﻿fabrication of glass fiber-reinforced polymer (gfrp) composite laminates by wet hand lay-up/vacuum bag (wlvb) method

Fabrication and Mechanical Property Assessment of Fiber Composite Laminates

Fiber reinforced polymer composite (FRP) is a type of high-strength material manufactured by mixing fiber reinforcement and polymer matrixes1,2,3. It is widely used in the aerospace4,5,6, construction7,8, automotive9, and marine10,11 industries due to its low density, high specific stiffness and strength, fatigue properties, and excellent corrosion resistance. Common synthetic fibers include carbon fibers, glass fibers, and aramid fibers12. Glass fiber was chosen for investigation in this paper. Compared to traditional steel, glass fiber reinforcement composite laminates (GFRPs) are lighter, with less than one-third of the density, but can achieve a higher specific strength than steel.
The preparation process of FRP includes vacuum-assisted resin transfer molding (VARTM)13, filament winding (FW)14, and prepreg molding, in addition to many other advanced fabrication processes15,16,17,18. Compared to other preparation processes, the wet hand lay-up/vacuum bag (WLVB) process has several advantages, including simple equipment requirements and uncomplicated process technology, and the products are not limited by size and shape. This process has a high degree of freedom and can be integrated with metal, wood, plastic, or foam.
The principle of the WLVB process is to apply greater forming pressure through vacuum bags to enhance the mechanical properties of the prepared laminates; the production technology of this process is easy to master, making it an economical and simple composite material preparation process. This process is completely manual, and it is greatly influenced by the skills of preparation personnel. Therefore, the products are prone to defects such as voids and uneven thickness, leading to unstable qualities and mechanical properties of the laminate. Hence, it is necessary to describe the WLVB process in detail, finely control steps, and quantify material proportion, in order to obtain a high stability of mechanical properties of laminates.
Most researchers have studied the quasi-static19,20,21,22,23 and dynamic behavior24,25,26,27,28, as well as the property modification29,30 of composite materials. The volume fraction ratio of fiber to matrix plays a crucial role in mechanical properties of FRP laminate. In an appropriate range, a higher volume fraction of fiber can improve the strength and stiffness of FRP laminate. Andrew et al.31 investigated the effect of fiber volume fraction on the mechanical properties of specimens prepared by the fused deposition modeling (FDM) additive manufacturing process. The results showed that when the fiber volume fraction was 22.5%, the tensile strength efficiency reached its maximum, and a slight improvement in strength was observed as the fiber volume fraction reached 33%. Khalid et al.32 studied the mechanical properties of continuous carbon fiber (CF)-reinforced 3D-printed composites with diverse fiber volume fractions, and the results showed that both tensile strength and stiffness were improved with the rise in fiber content. Uzay et al.33 investigated the effects of three fabrication methods-hand lay-up, compression molding, and vacuum bagging-on the mechanical properties of carbon fiber-reinforced polymer (CFRP). The fiber volume fraction and void of the laminates were measured, tensile and bending tests were conducted. The experiments showed that the higher the fiber volume fraction, the better the mechanical properties.
Voids are one of the most common defects in FRP laminate. Voids reduce the mechanical properties of composite materials, such as strength, stiffness, and fatigue resistance34. The stress concentration generated around the voids promotes the propagation of micro-cracks and reduces the interface strength between reinforcement and matrix. Internal voids also accelerate the moisture absorption of FRP laminate, resulting in interface debonding and performance degradation. Therefore, the existence of internal voids affects the reliability of composite and restricts their wide application. Zhu et al.35 investigated the influence of void content on the static interlaminar shear strength properties of CFRP composite laminates, and found that a 1% increase in void content ranging from 0.4% to 4.6% led to a 2.4% deterioration in interlaminar shear strength. Scott et al.36 presented the effect of voids on damage mechanism in CFRP composite laminates under hydrostatic loading using computed tomography (CT), and found that the number of voids is 2.6-5 times the number of randomly distributed cracks.
High-quality and reliable FRP laminates can be manufactured by using an autoclave. Abraham et al.37 manufactured low-porosity, high-fiber content laminates by placing a WLVB assembly in an autoclave with a pressure of 1.2 MPa for curing. Nevertheless, the autoclave is a large and expensive piece of equipment, resulting in considerable manufacturing costs. Although the vacuum-assisted resin transfer process (VARTM) has been in use for a long time, it has a limit in terms of the time cost, a more complicated preparation process, and more disposable consumables such as diversion tubes and diversion media. Compared with the WL process, the WLVB process compensates for insufficient molding pressure through a low-cost vacuum bag, absorbing excess resin from the system to increase the fiber volume fraction and reduce the internal pore content, thereby greatly improving the mechanical properties of the laminate.
This study explores the differences between the WL process and the WLVB process, and details the meticulous process of the WLVB process. The fiber volume content of laminates was calculated by the formula method, and the results showed that the fiber volume content of WL laminates was 42.04%, while that of WLVB laminates was 57.82%, increasing by 15.78%. The mechanical properties of laminates were characterized by tensile and impact tests. The experimental results revealed that with the WLVB process, the strength and modulus of the laminates were enhanced by 17.4% and 16.35%, respectively, and the specific absorbed energy was increased by 19.48%.

Author Spotlight: Enhancing Fiber Composite Laminate Quality with the Wet Hand Lay-Up/Vacuum Bag Process 

Measuring the Mechanical Properties of Glass Fiber Reinforcement Polymer Composite Laminates Obtained by Different Fabrication Processes

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Fabrication of Glass Fiber-Reinforced Polymer (GFRP) Composite Laminates by Wet Hand Lay-Up/Vacuum Bag (WLVB) Method  

﻿Fabrication of Glass Fiber-Reinforced Polymer (GFRP) Composite Laminates by Wet Hand Lay-Up/Vacuum Bag (WLVB) Method

Begin by heating the glass fiber fabric at 60 degrees Celsius for eight hours. Then, paste an isolation film on the acrylic sheet to prevent the resin from bonding. Position the mold in the laying area.Gently mix the resin and hardener for five minutes before placing it in a vacuum chamber to remove air bubbles. Place a non-porous release film on the mold and secure it with tape. Then, lay one Peel Ply on the non-porous release film.Pour epoxy resin on the film and evenly distribute it throughout using a scraper. Next, ply the first fiber fabric on the resin. Roll the fabric with a naked roller to ensure bubble removal and complete resin infiltration into the fabric.Again, pour and evenly spread the resin on the fabric using a scraper and continue till all the fabric has been used. Place a Peel Ply on the fabric and manually remove air bubbles by scraping in one direction. Lay a perforated release film followed by a breather fabric.Position the suction channel and breathable pad to one side. Secure a circular heat-resistant tape with an acrylic sheet outside the mold and attach the vacuum bag at the top of the device to create a closed space. Apply vacuum for 10 hours at room temperature, then switch off the vacuum pump and maintain quiescence for 14 hours.Cure the laminates completely at 80 degrees Celsius for 16 hours before measuring the fiber volume fraction. Comparison of laminates fabricated by the wet hand layup method with and without the vacuum bag process revealed that the vacuum bag assistance, the fiber volume fraction of the laminates increased by 15.78%and their average thickness reduced to 2.11 millimeters.

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The traditional wet hand lay-up process (WL) has been widely applied in the manufacturing of fiber composite laminates. However, due insufficiency in the forming pressure, the mass fraction of fiber is reduced and lots of air bubbles are trapped inside, resulting in low-quality laminates (low stiffness and strength). The wet hand lay-up/vacuum bag (WLVB) process for the fabrication of composite laminates is based on the traditional wet hand lay-up process, using a vacuum bag to remove air bubbles and provide pressure, and then carrying out the heating and curing process. 
Compared with the traditional hand lay-up process, laminates manufactured by the WLVB process show superior mechanical properties, including better strength and stiffness, higher fiber volume fraction, and lower void volume fraction, which are all benefits for composite laminates. This process is completely manual, and it is greatly influenced by the skills of the preparation personnel. Therefore, the products are prone to defects such as voids and uneven thickness, leading to unstable qualities and mechanical properties of the laminate. Hence, it is necessary to finely describe the WLVB process, finely control steps, and quantify material ratios, in order to ensure the mechanical properties of laminates. 
This paper describes the meticulous process of the WLVB process for preparing woven plain patterned glass fiber reinforcement composite laminates (GFRPs). The fiber volume content of laminates was calculated using the formula method, and the calculated results showed that the fiber volume content of WL laminates was 42.04%, while that of WLVB laminates was 57.82%, increasing by 15.78%. The mechanical properties of the laminates were characterized using tensile and impact tests. The experimental results revealed that with the WLVB process, the strength and modulus of the laminates were enhanced by 17.4% and 16.35%, respectively, and the specific absorbed energy was increased by 19.48%.

This paper describes a fabrication process for fiber-reinforced polymer matrix composite laminates obtained using the wet hand lay-up/vacuum bag method.

The traditional wet hand lay-up process (WL) has been widely applied in the manufacturing of fiber composite laminates. However, due insufficiency in the ...

Impact Property Characterization of Glass Fiber-Reinforced Polymer (GFRP) Composite Laminates

To begin the impact test of glass fiber reinforced polymer or GFRP composite laminates. Cut a set of 150 millimeter by 100 millimeter GFRP samples using a high precision cutting machine. Record the weight and size of each sample.Then mark the positions of the samples using positioning nails in their centers that the impactor can contact for every test. Fix the sample onto the impact support fixture using four rubber tips. Power on the drop hammer testing machine to conduct the impact test using a drop weight impact tower.After connecting it to the controller click home to do the beam reset and then click before test. Input the measurement thickness as 2.1 or 2.5 millimeters for WLVB or WL samples respectively. Set the additional mass to two kilograms and the impact energy to 10 joules.Click start to initiate the experiment. Record the impact response data such as force, deflection, and energy history. Remove the sample and document its morphology after the impact.Finally, calculate and compare the data of each sample. Great repeatability of the impact test was evident from the obtained force and absorbed energy curves of samples fabricated by WLVB and WL methods. The force time curve represented a typical non-piercing curve having a sine wave-like shape.The absorbed energy value increased first with the laminate absorbing and converting the impactor's kinetic energy into its internal energy. And then the value decreased over time as the laminate released elastic energy to rebound the impactor. The specific absorbed energy of the WLVB sample was larger than that of the WL sample.The WLVB laminates absorbed 19.48%more specific energy compared to the WL ones. Further, the higher impact energy absorption capacity of WLVB samples was confirmed by their relatively smaller damaged area.