Strength at Yield (Tensile)

Mechanical Properties of Plastics

What is tensile strength? Tensile strength is an ability of plastic material to withstand maximum amount of tensile stress while being pulled or stretched without failure. It is the point when a material goes from elastic to plastic deformation.

Elastic deformation – When the stress is removed, the material returns to the dimension it had before the load was applied. Valid for small strains (except the case of rubbers). Deformation is reversible, non-permanent

Plastic deformation – When the stress is removed, the material does not return to its previous dimension but there is a permanent, irreversible deformation.

Tensile strength is often referred to as ultimate tensile strength and is measured in units of force per cross-sectional area.

There are three types of tensile strength:

Yield strength (A) – The stress a material can withstand without permanent deformation

Ultimate strength (B) – The maximum stress a material can withstand

Breaking strength (C) – The stress coordinate on the stress-strain curve at the point of rupture

In other words, materials first deform elastically – when you release the stress they return to their original shape. Then with more force they deform plastically, this is yield – when you release the stress they have permanently been stretched into a new shape. Finally, they break; this is ultimately tensile stress, or breaking point.

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

Hence, it is one of the important mechanical properties for:

Material evaluation

Quality control

Structure design

Modeling, and

Failure analysis

Check out more on Tensile strength at yield:

» Difference between tensile strength and yield strength
» How to measure tensile properties of plastics?
» Significance of tensile properties and factors affecting tensile strength of plastics
» Strength at Yield (Tensile) – Property values for several plastics

Yield Strength vs. Tensile Strength

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

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 was completely linear

Typical Stress-Strain Curve

Stress-Strain Curve

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

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.

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

ε (strain) = ΔL/L₀; ΔL = L-L₀

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

Units to Measure Tensile Strength

In the International System, the unit of Tensile Strength is the pascal (Pa) (or megapascals, MPa or even GPa, megapascals), which is equivalent to newtons per square meter (N/m²).

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

NOTE: In engineering, strength and stiffness are concepts which are confuse often. For the right material classification, read about “Stiffness” here.

Stress-Strain Plots for a Typical Elastomer, Flexible Plastics, Rigid Plastic, and Fiber
(Source: Principles of Polymerization, Fourth Edition, George Odian)

How to Measure Tensile Properties of Plastics?

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 – Standard Test Method for Tensile Properties of Plastics

ISO 527-1:2012 – Determination of tensile properties. General principles

Of course, there exists several other methods as well as listed below, but they are not discussed here.

ASTM D638 and ISO 527 Test Methods

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.

The methods are used to investigate the tensile behavior of the test specimens.

Watch this Interesting Video on Micro Tensile Strength Test of Plastics per ASTM D638

Source: ADMET

And, the following calculations can be made from tensile test results:

Tensile strength (at yield and at break)

Tensile modulus

Strain

Elongation and percent elongation at yield

Elongation and percent elongation at break

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

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.

Significance of Tensile Properties

Tensile properties provide useful data for plastics engineering design purposes.

Tensile properties frequently are included in material specifications to ensure quality.

Tensile properties often are measured during development of new materials and processes, so that different materials and processes can be compared.

Finally, tensile properties often are used to predict the behavior of a material under forms of loading other than uniaxial tension.

Factors Affecting Tensile Strength of Plastics

The strength of polymers is further governed by their:

Molecular Weight: The strength of the polymer rises with 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, responsible for low strength, although crystallinity is present

At higher molecular weight polymer – The polymer chains become large and hence are crosslinked, giving strength to the polymer

Cross-linking: The cross-linking restricts the motion of the chains and increases the strength of the polymer.

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

Other than that velocity of testing, orientation level of fibers, temperature, filler content etc. also impact tensile strength values of thermoplastics.