laminate of resin. Ultrasonic C-scan nondestructive evaluation was used to assess the quality of the cured panels, as well as optical and electron microscopy and void content measurements via resin burn-out. The cured panels were mechanically tested using the short beam shear (SBS) method. The results showed that the water spray method proved to be the best in terms of producing noticeably different levels of porosity, although the panels required drying to remove residual water after cure. The voids from all three techniques were either oval or elongated in-plane between the plies, but they were not uniformly distributed in-plane. The use of C-scan proved to be helpful for characterizing overall uniformity of each panel, although the results could not be used to directly compare void content between panels. The use of SBS testing was successful for evaluating void dominated properties in panels with high void content, although it was not very sensitive to coupons with lower void contents. Several interesting observations are offered in this manuscript of the fracture surface details and their relation to the SBS load deflection curves. Overall, it was found that the failure mechanisms were mixed mode and the voids did not serve as failure initiation sites. However, the voids participated mainly in the horizontal propagation of cracks between layers, presumably making it easier when they were intersected by a crack and reducing SBS strength.
Composite structures are often cured in an autoclave to acquire the required space grade quality. Now the industry is focusing on the out of autoclave manufacturing method which leads to more voids inside laminate with respect to those manufactured in the autoclave. In the present work, the influence of voids on microcrack formation under thermal cycling and environmental conditions was analyzed. Thermal cycle experiments were performed using liquid nitrogen and oven, followed by microscopic observation of the polished cross-section of the 90° layered plies. Cracks were monitored, counted, and measured with respect to void and void free areas. Void content was characterized using microscopic and ImageJ software was used. It was observed that the microcracks will be formed both around the voids and in void free areas. As the number of thermal cycle increases, the number of microcrack around the voids increases much faster than compared to the void free areas. Also it was observed that most of microcracks were propagated in the transverse direction. Interlaminar shear strength was measured. Results indicate that interlaminar shear strength reduces as the number of cycle rises due to the increase in the microcrack density. Finite element method was used to simulate the process. The micro, meso, and macro model were created with respect to original samples voids and positions to calculate the stress distribution and its concentration. Good agreement between experiment and simulation was observed.
