Strength at yield (tensile)

What is tensile strength?

Tensile strength is the ability of a plastic material to withstand the maximum amount of tensile stress while being pulled or stretched without failure. It is the point at which a material transitions from elastic to plastic deformation.

• Elastic deformation: When the stress is removed, the material returns to its original dimensions. Valid for small strains (except in the case of rubbers). Deformation is reversible and non-permanent.
• Plastic deformation: When the stress is removed, the material does not return to its previous dimensions. This is a permanent and irreversible deformation.

Tensile strength is often referred to as ultimate tensile strength.

What are the types of tensile strength?

There are three types of tensile strength:
 

1. Yield strength (A): The stress a material can withstand without permanent deformation.
2. Ultimate strength (B): The maximum stress a material can withstand.
3. Breaking strength (C): The stress coordinate on the stress-strain curve at the point of rupture. Find more details about tensile strength at break.

Typical stress-strain curve representing Yield Strength (A), Ultimate Strength (B), and Breaking Strength (C)

What is the tensile strength at yield?

Materials first deform elastically when you release the stress, and they return to their original shape. Then, with more force, they deform plastically. This is yield, i.e., when you release the stress, they have permanently been stretched into a new shape. Finally, they break. This is ultimately tensile stress, or the breaking point.

Tensile strength (TS) at yield is also called tensile stress at yield. It measures the stress a plastic can withstand at the yield point, i.e., when an increase in strain is not provoked by an increase in stress. It is important for a material that is going to be stretched or under tension. 

Yield strength vs. tensile strength

Yield strength can be seen on a stress-strain curve as the point where the graph is no longer linear. Since it is quite difficult to determine an exact point where a line stops being linear, yield strength is usually the point where the value on the stress-strain curve is 0.2% off from what it would be if it were completely linear.

The main differences between yield strength and tensile strength are given below:

• Yield strength: It is the stress a material can withstand without permanent deformation or the point at which it will no longer return to its original dimensions (by 0.2% in length).
• Tensile strength: It is the maximum stress that a material can withstand while being stretched or pulled before failing or breaking.
    

 

  Stress-strain plots for a typical elastomer, flexible plastics, rigid plastic, and fiber 

(Source: Principles of Polymerization, Fourth Edition, George Odian¹)

Now that we know yield strength in detail, let's move on to understand how the stress-strain plots help interpret tensile strength.

Interpreting the stress-strain curve for tensile strength

When a stretching force (tensile force) is applied to an object, it extends, and its behavior can be obtained using a stress-strain curve in the elastic deformation region. This is known as Hooke's Law. The extension that a force produces is not only dependent on the material but also on other factors like the dimensions of the object (for example, length, thickness, etc.). 

 

What is stress?

Stress is defined as the force per unit area of plastic and has units Nm^-2 or Pa. The formula to calculate tensile stress is:   

σ (stress) = F/A

where,

• σ is stress (in Newtons per square metre or, equivalently, Pascals)
• F is force (in Newtons, commonly abbreviated N) and
• A is the cross-sectional area of the sample

What is strain?

Strain is defined as extension per unit length. Since it is a ratio of lengths, strain has no units.  

where, 

• L₀ is the original length of a bar being stretched,
• L is its length after it has been stretched,
• ΔL is the extension of the bar, the difference between these two lengths

Learn more about other mechanical properties in detail

Want to learn more about the properties of polymers? Get insights into the most important polymer properties available on our platform and master material selection.

In the next section, we will explore the factors that affect tensile strength and the areas where we can use this property.  

Which factors affect the tensile strength of plastics?

The tensile strength of plastics is influenced by several molecular and structural characteristics. They determine how polymer chains interact and respond to applied stress. These factors collectively define the material’s ability to resist deformation and failure under load. The strength of polymers is further governed by the following factors:  

Molecular weight

The strength of the polymer rises with an increase in molecular weight and reaches the saturation level at some value of the molecular weight. 

• At lower molecular weight: The polymer chains are loosely bonded by weak Van der Waals forces, and the chains can move easily. This is responsible for low strength, although crystallinity is present.
• At higher molecular weight: The polymer chains become large and hence are crosslinked, giving strength to the polymer.  

Crosslinking

The crosslinking restricts the motion of the chains and increases the strength of the polymer.

Crystallinity

The crystalline phase of the polymer increases strength. Hence, the intermolecular bonding is more significant. Therefore, the polymer deformation can result in higher strength, leading to oriented chains.

Other factors

Other factors that impact tensile strength values of thermoplastics are listed as follows:

Tensile strength of plastics

Applications of yield strength

For structural applications, the yield stress is usually a more important property than the tensile strength, since once it is passed, the structure has deformed beyond acceptable limits. The tensile stress at yield is one of the important mechanical properties. Its applications are discussed in detail below.

Material selection

Engineers use yield strength to select suitable materials for structures and parts that must withstand specific loads without permanent deformation. It helps in designing safe and functional components, such as beams, fasteners, and thin films³. 

Are you still struggling to pick the right polymer with the desired yield strength values? Our master catalog brings you 100K+ commercial plastics and elastomer grades. We are here to help you navigate our platform and ease your selection journey:

Steps to follow for quick material selection

Structural and mechanical engineering

Tensile properties provide useful data for plastics engineering design purposes. It is often measured during the development of new materials and processes, so that different materials and processes can be compared. Used to design bridges, buildings, automotive parts, aircraft structures, and machinery components. Here, yield strength ensures the handling of operational stresses safely. It defines load-bearing capacity and prevents failure under working conditions.

Quality control and analysis

Manufacturers test yield strength to ensure batch-to-batch consistency in plastics, composites, and metals. It detects defects or improper processing (for example, in heat-treated steels or molded polymers). It is frequently included in material specifications to ensure quality.

Performance prediction in end-use

It is used to predict the behavior of a material under forms of loading other than uniaxial tension. Enables failure analysis and determines how products perform under tension, impact, or bending, such as in packaging films, plastic components, or protective coatings. Aids in balancing strength versus ductility for applications requiring both toughness and formability.

Applications of tensile strength at yield

Units to measure tensile strength

Tensile strength is measured in units of force per cross-sectional area. Its unit is Newtons per square meter (N/m²). In the International System, the units of tensile strength are the following:

• Pascal (Pa)
• Megapascals (MPa), or
• Gigapascals (GPa) 

 
In the US, the commonly used units for convenience are pounds-force per square inch (lbf/in² or psi), or kilo-pounds per square inch (kpsi).

NOTE: In engineering, strength and stiffness are concepts that are often used. For the right material classification, read about stiffness here. 

 

Standards to measure tensile strength

Tensile tests measure the force required to break a specimen and the extent to which the specimen stretches or elongates to that breaking point. In general, tensile test methods are applied to measure the tensile properties of plastics. The common methods used are ASTM D638 and ISO 527-1:2012.

Both ASTM D638 and ISO 527 test methods cover the determination of the tensile properties of plastics and plastic composites under defined conditions in the form of standard dumbbell-shaped test specimens. The defined conditions can range from pretreatment, temperature, humidity, to testing machine speed.

There exist several other methods as well, but they are not discussed here.

 

ASTM D638

ASTM D638 is the standard test method for tensile properties of plastics. The test speed for ASTM D638 is determined by the material specification. 
 

ISO 527

ISO 527-1:2012 helps to determine the general principles of tensile properties. The test speed of ISO 527 is typically 5 or 50 mm/min for measuring strength and elongation, and 1 mm/min for measuring modulus.

The above-mentioned test methods are used to investigate the tensile behavior of the test specimens. An extensometer is a device that is used to measure changes in the length of an object. It is useful for stress-strain measurements and tensile tests.