The tensile strength test is simple and easy to operate, and the sample preparation is convenient, so it is one of the most commonly used tests in the mechanical properties of materials. Elastic deformation, plastic deformation, fracture and other stages in the tensile test can truly reflect the whole process of material resistance to external forces. Therefore, the tensile test is of great reference value for the testing of metal materials, rubber materials and plastic materials. There are many indicators that can be tested in test tensile properties of plastics. but the summary of two points is actually the material strength and plasticity data. The key indicators of these two points are tensile strength and elongation at break. Today we will learn more about these two indicators.

    I、 tensile strength, elongation at break of the respective definitions 1, tensile strength is the maximum uniform plastic deformation of the material stress. In the tensile test, the tensile strength of the sample until fracture of the maximum tensile stress is the tensile strength. 2, elongation at break is expressed as a percentage (%), usually refers to the ratio of the displacement of the sample to the original length at break

    II、the difference between elongation at break and elongation The stretching process of a material usually involves a plastic deformation phase, where plastic deformation occurs after the yield point and fracture occurs after the breaking point is reached. Therefore, elongation at break is usually the elongation of the entire process, while elongation is usually just the percentage of elongation at the stage where plastic deformation occurs.

    III、tensile strength, elongation at break testing considerations 1, the sample length of the tensile test: the longer the length, the greater the chance of weak rings, the lower the strength. Because the strength along the length of the fiber is not uniform, so the fiber is always broken at the weakest point. The longer the sample, the greater the probability of the thinnest weak-loop knot, the greater the possibility of damage, and the strength decreases 2, the number of samples in the tensile test: the more the number of roots, the lower the strength of a single fiber. Because the more the number of fibers in the bundle, the lower the average single fiber strength calculated from the strength of the bundle, and the average strength is lower than a single measurement. 3, the tensile speed of the tensile test: the greater the speed, the greater the strength and the greater the initial modulus. Under normal conditions, as the tensile speed increases, the strength at break, initial modulus and yield stress increase, and there is no regularity in the elongation at break.

    IV、the factors affecting the tensile properties of fibers (a) the impact of internal structure on tensile strength 1, macromolecular structure (macromolecular flexibility, macromolecular polymerization): fiber fracture depends on the relative slip of macromolecules and molecular chain fracture. The smaller the average polymerization degree of macromolecules, the smaller the bonding force of macromolecules, the easier the slip, the lower the fiber strength, the greater the elongation; on the contrary, the larger the average polymerization degree of macromolecules, the greater the bonding force of macromolecules, the less the possible slip, so the higher the fiber strength, the smaller the elongation rate. 2, molecular structure (orientation, crystallinity): the higher the orientation, the more parallel the arrangement of macromolecules, the more macromolecules subjected to stress during the stretching process, the larger the fiber, the greater the strength, the smaller the elongation at break. Cracked hole defects, morphological structure and inhomogeneity in the fiber lead to a reduction in strength. (ii) The effect of external environment on tensile strength Temperature and humidity: The temperature and humidity of the air affect the temperature and humidity of the fiber as well as the moisture return, which affects the strength of the fiber. The effect of temperature on various fibers is not the same, but they all have a general rule: under the conditions of high fiber moisture return, high temperature, and high thermal energy of fiber macromolecules, the flexibility of macromolecules is improved, the bond strength between the molecules is weakened, fiber strength is reduced, elongation at break is increased, and tensile modulus is reduced. Most fibers increase with relative humidity, the moisture content in the fiber increases, the weaker the intermolecular bonding, the looser the crystalline zone, and therefore the fiber strength decreases, the elongation increases, and the initial modulus decreases. However, the strength at break and elongation at break of natural cellulose wool and hemp increase with increasing relative humidity. Among the chemical fibers, polyester and polypropylene are essentially non-hygroscopic and their strength and elongation are hardly affected by relative humidity. The effect of relative humidity on fiber strength and elongation varies according to the strength of each hygroscopic property. The greater the hygroscopic capacity, the more significant the effect, and the smaller the hygroscopic capacity, the less important it is.

    V、Mechanism of tensile fracture and elongation When the fiber begins to be stressed, its deformation is mainly the fiber stretching macromolecular chain itself, that is, the bond length and bond angle deformation. Tensile curve is close to a straight line, basically in line with Hooke's law. When the external force increases further, the macromolecular chains in the amorphous region overcome the subvalent bonding forces between the molecular chains and are further stretched and oriented. At this point, part of the macromolecular chain is straightened and the tension may be pulled apart and may be irregular. Extraction of the crystal part. The breakage of subvalent bonds leads to gradual dislocation slip of the macromolecules in the amorphous region, relatively significant fiber deformation, and gradual decrease in modulus, which leads to the fiber entering the yield zone. When the dislocation-slip fiber macromolecular chains are essentially parallel in elongation, the macromolecular spacing is close and new subvalent bonds can be formed between the molecular chains. At this point, the fiber is continuously stretched and deformed mainly by the bond lengths of the molecular chains, the change of bond angles and the breakage of secondary bonds. When entering the strengthening zone, the fiber modulus increases again until it reaches the breakage of a large number of valence bonds in the fiber macromolecular backbone, which leads to fiber disintegration. Fiber breakage is caused by: breakage of the macromolecular backbone; sliding loss between macromolecules. Fiber elongation is caused by: straightening and elongation of the macromolecules (change in bond length and bond angle); improvement in orientation; and sliding between macromolecules.

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