Bending Failure Behavior of the Glass Fiber Reinforced Composite I-Beams Formed by a Novel Bending Pultrusion Processing Technique

2.1 Materials

The applied material is a kind of I-shaped cross-section fiber-reinforced composite bending pultrusion profiles. Epoxy resin is selected as the matrix, and the reinforcement is made of glass fibers. Moreover, the glass fibers are arranged axially in parallel inside the profile.

2.2 Material processing technology

The composite materials were prepared by bending pultrusion molding process. As shown in Figure 1, the yarns on the creels 1 and 2 pass through the yarn guide, the dehumidifying device, the roller, the comprehensive reaction mold, the traction device, and the cutting device in sequence, and can be processed into the bending pultruded composite I-beam.

Figure 1

The bending pultrusion processing technique.

The entire preparation process was completed in an open pultrusion machine. First, each fiber was controlled by the tension creel, the tensile load was controlled to 0.5 kN, and the yarns were released at an uniform speed. Second, after the fibers were led out from the creels, they entered the dehumidification device to reduce the moisture content, and the dehumidification temperature was controlled at 60°C. The dehumidified fibers passed through the threading device and entered the dipping tank. After the fibers were fully infiltrated in the dipping tank, they entered the pultrusion mold. After that, the mixture of fibers and resin entered the preheating zone (125°C) for heating, then entered the gel zone (165°C), where the resin began to undergo an exothermic reaction, changing from a viscous state to a gel state; then it entered the curing zone (180°C), where the resin got cured and shrank. Because of the release agent, the solid mixture of fibers and resin no longer adhered to the inside of the mold. Finally, the traction device pulled the cured profile out and cut it into testing pieces, as shown in Figure 2. For the composite I-beam testing specimens, the fiber volume content was approximately 55%. Besides, the number of testing specimens was 5, and the bending performance tests were performed.

Figure 2

The glass fiber reinforced composite I-beam specimen (A) sizes of the I-beam (mm), (B) I-shaped cross-section, and (C) front view of the I-beam.

2.3 Three-point bending tests

According to the testing standard of GBT 13096-2008, the INSTRON^®-8801 mechanical tester was used to perform the three-point bending tests, as shown in Figure 3. Furthermore, the upper loading indenter was set to contact with the upper surface of the testing specimen. There were two supporting points on the lower surface of the specimen with a span of 66 mm, which were symmetrical with each other, and the loading indenter's speed was 10 mm/min. When measuring the bending strength of the specimen, the load was applied continuously until the composite was damaged and could no longer bear the applied load, and then the test was terminated.

Figure 3

Three-point bending testing (A) experimental setup and (B) schematic diagram.

Further, to explore the failure mode of the damaged composite I-beams, the ZEISS Xradia610/Versa610 X-ray three-dimensional microscope was used.

3.1 Bending stress-deflection curves

During the three-point bending testing process, as the indenter moves slowly downwards at a uniform speed, the composite specimen will change mainly in two aspects. On the one hand, it is manifested in the externally visible deformation, i.e., the accumulation of deformation; on the other hand, it is manifested in the internal stress, i.e., the change of stress reflects the status of structural failure. Figure 4 shows the bending stress-deflection curves.

Figure 4

Bending stress-deflection curves.

By investigating the stress status shown in Figure 4, it can be found that the stress gradually linearly increases after the specimen is bent and deformed under compression. As the deformation gradually increases during the loading process, a larger load is needed for the specimen to generate the continuous bending deformation. In particular, when the structural stress reaches the failure stress threshold of the resin matrix, a small yield occurs. At this moment, the structural failure has begun to occur. It is manifested in the corresponding curves that the stress has a buffer zone and the rate of increase slows down. However, the stress is still gradually increasing. The highest value occurs when the stress reaches the failure threshold of the fibers. After that, the regional failure of resin and fibers continued to occur, which is manifested as small amplitude oscillations in stress. At this moment, the composite had failed and could no longer bear external loads.

3.2 Damage mode

By observing the failure morphologies of the tested composite I-beam specimens, it is found that the specimens have the similar fracture failure mode, as shown in Figure 5. Besides, it indicates that the central part of the upper layer (top flange) of the testing specimen is significantly damaged due to the direct load bearing. The cracks mainly extend vertically downwards from the direct loading point and simultaneously propagate along the length direction of the upper surface of the specimen, i.e., expanding along the length direction of fibers to the end face.

Figure 5

The failure morphologies of the tested composite I-beam specimens.

Moreover, to further explore the specific failure mode of the composite T-beam subjected to three-point bending load, Figure 6 shows the typical final failure mode and the microscopic images. Note that when the vertically downward cracks reach the I-shaped neck position, i.e., the junction area between the top flange and web, the lateral bifurcation occurs, and the resulting secondary cracks further propagate in two directions, and thus the fracture failure occurs.

Figure 6

The typical failure morphology of the damaged composite I-beam. (A) and (B) the damaged specimen #2, (C) and (D) microscopic images of the significant locally damage at the junction area between the top flange and web (specimen #2).

Therefore, the typical failure mode of the composite I-beam subjected to the continuous bending stress is that the upper layer cracks mainly propagate along the length direction of fibers and simultaneously propagate downward along the vertical direction. Moreover, the bifurcation cracks occur at the junction area between the top flange and web, and the structure of bottom flange is slightly damaged. The reason for the occurrence of this phenomenon is closely related to the arrangement of fibers in the specimen, i.e., there is no binding effect of fibers through the thickness direction between the abovementioned three parts (top flange, web, and bottom flange), which causes the cracks to easily germinate and propagate.

3.3 Damage mechanisms

As mentioned above, the structure of the composite I-beam is obviously damaged along the load direction under the three-point bending loading condition. The main failure modes are the cracking along the length direction of the fibers and the vertical direction from the top flange to the web.

The main failure mechanisms of the glass fiber resin matrix composite I-beam under three-point bending loading are cracking germination, cracking propagation, and the final fracture. First of all, during the loading process, as the failure stress of the matrix is significantly smaller than that of the glass fiber, when the matrix at the direct loading part reaches the stress threshold, the formation of matrix micro-cracks indicates the beginning of the initial stage of progressive damage. Then, as the composite is further stressed, the cracks propagate in the internal structure of the composite, i.e., the cracks further propagate along the length direction of fibers as well as the vertical direction. Finally, when the downwardly extending vertical cracks reach the junction area between the top flange and web, the lateral bifurcation occurs, and the resulting secondary cracks further propagate in two directions. Thus the ultimate fracture failure of the composite I-beam will occur in a short period of time.

3.4 The suggestions for improving bending resistance performance

Under bending load, the resin cracks and fiber fractures mainly appear on the top flange of the composite I-beam. Because the fibers are distributed at 0°, the cracks propagate along the fiber direction to form axial cracks, while the cracks also propagate downward to generate longitudinal cracks. Furthermore, the cracks propagate in the junction area of top flange and web, forming multiple main cracks to expand separately until the ultimate fracture failure occurs.

Generally speaking, the top flange is the key area for the bending-induced damage of composite I-beam, whereas there is less visible damage on the web and bottom flange. When designing the composite I-beams to resist three-point bending loads, in addition to selecting the high-performance fibers and resin materials with excellent mechanical properties, the structural optimization for flanges should also be well considered. For example, the three-dimensional fabrics can be used as the reinforcing phase. This proposed structure can effectively prevent the generation and further propagation of cracks. In addition, the junction area between the top flange and web can be locally strengthened to avoid or slow down the propagation of lateral cracks.