Elongation at Yield

Elongation at Yield is the ratio between increased length and initial length at the yield point. In an ASTM test of tensile strength, the test specimen is pulled from both ends. As the pulling progresses, the specimen bar elongates at a uniform rate. This elongation is proportionate to the rate at which the load or pulling force increases. 

Beyond the proportional limit & elastic stress limit, further pulling of the specimen in the opposite direction causes: 
 

• permanent elongation or
• deformation of the specimen

There is a point when an increase of strain is not provoked by an increase of stress on the test specimen i.e., beyond which the plastic material stretches briefly without a noticeable increase in load. This point is known as the yield point. Elongation at yield is the ability of a plastic specimen to resist changes of shape before it deforms irreversibly.

Graph depicting the typical stress vs. strain curve of plastics

Elongation at yield is the deformation of plastic material at the yield point. It is the relative increase in length. 
 

Where: 
 

• ɛ is the elongation
• ΔL is the final length
• L is the initial length

We can measure Elongation at Yield in % (% of elongation vs. initial size at yield point). It is also called tensile elongation at yield. 
 

Elongation at Yield is an important mechanical property of materials. 
 

• It measures the load a material can withstand at the yield point before breaking.
• Used in components that absorb energy by plastic deformation.

Ultimate elongation values of 100% are common for elastomers and film/packaging polyolefins. Rigid plastics, especially fiber-reinforced ones, often exhibit values under 5%. Materials that show high elongation are:
 

1. Thermoplastics with High Elongation – View Products
2. TPEs/TPVs with High Elongation – View Products
3. Rubbers with High Elongation – View Products
4. Thermosets with High Elongation – View Products

• Velocity of Testing: Slow testing allows for polymer relaxation and higher elongation values.

• Orientation Level: Fibers that are less oriented tend to exhibit greater degrees of elongation.

• Temperature: In general, the elongation increases with an increase in temperature.

• Filler Content: The elongation of composites decreases with an increase in the filler content.

Tensile tests measure the force required to break a specimen. It also determines the extent to which the specimen stretches or elongates to that breaking point.

In general, 'tensile test methods' measure the modulus of elasticity of materials. The common methods used are:
 

• ASTM D638 - Standard Test Method for Tensile Properties of Plastics
• ISO 527-1:2012 - Determination of tensile properties. General principles

These methods determine the tensile properties of plastics and plastic composites. This is done under defined conditions that can range from:
 

• pretreatment,
• temperature,
• humidity, and
• machine speed

The test specimens are in the form of a standard dumbbell shaped. 

For ASTM D638, the test speed is determined by the material specification. For ISO 527, the test speed is typically 5 or 50 mm/min for measuring strength and elongation, and 1 mm/min for measuring modulus. 

Apart from Elongation at Yield, the tensile test results can also calculate:
 

• Tensile strength at yield
• Tensile strength at break
• Young's modulus
• Tensile modulus
• Strain
• Elongation and percent elongation at break

An extensometer determines the elongation and tensile modulus. It is a device that measures the changes in the length of an object. It evaluates the stress-strain curve values.

The two main types of extensometers are contact and non-contact.
 

1. Contact extensometers are further divided into two types:
    

   • Clip-on extensometer: They can measure displacements from very small to relatively large. That is from less than 1 mm to over 100 mm. Used for applications requiring high-precision strain measurement (most ASTM-based tests). Major advantages include:
      
     • Low cost
     • Easy to use

    

   • Automated testing clip-ons: They replace digital "sensor arm" extensometers. They can be applied to the specimen automatically by a motorized system. They produce much more repeatable results than traditional clip-on devices. They measure very high extensions (up to 1000 mm) without losing any accuracy. Major advantages include:
      
     • Better linearity,
     • reduced signal noise, and
     • synchronization with the corresponding force data.
        
2. Non-contact extensometers: These devices are beginning to bring advantages for certain applications. Especially, in industries where it is impractical to use contact extensometers.

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