What Is a Tensile Bar?

Stretching refers to a method of orienting the polymer chains in a polymer along the direction of an external force to improve the structure and mechanical properties of the polymer. Stretching can be divided into two types: uniaxial stretching and biaxial stretching. The former causes the chains to be aligned in one direction, and the latter allows the chains to be aligned along a plane.

Stretching is usually above

Uniaxial stretching is improved

Stretch through

Silicon carbide particle reinforced aluminum-based composites (SiC _p / AI) have excellent comprehensive properties such as high specific strength, high specific modulus, high temperature resistance, electrical conductivity, thermal conductivity, small thermal expansion coefficient, and good dimensional stability. AI composites have poor plastic toughness, and their applications in the aerospace industry are limited. In addition to heat treatment and thermal processing, SiC _p itself has a great influence on the properties of composite materials such as size and distribution. Considerable work has been done by this researcher, but no consensus has been reached. ^[2] In the powder metallurgy method, reasonable selection of the size of SiC _p is significant for the preparation of SiC _p / AI composites with excellent performance and good processability.

The fracture behavior of different SiC _p- size reinforced aluminum matrix composites was discussed under the existing process conditions, in order to provide some basis for designing materials.

Discussion of tensile dimensional changes on tensile properties

The composite materials used in this experiment were prepared using the same process conditions, so it can be considered that the interface strength of composite materials reinforced by particles of different sizes is the same. Even if there is agglomeration of particles, no poor bonding between the particles and the matrix was found in the experimental observation. It can be considered that the interface between most particles and the matrix is good. Therefore, the load on the composite material can be effectively transferred to the reinforcing particles through the interface. Large-sized SiC _p has many crystal defects such as stack faults, and the larger the particle, the larger the interfacial area between the matrix and the particle, and the larger the load transferred to the particle through the interface, so the cleaving tendency is larger. In addition, during the plastic deformation of the composite material, it is difficult for large-size particles to flow in coordination with the matrix alloy, which is likely to cause stress concentration and cause particle cracking. Small-sized SiC _{p is} easy to plastically flow with the base alloy, and because of its small size and fewer internal defects, the tendency for cleavage is small. Under the premise of good interface bonding, crack propagation will always proceed in the base alloy until The specimen finally broke.

The above experimental results show that small-sized reinforced particles can improve the tensile strength and yield strength of composites. According to Arsenault's theoretical research, the yield strength of particle-reinforced metal matrix composites is closely related to the interaction of particles and dislocations, which is explained by the Orowan mechanism

among them,

Is the shear yield strength,

Is the critical shear yield strength, b is the Burgers vector, T is the dislocation line tension, and L is the average particle spacing. L can be calculated with the following formula

among them,

Is the volume fraction and d _m is the average particle diameter. The dislocation thread tension T is expressed by the following formula

Among them, is the shear modulus.

When the particle size is at the micron level, the mechanism by which its size affects the strength of the composite cannot be explained by the Orowan mechanism. In other words, the interaction between dislocations and SiC _p is not the main way to strengthen the particles. In the composite materials used in this experiment, the strengthening mechanism of SiC _p may be the main strengthening method through interface load transfer. Since large particles are easy to crack and may become the source of cracks, they cannot play a good strengthening role. On the other hand, at the same volume fraction, compared with small particles, large particles have a larger spacing in the base alloy, and the dislocation density caused by the difference in thermal expansion coefficient between the reinforced particles and the base alloy is lower during heat treatment, and The distance between dislocations is large, and the interactions such as cut order and plugging are weak, so the yield strength is low, but the plasticity is good. In addition, the difference in dislocation density will obviously lead to different peak aging time of the composite. In this experiment, the heat treatment process is the same for all composite materials. When the small particle reinforced composite material reaches the peak aging, the large particle reinforced composite material is still under-aged, resulting in performance differences. Finally, under the premise of good interface bonding, the latter has fewer interfaces than the former, which is why it has low strength and high plasticity. More microstructure observations are needed to prove this.

In this experiment, the 7 m SiC _p reinforcement effect is the most ideal. This may be the result of the combination of all the above factors. The 7 m particles are closer to aluminum alloy powder in size than the 3.5 m SiC _p , and the distribution may be more ideal. It avoids problems such as poor interface bonding due to particle accumulation, so the corresponding composite material strength is higher than that of 3.5 m SiC _p reinforced composite material. And because of the above reasons, the strength of the composite material is higher than that of 10 and 20 m SiC _p reinforced composite materials.

Tension study conclusions

The tensile strength of SiC _p- reinforced 2024AI-based composites decreases with increasing SiC _p size. Since the distribution of small-sized particles in the matrix alloy is not as uniform as that of large-sized particles, the plasticity increases as the particle size increases. In addition, the fracture of small-sized reinforced composites is mainly based on matrix tearing near the interface, while the large-sized reinforced composites are mainly based on cleavage and cracking of particles. ^[3]