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

Summary

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

Abstract

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%.

Introduction

Fiber reinforced polymer composite (FRP) is a type of high-strength material manufactured by mixing fiber reinforcement and polymer matrixes^1,2,3. It is widely used in the aerospace^4,5,6, construction^7,8, automotive⁹, and marine^10,11 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 processes^15,16,17,18. 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-static^{19,20,21,22,23} and dynamic behavior^{24,25,26,27,28}, as well as the property modification^29,30 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%.

Protocol

1. Material preparation

1. Cut eight pieces of 300 mm x 300 mm woven glass fiber fabric with scissors. Tape the cut first to prevent the fiber filaments from falling off.
   NOTE: Wear a mask and gloves to prevent finger pricking and filament inhalation when cutting the fabric. Not only the woven glass fiber fabric, but unidirectional fabric and other types of fiber, such as carbon fiber and aramid fiber, are also available.
2. Weigh out 260 g of epoxy resin and 78 g of hardener according to the mass ratio of 10:3.
   ​NOTE: The ratio of fiber fabric and resin system is recommended to be 360 g of epoxy resin system per square meter of single layer fiber fabric.

2. Fabrication process

NOTE: Figure 1 shows the schematic of fabrication of composite laminate for the hand lay-up process, which is shown in section 2.

1. WLVB
   1. Put the fabric in an oven at 60 °C for 8 h.
   2. Paste isolation film on the acrylic sheet to prevent the resin from bonding.
   3. Place the mold on the laying area.
   4. Mix the resin and hardener slowly for 5 min, and then put it into a vacuum chamber to draw out the air bubbles inside.
   5. Lay the non-porous release film on the mold and fix it with tape around it.
   6. Lay one peel ply on the non-porous release film.
   7. Pour in the epoxy resin and use a scraper to distribute the resin evenly throughout the film.
   8. Ply the first fiber fabric, roll with a naked roller to make sure the resin fully infiltrates the fabric and the bubbles are extruded, and then pour the resin, using a scraper to scrape the resin evenly.
   9. Repeat steps 2.1.7 and 2.1.8 until all the fabric has been used.
   10. Lay one peel ply on the fabric and squeeze out the air bubbles manually.
   11. Lay one perforated release film and one breather fabric successively.
   12. Place the suction channel and breathable pad on one side.
   13. Stick a circle of heat-resistant tape with an acrylic sheet outside the mold and attach the vacuum bag around with the tape to form a closed space.
   14. Turn on the vacuum pump to press 1 bar of pressure for 10 h at room temperature. Then, close the vacuum pump and maintain quiescence for 14 h.
   15. Put the laminates into the oven at 80 °C for 16 h to cure completely.
   16. Measure the fiber volume fraction using equations (1-3)^38,39.
           (1)
           (2)
           (3)
       n ​is the number of layers of the laminate, ρ_fiber is the areal density of the fiber fabric from the manufacturer, A_fiber is the area of the laminate, m_fiber is the mass of fiber fabric, m_total is the mass of the laminate, ρ_epoxy is the density of the epoxy, v_epoxy and v_total are the volumes of epoxy and laminate, respectively, and is the fiber volume fraction.
2. WL
   1. Put the fabric in an oven at 60 °C for 8 h.
   2. Mix the resin and hardener slowly for 5 min and then put it into a vacuum chamber to draw out the air bubbles inside.
   3. Lay the non-porous release film on the mold and fix it with tape around it.
   4. Lay one peel ply on the non-porous release film.
   5. Pour in the epoxy resin and use a scraper to distribute the resin evenly throughout the film.
   6. Ply the first fiber fabric, roll with a naked roller to make sure the resin fully infiltrates the fabric and the bubbles are extruded, and then pour the resin, using a scraper to scrape the resin evenly.
   7. Repeat steps 2.2.5 and 2.2.6 until all the fabric has been used.
   8. Lay one peel ply on the fabric and squeeze out the air bubbles manually.
   9. Lay one perforated release film, one breather fabric, and one non-porous release film successively.
   10. Place one aluminum plate with the same size as the fiber fabric on top.
   11. Maintain quiescence at room temperature for 14 h.
   12. Put the laminates into the oven at 80 °C for 16 h to cure completely.

3. Characterization of impact properties

NOTE: There are many methods for impact testing of composite laminates. Under low-velocity impact conditions, the commonly used method is the drop-weight impact test, while under high-velocity or ultra high-velocity impact conditions, the frequently used method is the bullet impact method. In this study, the drop-weight impact test was applied. The equipment is shown in Figure 2.

1. Cut a set of 150 mm x 100 mm samples from GFRP for an impact test, according to ASTM D7136, using a high-precision cutting machine.
2. Measure the weight and size of each sample.
3. Fix the positions of the samples using positioning nails in the centers of the samples that the impactor can contact for every test.
4. Fix the sample on the impact support fixture with four rubber tips.
5. Conduct the impact test using a drop-weight impact tower at the 10 J energy level. Turn on the drop hammer testing machine and click Connect to connect the controller with the dropdown, then click Home | Before test. Set the impact energy to 10 J and the additional mass to 2 kg. Input the measurement thickness, 2.1 mm for the WLVB samples and 2.5 mm for the WL samples, to determine the height of the impactor, and click Start to start the experiment.
6. Record the impact response data, including force, deflection, and energy history.
7. Take out the sample. Record the morphology of the sample after impact.
8. Repeat the impact test five times to ensure the repeatability of results.
9. Calculate and compare the data of samples.

4. Characterization of tensile properties

1. Cut a set of 250 mm x 25 mm samples from the laminates for a tensile test, according to ASTM D3039, using a high-precision cutting machine with a diamond cutter.
2. Measure the size of each sample with a vernier caliper.
3. Use epoxy adhesive to bond four 50 mm x 25 mm x 2 mm aluminum tabs on both ends of the sample to avoid stress concentration.
4. Spray a thin layer of white paint on the front of the sample, then spray black speckles.
5. Place the sample in the center of the clamps of the tensile testing machine and set up the image acquisition system, as shown in Figure 3.
6. To ensure the accuracy of the strain data, vary the position of the sample so that it is in the middle of the camera shooting area and in a vertical position. Additionally, adjust the camera focal length and exposure rate to ensure that the spots on the sample are clearly recorded.
7. Perform the tensile test. Click CONFIGURE TEST. Set the test speed to 0.5 mm/min. Click Specimen data. Input the measurement width and thickness of the samples. Click Start, then click Accept current position. Record the load-time data.
8. Take out the sample. Record the morphology of the sample after the tensile test.
9. Repeat the tensile test five times to ensure the repeatability of results.
10. Use digital image correlation (DIC) software to measure the nominal strain of the sample during the tensile test.
11. Click Length Scale Image to calibrate the length of the pixels, click Reference Image, and choose the first image as the reference image. Click Deformed Image and choose the remaining images as the deformed images. Click Drawing Tools | Select rectangle to select the measurement area. Click Extensometer and set the length of the extensometer to 100 mm, then click Processing | Start Correlation.
12. Divide the load by the cross-sectional area to get the nominal stress.
13. Combine the nominal strain from the DIC measurement and the nominal stress from the tensile testing machine.
14. Choose the slope of the linear segment of the strain-stress curve as the elastic modulus. Choose the peak value of the tensile force-time curve as the strength.
15. Compare the elastic modulus and the strength of samples.

Results

Table 1 shows the fiber volume fraction, average thickness, and fabrication process of the samples. The G8-WLVB and G8-WL represent the laminates consisting of 8-ply glass fabric manufactured by wet hand lay-up with and without the vacuum bag process, respectively. Obviously, with the vacuum bag assistance, laminates have an increase of 15.78% in fiber volume fraction, as well as an reduction of 16.27% in average thickness.

Strain-stress curves obtained by the tensile test of ...

Discussion

This paper focuses on the two different fabrication processes for the hand lay-up method with low cost. Therefore, two fabrication processes were selected to be carefully described in this paper, which are simpler, easier to master, lower in investment cost, and suitable for production with material modification in laboratories and small-scale factories. During the cure of laminates, high consolidation pressure plays an important role in manufacturing laminates with high quality. The adoption of the traditional WL proces...

Materials

Name	Company	Catalog Number	Comments
breather fabric	Easy composites	BR180
drop-weight impact testing machine	Instron	9340
Epoxy matrix	Axson Technologies	5015/5015
glass fiber	Weihai Guangwei Composites	W-9311
non-porous release film	Easy composites	R240
Peel ply	Sino Composite	CVP200
perforated released film	Easy composites	R120-P3
test machine	ZwickRoell	250kN
vacuum film	Easy composites	GVB200