Carbon fiber is an inorganic polymer fiber having a carbon content of more than 90%. The graphite fiber is contained in which the carbon content is higher than 99%. The microstructure of carbon fiber is similar to that of artificial graphite and is a layered graphite structure. The spacing between the carbon fiber layers is about 3.39 to 3.42 A. The carbon atoms between the parallel layers are not as regular as the graphite, and the layers are connected by van der Waals force.
The structure of carbon fiber is also generally considered to consist of two-dimensional ordered crystals and pores, wherein the content, size and distribution of pores have a great influence on the properties of carbon fibers.
When the porosity is below a certain critical value, the porosity has no significant effect on the interlaminar shear strength, flexural strength and tensile strength of the carbon fiber composite. Some studies have pointed out that the critical porosity that causes the mechanical properties of the material to decrease is 1%-4%. When the pore volume content is in the range of 0-4%, the interlaminar shear strength is reduced by about 7% for every 1% increase in the pore volume content. Through the study of carbon fiber epoxy resin and carbon fiber double maleimide resin laminate, it is seen that when the porosity exceeds 0.9%, the interlaminar shear strength begins to decrease. It is known from experiments that the pores are mainly distributed between the fiber bundles and at the interface between the layers. And the higher the pore content, the larger the pore size and the significantly reduced the area of the interlayer interface in the laminate. When the material is stressed, it is easy to break along the interlayer, which is also the reason why the interlaminar shear strength is relatively sensitive to the pores. In addition, the pores are stress concentration areas, and the bearing capacity is weak. When the force is applied, the pores expand to form long cracks, which are destroyed.
Even with two laminates of the same porosity (using different prepreg methods and manufacturing methods during the same curing cycle), they exhibit completely different mechanical behaviors. The specific values of mechanical properties decrease with increasing porosity show that the effect of porosity on mechanical properties is large and the repeatability is poor. The effect of voids on the mechanical properties of composite laminates is a complex problem due to the large number of variables involved. These factors include: shape, size, location of the pores; mechanical properties of the fibers, matrix and interface; static or dynamic loads.
The pore size and distribution have a greater influence on the mechanical properties than the porosity and pore aspect ratio. It is also found that large pores (area > 0.03 mm2) have an adverse effect on the mechanical properties, which is attributed to the influence of pores on the crack propagation in the interlaminar rich zone.