Elongation & Yield Strength
Elongation is a term widely used with elastic materials. When a material is subjected to a tensile load, there is a tendency for the material to deform. The measure of deformation that occurs before a material breaks when subjected to tensile load is called Elongation.
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When the tensile load is applied, there is an increase in the length of the material and a uniform reduction in its cross-sectional area. Elongation of an SAE Flange material that is caused by expansion can also be due to an increase in temperature, or if both an axial force and a high temperature acts on the material at the same time. Elongation occurs at two major levels of expansion, elongation at fracture and elongation at the yield point.
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The elongation at fracture is simply the ratio of the change in length and the initial length of the material after the breaking limit has been passed. On the other hand, the yield strength is the stress the material can withstand just before surpassing its elastic limit.
1. What is Elongation at Fracture ?
This is also regarded as elongation at break or fracture strain. It is defined as the ratio of the changed length (elongated length) and the initial length after breakage of the material or specimen. It is measured in percentage (%). For example, making use of natural plant fiber, elongation at fracture expresses the capability of the material to resist changes of shape without the formation of cracks.
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This process can be determined by using tensile testing following EN ISO 527. Considering synthetic fibers as the test specimen when it is compared with natural fibers, they show better physical and mechanical properties when it undergoes elongation at fracture. But looking at specific modulus as a factor, elongation at fracture is better in natural fiber than the synthetic fiber.
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Elongation at Fracture (%) = change length ∕ initial length × 100%
2. What is Elongation at Yield Strength ?
Elongation at yield strength is regarded as the ratio between the increased length and initial length at the point of yield. It is also known as tensile elongation at yield. In an ASTM test of tensile strength, the test material is pulled from both ends. As the pulling and expansion continue, the material elongates at a uniform rate that is proportional to the rate at which the pulling or load force increases.
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Looking beyond the elastic stress limit and proportional limit, further pulling of the material in the opposite direction causes a permanent elongation of deformation of the test material. There is a point when the increase of tensile strain is not caused by an increase of stress on the material and that is when the material stretches briefly without a noticeable increase in load. The point is called the Yield point.
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Elongation at yield strength can be related to the ability of a plastic test material to resist changes of shape before the material deformation cannot be reserved.
3. How to calculate Elongation at Yield strength?
Since Elongation is defined as the deformation of a thermoset or thermoplastic material at this yield point, it is therefore calculated as the relative increase in length. It is calculated in percentage.
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Elongation = (ΔL/L) × 100%
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Where;
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ΔL = change in length
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L = initial length
4. How to measure Elongation at Yield strength?
The tensile tests method is used to measure the force required to break a specimen and the extent to which the material elongates or stretches to the point of breaking. In general, the tensile tests methods are applied methods used to measure the elongation at break of materials. Some common tensile tests methods are:
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According to ASTM D638: this is a standard test method used for the tensile properties of materials.
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According to ISO 527-1:2012: this is a method used in the determination of tensile properties. They are used based on the general principles of elongation.
5. What are the factors affecting Elongation at yield strength?
Some factors affect the elongation at a yield strength of materials, especially thermoplastics. They are:
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Orientation level of fibers: fibers that are less oriented tend to have a greater level of elongation while fibers that are more oriented tend to have lower degrees of elongation.
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The velocity of Testing: Slow testing process allows for materials to undergo polymer relaxation which leads to a higher elongation value.
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Temperature: from a general view, an increase in temperature leads to an increase in elongation materials.
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Filler content: The elongation of composite materials decreases when there is an increase in filler content.