Research On The Stability Of Carbon Fiber Tube Axial Under Compression

The most prominent performance of carbon fiber composite materials is light weight and high strength. Compared with steel, it can be found that in addition to the general properties of steel, it also has high tensile strength, high natural vibration frequency, large internal resistance, chemical resistance and other superior properties. characteristic. Carbon fiber tubes are made of carbon fiber and resin, and are mainly used as structural parts and load-bearing parts. In this article, the editor will talk about the research on the axial compression stability of carbon fiber composite tubes.  

Carbon fiber reinforced composite materials have the advantages of light weight, high specific strength, high specific modulus, good corrosion resistance and anisotropy.

They also have good energy-absorbing properties and can be used in collision energy-absorbing parts. Composite materials are composed of a variety of materials and are anisotropic materials. Under compression, they show more complex damage patterns. Damage forms such as fiber breakage, matrix fragmentation, and interlayer delamination work together to absorb energy during the crushing process. This chapter takes thin-walled circular tubes as the research object, conducts axial crush tests, observes the damage pattern of carbon fiber composite circular tubes during the axial crushing process, obtains the displacement-load curve during the crushing process, and provides benchmark data for subsequent numerical simulations. .

The round tube structure is a good energy-absorbing structure. During the axial crushing process, the energy absorption of the composite round tube is higher than that of the rectangular tube. Therefore, this article uses the carbon fiber epoxy resin material produced by Jiangsu Boshi Carbon Fiber Technology Co., Ltd. The round tube is the research object and the axial crush test is carried out. Determine the size of the circular tube to be 38.2mm in inner diameter and 80mm in length. The different structural forms of single-layer composite materials have a great impact on the energy absorption characteristics of composite structural parts. A group of unidirectional belt-laminated circular pipes and a group of woven cloth-laminated circular pipes were selected for the test.

In this article, an electronic universal testing machine was used to conduct an axial crush test on the carbon fiber composite round tube. This test adopted a displacement loading method and set a loading speed of 3mm/min for the axial crush test. The set crush length was 45mm, which is slightly larger than the round tube. Total length 1/2. The selected carbon fiber composite round tube has one end with an external chamfer as the weak link. When the specimen is clamped before the axial crush test, the end with the chamfer is placed upward.

An axial crush test was conducted on the woven cloth round tube. It can be observed that the carbon fiber round tube failed and damaged starting from the upper end. At the beginning, the upper end of the round tube was pressed, and the fibers and matrix at the end were fractured and broken. As the pressure increased, As the head gradually moves downward, cracks appear between the carbon fiber layers and delamination failure occurs. The inner layer bends toward the inside of the circular tube, and the outer layer bends toward the outside of the circular tube, and is torn into several fiber bundles. The upper indenter continues to move downward, and the cracks in the layers and between the layers continue to expand. The inner layer “wraps” toward the middle, and the outer layer bends toward the outside, showing a bursting “flowering” shape. The entire crushing process is accompanied by a small amount of fiber and The matrix breaks and falls off.

The displacement and load values during the axial crushing process are collected through the testing equipment to obtain the displacement-load curve. Taking the displacement-load curve of specimen 1 as an example, the initial stage is the elastic stage. As the crushing distance increases, the output load gradually increases. , has an approximately linear relationship with the crushing distance. When the crushing force reaches the maximum value of 31.1kN, which is the peak value of the displacement-load curve, then the load value drops rapidly to 20.4kN. Then the load value increases and produces small fluctuations. The carbon fiber composite round tube undergoes stable deformation. Progressive failure, the average load is 21.7kN.

Under exactly the same test conditions, the axial crush test was conducted on specimens 4, 5, and 6 with unidirectional round tubes. It can be observed that with the downward movement of the upper indenter, in the initial stage, the weak spots containing chamfers At one end, that is, the upper end of the round tube, there is obvious matrix fragmentation, accompanied by a small amount of fiber breakage. During the axial crush test process of the unidirectional belt circular tube, it can be clearly observed in the initial stage that the carbon fibers in the outer layer are torn into several carbon fiber bundles due to the fracture of the matrix. At the same time, the matrix between the inner layer of carbon fiber and the outer layer of carbon fiber that make up the circular tube failed, and interlayer cracks appeared. As the upper indenter continues to move downward, the cracks within the layer and the cracks between the layers continue to expand. The inner layer “wraps” towards the middle and the outer layer bends outward, showing an explosion “blossoming” shape.

The displacement and load values during the axial crushing process are also collected through the testing equipment to obtain the displacement-load curve. Taking the displacement-load curve of specimen 5 as an example, the initial stage is the elastic stage. As the crushing distance increases, the output load gradually increases. The rise has an approximately linear relationship with the crushing distance. When the crushing force reached the maximum value of 44.7kN, the crushing load dropped rapidly to 41kN, and then the load value increased with small fluctuations. The carbon fiber composite round tube suffered stable progressive damage, and the average load is 34.3kN.

To sum up, when the geometry of carbon fiber composite circular pipes is similar and the axial crushing conditions are exactly the same, the unidirectional belted circular pipe has a larger average crushing load and specific energy absorption. During the crushing process It absorbs more energy, and the corresponding unidirectional round pipe also has a larger crushing load peak and a larger initial impact force. The ratio of the load average to the load peak is another important evaluation parameter to measure the energy absorption characteristics of structural parts. The closer the ratio is to 1, it means that the load peak is close to the load average, and the energy absorption characteristics are better. The ratio of woven cloth round tube is 0.69, and the ratio of unidirectional tape round tube is 0.77, which is closer to 1.

In order to study the axial compression stability of carbon fiber composite pipe, China Be-cu Prototype Material Technology Co., Ltd. analyzed the effect of carbon fiber pipe with different thicknesses, different layup aspects, layup sequence and component slenderness ratio of CFRP-aluminum through experiments and finite element analysis. The influence of alloy composite tube buckling capacity, and the following conclusions are drawn:  

The buckling load increases with the increase of the layer thickness.When the layup angle is 0°, the buckling load is the largest and the smallest when it is 90°.The layup sequence has little effect on the buckling load.  

When the slenderness ratio is greater than 60, the pure carbon fiber composite tube, pure aluminum alloy tube, and composite tube all undergo elastic buckling; when the slenderness ratio is less than 60, the pure carbon fiber composite tube undergoes elastic buckling, and the pure aluminum alloy tube undergoes elastic buckling. Plastic buckling, while the composite tube is between pure carbon fiber tube and pure aluminum alloy tube , with the increase of the number of layers of carbon fiber composites, the stability coefficient increases continuously.  

As a well-known manufacturer of China Be-cu Prototype Material Technology, we use the rolling process to produce carbon fiber tubes with a maximum length of 3.1m and a maximum diameter of 200mm, with high strength and good stability. In addition, for customers who need large-diameter pipes, other pipe forming methods can theoretically produce 500mm diameter pipes, which can basically meet the current industrial demand for carbon fiber pipes.

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What Is Carbon Fiber?Carbon fiber is made of polyacrylonitrile (PAN) (or pitch, viscose) and other organic fibers by carbonization (removal of most elements except carbon) by pyrolysis method under inert gas at high temperature above 1,000 °C. Inorganic polymer fibers with a carbon content of more than 90%.