Pultrusion Technology Realizes Multi-directional Fiber Structure of Composite Pultruded Profiles

 

 

Research Background

 

I am a pultruded profile, which is lightweight, high-strength and corrosion-resistant.

 

 

But you are anisotropic and your lateral performance is too poor.

 

 

That is because the unique unidirectional pultrusion process results in a single internal fiber direction. Despite this, my application range has become increasingly wider in recent years, and I can be found in large buildings, new energy vehicles, and wind turbine blades.

 

 

But you are anisotropic and your performance is not very designable

 

 

I come in a wide variety of cross-sectional shapes and offer great design flexibility!

 

But the fibers inside you are all “single-braided”, while the inside of my “pultruded profile” is “connected in all directions”, and the performance can be designed through the multi-directional fiber structure. You have to call me “big brother”.

 

Pultruded profiles and pultruded profiles are in dispute, so we have conducted in-depth research on the mechanical properties of the two:

 

Pultruded Process

The pultruded process (Figure 1a) requires the use of multiple sets of yarn frames to cooperate. First, the innermost layer of longitudinal fibers forms the core, and then the middle layer of winding fibers is wound around the core material in a unidirectional (Figure 1c) or bidirectional (Figure 1d) manner. Finally, the outer layer of longitudinal fibers is added and the surface felt is laid. Through the above process, a pultruded profile with a multi-directional fiber structure (referred to as a “pultruded profile” in this article) can be produced.

 

Compared with the pultrusion process (Figure 1b), the main feature of the pultruded process is the winding machine, which can add one or more layers of non-longitudinal fibers inside the profile during the pultrusion process. In relevant literature at home and abroad, the maximum winding angle of existing pultruded profiles is only 55°, and the winding fiber content (i.e., the volume ratio of winding fiber to total fiber) is less than 30%.

 

This study has realized the large-angle winding process for the first time, that is, the winding angle has reached 84° and 72°, and the winding fiber content has reached a maximum of 43.4%. The winding angle is controlled by the pultrusion speed and the winding speed, so the theoretical winding angle must be less than 90°. The maximum angle that can be achieved by the current process level is about 85°, so the winding angle achieved in this study is close to the process limit.

 

 

Figure 1 Multi-directional fiber structure of pultrusion process, pultrusion process and pultrusion profile

 

Experimental Study

This study conducted mechanical property tests on four pultrusion profiles (Box-1, -2, -3 and -4) with different winding angles and different winding fiber contents, and used a pultruded profile (Box-5) with the same total fiber content as the control group. The specific information of the specimens is shown in Figure (Table) 2, where the geometric dimensions are measured values ​​and the fiber composition is provided by the manufacturer.

 

Table 1 Composition and geometric dimensions of the specimens

 

 

First, this study conducted small-scale material property tests to study the differences in basic properties between pultruded profiles and pultruded profiles at the material level. Specifically, five types of tests were conducted according to the relevant ASTM test methods, namely longitudinal tension, longitudinal compression, transverse compression, longitudinal bending and in-plane shear tests. The test results are shown in Figures 2 and 3.

 

The material property test results show that the longitudinal tensile strength and longitudinal bending strength increase with the increase of longitudinal fiber content (Box-5 is the highest, Box-2 and Box-3 are the lowest).

 

The trend of compressive strength is not clear: Box-1 and Box-5 have the highest longitudinal compressive strength, Box-4 is the lowest; Box-1 has the highest transverse compressive strength, Box-3 is the lowest. Box-4 has the highest in-plane shear strength, followed by Box-1 and Box-5, while Box-3 has the lowest in-plane shear strength.

 

On the other hand, the elastic modulus and strength of the material have similar changing trends. In summary, the relationship between the tensile, compressive, bending and shear properties of pultruded profiles and pultruded profiles and the fiber content and structure is relatively complex. Therefore, in practical applications, it is recommended to use the preferred test method to determine the material strength and modulus.

 

Figure 2 Summary of material strength test results

 

Figure 3 Summary of material elastic modulus test results

 

This study conducted four-point bending tests at the large-scale component level to study the performance differences between the tension-wrapped profile beam and the pultruded profile beam. The test results show that Box-5 has the greatest bending strength, and its maximum load in four-point bending reaches 20kN, followed by Box-1 (18.7kN), Box-4 (16.0kN), Box-2 (15.8kN), and Box-3 (15.5kN).

 

The test load-displacement curve and load-flange strain curve are shown in Figure 4, and the test results are shown in Table 2. Some of the values ​​in Table 2 are lower (such as Box-4-2, Box-4-3, and Box-5-2), which is caused by the initial defects of the components and the stress concentration at the loading point.

 

Figure 4 Test results of four-point bending test

 

Table 2 Summary of four-point bending test results

 

 

Since the winding fibers can inhibit the propagation of cracks, the pultruded profile only shows limited crack propagation when it is destroyed, while the pultruded profile expands rapidly after the crack appears, as shown in Figure 5

 

Figure 5 Comparison of failure modes

 

Theoretical Research

This paper predicts the ultimate bearing capacity of flexural members of pultruded profiles. In the experimental study, it was found that all specimens were destroyed by “web crippling”. Indeed, pultruded profiles have a low bearing capacity for the concentrated force load mode, and web crippling is one of the most typical failure modes of pultruded profiles. This paper investigates the existing design methods in the literature, selects the method closest to the failure mode of this paper, and calculates the bearing capacity of each component, as shown in Table 2 (i.e. analytical results).

 

Secondly, this paper analyzes the relationship between the basic mechanical properties of pultruded profiles (including elastic modulus and strength) and the winding angle. The trend of the elastic modulus of composite unidirectional plates with the winding angle is shown in Figure 6.

 

Among them, the longitudinal tensile modulus first decreases significantly and then increases slightly with the increase of the winding angle; the transverse tensile elastic modulus first decreases and then increases with the increase of the winding angle; the shear modulus first increases and then decreases with the increase of the winding angle, reaching the highest value at 45°.

 

It is worth pointing out that the transverse modulus can only be effectively improved after the winding angle reaches 68°, so it is recommended that the winding angle should not be lower than 75° in practical applications.

 

Figure 6 Elastic modulus vs. winding angle

 

This paper also analyzes the relationship between the longitudinal tensile strength and transverse tensile strength of the material and the winding fiber content and winding angle, and the results are shown in Figure 7. It can be seen that the longitudinal and transverse strength of the material is monotonically increasing or decreasing with the winding fiber content and angle.

 

The general trend is that the higher the winding fiber content and the larger the angle, the lower the longitudinal strength of the material and the higher the transverse strength; vice versa.

 

 

Figure 7 Longitudinal and transverse tensile strength vs. winding fiber content and angle

 

Numerical Simulation Study

In order to deeply analyze the stress state of the pultruded member during bending failure, this study used the finite element software ABAQUS to perform numerical simulation and analysis, and used the Hashin failure criterion to determine the ultimate bearing capacity of the member, as shown in Figure 8.

 

The numerical simulation results are basically higher than the test results (as shown in Table 2). This is because the finite element model removes the initial defects of the material and relieves the stress concentration under the load point, avoiding the failure mode of web buckling.

 

Through finite element simulation, it is found that the ultimate bearing capacity of the pultruded profile is higher than that of the pultruded profile, Box-1 is 32.2kN, Box-2 is 27.7kN, Box-3 is 29.6kN, Box-4 is 27.3kN, and Box-5 is 23.2kN.

 

Further comparison of the stress components in the finite element model with the ultimate strength of the material revealed that the failure of the specimens was mainly due to the longitudinal compression failure of the flange plate at the load point. At this time, the other stress components generally did not reach the strength limit, but the transverse tensile stress of the pultruded profile (Box-5) reached its material strength, proving that there was transverse tensile failure of the resin at the failure point. The pultruded profile has a higher ultimate bearing capacity due to its higher transverse strength.

 

 

Figure 8 Finite element analysis of four-point bending test

 

This paper also studies the buckling strength of the tension-wound profile through finite element numerical simulation. Figure 9 shows the first-order buckling critical bearing capacity of the tension-wound profile and the pultruded profile. It is found that a larger winding angle (84°) significantly increases the buckling load of the component, such as Box-1, Box-2, and Box-3, while a smaller winding angle will reduce the buckling load, such as Box-4.

 

Figure 9 Summary of buckling bearing capacity

 

This study further conducted a parameter analysis on the effects of winding angle and winding fiber content on four-point bending strength and critical buckling load, and the test results are shown in Figure 10. The pultruded profiles all showed a higher bending strength than the pultruded profiles, with an increase of about 12%~23%, but their bending stiffness decreased by about 4%~21%.

 

At the same time, it was found that the optimal value of the winding fiber content was about 25%, and the bending strength of the pultruded profiles with a winding fiber content exceeding 25% decreased, which is consistent with the phenomenon observed in the previous test, that is, Box-1 (winding fiber content 11.6%) has higher bending strength than the other pultruded profiles (winding fiber content 32.5%~43.4%). On the other hand, with the increase of winding angle and winding fiber content, the critical buckling load has also been greatly improved.

 

Figure 10 Parameter analysis of winding angle and winding fiber content

 

This study inevitably used empirical formulas to predict the mechanical properties of the material in order to carry out numerical simulation analysis, but the accuracy of the empirical formula is limited, resulting in certain differences between the finite element simulation results and the test results. However, it is worth pointing out that the influence trend of winding fiber content and angle on the mechanical properties of the tension-wound profile found in the parameter analysis is credible.

 

Design Method

The parameters of the tension-wound profile are varied. In order to guide engineering design, this paper proposes a design process for the tension-wound profile, as shown in Figure 11. The basic process is: first, determine the required longitudinal, transverse and shear strength of the material based on the design bending moment and design shear force; secondly, obtain the required winding angle and winding fiber content of the tension-wound profile by checking the design chart (Figure 12); then further verify whether the tension-wound profile can meet the design strength and deflection requirements. If not, adjust the winding angle and winding fiber content again until the tension-wound profile meets the design requirements.

 

Figure 11 Design flow chart and design diagram of pultruded profile

 

(Normalized Relative to Pultruded Profile with the Same Fiber Volume Fraction)

 

In order to better demonstrate the design process of pultruded profile, we give a preliminary example, as shown in the figure below (slide to view):

 

Figure 12 Design example of a pultruded profile

 

Conclusion

This paper explores the mechanical properties of pultruded profile bending members by combining experimental research, theoretical analysis, and finite element numerical simulation. The pultrusion process can not only improve the failure mode of the profile, but also bring potential mechanical performance improvements.

 

When the concentrated load action mode changes, the pultruded profile can give full play to the advantages of transverse mechanical properties compared with the pultruded profile.

 

The study found that the winding fiber content and angle are the most important design parameters. This paper recommends that the winding angle of the pultruded profile should be greater than 70°, and the larger the winding angle, the better (up to about 85°); secondly, this paper recommends that the winding fiber content should not be too large, and it is recommended to control it between 12 and 25%.

 

Finally, it is worth pointing out that from the perspective of scientific research, this paper keeps the total fiber content of the pultruded profile consistent with that of the pultruded profile, so as to carry out research by controlling variables.

 

In actual engineering applications, the longitudinal stiffness of the material should first meet the design requirements, and secondly, the comprehensive performance of the component, especially the failure mode and buckling bearing capacity, can be further improved or even improved through the pultrusion process. When the longitudinal stress mode plays an absolutely dominant role, traditional pultruded profiles can meet the design requirements.