The spacing between the layers of the carbon fiber rod is about 3.39 to 3.42A. The carbon atoms between the parallel layers are not arranged as neatly as graphite, and the layers are connected by Van der Waals force.
In general, the structure of carbon fiber rods is considered to be composed of two-dimensional orderly crystallization and voids, in which the content, size and distribution of voids have a great influence on the performance of carbon fiber rods.
When the porosity is lower than a critical value, the porosity has no significant effect on the interlaminar shear strength, flexural strength and tensile strength of carbon fiber reinforced bar composites. Some studies indicate that the critical porosity is 1%-4% which leads to the decrease of mechanical properties of materials.
When the pore volume content is in the range of 0-4%, the interlayer shear strength decreases by about 7% with the increase of pore volume content by 1%. Through the study of epoxy resin and bismaleimide resin laminates of carbon fiber rods, it is found that when the porosity exceeds 0.9%, the interlaminar shear strength begins to decrease.
The results show that the voids are mainly distributed between the fibre bundles and the interlayer interface. And the higher the pore content, the larger the pore size, and significantly reduce the interlaminar interface area in the laminate.
When the material is loaded, it is easy to destroy along the interlayer, which is also the reason why the interlayer shear strength is relatively sensitive to the porosity. In addition, the pore is a stress concentration area, and the bearing capacity is weak. When the stress is applied, the pore enlarges to form a long crack, which is destroyed.
Even if two kinds of laminates with the same porosity (using different pre-immersion methods and manufacturing methods in the same curing cycle), they also show different mechanical behavior.
The specific values of mechanical properties decrease with the increase of porosity are different, which shows that the effect of porosity on mechanical properties is discrete and repeatable. The effect of voids on the mechanical properties of composite laminates is a complex problem because of a large number of variable factors.
These factors include the shape, size and location of the voids, the mechanical properties of the fibers, matrix and interface, and static or dynamic loads.
Compared with the porosity and the ratio of pore length to width, the influence of pore size and distribution on mechanical properties is greater. It is also found that large pore size (area > 0.03mm2) has adverse effect on mechanical properties, which is attributed to the effect of pore size on crack growth in interlayer rubber-rich zone.